Symposium NM4 : Nanomaterials-Based Solar Energy Conversion

New nanostructured semiconductor materials, manufactured by chemical methods are a broad class of materials that are of ever growing interest for energy device applications. Although the efficiencies of LEDs and solar cells based on such materials have been successfully improved [1,2], energy efficiencies must still increase for commercial application.

Fundamentally, all forms of energy loss do ultimately transform available free energy into heat, i.e. phonons carrying the vibrational energy. In semiconductors this happens through charge carrier processes, such as non-radiative recombination, trapping, and transport of charge carriers. In nanostructured semiconductors, we have recently shown that this coupling between electrons and phonons is generally strong due to the soft surfaces of the materials, which can lead to large non-radiative transition rates of electrons [3].

Here, we show that the phonon spectrum in nanostructured semiconductors can be systematically modified and can be used to modify the opto-electronic properties of the material. Using recent advantages in the understanding of the surface chemistry of PbS nanocrystals, we are able to gradually remove surface ligands without replacement to destabilize the nanocrystal surface and increase the strength of phonon interactions.
We quantify the changes to the phonon properties of the material directly by light absorption at THz frequencies and by determining the atomic mean square displacement through x-ray diffraction. We further probe the changes to the phonon properties indirectly by its influence on the optical properties of the semiconductor, i.e. the near infrared absorption and emission spectra and the luminescence lifetime.

Finally, we discuss how the engineering of the phonon can be used to reduce energy loss processes and improve device performances.

[1] Mashford, B. S. et al., Nature Photonics, 7, 407 (2013)
[2] Chuang, C.-H. M. et al. Nature Materials 13(8), 796 (2014)(2014)
[3] Bozyigit, D., et al. Nature 531, 618 (2016)

Alloyed metallic nanostructures represent a promising alternative to improve light absorption in solar cells, potentially removing the constrains imposed by the well defined optical response of pure metals. To date, different fabrication methods have been used to fabricate and apply metal-alloyed nanoparticles (NPs) to photovoltaic devices. Despite the success of the bottom-up nanolithography fabrication method, its high cost and limited scalability constrain their application in solid-state devices. Thus, the dewetting process of metallic thin films has been sough as straightforward route to fabricate nanoparticles. In this work, we fabricated Ag_xAu_1-x NPs with different chemical compositions by dewetting alloyed thin films and characterized their optical response at the macro- and nano-scale. We show that by varying the chemical composition of this nanoscale system, from Ag to Au, the optical response can be tuned from the visible range of the spectrum to the NIR. Moreover, using near-field scanning optical microscopy (NSOM), wavelength dependent maps of Ag_0.5Au_0.5 NPs showed a strong variation in the electric field profile within and around the nanostructures. Using finite-difference time-domain (FDTD) simulations we characterized the localized surface plasmon resonance (LSPR) and its spatial distribution, which corroborate our experimental results. The far- and near-field light-matter interactions of the alloyed nanostructures will be discussed in details. Further, we will present the potential to use Al-based alloys to increase light absorption in solar cells over a broad range of the spectrum, from UV to visible.

Microscale multijunction solar cells and concentrating optics provide a scalable path to cost effective solar energy conversion in terrestrial settings. This talk describes two recent advances in this technology. The first involves strategies for harvesting diffuse illumination, with demonstrated ability to improve the module level efficiencies by more than 2% absolute, even in geographic regions that offer the highest levels of direct normal irradiance. The second is in the development of two classes of nanoscale materials that can serve as broadband antireflection surfaces for use on planar glass substrates and on concentrating microlenses. The results enable exceptional optical throughput in telescopic concentrators with magnification factors that reach as high as 1600x. A concluding discussion outlines a pathway for combining these advances with future types of multijunction cells in modules that have the potential to reach efficiencies of greater than 50%.

Quantum dots (QDs) are very attractive materials and have been used for biological labelling, light-emitting diodes and photovoltaic devices due to their high absorption coefficients, size dependence and easy tunability of their optical and electronic properties due to quantum confinement. Furthermore, semiconductor QDs offer the possibility of multiple exciton generation,¹ which could allow them to overcome the Shockley−Queisser limit in solar cells.² Power conversion efficiency of more than 11% was achieved with QD based solar cells³ and are widely considered as a viable alternative to silicon and inorganic thin film based cells.⁴ However, typically colloidal quantum dots used for solar cells contain toxic heavy metals (Cd or Pb), which limits their industrial applications. So it is very important to explore alternative non-toxic materials, such as CuInS₂ derivatives. In our study we used “eco-friendly” quantum dots (CuInS₂ and CuInS_xSe_2-x) along with either n-type (TiO₂ or ZnO) or p-type electrodes (NiO or CuSCN nanowires) for solar cell fabrication.
One of main barriers which limits the efficiency of QD sensitized solar cells is efficient charge transfer at interface. In the classical QD sensitized solar cells the light is absorbed by the QDs, which then inject electrons into an n-type material (TiO₂ or ZnO), while the hole is regenerated by the liquid electrolyte.⁵ The main limiting step in such cells is hole transfer, which occurs slower and less efficiently than electron transfer. One way to overcome this is to invert the configuration of the cell to benefit from photoinduced hole injection from the QDs into a p-type material (NiO or CuScN nanowires). We found that hole injection rate (10⁸ s^-1) improves significantly in inverted configuration and becomes comparable to electron injection rate in conventional n-type QD based solar cells. Furthermore, this hole transfer rate becomes an order of magnitude higher for the case of CuInS_xSe_2-x compared to CuInS₂ when we used them with NiO. After optimization of electron and hole transfer separately in the presence and absence of electrolyte, we measured combined charge transfer in the presence of both electron and hole quenchers. It is known that charge recombination is one of the main factors limiting the power conversion efficiency of such devices. To overcome the recombination, we introduced Al₂O₃ passivation layer, which improves the charge transfer rates by 50%. Inverted solar cells fabricated with various QDs demonstrate high power conversion efficiencies which are 4 times higher than the best values for previous inverted QD sensitized cells.⁶
1 A. J. Nozik, et al., Chem. Rev., 2010, 110, 6873–90.
2 J. B. Sambur, et al.,, Science, 2010, 330, 63–6.
3 Zhong, X. et al. J. Am. Chem. Soc. 2016, 138, 4201.
4 P. V Kamat, J. Phys. Chem. Lett., 2013, 4, 908–918.
5 D. Aldakov , et al., J. Mater. Chem. A 2015, 3, 19050
6 J. Park, et al., J. Mater. Chem. A, 2016, 4, 827-837

Here, we propose a new n-layer platform for fabricating high performance depleted-heterojunction colloidal quantum dot (CQDs)-based solar cells. The gold-silver core-shell nanocubes (Au@Ag NCs) were embedded in low temperature-processed TiO₂ matrices (Au@Ag NCs-HL); their spectral response of plasmonic scattering was extended to near-infrared wavelength region due to the high refractive index of TiO₂, consistent with theoretical calculations. Furthermore, Au@Ag NCs-HL platforms significantly improved the carrier mobility approximately 20-fold. The improved mobility facilitated the carrier extraction from the p/n junction to electrode and reduced the recombination, enhancing the fill factor (FF) of the solar cells. With the broadband absorption enhancement and electrical improvement, the power conversion efficiency (PCE) of PbS CQD-based solar cells was enhanced from 6.9 % to 8.4 %¹.

In thermo-photovoltaics, high energy photons from a thermal source are converted to electricity. The question is what to do about the majority of low energy infrared photons? Actually, the semiconductor band-edge itself can provide excellent spectral filtering for thermo-photovoltaics, permitting efficient reflection of the unused infrared radiation back to the heat source. Exactly those low energy photons that fail to produce an electron-hole pair, are the photons that need to be recycled. This enables conversion from heat to electricity with >50% efficiency. Such a lightweight “engine” can provide power to electric cars, aerial vehicles, spacecraft, homes, and stationary power plants.

Solar thermophotovoltaics (STPVs) promise to harness the sun for efficient energy conversion while allowing for thermal energy storage for continuous operation. This approach aims to convert broadband sunlight to narrow-band thermal radiation tuned for a photovoltaic cell with a spectral converter. While STPVs have received significant interest, it has been challenging to experimentally demonstrate spectral enhancement with various absorber/emitter designs. In our work, we investigated a three-component thermally based spectral converter for our STPV device. This consisted of a carbon nanotube absorber, one-dimensional Si/SiO₂ photonic-crystal emitter, and a tandem plasma-interference optical filter. Accordingly, we were able to suppress 80% of the unconvertible photons to achieve a solar-to-electrical conversion efficiency of 6.8%. Meanwhile, we showed that the STPV device exceeded the performance of the underlying PV cell, indicating a successful enhancement strategy from the spectral conversion process. The heat generation rates in the photovoltaic cell were also reduced by 2x for the same output power densities. These results are an important step towards making STPVs a viable technology for solar power generation in the future.

Thermophotovolatics (TPVs) are a promising technology for applications in concentrated solar power and distributed combined heat and power because they are solid-state, scalable and compatible with a variety of high-temperature heat sources. To achieve high efficiency in the presence of realistic losses, it is critical to suppress radiative exchange between the emitter and the cell at energies below the bandgap of the cell (i.e., at long wavelengths)^1–4. In this work, we show how a scalable and thin-film cell design promotes spectral selectivity using a back-side surface reflector to return sub-bandgap radiation back to the hot emitter. Spectroscopy measurements confirm that the reflectance below the bandgap for these thin-film cells is significantly higher than for cells on bulk substrates. Based on the spectral properties we show that the heat engine efficiency can be significantly higher than the state-of-the-art, exceeding 30%.

1. Bierman, D. M., Lenert, A. et al. Enhanced photovoltaic energy conversion using thermally based spectral shaping. Nat. Energy 1, 16068 (2016).
2. Lenert, A. et al. A nanophotonic solar thermophotovoltaic device. Nat. Nanotechnol. 9, 126–130 (2014).
3. Lenert, A., et al. Role of spectral non-idealities in the design of solar thermophotovoltaics. Opt. Express 22, A1604 (2014).
4. Seyf, H. and Asegun, H. Thermophotovoltaics: A Potential Pathway to High Efficiency Concentrated Solar Power. Energy Environ. Sci. 9, 2654-2665 (2016).

Solar thermophotovoltaic (STPV) systems rely on an engineered nanophotonic absorber/emitter layer to convert broadband sunlight to narrow-band thermal emission for band-matching photovoltaics. Recent research has focused on absorber/emitter fabrication by nanopatterning of refractory metals based on lithography and reactive ion etching processes. However, in order to establish a scalable technology, large-area nanopatterned absorber/emitter surfaces are needed, and these surfaces must have long-term thermal stability at elevated temperatures. At high temperatures (over 1500 K), surface diffusion of refractory metals can result in structural degradation, causing loss of selectivity in thermal emission.
Carbon nanotubes (CNTs) are potentially attractive for use in TPV systems because of their high melting temperatue (exceeding 3,500 K under inert atmosphere) and surface stability, and because CNTs can be manufactured by scalable chemical vapor deposition (CVD) methods. We demonstrate the fabrication and characterization of honeycomb patterned CNT arrays (CNT ‘forests’), with 540 nm diameter cylindrical cavities and 660 nm spacing. The surface is designed using ab-initio FDTD calculations (physical parameters obtained from Lorentz-Drude model), and is fabricated by catalyst patterning using interference lithography, followed by low-pressure CVD using alcohol as the carbon source. After CNT growth, the honeycomb is coated with with Al₂O₃ and W by atomic layer deposition to form a core/shell structure. We study the thermal emission spectrum and thermal stability of the nanopatterned CNT structures, and discuss means of integration with high-efficiency STPV systems.

Nanostructured materials comprised of metals, metal oxides, and mixtures thereof hold particular promise in both the oxygen evolving reaction and the CO2 reduction reaction. Understanding continues to deepen on how materials morphology at the nanoscale influences reaction rates, and also selectivity, alongside atomic-scale effects such as metal modulators and grain boundaries. We will review some recent advances and insights into how nanoscale materials synthesis is contributing to carbon-based energy storage catalysts and solutions.

In order to solve the global energy issue, effective harvesting and converting solar energy to the form of chemical bonds is a valid way to achieve the supply of the energy consumption. [1]
Hydrogen evolution under sunlight has been focused on in the past a few years, for its wide reactants source and clean and low cost production of hydrogen. [2] Recently, in order to replace noble metal to enhance hydrogen evolution performance, a variety of transition metal oxides and hydroxides are widely used as effective co-catalyst. [3] In this work, we successfully synthesized multi shelled TiO₂-Cu_xO hollow spheres by the sequential templating method. [4-6] The unique multi-shell hollow spheres exalt light scattering within the intervals between different shells. Morever, it facilitates the separation of electrons and holes and promotes hydrogen bubbles overflowing from the surface. With the increase of number of shells, hydrogen evolution performance is also enhanced. The quadruple shell TiO₂-Cu_xO hollow spheres with closed double shells in the exterior presents the highest hydrogen evolution rate of 102mmol/h/g which is the best of TiO₂ based material. Furthermore, the recycle stability reaches 80h without obvious loss of hydrogen evolution rate. This work opens up a new method to enhance hydrogen evolution performance by structure design.
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Colloidal semiconductor nanoparticles hold great promise for efficient solar fuel generation.[1] In many systems the efficiency of photocatalytic water splitting is limited by the transfer rate of the photogenerated holes. In addition, in cadmium chalcogenides, a failure to quickly remove the hole leads to photooxidation, even in presence of a sacrificial electron donor. Here, we demonstrate a highly efficient system which achieves over 50% external and over 70% internal quantum yield of hydrogen generation using CdS nanorods decorated with nickel co-catalysts.[2] The process relies on mediated hole transfer from the semiconductor to the scavenger present in solution using a redox pair OH^*/OH^- as a shuttle. The fast transfer of the holes not only markedly increases the efficiency, but also confers long-term stability on the photocatalyst. We further analyse the electron and hole transport pathways to improve the quantum efficiency of photocatalytic H₂ production by Pt-decorated CdS nanorods to reach almost 90% [3]. We also report a CdS nanocrystalline photocatalyst with combined reduction and oxidation catalysts. We show that it is possible to efficiently transfer the hole to the chemisorbed Ru-based oxidation catalyst to produce oxygen while at the same time transferring the electron to the reduction catalyst for generation of H₂.[4] The proposed system is shown to split water without the use of sacrificial agents under visible light illumination confirming that ensuring sufficient rates of hole transport can lead to efficient and viable photocatalytic systems.

1. S.N. Habisreutinger, L. Schmidt-Mende, J.K. Stolarczyk, Angew. Chem. Int. Ed. 2013, 52, 7372-7408.
2. T. Simon, N. Bouchonville, M.J. Berr, A. Vaneski, A. Adrović, D. Volbers, R. Wyrwich, M. Döblinger, A.S. Susha, A.L. Rogach, F. Jäckel, J.K. Stolarczyk, J. Feldmann, Nature Mater. 2014, 13, 1013-1018.
3. T. Simon et al., to be submitted.
4. C.Wolff et al., to be submitted.

Nanoplasmonics is recognized to be potentially transformative in the field of heterogeneous catalysis. Localized surface plasmons (LSPs) are produced from the interaction of strongly plasmonic metal nanoparticles and light with wavelength comparable to the particles’ dimension. LSPs induce a strong local surface electric field, may decay into long-lived energetic electrons and/or holes, or may couple to phonons leading to local heating.¹ The aforementioned physical processes may greatly influence chemical processes occurring on the metal’s surface. To determine the feasibility of light-driven catalysis on metallic nanoparticles, we explore the excited-state energetics of select heterogeneously catalyzed chemical reactions, from N₂ dissociation to methane activation, via the embedded correlated wavefunction method² (e.g., multi-configurational second order perturbation theory embedded in a density-functional-derived embedding potential). Finally, we evaluate if these excited-state reaction pathways are accessible via plasmon resonance and decay.

[1] M. L. Brongersma, N. J. Halas and P. Nordlander. Nat. Nanotechnol. 10, p. 25 (2015)
[2] F. Libisch, C. Huang, and E. A. Carter, Acc. Chem. Res., 47, p. 2768 (2014).

Photoelectrochemical H₂ production through hybrid organic/inorganic interfaces exploits the capability of polymeric absorbers to drive photo-induced electron transfer to an electrocatalyst in water environment. A working photocathode was recently demonstrated exhibiting substantial performances¹ and research in organic photo-electrochemistry is now moving its first steps.
Hybrid photocathode’s design poses the need to manage simultaneously charge injection, selective contacts, bands engineering together with orthogonalization of light absorption and photogenerated carrier collection. These multiple requirements naturally drive the development of multi-layer architectures based on structured absorbers.
This work reports on the synthesis and characterization of host-guest organic photocathodes based on vertically aligned MoO₃ layered-structures. Scaffolds are synthetized using a pulsed laser deposition ballistic approach exploiting gas dynamic of the nanoclusters-inseminated supersonic jet flow field. Control over nanoclusters’ kinetic energy allows versatile synthesis of nanostructured, layered films. Resulting morphologies range from densely packed layered films to spatially separated, vertically aligned layered nano-walls. Scaffolds are conformally coated with 30 nm thick P3HT:PCBM polymers blend, a TiO₂ selective contact and a Pt electrocatalyst. The resulting architecture allows for the design of a new generation of 3D nanostructured electrodes combining optimized opto-electrical properties and structural/morphological features resulting in an hybrid photocathode showing a photocurrent density of 2 mA/cm² at +0.2 V_RHE and an onset potential of +0.65 V_RHE. Light absorption and photon/electron conversion efficiency dominate device performances especially in the case of short exciton diffusion length and thin film, absorbers like rr-P3HT in flat stacked architectures often trades-off charge transport for increasing light harvesting. ns-MoO₃ scaffolds covered with 20 nm P3HT:PCBM (plus TiO2/Pt overlayer) guest layer allows for comparable H2 evolution performances with flat devices covered with 200 nm thick flat P3HT:PCBM (plus TiO₂/Pt overlayer). Photons trapping enhancement originating by internal multi reflection/refraction of the incident light owing to the nanostructured 2D nano-walls morphology was investigated by means of femtosecond-laser spectroscopy. More importantly, nanostructured photocathodes demonstrated a performance improvement with respect to flat systems suggesting that out-of-plane films structuration may disclose potential for effective devices architectures beyond the stacked layers approach.
^[1] F. Fumagalli, J. Mat. Chem. A 2015, 10.1039/C5TA09330A.

The scale of renewable energy challenges not only calls for highly efficient photovoltaic (PV) or photoelectrochemical (PEC) technologies but also abundant, inexpensive, and robust materials. However, significant challenges in material performance, bulk and interface defects, and controlled physical properties must be overcome before the potentials of earth-abundant materials can be fulfilled. New semiconductors and highly active electrocatalysts must be discovered to enable the most efficient and sustainable production of energy using solar cells and PEC water splitting cells. Nanomaterials can mitigate the poor properties of earth-abundant semiconductors and catalysts to enhance solar energy conversion. We report nanostructures of several new earth-abundant electrocatalysts, such as nanowires of metal pyrites and exfoliated nanosheets of MoS₂ and layered double hydroxides (LDHs), for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) and significantly enhanced their catalytic performance. We established nanotructures of ternary pyrite-type cobalt phosphosulfide (CoPS) as the best earth-abundant HER catalyst to date that does not contain expensive noble metals. These earth-abundant catalysts have been integrated with nanostructured silicon and other semiconductors to enable the most efficient solar-driven hydrogen generation devices using earth-abundant materials.

Constructing an efficient solar energy convertor which can absorb light over broad range of wavelength is of fundamental importance, as well as critical for many applications from thermos-photo-voltaics to light/thermal detectors and solar steam generation [1]. These solar collectors should have spectrally selective absorber surface which provides high absorbance in the visible range and low emittance in the infrared. To achieve this, appropriate materials with well-designed optical structure are required. Recently, researchers are using two approaches to get broadband solar absorbers; multilayer and nanocomposite absorbers. The first approach uses different dielectric materials with various thickness in order to achieve the impedance matching of the layers to get effective absorber medium [2]. While second design achieves broadband absorption by depositing nano-composite layers on dielectric coated metallic substrate. These methods have the advantages of constructing new materials, however it is challenging to control the concertation of the material in the nanocomposite.
In this study, we design and fabricate a broadband plasmonic absorber comprising of nano-islands of metallic nanoparticles on SiO₂-coated metallic substrate. Compared with previous published studies, it provides simplicity of design and lower fabrication cost. The nano-island with small nanoscale gaps exhibit intense localized field, and the interaction between nano-islands and metallic film at the bottom results in the broadband light absorption. For the design of absorber, we vary the size of decorated metal nanoparticles, spacer materials and thickness to maximize light absorption with the 3D finite difference time domain simulation. The absorber is fabricated using PVD fabrication approach. The surface morphology, topology, and the optical properties of the as-fabricated broadband absorber is further characterized to guide the fabrication processes. The proposed plasmonic absorber promises great potential in enhancing the overall efficiency of solar power systems and optoelectronic devices

Reference
[1] K. Cushing and Nianqiang Wu., "Plasmon-Enhanced Solar Energy Harvesting," The electrochemical Society, pp. 63-67, 2013.
[2] Dong Liu, Haitong Yu, Zhen Yang, Yuanyuan Duan, "Ultrathin planar broadband absorber through effective medium design," Nano Research , 2016.
[3] Yun Zhang, Tiaoxing Wei, Wenjing Dong, Kenan Zhang, Yan Sun, Xin Chen, Ning Dai, "Vapor deposited amorphous metamaterials as visible near-perfect absorber with random non-prefabricated metal nano-particles," Scientific Reports, 2014.

An important challenge in solar energy harvesting and utilization is how to store it at a terawatts scale. Direct conversion from solar to chemical energy answers this challenge. Among the chemistries studied, solar water splitting promises a simple and elegant solution for solar energy storage but has been proven exceedingly difficult due to materials related issues. For instance, most high-efficiency materials are either too costly or too unstable or both for solar water splitting applications. Most low-cost and/or stable materials are too inefficient. In this talk, we examine the challenges in great details. We will further focus on issues at the solid/liquid interface and discuss strategies that have the potential to solve or mitigate some of the challenges. Using iron oxide and tantalum nitride as two prototypical study platforms, we will show that high photovoltage and high photocurrent can indeed be obtained. These results will likely lead to the eventual realization of practical solar fuel synthesis.

Thin-film solar blankets have greater power/weight ratios than traditional heavy glass-bearing crystalline solar cells, and competitive cost in specialized markets. Thin film absorbers are typically thinner than one micron, have poor absorption in the red part of the visible light spectrum, and canno5t be textured like thicker solar cells.

Near-field field enhancement around plasmonic structures, and far-field effects of scattering from individual or arrayed nanoparticles, are well understood.^1,2,3 To investigate the role of intermediate-range scattering on thin-film absorption, two-dimensional sub-monolayer metamaterial arrays, consisting of patterned aluminum pillars on a square submicron lattice and semi-random distributions of silver icosahedra⁴ and nanoprisms⁵ cast in polymer films, were deposited on undoped amorphous silicon thin films on glass substrates. A large spectral shift between scattering and absorption was found for the silver icosahedra in the 120 nm – 200 nm range,⁵ while reflectivity and transmission for the Al nanopillars agreed well with Finite Difference Time Domain simulations. Simulations of a single particle’s scattering of incident broadband light showed the same qualitative spectral shift with respect to the reflectivity. Total absorption in nanopillar array-coated amorphous silicon thin films increased by a sizable fraction compared to uncoated amorphous silicon, according to measurements and simulations; total efficiency improvement (taking into account plasmonic absorption) is being evaluated. We also considered low-absorption polymeric fibers for scattering visible light and controlling infrared emission/transmission. Polymer fibers with diameters in the 10 – 500 nm range (dependent on the draw ratio) are nucleated and lie in thin films when extruded, and form polymeric “fibrous films”.

We explored the scattering properties of plasmonic arrays and fibrous polymer films using integrating sphere (measures total forwards and backwards scattering) and angle-dependent laser-based spectrophotometry. Scattering was also measured from positive (arrays of fibers and large plasmonic particles) and negative (simple polymer and amorphous silicon) control samples. Our experiments and modeling will contribute better understanding and control of scattering from nanoparticles and nanostructures in the complex intermediate-field regime where the substrate thickness is comparable to a wavelength of visible light – the regime most relevant to thin-film solar modules.

[1] F. Luekermann, et. al., Appl. Phys. Letts. 100 (2012) 253907.
[2] “Plasmon Enhanced Absorption in Photovoltaic Cells”, PhD Thesis of Jeffrey Philip Clarkson, University of Rochester (2010).
[3] I. Diukman and M. Orenstein, Solar Cell Mats. 95 (2011) 2628.
[4] M. K. Kinnan and G. Chumanov, J. Phys. Chem. C.114 (2010) 1796.
[5] R. M. Osgood III, et. al., Proc. SPIE 8096, 809610 (2011).

Quantum dot particles (CdS, CdSe, PbS, PbSe and ZnSe) are used as the sensitizers (due to their suitable band gap) in a titania based quantum dot sensitized solar cell (QDSSC). Synthesis of sensitizers on the titania is critical step for efficient anode materials. Here, we introduced new synthesis approach to fabricate anode electrode for QDSSCs. In Molten Salt Assisted Self-Assembly (MASA) approach, the formation of CdSe on titania is more controllable and more uniform compared to other classical method (successive ionic layer absorption and reaction and chemical bath deposition). MASA is a process that uses salts in their molten phase as a solvent and a metal ion source. Titanium (IV) butoxide (TTB) or tetraethylorthosilica (TEOS), as polymerizing agents, and two surfactants, cetyl trimethyl ammonium bromide (CTAB) and 10-lauryl ether (C₁₂EO₁₀), as templating agents, are used in this self-assembly process. The MASA solution is a clear solution of all ingredients that solidify into mesostructured film upon spin coating over any substrate and ready for calcination to convert into mesoporous thin films. Further reaction under a mixture of H₂Se and N₂ gases produces meso-CdSe-TiO₂ or meso-CdSe-SiO₂, depending on the polymerization agent, TBB or TEOS, respectively. The same MASA solution can be infiltrated or coated over a titania film (FTO glass coated with P25 paste, 20-25 nm TiO₂ nanoparticles) film. Similarly, the calcination and H₂Se reaction produces meso-CdSe-TiO₂ or meso-CdSe-SiO₂ in the pores of P25 titania films and can be used as photo-anode in a QDSSC. The anode materials, in all stages of fabrication, were characterized using different diffraction and spectroscopic techniques and tested for their solar performance using a solar simulator. The cell display V_oc=557 mV, I_sc=11.5 mA/cm² and 0.52 FF and 3.3 % efficiency. This efficiency is quite well compared to what has been accomplished experimentally in the literature.

As the share of intermittent renewable energy sources on the electricity grid increases, the need for large scale energy storage solutions to balance the grid will become increasingly important. Concentrated solar thermal power (CSP) systems convert sunlight to heat and subsequently to electricity through a heat engine, e.g. a steam turbine. The main advantage in CSP systems is that the heat from the intermediate step can be stored at a significantly lower cost than storing electricity in batteries being stored in batteries. In traditional CSP systems, heat losses from the high temperature receiver are minimized using two methods: 1) by placing a spectrally selective coating on the absorber tube to minimize thermal radiation losses; and 2) by placing glass vacuum tube around the absorber tube to minimize conduction and convection losses. Both of these contribute considerable cost, complexity, and limitations to the system.
This work reports the first outdoor on-sun experimental demonstration of a new type of concentrating CSP receiver that aims to significantly lower the cost of electricity compared to state of the art systems. The new receiver concept utilizes an optically-transparent thermally-insulating nanoporous silica aerogel to effectively trap the heat generated by the absorbed solar energy. The aerogel eliminates the need for both the spectrally selective coating and the vacuum tube, serving a dual function and simultaneously minimizing all modes of heat transfer. Most importantly, elimination of the vacuum significantly increases the design flexibility in the receiver geometry. Larger receivers with a flat planar geometry factor can be realized, which in turn facilitates more flexibility in the design of the concentrator and the system as a whole. This flexibility enables significant system-wide cost reductions. Specifically, we couple the planar aerogel receiver to a linear Fresnel reflector (LFR) collector, which utilizes an array of mirrors which are smaller, flatter and less expensive compared to conventional concentrating optics.
In this presentation, we will detail the design, fabrication, and first experimental demonstration of an outdoor 3 kW prototype system constructed on MIT campus. The system LFR collector comprises an array of eleven 6-m long mirrors which each track the motion of the sun and focus incident sunlight to a common focal line. Along the focal line, a 1-m long 10-cm wide areogel receiver prototype is placed. The absorbed concentrated solar energy is used to heat up a high-temperature heat transfer fluid to temperatures approaching 400 °C. First results, including achieved optical and thermal efficiency under real on-sun operating conditions, will be discussed.
This work is supported by the U.S. Department of Energy through the Advanced Research Projects Agency−Energy (ARPA-E) FOCUS program under Award No. DE-AR0000471.

Molecular functionalization of nanocrystals is a successful strategy to combine optoelectronic properties of dye molecules, catalysts and oxide semiconductors towards the design of solar energy conversion devices. Photogenerated charge carrier recombination is a critical process in these systems. A lot of effort has been focussed in previous studies to control charge recombination in order to maximize the fraction of free energy per photon that can be extracted from solar cell and solar fuel devices. Long lifetimes are desirable, and several routes to effective interfacial charge separation have been proposed in the past, often based on the idea that the distance between the photgenerated charges is key to the determination of charges' lifetime.¹ The recombination process seems however to show unpredictable behavior in several instances, suggesting that other variables may be playing a role in the determination of photo-generated charge carriers relaxation.

We present a joint experimental and computational investigation of the electron hole recombination dynamics of dye sensitized mesoporous titanium dioxide films immersed in inert electrolytes. By using simultaneous transient absorption and transient absorption anisotropy measurements we find correlation between the ability of dyes anchored to the TiO₂ surface to exchange photogenerated holes and fast electron-hole recombination kinetics. This is observed in systems where either the dye surface coverage² or the surrounding electrolyte are varied to control the hole transport properties of the dye layer.

In addition, we implemented a Monte Carlo model to describe both the transport of holes on the TiO₂ particles' surface and their recombination with electrons in the TiO₂. We show that, in order to describe the observed dispersive character of the transient anisotropy decays, a significant degree of energetic disorder has to be included to model hole transfer betweeen neighboring dyes. Insterestingly, we also show that energetic disorder in the dye monolayer and dispersive hole transport can contribute to the stretched character of the electron hole recombination dynamics, potentially explaining the observed transient absorption decays. Based on our previous findings,³ our simulations suggest that inter-dye hole transport, combined with the experimentally observed energetic disorder in the TiO₂ mesoporous film, reproduces the observed dependence of the experimental half-life of photogenerated holes and the electron density in the TiO₂ scaffold.

To summarize, this work presents a complete description of the hole hopping process between dyes and reveal some key evidence emphasizing the importance of such lateral process in determining photogenerated carrier lifetime in dye sensitized solar cells and solar fuel systems.

1 Clifford, J. N. et al. J. Am. Chem. Soc., 2004, 126, 16, 5225–5233
2 Moia, D. et al. J. Phys. Chem. C, 2015, 119, 33, 18975–18985
3 Moia, D. et al. Chem. Sci., 2014, 5, 281-290

Energetic disorder in nanocrystal solids adversely impacts charge carrier transport in nanocrystal solar cells and other optoelectronic devices. Here, we use ultrafast transient absorption spectroscopy to show that homogeneously broadened PbS nanocrystal arrays (σ_hom²:σ_inhom² > 30:1, σ_inhom/kT < 0.25) can be achieved if nanocrystal batches are sufficiently monodisperse. Furthermore, we show that intrinsic charge carrier hopping rates are faster for smaller nanocrystals. This finding is opposite the mobility trend commonly observed in device measurements, but is consistent with theoretical predictions. Fitting our data to a kinetic Monte Carlo model, we extract charge carrier hopping times ranging from 40ps for the smallest nanocrystals to over 1ns for the largest, with the same ligand treatment. Additionally, we make the surprising observation that in more polydisperse nanocrystal solids, structural disorder has a greater impact than energetic disorder in inhibiting charge carrier transport. These findings emphasize how small improvements in batch monodispersity can have a dramatic impact on intrinsic charge carrier hopping behavior, and will stimulate further improvements in nanocrystal device performance.

In this work, a novel method has been introduced to fabricate a large-area highly ordered array of noble metals (Ag, Au) nanoparticles with size ranging from 30 nm to 500 nm using polystyrene and silica microspheres with diameters between 27 nm to 1μm . The fabrication was done in few steps and on different substrates by using the popular self assembly process method, the dip coating. Different characterization methods were used to analyze the surface and optical properties of the fabricated samples. Concerning the optical properties, the light scattering , and light trapping for absorption enhancement were especially researched.

The importance of titania nanotubes cannot be understated since they have been at the forefront in various applications such as photocatalysis, lithium batteries, solar cells, and sensors because of their extraordinary structural, electrical and optical properties. These extraordinary properties depend on the charge carrier properties and our ability to tune them. Having the ability to precisely control the doping concentration and tune the type of dominant carrier allows for a much greater control on optimizing the titania nanotube performance in the aforementioned applications. Here we present charge carrier characterization of titania nanotubes that were prepared by anodic oxidation of titanium foil in fluorine containing organic electrolytes and were doped in gas environment. We used a suspended micro fabricated device to measure the temperature dependent thermoelectric properties of these individual titania nanotubes and by fitting the data to thermoelectric models, we were able to characterize the charge carrier properties of titania nanotubes with varying doping concentrations. More importantly for the first time we report an n- to p- transition in charge transport mechanism in individual titania nanotubes. To link the obtained properties to the material structure we performed transmission electron microscopy on the same nanotubes and obtained the true thermal-electrical-structural relationship for individual titania nanotubes of various doping levels.

Recent investigations involving nanoscale energy conversion using conducting polymers have recently attracted enormous attention in photovoltaic research, due to their unique potentials including high throughput, solution phase processing, which will lead to low cost electricity production. The hybrid inorganic-organic solar cell devices are designed by interfacing n-type ZnO layers with (Poly(3-hexylthiophene-2,5-diyl) P3HT) as p-type semiconductor as well as donor molecules with poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) as the hole transporting layer. The photovoltaic cell produced here are characterized via Raman, photoluminescence, Fourier transform infrared spectroscopy and X-ray diffraction analysis. The devices show excellent photovoltaic performance under AM 1.5G illumination.

Hybrid semiconductor heterojunction thin films have drawn much attention for various flexible optoelectronic devices such as solar cells, photodetectors, and light emitting diodes due to their low processing cost, low power consumption and efficient charge separation. However, absorption in thin film is much lower than its bulk counterpart. Several techniques have been employed to improve absorption and enhance interfacial area in heterojunction devices including use of photonic crystals (PC)[1], microcavities, microlenses[2], 1D and 2D bragg diffraction grating within the active layers[3]. Patterned structures can be fabricated easily in organic materials by low cost soft lithographic techniques rather than inorganic materials. Here, we present the fabrication of patterned PEDOT: PSS layer with tunable periodicities using solvent assisted imprinting method, and used it as a platform for fabrication of patterned hybrid optoelectronic devices. We demonstrate an enhanced photoluminescence and external quantum efficiency of CdS/ZnO heterostructure thin films prepared on patterned PEDOT: PSS/glass substrates by pulsed laser deposition (PLD). In order to investigate the effect of pattern periodicity, we have fabricated ZnO/CdS devices on 1.5 μm and 750 nm periodicity PEDOT: PSS / glass substrates with the same interfacial enhancement factor of ~1.0 along with flat PEDOT: PSS surface. Near band edge emission of CdS is found to be enhanced for 750 nm patterned structure due to enhanced absorption within the active layer. It has observed that the reflectance is reduced to 5% at near band edge absorption of CdS for devices prepared on 750 nm patterned structure as compared to that on 1.5 μm (~14%) and flat (~36%) structures. External quantum efficiency (EQE) for all devices including flat device shows a broad peak in the visible region due to the absorption by CdS. The device with 750 nm pattern exhibited an enhanced EQE as compared to other devices because of its enhanced absorption within the active layer, which is due to low reflectance for devices of 750 nm pattern structures.
References:
[1] Carl J. Barrelet, Jiming Bao, Marko Loncar, Hong-Gyu Park, Federico Capasso, and Charles M. Lieber, Nano Letters, 2006, 6, 11.
[2] Yuqing Chen, Moneim Elshobaki, Ryan Gebhardt, Stephen Bergeson, Max Noack, Joong-Mok Park, Andrew C. Hillier, Kai-Ming Ho, Rana Biswas and Sumit Chaudhary, Phys. Chem. Chem. Phys., 2015, 17, 3723.
[3] Lucimara Stolz Roman, Olle Inganäs, Thomas Granlund, Tobias Nyberg, Mattias Svensson, Mats R. Andersson, and Jan C. Hummelen, Adv. Mater. 2000,12,189.

Inefficient light absorption and poor charge separation are considered as two major bottlenecks for achieving highly efficient bulk heterojunction organic solar cells (BHJ OSCs). In the present study, we have applied Ga doped ZnO as an electron transport layer for improving the charge collection in one of the promising donor: acceptor system comprised of Poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th): phenyl-C71-butyric acid methyl ester (PC₇₀BM) which resulted in efficiency enhancement from 6.89 to 7.24 %, as compared to a value of 6.89 % when only ZnO was used. Devices were fabricated with inverted geometry having a structure ITO/ZnO or Ga-ZnO (40nm)/ PTB7-Th: PC₇₀BM (~100nm)/MoO₃ (10nm) /Ag (100nm). Additionally, V-grooved textured PDMS films were attached to the backside of OSC substrates which has further improved PCE to 8.35 %. Our study suggests this performance enhancement as observed in OSCs with V-grooved textured PDMS films could be due to increased total optical path length of the incident light within the device.

We have shown that coating Graphene-Silicon (Gr/Si) Schottky junction based solar cells with graphene oxide (GO) improves the power conversion efficiency (PCE) of the cells, while demonstrating unprecedented device stability. The PCE has been shown to be increased to 10.6% (at incident radiation of 100 mW/cm²) for the Gr/Si solar cell with an optimal GO coating thickness compared to 3.6% for a bare/uncoated Gr/Si solar cell. The p-doping of the graphene by the GO, which also serves as an antireflection coating has been shown to be a main contributing factor to the enhanced PCE. The doping and the antireflection effect of GO coating which is shown for the first time, have been verified with Raman spectroscopy and reflectance measurements, respectively. A simple, cost effective spin coating process has been used to apply the GO with thickness commensurate to an antireflection coating and indicates the suitability of the developed methodology for large-scale solar cell assembly.

Organic solar cells (OSCs) have attracted great attention due to their advantages in inexpensive fabrication and flexibility. However, organic semiconductor has a disadvantage of low charge carrier mobility which limits the thickness of active layer, resulting in poor light absorption and low photocurrent. To enhance the photocurrent in OSCs, tremendous works have been conducted on developing plasmonic light trapping methods. Various geometries of silver (Ag) and gold (Au) NPs such as nanowires, nanoclusters, nanocubes, nanoprisms, nanostars and nanopopcorns were blended in the active layer. Although photo-conversion efficiency (PCE) can be enhanced by bleding the plasmonic NPs in active layer, dispersion of NPs in active layer is too sensitive to concentration and size of NPs.
To overcome these problems, 3-dimensional (3D) plasmonic structure have been demonstrated by attacting the plasmonic NPs on ZnO nanorods (NRs), TiO₂ NRs, carbon nanotubes (CNTs) and polymeric nanofiber. Because NPs were attached on the nanostructure, phase seperation or segregation could be prevented. For instance, Au NPs were formed by evaporation of Au on zinc oxide nanorods (ZnO NRs) and thermal annealing at 650 C. The plasmonic Au NPs had smaller size, higher density and stronger plasmonic electric field enhancement than the ones annealed on flat substrate. However, several problems still remained:. (1) previous works did not explain the mechanism why high-density and small plasmonic NPs were spontaneously formed on ZnO NRs or CNTs. (2) NPs had hemispherical shape. Hemispherical shape of metallic NPs block the incident light from the substrate and exhibit weaker plasmonic light absorption than sperical shape of NPs. So, geometry of NPs must be controlled to get strong plasmonic light trapping. (3) Density of NPs was too low. Since plasmons on the closely packed NPs have strong interaction among them, a method to increase the density of NPs is a key technology for improving the plasmonic light absorption. (4) ZnO, TiO₂ NRs and the CNTs have lower electrical conductivity and optical transmittance than inditum-tin-oxide (ITO). They decreases PCE when used as a transparent electrode in OSCs.
Here, we propose a new design rule to fabricate a novel 3D plasmonic structures of ITO nano-branches (ITO BRs) and Ag NPs. ring the annealing process, Ag NPs were dewetted from surface of ITO BRs and aggregates each other. Therefore, shape, density and size of NPs can be easily controlled by annealing temperature. The Ag NPs on ITO BRs have several geometrical advantages such as small size, high density, and spherical shape. Finite-domain time-difference optical simulation provide an evidence of strong plasmonic light scattering and electric field enhancement of Ag NPs. As a result, synergistic effects of ITO BRs and Ag NPs enhanced photocurrent by 14% in PTB7:PCBM OSCs.

As the power conversion efficiency of solar cells based on organo-lead halide perovskites has skyrocketed to levels of >20%, it turned out, that these perovskites are in many aspects almost at par with some of the most prominent inorganic semiconductors used in optoelectronics, e.g. GaAs. For example, both are not only good solar materials but also excellent light emitters. Other than epitaxial GaAs, organo-metal halide perovskites can be prepared by a number of low-temperature deposition techniques, which typically afford relatively rough, poly-crystalline thin films. For the application in a perovskite laser, the propagation of an optical mode in a slab waveguide based on rough layers will suffer from severe losses due to scattering, and thus elevated laser thresholds will result. That is why, the first perovskite lasers have predominantly been reported in vertical resonator structures.^{2, 3} In addition, the chemical instability of organo-metal halide perovskites prevents the use of conventional semiconductor wet-chemical patterning techniques for the preparation of photonic nano-structrures, like photonic crystals, etc. As a result, the perovskite layer is typically deposited on top of substrates pre-patterned with the resonator structure.^{4, 5} In this work, we demonstrate that organo-metal halide perovskites provide the excellent optoelectronic properties of crystalline inorganic semiconductors, but at the same time, they can be patterned⁶ by thermal nanoimprint lithography (NIL) at temperatures as low as 100°C – impossible for semiconductors like GaAs. For the first time, we create two-dimensional distributed feedback (DFB) gratings with a periodicity of Λ = 450 nm directly into the perovskite active layers. Notably, the optical properties of the perovskite are not deteriorated due to the imprint process, rather the material appears to be recrystallized during the imprint affording smoothened layers and almost perfect replication of the NIL-stamp into the perovskite. Upon optical excitation, our structures show second order DFB lasing, with an emission wavelength (λ) according to the Bragg condition λ = n_eff × Λ, where n_eff is the effective refractive index of the optical waveguide. We demonstrate tunable emission between 788-796 nm by a variation of n_eff. The laser threshold is 22 μJ/cm², which is a factor of 3 lower than that reported for 2D perovskite DFB laser.⁴ The direct NIL patterning of photonic structures into perovskite layers is expected to impact not only the field of perovskite lasers but also that of other perovskite devices, such as LEDs and solar cells.
1. M. Graetzel, Nat. Mater., 2014, 13, 838.
2. F. Deschler et al. J. of Phys. Chem. Lett., 2014, 5, 1421.
3. G. Xing et al., Nat. Mater., 2014, 13, 476.
4. S. Chen et al., ACS Nano, 2016, 10, 3959.
5. M. Saliba, et al., Adv. Mater., 2016, 28, 923.
6. S. Wang, et al., J. Vac. Sci. Technol., 2015, B33, 06F602.

Water splitting using sunlight is an important process for future energy supplies. TiO₂ is widely used as photoanode, but has a limited light absorption range. Here, ternary Ti–Mo–Ni mixed oxide nanotube arrays were fabricated via electrochemical anodization of Ti–Mo–Ni alloy in formamide-ethylene glycol-based electrolytes, to extend the absorption range into visible light. The electrolyte composition and anodization time were found crucial in controlling the structural features of the nanotubes. By tuning these parameters, arrays of thin walled (∼9 nm) and ∼8 μm long nanotubes were obtained. In photoelectrochemical water splitting, the mixed oxides showed incident photon conversion efficiency (IPCE) up to 65% for wavelengths from 300 nm to 450 nm. This enhancement in the IPCE of the mixed oxide nanotubes, compared with pure titania, can be related to synergistic effects of Mo and Ni oxides as well as to the unique structural properties of the fabricated mixed oxide nanotubes.

Colloidal quantum dots (QDs) have attracted great attention due to their size- and shape-tunable optical and electronic properties, which allow to realize a new class of devices in optical, electronic, and optoelectronic applications. While organic capping ligands facilitate colloidal synthesis of monodisperse QDs and good solubility in solvent, in the case of QD solids, these long-chain insulating organic ligands prevent carrier transport between QDs. Therefore, original long ligands should be replaced with short ligands to improve electronic properties of QD solids, which is generally required for QD-based devices. Solution-phase ligand exchange has been recently proposed as a promising strategy, enabling direct processing of conductive QD solids in a single step, whereas conventional solid-state ligand exchange consists of multiple layer-by-layer deposition steps. Several reports showed that short organic ligands and inorganic anions can be successfully applied as capping ligands in highly polar solvents, while preserving the colloidal stability. However, it is still challenging to produce uniform, thick quantum dot films from short-ligand-capped colloidal QDs, because solution-phase ligand exchange only works in highly polar solvents, which makes it difficult to apply conventional solution processing such as spin-coating. Here, we report single-step fabrication of uniform quantum dot films from a colloidal quantum dot ink solution via ultrasonic spray-coating. With precisely controlled uniform distribution of droplet size, we successfully developed a strategy to achieve homogeneous QD thin film considering the following points. First, sprayed micro-droplets should be merged into a wet film before drying to construct continuous QD film. Second, the final film morphology highly depends on the evaporation rate of the solvent in colloidal solutions. By tailoring the evaporation rate, agglomeration of colloidal particles is suppressed, leading to the uniform film deposition with strong coffee ring effect at the edge regions. On the basis of these two issues, we obtained uniform thin QD films by tuning the spraying condition and the temperature of substrate. The large-scale QD film deposition is also demonstrated by zigzag moving of spray nozzle along the substrate. By controlling overlapped portion of spray pattern, uniform QD film with size of 8 cm x 8 cm was obtained. Furthermore, the QD solar cells fabricated with spray-deposited QD film exhibited a power conversion efficiency of 4.29%, which is comparable result of QD solar cells with spin-coated QD film.

Organic solar cells (OSCs) have drawn much attention as next-generation light harvesting devices for their clean and efficient nature with reported power conversion efficiencies reaching 10% in single cell devices. Their renowned advantages among many include low-cost, flexibility, and light-weight. However, they rely on a transparent electrode, indium tin oxide which has a critical drawback of limited availability. Many researchers have been searching for an alter- native, yet it has not been superseded by any stack-up replacement to date. There are many proposed materials, and the fluorine-doped tin oxide is one of them. Unlike ITO, it can withstand temperatures above 300 °C without undermining its properties. Nevertheless, its transmittance and conductivity are lower than those of ITO. Aluminum-doped zinc oxide (AZO) is another viable option, but ITO outperforms AZO in terms of stability to moisture which is detrimental to device performance. Carbon-based electrodes like graphene are good candidates with exceptional flexibility and resilience, but they require designated dopants to improve their performance and wettability.
Recently, anatase niobium-doped titanium oxide (TNO), Ti1-xNbxO2 has been developed. As a transparent conductive oxide, TNO meets the requirement of 80% above transparency and possesses the resistivity of minimum 2 × 10−4 Ωcm in epitaxial thin film form. Also, TNO has a higher dielectric constant which leads to a larger effective mass in the infrared region. In addition, TNO can tolerate temperatures above 300 °C, which, in fact, enhances its Hall mobility. Despite such advantages, application of TNO has been focused mainly on dye-sensitized solar cells, and very few OSCs have been reported.
In this work, TNO electrode films were prepared in a manner such that both the transmittance and sheet resistance are optimal for OSC application. For this purpose, niobium dopant stoichiometry in TNO was reduced to 2 at%. Additionally, TNO was annealed at a temperature over 600 °C under H₂ to optimise oxygen stoichiometry for the optimal conductivity. Moreover, UV–ozone (O₃) treatment was applied to TNO films for a period of time to induce surface oxidation. The intrusion of oxygen species caused Nb[IV] and Ti[III] at the TNO surface to oxidise to Nb[V] and Ti[IV], losing free electron carriers as evidenced by analyses in this work. Thus, oxidised surface (Nb_0.02Ti_0.98O_2+y) manifested semiconducting behaviour, which functioned as an electron-transporting layer. Air- processed inverted type OSCs fabricated using O3-treated TNO without an ETL deposition process produced a PCE comparable to ITO-based OSCs, where ZnO had to be deposited as the ETL. Finding of such phenomenon offered eradication of the ETL deposition process in the inverted OSC fabrication. We anticipate cost reduction, reproducibility enhancement, and a breakthrough in the fabrication of solar cells.

Recently, the organohalide lead perovskite solar cells have attracted tremendous attractions in the past three years. In this work, we propose a new vacuum approach for fabrication of perovskite solar cell. Herein, the perovskite film is fabricated using a multi-step vacuum process after layer by layer deposition of PbI₂ and methylammounium iodide (MAI) materials, sequentially. In order to have a better understanding of this process, we compare the devices fabricated by multi-step and two-step evaporation processes. After studying the morphology and composition of perovskite films, we find that in the multi-step method, the film has a higher quality with a uniform composition. The results of x-ray photoelectron spectroscopy (XPS) and inductively coupled plasma mass spectroscopy (ICP-MS) demonstrate that the Pb/I ratio in the multi-step process is closer to 3 than the two-step technique and thus, using the two-step process, there is a substantially higher amount of metallic lead in the perovskite film when compared to the multi-step method. The photovoltaic measurements of perovskite devices illustrate that we can enhance the fill factor (FF) and current density of the device fabricated by the multi-step process. Finally, a power conversion efficiency (PCE) of 15.7 % was achieved by using a multi-step process.

This paper presents the results of a single-step process to produce and deposit few-layer graphene on copper substrates. The ball milling process is a mechanical alloying technique that has been employed here to produce the graphene deposit from a starting material of high purity graphite. The effect of varying different parameters in the process (rotational speed, crucible material, with/without balls and milling time) and material (graphite chunks and graphite powder) is also studied, and the resulting materials are characterized using Raman spectroscopy, atomic force microscopy, scanning electron microscopy and optical spectrometry. The resulting materials show distinctive variation from the standard graphite characteristics, indicating the successful synthesis and deposition of few-layer graphene across all tested specimens. The significance of this work is that it takes graphene synthesis and deposition a step closer to full automation.

Over the last decades the interest in nanostructured materials increased. Many of these materials have interesting properties and are already used in different applications, e.g. in solar cells or for catalytic reactions. Since the trend in energy production is shifting away from fossil fuels towards green energy it is very important to search for suitable materials for such applications. Moreover, synthesis methods need to be developed which allow the fabrication of these materials in a cheap and simple way.
CuInS₂, copper indium disulfide, is a Chalcopyrite-type semiconductor with a band gap of 1.5eV and high absorption coefficient.¹ Therefore it is well suited for solar-driven applications. In this work CuInS₂ films are grown directly on Fluorine-doped tin oxide coated glass substrates via a solvothermal method developed by Peng et al.² and modified in our group.^{3, 4} This study focuses on the use of different sulfur sources and the influence of reaction parameters on growth and film morphology. Furthermore the growth mechanism itself is investigated. Standard reaction conditions for the solvothermal synthesis are 150°C for 18h in Ethanol. Analysis is mainly done with electron microscopy and related techniques.
The use of two sulfur sources, Thioacetamide and l-Cysteine, leads to different types of surface topology. For Thioacetamide a flower-like surface is obtained whereas l-Cysteine gives a more compact CuInS₂ layer. By varying reaction time and temperature effects on the synthesis could be observed. For Thioacetamide 3h are sufficient to form crystalline nanostructured CuInS₂ films but for l-Cysteine at least 6h are needed. When l-Cysteine was used a minimum temperature of 140°C forms a CuInS₂ film while in the case of Thioacetamide 120°C are enough to form crystalline CuInS₂.
To investigate the growth process the reaction between Cu and S source only was conducted at RT. For Thioacetamide no reaction was observed during 18h of stirring but for l-Cysteine a grey solid could be extracted. Analysis in the transmission electron microscope revealed an agglomeration of nanoparticles in an amorphous matrix which consist mainly of copper sulfides and copper l-Cysteine/Cystine complexes. Electron energy loss spectroscopy showed that the copper in this sample was reduced from oxidation state +II from the precursor compound to +I as needed for CuInS₂. That means that l-Cysteine is capable of reducing copper at RT whereas Thioacetamide seems to need an elevated temperature.
For both S sources highly crystalline nanostructured thin films were obtained which possess different surface morphologies. Future performance test using hybrid solar cells as well as solar driven water-splitting devices will shed light on which morphology is better suitable for the different applications.
1 B. Tell, et al., Phys Rev B 1971, 4, 2463.
2 S. Peng, et al., J Alloys Compd 2009, 481.
3 A. Wochnik, et al., J Mater Sci 2012, 47.
4 A. S. Wochnik, et al., Solid State Sci 2013, 26.

Atomically thin transition metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂) are attractive because of their ultrathin structure and inimitable electronic band structures with unique functionalities: indirect-to-direct bandgap transition, semiconductor-to-metal phase engineering and the large excitonic effect. Moreover, the TMDs with optical bandgaps in the near-infrared to visible spectral range can exhibit extremely strong light–matter interactions suitable for energy harvesting devices. Most of the present WS₂-based solar cells are limited to micromechanical exfoliation or transfer techniques. However, the large-scale growth of direct, transfer-free WS₂-on-silicon solar cell with uniform and continuous coverage is still a challenge. Here, we report a scalable, reproducible and single-step vapor phase chemical growth process for the fabrications of large-area WS₂ films on p-type Si substrates confirmed by Raman, photoluminescence and X-ray photoelectron spectroscopic analysis. Owing to the n-type conductivity of WS₂, it exhibits photovoltaic characteristics with p-Si when measured under AM 1.5G illuminations.

We fabricated Si nanopillar (NP) arrays using e-beam lithography and coated them with poly(3-hexylthiophene-2,5-diyl) (P3HT) organic semiconductor layers. Optical reflection spectra and spatial distribution of the optical generation rate (G) showed that Mie resonance significantly increased the scattering cross-sections of the NPs and strongly concentrated incident light in the NPs. Such concentrated light should generate numerous charge carriers and influence the subsequent drift/diffusion of the carriers. Surface photovoltage (SPV), defined as the difference between the surface potential in dark and under illumination, could reveal the formation and separation behaviors of the photo-generated carriers. Especially, Kelvin probe force microscopy technique allowed us to obtain real space SPV maps with nanoscopic spatial resolution. Under red light, the SPV values at the NP top was much larger than planar sample due to the Mie resonance. Since the Si NPs provide pathways for efficient carrier transportation, high collection probability of the photogenerated carriers near the NPs can be expected. This suggests that the optical resonance in organic/Si hybrid nanostructures benefits not only broad-band light trapping but also efficient carrier collection.

Investigation of plasmonic nanostructures has attained a great attention over the past few years due to their potential of manipulating electromagnetic field at sub diffraction limits. Availability of advanced lithography techniques and hence the liberty of controlled fabrication at nanoscale, has opened a new window for the in depth study of plasmonic properties of even very complicated metallic nanostructures. Here we report on the investigation of tunable and unique plasmonic properties of Au plasmonic nanostars (PNSs) in the Infrared range by changing various geometrical aspects of PNSs. Plasmonically assisted enhanced upconversion of 1500 nm light in an Er³⁺ doped TiO₂ layer is demonstrated in order to improve the light harvesting for Si-based solar cells. By varying overall diameter of the stars, shape of star’s leg and size of central core it was possible to excite and tune the plasmonic resonance over broad spectral range starting from visible to infrared. Furthermore, critical dependence of plasmonic excitation on periodic and random arrangement of PNSs is investigated to tune far-field extinction efficiency. Au-PNSs were theoretically studied by FEM simulation technique. Top-down fabrication method i.e. electron beam lithography technique (EBL) was used to fabricate 2D arrays of PNSs on Er³⁺ doped TiO₂-coated quartz templates.

The photocatalytic water splitting with semiconductors to store solar energy in the simplest chemical bond in the H₂ molecule, is considered to be the “Holy Grail” of solar energy conversion and storage.¹ However, the performance of a given photo-catalyst is largely limited by four conditions: 1) long term stability with respect to photo corrosion, 2) a narrow band gap, coincident in energy with the visible/UV and matching band edges/maximum-minimum with the redox potential/levels of water, 3) environmentally friendly, and 4) and cost effective. The artificial Z-scheme based photocatalytic systems have been widely investigated in this regard due to its unique features². In this work, we have investigated a system initially reported by Rao et al.,³ in which Cd(Zn)S solid solution is deposited on top of Pt/ZnO to further probe into the system and understand the effect of its different components in search for new catalyst formulations. The materials were synthesized and tested, and characterized by HR-TEM, UV-vis, XPS, ICP and XRD. The system works in the presence of a complex medium composed of acetic acid and benzyl alcohol; the effects of which on catalysts stability and performance are studied. TEM images showed that the sub-nano Pt particles were uniformly dispersed. ICP and XPS analysis indicate that Pt is sandwiched between ZnO and Cd(Zn)S This is because 1 wt.% Pt, detected by ICP and TEM, is not detected by XPS in the same hybrid system, even though XPS could detect concentrations down to 0.1 wt.%Pt in the absence of ZnO.(the non-hybrid system). We have also measured the apparent quantum yields (AQY), from the initial reaction rates, of ZnO/Pt/Cd(Zn)S and Pt/Cd(Zn)S under both UV (360nm) and visible light (460 nm). AQY of 34% was obtained from ZnO/1wt.%Pt/Cd(Zn)S system at 360 nm, significantly higher than that of 1%Pt/Cd(Zn)S (14%). Furthermore, a AQY of 16% was also observed using ZnO/1wt.%Pt/Cd(Zn)S which is comparable to that of 1wt.% Pt/Cd(Zn)S (10%) at 460 nm. Therefore, we propose a charge transfer mechanism. These results suggest that z-scheme is formed under UV irradiation while charge separation occurs under visible light on ZnO/1%Pt/Cd(Zn)S.
References
1. Bard, A. J.; Fox, M. A., Artificial Photosynthesis: Solar Splitting of Water to Hydrogen and Oxygen. Accounts of Chemical Research 1995, 28 (3), 141-145.
2. Srinivasan, N.; Sakai, E.; Miyauchi, M., Balanced Excitation between Two Semiconductors in Bulk Heterojunction Z-Scheme System for Overall Water Splitting. ACS Catalysis 2016, 6 (4), 2197-2200.
3. Lingampalli, S. R.; Gautam, U. K.; Rao, C. N. R., Highly efficient photocatalytic hydrogen generation by solution-processed ZnO/Pt/CdS, ZnO/Pt/Cd_1-xZn_xS and ZnO/Pt/CdS_1-xSe_x hybrid nanostructures. Energy and Environmental Science 2013, 6 (12), 3589-3594.

High-quality thin films of bismuth ferrite BiFeO₃ (BFO) were grown by ozone assisted Atomic Layer Deposition (ALD). The solid precursors Bi(TMHD)₃, and Ferrocene (C₁₀H₁₀Fe) with better than 99% purity acquired from Strem Chemicals USA were utilized for the ternary oxide thin film deposition along with the ozone oxidizer. The thickness of the BFO layers was controlled by molecular sizes of precursors and chemical absorption between precursors and hydroxyl groups of Bi-O and Fe-O layers. Spectroscopic ellipsometry studies were carried out to determine the growth per cycle, ALD saturation window, as well as the optical parameters of the films. X-ray photoelectron spectroscopy (XPS) analysis of the BFO layers confirmed that the film is stoichiometric with a Bi³⁺:Fe³⁺ atomic ratio of 1:1 and no diffusion of Bi atoms were found at low deposition temperatures (~250 °C) employed in the ALD process. The evolution of grains size of BFO thin films was studied as a function of post-deposition annealing temperature in the range of 450-650 °C in the oxygen environment. The ferroelectric phase formation of the films was confirmed by X-ray diffractometry and Raman spectroscopy. The ferroelectric behavior of the films was characterized by hysteresis loop measurements using a Sawyer-Tower configuration. An improved ferroelectric nature of ALD grown BFO thin films opens up the possibility of fabricating ferroelectric photovoltaic devices for light harvesting applications.

In liquid-phase photocatalytic dye degradation and water splitting, it was recently found that nanocrystal superstructure based semiconductors exhibited improved spatial separation of photo- excited charge carriers and enhanced photocatalytic performance. Nevertheless, it remains unknown whether this strategy is applicable in gas-phase photocatalysis. Our group recently reported that indium oxide nanocrystals with surface defects in the form of oxygen vacancies and hydroxyl groups, denoted In2O3−x(OH)y, function as a single-component photocatalyst that activates the reverse water−gas shift (RWGS) reaction. The RWGS reaction occurs at frustrated Lewis pair (FLP) sites located on the In2O3−x(OH)y surface. These FLP sites comprise a Lewis acidic, coordinately unsaturated surface indium site proximal to an oxygen vacancy and a Lewis basic surface hydroxide site on In2O3−x(OH)y. Very recently, we uncovered that photo- excited charge carriers can lower the activation energy at these FLP sites, thereby accelerating the RWGS reaction rate.

As a result of the deep understanding of the reaction mechanism, we reported porous indium oxide nanorods with lengths ranging from a few hundred nanometers to great than 1 μm and demonstrated that assembling semiconductor nanocrystals into superstructures can also promote gas-phase photocatalytic processes while the population of active sites diminishes with increasing length of the nanorods. Transient absorption studies prove that the improved activity is a result of prolonged photoexcited charge carrier lifetimes due to the charge transfer within the nanocrystal network comprising the nanorods. It is the unique combination of nanocrystals assembled into a nanorod superstructure together with transport of photogenerated charge carriers from nanocrystal-to-nanocrystal therein that ultimately controls the charge relaxation dynamics and surface chemistry responsible for the observed nanorod length dependence of the hydrogenation rate of carbon dioxide to carbon monoxide. Our study sheds light on the physicochemical design principles of nanocrystal superstructure based photocatalysts that will enable more efficient utilization of solar energy.

Lead sulfide quantum dots (PbS QDs) are an attractive material for the development of solution-processable photovoltaics (PV) due to their ease of processing, low thermal-budget and unencapsulated stability in air, with certified power conversion efficiencies above 10%. However, even the best PbS QD PV cells are limited by diffusive transport, as the optical absorption length for most wavelengths exceeds the minority carrier diffusion length in the QD film, which is typically <100 nm.

In this work we use electron beam-induced current (EBIC) with a low-energy electron probe (E ≤10 keV) to reduce the interaction volume such that the nanoscale minority carrier diffusion length can be resolved. Monte Carlo simulations confirm that the electron interaction volume is not resolution-limiting. Low-energy cross-sectional EBIC was performed on focused ion beam (FIB)-polished planar PbS QD PV devices. The EBIC signal is primarily localized at the junction between tetrabutylammonium iodide (TBAI)-treated and ethanedithiol (EDT)-treated PbS QDs rather than the ZnO/PbS-TBAI interface, indicating that holes are the minority carriers in the TBAI-treated film. The effect of surface recombination at the FIB-polished surface is also considered. Fitting the deconvoluted EBIC profile as a function of the distance from the junction yields a lower bound of 90 nm for the hole diffusion length. Therefore, low-energy EBIC is a useful tool for measuring nanoscale minority carrier transport in emerging PV materials.

Using high quality quantum dots (QDs) and employing wide range of morphologies for TiO₂ photoanodes are few of several methodologies used to improve the performance of quantum dot sensitized solar cells (QDSSCs). The former include using high quality tunable low band-gap semiconducting materials such as CdSe, CdS, ZnSe and PbSe as sensitizers and the latter includes using TiO₂ nanotubes (TNT), TiO₂ micro-spheres (TMS) and TiO₂ nanoparticles as photoanode in QDSSCs.¹ Efficient absorption of all the region of solar spectrum is very crucial factor for making high efficiency solar cell. There has been rapid increase in efficiency of CdSe based QDSSCs in the past few years but still it lags far behind the highest efficiencies (~11 %) demonstrated in dye-sensitized solar cell (DSSCs).² CdSe based QDs are highly stable and the tunable band-gap all through the visible region of solar spectrum renders them to be a promising material for solar cell applications.³ However many challenges remain to be addressed to achieve the best device performance: optimization of bandgap and its band alignment with TiO₂, effective loading of sensitizer and effective way of extracting charge carriers are few important factors.
In this presentation we show the studies on optical and microstructural properties of TiO₂ photoanode and (Cd,Zn)Se QD. Detailed absorption spectral studies, photoluminescence and microstructural studies using SEM and TEM will be presented. The investigations on solar cell characteristics of (Cd,Zn)Se sensitized QDSSCs will be corroborated to establish the device performance on the bandgap of QDs. Detailed photovoltaic characteristics carried out using I-V, IPCE s and impedance spectra measurements are used to substantiate the results on various solar cell devices. The effect of Zn alloying in CdSe on the efficiency of QDSSCs will be elaborated.
1. P. V. Kamat, The Journal of Physical Chemistry Letters 4 (6), 908-918 (2013).
2. B. O'Regan and M. Gratzel, Nature 353 (6346), 737-740 (1991).
3. A. Kongkanand, K. Tvrdy, K. Takechi, M. Kuno and P. V. Kamat, Journal of the American Chemical Society 130 (12), 4007-4015 (2008).

Luminescent solar concentrators (LSCs) transform the solar spectrum by both downshifting incident sunlight and directing the shifted photons onto an adjacent solar cell. In a standard LSC design, incident sunlight is absorbed by a luminophore embedded within a plastic sheet, re-emitted at longer wavelengths that are trapped by total internal reflection, and eventually concentrated to an edge-mounted solar cell. Although promising, such devices suffer from losses including reabsorption of the emitted light by the luminophore, non-unity quantum yields, and incomplete light guiding to the solar cell. Recent advances have been made by integrating luminescent nanocrystals with large Stokes shifts into LSCs, utilizing the tunable absorption and emission spectra, high quantum yield, and narrow emission bandwidth in tailored nanocrystals. Furthermore, these downshifters can be easily integrated with photonic designs to enhance light guiding toward edge-mounted solar cells.
This talk will discuss several different design considerations for photonic LSCs. First, we will discuss the effect of luminophore size, shape, and optical properties on the performance of the LSC. We then show that by combining high quantum yield nanocrystals with a wavelength-selective photonic mirror on the top interface, escape cone losses are suppressed and concentration factors over 30 are achieved. Although this route promises high concentration factors, it also requires very high quantum yields and high mirror reflectivities to operate effectively. We show quantitatively how photon propagation in these devices is reduced when non-ideal materials are used.
We then show that metamaterial mirrors integrated onto the back surface of the LSC may be used to overcome these optical transport difficulties, as light is shifted into modes with more significant propagation. The ideal designs for these metamaterial mirrors, their effect on the concentration factor, and the relationship to luminophore properties will be discussed. We find that by controlling the light propagation path through the LSC effectively, lower quality luminophores and less reflective mirrors can be used, opening a pathway toward non-toxic and earth abundant materials systems.

Concentrating lenses are one of the most important components of concentrating photovoltaic cells. Using subwavelength grating structures, various flat micro lenses, ranging from the traditional dielectric lenses to plasmonic lenses, have been proposed. Among all such micro lenses, the subwavelength grating structures based on recently-proposed refractive index contract structures have gained the most attention due to its compatibility with planar CMOS integration processes while retaining excellent light-focusing properties.
Conventional subwavelength grating concentrating lenses are designed based on calculated phase overlap, wherein the phase change is fixed by the grating thickness, bar-width and airgap, and therefore the focus. In this study, we found that certain concentration effects can still be maintained by changing the grating thickness with the same bar-widths and airgap dimensions. We have discovered the existence of the grating thickness threshold: light concentration intensity spikes upon exceeding this limit. However, the light concentration property does not change continuously with respect to a steady increase in grating thickness. This observation indicates that there exists a concentration mode self-interference effect along the light propagation direction inside the gratings.
Our results may provide guidance in designing and fabricating micro lenses in a potentially easier and controllable manner. Such approaches can be utilized in various integrated nanophotonics applications ranging from optical cavities, read/write heads and concentrating photovoltaics.

We demonstrate the fabrication of a low reabsorption emission loss, high efficient luminescent solar concentrator (LSC) by incorporating near infrared (NIR) core/shell quantum dots (QDs) with a polymer matrix. Upon illumination of the LSC surface, solar radiation is down-converted to longer wavelengths and concentrated due to total internal reflection by the polymer matrix, then subsequently emitted through the edges of the LSC to the integrated PV devices. Although many types of QDs have been used in LSCs, it is still very challenging to achieve large-area, high efficiency LSCs due to the re-absorption issue, which could be addressed by engineering the Stokes shift (the difference in wavelength between the positions of the band maxima of the first excitonic absorption and emission spectra) of QDs. An engineered Stokes shift in NIR core/shell PbS/CdS QDs has been successfully achieved via a cation exchange approach by varying the core size and shell thickness through the refined reaction parameters. The as-synthesized core/shell QDs with high quantum yield (QY) and excellent chemical-/photo-stability exhibit a large Stokes shift with respect to the bare PbS QDs due to the strong core-to-shell electrons leakage. The large-area planar LSC based on core/shell QDs exhibits the highest value (6.1% with a geometric factor of 10) for optical efficiency compared to the bare NIR QD-based LSCs and other reported NIR QD-based LSCs.The suppression of emission loss and the broad absorption of PbS/CdS QDs offer a efficient approach to integrate LSCs and photovoltaic devices with approprioate spectral range, indicating that the proposed core/shell QDs are promising candidates for fabricating high efficient semi-transparent large-area LSCs.

Solar thermal systems, or systems which convert sunlight to heat, commonly use spectrally selective surfaces in order to achieve high efficiency. Spectrally selective surfaces are designed to be absorptive in the solar spectrum, but reflective (and therefore non-emitting) in the blackbody spectrum of their intended operating temperature. Recently, attempts to improve the performance of spectrally selective surfaces have included using a variety of nano-scale features such as pyramids or inverse-pyramid geometries. Ideal selective surfaces should have selectivity change as abruptly as possible, switching sharply from absorbing to reflecting at a cutoff wavelength. However, real selective surfaces fall far short of such an ideal structure, with the switch from absorbing to reflecting occurring over a band of wavelengths. Since the response of a material to light must be causal, the Kramers-Kronig relations provide constraints on the real and imaginary parts of a material’s index of refraction (or equivalently its susceptibility), precluding an emittance profile with a perfect step function. In this work, we establish the fundamental limit on the abruptness of spectral selectivity change, starting from the Kramers-Kronig relations. This limit sets the ultimate level of performance that fabricated spectrally selective surfaces should aspire to.

This material is based on work supported as part of the Solid State Solar-Thermal Energy Conversion Center (S3 TEC), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under award DE-SC0001299/DE-FG02-09ER46577.

Aerogels are widely known for their thermal insulating properties due to its nanoporous structure. Monolithic silica aerogels that are both optically transparent and thermally insulating can significantly improve the efficiency of solar absorbers by acting as a spectral selective cover that allows solar radiation to transmit through but minimizes the absorber losses in the infrared spectrum. Heat losses due to solid and gas conduction can also diminished due to the high porosity (>90%) and pore sizes smaller than the mean free path of gas molecules which obviates the need for operation in vacuum. However, the transparency of silica aerogels in the solar spectrum is typically <85%, which has prevented its adoption in solar-thermal applications. We report optical and thermal characterization of highly transparent silica aerogels optimized for solar-thermal applications. We measured a solar-weighted transmittance of 96% for an 8 mm sample thickness, which exceeds that of glass. The high transparency was obtained by carefully controlling the aerogel nanostructure to minimize the scattering losses at low wavelengths. Meanwhile, the aerogel thermal properties were characterized using a custom-built cooler bridge setup based on the steady state method. The measured heat transfer coefficient for a temperature difference between 400 °C and 250 °C was 6 W/m²K for an 8 mm thick sample. The thermal measurements were compared with a detailed numerical model based on the spectral equation of radiative transfer that uses the measured extinction coefficient as an input. This work shows the promise of simultaneously achieving high transparency and low heat transfer coefficients in silica aerogels, which can significantly improve the energy conversion efficiency of various solar-thermal systems.

To match increasing computational speeds of materials discovery, materials synthesis and characterization are undergoing a transformation. Combinatorial and solution-growth techniques have accelerated synthesis times per unique sample by >20×, and novel computationally-assisted characterization methods are accelerating extraction of performance-relevant materials parameters by >200×. In this talk, we will address the outstanding challenge to determine the “intrinsic” performance potential of a material, in the presence of process-induced nanodefects including extended structural defects, intrinsic point defects, and extrinsic defects (impurities). A structured approach to quantify the relative impacts of different defect classes will be presented. Across several novel optoelectronic materials classes, including bismuth- and tin-containing compounds, we observe that the bulk lifetimes of early-stage materials are typically not limited by extended defects (grain boundaries and intragranular dislocations); rather, these early materials appear to be limited by point defects, which can be controlled by a combination of judicious feedstock quality and growth conditions. By demonstrating an approach to identify and/or avoid common “false negatives” during materials screening, we take a step toward defining minimum requirements for accelerated experimental materials searches.

Historical thin film photovoltaic (PV) performance indicates that minority carrier lifetimes >1 ns are typically required to achieve >10% efficiencies¹. Borrowing lessons from established semiconductor materials such as Si, there are several factors that can reduce the carrier lifetime, particularly recombination through structural, and intrinsic and extrinsic point defects. Thus it is critical for new PV materials to evaluate the dominant recombination mechanisms.

We develop a combined modeling and experimental framework for lifetime-limiting defect identification and mitigation in emerging thin-film semiconductors. We apply this framework to tin (II) sulfide (SnS), an emerging solar absorber hampered by short minority carrier lifetimes (≤40 ps) limiting device efficiencies to ~5%¹. Device modeling indicates that 1 ns lifetime in SnS could enable 10+% efficient devices². Prior work in our group indicates that structural defects may not limit the lifetime. Thus we focus on point defect management to SnS, combining computational modeling and experiment to down-select from all possible point defects to only four. We manage these defects by targeted growth parameters, and successfully achieve >1 ns carrier lifetimes in SnS.

We use a Shockley-Read-Hall statistical model to predict the impact of specific point defects in SnS. Defect energies are calculated using density functional theory (DFT) with hybrid HSE functionals. Capture cross sections are estimated based on a Coulombic attraction/repulsion model. Our model implicates 4 most likely lifetime-limiting point defects: the intrinsic V_S and extrinsic Fe_Sn, Co_Sn, and Mo_Sn defects. This allows us to predict recombination activity at point defects without having detailed experimental information on defects (as is common in most emerging PV materials).

We grow SnS by sulfurizing in an S-rich environment using feedstocks with ~1 ppb Fe, Co, and Mo and achieve >2.5 ns lifetimes. Growth parameters for achieving phase-pure SnS in H₂S are guided by FactSage³ thermodynamic modeling software. Phase (XRD), morphology (SEM), stoichiometry and contamination levels (TOF-SIMS), and carrier lifetime (PL, TCSPC), are reported for various growth conditions.

Finally, we fabricate devices from high-lifetime films using our previously developed PV device architecture⁴, yielding efficiency improvements. Thus through this work, we demonstrate material and device improvement through deep defect understanding and highly targeted defect management.

[1] Jaramillo, et al., J. Appl. Phys., 119, 2016, p. 035101.
[2] Mangan, et al., 40th IEEE Photovoltaic Specialist Conference, 2014, pp. 2373-2378.
[3] Bale et al., Calphad, 26, pp. 2002, 189-228.
[4] Steinmann, et al., Adv. Mater., 26, 2014, pp. 7488-7492.

Optimum capture of the incident light through efficient photon management is considered a crucial requirement for realization of ultra-efficient photovoltaic devices. For this purpose, diffraction gratings have been applied for photon management in solar energy conversion. Specifically, due to the hexagonal arrangement of the grating structure, hexagonal gratings can be favorable in integrated optics since they occupy the least space compared to any other periodic arrangement. Here, our recent progress in development of nanostructured hexagonal diffraction gratings for application in light absorption enhancement in thin-film solar cells is presented. Leveraging the photon management ability of polystyrene nano-sphere arrays, the simplicity of the self-assembly fabrication process, and highly elastomeric properties of polydimethylsiloxane (PDMS), a novel stretchable transmissive hexagonal diffraction grating is introduced. Thanks to its unique flexible yet hexagonal structure, the proposed grating is capable of reproducible in-situ tuning of both diffraction efficiency and spectral range. The developed grating exhibits highly efficient and broadband light diffraction fairly independent of incident light polarization and angle of incidence while concurrently being able to achieve high diffraction efficiencies of about 80%. Because of its efficient and tunable diffraction capabilities, one potential application of the developed diffraction grating can be broadband photon management in nanostructured thin-film solar cells through significant enhancement of the light path length inside the light-absorbing thin-films of these devices. As a proof of concept, the proposed hexagonal diffraction grating is utilized for light absorption enhancement in colloidal quantum dot semiconductor thin-films. In addition, taking advantage of the same qualities of hexagonal arrangement induced by nano-sphere lithography, metallic reflective hexagonal diffraction gratings are also demonstrated to have a great potential for photon management through plasmonics. By careful engineering of the geometry of the gratings through controlling the size and the distance between grating components, both forward and backward propagating surface plasmon polaritons (SPPs) and localized surface plasmons (LSPs) were observed in a hexagonal diffraction grating for the first time. Simultaneous excitation of SPPs and LSPs demonstrated here can have a considerable impact on light absorption enhancement in nanostructured thin-film solar cells through both local and interfacial light confinement. The demonstrated capabilities of hexagonal diffraction gratings can enable integration of cheap and widely used materials with simple cost-effective fabrication for efficient photon management in solar energy conversion.

The ability of efficiently converting incoherent infrared light to visible wavelengths has the potential to enable new technologies. Applications include extending current commercial silicon photovoltaics beyond the limits of the silicon bandgap, as well as cost-effective short-wave infrared cameras (SWIR: 1-3µm). Recent studies have shown effective up-conversion of SWIR wavelengths to visible light in hybrid excitonic devices composed of organic semiconductors and colloidal nanocrystals.[1,2,3]
In these hybrid devices, PbS nanocrystals are directly excited in the SWIR. The resulting photoexcitations undergo Dexter transfer to sensitize the triplet state in rubrene. Diffusion-mediated triplet-triplet exciton annihilation results in higher energy singlet excitons which are readily emitted from a dopant dye (DBP).
To extract the dynamics we investigate the effect of the interfacial geometry on the triplet transfer, by varying the ligands on the surface of the nanocrystals. Observing the triplet transfer kinetics by transient photoluminescence spectroscopy, the transfer rate accelerates by an order of magnitude (from τ≈700ns to τ≈100ns), when reducing the ligand length from 18 carbon atoms to 4 carbon atoms. Interestingly, the transfer rate appears to saturate at shorter distances, implying that assuming a simple 1D Dexter model is insufficient.

[1] Wu, Congreve, Wilson et al. (Bulović, Bawendi, Baldo) Nature Photonics 10, 31–34 (2016)
[2] Huang et al. (Tang, Bardeen) Nano Lett., 15, 5552–5557 (2015)
[3] Mongin et al. (Zamkov, Castellano), Science, 351, 369-372 (2016)

There is tremendous growth in fields where small functional molecules and nanomaterials act as the key component of energy devices, which often requires the fabrication of thin-films for incorporation into the solid-state. Notably, many of such constituents are synthesized in solution and there often exists a significant barrier to transitioning them to the solid-state in an efficient and versatile manner. In our present work we report a co-polymer electrodeposition method that is applicable to a wide scope of small molecules for energy applications and provides unprecedented flexibility for their incorporation into the solid-state. This novel approach is applicable to systems such as photon upconversion, micro-switches, micro-actuators, photovoltaics, photosensing, light emission, solar thermal fuels and beyond.
Here have focused on recently reported solid-state solar thermal fuels¹ that have tremendous potential as materials that can both harvest and store solar energy. We design a novel co-polymer employing charging units that may be electrodeposited onto a variety of substrates with three-dimensional geometries. This approach also enables us to envision novel implementation of solar thermal fuels into solar thermal fibers that may be employed in fabrics, which are successfully demonstrated using our method. Importantly, the co-polymer method allows for excellent retention of the small-molecule properties, and presents a robust avenue to forming solid-state films with minimal compromises.
These novel solar thermal fuel materials exhibit solid-state UV chargeability, thickness tunability from hundreds of nanometers to tens of microns, exceptionally high materials utilization efficiency, a gravimetric energy density of approximately 100 J/g (~28 Wh/kg), and the ability to be deposited on a wide range of structured conducting substrates.
We see this work as an important milestone for transitioning small-molecule energy materials in an efficient and versatile way into the solid-state, offering additional processing and cost saving benefits.

1. Zhitomirsky, D., Cho, E. & Grossman, J. C. Solid-State Solar Thermal Fuels for Heat Release Applications. Adv. Energy Mater. 6, n/a–n/a (2016).

Abstract: Optoelectronic properties of quantum dot (QD) nanomaterials have been intensively investigated during last two decades. MBE grown structures and colloidal materials demonstrate selective photon-electron coupling associated with electron transitions in QDs. Photon-induces intersubband transitions cover wide spectral range from THz to infrared and can be effectively controlled by size and form of QDs. Operation of various photoelectron devices, such as THz/Infrared photodetectors and solar cells with intermediate states in QDs, are based on intersubband transitions in QD nanomaterials. It is well understood that doping of QDs plays a crucial role for corresponding photon-electron coupling. At the same time, the doping substantially increases the dark current, which deteriorates performance of all optoelectronic devices. In particular, large dark current reduces sensitivity of photodetectors and decreases the open circuit voltage in solar cells. Tradeoff between the intersubband absorption in QDs and the dark current is key issue for further development of QD optoelectronic devices.
Our approach to optimization of QD materials for specific optoelectronic applications is based on engineering of nanoscale potential profile, which is created by charged QDs. The nanoscale barriers prevent capture of photocarriers and drastically increase the photoelectron lifetime (the lifetime is proportional to the exponent of the barrier height). This in turn strongly improves the photoconductive gain, responsivity, and sensitivity (noise equivalent power and detectivity) in detectors and decreases the nonradiative recombination losses in photovoltaic devices. QD charging may be created by various types of selective doping. To investigate how selective doping affects photoelectronic properties of QD materials, we model, fabricated, and characterized AlGaAs/InAs QD structures with the n-doping of QD layers, doping of interdot layers, and bipolar doping, which combines the p-doping of QD layers with strong n-doping of the interdot space. We have measured spectral characteristics of photoresponse, temperature dependence of photocurrent, dark current, and spectral density of the noise current. In agreement with the modeling results, the experimental data show that providing the same electron population of QDs, the bipolar doping creates the most contrasting nanoscale profile with the highest barriers around dots. The bipolar doping allows us to realize the decoupled control of QD population and photocarrier lifetime, i.e. the decoupled control of optical and kinetic properties of QD nanomaterials. While the reported results have been obtained using MBE grown QD structures, the same approach may employed for tuning intersubband absorption and phototoelectron kinetics in colloidal QDs.

In recent years, the increase of carbon dioxide (CO₂) in the atmosphere has become a global environmental issue because of the serious problems, such as the “greenhouse effect”. The idea of mimicking the overall natural photosynthetic cycle of chemical conversion of CO₂ into useful fuels has been consistently gaining attention for more than thirty years. Such artificial photosynthesis allows direct conversion of CO₂ and water on photocatalysts into valuable hydrocarbon using sunlight at room temperature and ambient pressure to serve to reduce atmospheric CO₂ concentrations while providing on a renewable carbon fixation and energy storage.
In this presentation, we will report the utilization of solar energy to highly efficient conversion of CO₂ into renewable hydrocarbon fuel over structured nanomaterials. The geometric shape and exposure of specific crystal planes of the nanostructures as well as combination of graphene as a good electron collector and transporter are a requisite for the high level of photocatalytic reduction of CO₂.
References
(1) P. Li, Y. Zhou, Z. Zhao, Q. Xu, X. Wang, M. Xiao, and Z. Zou, J. Am. Chem. Soc. 2015, 137, 9547.
(2) W. Tu, Y. Zhou, and Z. Zou, Adv. Mater. 2014, 26, 4607
(3) H. Li, Y. Zhou, W. Tu, J. Ye, and Z. Zou, Adv. Funct. Mater. 2015, 25, 998.
(4) W. G. Tu, Y. Zhou, Z. G. Zou, Adv. Funct. Mater. 2013, 23, 4996
(5) W. Tu, Y. Zhou, Q. Liu, S. Bao, X. Wang, M. Xiao, and Z. Zou, Adv. Funct. Mater. 2013, 23, 1743
(6) Q. Liu, Y. Zhou, J. H. Kou, X. Y. Chen, Z. P. Tian, J. Gao, S. C. Yan, Z. G. Zou, J. Am. Chem. Soc. 2010, 132, 14385.

The technology of radiative cooling dissipates heat passively from Earth into outer space, through the so-called atmospheric transparency window in the spectrum of 8 to 13 micron. Effective radiative cooling could find various applications ranging from passive building cooling [1, 2] to renewable energy harvesting [3]. However, the temperature reduction experimentally demonstrated thus far has been relatively modest.

Here we theoretically show that ultrahigh performance radiative cooling with up to 60 ^oC below ambient is achievable, and experimentally demonstrate a temperature reduction that far exceeds previous works. In a populous area at sea level, such as Stanford at California, we have achieved an average of 37 ^oC below the ambient air temperature through a 24 hour day-night cycle, with a maximal temperature reduction of 42 ^oC that occurs at peak solar irradiance.

References:

[1] Raman, A. P., Anoma, M. A., Zhu, L., Rephaeli, E. & Fan, S. Passive radiative cooling below ambient air temperature under direct sunlight. Nature 515, 540–544 (2014).

[2] Advanced Research in Dry Cooling, Advanced Research Project Agency – Energy (2014).

[3] Byrnes, S. J., Blanchard, R. & Capasso, F. Harvesting renewable energy from Earth’s mid-infrared emissions. Proc. Natl. Acad. Sci. 111, 3927–3932 (2014).

Development of nanomaterials with controllable structures has been the focus of intense research related to their self-assembly, properties and possible applications. Electrospinning is a simple, low cost and scalable technique for obtaining one-dimensional nanostructures. The feasibility of shaping structure and morphologies of the fibers and the suitability of polymers for producing composites with other materials represents a promising approach for their exploitation in several applications, among them those for energy conversion devices. Accordingly, herein we show the versatility of electrospun nanofibers for their use in different components of photoelectochemical (PEC) systems. A scattering layer composed of TiO₂ nanocrystals assembled into a highly packed three-dimensional network of nanofibers was developed. This structure was employed as a nanofiber scattering layer (NFSL) on top of a transparent active TiO₂ layer composing the photoanode of dye-sensitized solar cells (DSSCs). The performance of the bi-layered photoanode achieved an 18% enhancement in the power conversion efficiency (PCE) compared to that of DSSCs with only active layer photoanodes. In another experiment we produced Pt/Pd hollow nanofibers by sputtering a Pt/Pd alloy (80/20 wt.%) onto polymer nanofibers (used as sacrificial template) for their employment as counter-electrodes (CEs) in DSSCs. The optimized device lead to a ~15% enhancement in power conversion efficiency (PCE), when compared to the commonly used flat Pt/Pd CEs with the same thickness. Additionally, a similar approach was employed to construct a hierarchically assembled hybrid CE based on copper sulfide (CuS) nanoplatelets grown on polymer nanofibers. The resulting CE was applied in a quantum dot-based PEC system for hydrogen generation in presence of sacrificial agents (S^2-/SO₃^2-). The nanofiber-supported CuS CEs achieved similar PEC response to that of Pt foil and stability tests showed that 85% of the initial photocurrent density was maintained after ~ 1 h, which is similar to that obtained with the Pt foil (86%). According to each application the hybrid nanostructures allow enhanced light harvesting efficiency within the photoanode, increased surface area for charge transfer and/or stability for long-term operation of the devices. These results suggest the nanofiber composites we have developed represent a simple, straightforward and promising strategy to tailor the components structure of PEC systems to boost their functional properties, due to the advantages afforded by the designed hybrid morphologies.

Harvesting solar energy as heat has many applications, such as power generation, residential water heating, desalination, distillation and wastewater treatment. However, the solar flux is diffuse, and often requires optical concentration, a costly component, to generate high temperatures needed for some of these applications. Here we demonstrate a floating solar receiver capable of generating 100°C steam under ambient air conditions without optical concentration. The high temperatures are achieved by using thermal concentration and heat localization, which reduce the convective, conductive, and radiative heat losses. This demonstration of a low-cost and scalable solar vapor generator holds the promise of significantly expanding the application domain and reducing the cost of solar thermal systems.

One of the challenges associated with making semitransparent perovskite solar cells for tandem applications is depositing a transparent electrode without damaging the perovskite. We have demonstrated that it is possible to sputter indium tin oxide if a buffer layer is used to protect perovskite. The indium tin oxide not only enables the fabrication of an efficient device, but also substantially improves stability by preventing the ingress of water, preventing the egress of the methylammonium iodide and preventing chemical reactions between halides and the electrode. Indium tin oxide improves the stability under one-son illumination at 100°C by more than four orders of magnitude. We have also been able to improve the thermal stability of perovskites by replacing methylammonium with cesium. We will show that packaged perovskite solar cells can perform well in a damp heat test at 85 degrees Celsius and 85% humidity, under prolonged one-sun illumination and in temperature cycling tests. We will also show that two-terminal tandems with perovskites on top of heterojunction silicon solar cells power conversion efficiency greater than 22%.

Tandem dye-sensitized solar cells (DSSCs) can theoretically use n-type and p-type cells connected in series to substantially boost efficiency compared to a single-cell devices. However, n-type DSSCs based on TiO₂ outperform counterpart p-type DSSCs based on NiO by at least a factor of five in power conversion efficiency. This mismatch limits the performance of tandem cells and highlights the need for a new material. Here, PbTiO₃, a perovskite, was identified by a materials-informatics approach as a promising new material due to the large bandgap and high dielectric constant. The dielectric constant describes the screening of charges within a material where a high dielectric constant could play a role in reducing the charge recombination by reducing the electrostatic attraction between separated electrons and holes. This effect could improve the fill factor which is the most problematic device performance metric in p-type DSSCs.
Devices were fabricated using the standard P1 chromophore and an I^-/I₃^- electrolyte. Both photoanodic and photocathodic performance were obtained by tuning the electrolyte solvent from an aprotic acetonitrile to a protic water, respectively. The switch from an aprotic to protic solvent increases protonation of the semiconductor surface, which in turn modulates the flatband potential. Upon the addition of 30% water to the acetonitrile electrolyte, the flatband potential shifted from being energetically favorable for electron injection from the P1 chromophore to being energetically favorable for hole injection from the P1 chromophore. This tuning of the semiconductor-electrolyte interface also decreased the charge transfer resistance by two orders of magnitude upon switching to a fully aqueous electrolyte, causing an increase in injection and J_SC. The resulting aqueous p-type DSSC has remarkably high fill factors of above 50%. This is the highest fill factor reported for p-DSSCs and is also the first report of an aqueous p-DSSC. With further investigation, PbTiO₃ could potentially replace NiO as the standard photocathode material.

Lanthanide-doped upconverting nanoparticles (UCNPs), with long-lived, ladder-like excited states capable of efficiently converting multiple low-energy photons into a higher-energy photon, show unique promise across a range of applications from single-molecule imaging¹ to solar energy harvesting. Two key attributes that limit their performance, however, are miniscule absorption cross-sections and narrow absorption bandwidths. To overcome these shortfalls, organic dye molecules with significantly stronger optical absorption can be attached to the surface of the UCNP to form hybrid antenna-upconverter architectures whereby optical energy is absorbed by the dye molecules and funneled to the lanthanide ions within the UCNP^{2, 3}, greatly enhancing both the effective absorption cross-section and bandwidth. However, the precise nature of the dye-UCNP energetic coupling mechanism is not known, and while promising, this hybrid system suffers from poor photostability and rapid degradation. Using a comprehensive suite of steady-state and time-resolved spectroscopies on model dye-UCNP complexes, we elucidate several key aspects of the energy transfer mechanism, revealing critical insight and design rules towards maximizing the coupling efficiency and photostability in these systems. Evidence herein indicates that the excited triplet state of the dye mediates efficient energy transfer between the dye molecules and the UCNPs, where intersystem crossing between the singlet and triplet manifolds in the dye is enhanced by the heavy atoms of the UCNP. Because of the increased role of long-lived triplet states, oxygen-mediated quenching is suspected as a primary factor in photodegradation, and increased photostability can be obtained in oxygen-free environments. This new understanding could lead to substantial improvements in the efficiency and stability of photon upconversion, a viable route to improving photovoltaic efficiency by utilizing sub-bandgap infrared photons that would otherwise go unused.


¹ Gargas, D. J.; Chan, E. M.; Ostrowski, A. D.; Aloni, S.; Altoe, M. V. P.; Barnard, E. S.; Sanii, B.; Urban, J. J.; Milliron, D. J.; Cohen, B. E.; et al. Engineering Bright Sub-10-nm Upconverting Nanocrystals for Single-Molecule Imaging. Nat. Nanotechnol. 2014, 9, 300−305.

² Zou, W. Q., Visser, C., Maduro, J. A., Pshenichnikov, M. S. & Hummelen, J. C. Broadband dye-sensitized upconversion of near-infrared light. Nat. Photonics 2014, 6, 560–564.

³ Chen, G.; Damasco, J.; Qiu, H.; Shao, W.; Ohulchanskyy, T. Y.; Valiev, R. R.; Wu, X.; Han, G.; Wang, Y.; Yang, C.; Agren, H.; Prasad, P. N. Nano Lett. 2015, 15, 7400–7407.

In recent years, colloidal quantum-dots based light-emitting diodes (QD-LEDs) have been considered as the attractive display device because of remarkable electrical/optical characteristics of colloidal quantum dots. QD-LEDs are of particular interest due to their wide-range color tunability, high brightness and good color purity by narrow emission bandwidth. Challenges remain, however, in achieving the necessary multilayer device structures using printing process.
In this study, for the first time, quantum dots and silver electrode as a cathode was printed by electrohydrodynamic (EHD) Jet printer and was applied to all solution-processed QD-LED (quantum dot light-emitting diodes). The J-V-L characteristic of the QD-LED with silver cathode (work function, 4.26-4.74) was compared with the QD-LED with an evaporated aluminum cathode (work function, 4.06-4.26). The QDs as the emitting layer was also printed by EHD-Jet, and ZnO NPs and TiO₂, as the carrier transporting layers were synthesized using solution mediated process. The optimized QD-LED device showed a luminance of 5,710 cd/m², current efficiency of 1.75 cd/A, and EQE of 1.51 %.

One of the efficient strategies to improve the performance of the solar cell devices is interface engineering. Herein, we synthesize a new structure of graphene, i.e., three-dimensional (3D) reduced graphene oxide scaffold (rGS) using a novel and low-cost solution process. In this regard, we first synthesize a core-shell structure of ZnO-graphene QDs, with an average size of 4 nm. Then, an acidic solution is employed to remove the ZnO cores. As a result of this process, a hollow structure of graphene is achieved. Afterward, rGS layer is deposited on the surface of TiO₂-coated FTO glass using electrophoretic method. Later, this 3D structure of graphene film with a huge surface area and an excellent conductivity is employed as an interface layer in perovskite solar cell device. The results of characterization tests and photovoltaic measurement show that rGS improves the carrier transportation, yielding a 27% improvement in device performance as compared to conventional devices. Finally, a power conversion efficiency of 17.2% is achieved for device based on the rGS. In addition, rGS improves the stability and reduces the hysteresis effect of the solar cell device.

Portable electronic devices have become increasingly widespread. Because these devices cannot always be tethered to a central grid, powering them will require low-cost energy harvesting technologies. As a response to this anticipated demand, we demonstrate transparent organic solar cells fabricated on flexible substrates, including plastic and paper, using graphene as both the anode and cathode. Optical transmittance of up to 69% at 550nm was achieved by combining the highly transparent graphene electrodes with organic polymers that primarily absorb in the near-IR and near-UV regimes. To address the challenge of transferring graphene onto organic layers as the top electrode, we develop a room temperature dry-transfer technique using ethylene-vinyl-acetate as an adhesion-promoting interlayer. The power conversion efficiency achieved for flexible devices with graphene anode and cathode devices was 2.8-3.8% at for optical transmittance of 54-61% across the visible regime. To the best of the authors' knowledge, this is one of the best-performing devices in literature with all-carbon electrodes and the first demonstration of a transparent flexible solar cell. In addition, these results demonstrate the versatility of graphene in optoelectronic applications and is important step towards developing a practical power source for distributed wireless electrical systems.

Semiconductor quantum dot (QD) is one of the most attractive nanomaterials for photovoltaic devices. With their relatively large extinction coefficients and a wide light absorption range over visible wavelengths, QDs can be effective light absorbers. However, despite these attractive properties, their solar cell function has not been well understood. In particular, their exciton states, charge separation and recombination processes with the TiO₂ electron acceptor and an electrolyte in relation to the solar cell performance have been poorly understood. For example, the excited electron and hole can readily be trapped by the surface states, losing initial excited energy, however their relations to the solar cell function is not clear. Charge separation of QD-TiO₂ interface is relatively slow compared to dye sensitized system, which may compete with charge trapping process or exciton state decays. Furthermore, charge regeneration process with the electrolyte can be important compared to the charge recombination dynamics. In this presentation, we will demonstrate quantitative analysis of charge trapping in QDs and relationship of the QD nanostructures with the interfacial electron transfer reactions.

Several types of QDs with a narrow size distribution are synthesized to control the potential energy levels of the conduction and valence bands. We analysed influence of QD surface states on the interfacial charge transfer dynamics, and their interfacial structure is modified to control the dynamics. The relationship between the QD surface/interfacial structure and solar cell performance will be discussed.

This work was financially supported by JST PRESTO program (Photoenergy Conversion Systems and Materials for the Next Generation Solar Cells) and JSPS KAKENHI Grant Number 16K05885, Japan. The author also acknowledges Australian Research Council (ARC) LIEF grant (LE140100104) and the Office for University-Industry Collaboration, Osaka University, for the financial supports.

Highly inter-aligned carbon nanotubes (CNTs) demonstrate excellent mechanical, electronic, and catalytic properties, which exhibit high surface area with enhanced optoelectronic and electronic performance. An artificial photovoltaic neural cells has been developed using coaxial, inter-aligned, ultrastrong and highly conductive CNT yarns (CNY) in both electrodes with solid phase electrolyte. The hydrothermally crystallized mp-TiO₂ film along with a thin (50nm) underlayer interfaced with the high surface area CNY has been grafted around the working electrode (WE), which has been found capable in wide range of structural flexibility (30-330⁰) without changing signal transmitting characteristics. The photoenergy conversion efficiency (~5%) is independent of cells shape or position. Due to the yarn shape, the CNY sensors can easily be woven into reinforced carbon fabric for aerospace composite application. Moreover, this embeddable intrinsic sensor network opens an innovative approach of aerospace system’s structural health monitoring.

Solar cells comprised of densely packed films of semiconductor nanocrystals (NC-solids) hold promise for third-generation solar cells, with certified efficiencies recently surpassing 11%. While the tunability of the optical, electronic, and phononic properties of NC-solids is one of their most attractive features, these many degrees-of-freedom present a challenge to understanding the carrier dynamics in these materials.
We use temperature dependent transient techniques on various solar cell device architectures to investigate the carrier dynamics in PbS NC-solids. We show that electron and hole transport occurs through temperature activated hopping between NCs, where the activation energy and prefactor to the carrier mobility results from a complex interplay between the NC size and choice of surface terminating ligand. Results are compared to analytical models and DFT calculations. We use our findings to discuss steady state transport properties, providing guidelines to further increase the performance of NC-solid solar cells.
Relevant Literature
[1] Yazdani, N. A., Bozyigit, D., Yarema, O., Yarema, M., & Wood, V. (2014). Hole Mobility in Nanocrystal Solids as a Function of Constituent Nanocrystal Size. The Journal of Physical Chemistry Letters, (5), 3522–3527.
[2] Bozyigit, D., Lin, W. M. M., Yazdani, N., Yarema, O., & Wood, V. (2015). A quantitative model for charge carrier transport, trapping and recombination in nanocrystal-based solar cells. Nature Communications, 6, 6180.

There is considerable interest in tin sulfide (SnS) as an alternative PV material to toxic lead and cadmium containing absorber materials in solar cells. Both tin and sulfur are abundant in nature, environmentally benign and inexpensive. To date literature reports of SnS PV power conversion efficiencies have been less than 5% [1]. However, Orthorhombic SnS with a band gap of 1.4 eV theoretically could reach efficiencies as high as 24% [2]. The field is currently held back by a lack of understanding of the relationships between the chemistry and structure with optoelectronic properties. In particular where the conduction and valence bands lie relative to the vacuum level and the presence and effect of defects are considered here.
In this paper the synthesis of SnS is studied as a function of temperature, time and precursor ratio to control crystal structure and stoichiometry. Inorganic and organic surface treatments are used to control electronic properties in thin films. In particular the relationships between synthesis condition with band-gap and Fermi energy position are studied. Preliminary results of solar cells are also presented and an understanding of the challenges the field faced considered.
References
[1] Sinsermsuksakul, P.; Sun, L.; Lee, S. W.; Park, H. H.; Kim, S. B.; Yang, C.; Gordon, R. G. Advanced Energy Materials 2014, 4
[2] Loferski, J. J. Journal of Applied Physics 1956, 27, 777

Hybrid solar cells based on n-type crystalline silicone (n-Si) with a solution processed polymer emitter layer are emerging as a cost effective alternative to common inorganic heterostructure solar cells. However, the design of these devices needs to accommodate for the deviating physical properties of these soft materials. Similar to the a-Si:H/c-Si heterojunction, in which sputter damage during the deposition of the transparent conductive oxide degrades the a-Si:H/c-Si interface,¹ thin organic layers suffer damage when the contact layers are deposited. As a consequence the device efficiency is severely diminished.² To overcome this problem in polymer/n-Si hybrid solar cells a 5 nm thick molybdenum oxide (MoO_X) anode buffer layer was introduced between the polymer poly-(3-hexylthiophene-2,5-diyl) (P3HT) and the semitransparent gold contact.³ MoO_X has the advantage that very thin, stable, and homogeneous layers can be deposited. As a consequence, the open circuit voltage increased from less than 400 mV to 612 mV. This is due to the fact that MoO_X prevents the diffusion of Au atoms into the polymer layer. Furthermore, the additional layer made it possible to successfully scale the cell size from 0.16 cm² up to 1 cm² while the overall efficiency increased to 10.1 %.


References:
1 B. Demaurex, S. De Wolf, A. Descoeudres, Z. Charles Holman and C. Ballif, Appl. Phys. Lett., 2012, 101.
2 M. Zellmeier, J. Rappich, M. Klaus, C. Genzel, S. Janietz, J. Frisch, N. Koch and N. H. Nickel, Appl. Phys. Lett., 2015, 107, 203301.
3 T. Hori, T. Shibata, V. Kittichungchit, H. Moritou, J. Sakai, H. Kubo, A. Fujii and M. Ozaki, Thin Solid Films, 2009, 518, 522–525.

The limitation imposed on light harvesting applications due to higher reflectivity of silicon can be minimized by reactive ion etching. This surface modification creates a forest of silicon micro-spikes and increases surface area of sample. When the spikes’ height exceeds the wavelength of incoming light, light is trapped on the surface through multiple pathway scattering and black silicon (bSi) is formed.

Cerium oxide (CeO₂) is believed to have good photoactivity, and finds many applications including photoelectrolysis. However, higher band gap limits the efficiency of water splitting process. We suggest black silicon surfaces as substrates for CeO₂ sputter coating to increase photon-material interaction. An additional catalytic layer of platinum is deposited to create highly energetic electrons as a result of plasmonic resonance and enhances incident photon to current efficiency (IPCE). The difference of surface current for laser on and off condition is found to be 32 times more in black silicon substrate than silicon when similar nanolayers were deposited.

Fabrication of bSi surface was done by deep reactive ion etching which is a single-step, nonlithographic process capable of patterning the whole wafer. CeO₂ nanolayer of thickness 40 nm was deposited on it by magnetron sputtering process. The thickness was optimized for optical absorption by finite domain time difference (FDTD) method. The resulting structure was coated with 10 nm layer of platinum. Optical simulations showed the metal thickness should not exceed the skin depth for better light harvesting. Comparison between reflection from silicon and bSi surfaces indicates huge scaling (by factor of 8) of reflection as the result of modified surface. A 650 nm wavelength laser was used to illuminate samples of size 1 cm x 1 cm with approximate power of 20 mW. The samples were biased with external voltage of 0.2 mV. A four point measurement was carried out by probing the sample from 4 corners to measure the sheet resistance and surface current on the samples for both on and off condition of the laser.

The resistance on silicon substrate sample was around 11 Ω for laser off state and the value decreases slightly to 9 Ω when the laser was turned on. The photocurrent rises from 0.017 mA to 0.021 mA in those conditions. On the other hand, the black silicon substrate sample produced higher resistance of 70 Ω in dark which decreased to 1.5 Ω for laser on state. Due to surface etching, the bSi samples were found to have much higher resistivity than silicon when Hall effect measurement was performed. The significant drop in on state resistance was due to the higher photoactivity and photocurrent was observed to increase from 2 μA to 0.13 mA for off and on state respectively. The results could be reproduced for many cycles suggesting stability of the materials as well as structure

Nanotubular array photoanodes are widely used recently in dye-sensitized solar cells (DSSCs) applications. However, the high performance expected from utilizing these one-dimensional (1-D) structures is quite limited by the low dye adsorption. This is mainly because of their smooth surface, which renders them lower efficiency compared to nanoparticales and nanochannels counterparts. In this work, we demonstrate a straight forward approach to fabricate perovskite electrospun nanofibers to be used as efficient photoanodes in DSSCs. Highly porous perovskite nanofibers can be obtained by increasing the sintering temperature over 600 C. This increases the dye adsorption over the nanofibers surface, with increased conversion efficiency compared to the traditional nanotubes counterparts. The thermal annealing temperature was optimized by TGA analysis. Also, XRD, SEM, TEM, and XPS measurements are performed to verify the crystallinity and the chemical characteristics of the fabricated NTs. Moreover, UPS measurements are performed to estimate the band edge positions for the perovskite structures with respect to the vacuum energy level. Full device is demonstrated based on the fabricated nanofibers. The photocurrent-voltage curves revealed the enhancement of the solar cell performance. The concept of highly porous electrospun nanofibers as photoanodes should be useful for the future development of DSSCs devices.

We report on the effect of annealing at different temperatures and durations on the structure, optical properties and wettability of layered TiO₂-based water-splitting devices. The water-splitting device used in this study is composed of three different layers (TiO₂, Al₂O₃ and SiO₂) on top of a silicon wafer. A layer of 200 nm SiO₂ buffer layer was deposited using a Plasma Enhanced Chemical Vapor Deposition (PECVD) system, followed by 30 nm thick Al₂O₃ film using an Atomic Layer Deposition (ALD) system, then 30 nm of amorphous TiO₂ using ALD. Gold plasmonic nanoparticles (Au NPs) were then deposited using Thermal Evaporator Deposition system on top of the obtained multilayers specimen to evaluate the influence of Au NPs presence as a Localized Surface Plasmon Resonance (LSPR) material to extend optical absorption to the visible region. The fabricated samples were then annealed at 450^oC and 600^oC for different durations. The structure and phase identifications were determined using X-ray Diffractometer (XRD) and High Resolution Transmission Electron Microscope (HRTEM). UV-Vis spectroscopy and Tauc bandgap analysis were used to study the influence of annealing on absorption spectrum and optical bandgap. Besides, wettability alteration was screened both in situ using an Environmental Scanning Electron Microscope (ESEM) and ex situ using sessile drop technique to measure the contact angle with respect of the sample microstructure. Our findings revealed that the heat treatment led to the transformation of amorphous TiO2 to its more stable phases anatase and rutile. This transformation enhanced significantly the optical properties and increased the hydrophilicity of the device. More importantly, the presence of the LSPR material allows the photoactivity of the device to be extended to visible range

Self-assembled III-V-IV alloys, such as (AlP)_xSi_5x-3, have recently emerged as promising Si-compatible absorbing materials for next generation PV materials due to their increased absorption in the visible wavelength range, and their excellent lattice matching to silicon.¹ While controlled low-temperature processes based on the self-assembly of "Al-P-Si₃" molecular cores yield a limiting AlPSi₃ composition, the properties of lower Si concentration analogs such as AlPSi₂ are less explored. Here we present density functional theory investigations of the structural, thermodynamic and optical properties of AlPSi₂ alloys containing a range of specifically tailored nanostructures, including AlP/Si super-lattices and alloys comprised of intact "Al-P-Si₂" building blocks possessing a silicon-like structure. Like their AlPSi₃ counterpart the AlPSi₂ alloys are found to be stable relative to their elemental reference states (-100 eV/atom) but metastable relative to zincblende AlP and silicon (+200 eV/atom). The electronic, dielectric and optical properties of the alloys in their ground state structures are computed using a meta-GGA (TB09) approach which yields excellent results for Silicon. Our results confirm the expectation that materials in the AlPSi₂ composition range possess even lower energy absorption tails than their AlPSi₃ analogs, extending above l ~ 500 nm. Extensions to several technologically relevant III-V-Ge2 alloys are also briefly discussed. ¹ L. Jiang, T. Aoki, D.J. Smith, A.V.G. Chizmeshya, J. Menendez and J. Kouvetakis, Chemistry of Materials 26, 4092 (2014).

There is a critical need for photocatalytic materials that efficiently convert solar energy into chemical fuels (e.g., hydrogen by solar water splitting) to achieve long-term energy sustainability. The perovskite cubic phase of SrTaO₂N is a promising photocatalyst on account of its ability to absorb visible light and long-term stability. Its photoactivity can be enhanced through control of crystal shape and single crystallinity, but achieving such control is a synthetic challenge. Here, we demonstrate that single-crystalline SrTaO₂N nanoplates can be synthesized by topotactic nitridation of single-crystalline Sr₂Ta₂O₇ nanoplates, providing a strategy towards oxynitride single crystals with non-equilibrium shape. Specifically, Sr₂Ta₂O₇ nanoplates with (010) facets expressed are obtained by aerosol-assisted molten salt synthesis (AMSS). Subsequently, nitridation yields SrTaO₂N nanoplates with (100) facets exposed. In contrast, cubic-like SrTaO₂N single crystals were obtained when SrCl₂ was added as molten media during nitridation. With co-catalysts deposited, the photocatalytic performance of the SrTaO₂N nanoplates for oxygen evolution reaction (OER) is superior to both cubic-like SrTaO₂N and polycrystalline SrTaO₂N reference. Diffuse reflectance spectroscopy (DRS) indiacated much less absorption around 700 nm for the SrTaO₂N nanoplates compared to the other two samples, which is ascribed to detrimental defect sites. Fewer defect sites in SrTaO₂N nanoplates may account for the enhanced photoactivity. Significantly, other oxynitride single crystals with non-equilibrium morphology and enhanced photoactivity should be possible through topotactic nitridation of metal oxide templates derived by AMSS.

Electron transfer in molecular wires are of fundamental importance for a range of optoelectronic applications. The impact of electronic coherence and ionic vibrations on transmittance are of great importance to determine the mechanisms, and subsequently the type of wires that are most promising for applications. In this work we use the real-time formulation of time-dependent density functional theory to study electron transfer through oligo-p-phenylenevinylene (OPV) and the recently synthesized carbon bridged counterpart (COPV). A system prototypical of organic photovoltaics is setup by bridging a porphyrin-fullerene dyad, allowing a photo-excited electron to flow from the porphyrin chromophore to the C60 electron acceptor through the molecular wire. The excited state is described using the fully self-consistent delta-SCF method. The state is then propagated in time using the real time TD-DFT scheme, while describing ionic vibrations with classical nuclei. The quantum mechanical currents are calculated and correlated in time with bond-distances and changes in local potential and charge. This provides a microscopic insight to vibrational contributions to electron transport, and steric effects on tunneling effects in liked porphyrin-fullerene dyads.

Carbon is the key to many technological applications that have become indispensable in our daily life. Altering the periodic binding motifs in networks of sp³-, sp²-, and sp-hybridized C-atoms is the conceptual starting point for a wide palette of carbon allotropes. To this end, the past two decades have served as a test-bed for measuring the physico-chemical properties of low-dimensional carbon with the advent of fullerenes (0D), followed in chronological order by carbon nanotubes (1D), carbon nanohorns, and, most recently, by graphene (2D). These species are now poised for use in catalysis.

Expanding global needs for energy have led to a significant effort to develop alternatives to fossil fuels. While alternative sources for energy are already in use, they comprise a small percentage of the energy demands needed to carry us through the 21st century. No single source will solve the global needs, but the development of photocatalysis has vast potential as a point-of-use power source.

I report on our efforts regarding a unifying strategy to use the unprecedented charge transfer chemistry of 0D fullerenes, the ballistic conductance of 1D carbon nanotubes, and the high mobility of charge carriers in 2D graphene, together in a groundbreaking approach to solving a far-reaching challenge, that is, the efficient use of the abundant light energy around us. For example, hybrid materials based on nanocarbons and metaloxides are the ideal design for realizing breakthroughs in high photon conversion efficiencies suitable for the catalysis of water.

The reverse water gas shift (RWGS) reaction driven by Nb₂O₅ nanorod-supported Pd nanocrystals without external heating using visible and near infrared (NIR) light is demonstrated. By measuring the dependence of the RWGS reaction rates on the intensity and spectral power distribution of filtered light incident onto the nanostructured Pd@Nb₂O₅ catalyst, it is determined that the RWGS reaction is activated photothermally. That is the RWGS reaction is initiated by heat generated from thermalization of charge carriers in the Pd nanocrystals that are excited by inter-band and intra-band absorption of visible and NIR light. Taking advantage of this photothermal effect, a visible and NIR responsive Pd@Nb₂O₅ hybrid catalyst that efficiently hydrogenates CO₂ to CO at an impressive rate as high as 1.8 mmol gcat^-1 h⁻¹ is developed. The mechanism of this photothermal reaction involves H₂ dissociation on Pd nanocrystals and subsequent spillover of H to the Nb₂O₅ nanorods whereupon adsorbed CO₂ is hydrogenated to CO. This work represents a significant enhancement in our understanding of the underlying mechanism of photothermally driven CO₂ reduction and will help guide the way towards the development of highly efficient catalysts that exploit the full solar spectrum to convert gas-phase CO₂ to valuable chemicals and fuels.

Photocatalytic water splitting is a promising technology that permits H₂ production directly from water and solar light, which are clean, renewable, and abundant resources. Carbon materials have been extensively used for photocatalytic water splitting because they possess special physicochemical properties. Carbon nanomaterials can act as supporting materials with high surface areas, catalysts for the reduction reaction, and efficient electron-transfer mediators. Previously, we studied the application of a carbon nanofiber (CNF)/photocatalyst composite in photocatalytic H₂ production. We first applied a flexible mat of CNFs for H₂ production under UV irradiation. We further investigated the fabrication of CdS and CNF mats and applied this composite to visible light-induced H₂ production. In this study, to investigate the role of CNF, we fabricated a trilayer heterostructure of CdS/CNF mat/Pt-nanoparticle-deposited TiO₂ (Pt-TiO₂) composite. We successfully fabricated trilayer heterostructures of CdS/CNF mat/Pt-deposited TiO₂ (Pt-TiO₂) for solar H₂ production. The CNF mat was prepared by electrospinning and carbonization. CdS and Pt-TiO₂ were coated on the front and back of the CNF mat by doctor blading. Under visible-light irradiation on the CdS side, the addition of the Pt-TiO₂ coating improved the H₂ production by a factor of 3.4. This suggests that the H₂ production reaction could occur on Pt-TiO₂, which is not active under visible irradiation; therefore, the CNF mat could act as an efficient photogenerated electron-transfer mediator from CdS to Pt-TiO₂. The H₂ production rates of the trilayer CdS/CNF/Pt-TiO₂ heterostructures were strongly affected by the thickness of the CNF mat and the carbonization temperatures used in production, which affect the resistance of the CNF mat between the CdS and Pt-TiO₂ sides. The results clearly demonstrated that the CNF acted as an efficient electron-transfer mediator as well as a support material.

In conventional DSSCs, the charge carrier recombination takes place at TiO₂/dye/electrolyte and transparent conducting oxide (TCO)/electrolyte interfaces. The interface between the TiO₂ nanoparticles and the TCO is exposed to the electrolyte due to the porous structure of the TiO₂ layer. The electron leakage by backward transfer takes place from the TCO layer to the electrolyte. The presence of an electron blocking layer (EBL) is significant for reducing undesirable charge carrier recombination. Generally, a TiO₂ thin layer deposited on TCO causes lower transmittance spectra resulting in reduced interior light inside the cell. Therefore, several metal oxides, such as Al₂O₃, ZnO, CuO and Nb₂O₅ which have larger band gaps than TiO₂, have been employed as EBL on TCO layer.

The present study is the premier example of investigation into EBL performance of ZrO₂. The work also highlighted the need for highly transparent, the lowest charge carrier resistance experimental condition for ZrO₂ – EBL, thus, DSSC efficiency loss could be minimized. In addition, optical properties at the interfacial region between mesoporous TiO₂ and ZrO₂ EBL have been investigated using UV-Vis. Current-density–voltage characteristics (J–V), electrochemical impedance spectroscopy (EIS) and incident photon-to-current efficiency (IPCE) have been studied for better understanding of the kinetics governing the photovoltaic properties.

A ZrO₂ thin film was deposited on a fluorine-doped tin oxide (FTO) electrode by hydrothermal treatment and its application as a new blocking layer material for dye-sensitized solar cells (DSSCs) was investigated. According to current-voltage (I-V) characteristics and electrochemical impedance spectra (EIS), it was found that the ZrO₂ layer functioned as both a blocking layer and a heat treatment protector for transparent conducting oxide (TCO) layer. The use of ZrO₂ layer as blocking layer increases the electron lifetime and decreases the recombination from TCO to the electrolyte. In addition to photovoltaic performance, ZrO₂ keeps resistivity of TCO stable after heat treatment compared to TiO₂ blocking layer. As a result, the overall energy conversion efficiency of the DSSC with ZrO₂ blocking layer was enhanced by 47 % for front side illumination compared to that of bare FTO substrate and 30 % compared to that of commercial TiO₂ blocking layer for backside illumination. This study demonstrated that ZrO₂ could be a promising alternative to the conventional TiO₂ blocking layer for high efficiency DSSCs.

Developing new materials to capture the sunlight have gained tremendous attention to meet the increasing demand of energy. In order to enhance the photoconversion efficiency of well-established TiO₂ nanotubes photoanodes, Niobium (Nb) is considered for its ability to increase TiO₂ conductivity. Accordingly, Nb₂O₅-TiO₂ ultrathin wall nanotube arrays was successfully grown on Ti-Nb alloy via anodization process in glycerin based electrolyte. The obtained nanotubes were characterized for their morphological, compositional, structural, and photoelectrochemical properties using field emission scanning electron microscope (FE-SEM), energy dispersive X-ray (EDX), Raman, glancing angle X-ray diffraction (GA-XRD), UV-Vis spectroscopy, and three electrode cell configuration along with a potentiostat and Xenon lamp. XRD patterns revealed the formation of mixed oxides of TiO₂ and Nb₂O₅, where this combination of oxides showed higher stability at elevated temperatures up to 650 ^oC. For samples annealed at different temperature, further analysis showed minor change in the crystallite size compared to that of TiO₂ and hence, very small microstrain was induced. While UV-vis spectrum showed slight or negligible enhancement in the absorption, photoelectrochemical indicated higher performance in terms of larger photocurrents (0.3 mA/cm²) for TiO₂-Nb₂O₅ in comparison with bare TiO₂ (0.18 mA/cm²). Also the cut-off of incident photon to current efficiency (IPCE) for the mixed oxide nanotubes was shifted to higher wavelengths (370 nm) than TiO₂ (350 nm), therefore such enhancement in the photoresponse is attributed to improved electron mobility.

Nanoflowers and nanospheres of Pd4S have been prepared for the first time from a single source precursor complex, [PdCl2(PhS-CH2CH2CH2-NH2)] (1), by its one pot thermolysis at 195 °C with oleylamine and the mixture of oleic acid and octadecene. In oleylamine, flower shaped nanoparticles of Pd4S were formed but in an oleic acid and octadecene mixture (1:1) the product was nanospheres of Pd4S (size in the range ~23–38 nm and 15–28 nm, respectively). Nanocomposites of Pd4S were also prepared with graphene oxide (GO) at room temperature. The synthesized nanoflowers of Pd₄S, nanospheres of Pd₄S and their naoncomposites with graphene oxide (GO-Pd₄S) have been authenticated with HRTEM, powder X-ray diffraction (PXRD) and TEM-EDX. The XPS of Pd4S nanoparticles indicates the oxidation states of Pd and S are both zero with a PdS ratio 4.1 : 0.9. For the catalysis of Suzuki–Miyaura coupling reactions the nanoflower of Pd₄S and nanospheres of Pd₄S individually and in the form of composites with graphene oxide (GO-Pd₄S) were explored. The nanoflowers of Pd₄S are superior than the nanospheres of Pd₄S and increasing in graphene oxide grafted nanoflowers /nanospheres of Pd₄S. The catalysis were carried out in aqueous ethanol. The catalytic efficiency follows the order GO–Pd4S-nanoflowers > GO–Pd4S-nanospheres > Pd4S nanoflowers > Pd4S nanospheres. The conversion was ~99% (in 5 h; at 80 °C) for the composite of GO-Pd₄S nanoflowers (Pd: 0.2 mol%). The recyclability of the GO–Pd4S nanoflower catalyst was examined for the coupling reaction and conversion was found to be ~46% in the fourth run even after increasing the reaction time to 12 h. To understand whether the catalytic process with the GO–Pd4S nanoflowers was homogeneous or heterogeneous mercury poisoning, triphenylphosphine and three phase tests were carried out. They suggest that active Pd leached from GO–Pd4S nanoflowers does the catalysis significantly in a homogeneous fashion. Overall the catalysis appears to be a cocktail of homogeneous and some heterogeneous nature.

In this work, we developed a novel metal contact design for organic-inorganic Si nanostructure hybrid solar cells where the electrode is placed between poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and Si substrate, compared to conventional front contact design for hybrid solar cells. Furthermore, for the effective carrier collection, the selective carrier contact such as vanadium oxide (V₂O_x) film was inserted between the Si substrate and the front metal electrode. Photogenerated hole carriers can be selectively collected through the front contact thanks to the inversion layer of V₂O_x/Si junction, leading to the significant improvement of open-circuit voltage and fill factor compared to conventional metal contact. In addition, the microscale metal mesh electrode with high transparency and conductivity were fabricated via the lithography and electroplating processes. In particular, our hybrid solar cells show the remarkably high value of external quantum efficiency over the entire wavelength of 500 to 1000 nm, indicating that the photocarriers generated in Si nanostructure can be effectively collected through the mesh electrode.

ZnS-CuInS₂ (ZCIS) alloy nanostructures are becoming increasingly important materials because of their low toxicity and photoluminescence properties. Here we explore the emission properties of ZCIS QDs capped with different ligands, which exhibit Zn:Cu dependent emission properties. Absorption and photoluminescence excitation spectra indicate band gap transition and composition independent light absorbing state. The emission spectra point out the existence of two emissive states with lifetimes of ~10 ns and ~100 ns. The photoluminescence and time resolved emission analysis provide insight into the synergy between the two intraband states and the possibility of modulating the emission through variation in the Zn/Cu ratio. Lastly we examine the effect of ligand type on charge transfer phenomena between ZCIS nanoparticles. Better understanding of light absorbing and emission mechanisms in alloyed nanostructures is essential for future development of photoelectric and display devices.


Zaiats, G.; Kinge, S.; Kamat, P. V. Origin of Dual Photoluminescence States in ZnS-CuInS₂ Alloy Nanostructures. J. Phys. Chem. C 2016, DOI: 10.1021/acs.jpcc.6b01905

Thermal energy delivered at temperatures of 80 °C to 400 °C is in significant demand in everyday life for domestic, industrial and power generation purposes. Supplying useful thermal energy from solar radiation holds great promise to further displace fossil fuel usage. However, efficient and cost effective solar thermal receivers are crucial to promote solar thermal energy. In this work, we developed and characterized a lab-scale silica aerogel based solar thermal receiver and demonstrated improved solar thermal conversion efficiency. We incorporated tailored optically transparent, thermally insulating silica aerogels, which enables the use of simple black paint (e.g., Pyromark) instead of expensive selective surfaces to achieve high efficiency. The nanostructure of the silica aerogel was tuned such that it can transmit 96% of the solar radiation and at the same time, block heat losses via conduction, convection and radiation. The effective solar absorption and thermal emissivity of an aerogel covered with the black paint was measured to be 0.93, and 0.2 at 400 °C, respectively, which makes it comparable to state-of-art selective surfaces. A custom-built solar thermal receiver test setup was used to study the performance of the aerogel-based receiver under simulated solar fluxes (concentration ratios from 1 to 30) in the temperature range of 80 °C to 400 °C. The aerogel receiver efficiency is expected to increase from 60% (black paint absorber) to greater than 85% at 400 °C under 30 suns. Our results show that a silica aerogel-based solar thermal receiver is a viable technology to provide useful and cost effective thermal energy for a broad range of applications.

Semiconductors are used as light harvesting materials in photovoltaic devices. Materials used for this purpose should satisfy two major optical requirements: absorb the maximum amount of sunlight while having long exciton decay kinetics. Type II nanocrystals (NCs) have anomalously long exciton lifetimes due to spatial separation of electron and hole wave function. It is believed that this spatial separation of carriers should also cause these materials to absorb poorly at the band edge. Here we discuss this popular misunderstanding.
We synthesize ZnTe/CdS type II NCs. Their absorption properties are examined and interpreted. We find that these materials can indeed exhibit absorption cross sections as large as or larger than type I NCs, while still having very weak transition dipole moments. An explanation for these curious observations is provided.

Power conversion efficiency in perovskite-based solar cells has recently improved to ≥20%, however, there is insufficient understanding of the underlying optoelectronic device function. Among all perovskite materials as candidates for the light harvesters in such solar devices, organolead halide perovskites, MAPbX₃ (X=I, Br, Cl), have stood out with their outstanding optoelectronic properties such as tunable bandgaps, long electron-hole diffusion length and high electron/hole mobility. Indeed, replacement of ETL/HTL with graphene-derived materials (graphene oxide, reduced graphene oxide, n/p-doped graphene, etc.) has emerged as a pathway to improve device performance, and replaced traditional use of TiO₂ and Al₂O₃ in these devices. Nevertheless, unclear film growth, nucleation and degradation mechanisms at the graphene/perovskite hybrid interfaces require understanding of interfacial working mechanisms and perovskite film formation, particularly over oxide-based electron/hole conductors. Moreover, graphene/perovskite structure-property relationships are not well understood due to unclear chemistry/poor characterization at the interfaces of ETL/perovskite/HTL hybrids. To explore interfacial working mechanisms and perovskite film formation in graphene-derived perovskite solar cells, we performed variable temperature (≤600°C) in situ spectroscopy (infrared absorption, micro-Raman, UV-vis-NIR, x-ray photoelectron and luminescence). Our studies targeted perovskite/graphene interfaces and perovskite growth mechanisms to overcome detrimental effects of stability factors such as incomplete lead precursor conversion, inconsistent crystallite formation/film uniformity, and weak cation-anion-solvent coordination. Effect of film thickness, lead content, stoichiometry control, underlayer/overlayer composition, and perovskite growth temperature were optimized for better film efficiency and charge transport. To address film scalability and stability, we studied opto-thermal changes in reduced graphene/graphite oxide (RGO) upon halide-based (CH₃NH₃PbI₃, CH₃NH₃PbBr₃, CH₃NH₃PbCl₃) perovskite deposition, and performed spectroscopic analysis derived from the intensity and peak areas of perovskite vibrational normal modes of C-H (~2800-3200 cm^-1) and N-H (~2000-2800 cm^-1) and their interfacial reactions with oxygen functional groups in RGO. Controlled perovskite formation was achieved at room temperature for bromide/chloride-based perovskites resulting improved chemical stability with heat (vs. iodide derivative) that were decomposed at ≥150°C. Poor perovskite formation was monitored on RGO resulting in film degradation in air (O₂, H₂O) by in situ characterization; additional insights were derived from defect analysis from I_D/I_G ratio variation at perovskite/RGO interfaces. Film morphology and composition was examined by ex situ XRD, SEM, TEM, and AFM studies. 1. Acik et al. J. Mater. Chem. A, 4, 6185, 2016. 2. Acik et al. Nature Mater. 9, 840, 2010.

Use of the Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The abstract has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. Office of Science User Facility under Contract No. DE-AC02-06CH11357. M.A. also acknowledges support from the Joseph Katz Named Fellowship at Argonne National Laboratory.

ZnS/CIGS solar cells have been developed rapidly and it also shows efficiency as high as 22.3%, which was comparable to the CdS/CIGS solar cells. But, the zinc-based buffer layer has two drawbacks such as low deposition rate and insufficient coverage, which must be met for large mass production. In this paper, Zn(S, O) buffer layers for CIGS solar cells fabricated by nano-particle process were prepared by a “high rate” chemical bath deposition process. In this process, a 30nm Zn(S, O) thin film could be prepared within 10 minutes. Scanning electronic microscopy and UV–visible–NIR spectrophotometer were employed to characterize the thin film quality. The Zn(S, O) thin films showed good uniformity, good coverage and high transmittance. The optical bandgap of the as-grown Zn(S, O) thin films were measured to be 3.62eV. Besides, the efficiencies of Zn(S, O)/CIGS solar cells were also measured under standard condition. The efficiencies of Zn(S, O)/CIGS solar cells could reach 13.0% with area of 45 cm², which was the higher to a reference CdS/CIGS solar cell(12.6%). The results hinted that the new process showed promising potential to get good film quality and high throughput, which could be suitable to mass production for CIGS thin film solar cells.

Efficient utilization of the solar energy in the near infrared region is one of the keys to achieve ultra-high efficiency solar cells such as multi-junction solar cells. Colloidal quantum dots (CQDs) such as PbS then have been gaining much attention as promising constituent materials for the solar cells due to the unique optical properties originating from quantum size effect, which enables us to tune exciton absorption band so as to cover a wider range of solar spectrum. However, short exciton and/or carrier diffusion lengths of PbS QD films limit the PbS QD active layer thickness, therefore limiting light harvesting efficiency. There is then a trade-off between carrier transport efficiency and light harvesting efficiency. In the light of this situation, we have developed PbS CQD-based solar cells with ZnO nanowire (NW) arrays not only to establish carrier pathways but also to increase light harvesting efficiency, and achieved EQE values of approximately 60% at a first exciton absorption peak (1020 nm) and over 80% in the visible region by optimizing the morphology of ZnO NW arrays [J. Phys. Chem. Lett., 4, 2455 (2013)]. Recently, we studied photocurrent properties of PbS QD/ZnO NW solar cells by focusing on the cell structure, and found out that, once good electron pathways are established by incorporating ZnO NW structure, holes in the PbS QD region can diffuse a distance over 1000 nm [J. Phys. Chem. C, 119, 27265 (2015)].
To enhance solar cell performance by extending absorption band farther in the near infrared region, nine different colloidal PbS QDs, giving different exciton absorption peaks between 700 nm and 1500 nm, were synthesized. Solar cells were constructed by combining these PbS QDs with the ZnO NW structure, which had been optimized for PbS QDs showing a 1020-nm exciton peak [J. Phys. Chem. Lett., 4, 2455 (2013)]. The cell fabricated with QDs showing a first exciton absorption peak at 1200 nm reached a high EQE of 40% at its peak. A relatively high EQE of 26% was also obtained for a solar cell made up of QDs giving a 1300-nm exciton peak.

ZnO is a wide band gap metal oxide semiconductor well-known for its potential applications in optoelectronics and photovoltaics. One of the crucial advantages of the ZnO technology is a wide range of available ZnO synthesis techniques. Vacuum based methods (MBE, CVD, ALD) allow to deposit high grade ZnO films with low concentration of structural defects. Such techniques are suitable for industrial microelectronics, but became considerably expensive for the photovoltaic application with large scale and low-cost substrates such as organic materials with melting point below 200°C. Low-temperature liquid phase techniques (electrochemical, hydrothermal or sol-gel deposition) are advantageous in the sense of low thermal budget, but crippled by low quality of the deposited ZnO. Combination of techniques from both groups for deposition of ZnO can benefit in numerous ways. Hence the aim of this work was to try alternative approaches of ZnO thin films synthesis by combination of hydrothermal and ALD techniques.
We have studied the hydrothermal deposition of ZnO films on silicon oxide covered and pristine Si wafers with ALD ZnO seed sublayer. The thickness of seed layer varied from few nanometers to 20 nm. Hydrothermal deposition was carried out at 85°C from zinc nitrate solution with HMTA additive. It was shown that depending on the thickness of seed layer, morphology of the deposited ZnO film significantly varied. Separate ZnO crystals scattered across the surface of the substrate were obtained on the thin seed layer. On the 20 nm seed layer compact uniform continuous films consisting of closely packed ZnO rods were formed. It was noted that morphology of the deposited films was determined only with the thickness of the seed layer and was the same for Si and silicon oxide covered substrates. Characterization of the obtained continuous ZnO films with XRD, XPS, and PL revealed that they have wurzite crystalline structure with preferential (0002) orientation, but contained structural defects related to oxygen interstitial atoms and vacancies. Oxygen defects lead to appearance of broad PL band in the visible range. However, ZnO films also demonstrate band-to-band PL in near UV range with maximum at 380 nm. ZnO films resistivity measured by four probe method was up to 0.7 Ohm cm. The resistivity increases with the thickness of the hydrothermal ZnO film while at the same time the concentration of structural defect dropped. It was concluded that conductivity of hydrothermal ZnO films caused by intrinsic structural defects inherited from seed layer as the last has low resistivity of 2 micro Ohm cm. Several technological attempts to improve the crystalline structure of the ZnO films for favorable photovoltaic application will be presented.
This work is part of the Belarus Government Research Programs “Photonics, opto- and microelectronics”, Grant 2.1.02, and “Physical materials science, novel materials and technologies”, Grant 2.21.

Recently, development of Ag nanowire based transparent electrode has received much interest as a potential candidate to replace the commonly used material such as ITO(Indium Tin Oxide) and AZO(Aluminum doped Zinc Oxide) for flexible solar cells and other optoelectronic devices. Ag nanowire network electrode has been previously demonstrated to have excellent optical and electrical performance as well as being low cost in comparison to the conventional ITO thin film. However, Ag nanowire network has unresolved issues such as the chemical instability against oxidation as well as percolation and less efficient transport of charge carriers that arrive at the electrode on the surface of the solar cell. Therefore, there is a need for development of conductive layer that can be coated on top of the Ag nanowire that can resolve the aforementioned issues. In this study, the combustion reaction followed by a post-annealing process was used to form sol-gel based AZO thin layer that helped in enhancing the performance and the lifetime of the Ag nanowire based transparent electrode for a solar cell. Unlike the traditional sol-gel process, the oxide phase was efficiently formed at relatively low temperature of 200^oC by utilizing emitted heat from combustion reaction. AZO thin film fabricated by the combustion reaction showed relatively low conductivity value of 4x10^-2 S/cm, but is sufficient to serve the role of transporting the charge carriers from active layer to the Ag nanowire network. It should be noted that the low processing temperature of below 200^oC is required for depositing AZO layer on top of already existing Ag nanowires that are on flexible polymeric substrates for flexible solar applications. Using the optimized reaction conditions, a uniform coating on top of the Ag nanowires was obtained and the chemical stability was also enhanced due to the presence of AZO layer, which acts as a diffusion barrier for oxygen. The Ag nanowire/AZO composite transparent electrode developed in this study was demonstrated to have excellent electrical and optical performance of 10ohm/sq and 90% at 550nm.

Cu₂ZnSnS₄ (CZTS), a quaternary compound semiconductor, is an earth abundant and sustainable semiconductor material that is used as an absorber layer for thin film solar cells. In the present work, CZTS thin films were deposited onto soda lime glass (SLG) by RF magnetron sputtering of alternating stacking orders of CuS, ZnS, SnS as a function of substrate temperature. X-ray diffractograms confirmed the formation of polycrystalline CZTS films, where the degree of crystallinity was determined by the prominence of diffraction peak corresponding to the <112> direction of CZTS, which was found to increase with substrate temperature. The stoichiometry of the films were investigated using X-ray Photoelectron spectroscopy (XPS). The film deposited at 300^oC with the stacking order ZnS/CuS/SnS was found to be nearly stoichiometric, exhibiting an optimum band gap (1.56 eV) and p-type conductivity. Scanning electron microscopy (SEM) analysis revealed smooth morphology and increasing grain size with increasing substrate temperature. The optical band gap of CZTS thin films was found to vary between 1.3 and 1.5 eV. XPS analysis showed that the stoichiometry of the CZTS strongly depends on the stacking order; CuS/ZnS/SnS, SnS/ZnS/CuS, etc., indicative of diffusion limitations within the stacks.

In order to collect the hot electrons, light trapping using cylindrical glass substrates for ultra-thin aSi:H photovoltaic cells with no doped layers is introduced. The photovoltaic cell has the structure of 5 nm MoO_x (hole collection layer) – 10nm intrinsic a-Si:H (photoactive layer) – 1.5 nm LiF / 100 nm Al (electron collection layer & back electrode), all deposited in that order onto a cylindrical quartz substrate covered with a 100 nm ITO layer, which is acting as the transparent front electrode for the cell. By rotating the cell with respect to the incoming light, the angle of incidence of the incoming light for which the cell efficiency is at its highest value, is determined.

Nanocrystalline Si-based materials have attracted much attention since they can be used in the next generation solar cells to extend the spectral response range of devices. In order to improve the device performance, doping control is a fundamental issue. However, it looks like that doping in Si nanocrystals (Si NCs) is quite different from their bulk counterparts. Theoretically, the formation energy of impurities in Si NCs is larger which results in the so-called “self-purification” effect. In the present work, phosphorus-doped (P-doped) Si NCs/SiO₂ multilayers were prepared by annealing P-doped amorphous Si/SiO₂ stacked structures at various temperatures. The doping behaviors of P dopants in Si NCs were systematically studied. It was found that the P atoms tend to passivate the surface states (such as dangling bonds) of Si NCs during the formation of Si NCs via annealing process. Meanwhile, the low temperature electron spin resonance (ESR) spectroscopy, the appearance of ESR signal originating from the conduction electrons demonstrated that part of the P dopants can be incorporated into the Si NCs at the substitutional sites. Consequently, the conductivity of doped Si NCs is enhanced obviously and the localized surface plasmon resonance effect was observed which can be modulated by controlling the doping ratio. Furthermore, the photoluminescence band centered at ~1300nm was observed at room temperature from P-doped Si NCs samples with ultra-small sizes which may indicated that P doping in Si NCs also generate deep levels in gap which is responsible for the sub-band light emission. This work was supported by “973 program” (2013CB632101), NSFC (No.11274155) and “333 project” of Jiangsu Province (BRA2015284). Mr Wei Tong in High Magnetic Field Laboratory of the Chinese Academy of Science was thanked for the help in the electron spin resonance measurements.

Building on increasing progress and interest in plan-view and cross-sectional milling for sub-surface studies, Computed Tomographic (CT)AFM is developed and implemented to investigate transport pathways in polycrystalline CdTe solar cells. Based on tens to hundreds of images through the thickness of a specimen, nanoscale features and materials properties are uniquely fully resolved throughout all 3-dimensions. Examples include the first nanoscale maps of photoconduction pathways and interconnections within operating polycrystalline solar cells as a function of depth. Measurements are made at Isc as well as Voc conditions, during 0.5-5 suns of illumination. The results indicate enhancements for certain grains, suggest a strong influence of grain and boundary mediated current percolation pathways, and directly reveal profound influences of point, line, and planar defects on solar cell performance parameters.

Inorganic Quantum dot (QD) solar cells have the potential to improve the efficiency for solar energy conversion by using photocurrent generated in QDs inserted in the active layer. On the other hand, organic thin film solar cells have attracted much attention due to the features such as low cost, large variety of materials and flexibility. Therefore, the solar cells having both characteristics are very attractive. One example of such a solar cell is the hybrid one which embedded inorganic QDs in the interface of organic thin film heterojunction. In this paper we prepared CuPc/C₆₀-based thin film organic solar cells with Si QD, and investigated photocurrent characteristics of the solar cells.

The sample structure is ITO/CuPc(20nm)/Si QDs(11nm)/C₆₀(40nm)/BCP(5nm)/Al. The mean diameter of Si QD is 5nm. An indirect- and direct- optical band gap of Si QD obtained from UV-VIS absorption spectroscopy are 2.0eV and 3.5eV, respectively. An open-circuit voltage of 0.3V, a short-circuit current density of 0.032mA/cm² and fill-factor of 0.23 are obtained from current-voltage measurements under AM1.5G sunlight irradiation. The result shows that the sample acts as a solar cell though the performance is not so good. Photocurrent characteristics are investigated by spectral responsivity measurements. The clear enhancement of resposivity at a photon energy of 3.5eV are observed, compared with a reference CuPc/C₆₀ solar cell which does not include Si QD. These results shows that this inorganic-organic QD solar cell structure has a one of a new type of candidates of the QD solar cells.

Bismuth triiodide (BiI₃) has a band gap of 1.7 eV and a large dielectric constant, which makes it a promising material for photodetector devices and solar cells. We have fabricated photodetector devices using triangular shaped BiI₃ nanoplates that display remarkably fast and sensitive photoresponse behavior. We observe a four-fold increase of photocurrent when measured under vacuum (P < 10^-3 Torr) conditions. In stark contrast to single activation energy level shown in typical semiconductors, our devices show two dominant activation levels of 456 meV and 77 meV at high temperature (200 K ~ 300 K) and low temperature (100 K ~ 170 K) regimes, respectively. We have used aberration-corrected scaning transmission electron micrscopy (STEM) to image the atomic-scale structure of the nanoplates. Simulatenously acquired electron energy loss spectra (EELS) shows the presence of oxygen dopants in BiI₃. We have combined the STEM-EELS results with first-principles density functional theory calculations to identify the preferred location of oxygen dopants and their effect on the electronic properties of BiI₃. We have calculated the thermodynamic transition levels of native point defects and oxygen dopants, which in correlation with the experimental results is used to obtain an atomic-scale understanding of the role of defects on the optoelectronic properties of BiI₃. Our combined study shows the promise of BiI₃ as a defect-tolerant material for photodetector and solar cell applications.

Dye pollutants contaminate water bodies when disposed into the ecosystem. Effects caused the disposal of these dye pollutants include aesthetic pollution, eutrophication and perturbations in aquatic life. This project aims to mitigate this water contamination problem by studying possible degradation of these pollutant compounds. Semiconductor photocatalysis process using nanocrystalline TiO₂ as the semiconductor catalyst was adopted in this project to photodegrade rhodamine B dye and Sudan III dye solutions by exposing it to UV light.
In this project, we explored the synthesis of porous TiO₂ nanostructured powders via chemical means such as the Sol gel and Hydrothermal techniques. The as-produced nanoparticles were then characterized via X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Brunauer–Emmett–Teller (BET) surface area analysis, Fourier Transform Infra-Red, and Raman Spectroscopy for their microstructure, morphology, optical, porosity and absorption properties. The photocatalytic degradation of rhodamine B and Sudan III dyes were studied at various exposure times to the UV light up to 150miutes with 30 minutes interval.
The XRD and Raman spectra of the samples showed that, the anatase phase of TiO₂ was produced when Sol gel and Hydrothermal methods were used in the nanoparticles synthesis except the sample synthesized by Hydrothermal and calcined at 600°C which showed about 15% rutile phase.
Fourier Transform Infra-Red spectrum on the TiO₂ prepared by Sol gel method showed band at 3228cm^-1 and 1635 cm^-1 which is attributed to O-H stretching and stretching of titanium carboxylate respectively and these peaks were seen disappearing after calcining at different temperatures. Only the strong absorption between 800cm^-1 and 410 cm^-1 remained, which were attributed to obtained TiO₂ nanoparticles.
The highest BET surface area was reported to be 207.7 m²/g which was assigned to the as-prepared TiO₂ nanoparticles synthesized by the Hydrothermal technique. The adsorption and desorption isotherms of this sample exhibited typical type IV pattern with hysteresis loop, characteristic of mesoporous material according to the classification of IUPAC.
94% degradation of Rhodamine B dye was observed after 150 minutes irradiation of UV light when TiO₂ catalyst synthesized by Sol gel technique and calcined at 300°C was used in photodegrading Rhodamine B dye solution and a 100% degradation was seen after 150 minutes UV light irradiation in Sudan III dye solution when Hydrothermal prepared catalyst calcined at 300°C was used.
The SEM micrographs obtained also showed that nearly spherical TiO₂ nanoparticles were produced. The present results obtained showed that a mesoporous spherical anatase TiO₂ nanoparticles with high photocatalytic activity and high surface area were synthesized. They were highly crystalline with grain size ranging from 2nm to 30nm. These synthesized TiO₂ nanoparticles can be applied in the degradation of wide range of dye pollutants.

Solar energy and terrestrial emission have great potential to satisfy the energy demand of humans: their energy contents exceed the global energy usage by three orders of magnitude. In order to utilize the solar resource, photovoltaic technology has been widely adopted to convert solar energy into electricity, and the crystalline silicon material occupies ~90% of the photovoltaic market. However, the silicon material in a typical photovoltaic module contributes more than 30% of the overall cost. Further cost reduction is possible by reducing the silicon thickness as long as light can be efficiently trapped to overcome the weak absorption caused by the indirect bandgap of silicon. We have carried out combined optical and electrical simulations to design crystalline silicon thin-film solar cells, and fabricated 10-µm-thick crystalline silicon photovoltaic cells with a peak efficiency of 15.7%. Our simulation shows even higher efficiency can be achieved. We further show that random structures on crystalline silicon thin films can achieve similar solar absorptance as periodic structures. While a photovoltaic cell works by absorbing photons to generate electricity, a p-n junction maintained at above ambient temperature can work as a heat engine, converting some of the supplied heat into electricity and rejecting entropy by interband emission. Such thermoradiative cells have potential to harvest low-grade heat into electricity. By analyzing the entropy content of different spectral components of thermal radiation, we identify an approach to increase the efficiency of thermoradiative cells via spectrally selecting long-wavelength photons for radiative exchange. Furthermore, we predict that the near-field photon extraction by coupling photons generated from interband electronic transition to phonon polariton modes on the surface of a heat sink can increase the conversion efficiency as well as the power generation density, providing more opportunities to efficiently utilize terrestrial emission for clean energy. An ideal InSb thermoradiative cell can achieve a maximum efficiency and power density up to 20.4 % and 327 Wm^-2, respectively, between a hot source at 500K and a cold sink at 300K. However, sub-bandgap and non-radiative losses will significantly degrade the cell performance.

The goal of our work is to increase light-matter interactions in organic photovoltaics. As first design we implemented a self-assembled nano-gap antenna. The nano-gap antennas are linked at a controlled distance of a few nanometers by Dithiothreitol molecules (self-assembly method). The spacing molecules ensure a minimum distance that plays a fundamental role in the formation of intensity hot spots in the nanogap as well as large and red-shifted scattering peaks. This device exhibited an efficiency 14% higher than the reference one showing a relevant enhancement in the red part of the EQE measurements*. As second design we build up a photonic crystal and a metal cavity around a transparent organic solar cell. We enclosed the active material in between two metal electrodes forming an optical cavity designed to optimize photon trapping inside the cell. To increase near IR light trapping, while maintaining transparency in the visible, an anti-reflection coating was deposited on top of the front metal contact while a non-periodic multi-layer was inserted in between the back metal contact and the substrate. The cavity configuration was designed specifically for the cell architecture used and we achieved semi-transparent cells with 5.3% PCE, corresponding to 90% the PCE of the opaque cell.


Francesco Pastorelli, Sebastien Bidault, Jordi Martorell, Nicolas Bonod, DOI: 10.1002/adom.201300363

Francesco Pastorelli, Pablo Romero Gomez, Rafael Betancur, Alberto Martinez-Otero, Nicolas Bonod, Jordi Martorell, DOI:10.1002/aenm.201400614.

Francesco Pastorelli, DOI: 10.1002/aenm.201570008

Silicon solar cells have been dominating the PV industry. Thin silicon represents an exciting direction for future solar cells. Here I will show our recent progress on thin single-crystal Si down to 2 micron: fabrication of thin Si, nanoscale photon management, device physics and remarkable mechanical flexibility and robustness. We demonstrate 10 micron thick Si solar cells with 13.7% power efficiency. We also developed an easy integration of transparent contact with Si solar cells.

As a transparent top electrode/back reflector material, metal nanomeshes (NMs) have demonstrated excellent electrical and optical properties as well as light trapping behavior. We systematically investigate the behavior of integrating metal NMs for light trapping onto the top and/or bottom of ultrathin film crystalline Si (c-Si) solar cells. Metal nanoparticles (NPs) on the frontside couple incident light to localized surface plasmon polaritons, Fabry-Perot modes and waveguide modes, whereas a NM excites Fabry-Perot modes and waveguide modes as well as surface plasmon polaritons that concentrate the incident light into the c-Si/metal interface. The backside metal NM functions as a back reflector supporting both Fabry-Perot resonances and waveguide modes in the c-Si. The excitation of cavity modes within the holes of metal NM may also lead to enhanced absorption in the c-Si. We illustrate how surface plasmon polaritons, Fabry-Perot resonances, and waveguide modes are excited by NMs sandwiching the c-Si thin film. With an appropriate antireflection coating, a 178% enhancement in short-circuit current may be achieved compared with that of a bare 300 nm thick c-Si solar cell.

While halide perovskite semiconductors have been pursued for many years, the recent interest in CH₃NH₃PbI₃ for use as an absorber in a high-performance thin-film solar cell has grown dramatically since the first demonstration of perovskite photovoltaics in 2009 [1]. The unprecedented rise in performance over the following years derived in part from the outstanding optoelectronic properties of this materials class, but also from the self-assembling nature of CH₃NH₃PbI₃ and the resulting relative ease with which high quality perovskite materials can be prepared using a wide range of vacuum and solution-based approaches. In this talk we will explore the richness of the perovskite family beyond CH₃NH₃PbI₃ [2] and examine how these systems might impact PV device application.

[1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Amer. Chem. Soc. 131, 6050 (2009).
[2] B. Saparov, D. B. Mitzi, Chemical Reviews 116, 4558 (2016).

The Cu(In,Ga)Se₂ solar cells are reported to have high efficiencies, but the cost of processing remains high due to the necessity to use vacuum based processes for thin film deposition. In attempt to reduce the cost of the solar while maximizing the efficiency, we report a Ag nanowires+PEDOT:PSS hybrid transparent electrode that was deposited using all-solution processed, low cost, scalable methods alternative to the conducting oxide deposited by vacuum processes. The Ag nanowires network with transmittance and sheet resistance of 85 and 17.5 ohm/sq., respectively, was determined to have the optimum areal density of the Ag nanowires network for optimized J_sc and V_oc. When bare Ag nanowires network was deposited on the CdS film, the cell showed a small J_sc due to insufficient contact between the long nanowires and the rough CdS surface. Besides, there are empty spaces between Ag nanowires network which causes decrease of collection efficiency of free electrons. To increase the J_sc of the cell, PEDOT:PSS conducting polymer was used as a filler conducting polymer that can increase the collection efficiency of the charge carriers arriving at the CdS surface and PEDOT:PSS also help electrical contact between Ag nanowire and CdS layer. As a result, the J_SC significantly increased from 15.4 to 26.5 mA/cm². However, the V_oc was too low and the R_sh was high, which indicated that there was a significant amount of leakage current presumably due to interfacial recombination at the Ag/CdS interface. To
reduce the leakage current at the interface, a wide-band-gap Zn(S,O,OH) film was deposited between the CdS film and Ag NW network. The Zn(S,O,OH) film can provide a high barrier for blocking hole transport at the interface, and as a consequence Voc was measured to have sharply improved from 0.35 to 0.55 V. In addition, the Zn(S,O,OH) film annealed at 300 °C was shown to have reduced OH groups and that induced a further increase in the VBO at the interface to reduce surface recombination further. Through optimization of the fabricating process, the best efficiency of 11.6% was achieved. Our study is the first report on an Ag NWs + PEDOT:PSS hybrid transparent electrode applied to CIGS solar cells with high efficiency.

Thin-film solar cells based on inorganic absorbers permit a high efficiency and stability. Among or those absorber candidates, van der waals material antimony dichalcogenides (Sb2S(Se)3) had attracted extensive attention because of its suitable band gap (1.2eV ~ 1.7 eV), strong optical absorption, non-toxic, low-cost and earth-abundant constituents. Currently high-efficiency Sb2S3 solar cells adopts the structure of solid-state dye-sensitized solar cell: absorber layer deposited on nanostructured TiO2 electrodes in combination with organic hole transport material (HTM) on top. However, it’s challenging to fill the nanostructured TiO2 layer with Sb2S3 and subsequently by HTM, this could lead to uncovered surface permits charge recombination. And also the existing of Sb2S3/TiO2/HTM triple interface will enhance the recombination due to the surface trap state. Therefore, a planar junction cell would not only have simpler structure with less steps to fabricate but also ideally also have a higher open circuit voltage because of less interface carrier recombination. By far there is limited research focusing on planar Sb2S3 solar cell, so the feasibility is still unclear. In order to fabricate Sb2S3 thin-film, normally chemical bath deposition (CBD) is adopted, but it inevitably form antimony oxide as impurity which providing deep trap states. Recently, atomic layer deposition (ALD) has also been used to successfully produce high quality Sb2S3 film, but it’s hard to be scale-up in the manner of cost and speed. Here, we developed a low-toxic solution based technique to fabricate Sb2S3 thin film solar cell. The planar device with a structure of FTO/TiO2/Sb2S3/P3HT/Ag has a PCE over 5% which is similar or higher than yet the best nanostructure devices with the same HTM. Furthermore, the morphology of Sb2S3 layer have been studied. We showed that, Sb2S3 tends to form isolated islands on TiO2 surface, but a continuous film can be formed via solution engineering. Also strong coupling molecule was introduced as an interface modifier, as result, the charge transportation at the interface, the open-circuit voltage and filling factor dramatically improved and reached a record PCE for planar Sb2S3 solar cell.

Intermediate band (IB) materials are a novel class of materials that, like semiconductors, have a band gap but also have an extra set of allowed electronic levels entirely contained inside the semiconductor band gap. Solar cells made from such materials have the potential to radically improve photovoltaic efficiencies, similar to triple-junction cells. Current intermediate band devices are hampered by short carrier lifetimes, limiting efficiency improvements. I will describe theoretical and experimental work to understand carrier lifetimes and their impact on device efficiencies. I will introduce a figure of merit, which predicts the potential effectiveness of candidate IB materials in advance of device fabrication. This figure of merit captures the tradeoff between enhanced absorption and enhanced recombination within an IB material, and it suggests a path toward efficient IB materials. I will give examples of measurements of the figure of merit and theoretical predictions for new systems. I will conclude with a list of appealing and unappealing candidate material systems.

Copper Zinc Tin Sulphide (CZTS, Cu2Zn2Sn4S8) is a promising earth-abundant thin film photovoltaic material. Current devices have disappointing open circuit voltages considering the band gap of the material. At open circuit all charges are recombining; the open circuit voltage is set by the degree of recombination.

From studies of the photoluminescence, there appears to be significant band-gap tailing even in high quality bulk CZTS material. Independent of the source of disorder, the device physics are particularly affected due to the low dielectric constant of this material.

In this work we combine electronic structure calculations with different representations of disorder. We write custom Monte Carlo codes to simulate both substitutional disorder; and sample positional disorder with both molecular dynamics and lattice dynamics (phonons).

With these models of disorder we simulate the band tailing observed in bulk material, and estimate its effect on the device physics, and predict what experimental observables would agree with the different models.

Solar cells comprised of organic materials as active layers are promising candidates to help supply the growing worldwide energy demand. Due to the high absporption coefficient of certain organic compounds, layer thicknesses of less than several hundred nanometers are sufficient for good light incoupling. In combination with suitable large-scale fabrication techniques, organic solar cells have the possibility to achieve a good $/Watt ratio.
While the efficiency of organic solar cells has made continuous progress in the last decade and reaches values of up to 11.7% [1], further improvement is needed in order to reach commercial competitiveness.
Still the most favorable device architecture is the bulk-heterojunction structure, a blend between organic donor and acceptor materials to separate the strongly bound excitons in the usually low-permittivity organic materials (epsilon ~3-5). A blend is advantageous due to simple available fabrication techniques and provides a tradeoff between efficient exciton splitting and good charge transport.
However, there is still a lack in understanding about the internal processes, including the dynamics of charges after an excited state was separated at a heterojunction into an electron and a hole. The two oppositely charged particles still feel their mutual electrostatic interaction across the heterojunction and stick together at the interface before they eventually either recombine geminately or become separated and are transported though the intertwined morphology to the contacts. The strength of the mutual interaction in interplay with the energetically disordered landscape is expected to be a key determining factor in the splitting dynamics and efficiency. Inefficient charge splitting increases recombination and limits the efficiency of the cell.
We set up a three-dimensional full-device model based on the kinetic Monte Carlo (kMC) algorithm of an organic bulk-heterojunction solar cell, including a representation of the blend with both spatial and energetic disorder and a sophisticated treatment of the Coulomb interaction by evaluation of an Ewald sum.
Using the kMC model we extract the local charge density distributions within and track charge trajectories through the blend. With these data we can investigate the effect of energetic disorder and permittivity/Coulomb interaction on interface effects such as charge accumulation and interface densities, recombination (geminate vs. nongeminate), and the charge separation dynamics, to get a deeper understanding of the internal mechanisms.
Our results can support other theoretical expectations [2, 3] of reduced recombination with increasing permittivity.


[1] Zhao, Jingbo, et al., Nature Energy 1 (2016): 15027.
[2] Koster, L., Sean E. Shaheen, and Jan C. Hummelen. Advanced Energy Materials 2.10 (2012): 1246-1253.
[3] Cho, N. et al. Advanced Energy Materials 4, no. 10 (2014).

Singlet Fission (SF) is a spin-allowed process to convert one singlet excited state into two triplet excited states, namely a correlated triplet pair. The ability to effectively implement SF processes in solar cells could allow for more efficient harvesting of high-energy photons from the solar spectrum and allow for the design of solar cells to circumvent the Shockley-Queisser performance limit. Indeed, several recent studies have demonstrated remarkably efficient solar cell devices based on SF.
In the present work, we show unambiguous and compelling evidence for unprecedented intramolecular SF within a set of three different regioisomeric pentacene dimers at room temperature and in dilute solution. To this end, pump-probe experiments are employed and complemented by theoretical calculations using high level ab-initio multireference perturbation theory methods. The observed triplet quantum yields reach remarkable values, which are as high as 156 ± 5%. The latter should be compared to triplet quantum yields of 10% as they are typically found in pentacene derivatives due to slow intersystem crossing dynamics. The collision of a photoexcited chromophore with a ground state chromophore has been shown to give rise to SF. Here, we demonstrate that the proximity and sufficient coupling through bond or through space in pentacene dimers is sufficient to induce intramolecular SF. As a consequence, two triplets are generated on the same molecule. This work constitutes a conceptional breakthrough, in that it documents the great potential of synthetically tailored acenes to surpass the 30% Shockley-Queisser limit and its impact in terms of easing device fabrication.

Recently, bio-organic hybrid devices using DNA-based materials have been received great attentions due to their advantages, such as inherently biodegradable, eco-friendly, replenishable, and abundant resources. Among them, nucleobases such as cytosine, adenine, guanine and thymine, are able to deposit as a thin film by thermal evaporation. However, nucleobases are difficult to be used in electronic devices because they have electrically insulating characteristics. For this reason, nucleobases including DNA-based materials have been reported in limited cases, e.g. dielectric layers in field effect transistors.
In this study, we demonstrate the performance of cytosine interlayer inserted between C₆₀ electron transport layer and cathode in various photovoltaics containing small molecule-, polymer- and perovskite-based active layers. The device results show enhanced efficiencies due to the increased open circuit voltage and short circuit current when cytosine is inserted at the cathode/C₆₀ interface. To understand the enhancement mechanism of the cytosine interlayer, the electronic structures of the Al/C₆₀ and Al/cytosine/C₆₀ interfaces were studied using in situ ultraviolet photoelectron spectroscopy and inverse photoelectron spectroscopy. The cytosine interlayer behaves as an exciton blocking layer (EBL) and charge extraction layer (CEL) at the same time. We suggest that cytosine be an alternative EBL/CEL material which can replace the conventional EBL/CEL such as bathocuproine (BCP) and bathophenanthroline (BPhen).

Nobel-metal-free photocatalyst for H₂ evolution from water, which acts as an effective catalyst for sustainable and cheap conversion of solar to fuel energy, has been actively pursued for long time. Copper-based chalcogenide of earth-abundant elements, especially Cu₂ZnSnS₄ (CZTS), has recently arisen as low cost and environment friendly material for photovoltaics and photocatalysis. Despite these superior properties, great challenge has presented to further increase the photocatalytic activities of CZTS. Herein, we report a new composite material consisting of CZTS nanoparticles decorated on MoS₂-reduced graphene oxide (rGO) hybrid. The synthesis, characterization, and enhancement mechanism of CZTS/MoS₂-rGO composite will be discussed. In photocatalytic H₂ generation experiment with as-synthesized CZTS/MoS₂-rGO composite, the presence of MoS₂-rGO cocatalysts resulted in synergistic effect in photocatalytic H₂ generation rates, which can be attributed to the increased charge separation by rGO and more specific photocatalytic activity site of MoS₂. Through the optimization of each component proportion, the CZTS/MoS₂-rGO composite enhanced the photocatalytic H₂ generation for more than 3 times when the content of the MoS₂-rGO cocatalyst is 10 wt % and the weight ratio of MoS₂ to rGO is 9:1, compared to bare CZTS nanoparticles. Furthermore, this CZTS/MoS₂-rGO composite showed much higher photocatalytic activity than both Au and Pt nanoparticle-decorated CZTS (Au/CZTS and Pt/CZTS) photocatalysts, indicating the MoS₂-rGO hybrid is a better co-catalyst for photocatalytic hydrogen generation than the precious metal. The CZTS/MoS₂-rGO system also demonstrated stable photocatalytic activity for a continuous 20 h reaction.

Solar hydrogen is an ideal alternative to fossil fuels due to its environmentally friendly and clean nature. Therefore lots of groups have studied the photocatalytic reaction to increase the efficiency of water splitting by using various materials such as metal oxide, metal chalcogenides, or metal-semiconductor hybrid nanostructures. Especially metal-semiconductor hybrid nanocatalyst has been regarded as a strong candidate for photocatalytic hydrogen generation because the metal helps photo-induced carrier separation as well as the reactive metal surface with a large surface area can serve as the suitable catalytic substrate.
Here, for the first time, we synthesized the well-defined Au-CdSe-Pt asymmetric nanodumbbells which include two kinds of metal-semiconductor interface in a single nanoparticle. We found that the amount of hydrogen generation exhibits significant dependency on the choice of the metal tips. This phenomenon was explained by the photoelectrochemical method, and it demonstrated that the role of metal as a recombination center is also important in determining photocatalytic efficiency.

Organic-inorganic solar cells such as dye-sensitized solar cells (DSSCs) use transparent metal-oxide semiconductor materials (e.g. TiO₂, ZnO) as a photoelectrode. In these types of devices, the photoelectrode plays a critical role because excited electrons are transported from the absorption layer to the collector, via the photoelectrode. Photoelectrode materials with distinct morphologies and optoelectrical properties can be synthesized by using various synthesis methods. However, commonly used photoelectrode (i.e. ZnO) and absorption material (i.e. organic dye) in DSSCs, do not absorb light with a wavelength longer than 800 nm. Since near-infrared light (NIR) accounts for 46% of available sunlight energy, the inability to absorb in the NIR range limits the overall efficiency of DSSCs. Therefore, a secondary absorption layer can be introduced between the dye and the photoelectrode to improve the conversion efficiency, allowing for the widening of the absorption range into the NIR region (800 to 1000 nm).
In this work, a CuO thin layer synthesized on ZnO nanorods (NRs), in a core-shell structure, is used as a secondary absorption material. The crystallinity of the synthesized CuO and ZnO is analyzed by X-ray diffraction. In order to determine optical properties of the CuO on ZnO NRs, UV-Vis-NIR photospectrocopy is used. The CuO on ZnO NRs shows lower transmittance range from 400 to 1300 nm than ZnO. DSSCs using ZnO NRs with and without CuO layer are fabricated using a commercial dye (N719), an iodide-based electrolyte, and Pt layer as a counter electrode. The current-voltage characteristics show the solar cell devices with CuO on ZnO NRs have higher efficiency compared to ZnO photoelectrode without CuO shell. Additionally, the resistivity at each interface layer is measured by electrochemical impedance spectroscopy.

Solar panels with silicon photovoltaic cells dominate the number of installed solar panels. All of these silicon photovoltaic cells use a single junction structure which limits the maximum energy conversion efficiency to 33%. One approach to overcome this limit is a tandem junction structure. The maximum energy conversion efficiency could be increased by twelve percentage points to a limit of 45% if a silicon compatible material can be found with a band gap of 1.7 eV. Additionally, the material has to have a lattice constant similar to silicon, or have a novel method of overcoming strain between the two junctions. A material that may for fill these requirements is silicon diselenide (SiSe₂). SiSe₂ has been reported to have a band gap of 1.7eV, measured using optical absorption spectroscopy. Additionally, SiSe₂ is considered a one dimensional material, therefore the SiSe₂ could bond to the bottom silicon junction using van der Waals forces.
In this study, bulk samples to SiSe₂ were grown by melt growth to confirm the results from past publications. However, the main focus was on develop thin films of SiSe₂. Thin films were grown by deposition Se on silicon and silicon coated quartz using a liquid phase deposition from a solution of selenium dissolved in sodium sulfite. Selenium was collapsed out of solution using citric acid and left of settle on the wafers. These samples were sealed in quartz ampules and then annealed at temperatures from 300 to 1000°C for up to 12 hours. Additionally, silicon wafers were loaded into quartz ampules with pure selenium to act as a selenium sources. The formation of SiSe₂ was confirmed by Raman spectroscopy. Optical properties were investigated using photoluminescence and optical absorption. All of these measurements were done with the samples still sealed in the ampules due to the instability of SiSe₂.
SiSe₂ is a very unstable material when exposed to water and humid environments and releases H₂Se gas. Samples of SiSe₂ rapidly decompose when placed in water suggesting that no protective silicon oxide layer is formed that would naturally passive the material. To make it feasible to remove the SiSe₂ samples from the quartz ampules, this ampules were opened in a glove box with an H₂Se detector. Samples of SiSe₂ were then placed in SU-8 and PDMS to encapsulate the SiSe₂. The SU-8 sample had significant discoloration while the PDMS did not show a significant reaction. H₂Se was not detected on the gas detector, while Raman analysis showed that the material was still SiSe₂ when encapsulated.

Solution-processed emerging thin film solar cells, such as devices based on organic and colloidal quantum dot (CQD) light absorbers, offer low-temperature processing, mechanical flexibility and conformability, lightweight modules, and compatibility with continuous roll-to-roll manufacturing. Given each of these is today limited to ca. 10% power conversion efficiency (PCE) in single-junction devices, multi-junction solar cell architectures that can harvest a broader portion of the solar spectrum are garnering significant attention across both the CQD and the polymer solar cell communities.
In this study, we investigate and demonstrate the hybrid tandem solar cells that rely on the combination of solution-processed CQD and bulk heterojunction polymer:fullerene subcells which CQD active layer consists of only one or two layers showing high power conversion efficiency. The hybrid tandem solar cell is monolithically integrated and electrically connected in series with a suitable p-n recombination layer including metal oxide and thin metal layers. We discuss the monolithic integration of the CQD and polymer subcells and an adequate device configuration for the efficient device performance with the optical matching.

Copper (I) oxide (Cu₂O) is a very favorable p-type semiconductor, which finds potential applications in areas like solar water splitting. Here we report the fabrication of dendritic gold and Cu₂O composite layers as photocathode for solar water splitting. The dendritic gold nanostructures were deposited on indium doped tin oxide (ITO) substrate via electrodeposition. This deposition was carried out by using a bath of gold chloride solution and L-cysteine. The morphology of the deposited gold was controlled by varying the L-cysteine concentration and the deposition potential. Further, p-type Cu₂O was deposited on the gold underlayer to complete the fabrication of the photoelectrode. The Cu₂O layers were also deposited by electrodeposition using a bath of lactic acid stabilized copper (II) sulfate. The gold nanostructures and the Cu₂O layers were characterized by scanning electron microscopy (SEM), X-ray diffractometer (XRD) and photoelectrochemical (PEC) measurements. A control sample of Cu₂O layer was also prepared. The PEC measurements of the control sample and various other dendritic gold and Cu₂O samples were compared.

Hollow nanoparticles (HNPs) are considered to be a promising structure for drug delivery systems, catalysts, chemical and biological sensors, and solar cells. The inner cavity of the hollow particles offers a large volume for transport of drugs, DNA, and cosmetics, which is essential for drug delivery systems. HNPs exhibit high catalytic and sensor activities, because their inner- and outer-shell surfaces facilitate contact with reactant molecules. The light-scattering effect can be enhanced by increasing the difference between the refractive indices of the empty inner cavity and solid shell of HNPs. Although HNPs demonstrate these advantages, design of an optimized structure for specific application fields to further enhance their performance remains challenging.
To enhance the advantages of the hollow structure, multi-shell HNPs have recently attracted interest due to their outstanding light-scattering effect and large surface area. The multi-shell structure provides enhanced light scattering by repeated reflection and scattering events between the inner and outer shells. Moreover, the active surface area is larger, compared with that of a single shell, due to the surface area of the additional inner shells.
In recent years, much research has aimed to develop a method of fabricating multi-shell HNPs to improve their performance in various applications. Previous approaches to manufacturing multi-shell hollow structures involved mainly hydrothermal reactions using an autoclave, which limited the ability to control the size and aggregation of particles due to high reaction temperature. Thus, there is a growing demand for methods of fabricating multi-shell hollow particles at the nanoscale, with high surface area and monodispersity.
Herein, we suggest a simple fabrication method for SiO₂/TiO₂ double-shell HNPs (DS HNPs) based on the sol-gel reaction and sonication-mediated etching. In this work, the particle size can be easily controlled over the range 120 to 240 nm using silica core templates of various sizes. The pore distribution and surface area of DS HNPs were investigated using nitrogen adsorption/desorption isotherms. DS HNPs, 240 nm in size, exhibited a high surface area of 497 m² g^-1 due to the presence of pores within the inner and outer shells. The double-shell structure of DS HNPs exhibited enhanced light-scattering compared with SS HNPs via multiple scattering events between the inner and outer shells. Additionally, the photovoltaic performance of the DS HNPs, as a light-scattering material, was estimated in the anode electrode of a DSSCs.

The critical and multistep complex procedures involved in fabricating PbS colloidal quantum dot solar cells offset the rapid progress in efficiency of these devices. While affordable and scalable routes are being explored, the device architecture remains critical in achieving high efficiency. In this work, we demonstrate a scalable, low cost and less toxic synthesis route for the fabrication of PbS/CdS thin film solar cells with efficiencies as high as ~5.59%, which is the highest efficiency obtained so far for the PbS based solar cells not involving quantum dots. The devices use a stack of two band-aligned junctions that facilitates absorption of a wider range of solar spectrum and an architectural modification of the electron accepting electrode assembly consisting of a very thin CdS layer (~10 nm) supported by vertically aligned ZnO nanorods on ~50 nm thick ZnO underlayer. Compared to a planar electrode of a 50 nm thick CdS film, the modified electrode assembly enhanced the efficiency by 39% primarily due to a significantly higher photon absorption in the PbS layer, as revealed by a detailed 3D finite difference time-domain optoelectronic modelling of the device.

The broad energy distribution of solar spectrum hinders the effective use of solar energy with commonly used photocatalysts which are large-bandgap semiconductors. We show an organic-inorganic hybrid material to realize a fluorescence resonance energy transfer (FRET)-assisted upconversion for efficiently utilizing a significantly wasted portion of the sunlight. In this process, rhodamine B (RhB) acts as a donor in FRET pair with naked organic quantum dots of 3.4 nm as an acceptor, which are not passivated by any insulating molecules and have a sp² graphitic moieties. The FRET effect enhances the photosensitization and upconversion properties of organic quantum dots. The upconverted photons via organic quantum dots are subsequently used to employ Ag₃PO₄ particles and thus generate 18 times higher photocurrent at λ_ex > 500 nm than before adding the RhB molecules. To elucidate the reason for increase in photocurrent, steady-state and time-resolved photoluminescence (PL) spectroscopies are used. The improved PL quenching of RhB compared to methylene blue in the FRET-upconversion system, at λ_ex > 500 nm, indicates RhB formed a favorable charge transport path enhancing the charge separation while methylene blue is not able to assist the photosensitization because its upconverted emission wavelength is too long to excite Ag₃PO₄. Furthermore, time-resolved PL studies show that electrons transferred to Ag₃PO₄ efficiently quench the remaining holes in HOMO of organic quantum dots, which results in the increase in the FRET efficiency. The findings provide a novel concept for designing an energy cascade to utilize light energy in visible and near-infrared region.

Structurally well-defined small molecule (SM) donors are promising candidates in bulk-heterojunction (BHJ) solar cells where p-conjugated polymer donors are commonly used in conjunction with fullerene acceptors (e.g. PC₆₁/₇₁BM). Taking advantage of their synthetic modularity through the combination of a variety of electron-donating and -accepting motifs, SM donors can lead to a wide range of optical, electronic and self-assembling properties which may impact material performance in BHJ solar cells. In recent studies, we found that changing the sequence of donor and acceptor units in benzo[1,2-b:4,5-b′]dithiophene–6,7-difluoroquinoxaline SM donors affects: (i) molecular packing, (ii) propensity to order and preferential aggregate orientations in thin-films, and (iii) charge transport in BHJ solar cells. Using a set of three SM donor systems, we show that the lower-bandgap analogue, with the 6,7-difluoroquinoxaline ([2F]Q) motifs directly appended to the central benzo[1,2-b:4,5-b′]dithiophene (BDT) unit, exhibits that most favorable molecular packing and aggregation patterns in thin films, and can yield BHJ device efficiencies of >6%. ¹H-¹H DQ-SQ NMR analyses confirms that this derivative and its counterpart with [2F]Q motifs substituted as end-group possess distinct self-assembly patterns. Their charge transport properties are also very distinct in thin films, resulting in striking BHJ device efficiency differences ranging from 2% to over 6% across the set of SM donors.

SiO₂-MgF₂/TiO₂ double-layer films with antireflective, self-cleaning and adherent properties were prepared by spin-coating SiO₂-MgF₂ and TiO₂ sol on glass substrate successively and subsequently being calcined at 250^OC. The optical and structural properties of films have been investigated by visible spectrophotometer and field emission scanning electron microscope, respectively. At the same time, self-cleaning property generated from superhydrophilicity and photocatalysis was obtained. The results indicated that the as-prepared SiO₂-MgF₂/TiO₂ double-layer films show a maximum increase in transmittance near 520 nm wavelength of 2.8% and phootocatalytic property with the R value of 4.7(JIS R 1703–2).. It has been demonstrated that high transmittance, self-cleaning and adherent composite has been obtained by a simple sol–gel route presenting good potential to be applied on photovoltaics systems.

Among next-generation photovoltaic devices, colloidal quantum dot photovoltaics (CQDPV) are gaining much attention due to its unique properties originated from quantum confinement effect such as band tunability and multiple exciton generation, which are very advantageous to overcome the limitation of conventional photovoltaics. To improve the efficiency of selective charge extraction, adopting a good hole transport layer (HTL) is very important. Although molybdenum oxide is the most commonly used as HTL in CQDPV, it has issues of insufficient hole conductivity and degradation issue. Due to its poor hole conductivity, short circuit current density is limited and thus a precise thickness control is required. Degradation problem of molybdenum oxide under air environment is also severe to the degree that a device kept in air for several days shows a rapid falling of photon conversion efficiency (PCE) by more than 50%. Here, we adopted hetero-bilayer of HTL consisting of one polymer layer with a higher hole conductivity and another high energy-band gap material with oxidation resistance. As a result of adopting the novel HTL structure, the short circuit current, open circuit voltage and PCE of our CQDPV device are improved from 20.4mA/cm² to 25.2mA/cm², 0.50V to 0.56V and 5.8% to 8.06%, respectively. Reliable ohmic-contact formation capability with the top electrode as well as high hole conductivity of the polymer layer contributes to short-circuit current improvement. Also, the efficient leakage current blocking by the high energy-band gap material contributes to the increase of open circuit voltage. Moreover, the device with hetero-bilayer of HTL is stored in air for 15 days without a meaningful degradation of PCE.

Recently accomplished landmark power conversion efficiency (PCE) over 10% of polymer solar cells (PSCs) presents promising potential for the practical application of the printable photovoltaics. However, for achieving the top-level performances, particular designs and huge efforts have to be done at the interlayers. Most of them comprise organic molecules, composites, hybrid materials or multiple-layers, which is usually complicated and costly in synthesis and fabrication, and lack of stable. Moreover, the limited thicknesses of the interlayers (a few to tens of nanometers) have to be avoided for printing photovoltaics in commercial production as the stable and reproducibility issues. In this communication, we report high performance PSCs by preparing the cathode interlayer with well-studied inorganic semiconductor aluminum doped zinc oxide (AZO), spin-coated from a solution of our lab made AZO nanocrystals, based on PTB7-Th:PC₇₁BM blend. PCE of 10.42% was achieved with a ~10 nm AZO interlayer. And prominent PCE approaching 9% was accomplished even with a 120 nm AZO interlayer. Besides, high PCE of 8.93% (average 8.39±0.35 %) was also demonstrated for flexible devices on PET/ITO substrate. Moreover, well distributed efficiency and good storage/mechanical stability are demonstrated for our devices as well.

Inorganic materials synthesis techniques that can approach low temperature routes akin to chemical solution processing are attractive for their ability to prepare nanocrystalline oxides. These methods can offer thin film integration options more compatible with modern device platforms such as spray coated or printed devices (2D, 3D) and flexible electronics. A chemical solution processing method based on sol-gel chemistry was used to obtain a set of perovskite compounds of the formula [KNbO₃]_1-x[BaNi_0.5Nb_0.5O_3-δ]_x, called KBNNO, known to be a class of visible light absorbing ferroelectric photovoltaic materials, with a tunable bandgap as a function of x. The materials produced were fully crystallized with average nanoparticle sizes of 50 nm (KNO) and 20 nm (KBNNO). Control over the composition of KBNNO was based on the synthesis of nanocrystalline potassium niobate KNbO₃ (KNO) via potassium and niobium ethoxides, with subsequent chemical reaction of complimentary barium and nickel alkoxides and methoxyethoxides. Characterization by Raman, TEM, SEM, XRD, and EDS confirms structure and composition. Following the introduction of Ba and Ni, a transition from the original orthorhombic Amm2 unit cell (x = 0) to a more complex atomic arrangement in cubic Pm3m (x > 0.1) is observed. This synthetic route to KBNNO, previously only synthesized by solid state processing at 1050-1200 °C, provides a lower temperature (< 525 °C) approach to doping ferroelectric KNbO₃ with Ba and Ni, which inserts Ba²⁺ onto the A-site, and Ni²⁺ onto the B-site with the addition of oxygen vacancies for charge compensation. Frequency dependent dielectric measurements, performed on KNO-PFA (poly furfuryl alcohol) and KBNNO-PFA nanocomposites, show stable effective dielectric constants of 41.2, 70.8, 94.0, and 108.3 for KNO, KBNNO x = 0.1, 0.2, and 0.3 respectively at 1 MHz. Using full error analysis and the modified interphase model, a Maxwell-Garnett based micromechanics approach, the dielectric constant of the individual nanoparticles of KNO, KBNNO x = 0.1, x = 0.2, and x = 0.3 were calculated to be 154, 180, 225, and 255 respectively. The decrease in observed values relative to bulk films is attributed to a potential particle size suppression of the ferroelectric behavior.

In this presentation, we report a first hand study of gold nanoparticles enhanced photocurrent observed in nanostructures based heterojunction solar cell. The heterojunction solar cell was fabricated, using chemically synthesized narrow gap, IV-VI group semiconductor nanoparticles (PbS and PbSe) of 3~6nm diameter, wide gap semiconductor ZnO nanowires of ~1 μm length and ~50nm diameter, and gold nanoparticles (~5nm to 100nm), by spin-coating (~20cycles) onto FTO glasses, in ambient conditions (25C, 1atm). The synthesized nanostructures were characterized by XRD, UV-VIS absorption, SEM, AFM, TEM, solar simulator, etc. Nanostructures of variant sizes were integrated in to the heterojunction devices to study the effects on photocurrent and solar cell performance. The sizes, lengths, thickness of nanostructures were optimized to have best solar cell devices. The effects of fabrication conditions (such as growth temperature, growth time, anneal temperature, ligand treatments, in air or in N₂, etc.) on device performance were also studied. The architecture of film stack, i.e., the positions of Au nanoparticles and PbS, PbSe nanoparticles were also studied. It is confirmed that introducing Au nanopartiles with proper size will lead to increase of light absorption of film stack and photocurrent of solar cell devices. Further study is ongoing to confirm it’s the plasmonic effects from Au nanoparticles that increases the photocurrent.

A well known concept for further improvement of the efficiency of solar cells are tandem solar cells. The most common representative of this concept are serial tandem solar cells (sTSC) where the subcells are connected electrically in series. Due to Kirchhoff’s law this leads to a current limitation of the whole device by the subcell generating less current under a specific spectrum. This requires a precise layer thickness adjustment for both subcells to achieve current matching and to minimize losses due to the current limitation.
An alternative approach are parallel tandem solar cells (pTSC) utilizing a transparent electrode as third terminal in between of both subcells. The parallel connection leads in this case to a voltage limitation by the subcell with the lower open-circuit voltage. This enables the easy current optimization for each subcell by adjustment of the layer thicknesses without any restriction due to a current limitation. Furthermore it is possible to combine single cells with different polarity in contrast to sTSC where they need to have the same polarity. This grants more flexibility in combining single cells of different materials with each other since inverting a stack typically leads to a very different performance of the solar cell.
We show the feasibility of such devices for different combinations of evaporated small molecule material systems, i.e. combining a thiophene bulk heterojunction (BHJ) as first subcell with a NIR absorbing bodipy BHJ as second subcell reaching a PCE of 7 %. Thin metal layers consisting of a 1 nm Al seed layer and 10 nm Ag are used as transparent intermediate electrode.

Quantum dots (QDs) are semiconductor nanocrystals with diameters in the range of 2-10 nm (10 to 50 atoms) and which exhibit electronic properties between bulk materials and molecules [i]. Considering their optical properties, fluorescence processes exhibit real applications in sensor and biosensors fields. In fact, the fluorescent properties of these materials can be tuned depending on the particle size. Generally, a decrease of the QDs size produces an increase in the energy gap between the valence and the conduction band, which induces to a shifted of the emission wavelength toward shorter values [ii].
In this work we examined an optimization of the synthesis parameters of CdTe QDs using a Doehlert experimental design. The synthesis employed Na₂TeO₃ and CdCl₂ as precursors, mercaptosuccinic acid (MSA) as capping agent, NaBH₄ as reducing agent and Borax/Citrate as buffer in a reflux system. The synthesis parameters optimized were: temperature, pH, molar ratio of precursors and reaction time.
After, an analysis of the optical and electrochemical changes after the interaction with hydroxyl radicals was carried out. The optical changes were studied by time-resolved fluorescence measurements evaluating the fluorescence lifetime observing different quenching processes. On the other hand, changes in the differential capacitance of the QDs deposited on different electrodes were studied by means of Electrochemical Impedance Spectroscopy (EIS). The changes in these properties were studied in absence and presence of hydroxyl radicals produced by a Fenton reaction.


[i]Ekimov, A. I. & Onushchenko, A. (1981). "Quantum size effect in three-dimensional microscopic semiconductor crystals". JETP Lett. 34: 345–349.

[ii] Medintz I L, Clapp A R, Brunel F M, Tiefenbrunn T, Uyeda H T, Chang E L, Deschamps J R, Dawson P E and Mattoussi H. Nature. Mater. (2006) 5: 581.

We compared the electrical, optical, structural, and morphological properties of spin-coated and RF magnetron sputtered ZnO films to use as an electron transport buffer layer for inverted organic solar cells (IOSCs). To remove the additional spin-coating process for the ZnO buffer layer, we developed buffer and transparent cathode integrated electrodes (ZnO/ITO). By continuous sputtering of the ZnO and ITO films without breaking vacuum, we fabricated the ETL and cathode integrated ZnO/ITO cathodes for IOSCs. To compare the performance of IOSCs fabricated on spin-coated ZnO buffer layer and sputtered ZnO buffer layer, we fabricated IOSCs using a PV-D4610:PC₇₀BM organic active layer. Although the RF magnetron sputtered ZnO buffer layer showed similar optical transparency, surface morphology, and work function to spin-coated ZnO buffer layer, the power conversion efficiency (PCE) of the IOSCs with sputtered ZnO buffer layer was fairly lower than that of the IOSC with spin-coated ZnO buffer layer. Regardless of thickness of the ZnO, all IOSCs with sputtered ZnO buffer layer showed lower PCE value than reference IOSCs. Using TEM examinations, we observed that the effective electron extraction from the organic active layer is difficult because the sputtered ZnO cannot provide effective sites for organization of the PV-D4610 chains. Therefore, the phase separation between D4610 and PC₇₀BM led to a low PCE value of the IOSC with sputtered ZnO buffer layer. However, the IOSC with thermal annealed ZnO buffer layer or after supplying high bias voltage showed a significantly improved PCE value. This indicates that better crystallinity of ZnO buffer layer is critical to obtain higher PCE value of the IOSCs. Based on XAS, UPS, and TEM examinations, we suggested possible mechanism to explain different performances of IOSCs fabricated on spin-coated ZnO and sputtered ZnO buffer layer. In addition, we investigated the feasibility of ZnO/ITO integrated electrodes fabricated by continuous sputtering process for cost-effective IOSCs.

Photovoltaic devices based on colloidal lead sulfide (PbS) quantum dots have been fabricated. Using spray deposition via an airbrush technique, PbS active layers and selective contacts were deposited (where the contact was composed of zinc oxide nanocrystals, while the PbS layer was treated to a halide ligand exchange). Structural characterization (photoelectron spectroscopy – XPS/UPS, scanning probe techniques) shows that solution concentration of the PbS active layer, annealing parameters, and deposition process all have significant effects upon film quality, and ultimately device performance. Further, interface modification with small-molecule phosphonic acids, in addition to standard oxide treatments (e.g., plasma cleaning, basic solution etching, etc.) showed that effects between the zinc oxide n-selective contact and the n-PbS varied greatly depending on the push-pull nature of the surface modifiers and the state of the surface after treatment. These results show that sprayable inorganic nanocrystal photovoltaic devices can be realized, with the ultimate goal of producing a fully sprayable optoelectronic device.

Transition Metal Oxides (TMOs) have recently demonstrated to be a good alternative to boron/phosphorous doped layers in crystalline silicon heterojunction solar cells. In this work, the interface between n-type c-Si (n-Si) and three thermally evaporated TMOs (MoO₃, WO₃ and V₂O₅) was investigated by High-Resolution Transmission Electron Microscopy (HRTEM), Time-of-Flight Secondary Ion Mass Spectrometry (TOFSIMS) and X-ray Photoelectron Spectroscopy (XPS). For the oxides studied, surface passivation of n-Si was attributed to an ultra-thin (1.9 – 2.8 nm) SiO_x (x~1.5) interlayer formed by chemical reaction, leaving oxygen-deficient species (MoO, WO₂ and VO₂) as by-products. Carrier selectivity was also inferred from the inversion layer induced on the n-Si surface, a result of Fermi level alignment between two materials with dissimilar electrochemical potentials (work function difference Δφ ≥1 eV). Therefore, the hole-selective and passivating functionality of these TMOs, in addition to their ambient temperature processing, could prove an effective means to lower cost and simplify solar cell processing.

Gallium oxynitride (GaON) nanosheets for photoelectrochemical (PEC) analysis are synthesized via direct solvothermal approach. Their FE-SEM revealed nanosheets morphology of GaON prepared at a reaction time of 24 hrs at 180°C. The elemental composition and mapping of Ga, O and N are carried out through electron dispersive X-ray spectroscopy (EDX). The cubic structure of GaON nanosheets is elucidated by X-ray diffraction (XRD) analysis. The X-ray Photoelectron Spectroscopy (XPS) further confirms Ga, O and N in their respective rations and states. The optical properties of GaON nanosheets are evaluated via UV-Visible, Photoluminescence (PL) and Raman spectroscopies. The bang gap energy of ~1.9 eV is calculated from both absorption and diffused reflectance spectroscopies which showed stronger p-d repulsions in the Ga (3d) and N (2p) orbitals. This effect and chemical nitridation caused upward shift of valence band and band gap reduction. The GaON nanosheets are investigated for PEC studies in a standard three electrode system under 1 SUN irradiation in 0.5 M Na₂SO₄. The photocurrent generation, oxidation and reduction reactions during the measurements are observed by chronoampereometry, linear sweep voltammetry (LSV) and cyclic voltammetry (CV) respectively. Henceforward, these GaON nanosheets can be used as potential photocatalyts for solar water splitting.

Recent studies have shown the role of block copolymer as compatibilizer in tuning the phase separated morphology of the active layer so as to improve the overall photovoltaic efficiency of the OPV devices. Here, we substantiate this observation by investigating the role of rod-coil block copolymer P3HT-b-PS (Poly3hexylthiophene-b- Polystyrene) as compatibilizer in influencing the blend morphology and device performance of several polymer:fullerene blend systems. Fullerene derivatives N-(3-methoxypropyl)-2-(carboxyethyl)-5-(4-cyanophenyl) fulleropyrrolidine (NCPF) and N-(3-methoxypropyl)-2-(carboxyethyl)-5-(5, 5-difluorobenzo-dioxole) fulleropyrrolidine (FFNCPF) were synthesized using Prato reaction. The UV-Vis spectra of NCPF & FFNCPF exhibit a band at 430 nm which is characteristic of fulleropyrrolidine derivatives. The NCPF and FFNCPF show good solubility in polymer casting solvents such as chlorobenzene and 1, 2 dichlorobenzene. P3HT-b-PS copolymer was successfully synthesized using the combination of Grignard metathesis, ATRP, and click chemistry. The emergence of a new peak at δ 7.71 in ¹H NMR spectra indicate the successful coupling of PS and P3HT block. P3HT-b-PS block copolymer was found to effectively alter the thin film nanostructure of polymer/fullerene blends and polymer crystallinity. Maximum enhancement of PV properties of compatabilized P3HT/PCBM blend system was noticed followed by P3HT/FFNCPF and P3HT/NCPF. The significant improvement in photovoltaic properties can be correlated with face on orientation of P3HT crystallites and the segregation of the fullerene domains at the BHJ cathode interface.
Acknowledgement : U.S. Department of Energy grant (DE-FG02-10ER4779)

There has been a growing attention for development of earth abundant photovoltaic materials for cost-effective way electricity production in future. Tin monosulfide (SnS) is one such perspective and relatively new photoabsorber material for thin film solar cells. In the present study, SnS/CdS heterojunctions were prepared by evaporating CdS and SnS with high vacuum method in superstrate structure. Thin films of SnS absorber were deposited at a substrate temperature of 300 °C with a thickness of 500 nm onto the CdS/ITO and CdS/ i-ZnO/AZO structures. The prepared structures were annealed in vacuum at 450 °C for 1h in sealed ampoules to improve their photovoltaic properties. The quality of as-grown as well as annealed SnS thin films was studied by SEM, EDS, Raman, XRD and photoresponse measurements, etc. The crystal structure and phase composition analysis confirms the stoichiometry composition and orthorhombic structure of evaporated and annealed films. The fabricated heterostuctures were characterized by current-voltage measurements and showed the best solar conversion efficiencies of 0.63% and 0.78% for annealed SnS/CdS/ITO and SnS/CdS/i-ZnO/AZO, respectively.

This work describes the electrical behavior of dye sensitized solar cells manufactured with nanocrystalline TiO₂-semiconductor in its anatase phase and sensitized with diverse natural tints. A number of natural sensitizers have been tested, including red fruits as blackberries, hibiscus and beet in order to comprehend the relationship between anthocyanin and electron transfer and green vegetables as spinach and grass, as well as for known the relationship between chlorophyll and electron transfer. The nanocrystalline semiconductor was characterized by DRX, FTIR and SEM. The bands observed at 931, 667 and 514 cm-1 in FTIR analysis confirmed the presence of of Ti-O-Ti bonds. From DRX analysis it is confirmed the presence of TiO₂ in its anatase phase. This study confirms the great potential of the use of organic dyes for sensitized of the TiO₂-semiconductor. Principally in blackberries reaching values around 300mV owing to high concentrations of purple pigment due to the molecule called anthocyanin and the anchoring properties of the anthocyanin with the semiconductor studied.

Potassium niobate (KNbO₃) is technologically one of the most important lead-free and environmentally friendly ferroelectric materials that possess excellent properties, such as large electro-optical coefficients, electromechanical coupling, high nonlinear optical coefficients for various electronic and photonic device applications such as ferroelectric memories, electronic resonators, electrostrictive actuators, sensors, optical harmonic generators, etc. The electro-physical properties like crystal structure, optical bandgap, dielectric constant, Curie temperature, electrical conductivity, and ferroelectric coercitivity of KNbO3 can be modified by doping as well as by substituting isovalent or heterovalent ions into K and/or Nb sites. Due to the recent increasing interest in ferroelectric photovoltaic studies, we made systematic investigations on structural, microstructural, optical, dielectric, charge conduction and ferroelectric properties of the solid solution, [KNbO3]_0.1[(BaNi_1/2Nb_1/2O₃)]_0.9 (KNBNNO). KNBNNO electro-ceramics were prepared by standard solid-state reaction technique using K₂CO₃ (99.5%), BaCO₃ (99.95%), Ni₂O₃ (99.9%), and Nb₂O₅ (99.9%) as starting precursors. Formation of ferroelectric phase of the as synthesized sample was confirmed using x-ray diffractometry and its structural phase transitions were probed by damped oscillator modeling of temperature dependent Raman spectra. Dielectric properties of KNBNNO ceramics were studied as a function of temperature (80–500 K) and frequency (100 Hz–1 MHz) in metal-ferroelectric-metal (MFM) capacitor configuration. The dielectric constant and loss tangent at 100 kHz were 280 and 0.01, respectively, at ambient conditions without any significant temperature dependence. The frequency dependence of ac conductivity showed features typical of universal dynamic response (UDR) and obeyed a power law, σ_ac=σ_{dc +} Aωⁿ.

Squaraines represent a family of small molecules with high potential for OPV devices because they absorb strongly in the NIR and offer many chemical tuning options. In particular, side groups can be modified to change the intermolecular packing arrangement in deposited crystalline thin films, such that exciton diffusion and charge mobility might be optimized. However, changing the molecular structure to modify crystal packing also impacts the monomeric electronic structure. It then becomes almost impossible to isolate the true influence of the packing geometry on device efficiency. Nevertheless, in this work we provide evidence for multiple polymorphs formed by a single type of squaraine molecule, using a mixed solvent microemulsion approach. We obtain an individual aggregate spectrum for each suspended polymorph and we investigate its selective transferal to thin film layers used in devices. We also assign the spectra of each polymorph with computational simulation using an essential states model, which takes into account different contributions from intermolecular charge transfer. When the impact of different aggregates upon device efficiency is fully understood, we can rationally design future squaraine molecules with specific molecular electronic properties and packing geometry targeted for OPV success.

Self-assembly of semiconductor quantum dots (QDs) is currently under intense investigation because these structures have interesting photovoltaic and photonic properties, which are provided by quantum confinement effect. In this work, we reported the self-organization of Lead(II) Sulfide (PbS) QDs in thin films with 3D superlattice. These films were prepared by dropping a solution of QDs in n-Octane (C₈H₁₈) stabilized by oleic-acid on glass substrates heated at selected temperatures. This procedure has created films with different structures. Here, we studied the effects of the solvent drying temperature, ranging from 25°C to 140°C, on the self-organization of the QDs films. X-ray diffractograms of the films dried at 25°C and 40°C showed a set of sharp and intense peaks that are higher order satellites of a unique peak at 1.8 degrees (two theta), which characterize a superlattice arrangement. On the other hand, to the highest drying temperature (140°C) the superlattice peaks were not observed. It suggest that the level of organization decreases with increasing temperature. The superlattice periodicity was calculated using the Bragg’s Law, it gave us an interplanar spacing of 5.3 nm. This result is in agreement with the mean particles diameter, calculated through the broadening of (111) peak of PbS using the Scherrer’s formula, together with the length of the molecule of oleic-acid. The films with superlattice structure were also investigated by transmission electron microscopy (TEM), it showed a long-range ordering of QDs. The photoluminescence (PL) peak of the PbS-QDs films with superlattice arrangement showed a shift toward lower energy compared to that obtained from a colloidal solution. This effect was attributed to the fluorescence resonant energy transfer (FRET) between neighbors QDs. Moreover, we observed greater redshift of the PL peak for film with lower drying temperature, suggesting it has a more organized structure.

Plasmonic metal nanostructures have received increasing attention as promising light-harvesting materials for sustainable and efficient solar water splitting due to their excellent chemical stability and strong light absorption via localized surface plasmon resonance (LSPR). The metal nanostructures can convert the absorbed solar energy to high energetic charge carriers known as ‘hot carriers’ by the decay of LSP that can be subsequently utilized for photocatalytic reactions. The photocatalytic activities can be significantly affected by the structural and optical properties of the metal nanostructures that are strongly associated with the generation and injection of hot carriers. Here we introduce a plasmonic photoanode comprising three-dimensional (3D) porous network of colloidal gold nanoparticles (AuNPs) and iridium oxide (IrO₂) water oxidation catalysts with a titanium oxide (TiO₂) thin layer as an electron filter. The 3D catalyst-plasmonic anode is prepared by the layer-by-layer (LbL) assembly of negatively charged AuNPs and cationic polyethyleneimine (PEI). The multi-layered assembly enables the facile variation of the structural and optical properties of the Au nanostructures. In particular, silica coating of the multi-layered Au nanostructures is carried out to prevent the structural changes of the unique 3D porous LbL network of the AuNPs during thermal annealing. The silica layer is removed by a 30 % KOH solution at 50 ^oC. The silica-mediated control induces a broadband light absorption from visible to NIR region and greatly increases plasmonic photocurrents compared to large spherical AuNPs. These results indicate the importance of the structural factors for plasmonic catalysis. Furthermore, IrO₂ hydrosols are introduced to the multi-layered Au nanostructures as water oxidation catalysts. Increased photocatalytic activities are observed with the increased number of assembled AuNP layers due to the enhanced absorption in the full range of solar spectrum. However, the charge transfer behaviors are very complicated as shown by large variations in catalytic activities, indicating that the fine tuning of electronic structures is critically important for sustainable water oxidation by plasmonic nanostructures. Our study suggests that further understanding on the effects of structural factors of the plasmonic nanostructures on hot carrier generation and injection will lead to a promising light-harvesting platform for sustainable and efficient solar water splitting. This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2016R1A2B4013045).

Since photovoltaics (PVs) based on colloidal quantum dots (CQDs) offer the optoelectronic tunability and a powerful platform for the fabrication, CQDs are attractive promising materials for PV applications. To improve strategies for the efficient ability of light harvesting in PVs based on CQDs, a better understanding of the impact on surface chemistry of CQDs is essential.
The recent breakthrough in improvements of power conversion efficiency (PCE) and device stability of PbS CQD PVs has been achieved through halide treatment with a protic solvent including the short-chain alcohol as MeOH by Chuang et al. The protic character of MeOH in ligand exchange process can provide the proton needed to release a bound oleate ligand, which promotes exchanging surface ligands from native oleate ligands to halide atoms. On the other hand, Hassinen et al. found that MeOH leads to strip tightly bound X-type ligands from the CQD surface and quench photoluminescence (PL) of CQDs. The unpassivated sites which are induced on the surface of CQDs by the unbalanced concentration between protons and halide atoms during ligand exchange process lead to degrade the performance of optoelectronic applications due to the surface trap states.
In this study, we found that variation of surface traps and inter-particles coupling in PbS CQD assemblies by the PL measurement previously developed by Zhitomirsky et al. after the native surfactant ligands have been exchanged by multifunctional ligand exchange process. With multifunctional ligand exchange process, mobility and effective life time of charge carriers were increased due to the enhanced inter-particles coupling and surface passivation, respectively. UV photoelectron spectroscopy (UPS) indicated that the CQDs with multifunctional ligand exchange process induces a static valence-band maximum (VBM) level and a downshift of the Fermi energy level. X-ray photoelectron spectroscopy (XPS) to probe the the species on the surface of CQDs strongly supported the results of previous optical study and transition in band structures. Consequently, the CQD PVs treated by multifunctional ligand exchange process reached a high efficient PCE of 10.5 %.

In order to get highly efficient dye sensitized solar cells, the dye structures have been one of the most researched areas. Interestingly, coadsorbent molecules interact and gain to some extent certain control over the dye’s surroundings that give better efficiencies. Coadsorbent molecules bearing bulky groups have attained better suppression of the recombination and aggregation of the dyes, therefore, the study of different 3β-amino derivates of bile acids with bulky groups featuring naftyl, p-tert-butylphenyl, adamantyl, and diphenylacetyl amides, instead of the terminal hydroxyl group were tested.
All the studied compounds and the corresponding cells with chenodeoxycolic (CDOA) and deoxycholic (DOA) acid, showed to be effective as coadsorbents with reference dye N719 giving significantly better efficiencies than that cells without coadsorbent’s addition, with 0.8-2.2% efficiency increasing. Even if these compounds did not provide an efficiency similar to CDOA, it was demonstrated their ability as coadsorbents compared to DOA. The compounds gave better results diminishing recombination compared to CDOA, although they were also very dependent of the effect on the conduction band (CB) movement. Furthermore, all of them confirmed a valuable effect in dyes aggregation’s decrease by the increase of IPCE and short circuit current density (J_sc) compared to the reference cells without coadsorbents. Furthermore, all the compounds were capable of getting a J_sc as good as CDOA. According to the results of EIS and J-V curve’s parameters, the bulky groups as the quantity and position of the hydroxyl groups of the bile acid’s structural base, seems to be responsible of the different effects in the recombination and the CB movement processes observed.

The structure of silica aerogel allows the material to have high transparency in the solar spectrum and low effective thermal conductivity, making it well-matched to many solar thermal applications. Aerogels are formed by connecting aggregated nanoparticles to create an interconnected network of polydisperse nanopores. These pores make up the majority of the material (>90%) and are smaller than the mean free path of air which minimizes conduction and convection. Meanwhile, the solid network is highly absorptive at infrared wavelengths which reduces thermal radiative losses, and the small effective sizes of the particles and pores allow for high transmission of the solar spectrum. While the synthesis process determines the initial structure and properties, post-synthesis high temperature offers a simple and effective way to reduce solar transmission losses by modifying the structure and surface bonds. In this work, we investigated the effect of high temperature annealing to optimize the structure and properties of silica aerogel for various solar applications. Aerogels synthesized from methyl silicate 51 and tetramethoxysilane precursors were annealed at 400 °C and 600 °C and characterized using UV-Vis and FTIR spectroscopy, small angle x-ray scattering, and gas sorption to investigate structural change with respect to annealing time and post-annealing recovery time. Aerogels showed a reduction of up to a 60% in scattering and absorption losses for wavelengths in the solar spectrum, which represents a potential solar to thermal conversion efficiency increase of 2-5%. While a portion of this increase in solar transmission is due to the reversible desorption of water, a more significant and permanent increase is due to a reduction of the effective scattering center as well as a thermally-driven surface chemistry change. By optimizing the aerogel structure via high temperature annealing, we demonstrated solar transmission greater than 96% and a heat transfer coefficient less than 6 W/m²K at 400 °C in an 8 mm monolithic sample.

Organic photovoltaics (OPV) devices offer several advantages over the current inorganic devices, including fabrication with flexible substrates, lightweight, and production by inexpensive processes. For that reason, the killer applications suitable for OPV must be found to realize widespread commercialization of them. OPV devices are more productive in the slanting rays of light whether than when exposed to direct sunlight. For making of the best features of it, this work aimed to research on the development of a new class of three-dimensional OPV (3D-OPV). The 3D-OPV can be used effectively even if the altitude of sunlight is low, and high efficiency of power generation can be maintained as a whole.
The 3D-OPV composed of multi-layer with PCE-10 as p-type material and PC₇₀BM as n-type material can be fabricated by electrospray deposition method. In this ESD process, common organic solvents can be used in fabrication of each layer. We prepared both the dome-shaped organic thin-film solar cell and the plane-shaped solar cell fabricated by ESD method, and compared the performance of each electric power generation. Theoretical prediction for whole sky solar radiation intensity on the surface of solar cell indicated that the solar radiation intensity on the dome-shaped surface was higher than a plane surface in the all daytime. Actually, the photovoltaic efficiency of the dome-shaped solar cell has more stable than the plane-shaped one in response to change solar radiation angle.

Organic cyanine chromophores can be used in a variety of optical based applications such as light filtering, infrared labeling, bio- imaging, optical switches, photovoltaic devices due to their strong characteristic absorptions. The low thermal and photo stability of the cyanine chromophores impede their extensive application since the high processing temperature tends to degrade the chromophore. The high transparency of wide band gap ZnO in the visible leads to many applications ranging from photovoltaics, and nano energy generators to UV absorbing sunscreens. By binding the chromophore to ZnO nanoparticles, the thermal stability was increased greatly due to vibrational relaxation. The chromophore bound ZnO nanoparticles were generated via electrochemical deposition carried out under oxidizing conditions: the sacrificial anode Zn metal (99.999%) and the cathode stainless steel were immersed in a solution of tetrabutylammonium bromide and the cyanine chromophore. In this study, two cyanine chromophores were studied and both showed significant enhancement in the thermal stability. The linkage between the chromophore and ZnO nanoparticles was characterized by FT-IR spectroscopy. The increase in thermal stability from the unbound chromophores to bound chromophores was measured by thermal gravimetric analysis. The particle size and morphology were examined by scanning electron microscopy (SEM). UV/Vis spectroscopy showed that the nanocomposite maintained the characteristic absorptions when bound to the ZnO nanoparticles.

The significant challenge faced by our global society, from issues of climate change to question of energy security could be solved if we can find a champion catalyst that can convert atmospheric CO₂ to carbon based fuels. However, designing catalytic nanostructures that can thermochemically or photochemically convert gaseous CO₂ into fuels is a significant challenge which requires a keen understanding of the physical and chemical properties of complex materials and the processes happening on them at atomic and electronic level. In this context, this work will present our recent advancements in the area of gas phase heterogeneous catalysis achieved by using computational techniques in conjuction with experimental research. Specifically, in this talk I will highlight the insights provided by computational analysis into the surface chemistry of CO₂ reduction reaction on In₂O_3-x(OH)_y nanoparticles, in the presence and absence of light. This research resulted in the discovery of a new class of “frustrated Lewis pair (FLP) heterogeneous photocatalysts”, which among many reaction possibilities could enable efficient gas-phase hydrogenation chemistry of CO₂ to fuels and chemicals [1,2].

[1] Kulbir Kaur Ghuman, Laura B. Hoch, Paul Szymanski, Joel Y. Y. Loh, Nazir P. Kherani, Mostafa A. El-Sayed, Geoffrey A. Ozin, and Chandra Veer Singh, Photoexcited Surface Frustrated Lewis Pairs for Heterogeneous Photocatalytic CO₂ Reduction, Journal of the American Chemical Society 138, 1206 (2016).
[2] Kulbir Kaur Ghuman, Thomas E. Wood, Laura B. Hoch, Charles A. Mims, Geoffrey A. Ozin, and Chandra Veer Singh, Illuminating CO₂ Reduction: Investigating the Role of Surface Hydroxides and Oxygen Vacancies on Nanocrystalline In₂O_3-x(OH)_y, Physical Chemistry Chemical Physics, 17, 14623 (2015).

The explosive growth of photovoltaics over the last decade, has fundamentally altered the solar photovoltaic technology landscape, with implications for the research directions that can ultimately yield significant impacts on future photovoltaic technologies. Crystalline silicon has dominated all other existing photovoltaic technologies with high efficiency (cell: η = 17-25%; module: η =15-22%). These developments have thus moved the research frontiers towards concepts that can enable ultrahigh efficiency (η = 30-50% and beyond). Reaching these efficiencies requires addressing fundamental optical principles and full solar spectrum utilization. We discuss an approach combining i) fundamental principles governing limits to solar conversion efficiency ii) semiconducting optoelectronic materials with very high radiative efficiency) and iii) new electromagnetic design ideas such as metasurfaces, photonic crystals and transformation optics. We discuss photonic and semiconductor device design to increase open circuit voltage and ‘full spectrum’ spectrum splitting photonics to mitigate efficiency losses from carrier thermalization. I will discuss recent advances in spectrum-splitting concentrators, luminescent solar concentrators and high efficiency cells with effectively transparent contacts.

An efficient solar thermal device requires maximum solar absorptance but minimum thermal emissivity while at the same time a high operating temperature can improve overall efficiency. We describe a plasmonic metamaterial designed for high-temperature, solar-selective thermal absorbers. The structure consists of a thin gold under-layer, followed by a graded cermet of aluminum nitride and gold, and an alumina top layer. The choice of materials is dictated by their combination of high temperature stability and oxidation resistance, their complex refractive indices, and by the fact that the coating materials can be prepared by reactive co-sputtering. The detailed design of the coating is, however, non-trivial, and was established by numerical optimization. The graded gold film provides the necessary low emittance and the cermet composite provides absorptance over the solar spectral range. Finally, the top layer of Al₂O₃, which has a relatively low refractive index compared to the underlying cermet, serves as an anti-reflection layer.
High temperature ellipsometric measurements were taken to characterize and verify the performance of the coating. Tests of structural stability show that the microstructure of the coating is extremely stable, with the individual layers remaining intact up to 800°C. This new thin-film metamaterial has optical properties and thermal stability that suggest that it could be very useful in high temperature solar thermal collectors.

There has been growing interest in the solar illumination of nanoparticles and nanostructured materials that capture light energy efficiently, enabling localized, confined heating at the liquid-vapor interface to vaporize liquids at high efficiency[1,2]. Just as there are many fundamental aspects of this problem to investigate there are an increasing number of applications for this process, such as solar distillation. We will discuss how this process can be used for the all-solar production of cellulosic bioethanol and for water purification processes, and discuss the fundamental as well as examine the economic cost-analysis considerations of converting conventional ethanol distillation to an entirely off-grid process.
[1] Nate Hogan, Alex Urban, Ciceron Ayala-Orozco, Alberto Pimpinelli, Peter Nordlander and Naomi J. Halas, “Nanoparticles heat through light localization”, Nano Letters 14, 4640-4645 (2014).
[2] Oara Neumann, Albert D. Neumann, Edgar Silva, Ciceron Ayala-Orozco, Shu Tian, Peter Nordlander, and Naomi J. Halas, “Nanoparticle-mediated, Light-Induced Phase Separations”, Nano Letters 15, 7880-5 (2015).

Water splitting (WS) nanostructures loaded with metallic nanoparticles are being considered as ideal candidate for designing efficient water splitting devices. These devices are used to dissociate water molecules using solar energy and store the hydrogen part as a fuel. The metallic nanoparticles having plasmonic behavior plays a key role to extend the water splitting activity of wide band gap semiconductors from UV light to full spectrum. Here, we report a detailed Electron Energy Loss Spectroscopy (EELS) investigation of the Au-TiO₂ WS nanostructures to examine the presence of its plasmonic behavior. The deposited Au material varies in shape and size and is mostly in particulate form. The inelastic low loss obtained from the Au region is compared with the density functional theory (DFT) simulation, which indicates the presence of intraband transitions of the s plus d electrons in Au. Asymmetric structures of nontrivial fishing hook shape and selected asymmetric particles are selected for plasmonic investigation. Eigen modes of the plasmonic losses are observed over a wide range of energy (0.6 – 2.5 eV). Depending on the size and the shape of the particles, the modes vary from visible to near IR. This observation indicates the functionality of the metal dielectric photocatalysts (MDPhC) devices over a wide range of wavelength ranging from UV to near IR.

In general, semiconductors have predetermined bandgap energies, and solar cells have fix performance parameters. However, there are many situations where the ability to tune these values would be beneficial. For example, the ability to modify the semiconductor bandgap could improve device efficiency, while the ability to actively control light trapping could enable a solar cell to become transparent and act as a window when power generation is not necessary.

Here we will present on our recent work to (i) modify the bandgap of a semiconductor through photonic engineering and (ii) electrically change the light trapping properties of a solar cell to enable duel functionality as both a transparent window and a power generating device. First we will show how a photonic structure can be used to increase photon recycling to boost not only the operating voltage of a solar cell, but also the effective bandgap of a semiconductor. Experimental and theoretical results are found to be in agreement, yielding absorption and photoluminescence shifts of over 100 nm. Second, we fabricate and test a polymer dispersed liquid crystal (PDLC) device incorporating a solar light absorber (a-Si). We show that the device would be capable of generating enough power to switch between transparent and opaque states due to the low power dissipation of the device (<0.8 mW/cm^2). Together these results show the power of tunable photonic elements for photovoltaic applications.

Passive thermal emitters have been the focus of recently published work. Passive emitters exploit the existence of the atmospheric transparency window extending across the wavelength range of 8 to 13 μm. Within this range, the atmosphere has an insignificant absorption of thermal radiation, thus thermal radiation can be carried away towards outer space without heating the surrounding air. Such emitter requires both strong and selective emission of thermal radiation. Several selective infrared emitters, such as composite materials, white-pigmented paints and SiO films, demonstrate enhanced emissive properties within atmospheric transparency window. However, those structures are unable to achieve near unity and broadband emission within the entire 8–13 μm window. Most recently, conical anisotropic metamaterials have shown strong and selective emission; nonetheless they present a challenge in terms of fabrication because of their structural complexity. In this work we proposes a near perfect thin film emitter with a structure composed of mainly three effective layers setting on top of metallic substrate to diminish any transmission of IR radiation. Silicon Carbide represents the base layer for this design. On top of it there is a thin layer of boron carbide and finally a layer of Gallium arsenide. The broadband emission is made possible because of the use of the SiC layer. SiC has the ability of wide emission in the IR region originating from its lattice vibrations within that spectral range. Tailoring the thicknesses of the top two layers allows for the near perfect emission. By increasing the thickness of those two layers higher emission in the IR region is achieved however the broadband central wavelength will shift towards higher wavelengths. Thus a compromise must be made. The Finite-difference time-domain (FDTD) simulation results show that our proposed thin film emitter with optimized dimensions can achieve more than 90 percent emission extending across the majority of the transparency window.
We plan to fabricate the emitter in the near future using AJA sputtering tool and then use Fourier transform infrared spectrometer (FTIR) to characterize the performance of the emitter within the IR spectral range.

Tandem solar cells present an exciting means for improving the efficiency of solar modules, thereby reducing their balance of system cost (BOS) and cost per watt peak ($/Wp) of a module. Under one sun irradiation, the theoretical maximum efficiency can be increased from 33% up to 42% from a single to a double junction [1]. Highest efficiency tandem solar cells are typically based on III-V semiconductors and double junction cells have achieved up to 31.1% efficiency [2]. However, the high material cost and expensive epitaxial layer growth of III-V semiconductors limit tandem devices to special markets such as concentrated PV or extra-terrestrial applications. Tandem solar cells of printed Cu(In,Ga)(S,Se)₂ (CIGS) and hybrid-perovskite thin film solar cells have the potential to overcome this challenge and approach electricity prices as low as those from coal and natural gas [3].
Here, we present new results on solution-processed tandem solar cells. The bottom cells are low-bandgap CIGS (1.0 to 1.2 eV) formed from molecular-inks [4]. The top cells are NIR transparent solution-processed lead halide based perovskite cells with bandgaps from 1.5 - 1.7 eV [5]. We show device results for both four and two-terminal configurations with stabilized AM1.5 power conversion efficiencies of 18.8% and 18.5%, respectively. For the two-terminal case, we demonstrate that optimal current matching is obtained for devices with 1.6 eV and 1.0 eV bandgap. Further, we present exceptional low-light performance from the solution-processed CIGS with increasing fill factor and efficiency at reduced intensity and with illumination of only the NIR portion of the AM1.5 spectrum. The low-light performance of the bottom cell, the new NIR transparent architecture of the top cell, and the improved conversion efficiencies of the combined tandem devices, highlight the potential of solution-processed perovskite and CIGS tandem cells.

References
[1] A. De Vos, J. Phys. D: Appl. Phys., 13 (1980) 839.
[2] M.A. Steiner et al., IEEE Journal of Photovoltaics, 3 (2013) 1437-1442.
[3] M.A. Green, A. Ho-Baillie, H.J. Snaith, Nature Photonics, 8 (2014) 506-514.
[4] A.R. Uhl et al., Energy & Environmental Science, 9 (2016) 130-134.
[5] Z. Yang et al., Nano Energy, 22 (2016) 328-337.

Perovskite is a class of materials with the same crystal structure as calcium titanium oxide (ABO3). They are a special family of compounds because of their spontaneous polarization, showing outstanding performances in many applications like photovoltaics, photoelectrolysis, diodes, etc.

As for perovskite solar cell, efficiencies of devices rise from 3.1% in 2009, published by Kojima et al. to 22.1% in 2016. There are two types of perovskite materials, inorganic and organic-inorganic complex, the latter one being used in perovskite solar cell. The organic-inorganic complex has the benefit of fabrication at lower temperature. The other one, inorganic materials, however, are more stable than organic-inorganic materials. In our research, SrTiO3 is one of the few perovskite compounds maintaining cubic structure at room temperature. We take advantage of this characteristic to synthesize SrTiO3 nanoparticles in alkaline solution with TiCl4 and SrCl2 as starting materials under 60~80°C condition without making use of autoclave. The results represent a facile way to fabricate inorganic nanoparticles with advantage of organic-inorganic synthesis process. Furthermore, the particles morphology and size could be controlled through the intermediate products, reaction rate constant, different temperature and aging time. Cubes, octahedral and rhombic dodecahedra with particle size between 80~200nm have been synthesized. Effects of facets were found. The difference in photocurrents behaviors between cubes, bounded by six (100) facets, octahedral, which has only (111) facets and rhombic dodecahedron, surrounded with pure (110) facets is explained.

Recently, there have been extensive studies on optical properties of semiconductor nanostructures such as nanodots, nanorods, and nanowires for developing highly efficient solar cells, lasers, and light-emitting diodes. In these nanostructure devices, nonradiative Auger recombination rates of biexcitons (BXs) is extremely enhanced. Since Auger recombination determines the quantum efficiency of light-emitting devices, the manipulation of the Auger recombination rate for BXs and the enhancement of the BX emission are an important issue for improving the device performance. Recently, it has been reported that highly efficient BX emission can be achieved in new types of nanocrystals (NCs) consisting of a CdSe core coated with a thick CdS shell (so-called giant CdSe/CdS NCs). The enhanced BX emission efficiencies can be explained with suppression of Auger processes due to the thick shell structure [1]. An alternative explanation is the enhancement of the radiative recombination rate for the BX [2]. To further improve the BX emission efficiency, a detailed study on BX recombination dynamics is still required. Comprehensive studies on the BX emission in giant NCs provide important clues to understand the influence of Auger recombination and radiative recombination on BXs in semiconductor nanostructures.
In this study, we investigated the shell-thickness dependence of the BX recombination dynamics in giant CdSe/CdS core/shell NCs by means of single dot spectroscopy. Time-resolved photoluminescence (PL) and second-order photon correlation (g⁽²⁾) were measured at room temperature using the time-tagged time-correlated single photon counting method. We observed efficient BX emission, which can be explained by an increase of the radiative recombination rate induced by Coulomb interaction between carriers in the giant NCs.
The PL lifetimes of exciton (X) and BX, , were determined from the PL decay curves, while the ratio of the quantum yields of BX and X (Qxx / Qx) were evaluated from the g⁽²⁾ curve. We derived the degree of the enhancement in the BX radiative recombination rate (β) directly by measuring Qxx / Qx and . The observed values of β for thin-shell NCs are ~ 4 , consistent with the statistical scaling law, which assumes that the radiative recombination rate of the multiple exciton state is in proportion to the number of radiative recombination pathways [3]. On the other hand, for thick-shell giant NCs, the values of β are about around 10 ,which is much larger than 4. To interpret these results, we develop a new model, which explains that the radiative recombination rate of BXs with quasi-type-II confinement is enhanced by Coulomb interaction.
Part of this work was supported by JST-CREST.

[1] Y. –S. Park, et al., Phys. Rev. Lett., 106, 187401 (2011).
[2] S. Sampat, et al., ACS photonics, 2, 1505 (2015).
[3] Y. –S. Park, et al., ACS Nano, 8, 7288 (2014).

We report ordered, high aspect ratio, tapered Si microwire arrays that exhibit an extremely-low angular (0^o to 50^o) and spectrally averaged reflectivity of <1% of the incident 400 nm - 1100 nm illumination. After isolating the microwires from the substrate with a polymer infill and peel off process, the arrays were found to absorb 89.1% of angular averaged incident illumination (0^o to 50^o) in the equivalent volume of a 20 micron thick Si planar slab. The absorption was slightly below the 4n² light trapping limit of a 20 micron thick Si slab for most of the solar spectrum, and exceeded the limit at wavelengths near the Si band gap (1050 nm – 1100 nm), reaching 99.5% of the classical light trapping limit between 400 nm - 1100 nm. The remarkable optical characteristics – minimal reflection and high absorption-- of the tapered Si microwire arrays cannot be completely explained by either effective medium concept or ray optic analysis especially at wavelengths near band gap of Si, and requires a wave-optical analysis for quantification. Therefore we employed a combination of full wave electromagnetic simulations and analytic waveguide analysis to develop an understanding of the array optical properties. We explain the remarkable optical characteristics – minimal reflection and high absorption-- of the tapered Si microwire arrays broadband absorption by enhancement in coupling to waveguide modes due to the tapered microstructure of the arrays.

We performed carrier lifetime measurements in these peeled off arrays via time-resolved microwave photoconductivity decay. Lifetimes of 0.75 µs were measured in these arrays with 20 nm of Al₂O₃ deposited using atomic layer deposition as sidewall passivation and in situ 5.8M HCl back surface passivation after a 20 s back surface damage removal etch in 3.6M KOH at room temperature. An analytical model was implemented to estimate surface recombination velocity (SRV), implied V_oc, and the corresponding maximum efficiency achievable from the carrier lifetime measurements. The SRV achieved was estimated to be 150 cm/s, with the corresponding implied V_oc to be 0.655 V, and the maximum efficiency achievable to be 22.2%. The performance of the arrays is currently limited by surface recombination and therefore further improvement in surface passivation methods to these arrays can push the performance of these arrays to reach an implied V_oc > 0.7 V and maximum possible efficiency > 25%. The high absorption, long charge-carrier lifetimes, and high aspect ratio in these ordered microwire arrays make them an attractive platform for high efficiency thin-film crystalline Si solar cells and as well as for the photoelectrochemical production of fuels from sunlight.

This work reports single nanoparticle optical investigations of 0D-2D hybrids composed of cesium lead iodide (CsPbI₃) perovskite nanoparticles combined with single layer graphene (SLG) and with titanium dioxide (TiO₂), with the aims of (i) understanding the mechanism of light induced interaction in such hybrids and (ii) of demonstrating realization of hybrids with improved optoelectronic properties. By monitoring the time-resolved single nanocrystal photoluminescence emitted from optically excited perovskite nanoparticles (PNPs) coupled with SLG (PNP-SLG), we demonstrate efficient nonradiative energy transfer. By contrary, PNP-TiO2 hybrids do not exhibit light induced interaction in the form of energy or charge transfer. This study qualified single layer graphene as a good acceptor material compared to TiO₂ and PNP-SLG hybrids as new 0D-2D materials with improved optoelectronic properties.

Combinatorial quest for cost effective - environment friendly all-oxide photovoltaics raised several outliers that didn’t match the anticipated p-n junction mechanisms. Further investigation showed that Surface Plasmon Resonance (SPR) in the back contacts, causing hot electron injection, is the major performer in some outliers. SPR rich nanostructures formation at the oxides surface were not-intentional, the nanostructures formed naturally during physical depositions of the metals on rough surfaces and not via templating, patterning or encapsulation of plasmonic particles. The metals in such devices, deposited in one step, perform as both the electrons injectors to the semiconductor and as current collectors, showing stable regeneration and relatively high performance for such simple and direct systems (0.2% power conversion efficiency). Amongst the systems we found Ag|TiO2, Au|Sr:TiO2|TiO2, and Ag|In-W-O to be intriguingly performing with clear IPCE evidence of activity below the semiconductors bandgap.

Semiconducting nanowires have been recognized as promising materials for high-performance electronics and optics where optical and electrical properties can be tuned individually, where the nanowires due to excellent light absorbing properties [1] have been suggested for future high efficiency solar cells [2, 3]. Especially, the geometrical shape of the NWs offers excellent light absorption. Using NWs covering only about 12 % of the surface, record efficiencies has been reported for InP NWs of 13.8 % and for GaAs NWs of 15.3%.
In order to further optimize the performance of NWPV, and integrate them on Si in a tandem junction configuration, nanowires with dimensions corresponding to optimal light harvesting capability are necessary. We developed nano imprint lithography for patterning of catalytic metal particles with a diameter of 200 nm in a hexagonal pitch of 500 nm, for which synthesis was redeveloped since the metal particles were found to move during annealing, destroying pattern fidelity before nucleation. We found that a pre anneal and nucleation step was necessary to keep the particles in place during high temperature annealing to remove surface oxides. In Nano-Tandem, Horizon 2020 research and innovation programme (grant agreement No 641023) we intend to transfer these nanowires to a Si platform (existing PV), either by direct growth on Si PV, or by nanowire peel off in polymer, followed by transfer and elelctrical contacting, or by aerotaxy and alignment for transfer to Si. The optimal band gap in combination with Si is about 1.7 eV, where we identify GaInP and GaAsP as materials for development of nanowire pn junctions by doping, the heart in a solar cell.

1. J. Wallentin et al. Science, 339, 1057 (2013)
2. N. Anttu et al., Phys. Rev. B 83, 165431 (2011)
3. J. Kupec et al., Opt. Express 18, 27589 (2010)
4. Åberg et al, IEEE J. of Photov, 6, 185 (2016)

Photocatalysis is an active area of research with a potential of presenting a solution to critical problems such as sustainable energy production, the control of environment pollution, and even global warming. Therefore, in the recent years it has been studied intensely by the researcher. Unfortunately, traditional photocatalysts, especially semiconductors, have been suffering from inherent deficiencies in photocatalytic applications due to long-term stability, low charge carrier mobility, and high activation barrier. For the last 3-5 years, studies on the usage of plasmonic nanostructures directly in photocatalytic applications have been accelerated and acquired great advances. However, the Plasmonic photocatalysis applications realized in literature have generally been focused on the utilization of nanoparticles. In this work, we examined the utilization of platforms involving 3-D anisotropic plasmonic nanostructures in plasmonic photocatalysis applications. The anisotropic gold and silver based nanostructures were fabricated with the help of oblique angle deposition technique onto the silicon wafer surfaces having different thickness, surface density, and angle of inclination. In addition to these structures, core/shell materials were also prepared by coating gold for silver nanostructures (Au@Ag) and silver for gold nanostructures (Ag@Au) through physical vapour deposition (PVD) technique at different thickness levels. Plasmonic photocatalysis were then realized by using lasers at different wavelengths (532 nm, 655 nm and 808 nm) and energies (50mW-2W) and directions (parallel or anti-parallel according to the nanostructure orientation) for photocatalytic reduction reaction of aromatic nitro compounds.
This project was supported by the Scientific and Technological Research Council of Turkey (TUBITAK) Grant Number 114Z296.

Si quantum dots (QDs)-based multilayers have attracted much attention nowadays because the dot size can be well controlled in the multilayered structures and they can be potentially applied in many kinds of devices, such as Si-based light source and all Si-based solar cells [1-2]. However, due to the large difference of the refractive index between the Si and the dielectric layer materials (SiO₂, SiC etc), the light losses are quite serious due to the internal reflected in the interface as a consequence of Snell’s law and using nano-patterned structures is one of effective approach to improve the device performance[3]. Here, the Si nanowire (Si NW) arrays with various depths are obtained by the wet electroless etching method [4] and good anti-reflection behaviors are confirmed experimentally. It is found that the surface reflection of Si wafers can be obviously suppressed in a wide spectral range. By fabricating Si QDs-based multilayers on formed Si NW arrays to get Si QDs/Si NW hetero-junction solar cells, the enhanced photovoltaic properties are demonstrated compared with that of flat device. It is also found that the device performance is first improved with the depth of Si NW and then decreased with further increasing the depth. ESR measurements indicate the increased the surface defects with increasing the etching time. The device performance is improved again after inserting a thin layer of Al₂O₃ film to passivate the etched surface as revealed by ESR spectra. This work is supported by NSFC (No.11274155) and “973 Program” (2013CB632101).

[1] F. Priolo, T. Gregorkiewicz, M. Galli and T. F. Krauss, “Silicon nanostructures for photonics and photovoltaics,” Nature nanotechnology, vol. 9, no. 1, pp. 19-32, 2014.
[2] Y. Cao, J. Xu, Z. Ge, Y. Zhai, W. Li, X. Jiang and K. Chen “Enhanced broadband spectral response and energy conversion efficiency for hetero-junction solar cells with graded-sized Si quantum dots/SiC multilayers,” J. Mater. Chem. C, vol. 3, pp. 12061—12067, 2015.
[3] Y. Liu, S.H. Sun, J. Xu, L. Zhao, H.C. Sun, J. Li, W. W. Mu, L. Xu, K. J. Chen “Broadband antireflection and absorption enhancement by forming nano-patterned Si structures for solar cells,” Optic Express, vol. 19, no. S5, pp. 1051-1056, 2011.
[4] K. Sato, M. Dutta, and N. Fukate, "Inorganic/organic hybrid solar cells: optimal carrier transport in vertically aligned silicon nanowire arrays," Nanoscale, vol. 6, pp. 6092-6101, 2014.

One-dimensional nanoparticles or nanorods (NRs) of II-VI semiconductors have been known to possess robust optoelectronic properties compared to their zero-dimensional counterparts or quantum dots (QDs). Their unique morphology with one-dimensional elongation, however, has limited their utility in photovoltaic devices due to the difficulty of incorporating these entities as multi-layered light absorbers for quantum dot solar cells (QDSCs) or mono-layered sensitizers anchored on mesoporous oxide photoanodes for sensitized solar cells (SSCs). An intendedly open-structured titanium dioxide (TiO₂) film is shown to readily accommodate ~30 nm long NRs, effectively working as a photoanode of SSC device with over 3% efficiency. Further improvement of PVs utilizing 1-D sensitizers is investigated by co-sensitization strategy, where II-VI QD interlayer (e.g., CdS) pre-deposition and inorganic-organic post treatment on thereof were followed by NR post-sensitization.

[1] S. Lee, J. C. Flanagan, J. Kang, J. Kim, M. Shim, and B. Park, Sci. Rep. 5, 17472 (2015).

Graphene bears great potential as electrodes for photovoltaic devices. But graphene anodes usually have poor hole collection efficiency due to the mismatch of energy level between the anode and light-harvesting layers. We used molybdenum oxide (MoO_x) film to alter the work function of graphene and to improve the interfacial morphology, yielding highly efficient hole transfer. Such graphene/MoO_x anodes demonstrated low surface roughness and high electrical conductivity. Using the graphene/MoO_x anodes in PbSe quantum dot solar cells, we achieved 1-sun power conversion efficiency of 3.56%. Compared to the control devices with ITO anodes, the graphene/MoO_x based devices exhibit considerable performances.

Thin-film crystalline silicon (c-Si) solar cells are a good candidate to reduce the cell manufacturing cost by decreasing the amount of material and lowering the demand for material quality. However, when the Si absorber thickness is reduced, recombination at the contact becomes a more important concern in achieving a high open circuit voltage. Therefore, metal-Si contact needs to be passivated. Here, we demonstrated a novel design of the metal-interlayer-semiconductor (MIS) contact by sputtering NiO_x on top of p-Si to serve as a hole carrier selective contact. By inserting NiO_x between metal and p-Si, the Fermi level can be depinned and re-aligned with a large conduction band offset and a very small valence band offset, thus forming a hole-conducting and electron blocking contact to reduce the contact recombination.

In the first part of this work, transmission line measurements (TLM) were used to demonstrate that thin NiO_x layer deposited by sputtering can effectively block the conduction of electrons. 5 nm NiO_x layers were deposited on phosphorus-doped epitaxial silicon with a doping concentration of 1e¹⁷/cm³. A forming gas (5% H₂, 95%N₂) anneal was conducted after the contact metal deposition. TLM results indicated that pure metal on n-type silicon structure, the as-grown contact showed non-Ohmic I-V behavior, whereas the behavior became Ohmic after 350°C forming gas annealing. However the metal-NiO_x-Si MIS contact structure was not able to form Ohmic-contact even after annealed at 500°C in forming gas ambient, which clearly proved the electron blocking effect associated with the NiO_x layer.

In the second part of this work, NiO_x is integrated within the p-contact of a thin-film silicon solar cell in which the active region is 2 um thick. NiO_x is deposited by sputtering with thicknesses of 5nm and 8nm. The highest Voc of 605mV was obtained from 8nm NiO_x inserted between metal contact and p-type silicon, which is 7mV higher than that of pure metal contact cells. Our current results show NiO_x is a promising material to be integrated within thin-film Si solar cells to achieve higher Voc. However, further work needs to be conducted to improve the NiO_x film quality and optimizing the NiO_x thickness.

The ability to capture sunlight is of great importance for high-performance dye-sensitized solar cells (DSSCs). Nano-sized semiconductors have improved the light absorption ability of these solar cells, due to their enhanced surface-to-volume ratio compared with bulk materials. However, nanomaterials having a size less than ca. 50 nm, such as those used in the DSSC’s mesoporous layer, cannot sufficiently scatter or reflect sunlight. To overcome these issues, researchers have focused on synthesizing light-scattering materials, such as micro-sized particles, core/shell materials, and hierarchical structure materials. However, these light-scattering materials can hinder light absorption in the mesoporous layer, due to their low surface area. Therefore, efficient light absorption requires consideration of the material’s size, surface area, and light-scattering ability.
Recently, multi-shell hollow nanoparticles (MS-HNPs) have been highlighted as promising materials for DSSC applications, offering a high surface area and strong light scattering. The MS-HNPs have a beneficial configuration for multi-reflection of sunlight and redox reactions with the electrolyte. Moreover, the surface area of MS-HNPs is much larger than that of single-shell (SS)-HNPs having the same size. Various MS-HNPs have been developed, such as multilayered SnO₂ hollow microspheres coated to TiO₂, quintuple-shelled SnO₂ hollow microspheres, multi-shelled ZnO hollow microspheres, and shell-in-shell TiO₂ hollow microspheres. These MS-HNPs improve the power conversion efficiency of the DSSCs, due to their strong light-scattering effect and increased surface area. However, among the MS-HNPs, the largest surface area of multi-shelled ZnO hollow microspheres was only 47 m² g⁻¹, which is not sufficient for light absorption in the sensitizer layer. Additionally, most MS-HNPs are synthesized using a hydrothermal process, which limits the mass production of MS-HNPs. Therefore, the development of MS-HNPs with nano-size, high surface area, and strong light-scattering remains a challenge.
Here, we reported a synthesis method of multi-shell TiO₂ hollow nanoparticles (MS-TiO₂-HNPs) via sol-gel reaction, calcination, and etching process. The uniform morphology and anatase crystallinity of MS-TiO₂-HNPs were precisely fabricated by controlling amount of H₂O and calcination temperature. The prepared the MS-TiO₂-HNPs had a high surface area (ca. 171 m² g^-1), strong multi-reflectance, and facile electrolyte circulation and diffusion. The single shell TiO₂ HNPs and double shell TiO₂ HNPs were synthesized as a control. The power conversion efficiency the PCE of MS-TiO₂-HNP-based DSSCs improved to 9.4%, compared with 8.0% for SS-TiO₂-HNP-based DSSCs. This novel structure is expected to play an important role in photocatalyst, lithium-ion battery, supercapacitor, and drug delivery.

Organic semiconductors have witnessed a considerable development in recent years; research activity on this class of materials and their potential applications has increased quickly. The interest in this area is obtain new organics compound with properties of high efficiency for the manufacturing organic light emitting diode (OLED), organic photovoltaic cell (OPV), and thin film transistors (TFTs). The organic devices have a multilayered structure composed generally of functional organic layers sandwiched between two electrodes. Thin films of organic compound and electrodes are deposited by thermal evaporation onto glass or other rigid or flexible substrates if are small molecules. In the case of polymers the thin films are deposited by spin-coater onto the different substrates. We present the study the new molecular structures using the biosynthesis of natural organic compound with nanoparticles should be simple as much as possible, which can reduce the cost of organics devices massively due to a great quantity of use in the fabrication process. The interface state between two layers in organic device depends on the surface morphology of those layers and affects properties of device. The variation in the morphology of an organic thin film depends on the substrate, the contamination of the substrate, deposition rate and substrate temperature.
Thus, it is important analyze the variation of their morphology this hybrid material using as deposition technique the spin-coater. The nucleation and subsequent growth of the hybrid thin film on the substrates have been studied by atomic force microscopy (AFM). The results indicated an increase in island density and in the roughness in compared with the natural organic compound only. The OPVs fabricated with the hybrid material showed an increase in its efficiency.

C. Liang1, Z. He2 and W.C. H. Choy1,*
1Department of Electrical and Electronic Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong.
2. Key Laboratory of Luminescence and Optical Information, Ministry of Education, School of Science, Beijing Jiaotong University, Beijing 100044 China
E-mail: [email protected]

Organic–inorganic halide perovskites have interesting photovoltaic properties. The power conversion efficiency (PCE) of perovskite solar cells has been rapidly increasing in recent years and the theoretical prediction of PCE can reach about 31%. This remarkable performance triggered intensive investigation of the unique intrinsic properties of perovskites/ There is an intense debate on the material properties leading to the abnormal electrical properties particularly current-voltage hysteresis. Several competing mechanisms, such as trapping/de-trapping of charge carriers, ferroelectricity, and ion migration, were proposed to explain the hysteresis. More conclusive evidence is urgently needed to identify the main characteristics of perovskites.
In this work, we propose to investigate the electrochemical light emission properties of CH₃NH₃PbI_3-xBr_x perovskite nanomaterials and the shift of emission spectrum and ion migration of mixed bromide-iodide perovskite emission properties with the assist of impedance studies, we will provide direct evidence of halide ion migration, especially I− ions, are mobile in the perovskites. Under low bias, limited number of ions migrates to the interfaces. However, at higher bias, more ions, especially I− ions, drift toward the interface region, leading to the composition (I-to-Br ratio) change in the middle emission zone and thus the EL spectra shift. Our result suggests that the key to alleviating the hysteresis is to reduce the ion density at the interfaces. The accumulated ions at the interfaces results in a huge capacitance (~100 µF/cm²) which suggest a potential application of energy-storage devices, such as solid-state supercapacitors and batteries.
H. Zhang, H. Lin, C. Liang, H. Liu, J. Liang, Y. Zhao, W. Zhang, M. Sun, W. Xiao, H. Li, S. Polizzi, D. Li, F. Zhang, Z. He, W.C.H. Choy, Adv. Funct. Mater., vol. 25, pp.7226–7232, 2015.

Semiconductor photocatalysts have attracted much attention for their potential applications in energy conversion and environment protection. Among the efforts to improve the visible-light catalytic performance for practical use, inducing defects or controlling microstructures in catalysts has become an attractive issue. Herein, Bi₂MoO_6-x with surface disorder was synthesized through a facile redox method. The as-prepared Bi₂MoO₆ nanoplates were firstly reduced by CaH₂ and then annealed in air at low temperatures (<400 °C) to grow surface disorder. The material actually forms a crystalline core and an amorphous shell (Bi₂MoO_6-x@Bi₂MoO₆). The stoichiometric surface was confirmed by X-ray photoelectron spectroscopy (XPS), and oxygen vacancies in the bulk were detected by magnetic measurement. Formation of core-shell structure is observed by high resolution transmission electron microscopy (HRTEM). Solar absorption was measured by UV-Vis-NIR diffuse reflectance spectroscopy (DRS), and the visible-light photocatalytic activity was evaluated by monitoring degradation of methylene blue (MB). Bi₂MoO_6-x sample treated with CaH₂ exhibits remarkably enhanced solar absorption and improved photocatalytic activity toward dye degradation. And then the oxygen vacancies were gradually "healed" through low-temperature reoxidation, accompanied by the recovery of its pristine yellow color. Strong correlation between the content of oxygen vacancies and the enhancement of solar absorption can be easily found, indicating that the enhanced solar absorption of the reduced sample is induced by oxygen vacancies. During the reoxidation procedure, the surface disordered layer grows thicker and the reoxidized sample exhibits even higher photocatalytic activity than the reduced sample. We suggest that surface disorder contributes to enhancement of charge carrier separation and creation of more surface active sites, which are beneficial to photocatalysis. These results also present an understanding about the formation of surface disorder and roles of oxygen vacancies and surface disorder in photocatalysis, so as to develop highly efficient solar-driven semiconductor photocatalysts.

Vertically oriented TiO2 nanotube arrays have great charge transport and charge lifetime properties enabling them to be involved in many applications such as sensors, fuel cells, and light emitting diodes. This is attributed to their low number of grain boundaries, high internal surface area and ease as well as low cost of fabrication process. However, the large band gap of TiO2, 3.2 eV, leads to photo response in ultraviolet region only, which is only about 5% of the solar spectrum. There are several ways to expand light absorption. A promising method for enhancing light absorbtion is adding noble elements such gold or silver in their Nano particles form on the surface of the TiO2. The Plasmon excited hot electron in the Au is transferred to the conduction band of the TiO2 and causes the span of the schottcky junction between the Au and TNTs in to the conduction band which allows for higher efficiency for any electrochemical process. Herein, we report the fabrication of stable TiO2 nanotubes using an aqueous electrolyte with low water content by anodization technique the nanotube arrays are pure highly-crystalline anatase with wall thickness ranging from 7-9 nm (less than diffusion length of electrons), diameter from 90-120 nm, and tube length approx. 2 µm. the Titanium Nano Tubes decorated by gold Nano particles by magnetron sputtering in argon atmosphere. Plasmonic resonance showed Au characteristic absorbtion peak in the absorbtion spectrum. Bare and sputtered samples were investigated by photocurrent measurements and Mott- Schottky analysis showing enhanced I-V characteristics with very favorable onset potential compared to the reported state-of-art titania nanotubes which will decrease the needed amount of external applied potential and increased hydrogen evolution rate.

Organic phase-change materials such as paraffins and fatty acids have been employed to harvest and store solar heat, while solar thermal fuels that incorporate photochromic molecules such as azobenzene have been recognized as another highly attractive class of materials that convert and store photon energy in the strained chemical bonds. Herein, we demonstrate a superior hybrid to traditional phase-change materials and photochromic molecules via the synthesis of new diacetylene derivatives with azobenzene moieties and with varied alkyl spacers. We developed a series of azobenzene-functionalized symmetric diacetylenes and polydiacetylenes to take advantage of their self-assembly behavior and obtained high performance solar thermal fuel materials that can store up to 176.2 kJ/mol (or 200.2 kJ/mol, if completely charged); double that of pristine azobenzene. The photo-induced phase transition of close-packed crystalline structures of trans isomers to amorphous/liquid cis states under UV irradiation enabled the extra energy storage in the materials in addition to the isomerization enthalpy of azobenzene units.

A novelty hybrid Zinc oxide and in-situ thermal reduced graphene oxide (ITR-GO) hybrid cathode interlayer was fabricated by duel nozzle spray-coating technology for inverted organic solar cells (OSCs). We introduced a duel nozzle spray-coating technology to precisely control the content of two components (ZnO and GO) to form a hybrid film. Then, a facile, fast and one step in-situ thermal treatment was proposed to assemble ZnO with GO as well as reduce GO into ITR-GO simultaneously. It is elucidated that treatment leads to a superior hybrid CIL film with high electron mobility, well-organized morphology and suitable energy level which can enhance the electron transport and extraction between the cathode and the active layer and reduce carrier recombination for OSCs devices. Ultimately, PTB7: PC₇₁BM and PTB7-Th: PC₇₁BM based OSCs devices with hybrid CIL of ITR-GO/ZnO: ITR-GO structure exhibited a significant enhancement in their power conversion efficiency (PCE) from 6.16 to 8.04 % and 8.02 to 9.49 %, respectively. Moreover, the well-organized ITR-GO/ZnO: ITR-GO CIL can also acts as an internal shield against humidity, protecting the air sensitive polymers and improving the lifetime of the devices.

In this paper, we fabricated high-performance non-fullerene ternary blend solar cell showing over 6.8% power conversion efficiency (PCE) enhanced from binary blend all-polymer solar cell (all-PSC) up to 5.9% PCE. As enhancing the performance of all-PSCs, we synthesized and introduced 6,6’-dithiopheneisoindigo (DTI) as additional sensitizer. This non-fullerene ternary blend system was designed for enhancing the performance of all-PSCs, taking advantages of thermal and mechanical stability at the same time. Interestingly, although the small molecule sensitizer which was used in our ternary blend system did not have well-matched energy level with polymers, the ternary blend devices showed enhanced performance compared with reference devices. The maximum 20% of PCE enhancement was achieved when the 10 wt% of DTI was added. In order to found out the relationship between enhanced PCE and the considerations, we investigated the trends of optical properties and crystallinity via the addition of DTI. Combining with UV-Visible spectroscopy, external quantum efficiency (EQE), grazing incidence X-ray scattering (GIXS), atomic force microscopy (AFM), and measurement of light-dependence of open-circuit voltage (V_oc), short-circuit current density (J_sc), and photocurrent density (J_ph) were performed to examine the relationship between optical property, morphology, crystallinity and device performance. From the experimental results, our system is a new approach of enhancing the performance of all-PSC utilizing synergistic effect of polymers and high crystalline small molecule which could reduce the recombination in the crystalline phase of polymers.

Titanium dioxide is one of the most interesting photocatalysts for water splitting. However, one of the great problems that faces most of the studies in recent years is the limited absorption of solar light because of its large bandgap (∼3.2 eV) and low photocatalytic activity due to the fast recombination of charge carriers. In this work we used sol-gel method followed by hydrothermal treatment with H₂O₂ at relatively low temperature (180 c°) to prepare fully crystalline (sub 10 nm) anatase titanium dioxide nanoparticles with band gap of only (2.9 eV) and high surface area ( 99m²/g), with an increased amount of shallow defects to overcome the negative effects of bulk defects. This prepared Nano crystals have a notable enhancement in solar light harvesting and water splitting efficiency compared to commercial Degussa P25. The photoactivity, structure and electrochemical behavior of the prepared particles were investigated by XRD, TEM, photocurrent measurements, PL, XPS, positron annihilation, Doppler broadening and Mott-Schottky analysis.

Cu(ln,Ga)Se₂ (CIGSe) is a I-III-VI₂ semiconducting material that possess chalcopyrite structure, which is employed as an absorber layer in thin film solar cells. Solution-based methods for CIGSe nanoparticles synthesis are upscalable and serve as a template to produce low-cost solar cells. In this work, facile one pot method is used for the synthesis of Cu_yIn_0.7Ga_0.3Se₂ nanoparticles by mixing copper (I) chloride, Indium (III) chloride, gallium (III) chloride and elemental selenium in oleylamine under nitrogen atmosphere. The ratio of y= Cu/(In+Ga) was varied such as 0.4, 0.6, 0.8, 1.0 and 1.2. X-ray diffraction patterns revealed chalcopyrite crystal structure for all samples along with a peak shift to lower 2θ values with increasing Cu content. The lattice parameters a and c were increased with increasing Cu concentrations. Raman spectra exhibited A₁ optical phonon vibrational mode, which gradually shifted to lower wavenumber from 180 to 172 cm^-1 with increasing Cu content. The peak observed at 248 cm^-1 corresponds to CuSe phase which is due to excess Cu present in the sample (y=1.2). Transmission electron microscopy images showed irregular as well as hexagonal plate like morphologies with the increasing particle size from 20 to 100 nm as Cu content increases. The samples were slightly Ga rich and Se poor as the Cu content increases which was confirmed by energy dispersive X-ray analysis. Ultraviolet visible near infrared absorption spectra of the synthesized CIGSe nanoparticles revealed a tunable bandgap in the range of 0.9 to 1.3 eV by varying Cu content. From these results, we conclude that it is possible to vary the Cu content using solution-based process. It will be interesting to see the role of Cu content in thin film solar cells. Therefore, these CIGSe nanoparticles can be used as an absorber layer in low cost photovoltaic devices.

Silicon nanowires have been drawing attention for many years due to their potential in applications in semiconductor-based technology, especially in solar cells recently. The vapor-liquid-solid (VLS) growth mechanism are widely employed in the growth of Si nanowires while in controlling nanowires size, crystallinity, and morphology. However, contamination of metal catalysts, for example Au which creates mid-gap states in Si bandgap, in VLS grown nanowires remains a great concern on performance of solar cell devices. The use of indium instead of gold makes the growth process compatible with Si technology requirements. Indium nanoparticles (In-NPs) with produced several tens and hundreds of nanometers in size have been synthesized from physical and chemical methods, including ultra-sonication,¹ dispersing molten indium into paraffin oil, ² and reduction with sodium metal ³ etc. Two-steps synthesis to produce In-NPs 10-15 nm in size was carried out successfully via a phase transfer reaction.⁴ However, this method makes a challenging in scale-up production due to the efficiency of mixing two phase liquids at large scale. Thus, one-pot synthesis of In-NPs sub-10 nm in size would become necessary to be suited for large scale implement.
In this work, a simple and effective method is presented to produce In-NPs with a size of sub-10 nm. In-NPs size is controlled by reaction temperature and reducing agent concentration in the solution. First, 0.03 mM of InCl₃4H₂O and 0.44 mM of CTAC were filled in a flask together with 5 mL of water. The solution was heated to 50 °C with a water bath under a 400 rpm stirring. Subsequently, reductant solution of NaBH4 is rapidly injected to the transparent, colorless prepared water solution. It is found that different shape of nanoparticles are obtained by different reduction rate (In³⁺:BH₄^-). TEM and UV-vis spectrometer are used to character size and shape of the In-NPs. Shape control in results appears to be driven by reduction kinetics, the shape of nanoparticles is changed from cubic to spherical with an increase of reduction rate (In³⁺:BH₄^-). Meanwhile, particle size is sharply decreased up to 10 nm. An SPR peak centered around 294 nm in cubic particles, and another SPR peak centered around 289 nm in spherical particles. The blue-shift of SPR peak might be due to size decreasing of nanoparticles.

References
1. Li.Z, et al., Mater. Sci. Eng. , A 2005, 407,7.
2. Zhao, Y., et al., J. Phys. Chem. B 2003, 107, 7574.
3. Abtew, M., et al., Mater. Lett. 2005, 59, 1032.
4. Chou, N. H., et al., J.Am. Chem. Soc. 2008, 130, 8140.

Acknowledegement:
This work was supported by the MEXT, FUTURE－PV Innovation (FUkushima Top-level United center for Renewable Energy research – PhotoVoltaics Innovation) Project.

It’s been reported that sub bandgap photons absorbed with the plasmonic mode resonance on thin Au/TiO₂ metal/semiconductor photonic crystals could be injected to the semiconductor as hot electrons while photons absorbed with other modes could not [1,2]. In order to understand why plasmonic hot electrons could be injected across the metal/semiconductor interfaces, we have coated gold nanorods on the surface of the metal/semiconductor composite layer to incur plasmons at frequencies other than the plasmonic frequency of the photonic crystal structure (600nm) we made.

Au nanorods with 15nm diameter and 45nm length (aspect ratio 3:1) have localized surface plasmon (LSP) resonance frequencies at 530nm (transverse) and 700nm (longitudinal). We firstly used electrophoretic deposition method to deposit nanorods on the metal semiconductor composite layers at different suspension densities to get the optimum gold nanorod density. Under 10V electric field, positively charged gold nanorods at the concentration of 6.52x10¹³#/mL could deposit the metal semiconductor composite surface with the density of 230#/µm², which was reasonably uniform and sparse over the surface of the photonic crystal, all lying horizontally to the surface. Photocurrent generation tests with the horizontally coated gold nanorods show only transverse mode plasmons could be injected into the semiconductor. This confirms our study that internal electric field in the direction normal to the metal-semiconductor interface can generate more hot carriers with enough momentum component to cross the Schottky barrier and improve the device’s efficiency.

In order to see the hot electron injection at the longitudinal mode of LSP, we coated gold nanorods vertically, using electron beam lithography (EBL). The Au/TiO₂ composite substrate was coated with PMMA, followed by the electron beam writing. Gold nanorods were grown in the patterned nanocavities by evaporating Au at 2Å/s. The e-beam evaporation system can be set to rotation or zero-rotation mode. The latter improves the cylindrical shape of the nanorods since metal vapors could move straight to the target. A 3nm Ti adhesion layer helped to prevent the collapse of nanorods during lift-off process. Photocurrent measurement with vertically standing nanorods is in progress, which will confirm our study to explain the hot electron generation and injection process across the metal/semiconductor interface.

References
[1] Chou, J. B., et al. “Enabling Ideal Selective Solar Absorption with 2D Metallic Dielectric Photonic Crystals”, Advanced Materials, V. 26, Issue 47, p.7922, 2014.
[2] Chou, J.B., et al., “Broadband Photoelectric Hot Carrier Collection with Wafer-Scale Metallic-Semiconductor Photonic Crystals”, 42th IEEE Photovoltaic Specialist Conference, New Orleans, 2015.
[3] Elfaer, A., et al., “Gold Nanorods Coated Metallic Photonic Crystal for Enhanced Hot Electron Transfer in Electrochemical Cells”, MRS Advances, V. 1, Issue 13, p. 831-837, 2016.

Losses caused by the metal top contacts still remain as an issue in crystalline silicon solar cells. One approach to eliminate shading losses is to utilize transparent nanostructure networks synthesized through rapid and low cost processes. In this work, the potential of highly conductive silver nanowire (Ag NW) networks as transparent top electrodes for the elimination of metallization process in Si solar cells was investigated. Ag NW top contact cells were found to possess enhanced conversion efficiencies with respect to conventional metal contact reference cells. Increase in conversion efficiency was attributed to the elimination of shading losses, preferential scattering of light into the substrate by localized surface plasmon resonances of the Ag NWs and higher charge collection capability with respect to conventional metal contacts.

For a dye-sensitized solar cell (DSSC), the binding mechanism between the chromophore and semiconducting material can significantly dictate the device performance. The type of bond affects the charge transfer, which in turn impacts the injection and recombination dynamics of the cell. Understanding these relationships is an integral part of improving the efficiency of DSSCs.

Some studies have been done to identify binding modes for n-DSSCs, but there are no reports for p-DSSCs. Binding modes are typically found through a combination of FTIR, XPS, and Raman spectroscopy—all of these are predicated on high chromophore loading in the devices. However, p-type devices do not typically have as high dye loading as n-DSSCs, and therefore lack clear variation of the chromophore in the FTIR spectra. This hinders the ability to identify the presence of dye and its binding mode. Statistical-based spectral analysis must be done for p-type devices in order to identify binding modes that can help us understand how to improve the efficiency of the cell, but traditional methods have not worked.

Principal component analysis (PCA) is commonly paired with FTIR spectroscopy in other applications, such as classifying vegetable oils or determining the chemical components of woody plant cell walls. Here, we utilize PCA to identify chromophore-semiconductor binding modes for the first time. This multivariate method concentrates variations in large data sets within the first few components and identifies recurrences of variation in a primarily uncorrelated dataset. By identifying these repeated variations, PCA classifies groupings of spectra that can correlate to binding modes. This method is of particular interest because the computing programs used for PCA analysis are open-source, PCA analysis itself is very robust, and FTIR spectroscopy is easily accessible, making this method useable to a wide audience.

To test the robustness of this model, PCA analysis was done on three different p-type semiconducting materials at different points along the dye loading isotherm. Lead titanate and zinc cobalt oxide are emerging materials that were studied in conjunction with nickel oxide, the literature standard. Preliminary PCA models of lead titanate as a function of dye loading time indicate a correlation between principal component groupings and device performance. Further optimization of the data set is being done to develop a more robust model with clear PCA groupings that can be expanded to more semiconducting materials as well as confirming the chromophore binding bodes through XPS and Raman.

Photoanodes using CuInS2/CdS/ZnO nanorod arrays are fabricated by a solution-based process for hydrogen generation by solar water splitting.
P-type Cu(In,Ga)(Se,S)2(CIGS) chalcogenide materials have high absorption coefficients, tunable band gap energies(1.0–1.7 eV), so they are well known materials for photo-absorber in thin film solar cell. Recently, they have been applied as photocathodes in photoelectrochemical waster splitting cell due to its p-type property. but we use them as photoanodes with n-type CdS/ZnO nanorod arrays to make p-n junction in the photoanode. This p-n junction makes efficient charge seperation and improve photoelectrochemical performance of cell.
We make CuInS2(CIS) thin film to avoid the toxicity of Ga. Most of the efficient CIS electrodes were fabricated through vacuum methods, including co-evaporation, sputtering, and pulsed laser deposition (PLD). However, because these vacuum methods have drawbacks, such as a high production cost and difficulty in scaling up, non-vacuum spin coating method is used here.
Nanoparticle structure has too many interfaces between particles, which facilitate easy electron-hole recombination when charge transfer occurs in the structure. However, nanorod structure is directly connected to the substrate and prevents easy electron-hole recombination. Also, it has a higher surface to volume ratio, scatters much more light to absorb and has short length for a hole diffusion to an electrolyte. So, for efficient light harvesting and photoexcited charge collection, ZnO nanorod arrays were grown and co-sensitized with CdS and CuInS2(CIS). A CdS layer was deposited on the ZnO NW via successive ion layer adsorption and reaction (SILAR), and the CIS layer was prepared by depositing a molecular precursor solution onto the CdS/ZnO NW.
By perfoming XRD and EELS, we confirmed CIS, CdS, ZnO are well deposited. In optical absorption spectra, ZnO nanorod arrays can only absorb light below 400nm, but with CIS/CdS, they can absorb light up to 800nm.
The generated anodic photocurrent was increased with the subsequent deposition of the CIS and CdS layers. Ultraviolet photoelectron spectroscopy analysis revealed cascade type-II band alignments for the CIS/CdS/ZnO NW photoanodes, which enabled efficient electron collection. Our heterostructure photoelectrode has generated a greatly improved photocurrent density of 13.8mA/cm�2 at 0.3V vs. SCE under 1 sun illumination.

Antimony and bismuth-based solar absorbers are of interest due to similarities in the chemical properties of antimony/bismuth halides and the exceptionally efficient lead halide hybrid perovskites. Whilst they both experience the same beneficial relativistic effects acting to increase the width of the conduction band, bismuth and antimony are non-toxic and non-bioaccumulating, meaning the impact of environmental contamination is greatly reduced. Here, we use hybrid density functional theory, with the addition of spin orbit coupling, to examine a range of Sb and Bi containing V-VI-VII candidate photovoltaic absorbers.[1-5] We show that SbSI, SbSeI, BiSI and BiSeI possess electronic structures suitable for photovoltaic applications. Furthermore, we calculate band alignments against commonly used hole transporting and buffer layers, which indicate band misalignments are likely to be the source of the poor efficiencies reported for devices containing these materials. Based on this we have suggested alternative device architectures expected to result in improved power conversion efficiencies.

[1] K. T. Butler, J. M. Frost, and A. Walsh, Energy Environ. Sci. 8, 838 (2015)
[2] A. M. Ganose, K. T. Butler, A. Walsh, and D. O. Scanlon, J. Mater. Chem. A 4, 2060 (2016).
[3] A. M. Ganose, M. Cuff, K. T. Butler, A. Walsh and D. O. Scanlon, Chem. Mater. 28, 1980 (2016)
[4] D. S. Bhachu et. al., Chem. Sci. DOI: 10.1039/C6SC00389C (2016)
[5] K. T. Butler, S. McKechnie, P. Azarhoosh, M. van Schilfgaarde, D. O. Scanlon and A. Walsh, Appl. Phys. Lett. 108, 112103 (2016)

Due to low cost, ease of large-scale manufacturing and possibility of deposition on flexible substrates, dye sensitized solar cells (DSSC) have established themselves as promising photovoltaic technologies for market applications not available to conventional silicon-based or thin film cells, e.g. for solar panels with controlled shape and color for architectural applications. One of the top priorities in photovoltaic research is the increase in cell efficiency closer to the theoretical maximum. In DSSCs many routes are followed to achieve this goal with the development of novel dyes, new electrodes and electrolytes being the most discussed, closely followed by works focusing on encapsulation and stability.

In this report we focus on the application of novel nanocrystalline ZnO electrodes for photoanodes, synthesized via room temperature DC reactive magnetron sputtering of nanostructured Zn films with subsequent oxidation annealing. The material resembles coral with branches composed of ZnO nanocrystals of tens of nm in diameter. We show that by influencing process parameters, in particular gas flow values and gas flow ratios (10:1, 10:2) of argon and oxygen process gases, it is possible to control the microstructure and morphology of the Zn nanostructures. In particular, increasing the oxygen flow from 0.6 to 3 sccm leads to a decrease in mean crystallite size, enabling to optimize the electrode microstructure for DSSC efficiency.

The second important aspect of the work is the co-sensitization of the ZnO DSSC photoanodes using the standard Ru-based N-719 dye and Rose Bengal (RB). While commonly co-sensitization is regarded as resulting in the competition of different dyes for available electrode surface, lowering the efficiency, we propose to fabricate layered dye structures by sequential sensitization. We show, that when we obtain the cascade alignment of the HOMO/LUMO levels in the two dyes with the conduction band maximum and valence band minimum of ZnO (ZnO/N-719/RB), a 26 % increase of the cell efficiency (η = 1.26 % compared to η = 1% for N-719 only) is observed. Contrary, when the alignment will form a potential trap (ZnO/RB/N-719), the cell efficiency drops (η = 0.94 %). In both cases a thin amorphous film forms on the surface of ZnO, however its thickness is significantly different: 8.4 nm in cascade and 28.1 nm in trap alignment, which may be related to surface-based processes enhancing dye aggregation. We discuss these results using the structural data from scanning and transmission electron microscopy, photodiode modelling of the J-V curves measured under 1.5AM 1 sun and potentiostatic electrochemical impedance spectroscopy measurements.

This research was supported by the Ministry of Science and Higher Education in the frames of the Iuventus Plus programme through the project ‘Studies on the Influence of Dye and Quantum Dot Aggregation on the Efficiency of Nanocoral ZnO-based Dye Sensitized Solar Cells’, contract: 0038/IP2/2015/73.

This work reports on the use of vanadium pentoxide as a low-temperature alternative to conventional p-doped emitters for crystalline silicon (c-Si) solar cells based on n-type wafers. Vanadium pentoxide (V₂O₅) is a transition metal oxide, whose semiconducting properties are determined by oxygen-vacancies created during the deposition process. Due to its high work-function (> 5 eV) and wide energy band gap (> 3 eV),V₂O₅ acts as a transparent front side hole-selective contact when thermally-evaporated on a HF-cleaned c-Si surface. Due to chemical reaction, an ultra-thin SiO_x layer forms in the V₂O₅/c-Si interface, providing the necessary front-side passivation. As for the rear side of the n-type wafer, two different passivated back contact strategies were compared: one with locally-diffused point-contacts created by laser-firing, whereas the other used the heterojunction with intrinsic thin layer concept. Of both rear contact types, the heterojunction performed the best with an open-circuit voltage (V_OC) of 665 mV in a polished c-Si substrate. When repeating this design on a texturized wafer, a V_OC of 642 mV and a conversion efficiency of 16.5% was achieved. These results bring into view a new silicon heterojunction solar cell concept with advantages such as the elimination of toxic dopant gases and a simpler low-temperature fabrication process.

Light trapping is essential in photovoltaic devices with a thin photoactive layer. Several light trapping schemes have been developed for polymer solar cells; however, most involve sophisticated and expensive procedures, and thus, are not feasible for mass commercialization. In this study, we demonstrate a novel nanodome rear electrode that is placed on top of the photoactive layer to trap light in polymer solar cells. This nanodome rear electrode is fabricated using polystyrene nanospheres, and it is simple, fast, and cost-effective to manufacture. By analyzing dark-field microscope images, we demonstrate that the nanodome rear electrode has superior light-scattering properties as compared with the conventional flat rear electrode. We use the absorption spectrum to reveal the light-scattering nature of the nanodome rear electrode, which has an elongated optical path length in the photoactive layer, an enhanced light absorption, and yields better device performance. The enhancements in power conversion efficiency and the short-circuit current density of polymer solar cells with a nanodome rear electrode is 20% and 49%, respectively as compared with a solar cell with a conventional flat rear electrode. For comparison, we also fabricated devices with nanohole-array rear electrodes; however, the high transmittance of the nanohole-array rear electrodes results in poor device performance. The nanodome rear electrode enhances the performance of polymer solar cells and makes the fabrication of light-trapping nanostructures simple and suitable for mass production.

The size and bandgap tunability of colloidal quantum dots (CQDs) has made them attractive materials for various optoelectronic devices such as solar cells, light-emitting diodes and photodetectors. The Stokes shift, which is the redshift of emission energy with respect to absorption energy, is a fundamental phenomenon in light-matter interactions. For PbS CQDs, the Stokes shift is large, representing a significant channel for energy loss. Using time dependent density functional theory (TD-DFT), we have compared the effects from Franck-Condon relaxation and singlet-triplet energy splitting. The results indicate that the origin of the Stokes shift in PbS quantum dots is different from the dark excitonic states in other CDQs such as InAs and CdSe. In addition, we calculated the PbS Franck-Condon Stokes shift passivated with various organic and atomic ligands, and found it to be largely independent of ligand type. This is supported by our experimental absorption and photoluminescence measurements. By investigating the conduction band minimum and valence band maximum, we attribute the ligand independence to the delocalized nature of their charge density distributions. Our results allow us to suggest new ways to tune the optical properties of CQDs for photovoltaics applications.

Nanu et al. proposed a new concept for a compound semiconductor solar cell, three-dimentional CuInS₂ (3D-CIS) solar cell, which has nanoscale 3D p-n junction with CIS and nanoporous TiO₂ ¹⁾. It is expected that 3D-CIS solar cell has better conversion efficiency compared with general planer structured CIS solar cell because its large p-n junction areas and short diffusion path of carriers enable the efficient collection of photo-induced electron and holes. However, it is difficult to fill CIS deeply into nanoporous TiO₂ films by conventional solution method ²⁾.
Supercritical fluid (SCF) deposition is an advantageous technique for filling high-aspect nanostructure with metal film, because the properties of SCF, such as liquid-like solubility, gas-like diffusivity and zero surface tension, allow organo-metal precursors to interpenetrate nanostructure and enable to deposit metal thin film into nanostructure ³⁾. On the other hand, we developed SCF sulfurization technique which facilitates the conversion of metal films into chalcogenide semiconductor films⁴⁾. In this sturdy, we applied these supercritical fluid methods for filling CIS into nanoporous TiO₂ films.
We deposited CIS into nanoporous TiO₂ by two-step SCF process. First, Cu and In was deposited by reducing bis(2,2,6,6-tetramethyl-3,5-heptanedionato) copper (II) and tris(2,2,6,6-tetramethyl-3,5-heptanedionato) indium (III) in the mixture solvent of CO₂, EtOH and hydrogen. Subsequently, Cu, In deposited film was converted into a CIS film by sulfurization in supercritical EtOH dissolving elemental sulfur.
From the X-ray diffraction measurement, chalcopyrite structures originated from CIS thin films were confirmed. From the cross sectional SEM observation and Energy Dispersive X-ray spectrometry (EDX) analysis, it was conformed that CuInS₂ was deposited into nanoporous TiO₂ deeply and homogeneously. It was conformed fabricated CIS thin film had ideal bandgap of 1.5 eV. When we deposited CIS on three different size of TiO₂ particle composing nanoporous film, 25, 40, and 100 nm, it was found that the amount of CIS deposition increased with decrease in the TiO₂ particle size. This indicates that Cu and In depositions in SCF were derived from the surface reductive reaction, suggesting this process has a high potential for faster filling or a higher filling ratio for narrower-pore-structured materials.
In order to measure its optical properties, we fabricated Au electrode/CIS/In₂S₃/TiO₂/TCO electrode structure. The CIS films on the 25 and 40 nm TiO₂ films showed better light irradiation current compared with on the 100 nm TiO₂ film. These results suggest SCF methods are effective for realizing 3D nanostructure for photovoltaics application.
References:
1) M. Nanu, et al., Adv. Mater. 16, 453-456 (2004).
2) R. O’Hayer, et al., Adv. Funct. Mater., 16, 1566-1576(2006).
3) J. M. Blackburn, et. al., Science 294, 141-145 (2001).
4) Y. Nakayasu, et al., APEX, 8, 021201 (2015).

Solar water splitting has been widely investigated using semiconductors and organic dyes; however, their narrow absorption ranges and chemical instability in an aqueous milieu have limited their practical applications. Recently, light harvesting by plasmonic metal nanostructures has gained increasing attention as an attractive alternative to the aforementioned materials due to their wide absorption spectra and desirable chemical stability. Here we suggest Au/TiO₂ half-dome heterostructures as a new type of photoelectrode to harvest the light by surface plasmon resonance (SPR) for visible light-driven hydrogen evolution. Half-dome structures are fabricated by e-beam evaporation of Au and TiO₂ layers on cylindrical photoresist patterns. Pt is also introduced onto the surface of TiO₂ for hydrogen evolution. Hot electrons decayed from surface plasmon are transferred to Pt cathode via the TiO₂ electron filter layer, and sacrificial reagents (e.g., methanol, triethanolamine, etc) are oxidized by the remained hot holes in the Au nanostructures. Photocurrent and photovoltage of the heterostructures are measured under visible light illumination, and hydrogen gas from the photocatalytic reaction is quantitatively analyzed using gas chromatography. The heterostructures with various Au nanostructures prepared via thermal annealing drive the water reduction reaction in a sustainable manner by absorbing the visible light energy. Moreover, we found that incident light angle and orientation of nanostructures affect plasmonic photo-reactivity. The quantum efficiency depends on the angle between the electric field of irradiated light and the Au/TiO₂ interface, indicating hot electron injection is controlled by the electric field direction of light. The angle effect results in increased photocatalytic activities as observed in half-dome heterostructures with oblique Au/TiO₂ wall structures compared to a flat thin film heterostructures. This approach provides a new insight to improve SPR-based photolysis of water through the control of nanostructures and their orientations. This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2016R1A2B4013045).

A novel design of highly efficient modified grating crystalline silicon (c-Si) thin film solar cell is demonstrated and analyzed using 2D finite element method. The suggested grating has a double-sided pyramidal structure. The incorporation of the modified grating in c-Si thin film solar cell offers a promising route to harvest light into the few micrometers active layer. Further, a layer of silicon nitride is used as an antireflection coating (ARC). Additionally, the light trapping through the suggested design is significantly enhanced by the asymmetry of the top and bottom pyramids. The effects of the thickness of the active layer and facet angle of the pyramid on the spectral absorption, ultimate efficiency (η), and short-circuit current density (J_sc) are investigated. The numerical results showed 87.9 % efficiency improvement over the conventional thin film c-Si solar cell counterpart without gratings.

This paper will propose methods to use leaf vasculature formations to advance a material to act as an infrared block. The research shows the use of microfluidics based flows to direct the structural assembly of a polymer into a thermally functional material. To manage IR radiation stop-band to lower a polymer device phase transition temperature. This paper will determine this functionality by hierarchical multi microchannel network scaling, to regulate laminar flow rate by analysis as a resistor circuit.

Nature uses vasculature formations to modulate irradiance absorption by laminar fluidic flow, for dehydration and autonomous self-healing surfaces as a photoactive system. This paper will focus specifically on pressure drop characterization, as a method of regulating fluidic flow. This approach will ultimately lead to desired morphology, in a functional material to enhance its ability to capture and store energy. The research demonstrates a resistor conduit network, can define flow target resistance, that is determined by iterative procedure and validated by CFD. This algorithm approach, which generates multi microchannel optimization, is achieved through pressure equalization in diminishing flow pressure variation. This is functionality significant in achieving a flow parabolic profile, for a fully developed flow rate within conduit networks. Using precise hydrodynamics is the mechanism for thermal material characterization to act as a switchable IR absorber. This absorber uses switching of water flow as a thermal switching medium to regulate heat transport flow. The paper will define a microfluidic network as a resistor to enhance the visible transmission and solar modulation properties by microfluidics for transition temperature decrease.

ZnSe quantum dot sensitized CdS nanowires vertically grown on FTO glass substrates show improved photoelectrochemical performances due to the enhanced surface area and the type-II energy band structure. A mild solution based approach, the solution-liquid-solid (SLS) mechanism, enabled the growth of uniform and high-density CdS nanorods having a diameter of ~15-20 nm and a length of <1 μm. Bi was used as a catalyst for the SLS growth of CdS nanowires. ZnSe quantum dots were synthesized with high-crystallinity and an average particle size of ~3 nm by a hot-injection method. As-prepared ZnSe quantum dots showed strong light absorption and emission at ~395 and ~410 nm, respectively. As-synthesized CdS nanorods were uniformly coated with ZnSe quantum dots using a spin coating method. To consolidate quantum dots and nanorods, heat treatments were conducted and the ZnSe shell formed on the nanorod surface with a thickness of ~3 nm. Uniformly coated ZnSe shell with high-crystallinity was identified by HRTEM analyses. The photocurrent onset potential of CdS/ZnSe nanorod electrodes was almost equivalent to that of bare CdS nanorod electrodes, which implying their similar photovoltages. Thus, only the effective charge separation could induce the enhanced phtoelectrochemical reactions for splitting water. According to the energy band gaps of CdS nanorods and ZnSe quantum dots estimated by UV-Vis light absorption analyses, their energy band alignment was expected to be the type-II band structure. This type-II band structure could drive the charge separation of carriers between the two semiconductors and then accelerated the photocatalytic reactions for splitting water not only at lower but also at higher bias voltages.

Gold nanoparticles (Au NPs) were synthesized by the citrate reduction method to be used in the fabrication of photoelectrodes. The incorporation of Au NPs in photoelectrodes is expected to improve absorption of light in the photoelectrodes. Here we report on the evolution of the Au NP size and morphology with varying process parameters like temperature and citrate to gold precursor (Na₃Ct/HAuCl₄) ratios. The reaction temperatures were kept below 100 °C and were the focus of our study. A Na₃Ct/HAuCl₄ ratio range of 1.25:1 to 4.33:1 was investigated. The NP size and morphology were strongly influenced by the Na₃Ct/HAuCl₄ ratio, while the temperature played a subtle role. The reaction times were also monitored. The higher concentration samples required almost an order of magnitude longer reaction time compared to the lower concentration samples. The samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and UV-Visible spectroscopy.

Lead sulfide colloidal quantum dots are good candidates for photovoltaics because they are low coat, fabrication is relatively simple, and the band gap can be tuned across the solar spectrum. Having the energy levels appropriately aligned is crucial for good performance. In this work, the band structure of colloidal quantum dot (CQD) bilayer heterojunction solar cells is optimized using a combination of ligand modification and QD band gap control. Solar cells with power conversion efficiencies up to 9.33 ± 0.50% are demonstrated by aligning the absorber and hole transport layers (HTL). Kelvin probe, cyclic voltammetry, and UV-vis methods were used to determine QDs’ fermi levels, conduction and valence band edge positions. Key to achieving high efficiencies is optimizing the relative position of both the valence band and Fermi energy at the CQD bilayer interface. By comparing different band-gap CQDs with different ligands we find that a smaller band gap CQD HTL in combination with a more p-type-inducing CQD ligand is found to enhance hole extraction and hence device performance. We postulate that the efficiency improvements observed are largely due to the synergistic effects of narrower band-gap QDs causing an upshift of valence band position due to 1, 2-ethanedithiol (EDT) ligands and a lowering of the Fermi level due to oxidation.

Ternary solar cells show a way solving the problem of low Jsc and small absorption range in the field of organic thin film solar cells by widening absorption spectrum and promising a thicker film to intensify the absorption, simultaneously. However, one of the biggest problems is the compatibility between the addition materials, which is mainly small molecules, and the polymer active layer. This compatibility will have a profound impact on the morphology of blending layer and the crystallinity of the active donor mixture which will result in a variation of mobility. This article composed a new double donor system: oligomer-polymer with same monomer, promoting all aforementioned parameters, including absorption range and intensity, morphology, and crystallinity. Many pairs of oligomer and polymer have been tested and we found that materials in polymer state with smaller bandgap, whose absorption is more preferable to red light, cooperates with its oligomer better. This may because in this kind of materials, the scale of conjugated system adjusts the dislocation energy of molecule to form quinoid structure more conspicuously.

Yi Zhang, Linfei Zhang, Chun Cheng*

Department of Materials Science and Engineering, South University of Science and Technology of China, Shenzhen, Guandong – 518055, P.R. China

Corresponding author: [email protected]


Graphitic carbon nitride (g-C₃N₄) has drawn broad attention and become a new hotspot as a metal-free photocatalytic in the area of solar energy harvesting via water splitting. However, practical applications of bare g-C₃N₄ are still hindered by several obstacles due to the high recombination rate of charge carrier, low electrical conductivity and the lack of absorption above 460nm in bulk g-C₃N₄. Finding a new structure for g-C₃N₄ which can improve its properties is now a goal for many researchers. Here we developed a template-free synthesis route of g-C₃N₄ microrod with “zeolite” nanoarchitecture, simply doping P into the precursor of g-C₃N₄ via supermolecule self-assembly process. The as prepared P -modified g-C₃N₄ is platelike, filled with numerous nanosize holes, and gather together forming a microrod spontaneously. The specific surface area is largely increased as well as the photocatalysis efficiency. Meanwhile. The P-modified g-C₃N₄ also shows remarkable photocatalytic activity in both UV-Vis and NIR regions, which means full spectrum photocatalysis can be achieved. The fabrication process is temple free and eco-friendly. The P-modified g-C₃N₄ provides a promising solution for photocatalysis with high conversion efficiency and good full spectrum absorption. It also has a potential application as electrodes for oxygen reduction, reduction of CO₂ and catalyze for organic synthesis.

An ultra-thin LiF layer in conjunction with an Al layer is employed as the electron collector for a-Si:H based single-junction thin film photovoltaic cell. The cell has the structure of boron doped μ-SiO_x (hole collector) - intrinsic a-Si:H (photoactive layer) - LiF / Al (electron collectector & back electrode). The substrate used is U-type Asahi glass, which is also acting as the transparent front electrode. The optimum thickness for the LiF layer, and the effects of different post deposition annealing temperatures are investigated. The highest efficiency value is obtained for the cell with the 1.5 nm thick LiF layer, annealed at 120^oC. The open current voltage (V_oc) of 0.937 V, the short current density (J_sc) of 13.598 mA/cm², and the fill factor (FF) of 0.690 are achieved. The J_sc and V_oc values are comparable to the values measured for the a-Si:H based p-i-n reference cell, but the FF values is found to be lower, which is attributed to the losses due to recombination at the intrinsic a-Si:H / LiF / Al junction. The current versus voltage measurements are carried out under standard test conditions. The J_sc values are corrected according to the external quantum efficiency measurements of the cells in the AM1.5 spectrum region between 270 nm and 800 nm.

Copper-zinc-tin-sulfide-selenide (Cu₂ZnSnS_xSe_4-x, usually called CZTSSe) has drawn attention as a promising alternative to CuIn_xGa_1-xSe₂ (CIGS) and CdTe as a light absorber for thin-film solar cells. Compared to CIGS and CdTe, the key advantage of CZTSSe is that all of its elemental components are earth abundant and relatively non-toxic. Solar cells produced by printing and coating processes using nanoparticle inks could potentially be much less expensive than those produced by vacuum deposition processes. Solution processed CZTSSe photovoltaics have achieved a power conversion efficiency (PCE) of 12.6%, which is becoming competitive with other thin film solar cells. However, those record-setting devices were processed using hydrazine, a highly toxic solvent. Cells produced by another approach using copper zinc tin sulfide (CZTS) nanoparticle inks with post-treatment using Se vapor have reached an efficiency of 9.0%. Direct use of CZTSSe nanoparticle inks could, along with other developments in processing, eliminate the use of hydrazine and the separate selenization step. Toward this goal, we have synthesized kesterite CZTSSe nanoparticle inks by a hot-injection colloidal synthesis method. Different ligands have been utilized to control the shape of the CZTSSe nanoparticles. The shape and the size of the nanoparticles were examined using transmission electron microscopy (TEM). The composition uniformity of the particles was determined by high-angle annular dark-field imaging (HAADF) and energy-dispersive X-ray spectroscopy (EDX). The crystal structure was confirmed by X-ray diffraction (XRD). Other functional materials in the soda lime glass/Mo/CZTSSe/CdS/ZnO/ITO structure are also being produced by colloidal synthesis and characterized, in an effort to move toward fully solution processed devices. Preliminary device performance results will be presented along with the material synthesis efforts.

Since the discovery of photo-induced water splitting on TiO₂ materials by Fujishima and Honda in 1972, TiO₂ has been used as a promising material in the application of photovoltaics, photosynthesis, photocatalysis, and photodegradation. However, TiO₂ nanomaterials have two distinct disadvantages that limit the utilization efficiency of solar energy. One is the lack of effective absorption in the visible light region due to wide bandgap (3.2eV). The other is that the photogenerated electron-hole pairs can easily recombine. The black TiO₂ nanomaterial synthesized by hydrogenation techniques can improve the absorption in visible light band. In addition, several strategies, such as doping with metal or non-metal and coupling with narrow bandgap semiconductors, have been developed for improve the generation, separation, and transportation of electron-hole pairs. In this paper, black TiO₂ nanotubes (B-TNT) modified by CdSe quantum dots with gradient size distribution were prepared by anodization, electrochemical reduction, and dip coating process. This nanocomposite system has several advantages: (1) 1D tubular structure of B-TNT offers a straight pathway for electron-hole pairs transportation; (2) black color of TiO₂ nanotube enhance the visible light efficiency; (3) gradient size distribution of CdSe quantum dots increases full spectrum absorption range; (4) multi-size CdSe quantum dots act as sensitizers to form hierarchical bandgaps to help charge separation. The phase structure, surface morphologies, and photoelectrical performance of co-catalyst system have been studied by X-ray diffraction, scanning electron microscopy (SEM) and three-electrode electrochemistry system. We expect this low cost and easy preparation method could improve the utilization efficiency of solar energy.

Interest in Quantum dots (QDs) as photosensitizers in photocatalytic systems has increased in recent years as they have been shown to improve photostability and efficiency over traditional organic light harvesting molecules.¹ For example, vanadium clusters (V₆O₇^-) combined with Ru(bpy)₃²⁺ photosensitizers form an efficient system for photocatalytic water oxidation, however the harsh oxidizing conditions of the reaction causes the dye to degrade and the catalytic reaction to stop after only 2.5 hours.² The inherent robustness of QDs make them an ideal alternative to these molecular photosensitizers. In this presentation we will discuss studies where we combine the tunability of QDs with the reactivity of vanadium clusters for photocatalytic water oxidation. A wide range of possible capping and shelling variations for QDs will be discussed that optimize both photochemical stability as well as solubility, which is important as QDs must be water-soluble for use in this application. We have demonstrated that interaction of the glutathione capped CdZnS or CdSe/ZnSe QDs and V₆O₇^- clusters result in a concentration dependent quenching of the PL of the QDs, which demonstrates charge transfer from the QDs to the clusters through a nonradiative pathway. Concentration dependent quenching studies indicate that the interaction is static, meaning the cluster and QD are effectively bound. Importantly, no photochemical degradation of the QDs is observed after several days showing the inherent stability of the system. Photocatalytic water oxidation experiments will determine the efficiency and robustness, of this water-oxidation photocatalytic system.
(1) Han, Z., Qui, F., Eisenberg, R., Holland, P. L., Krauss, T.D. Science, 2012, 338, 1321-1324.
(2) Santoni, M.; La Ganga, G.; Nardo, V.M.; Natali, M.; Puntoriero, F.; Scandola, F.; Campagna, S. J. Am. Chem. Soc. 2014, 136, 8189-8192.

The chemically prepared ZnO electron transporting layer often produce interfaces unacceptable for efficient electron extraction in organic-inorganic hybrid photovoltaic devices and understate its performance. Herein, we propose a facile interfacial modification technique to enhance the charge collection efficiency of ZnO cathode electrode by efficiently bridging the superficial troughs and ridges of ZnO with the photoactive PCDTBT: PC₇₁BM polymer blend. The investigations show that vacuum sublimated C₇₀ interlayer efficiently fills the gaps between ZnO and the polymer blend reducing accumulation of the charges at the interface and thus minimizing the recombination probability. It also plays a very crucial role in passivating ZnO electrode against interfacial traps due to adsorbed chemical species. The inclusion of C₇₀ surface modifier into the devices led to a nearly twofold increase in device performance with PCE reaching close to 4%. In view of the emerging importance of low cost photovoltaics and significance of the interfacial effects in the device performance, we believe that this work can provide a deeper insight on the routes to tackle interfacial defects to achieve high performance and low cost photovoltaic devices.

The origin of the differences between the performance parameters found for dye-sensitized solar cells (DSCs) using liquid and poly(ethylene oxide)-based solid polymer electrolytes has been investigated. Limitations associated with poor polymer electrolyte penetration and ionic diffusion have been analyzed together with other effects such as the dye regeneration rate, the conduction band edge shift and the electron recombination kinetics occurring in the solid polymer electrolyte. We have found that dye-regeneration was faster for sensitized TiO₂ films fully wetted with polymer electrolyte than that with liquid cells. This new result was attributed to a 0.2 eV decrease in the dye HOMO energy yielding to a increase in the driving force for dye regeneration. These understandings may contribute the further increase in the energy conversion efficiency of DSCs employing solid polymer electrolyte.

Plasmonic thin-film solar cells have recently gained a great research interests due to the tunability of plasmonic nanoparticles with efficient light trapping and I-V characteristics. Although noble metals have shown great physical and optical properties in the visible region of the solar spectrum, the main drawback of using such materials is their cost and unavailability for large scale production and integration.

In this work, we show the scientific possibility to replace Au with refractory plasmonics, especially transition metal nitrides/oxynitrides such as TiN. The main advantage of TiN is the low cost that may allow mass production as well as availability over noble metals, along with the CMOS compatibility. The close behavior of the optical and electrical properties between TiN and noble metals (such as Au or Ag) makes this replacement possible.

The plasmonic effect of TiN across the scattering and absorption cross-section is demonstrated. We then show the I-V characteristics for a 2 microns thin-film solar cell with TiN nanoparticles with different orientations and configurations, targeting best –and easiest to fabricate- efficiency possible. Afterwards, multiple sizes of the nanoparticles are investigated, targeting the best scattering and coupling-to-substrate efficiency, and hence better overall efficiency. Moreover, different TiN nanostrutures have been synthesized and tested. Finally, we introduce the business model for the usage of TiN-based solar cells.

The recent advance in perovskite solar cell research has captivated much attention. Surfaces and interfaces in these solar cell devices can impact strongly on device performance. My group at OIST investigates relevant surfaces and interfaces for better understanding about perovskite materials and solar cells. Based on the findings, we develop strategies to further improve device performance. In this talk, I will introduce our recent research progress [1-5].
[1] Juarez-Perez, E. J.; Hawash, Z.; Raga, S. R.; Ono, L. K.; Qi, Y. B.*; Energy Environ. Sci. 2016, in press (DOI: 10.1039/C6EE02016J).
[2] Raga, S. R.+; Ono, L. K.+; Qi, Y. B.*; J. Mater. Chem. A 2016, 4, 2494-2500 (+ equal contributions).
[3] Jiang, Y.; Juarez-Perez, E. J.; Ge, Q.-Q.; Wang, S.; Leyden, M. R.; Ono, L. K.; Raga, S. R.; Hu, J.-S.; Qi, Y. B.*; Mater. Horiz. 2016, in press (DOI: 10.1039/C6MH00160B).
[4] Juarez-Perez, E. J.; Leyden, M. R.; Wang, S.; Ono, L. K.; Hawash, Z.; Qi, Y. B.*; Chem. Mater. 2016, 28, 5702.
[5] Leyden, M. R.; Jiang, Y.; Qi, Y. B.*; J. Mater. Chem. A 2016, 4, 13125 – 13132.

Copper-zinc-tin-chalcogenides (CZTSSe) has sparkled tremendous research interest due to its excellent electronic, optical and optoelectronic properties in addition to less environmental toxic and cost effectiveness candidature. In the present work, CZTS nanoparticles were prepared using copper (II) acetylacetonate, zinc acetate, tin (II) chloride and sulfur powder in oleylamine under nitrogen atmosphere by hot injection method. The reaction temperature was varied from 180°C to 240°C for 4 hours. Then, these nanoparticles were annealed at 300°C under Se atmosphere for different times such as 5, 10, 20 and 40 minutes. The CZTSSe nanoparticles were characterized by X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray analysis (EDAX) and Ultraviolet visible near infrared (UV-VIS-NIR) spectroscopy. From XRD spectra, the major diffraction planes of (112), (220) and (312) were observed which correspond to kesterite crystal structure of CZTS (JCPDS no.01-075-4122) in all cases. For the annealed nanoparticles, the peaks were shifted to lower 2θ values and the lattice parameters a and c were increased. Raman spectra showed the peak at around 173, 194 and 237 cm^-1 for the CZTSSe nanoparticles. FE-SEM images revealed that nanoparticle size varied from 65 to 85 nm as the annealing time increases. EDAX analysis revealed that the Se/(S+Se) ratio increases from 0.79 to 0.94 with increasing annealing time. From UV-VIS-NIR spectra, the bandgap of the samples were calculated and the bandgap was varied from 1.5 to 1.1 eV as the annealing time increases. Hence, the results suggest that bandgap can be tuned as the Se/(S+Se) ratio increases and the CZTSSe particles will be employed as an efficient absorber layer in low cost thin film solar cells.

The compound Cu₂ZnSn(S_xSe_1-x)₄ has tremendous potential as a photovoltaic material due to its adjustable band gap (1-1.5 eV), high absorption coefficient (>10⁴ cm^-1), and most importantly the abundance and non-toxicity of its constituent elements. Currently, solution-processable colloidal Cu₂ZnSn(S_xSe_1-x)₄ nanocrystals (NCs) have been successfully utilized to deposit the active layer of photovoltaic devices in which up to 9% efficiency has been achieved. However, these results were realized by annealing the NC layer at high temperature under Se or S atmosphere in order to reduce the surface defects and grain boundaries via grain growth. The fabrication of low-cost photovoltaic devices based on colloidal Cu₂ZnSn(S_xSe_1-x)₄ NCs without applying annealing step remains a daunting challenge due to the high defect density and poor charge transport properties of as-deposited Cu₂ZnSn(S_xSe_1-x)₄ NC films. We introduced a series of comprehensive approaches to address this problem including a novel strategy to improve the charge carrier hopping between NCs via synthesis of a conductive shell. Furthermore, rational ligand exchange schemes were employed to fully passivate complex quaternary NC surface in order to minimize photo-carrier recombination. Additionally, chalcogen ratio was varied to uncover the optimal composition that provides best photovoltaic properties. The effectiveness of these approaches was evaluated by measuring electrical properties of the Cu₂ZnSn(S_xSe_1-x)₄ NC films and finally fabricating solar cells.

Culn_1-xGa_xSe₂ (CIGSe) is a chalcopyrite structure belongs to I-III-VI₂ semiconductor material and it has been employed as an absorber layer in thin film photovoltaic devices. CIGSe nanocrystals synthesized by solution-based methods are upscalable and serve as basis to produce low-cost thin film solar cells. In this work, CIGSe nanocrystals were synthesized by mixing copper (I) chloride, Indium (III) chloride, gallium (III) chloride and selenium in oleylamine under nitrogen atmosphere at 260°C for 4 h using thermal decomposition method. The x= Ga/(In+Ga) ratio was varied across entire stoichiometric range from 0 to 1. X-ray diffraction patterns revealed chalcopyrite crystal structure for all samples along with a peak shift to higher 2θ values with increasing Ga content. The lattice parameters a and c decreased linearly with increasing Ga concentration which is consistent with Vegard’s law. Raman spectra exhibited A₁ optical phonon vibrational mode, which gradually shifted to higher wavenumber with increasing Ga content. Field emission scanning electron microscopy and transmission electron microscopy images showed irregular as well as hexagonal plate like morphologies in the size range of 15 to 25 nm. High-resolution transmission electron microscopy images showed well-defined lattice fringes and d spacing correspond to (112) plane which gradually decreases with increasing Ga content. The material compositions of synthesized CIGSe nanocrystals were very close to the desired stoichiometry, which was confirmed by energy dispersive X-ray analysis. Ultraviolet visible near infrared absorption spectra of the synthesized CIGSe nanocrystals revealed a tunable bandgap over the range of 1 to 1.7 eV by varying the Ga/(In+Ga) ratio. Therefore, the synthesized CIGSe nanocrystals can be used as an absorber layer in low cost thin film photovoltaic devices.

Pristine MoS₂, TiO₂/MoS₂ nanostructures of various morphologies including nanorods, nanowires, nanospheres, have been earlier synthesized for photocatalytic, and supercapacitor applications^[1-3]. Recently MoS₂ decorated TiO₂ nanostructures has been shown be a good material for Li-ion batteries^[4,5]. However, the challenge of fabricating/coating a uniform MoS₂ on TiO₂ mesoporous structures still remains to be widely explored. Atomic layer deposition (ALD), due to its self-limiting surface reaction mechanism allows for accurate control of thickness on nanostructures and hence a suitable technique to fabricate such hybrid nanostructures. We report the fabrication of hybrid TiO₂/ MoS₂ nanostructures utilizing polystyrene opal structure as the sacrificial layer to produce TiO₂ inverse opal nanostructure (TIO) followed by atomic layer deposition (ALD) of MoS₂ in order to fabricate hybrid TiO₂/ MoS₂ nanostructures. Water splitting properties of ALD fabricated TIO₂/MoS₂ nanostructures were also studied.

Opal structures were fabricated using polystyrene (PS) particles of different diameters. ALD TiO₂ was grown on PS opal structures using TiCl₄/H₂O vapor. ALD MoS₂ was performed on TIO nanostructures using Mo(CO)₆ and DMDS. PS opals structures showed perfect fcc arrangement. The presence of absorbing MoS₂ layer on TIO nanostructures interfered with stopband reflectance properties of TIO nanostructures. Annealing MoS₂ coated TIO however generated unique fiber like TiO₂/MoS₂ nanostructures. The crystallization of MoS₂ layer revealed unique light absorbing properties in a broad visible light range (400 – 700nm) as a function of particle size used for fabricating TIO nanostructures. Water splitting properties were measured using three-electrode cell configuration in 1M Na₂SO₄ using platinum wire as the counter electrode and Ag/AgCl as the reference electrode.

Growing global energy consumption trends and its impact on environmental sustainability urges the need for clean and renewable energy sources. Energy harvested directly from sunlight offers an alternative. Progress in photovoltaics has lowered costs and increased the efficiency attained by commercial and lab scale modules. As solar energy conversion is intrinsically periodic, energy storage is required to overcome peak demand and production offsets. Storage of electrical energy is still a concern and focus of intense research. Alternatively, energy storage in the chemical domain offers a potential economy of scale leveraging on current technology and business-models for the storage and commercialization of fuels. In this scenario much attention has been devoted to enhanced photo electrochemical hydrogen production by combining different materials in nanostructures tailored for increased photochemical conversion.
This work is reports on the fabrication and characterization of working electrodes for Photo-Electrochemical (PEC) water splitting based on black silicon mesoscopic scaffolds coated with noble metals and metal oxide nanolayers. Three metal oxides have been studied (TiO₂, CeO₂ and WO₃) to act as a photocatalysts, with optical absorption being enhanced by the black silicon microstructuring and the plasmonic effect from the noble metal-dielectric interface.
Dry current-voltage (I-V) curve sample characterization presenting either ohmic or rectifying behavior enables one to understand the interplay between semiconductor substrate doping type, selection of metal and metal oxide, individual thickness of different materials, structure design and post-fabrication thermal treatment. The study also presents quantum efficiency measurements in order to assist the optimization process towards the selection of the most efficient structure for PEC

Bringing 1 kg of material to Earth’s orbit costs approximately 20000 USD. This means that the solar energy harvested by space crafts and space bases is not for free. A rule of thumb for space-dedicated devices is thus “the lighter in weight, the better”. From this idea, we decided to test whether organic-based light weight electronics and photovoltaics are potentially viable for outer space.
The OSCAR¹ experiment (Optical Sensors based on CARbon-materials) aims to explore the use of novel generation organic-based solar cells for such extreme applications. This will be achieved through the in-situ investigation of devices’ performance during a stratospheric balloon flight. Alongside a variety of organic solar cells technologies, perovskite photovoltaic devices will also be studied.
The project fits in the framework of the student initiative REXUS/BEXUS, a collaboration between the Swedish National Space Board (SNSB) and the German Aerospace Center (DLR), also supported by the European Space Agency (ESA).
Ex-situ characterization will also be performed, in order to decouple the effect of the extreme stress conditions (low temperature, high radiation levels, low pressure...) on organic and hybrid perovskite layers. This will consist, among other approaches, of a study of their morphology and electrical properties at the nanoscale.
To assess such material characteristics, we can combine the effect of electron microscopy with Scanning Probe Microscopy. The latter allows to gain insights not only on the surface morphology, but also on local conductivity and surface potential. Examples of how this technique has been used in the determination of organic solar cells are vastly available in the literature^2,3, and its application to perovskite films has also been reported in a few recent articles^4,5.
Our presentation covers an introduction to the project, the first results after the flight, and an outlook to the next challenges for organic-based electronics towards aerospace applications.

The authors would like to acknowledge IMEC-Leuven for the preparation of the Perovskite Modules, the SNSB, DLR and ESA for their sponsorship and support with the REXUS/BEXUS campaign, and UHasselt-Dienst Onderwijs for the financial support of the OSCAR-project.
1. REXUS/BEXUS-OSCAR. at
2. Cardinaletti, I. et al. Toward bulk heterojunction polymer solar cells with thermally stable active layer morphology. J. Photonics Energy 4, 040997 (2014).
3. Shao, G., Rayermann, G. E., Smith, E. M. & Ginger, D. S. Morphology-dependent trap formation in bulk heterojunction photodiodes. J. Phys. Chem. B 117, 4654–4660 (2013).
4. Conings, B. et al. Intrinsic Thermal Instability of Methylammonium Lead Trihalide Perovskite. Adv. Energy Mater. 5, 1500477 (2015).
5. Zhao, Z., Chen, X., Wu, H. & Cao, G. Probing the Photovoltage and Photocurrent in Perovskite Solar Cells with Nanoscale Resolution. (2016). doi:10.1002/adfm.201504451

An intense effort is boosting the development of photoelectrochemical (PEC) H₂ generation toward fulfill the increasing demand for clean energy. TiO₂ and ZnO demonstrated to be promising photocatalysts, yet require UV-light activation due to their wide band gap (3.2 eV). [A. Fujishima, Nature, 1972, 238, 37-38.] To enhance their light absorption, quantum dots (QDs) have been developed as light absorbers to sensitize the metal oxides. However, a major challenge is the high charge recombination and limited photostability due to their surface sensitivity. An elegant solution to address this challenge consists in using core-shell QDs. To facilitate the charge separation, compositions and electronic band structures were suitably tailored [Rajesh Adhikari, Lei Jin (co-first author), Nano Energy, 2016, minor revision]. Despite the remarkable results obtained by QDs optically active in the visible range, much solar energy is not converted. We developed a hybrid PEC photoanode [Lei Jin et al, Adv. Sci., 2016, 3, 1500345.], which is composed of a TiO₂ mesoporous frame, functionalized by colloidal near infrared (NIR) core-shell QDs via electrophoretic deposition (EPD) [Lei Jin et al, J. Mater. Chem. A, 2015, 3, 847], and followed by an in situ CdS capping layer, acting as passivating layer and optically active component for light adsorption. The highest saturated photocurrent density value (~ 11 mA/cm²) was obtained in a stand-alone PEC system using NIR QDs as sensitizer and a dye-sensitized solar cell as external bias under one sun illumination. The PEC system showed a stable saturated current density under high intensity illumination (800 mW/cm²), demonstrating its high potential to be combined with a solar concentrator for H₂ production. Further development of PEC by using QDs can focus on the optimization of charge transfer by suitable engineering of the composition/thickness of the external shell and of the surface capping agents [Lei Jin et al, Advanced Functional Material, 2016, under submission].