Excitonic solar cells including dye-sensitized solar cells, quantum dot-sensitized solar cells, bulk heterojunction organic photovoltaics, are built upon nanostructures of various functional materials. Nanostructures are essential for the high power conversion efficiency, for example, in dye-sensitized solar cells and quantum dot-sensitized solar cells, mesoporous photoanodes made of nanoparticles offer large specific surface area for loading a large amount of dyes or quantum dots so as to capture a sufficient fraction of photons. However, the large surface area of such nanostructures also provide easy pathways for charge recombination, and surface defects and connections between adjacent nanoparticles may retard effective charge injection and charge transport, leading to a loss of power conversion efficiency. Surface facets and chemistry may also affect the conformal coating and adhesion of dye molecules and polymer layers. In this presentation, I will present and discuss our recent work on the design and control of (1) nanostructures and surface chemistry of photoanodes for quantum dot - sensitized solar cells, (2) nonstoichiometric composition, doping, and allignment of quantum dots in quantum dot-sensitized solar cells, and (3) the incorporation of plasmonic nanocrystals. Our research has demonstrated that the power conversion efficiency can be significantly enhanced with excellent device stability when both nanostructures and interface chemistry are properly enginnered.

A high efficiency device requires that the useful process, such as charge extraction in the case of a solar cell, be faster than the loss of energy as heat, which occurs as electrons release their energy in the form of mechanical vibration quanta (‘phonons&’). While electron-phonon coupling is well understood in bulk crystalline semiconductors, where electron and phonon density of states have been measured and calculated and coupling rates measured, electron-phonon coupling in nanomaterials is not understood.
Here, we perform inelastic neutron scattering (INS) and density-functional-theory calculations to study the electron-phonon interactions in nanocrystal-solids. We show that mechanical weakness of the nanocrystal surface enables the strong coupling of low-energy surface-phonon modes to electronic transitions. Using thermal admittance spectroscopy to quantify the electronic transition rates in nanocrystal-based diodes, we provide evidence that these low-energy phonon modes can drive large energy transitions (~1 eV) at unusually high rates, due to large entropy change in these transitions.
Our findings explain the role of surface defects and passivation on nanocrystal-based device performance and guide the bottom-up engineering of next generation semiconductors, where optical, electronic, and phononic properties can be tailored.

The efficiency and stability of emission from semiconductor nanocrystal quantum dots (QDs) is negatively affected by ‘blinking&’ on the single-nanocrystal level, i.e. random alternation of bright and dark periods. The timescales of these fluctuations can be as long as many seconds, orders of magnitude longer than typical lifetimes of exciton states in QDs. In this work we investigate photoluminescence from QDs ‘delayed&’ over microseconds to milliseconds, thus connecting the timescales of exciton decay to those of blinking. With single-QD as well as ensemble measurements, we examine the dynamics and spectral characteristics of this delayed emission, and quantify radiative and nonradiative decay constants by systematically varying the photonic environment of the QDs. Our results indicate that delayed emission and blinking have the same physical origin, namely charge carrier separation, storage for microseconds to seconds, and eventual recovery of the exciton state. A new microscopic model is developed that connects our new results on delayed emission to exciton recombination and blinking. While existing models for blinking have in common that they seek an explanation for uninterrupted optical cycling during long ON ‘periods&’ of up to many seconds, our results on delayed emission show that such long periods of uninterrupted cycling do not occur. The new blinking model, which assumes a significant (of order 10%) probability of charge carrier separation at all times during optical cycling, reproduces the photoluminescence decay dynamics of QDs, including the delayed component, as well as the blinking characteristics.

CdSe/CdTe heterojunction nanorods (HNRs) with type-II staggered band offset can allow directional and efficient separation of photogenerated charge carriers. However, CdTe nanocrystals can be easily oxidized with just post-synthesis processing in air, which can then lead to charge traps that negate the benefits of the type-II band offset. Here, we introduce a simple ligand exchange method immediately post-synthesis to replace the native ligands on CdSe/CdTe or CdSe/CdSe_xCdTe_1-x HNRs with 1-octanethiol, resulting in improved photoluminescence and good stability in air. Transient absorption measurements reveal that electron transfer from CdTe to CdSe remains fast and efficient (~400 fs) despite the hole trapping nature of thiol ligands for CdSe. Absorption bleach arising from CdTe-to-CdSe electron transfer can be observed out to 1 mu;s even after days of storage in air, an order of magnitude longer than HNRs with native ligands kept rigorously air-free and three orders of magnitude longer than HNRs with native ligands exposed to air. This improved environmental stability that preserves efficient charge separation and minimizes trapping translates to enhanced photocurrent generation, especially with respect to contribution from photoexcitation of CdTe transitions. When used as light harvesters in sensitized solar cells, CdSe/CdSe_0.4CdTe_0.6 HNRs capped with thiol ligands show an enhanced ability to extract photogenerated charges, resulting in a 3.02% power conversion efficiency.

Thin films comprised of nanocrystals continue to attract interest as functional layers in photovoltaic devices. The focus of this paper will be on films comprised of heavily doped ZnO nanocrystals for use as transparent conducting oxide (TCO) layer. Majority carrier transport, electrons in the case of ZnO, is imperative to performance of TCO layers; yet it remains relatively poorly understood in films comprised of nanocrystals. The interpretation of results from routine electrical characterization techniques, such as Hall effect measurements, has historically been difficult. In this experimental work, we present results from assemblies of ZnO nanocrystals embedded in Al₂O₃. The insulating matrix is required to remove acceptor defects from the particle surfaces and render them conductive. It is demonstrated that the key parameter controlling the electron transport mechanism is the contact resistance between nanocrystals in the assembly, which can be controlled by systematically adjusting the contact area between particles using atomic layer deposition. The magnitude of the contact resistance between particles compared to the quantum resistance determines the transport mechanism. If contact resistance is greater than the quantum resistance, transport is in the dielectric regime wherein a variable range hopping mechanism is observed. If contact resistance is less than the quantum resistance, disordered metallic conduction is observed with relatively high electron mobility of 5 cm² V^-1 s^-1 if nanocrystals are well-connected. In the dielectric regime, an anomalous hall effect is observed at moderately low temperature (100 ~ 250 K). This anomalous Hall effect has several unexpected features, which to our knowledge have not been reported before. Alternatively, if the nanocrystals are well-connected and the electron transport mechanism is metallic, the Hall coefficient displays explicable features of a disordered, heavily doped, n-type semiconductor. In the metallic regime, a negative hall coefficient is observed that is independent of temperature and corresponds to a reasonable value for the carrier concentration.

Chalcopyrite (Cu(In,Ga)Se₂) solar cells have achieved the highest stable efficiency amongst polycrystalline thin-film devices with a latest record of 21.7%. For increased competitiveness on the market and addressing energy production on the TW scale, a reduced consumption of Indium - their element with the highest supply risk - is desired. A reduction of absorber layer thickness to below 500 nm with efficiencies above 10% has proven possible, but still leaves significant room for increasing short circuit current density. For light absorption enhancement in the ultra-thin Chalcopyrite layers plasmonic and photonic nanostructures are highly promising due to their light concentrating and scattering behavior.
Plasmonic nanoparticles made from e.g. Ag or Au can be fabricated with a well-defined size and density distribution leading to a controllable spectral response. Integrating these nanoparticle distributions into thin-film solar cells allows the absorption to be significantly enhanced based on 1) high near-field enhancement around the nanoparticles and 2) scattering towards the absorber layer and leading to light trapping inside. These mechanisms however require a short distance of the optically active nanostructures from the absorber layer which has turned out to be challenging for Chalcopyrite solar cells. High temperature fabrication processes and the formation of e.g. Ag-Chalcopyrite compounds makes the direct integration of metallic nanoparticles into Cu(In,Ga)Se₂ absorbers impossible. Core-shell nanoparticles are a promising option for chemical and also electrical passivation; we found that a refractive index of the shell higher than the one of the surrounding medium is crucial for effective use of enhanced near-field and high scattering from the metal core.
Photonic nanostructures, made from insulating dielectric material only, are found to have high potential for replacing their metallic counterparts due to absence of parasitic absorption. They show comparable scattering cross sections of more than five times their geometrical cross section and can also be brought into direct contact with the Chalcopyrite absorber. We have proven an increase in short circuit current density of more than 2 mA/cm² in ultra-thin Cu(In,Ga)Se₂ solar cells by integrating SiO₂ nanopsheres and -patterns at the front or rear side of the structure. Going beyond the pure optical properties of nanostructures integrated into ultra-thin-film devices, electrical properties also need to be considered and optimized effectively. Point contact structures from e.g. Al₂O₃ at the absorber&’s back or also front interface are investigated with respect to reduced recombination leading to a combined electrical and optical benefit for the solar cell performance.

Hybrid solar cells are in sustained development among the photovoltaic (PV) technologies. Molybdenum diselenide (MoSe₂) is seen as a promising alternative semiconductor for hybrid PV due to its absorption in the near-infrared spectrum, its good carrier transport and its ability to form 2D structures. Scanning Spreading Resistance Microscopy (SSRM) is used for the electrical characterization of crystalline Nb-doped MoSe₂. A n-type nature of doping is demonstrated, with local accumulation of dopants in the crystal when substituting Mo atoms by Nb in a 0.01 % stoichiometric ratio. The study of the local electrical resistance of the Nb-doped MoSe₂ surface also shows more conductive spots with a nanometric size, most likely due to a local accumulation of dopants and therefore carrier density within the crystal. A mechanical exfoliation of the single crystals led to micrometric and nanometric fragments suitable for hybrid bulk heterojunction (HBH) fabrication. An intrinsic photo-electrical effect has been observed by PhotoConductive Atomic Force Microscopy (pc-AFM) in Nb-doped MoSe₂ dispersed in a poly(methyl methacrylate) matrix. A negative photo-current has been recorded, showing a spontaneous charge separation of the photo-generated excitons in the material, along with an efficient collection of the charges. On the basis of the SSRM measurements, a band structure for the probe-sample system is proposed.
Finally, a photoactive effect has been shown by blending the Nb-doped MoSe₂ “flakes” with a typical p-type semiconducting polymer: the poly(3-hexylthiophene) (P3HT). The photo-electrical properties of those Nb-doped MoSe₂:P3HT systems are studied at the nanoscale by pc-AFM. This study hence demonstrates the capability of MoSe₂ to be considered as a promising acceptor material with a very good stability in future hybrid photovoltaic devices. Having described this system, one could expect to get HBH with a more controlled nanomorphology by monitoring 2D MoSe₂ nanostructures. As the bandgap of the MoSe₂ crystal widens when the number of unit lamellae decreases, operating with 2D nanostructrures should improve the charge separation at the Nb-doped MoSe₂:P3HT interfaces, since a lowering of the valence band of the 2D MoSe₂ relative to the bulk can be expected.

In the near future, building integrated photovoltaics (PV) could revolutionize urban architecture by allowing one to reach the ambitious goal of net zero energy consumption buildings. Luminescent solar concentrators (LSCs) can play an important role in this transition as they provide a means to realize semitransparent PV windows that are able to convert the energy passive facades of urban buildings into distributed energy generation units^[1]. Wide use of LSCs has, however, so far been hindered by the lack of suitable emitters especially with regard to the limited coverage of the solar spectrum and significant optical losses associated with re-absorption of guided luminescence that prevent the realization of large area devices and lead to strong coloring of devices, limiting their usage in architecture. Colloidal quantum dots (QDs) can help overcome these limitations thanks to material engineering strategies for increasing Stokes shift, including heterostructuring^[2] and doping^[3]. Despite their great potential, LSCs fabricated using CdSe/CdS heterostructures or doped QDs still suffer from incomplete coverage of the solar spectrum due to a large energy gap of the absorber material. Furthermore, core/shell structures contain hazardous heavy metal ions and thus require expensive disposal/recycling protocols.
An interesting class of nontoxic materials that provide a large, hundreds of meV Stokes shift without the need for heterostructuring are QDs of ternary I-III-VI₂ semiconductors such as CuInS₂ (CIS), CuInSe₂ (CISe), and their alloys (CuInSe_xS_2minus;x or CISeS). An attractive feature of these QDs is that they do not contain heavy metals and can be fabricated in large quantities via high-throughput, non-injection techniques using inexpensive precursors. Furthermore, their large absorption cross-sections and a spectrally tunable, near-IR absorption onset are well suited for harvesting solar radiation^[4]. Here we use CISeS QDs coated with ZnS to realize the first large-area IR QD-LSCs with reduced re-absorption losses and extended coverage of the solar spectrum. We focus on CISeS QDs with emission at 960 nm (1.3 eV), which is near optimal for LSCs coupled to Si PVs and also allows for the realization of colourless QD-LSCs. By incorporating QDs into a photopolymerized PLMA matrix, we obtain freestanding, colorless polymer slabs of excellent optical quality that introduce no distortion to transmitted colors and are thus well suited for integration into buildings in the form of PV windows. Using this approach, we achieve an optical power conversion efficiency of 3.2% of the incident solar radiation, which is the highest value ever reported for large area LSCs with any types of chromophores.[5]
References
[1]M. G. Debije, et al., Adv En Mater 2012, 2, 12.
[2]F. Meinardi, et al., Nature Photonics 2014, 8, 392.
[3]C. S. Erickson, et al., Acs Nano 2014, 8, 3461.
[4]D. Aldakov, et al., J Mat Chem C 2013, 1, 3756.
[5] F. Meinardi, et al., Nature Nano 2015, under review

Luminescent solar concentrators doped with CdSe/CdS quantum dots provide a potentially low-cost and high-performance alternative to costly high band-gap III-V semiconductor materials to serve as a top junction in multi-junction photovoltaic devices for efficient utilization of blue photons. In this study, a photonic mirror was coupled with such a luminescent waveguide to form an optical cavity where emitted luminescence was trapped omnidirectionally. By mitigating escape cone and scattering losses, 82% of luminesced photons travel the length of the waveguide. Using quantum dots with a 60% luminescent quantum yield, this allows a concentration ratio of 30.3 for blue photons in a waveguide with a geometric gain of 61. Further, we study the photon transport inside the luminescent waveguide, showing unimpeded photon collection across the entire length of the waveguide.

Although organic dyes may be attractive alternatives to metal-containing dyes for dye-sensitized solar cells (DSCs) they are often less efficient and remain relatively less explored. We have examined two essential processes for the success of an organic DSC, electron injection from the sensitizer into TiO₂ and minimization of non-favorable charge recombination. The influences of surface-attachment functionality on electron-injection yield (Phi;_inj), inertness of dye-TiO₂ linkages, and photoelectrochemical performance of DSCs are reported. In addition, the electron lifetime (tau;_e) in a phosphonate-attached chalcogenorhodamine dye-DSC is compared to the tau;_e in a high-performing cis-bis(4,4'-dicarboxy-2,2'bipyridine)dithiocyanatoruthenium(II)-DSC (N3-DSC). The chalcogenorhodamine dyes were constructed around 3,6-bis(dimethylamino)selenoxanthylium and varied in the 9-substituent: 5-carboxythien-2-yl (1-Se) and 5-phosphonothien-2-yl (3-Se). Transient absorption spectroscopy (TA) revealed the Phi;_inj via carboxylates was 2-fold greater than via phosphonates [1]. The favorable Phi;_inj of 1-Se was offset by rapid desorption from TiO₂ into acidified CH₃CN. Incident photon-to-current efficiencies (IPCEs) of DSCs ranged from 66% to 91% and were initially higher for 1-Se. The performance of 1-Se degraded rapidly, whereas the performance of 3-Se remained stable [2]. Thus, utilizing phosphonic acids increased the inertness of chalcogenorhodamine-TiO₂ interfaces without greatly impacting the photoelectrochemical performance. Despite the high IPCEs achieved with these DSCs, photocurrent-photovoltage (J-V) experiments reveal overall power conversion efficiencies (eta;) of <1% for each chalcogenorhodamine-sensitized solar cell, compared to 5.9% for N3-based DSCs. The low eta; from J-V experiments under simulated solar illumination stands in sharp contrast to the high IPCEs under low-intensity monochromatic illumination and the high relative Phi;_inj from TA experiments. These differences led to the design of ongoing transient photovoltage (TPV) experiments, which aim to characterize charge recombination and dye regeneration at TiO₂-chalcogenorhodamine-electolyte interfaces, and will be emphasized in this talk. We compared tau;_e for 3-Se-DSCs and N3-DSCs with and without supporting electrolyte. Our results indicate the presence of an alternative non-favorable electron-recombination pathway in the 3-Se-DSCs compared to the N3-DSCs.
[1] Mulhern, K. R.*; Detty, M. R.; Watson, D. F., Effects of surface-anchoring mode and aggregation state on electron injection from chalcogenorhodamine dyes to titanium dioxide. Journal of Photochemistry and Photobiology A: Chemistry 2013,264, 18-25.
[2] Mulhern, K. R.*^Dagger;; Orchard, A.^Dagger;; Watson, D. F.; Detty, M. R., Influence of Surface-Attachment Functionality on the Aggregation, Persistence, and Electron-Transfer Reactivity of Chalcogenorhodamine Dyes on TiO₂. Langmuir 2012,28, 7071-7082.
*Now Kacie R. Liwosz ^Dagger;Indicates equal contribution.

Nanophotonics provides superb opportunities for tailoring the flow of light. This way, many novel physical phenomena can be enabled, as well as many important functionalities for novel energy applications (e.g. energy conversion, lighting). In order to make these phenomena useful for large systems, large-area nano-fabrication techniques have to be successfully implemented. In this talk, I will present some of our recent theoretical and experimental progress in exploring these opportunities.

Nanostrutures will modify the spectral and angular emissvity of solar materials. Consequently, the photovolatic voltage will be enhanced or suppressed. We will discuss the way to model such changes and describe a few examples. In particular, based on voltage enhancement, we will show how to go beyond one-sun Shockley-Quiesser limit using a single semiconductor material in a tandem configuration.

In crystalline silicon-based solar cells, a substantial part of the energy losses is related to the carriers&’ thermalization in the UV-blue range. This issue can be partially circumvented by a down-shifting process inside a luminescent rare-earth (RE) doped thin layer. However, due to the low absorption cross-section of the RE ions, the efficiency of the converting layer needs to be increased. In this study, we introduce a new concept which combines a rare-earth doped thin layer (Y₂O₃: Eu³⁺) with a 2D photonic crystal (SiN_x), in order to take control of the frequency conversion process. A Y₂O₃:Eu³⁺ 100 nm thick layer was first deposited by Pulsed Laser Deposition. Above this converter layer, a SiN_x thin film was then deposited and periodically patterned using laser interference lithography and reactive ion etching, to ensure an exaltation of the guided electromagnetic field inside the luminescent layer. This nanophotonic structure was designed and optimized using RCWA simulation in order to enhance separately absorption and emission of the rare-earth layer or both of them at the same time.
In this communication, we will discuss on the design, fabrication and measurements performed on these new architecture enabling the control of light-matter interaction. We will show that a substantial enhancement of the conversion yield was achieved, which could pave the way to the development of more efficient photovoltaic devices.

One dimensional photonic crystals (Bragg stacks) can be used to control the directionality of fluorescence emission. In this paper we report on how this property can be used to improve the operation of fluorescent solar concentrators. Fluorescent solar concentrators aim to concentrate light onto solar cells by trapping fluorescence through total internal reflection. In a good fluorescent solar concentrator the major obstacle to efficient photon transport is the loss of photons through the top and bottom escape cones. One possible method to decrease this loss and improve the efficiency of these devices is to fabricate one-dimensional photonic crystals doped with fluorescent molecules. If these photonic crystals are tuned to exhibit a photonic band gap in the escape cone directions and at the emission frequencies of the fluorescent molecules, a suppression of the escape cone emission and an enhancement of the edge emission is expected. In this paper, we detail the fabrication of the first one dimensional integrated photonic solar concentrator and show the suppression of the escape cone emission. This suppression and also enhancements in the edge emission will also be shown to correspond well with the calculated band structure of the device. The control of emission inside fluorescent solar concentrators opens up a number of additional possibilities for efficiency enhancements that will also be discussed.

In recent years the interest in solar energy conversion based on nanoparticles has surged due to their promising material and optical properties. One of the promising optical properties is that nanostructures are known to exhibit absorption cross sections that can be considerably larger than their own geometrical cross section, thereby effectively collecting current from a larger area. This phenomenon appears to be very similar to macroscopic concentrator photovoltaics, where the intensity of light incident on a solar cell is increased by means of a lens. As a result the carrier concentration increases, which results in higher voltages and larger conversion efficiencies. Due to the large optical absorption cross sections of nanostructures, this enhanced efficiency might thus be available without the use of external optics.
In this work we discuss how macroscopic and nanophotonic concentration are actually quite different, and that care needs to be taken when designing a nanophotonic solar cell. We show that large optical cross sections for nanoparticles help to reduce bulk recombination losses, but that this by itself never leads to efficiencies above the one sun Shockley-Queisser limit. To exceed this limit angular anisotropy in the absorption of the nanoparticle is required, but we show that in general this is actually very hard to achieve. As a numerical demonstration we discuss the voltage and current characteristics of single finite GaAs nanowire solar cells, a very promising building block of highly efficient photovoltaic devices. The angularly dependent absorption cross section of such a finite wire is complicated and involves Mie modes as well as the excitation of guided modes, but we derive simple yet accurate analytical approximations. We show that indeed defect recombination can be reduced with respect to bulk values, but that the angular anisotropy is low and therefore efficiencies do not exceed regular Shockley-Queisser values. Finally, we propose several approaches to actually achieve optical concentration with nanomaterials, for instance by utilizing array effects in large-scale systems.

Since the report on the long-term durable solid-state perovskite solar cell employing MAPbI₃ (MA = CH₃NH₃) perovskite in 2012, following two important pioneer works on perovskite-sensitized solar cells in reported 2009 and 2011, there has been a surge of interest in perovskite solar cell. We report here highly reproducible hysteresis-free perovskite solar cell with mean power conversion efficiency (PCE) exceeding 19% and best PCE exceeding 20% via well-defined acid-base adduct approach. The device includes basically a MAPbI₃ layer with small fraction of mesoprous TiO₂ layer and a spiro-MeOTAD hole transport layer. The Lewis base adducts of PbI₂ was found to yield high quality pinhole free MAPbI₃ film at ambient condition, leading to high photocurrent and voltage. Compared to conventional methods, Lewis base adduct approach showed higher charge extraction ability as evidenced by photo-CELIV measurement. Furthermore, molecular-level interfacial nanoengineering improved fill factor significantly. Hysteresis-free perovskite solar cell in a normal structure was realized by structural and electronic modifications.

Confocal photoluminescence (PL) microscopy and fluorescence lifetime imaging microscopy (FLIM) reveal significant local heterogeneity in non-radiative recombination rates in perovskite thin films, and point to the role of surface and grain boundary recombination in these films. We use PL spectroscopy, including FLIM, to study the effects of different passivating chemical treatments on carrier recombination in CH₃NH₃PbI₃ films. We both identify compounds with the potential to further reduce non-radiative recombination in perovskite films, and quantify the effects on local trap state density using kinetic models. Using picosecond PL spectroscopy, we further elucidate the various kinetic pathways that carriers traverse before repopulation of the ground state. Our study highlights the nature of non-radiative loss at the surface of perovskite films and points the way for further improvement in these materials for light emitting and solar cell applications.

We report here an effective methodology for MAPbI₃ (MA = CH₃NH₃) perovskite solar cell with power conversion efficiency (PCE) exceeding 20% via molecular-level interfacial nanoengineering. The basic cell configuration is FTO/thin film TiO₂/mesoporous TiO₂/MAPbI₃/spiro-MeOTAD/Ag, where 40-50 nm-sized TiO₂ particles are used for the mesoporous TiO₂ layer. MAPbI₃ layer are prepared via adduct approach, where Lewis acid PbI₂ is reacted with Lewis base polar aprotic solvent such as N,N-dimethylsulfoxie (DMSO) to form MAI-PbI₂-DMSO adduct film. The adduct approach yields average PCE of 18-19%. However, the open-circuit voltage is lower than 1.1 V despite the theoretical output voltage expecting 1.3 V. Moreover, fill factor is hard to approach 80%. Those are related to non-radiative recombination in bulk perovskite and/or interfacial charge recombination. We have introduced thin interfacial layer on the perovskite layer contacting either p-type or n-type layer. Molecular-level engineering by varying interfacial gap between perovskite layer and n- or p-type layer results in open-circuit voltage as high as 1.13 V and fill factor of 78%, which leads to a PCE of 20.7%. The molecular orientation and opto-electronic role of interfacial layer is investigated using AFM and spectroscopic tools, along with impedance at interfaces.

Hybrid organic inorganic lead halide perovskites emerged as attractive solar cell materials. Two-step sequential deposition is an important method to fabricate perovskite film. It usually involves depositing a uniform condense PbI₂ film, and subsequent conversion of the PbI₂ into perovskite. A homogenous perovskite upper layer on top of the TiO₂ is important to prevent the charge recombination between hole transporter and electron transporter, thus to get a high power conversion efficiency. However, the condense PbI₂ atop the TiO₂ inhibits its conversion into perovskite. In this presentation, we will show a new strategy to assist the conversion of the PbI₂. With the new method, a homogeneous condense perovskite upper layer on mesoporous TiO₂ with well-infiltrated perovskite can be produced. High power conversion efficiency up to 20% can be achieved.

The organic-inorganic metal halide perovskite CH₃NH₃PbI₃ has emerged as a new material for photovoltaic cells with power conversion efficiencies near 20%. CH₃NH₃PbI₃ and the related CH₃NH₃PbI_3minus;xCl_x exhibit long charge-carrier diffusion lengths [1], high charge-carrier mobilities and unusually low bi-molecular charge recombination rates [2]. Yet relatively little is known about how nano-morphology and structural phase transitions affect charge transport in photovoltaic cells.
We show that at room temperature, CH₃NH₃PbI_3minus;xCl_x films exhibit extraordinarily low energetic disorder, with an emission spectrum that is predominantly homogenously broadened by ~100meV, through coupling of charge-carriers to phonons [3]. However, the charge-carrier dynamics are likely to be affected by structural phase transitions that occur at ~160K and ~315K. An understanding of changes in optoelectronic properties occurring across tetragonal-to-cubic transition at 315K is particularly important since the actual working temperature of a high-efficiency perovskite solar cell in field conditions can exceed 70°C.
We have investigated the transient photoluminescence and photoconductivity in CH₃NH₃PbI₃ and CH₃NH₃PbI_3minus;xCl_x thin films from 8 K to 370 K across three structural phases. Analysis of the charge-carrier recombination dynamics reveals a variety of starkly differing recombination mechanisms. Evidence of charge-carrier localization is observed only at low temperature. Below the orthorhombic-to-tetragonal phase transition at 160K complex PL decay channels are observed [4] that may derive from nanoscopic inclusions of domains adopting the high-temperature phase. However, high mobility and diffusion length are maintained at high temperature beyond the tetragonal-to-cubic phase transition at ~310K. These results demonstrate that there are no fundamental obstacles to the operation of cells based on CH₃NH₃PbI₃ under typical field conditions.
[1] Stranks, Eperon, Grancini, Menelaou, Alcocer, Leijtens, Herz, Petrozza, Snaith, Science 342 (2013) 341.
[2] Wehrenfennig, Eperon, Johnston, Snaith, Herz, Adv. Mater. 26 (2014), 1584.
[3] Wehrenfennig, Liu, Snaith, Johnston, Herz, J. Phys. Chem. Lett. 5 (2014), 1300.
[4] Wehrenfennig, Liu, Snaith, Johnston, Herz, APL Materials 2 (2014), 081513.

A great deal of recent work has examined the optical properties of nanowire solar cells. It is now well known that properly designed nanowires concentrate incoming light, achieving high absorption with minimal material volume. Volume reduction can relax the requirements on material quality, as the distance electrical carriers must travel to reach the contacts is minimized. However, little attention has been paid to the thermal consequences of the nanowire design. In particular, the thermal conductivity of nanowires is known to be several orders of magnitude lower than for bulk. Moreover, the concentration of light absorption inside the nanowire implies more concentrated heat generation, as a portion of absorbed energy is converted to heat inside the nanowire. It is thus of crucial importance to consider how the operating temperature of a nanowire solar cell compares to that of a traditional, planar one.
In this work, we carry out a detailed simulation of the temperature rise of a GaAs nanowire solar cell under solar illumination. We compare the temperature to that of an equivalent height planar structure. Our model takes into account coupled thermal and optical effects. We simulate the detailed spatial absorption profile in the nanowires using the finite-difference time domain method. To obtain the spatial heat generation profile, we assume that some fraction of the absorbed energy (equal to one minus the cell efficiency) is converted to heat. We then calculate the IR emissivity of the structure using the rigorous coupled-wave equations. The results of the electromagnetic simulations are then incorporated into a full 3D simulation of the heat equation including both convective and radiative boundary conditions.
We show that in fact, the nanowire cell heats up slightly less than the planar structure. Physically, this is due to the higher emissivity of the nanowire structure at thermal wavelengths, which enhances the radiative cooling rate. We find that the thermal conductivity of the nanowires has only minimal effect on the operating temperature. In steady state, the temperature is determined by a power balance equation between input and output flux. Despite the concentrated heat source within the nanowire, the temperature variation over the unit cell of the nanowire is extremely small compared to the overall temperature rise. As a result, the temperature is nearly constant over the whole structure, and so to first approximation, the power balance equation is independent of thermal conductivity.
Further, we find that a much higher cooling rate can be obtained by infiltrating the nanowires with BCB (benzocyclobutene), a polymer commonly used for electrical insulation and structural support. For a heat input of 900 W/m^2, this results in a reduction of the operating temperature by approximately 7K relative to a structure without BCB.
We conclude that overall, from a thermal standpoint, GaAs nanowire solar cells have similar performance to their planar counterparts, and that BCB-infiltrated nanowire cells significantly outperform bare structures.

Photoelectrochemical (PEC) devices capable of converting sunlight into solar fuels hold out the promise of a renewable energy economic paradigm. However, in order to systematically engineer PEC devices, several key scientific questions need to be addressed: in particular, the carrier transport process from oxide or semiconductor materials, where electron-hole pairs are generated by incident sunlight, to the liquid environment. In this work a semi-classical method for understanding charge transfer processes in PEC devices is presented, building upon the energy band diagram approach common in the solid-state device community. Our approach is based upon integrating band theory theory with Gerischer&’s picture of heterogeneous electron transfer, coupled through Boltzmann statistics and variable dielectric electrostatics. We show that this approach is cable of integrating key concepts, such as solid-liquid Fermi level alignment and band-edge pinning/unpinning. In general the method elucidates an approach towards the systematic design of PEC devices.

Luminescent Solar Concentrators (LSCs) offer a means to reduce the cost of solar energy by concentrating incoming light. A LSC consists of a flat, transparent host matrix that is doped with fluorophores and has solar cells attached to its sides. Incident light is absorbed by the fluorophores and concentrated towards the sides of the host matrix via total internal reflection. LSCs are capable of absorbing diffuse sunlight which makes them very suitable for urban environments and building-integrated photovoltaics. Most commonly organic dye molecules are used as fluorophores; however, the absorption of dye molecules is spectrally quite narrow and therefore only a small part of the AM1.5 will be harvested. Quantum dots absorb over a wider spectrum but often suffer from lower quantum yields than organic dye molecules.
We have shown theoretically that linking quantum dots to dye molecules can enhance the performance of a LSC. The linkage induces Förster resonance energy transfer (FRET) from the quantum dot to the dye molecule. The quantum dot serves as an absorption center due to its spectrally wide absorption. Due to the proximity, the energy transfer will be efficient despite a potentially lower quantum yield of the quantum dot. The dye molecule will then emit the photon with a double Stokes shift which will reduce the probability of re-absorption by the quantum dot. This system exploits the advantages of both, the quantum dots and the organic dye molecules: the spectrally wide absorption of the quantum dots and the high quantum yield of dye molecules.
At the conference we will present our latest experimental results. CdSe/ZnS quantum dots functionalized with amine-derivatized PEG are used as the donor fluorophore. An Alexa Fluor dye molecule with a NHS ester reactive group serves as acceptor. The absorption profile of the dye is overlapping with the emission profile of the quantum dot to induce efficient FRET. The quantum dots and dye molecules are conjugated in a buffer solution and unconjugated dye molecules are removed using a centrifuge. The FRET efficiency is determined by comparing the decay times of quantum dots by themselves with conjugated samples. Quantum dot/dye conjugates in solution are then injected into a glass cell to determine their performance as LSC fluorophores. An experimental method developed by our group determines the optical efficiency of the quantum dot/dye conjugate LSC using an integrating sphere. Measurements are compared to unconjugated quantum dots and dye molecules to verify that FRET can boost the performance of LSCs.

Composite nanocables (CNCs) with thermoelectric NaCo₂O₄ cores and TiO₂ shells have successfully been prepared by electrospinning technique with a coaxial syringe needle. The CNCs were added into the TiO₂ nanocrystals to obtain a new thermoelectric-photoelectric composite photoanode, and TiO₂ shells of the CNCs achieve a good match with dyes while thermoelectric NaCo₂O₄ cores with high conductivity play a role of fast conduction channels of electrons. The different additive amount of TiO₂-NaCo₂O₄ CNCs caused different device performances. The DSSC with 10wt% CNCs in TiO₂ photoanode exhibits best photovoltaic performance with high J_sc of 21.16 mA cm^-2, V_oc of 0.754 V, FF of 68.09% and PCE of 10.86%, due to the reduced series resistance and improved electron transport. When the content of CNCs further increases to 20wt% and 40wt%, the DSSC performance decreases due to the reduced dye loading capacity of CNCs. In addition, the temperature gradient effect on photovoltaic performances of the DSSCs with 10wt% TiO₂-NaCo₂O₄ CNCs and without any CNCs were investigated. The negative temperature gradient of -5 K increases PCE of the DSSC with 10wt% CNCs from 10.86% to 12.07%, while the positive temperature gradient decreases its PCE to 8.58%, due to the built-in electric field inside thermoelectric materials according to thermoelectric Seebeck effect. The temperature gradient effect implies that a further performance improvement is possible for the DSSCs with the thermoelectric-photoelectric CNCs because a temperature gradient between two sides of solar cells is inevitably caused by irradiation differences on two sides of solar cells and heating effect of sunlight irradiation. These results demonstrate the positive effect of the new thermoelectric-photoelectric composite nanocables for high efficiency dye-sensitized solar cells. This work not only is significant for developing novel photoanode of DSSCs with high performance, but also provides a new idea for integrated utilization of different new energy technologies including photovoltaic devices and thermoelectric devices.

Internal light scattering in textured solar cells is a currently used strategy to increase the optical path length of incident photons within the absorber layer (light-trapping), which leads to increased charge carrier generation. The characterization of the light-trapping capability of surface-textured solar cells has not been an easy task, since the most commonly used methods based on electrical measurements, do not probe exclusively the optical properties of the films but are also affected by the films&’ electronic performance. However, it has been recently proposed that Raman spectroscopy is a particularly valuable tool for probing the light-trapping in the case of textured microcrystalline silicon solar cells deposited on front textured ZnO superstrates [1]. This exploits the direct correlation between intensity of the Raman spectrum from a textured film and its optical thickness. In this study, we apply a similar approach based on Raman spectroscopy to quantify light-trapping in superstrate type p-i-n amorphous silicon (a-Si) thin film solar cells fabricated on nanotextured surfaces. Our sub-wavelength scale nanotextures are quasi-periodic nano-wrinkled surfaces, with varying periodicity and size range of the features. The absolute Raman signal intensities of the amorphous Si absorber layer of the solar cells fabricated on flat and different nano-wrinkle surfaces were measured using three laser wavelengths of 455 nm, 514 nm and 785 nm, with which we are capable of probing different depths in the a-Si thin films. We successfully correlate our findings in terms of light-trapping performance of the nanotextured surfaces to the efficiency of the corresponding solar cells. Our experimental results show a significant photocurrent enhancement in the solar cells fabricated on these nano-wrinkle surfaces, consistent with an improved photon harvesting. The similarity in the variation of the measured absolute Raman signal intensities and the photocurrent values with respect to different nano-wrinkle features shows that Raman spectroscopy can indeed be used to predict the light-trapping efficacy of irregular nanotextures applied in solar cells.
[1] M. Ledinsky, E. Moulin, G. Bugnon, K. Ganzerová, A. Vetushka, F. Meillaud, A. Fejfar, C. Ballif, Appl. Phys. Lett. 105, 111106 (2014)

Silicon-based multijunction photovoltaics are desirable due to their high efficiency potential and leveraging of existing crystalline Si market dominance. III-V materials have the highest efficiency potential as top cell materials on Si, but numerous challenges- including lattice mismatch and antiphase domains- have hindered the development of high quality III-V/Si hybrid structures. Our approach to growing high-quality III-Vs on Si relies on selective area growth using nanopatterned templates. GaAs, which has a 4% lattice mismatch with Si, can be selectively nucleated in nanoscale holes in a SiO₂ layer. Below a critical feature width, defect propagation is blocked, enabling heteroepitaxial growth of high quality GaAs for Si-based photovoltaics. An additional benefit is the excellent surface passivation provided by SiO₂ on Si, compared to a typically defect-rich interface with a III-V layer. In this work, we will discuss the fabrication of Si templates patterned using substrate-conformal imprint lithography, an inexpensive and scalable technique for achieving nanoscale features. These patterned SiO₂ on Si structures are then used for growth of GaAs films. Ultimately, this platform enables the fabrication of GaInP/GaAs/Si triple junction solar cells with an efficiency potential of > 45% and costs compatible with one-sun photovoltaic installations

Solution-processable semiconductor nanocrystals as thin films are a promising low-cost alternative to vapor-deposited materials in electronic devices such as solar cells. The exchange of the synthesis-induced ligand shell of these nanocrystals for suitable semiconducting organic ligands can lead to coupled organic-inorganic nanostructures (COIN). These organic ligands are supposed to couple the inorganic nanocrystals first by stronger binding interactions and second by near-resonant alignment of energy levels.^[1] By this coupling the electronic properties such as conductivity or charge carrier mobility in nanocrystal thin films can be significantly optimized.^[2]
The goal of our work is to improve charge transport of CuInSe₂ nanocrystal thin films^[3] in regard to facilitate solar cell assembly. This should be accomplished by exchange of insulating ligands for semiconducting organic ligands. We have determined energy level positions of two thiadiazoles (2,5-dimercapto-1,3,4-thiadiazole and 5-amino-2-mercapto-1,3,4-thiadiazole) by cyclic voltammetry to confirm the required alignment of suitable energy levels between these organic ligands and the nanocrystals. In addition we performed the ligand exchange in a thin film and analyzed charge transport properties.
In our contribution we show the results of the cyclic voltammetry measurements in comparison to the energy level positions of CuInSe₂ nanocrystals to evaluate electronic coupling of this combination. Furthermore we present the effect of the ligand exchange on the nanocrystal thin film and charge transport properties.
[1] M. Scheele, W. Brütting and F. Schreiber, Phys. Chem. Chem. Phys.2015, 17, 97-111.
[2] M. Scheele, D. Hanifi, D. Zherebetskyy, S. T. Chourou, S. Axnanda, B. J. Rancatore, K. Thorkelsson, T. Xu, Z. Liu, L.-W. Wang, Y. Liu and A. P. Alivisatos, ACS Nano.2014, 8 (3), 2532-2540.
[3] J. Lauth, J. Marbach, A. Meyer, S. Dogan, C. Klinke, A. Kornowski and H. Weller, Adv. Funct. Mater.2014, 24, 1081-1088.

Solar cells made from colloidal nanocrystals have shown great potential because of their solution processability, tunable absorption spectra, and promising efficiencies. However, the most efficient nanocrystal solar cells use toxic elements such as lead, cadmium, or antimony.^(1-3) We will present colloidal AgBiS₂ nanocrystals based on nontoxic, abundant elements and their use in solar cells with efficiencies approaching 6%. To our knowledge, this is the highest efficiency of any nontoxic, abundant nanocrystal solar cell to date. Furthermore, AgBiS₂ nanocrystals can be solution processed close to room temperature, unlike many other ternary and quaternary chalcogenide systems such as CIGS and CZTS.
We have developed a synthesis for AgBiS₂ nanocrystals with high, tunable absorption spectra (bandgap = 1.0 - 1.4 eV) that are stable in colloidal solutions for many months. These nanocrystals can be incorporated into planar or nanostructured solar cells with record efficiencies. As no selenization is required, both the material synthesis and the solar-cell fabrication are performed at low temperatures (le;100°C). Furthermore, the highest-efficiency cells yield short-circuit currents of up to 20 mA/cm² with a very thin (~40 nm) active layer. These cells therefore offer the additional advantage of having an ultrathin active layer, which lowers material costs and paves the way for further improvement through incorporation of ultrathin absorbing layers in new device architectures such as tandem and sensitized solar cells.
References
1. C. M. Chuang, P. R. Brown, V. Bulovicacute;, M. G. Bawendi, Improved performance and stability in quantum dot solar cells through band alignment engineering, Nat. Mater. , 1-6 (2014).
2. I. J. Kramer, E. H. Sargent, The architecture of colloidal quantum dot solar cells: Materials to devices, Chem. Rev.114, 863-882 (2014).
3. Z. Pan et al., High-efficiency “green” quantum dot solar cells., J. Am. Chem. Soc.136, 9203-10 (2014).

By being one of the common forms of iron oxide and one of the main components of iron ore, hematite is of immense importance to our industrialized society. In the presence of rust it is also one of few semiconductors visible, although not appreciated, in everyday life. Despite this it turns out that behind the familiar rusty faccedil;ade our understanding of the materials optical and catalytic properties, it is still insufficient for an efficient technological application. Hematite has been suggested in application in everything from cancer treatment and water cleaning, but also as a non-toxic, cheap and abundant material for photocatalytic water splitting. Here, the optical absorption is analyzed in detail as a function of film thickness for 35 thin films of hematite ranging between 2 and 70 nm. The hematite was deposited by atomic layer deposition on FTO-substrates using Fe(CO)₅ and O₂ as precursors. For film thicknesses below 20 nm the optical properties are severely affected as a consequence of quantum confinement. One of the more marked effects is a blue shift of up to 0.3 eV for thinner films of both the indirect and direct transitions, as well as a 0.2 eV shift of the absorption maximum.[1] Raman measurements revealed no peak shift or change in relative intensity for vibrations for the thinnest films indicating that the decrease in indirect transition could not directly be assigned to depression of any specific phonon but instead seems to be a consequence of isotropic phonon confinement. Elemental diffusion of Si, Sn and F [2] from the Fluorine doped SnO2 substrate upon thermal treatment of the hematite electrodes and the implications for the catalytic activity in water splitting and the resulting photocurrents are also discussed.
[1] Fondell, M., Jacobsson, T. J., Boman, M., Edvinsson, T. Optical quantum confinement in low dimensional hematite, J. of Materials Chem. A, 2 , 2014, 3352-3363
[2] Diffusion of Sn, Si and F in Hematite: Implications for Photocatalytic Water Splitting Applications, Fondell, M., Jacobsson, T. J., Boman, M., Edvinsson, T., Gorgoi, M. , von Fiendt, L. , Boman, M., Edvinsson, T. and Lindblad. T., submitted

Hematite is one of the most stable and sustainable photoanode materials for solar water splitting that do not require any protection layer engineering. It has a suitable band gap of 2 eV and a well-aligned energetic position of its valence band for the oxidation of water. However, its breakthrough for application in a water splitting device is hampered by its poor charge collection efficiency. Charge transport limitations urge the photoanode design towards thin films deposited on high surface area nanostructured conductive scaffolds,^{1, 2} where recombination losses within the thin absorber material are minimized while light absorption can be adapted by varying the nanostructure thickness. Though thin hematite films deposited by, for example, ultrasonic spray pyrolysis have reached high photoactivities,^3-5 this method is not very suitable to homogenously coat within a porous nanostructure. Thus, the ideal method is atomic layer deposition (ALD) that produces highly ordered thin films in a layer-by-layer deposition.⁶ Here, we present a study on the nature of the charge transport processes in hematite on high quality conformal thin films deposited via a novel low-temperature ALD route. We show that these hematite films are phase-pure, crystalline as-deposited and, therefore, fully photoactive without the need for post-annealing treatments. Implementing a functional underlayer, hematite thicknesses of less than 10 nm will be shown to yield the best charge collection efficiency and, in addition, remarkable photocurrents for films this thin. We will discuss the dependence of the optimal film thickness on the space charge layer width and hole diffusion length and will present a general understanding for the prediction of the optimal hematite film thickness.
References
1. Sivula, K.; Le Formal, F.; Grätzel, M. Chem. Mater. 2009.
2. Stefik, M.; Cornuz, M.; Mathews, N.; Hisatomi, T.; Mhaisalkar, S.; Gratzel, M. Nano Lett. 2012.
3. Steier, L.; Herraiz-Cardona, I.; Gimenez, S.; Fabregat-Santiago, F.; Bisquert, J.; Tilley, S. D.; Gratzel, M. Adv. Funct. Mater. 2014.
4. Hisatomi, T.; Dotan, H.; Stefik, M.; Sivula, K.; Rothschild, A.; Gratzel, M.; Mathews, N. Adv. Mater. 2012.
5. Hisatomi, T.; Le Formal, F.; Cornuz, M.; Brillet, J.; Tétreault, N.; Sivula, K.; Grätzel, M. Energy Environ. Sci. 2011.
6. Martinson, A. B. F.; DeVries, M. J.; Libera, J. A.; Christensen, S. T.; Hupp, J. T.; Pellin, M. J.; Elam, J. W. J. Phys. Chem. C 2011.

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.

Increasing solar cell (SC) efficiency while keeping low cost process is one of the big challenges that the silicon solar industry is facing. One of the solutions consists in tuning the light received by the SC for absorbing more IR photons thanks to a frequency conversion layer. This work focus on down-conversion layers (DCLs) which was deposited on top of SC for increasing the number of IR photons that energetically match the Si SC band gap. In general, the single DCL absorbs UV photons and converts them in two IR ones. This new distribution of photons maximizes the number of IR photons at the expense of UV one and allows to maximize photogenerated carriers with energy slightly above the SC band gap at the expense of hot carriers. This results in a decrease of the energy losses due to the thermalization of hot carriers in the SC. Previous study of our group on SiO_xN_y: Tb³⁺-Yb³⁺ [1] had shown promising results with achieving a quantum internal efficiency up to 183%. However, a silicon SC industry compatible process is required to be integrated in any SC system improvement. From this stand point, we have fabricated a DCL constituted of a SiN_x matrix doped with Tb³⁺ and Yb³⁺ ions by reactive magnetron co-sputtering. Indeed, SiN_x is already used in the Si solar industry as antireflective material which ensures its compatibility with Si SC and offers low cost production. In addition, removing the oxygen from the matrix allows the incorporation of rare earths without clusters formation that would limit the rare earth ions luminescence efficiency.
The optical properties of the undoped SiN_x host matrix were investigated prior doping with Tb³⁺ ions. We have optimized the system obtained (via the deposition parameters) with the aim of achieving an intense photoluminescence (PL) emission from Tb³⁺ ions. Subsequently, Yb³⁺ ions were incorporated to generate an intense PL emission at 980 nm through a wide UV domain excitation. Quantum efficiency as high as ~200% has been obtained demonstrating the successful optimization of the excitation and energy transfer processes in developed DCL system. The cooperative energy transfer taking place between Tb³⁺-Yb³⁺ ions involved in the down-conversion is highly dependent on the distance between the two ions. Thus, in order to achieve an even higher quantum efficiency, a multilayer approach based on alternating stack of Tb- and Yb-doped sublayers has been developed to increase the number of excited Yb³⁺ ions. The effect, how this approach influences the properties of DCL frequency conversion process will be discussed in details during the presentation.
References:
[1] Y.-T. An, C. Labbé, J. Cardin, M. Morales, F. Gourbilleau, Highly Efficient Infrared Quantum Cutting in Tb³⁺- Yb³⁺ Codoped Silicon Oxynitride for Solar Cell Applications, Adv. Opt. Mater. 1 (2013) 855-862.

Such elements of the group IV as silicon and germanium are the most common building blocks of modern electronic devices due to their desirable band gap values, abundance and the relative ease of processing. Relatively small band gaps of these materials have ensured their applications in IR detectors and solar cells, despite the indirect nature of their band gaps, which gives rise to the reduced ability to absorb light. It has been shown from theoretical analysis that Si or Ge alloying with tin would produce better absorbing direct band gap materials. However, miniscule solubility of Si and Ge in Sn due to large lattice mismatch and much higher cohesion energies of Si and Ge compared to that of Sn make alloying difficult to achieve in bulk. Nevertheless, the high surface-to-volume ratios and degree of curvature, available in small NCs, open up a unique possibility for a structural relaxation that reduces the strain from lattice inhomogeneity, making a wider range of alloys stable in the nanocrystalline form. Towards the goal of creating better absorbing Group IV materials, we have already achieved¹ the synthesis of Sn_xGe_1-x nanocrystals with x up to 0.4 and now we report on the synthesis of alloy nanocrystals with the full compositional range where tin content reaches 95%. X-ray diffraction, TEM and EDX mapping analysis have confirmed the presence of homogenous alloys over the completely compositional range. Absorption spectroscopy of NCs of varying composition demonstrates a pronounced red-shift and increase in molar absorptivity as a true indicator of solar relevance with increasing Sn-content. Although definitive evidence of a transition to direct-gap behavior remains elusive, weak photoluminescence is observed in a substantial fraction of NC samples in which Sn-content exceeds 35%. These results suggest that solution-processible NCs based on alloys of non-toxic and abundant Group IV elements are a potentially promising material system for solar energy capture applications. The details of the synthetic approach together with results of the structural and optical characterizations will be presented and discussed. The mechanistic insight about the reaction pathways leading to the formation of Sn_xGe_1-x alloy was also used to attempt the synthesis of nanocrystalline Sn_xSi_1-x analogue. Preliminary results of this study will also be discussed.
References:
1. Ramasamy, K.; Kotula, P. G.; Fidler, A. F.; Brumbach, M. T.; Pietryga, J. M.; Ivanov, S. A. Chem Mater., DOI: 10.1021/acs.chemmater.5b01041.

Solar energy and optoelectronics applications of quantum confined silicon dots generally require an efficient means of collecting and distributing charge carriers. This has led us to consider quantum dots encapsulated within a hydrogenated amorphous silicon matrix that plays a critical role in photon collection, hot carrier cooling and transport. Cooling rates and transport mobilities are governed by phonon-assisted hopping events that we can now quantify in terms of nonadiabatic coupling (NAC). The approach provides significant advantages over more conventional methods of characterizing electron-phonon coupling for systems in which the electronic states are spatially localized. Dynamical effects associated with atomic motion and accurate excited electronic state energies are both accounted for. NAC is calculated using a new, parallelized Time-Dependent Density Functional Theory (TD-DFT) routine based on Density Functional Perturbation Theory (DFPT). This computational methodology is applied to predict cooling rates and transport mobilities for amorphous silicon, crystalline silicon and layered structures in which the morphologies are alternated. Tailoring the size and spacing of the nanocrystals modifies the interplay of electronic and vibrational states so as to optimize carrier transport.

Silicon heterojunction solar cells with open-circuit voltages (V_oc) above 700 mV and efficiencies above 25% have been reported and shown great potentials. The high voltages are due to the unique hydrogenated amorphous silicon (a-Si:H) layers that are capable of both passivating (intrinsic a-Si:H) and extracting (doped a-Si:H) the photogenerated carriers. Unlike the diffused emitters, the thin doped a-Si:H layer is not laterally-conductive enough for the charges to travel millimeters and be collected. As a result, a conductive window layer, usually transparent conductive oxide (TCO), is needed for on-site charge collection. Unfortunately, any conductive layer can absorb light parasitically and consequently, the high V_oc is partially offset by the loss of short-circuit current density (J_sc). Therefore, it is of significance to seek for better window electrode materials that are highly transparent and conductive. The authors have demonstrated that silver nanowire (AgNW) network can be such a candidate with superior conductance and transmittance to TCOs. However, a very thin spacer layer of TCO underneath the AgNW network is still necessary. It is for the same reason that the sub-micron gaps between randomly-arranged NWs make the charges difficult to travel laterally.
Thickness of the TCO spacer layer, diameter of the AgNWs and density of the AgNW network are the key parameters affecting the optical and electrical performance of the proposed AgNW-TCO hybrid electrode. For better understanding and optimizing, an effective optical model is necessary. However, the challenge of modeling is that multiple optical structures are involved in the device, e.g., NWs (~ 100 nm), TCO (1 - 100 nm), a-Si:H (1 - 50 nm), textured surface (1 - 5 mu;m), wafer substrate (180 mu;m), etc. These structures are of massive scale difference from a few nanometers to hundreds of microns, making individual modeling methods incapable, i.e., the device is too computationally intensive for electrodynamic modeling (e.g., Finite-Difference Time-Domain method, FDTD) and too coarse for ray-based simulators (e.g., ray-tracer).
In this paper, we use a novel angular matrix framework (AMF) to resolve the challenge, where matrices are used to describe the transition of the angular distribution of the light when it is reflected or transmitted at the interface, or absorbed in the bulk. Three steps are involved in the AMF: (i) ray-tracing is used for textured surface and light propagation in the wafer substrate. The FDTD is implemented on the NWs and the thin-film coatings. (ii) The inputs to the models are decomposed from the angular matrices, and the outputs are composed to the matrices. (iii) The interactions of the adjacent optical structures are realized by matrix operations. The framework is capable of effectively integrating ray-tracing and electrodynamic simulation for the hererojunction solar cells with AgNW-TCO hybrid electrode and quickly providing optimizing guidance.

The design of highly efficient, non#8208;biological, molecular#8208;level energy conversion “machines” that generate fuels directly from sunlight, water, and carbon dioxide is both a formidable challenge and an opportunity that, if realized, could have a revolutionary impact on our energy system. In the past five years, considerable progress has been made in scientific discovery of key materials and mechanisms needed to realize artificial solar fuels generators, and advances modeling and design have generated a now quite widely embraced conceptual paradigm for a solar fuels generator. While we still lack sufficient knowledge to design solar-fuel generation systems with the ultimate efficiency, scalability, and sustainability to be economically viable, considerable advances have been made, particularly for water-splitting solar fuels devices. I will also survey outstanding scientific challenges to realization of artificial photosynthesis for generation of fuels and chemicals by reduction of carbon dioxide.

Transition-metal-oxide semiconductors are promising photoanodes for solar water splitting due to their excellent chemical stability and appropriate band gaps. However, in absorbers such as BiVO₄, TiO₂, α-Fe₂O₃, and WO₃, charge carriers localize as small polarons, leading to low minority carrier mobilities. This limits the minority carrier collection efficiency in the quasi-neutral region of the light absorber, and lowers the overall photoactivity. In this work, we demonstrate that modestly elevating the temperature activates small-polaron hopping in monoclinic BiVO₄, significantly enhancing the saturation photocurrent without a substantial anodic shift of the onset potential. Specifically, using a Si/SnO₂/Mo:BiVO₄ tandem photoanode/photovoltaic, increasing the absolute temperature by 11% from 10 to 42 °C elevates the saturation photocurrent from 1.8 to 4.0 mA cm^-2. Concurrently, the onset potential shifts slightly from 0.02 V to 0.08 V versus the reversible hydrogen electrode (or equivalently, from -1.22 V to -1.13 V versus the equilibrium potential of oxygen evolution). Our observation contrasts with the prevailing understanding that decreasing photovoltage with temperature dominates the energy conversion efficiency. Thermally-activating minority carrier transport represents a general pathway towards enhancing the photoactivity of light absorbers exhibiting small-polaron conduction.

As an economically competitive pathway to clean chemical fuel (hydrogen gas), photoelectrochemical (PEC) water splitting is a kinetically and thermodynamically difficult process. Spontaneous reaction needs a large photovoltage of 1.23 eV plus the overpotentials and energy losses due to the multi-electron transfer kinetics and non-ideal interfacial thermodynamics. Hematite (α-Fe₂O₃), with a suitable bandgap ca. 2.1 eV, represents a promising photoanode semiconductor material which is stable, inexpensive and nontoxic. Previously, performance of hematite in photoelectrochemical water oxidation was mainly impeded by its low photovoltage production. It was reported that structural defects and surface trap states are the primary reasons for the unnecessary potential loss at photoelectrode and electrolyte surface. Herein, a facile re-growth method is developed to reduce structural disorders and modify hematite surface. More specifically, nanostructured hematite is synthesized by post annealing FeOOH nanowire at 800 °C for 5 min. Afterward, when growth and calcination are repeated under the same conditions, surface disorder and defects are gradually reduced. When coupled with a layer of robust amorphous NiFeO_x electrocatalyst, a greater photovoltage of 0.80 V and correspondingly low turn on voltage of 0.45 V (vs. reversible hydrogen electrode) are measured. X-ray diffraction, Raman spectrum and soft X-ray absorption analysis clearly show the structural difference of hematite with and without re-growth treatment in bulk as well as on the surface. This excellent performance allows us to construct unassisted PEC devices with hematite/NiFeO_x as photoanode and amorphous-Si as photocathode to split water at an overall efficiency of 0.91%. This work successfully pushes the performance limit of hematite and points to a research direction for the study of semiconductor electrode and electrolyte interface.

Bright and photo-stable quantum dots with narrowband emission can be used as downconverters in a new generation of luminescent concentrator. Such concentrators can be integrated with existing solar cell technologies to provide improvements in performance. This talk will describe recent programs in implementing such a design.

Solar thermal energy conversion is superior to traditional solar photovoltaics because it can utilize nearly the entire solar spectrum, thus enabling higher energy conversion efficiency. Solar absorbers are generally required for any solar thermal technologies in order to convert solar flux into heat. To maximize the conversion efficiency, ideal solar absorbers should exhibit near-blackbody absorption in the solar spectrum while suppressing infrared emission at elevated temperatures. However, the development of such solar selective absorbers has been hurdled by the challenges associated with their spectral performance, thermal stability for high temperature applications, and manufacturing cost. Here, we demonstrate large-scale nanophotonic solar selective absorbers with excellent solar selective absorptivity and thermal stability. The measured absorptivity/emissivity of the nanophotonic absorbers has spectrum selection with ~ 95 % solar absorptivity and ~10 % emissivity in the infrared range. The fabricated structures demonstrate stable spectral performance at 800 ^oC. Due to the three-dimensional nanophotonic design, the excellent selective absorption maintains for a wide range of incident angles +/- 50 degree , which indicates the omnidirectional absorption of the solar absorbers.

Two promising methods to convert sunlight to electricity are through the use of photovoltaics (PV) and concentrating solar power (CSP), however each method has distinct drawbacks. The large reduction in manufacturing costs for PV cells over the past few years is well-known; however, single-gap PV cells cannot utilize solar photons with energy below the band gap and convert photons far above the band gap very inefficiently. Additionally, storage of PV-generated electricity for use at off-peak times remains expensive using current technologies. On the other hand, CSP utilizes the entire solar spectrum and can be paired with inexpensive thermal energy storage; however, the overall cost and efficiency of CSP plants are not competitive with PV. We propose a novel, high efficiency, hybrid (combined PV and thermal) solar receiver to optimally utilize the full solar spectrum, which is enabled due to high performance silica aerogels and spectral splitting using a thin-film multi-layer coating. Silica aerogels are highly porous matrices made from agglomerated nanometer scale particles. Aerogels have exceptionally low thermal conductivity due to the low solid volume fraction and the small pore sizes suppressing convection and limiting gas molecule mean free path length. With proper synthesis, silica aerogels can also be made very transparent in the solar spectrum. The low thermal conductivity and high transparency allows sunlight in, but minimizes radiative and convective heat losses out, enabling efficient conversion of sunlight to high temperature thermal energy. For spectral splitting, a thin-film multi-layer stack of alternating high and low index of refraction materials allows targeted reflection of a narrow spectral band where PV conversion is most efficient. Our investigations into the proposed hybrid solar receiver suggest that it could produce electricity at a lower levelized cost of energy than current state of the art CSP technologies and advance the adoption of dispatchable solar energy.
This work was funded by Advanced Research Projects Agency - Energy (ARPA-E) under award number DE-AR0000471 for development of a Full Spectrum Stacked Solar-Thermal and PV Receiver.

Solar thermophotovoltaic (TPV) systems convert sunlight into electricity via thermal radiation. The efficiency of this process depends critically on both the selective absorber and the selective emitter, which are controlled by both the materials and the photonic design. For high concentration solar TPV applications, 2D photonic crystals made of refractory metals such as tungsten have demonstrated promising results. On the other hand, it is difficult to exceed world record efficiencies for photovoltaics with such an approach. Thus, we propose an integrated photonic crystal selective structure (IPSS), which combines 2D photonic crystals and filters into a single device to both collect solar heat and reradiate above-gap photons. Finite difference time domain (FDTD) and current transport simulations show that an IPSS can significantly suppress sub-bandgap photons. This increases sunlight-to-electricity conversion for photonic crystal-based emitters above 40% at 1000 suns concentration using a Ga_0.47In_0.53As PV diode with a bandgap of 0.74 eV (lattice-matched to InP). The physical basis of this enhancement is a shift from a perturbative to a non-perturbative regime that maximizes photon recycling. Furthermore, combining an IPSS with non-conductive optical waveguides eliminates a key difficulty associated with solar TPV: the need for precise alignment between the solar-heated selective emitter and the cool PV diode.

Recent investigations of semiconducting two-dimensional (2D) transition metal dichalcogenides have provided evidence for strong light absorption relative to its thickness. This can be attributed to high density of states in these atom-thing material systems. Stacking a combination of metallic, insulating, and semiconducting 2D materials enables functional devices while keeping the overall volume small. While photovoltaic cells based on 2D materials have been demonstrated, the reported absorption is still just a few percent of the incident light due to their sub-wavelength thickness leading to low cell efficiencies. Here we show that taking advantage of the mechanical flexibility of 2D materials by rolling a molybdenum disulfide (MoS₂)/graphene (Gr)/hexagonal boron nitride (hBN) stack to a spiral solar cell allows for optical absorption up to 90%. That is, we investigate the optical absorption of a 1 um-long hetero-material spiral cell consisting of the aforementioned hetero stack forming an optical nanocavity. Our results show that the absorption is about 50% stronger compared to a planar MoS₂ cell of the same thickness despite the volumetric absorbing material ratio being only 6%. We find that a core-shell structure exhibits enhanced absorption and pronounced absorption peaks with respect to a spiral structure without metallic contacts. A study of the eigenmodes and group-index of the nanocavity confirms the enhancement of the light-matter interactions by the optical cavity. However, a spectral analysis shows that the core-shell nanocavity design provides the aforementioned high absorption over a several hundreds of nanometer spectral bandwidth suitable for solar cell applications. In conclusion, we anticipate these results to provide guidance for photonic structures that take advantage of the unique properties of 2D materials in solar energy conversion applications.

Solar energy holds a great deal of promise as the best clean energy source for human beings, but its increasing utilization depends on the development of technologies that are cost-competitive with other energy sources. Cost reduction can be achieved by either incrementally improving existing technologies or by inventing disruptive new approaches. This talk will give a few examples from our research of solar technology approaches which aim at reducing the cost of solar energy systems using nanostructured materials. For example, efficient solar steam generation is possible with the addition of a porous absorber that localizes solar energy at the surface of water. This technology has potential applications in both clean water and power generation. The second technology is an aerogel-based solar receiver for concentrated solar power plants, with the aim of high-temperature operation under atmospheric conditions. The third example is solar thermoelectric generation devices, which are a solid-state technology that also have the advantage of utilizing the full solar spectrum. The final approach I will discuss is thermophotovoltaic energy conversion in both the near and far fields. By extending the concept of quantum-wells from electrons to photons, we show that thin-film based thermophotovoltaic cells can achieve high efficiency. The fundamental science guiding and stimulating the development of these technologies will be elaborated.

Plasmonic absorbers, which can absorb light efficiently through metallic nanostructures, have attracted a lot of attention due to various solar harvesting applications. However, up to now, most of the reported metallic absorbers are predominantly fabricated by the top down approach, the performance and scalability of which need to be further improved to enable wide spread applications. In this work, we report a plasmonic absorber fabricated by a bottom up fabrication process, which can enable an average measured absorbance of ~ 99% across the wavelengths from 400 nm to 20 µm, the most efficient and broadband plasmonic absorber reported so far. The bandwidth and absorption efficiency of the proposed absorber can be structurally tuned from visible to infrared region. An optimized absorber is fabricated through self-assembly of gold nanoparticles onto a nanoporous template by one step deposition process. Because of its efficient light absorption and strong field enhancement, it can enable solar steam generation with over 90% efficiency under solar irradiation of 4 kWm^-2. The proposed design of ideal plasmonic absorbers can be applied to other plasmonic metals, such as aluminum. The pronounced light absorption effect coupled with the high throughput self-assembly process can lead towards large scale manufacturing of other novel nanophotonic structures and devices. Our absorbers may be good candidate for many applications, such as solar steam generation, thermophotovoltaics, as well as light/thermal detectors.

We investigated the use of indium tin oxide (ITO), a semiconductor nanoparticle, as a spectral filter for use in concentrated GaAs photovoltaic (PV) cells. The synthesized nanoparticles filter high energy UV and infrared photons that usually degrade PV performances. The energy of the absorbed solar radiation will be transferred to a heat transfer fluid for photothermal energy conversion. Synthesis of ITO nanoparticles tailored to achieve optimal optical performance while remaining stable in a heat transfer fluid is ncessary. Particles were synthesized using a one-pot synthesis with indium acetylacetonate (In(acac)₃, tin bis (acetylacetonate) dichloride (Sn(acac)₂Cl₂) , and myristic acid (CH#8323;(CH#8322;)#8321;#8322;COOH) as the starting materials and oleylamine as a reducing agent and surface passivation ligand. In order to enhance the solubility and stability of the as-synthesized ITO particles in heat transfer fluids, different strategies such as ligand exchange, silica encapsulation, and synthesis with alternate passivating ligands were employed. Characterization of the nanoparticles was accomplished by Fourier transform infrared spectroscopy, UV-Visible spectrometry, dynamic light scattering, scanning electron microscopy, and solution stability tests at various temperatures.

Optical nanostructures are well known to for their ability to absorb and scatter light. However, by modifying the absorption in a semiconductor, one can also modify the electronic properties of the device, include the voltage response. Here we will discuss ways in which we can surpass the efficiency limit of traditional photovoltaic devices using novel new methods involving nanoscale optical concentration [1], photonic crystals to effectively modify the semiconductor bandgap [2], and modified radiative thermal emission [3]. We will show our recent experimental results involving the use of photonic materials to modify the effective semiconductor bandgap of a GaAs solar cell and how nanoscale optical concentration has the potential to increase cell efficiencies to >40%. Finally, we will show that by combining specifically designed radiative cooling structures with solar cells, higher power conversion efficiencies can be obtained for solar cells in both terrestrial and extraterrestrial environments. Specifically, we show that a solar cell can be improved by up to 4% absolute efficiency in an extraterrestrial environment in near-earth orbit using radiative emission. By combining these strategies, we will layout a pathway to achieving ultra-high efficiency solar cells based purely of photonic design considerations.
[1] Yunlu Xu and Jeremy N. Munday, IEEE Journal of Photovoltaics, 4, 233 - 236 (2014)
[2] Yunlu Xu, Tao Gong, and Jeremy N. Munday, Scientific Reports, in press (2015)
[3] Taqiyyah Safi and Jeremy Munday (in review)

Organic-inorganic perovskites have recently attracted great interest for use in solar cell technology, because of the high carrier mobilities, long minority carrier lifetimes, tunable band gaps and relatively benign defects and grain boundaries for systems based on Group 14 metals (e.g., Ge, Sn and Pb). Indeed, these materials have enabled unprecedented rapid improvement in device performance to levels above 20% power conversion efficiency and with open circuit voltages above 1V for a single junction photovoltaic (PV) device. This talk will explore beyond the current focus on three-dimensional (3-D) lead(II) halide perovskites, to highlight and review the outstanding chemical flexibility and potential of the broader class of 3-D and lower-dimensional organic-based perovskite family. Challenges for this materials class include replacing lead with more environmentally benign metals, improving PV device stability (moisture, UV and air) and controlling hysteresis. The concept of a multifunctional organic-inorganic hybrid, in which the organic and inorganic structural components are well ordered on the nanoscale and provide unique and hopefully synergistic features to the compound, represents another important contemporary target. Further exploration within this structural and chemical space is expected to yield important opportunities for broader energy materials design and fundamental understanding.

Colloidal quantum dots (CQD) are promising as the light absorbing species in next generation photovoltaic solar cells and as the luminophores in light emitting devices, and bio-labelling. However, in CQDs the photoluminescence quantum yield, charge extraction efficiency and lifetime of photo-generated charges are often limited by surface-mediated non-radiative recombination, which can occur on a sub-nanosecond time-scale. Treatment with halide ions has emerged recently as a particularly effective method of surface passivation resulting in significant improvements in device performance, such as the demonstration of record efficiency for a CQD-sensitised solar cell.
In this study ultrafast transient absorption spectroscopy is used to study sub-nanosecond charge dynamics in CdTe colloidal quantum dots. After treatment with chloride ions, these can become free of surface traps that produce non-radiative recombination. This enables direct observation of the effects of surface trapping by comparing CQD charge dynamics before and after treatment. The surface-traps typically increase the rate of electron cooling to the band edge by 70% and introduce a recombination pathway that depopulates the conduction band minimum of single excitons on a sub-nanosecond timescale, regardless of whether the sample is stirred or flowed. It is also shown that surface-trapping significantly reduces the peak bleach obtained for a particular pump fluence and introduces a rapid sub-nanosecond decay from the peak bleach, which has important implications for the interpretation of transient absorption data, including the estimation of absorption cross-sections and multiple exciton generation yields.

Inorganic semiconductors such as Cu₂ZnSnS₄(CZTS) have gained a lot of attention as low-cost solar cell materials and for optimal use of the visible spectrum, Cu₂ZnSn(S_1-xSe_x)₄(CZTSSe) has been proposed. Although the main advantage of this kind of solar cell materials is the use of earth abundant elements, the synthesis of highly pure CZTSSe through of chemical routes has been a challenge and the optimal Se/S ratio for suitable band gap has not been experimentally verified. Here, we report the results of the study undertaken to evaluate the composition, structure and optical properties of CZTSSe nanoparticles with different Se/S ratio using a solution-based technique. For synthesizing the semiconducting nanoparticles, metal precursors such as copper(II) acetate, zinc acetate dihydrate, tin(II) chloride dihydrate, and the chalcogenide sources, sulfur and selenide mixed at different molar ratios, were introduced in oleylamine and heated at 260#730;C for 300 min under an N₂ atmosphere. The nanoparticle diameters ranged between 10-15 nm and the Se/S ratio between 0 and 1. The as-synthesized CZTSSe particles with varying S/Se ratios exhibited band gap values lower, 0.7-1.1 eV, than the theoretical values. However, the discrepancy was reduced when the particles were heat treated at temperatures higher than 300 ^oC in sulfur rich-atmospheres. Thus, the band gap value shifted to high energy region, 1.14-1.27 eV, whereas the composition remained invariable. On the other hand, XRD pattern shows that as-prepared CZTSSe matched very well with stannite/kesterite type structure; although the (002) and (101) peaks were not observed, simulated measurements for perfect kesterite crystals indicate the presence of both peaks at around 17° and 18°, respectively. However, these peaks appeared when annealed and considered to have enhanced the crystallinity of the powders. Consequently, we found that the crystallinity of CZTSSe has a strong effect on the optical properties, and this could be achieved by subjecting these particles to annealing at higher temperatures.

As energy issue draw more and more attention, Dye sensitized solar cells (DSSC) had been reported to be a promising candidate for industrial application for its high efficiency as well as low cost. Yet the electrolyte using organic solvent suffer from problems like leakage and vaporization. Since working condition of solar cells would be under strong light intensity and temperature up to 60 °C, and encapsulation material would easily fail due to the aging, it would be a great challenge for stability of electrolytes in DSSCs. For fiber-shaped solar cell, which is most likely to industrialize thanks to the tide of wearable electronics, this problem is even more severe. Solid-state electrolytes are resistive to these problems in nature, and have been shown to have high stability. Herein, we reported a kind of solid state electrolyte made from eutectic melts of ionic liquid crystal, combining different ionic liquid, which is solid-state and non-volatile, providing good ionic conductivity without any volatile solvent. Besides acceptable efficiency, it provides extraordinary stability, solar cells using these electrolytes can recover full efficiency even after heated to 110 °C, and have excellent stability for different storage and working condition. Long time performance of the fiber-shaped solar cell was greatly improved.

TiO₂-graphene photoactive composites show promise for a wide variety of environmental applications¹, including pollutant degradation, water treatment, and energy/fuel production. When combined with graphene, TiO₂ shows increased photocatalytic activity, which has been attributed to a variety of reasons¹. Narrowing of the band gap of TiO₂ into the visible region may occur due to formation of TiO₂-C moieties. The large surface area of graphene may facilitate surface reactions between adsorbates and catalyst. Finally, increased charge separation (which lowers charge recombination rates, the opposite of photoexcitation) may occur as electrons are shuttled away by graphene. Still many questions exist for these materials. For instance, experimental evidence suggests that graphene with low number of defects makes for an ideal TiO2-graphene photocatalyst². However, many popular production methods involve reduction of graphene oxide, which may produce graphene with many defects. It is crucial to identify the role of such defects in these photocatalysts. Other work also suggests that intimate contact between TiO₂ and graphene will increase catalytic activity³.
We present density functional theory simulations of TiO₂-graphene systems in order to elucidate the nature of the interface between the two materials. We have specifically focused on TiO₂ clusters up to 1 nm in size and rutile (110) surfaces. We considered graphene as defect-free or with common defects such as hydroxyls, vacancies, or epoxides. Our results indicate that significant binding occurs at defect sites, much more so than over pristine graphene. Most notably defects on the graphene surface lower the band gap of TiO₂, indicating that new composite electronic states may arise and increase photoexcitation yield in such materials. We also analyzed how surface-surface distance (indicative of degree of contact between graphene and TiO₂) affects the binding and TiO₂ band gaps. Our results highlight the potential role of the graphene surface state in TiO₂-graphene photocatalyst materials, and suggest design strategies for graphene-based composite materials based on the TiO₂-graphene interface structure.
(1) An, X. Q.; Yu, J. C. Rsc Advances2011, 1, 1426.
(2) Liang, Y. T.; Vijayan, B. K.; Gray, K. A.; Hersam, M. C. Nano Letters2011, 11, 2865.
(3) Zhang, Y.; Zhang, N.; Tang, Z.-R.; Xu, Y.-J. Physical Chemistry Chemical Physics2012, 14, 9167.

The nanostructured Si solar cells are expected to overcome the limit of conventional light trapping strategies such as anti-reflection coating and surface texturing. In spite of superior optical absorption gain, the nanostructured solar cells often exhibit poor energy conversion efficiency, mostly due to the surface recombination-induced electrical loss. In this work, we have investigated influence of SiN_x dielectric layer coating on photovoltaic (PV) performance of Si absorbers with surface hexagonal nanoconical frustum (NCF) arrays. We fabricated the NCF array solar cells using nanoimprint lithography and achieved a very high photocurrent of 36.94 mA/cm². The SiN_x layer significantly reduced the surface recombination rate and improved the quantum efficiency QE of the nanostructured cells. The Mie resonance of the NCF array enhanced the optical absorption in broadband and also increased near surface optical intensity. Such high density of charge carriers near the surface may lead to large recombination losses, which often can lead to poor PV performance of the nanostructured cells, in spite of the notable optical gain. Therefore, proper surface passivation via the deposition of a dielectric layer is crucial to exploit the enhanced optical absorption of the nanostructured solar cells.

Despite the great market expectations for buiding-integrated photovoltaics sector, glass-on-glass mass production of dye-sensitized solar cells (DSSCs) is still under development and companies involved are trying to find compatible solutions of technical (device optimization in terms of production cost, long term stability and performances) and marketing issues (best product for smart windows in terms of aestethic and perfomances). The availability of affordable, efficient and easily up-scalable photovoltaic (PV) technologies is a pivotal challenge in view of an upcoming diffusion of a next generation of building-integrated energy conversion systems.In this respect, during the last two decades, DSSCs have attracted widespread academic and industrial interest because, in comparison to silicon based technologies, they can be manufactured using low cost materials and relatively easy fabrication processes, often borrowed from the printing industry. These issues alone, however, do not constitute a sufficient and effective driving force toward the full scale industrialization of DSSC technology. Competition with silicon-based PV technologies in terms of cost-effectiveness has become hardly sustainable. The natural technological evolution of this class of devices should be represented by an integrated photovoltaic powered electrochromic (PV-EC) window, also considering that the operational characteristics of both PV and EC technologies are highly compatible. A small area of PV cells could in fact provide sufficient electric power to operate a large-area EC window.
A photovoltachromic cell (PVCC) may potentially act as a complex artificial skin, by generating electric energy as a photovoltaic system but even ‘‘perceiving&’&’ small variations in external radiation and controlling the energy fluxes by means of a smart variation of their optical transmittance. Incoming solar light is partially harvested by a dye-sensitized photoelectrode made on the front glass of the cell which fully overlaps a bi-functional counter. When the cell is illuminated, the photovoltage drives electrons into the electrochromic stripes through the photoelectrochromic circuit and promotes the Li+ diffusion towards the electrochromic region, which thus turns in its colored state. At the same time an efficient photovoltaic functionality is executed by the catalytic region. A photocoloration efficiency of about 20 cm²*min^-1*W^-1 has been demonstrated at low illumination intensities along with fast response (coloration time < 30 sec and bleaching time < 60 sec). Regarding the PV functionality, a power conversion efficiency ranging from 2% to 5% is achievable depending on the light absorbing prerogatives of the dye-sensitized electrode as well as on the specific electrolyte&’s chemical composition.

TiO₂ has been considered as promising materials for optoelectrical applications including photovoltaic cells, and dye-sensitized solar cells, because it is low cost materials with appropriate bandgap, high photocatalytic activity, and chemical durability. TiO₂ has three major crystalline polymorphs : rutile(tetragonal, space group: P4₂/mnm), anatse(tetragonal, space group: I4₁/amd), and brookite(orthorhombic, space group: Pbca). Rutile is a stable phase, whereas anatase and brookite are metastable phases which can be transformed to rutile when annealed. Research on Brookite has been less focused on compared to anatase and rutile, because brookite rarely exists in nature, and it is difficult to synthesize.
In this research, various TiO₂ nanostructures were synthesized on titanium foil, by facile one-step hydrothermal reaction. Cleaned titanium foil was placed on Teflon-lined stainless steel autoclave filled with 0.5M NaOH aqueous solution. After the reaction at 220&’C, the titanium foil was immersed in 0.1M HCl solution to replace Na+ ion to H+ ion. Then, the titanium foil was put in a muffle furnace for heat treatment. Synthesized TiO₂ can have four morphologies which are sheet, tube, wire, and bullet shape, depending on the experimental parameter such as reaction temperature, reaction time and concentration of the additive. Nanowire structure was synthesized at 220&’C with 0.5M NaOH. Nanosheet structure was synthesized at lower temperature, and nanotube and bullet structure was synthesized at lower concentration of additive.
The morphology was observed using SEM and TEM. Nanosheet and nanotube structure were transformed to nanowire after appropriate reaction time. Average length of nanowires could be modified by changing the reaction time, while the bullet structure didn&’t change much depending on the reaction time. The crystalline structure was confirmed by SAED pattern of TEM and XRD analysis. As a result of TEM and XRD analysis, we found wire, sheet, and tube structures were anatase, and bullet structure was brookite.
The photoelectrochemical properties of various TiO₂ nanostructured electrodes were studied in a three electrode cell with Ag/AgCl and Pt as the reference and counter electrodes. Linear sweep was conducted under chopped illumination (AM 1.5G, 100mW/cm²). Photocurrents had different values depending on their morphologies. The brookite had the highest photoelectrochemical properties compared to other morphologies of anatase with similar length.
The field emission properties which can be affected by the morphologies of emitter of various TiO₂ nanoarrays were measured in vacuum chamber. Diverse length of the TiO₂ nanowires and nanotubes showed different field emission properties, and they were analyzed and optimized by theoretical calculation with “Zero Thickness Charge Disc (ZTCD)” model.

Lead sulfide (PbS) colloidal nanocrystals (NCs) are a promising material for infrared optoelectronic devices. Understanding charge transport in solids made from these materials is essential to designing more efficient devices. Current devices are believed to be limited by surface defects that act as recombination centers and trap charge carriers before they can be extracted. This work uses transient absorption and photoluminescence spectroscopies to monitor the trap state and band edge charge carrier populations in films made from highly monodisperse (<5%) PbS NCs. Charges in strongly coupled solids are found to escape trap states that are located below the band gap by several times the thermal energy. The effects of NC size, sample temperature, and ligand length on the TA and PL spectra are studied to understand the activation mechanism and role of charge transfer in allowing charge carriers to escape trap states.

Simplifying the process of fine-tuning the electronic and optical properties of graphene oxide (GO) is of importance in order to fully utilize it as the hole interfacial layer (HIL). We introduced silver trifluoromethanesulfonate (AgOTf) inorganic chemical dopant that tune and control the properties of single-layer GO films synthesized by chemical vapor deposition. The morphology, work function, mobility, sheet resistance, and transmittance of the GO film were systematically tuned by various doping concentrations. We further developed solution-processable low-temperature hole interfacial layer (HIL) poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS):AgOTf-doped GO HIL in highly efficient perovskite solar cells. The PEDOT:PSS:AgOTf-doped GO HIL grants desirable charge-collection in the HIL allowing the entire device to be prepared at temperatures less than 120 #730;C. The fabricated perovskite solar cells utilize rigid substrate demonstrate compelling photovoltaic performance with a power conversion efficiency (PCE) of 11.90%. Moreover, flexible devices prepared using a polyethylene terephthalate (PET)/ITO demonstrate a PCE of 9.67%, while ITO-free flexible devices adopting PET/aluminum doped zinc oxide (AZO)/silver (Ag)/AZO demonstrate a PCE of 7.97%. This study shows that PEDOT:PSS:AgOTf-doped GO HIL has significant potential to contribute to the development of low-cost solar cells.

Colloidal quantum dots (CQDs) are promising materials for novel light sources and solar energy conversion [1-3]. The efficacy of CQDs in applications such is light emitting diodes and solar cells is reduced by the non-radiative recombination associated with surface traps [4,5]. Thus understanding and suppression of “unwanted” surface-related recombination is essential in order to maximize the performance of devices incorporating CQDs. With a post-synthetic ion chloride treatment (without formation of any thick inorganic shell) of CdTe CQDs we succeed a near-complete suppression of surface trapping that resulted in almost completely suppression of non-radiative decay pathways and near-unity photoluminescence (PL) quantum yield [6]. We performed a systematic investigation where the effect of the treatment is characterised by absorption and PL spectroscopy, PL decay, scanning transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The chloride was shown to reside largely on the surface of the quantum dots, and does not affect crystallinity.
The effect on the recombination dynamics was also investigated with or without chloride passivation and after air exposure [7]. Radiative recombination was found to be dominant in the passivated CQDs and the radiative lifetime scales linearly with the CQD volume. Non-radiative recombination contributions appear for non passivated sample cases and after air exposure. Sample-dependent lifetimes ranged from less than 1 ns to 10s of ns, while the non-radiative dynamics can be explained by an atomistic model for Auger-mediated trapping of holes in surface oxidized CQDs. The chloride treatment also significantly increased the stability of the CdTe quantum dots when exposed to the air, which will facilitate their inclusion in many optoelectronic devices.
References
[1] A. H. Ip , S. M. Thon , S. Hoogland , O. Voznyy , D. Zhitomirsky , R. Debnath , L. Levina , L. R. Rollny , G. H. Carey , A. Fischer , K. W. Kemp , I. J. Kramer , Z. Ning , A. J. Labelle , K. W. Chou , A. Amassian , E. H. Sargent , Nat Nano 2012 , 7 , 577.
[2] E. H. Sargent, Nat Photon 2012 , 6 , 133.
[3] P. V. Kamat, J. Phys. Chem. Lett. 2013 , 4 , 908.
[4] K. Katsiev, A. H. Ip, A. Fischer, I. Tanabe, X. Zhang, A. R. Kirmani, O. Voznyy, L. R. Rollny, K. W. Chou, S. M. Thon, G. H. Carey, X. Y. Cui, A. Amassian, P. Dowben, E. H. Sargent, O. M. Bakr, Adv. Mater. 2014, 26, 937- 942.
[5] D. Zhitomirsky, O. Voznyy, S. Hoogland, E. H. Sargent, ACS Nano 2013, 7, 5282 -5290.
[6] Robert C. Page , Daniel Espinobarro-Velazquez , Marina A. Leontiadou , Charles Smith , Edward A. Lewis , Sarah J. Haigh , Chen Li , Hanna Radtke , Atip Pengpad , Federica Bondino , Elena Magnano , Igor Pis , Wendy. R. Flavell , Paul O&’Brien , and David J. Binks, Small 2015, 11, No. 13, 1548-1554.
[7] Daniel Espinobarro-Velazquez, Marina A. Leontiadou, Robert C. Page, Marco Califano, Prof. Paul O'Brien and David J. Binks, ChemPhysChem 2015, 16,1239 -1244.

Dye sensitized solar cells (DSSC) have received growing interest due to their potential for low cost, easy fabrication and high efficiency photovoltaic (PV) device. Usually, a conventional DSSC utilize Platinum (Pt) as a counter electrode (CE) to catalyse the electrolyte (e.g. iodine redox couple). Nevertheless, the development of low-cost DSSCs is still hindered due to the use of expensive and easily corroded Pt CE. With this in mind, alternatives for Pt as CEs in DSSCs are highly desirable. Ideally, a CE material should have a combination of excellent catalytic activities, high electrical conductivities, and superior electrochemical stabilities. Recently, carbon nanomaterials (such as Carbon black, carbon nanotubes (CNTs) and graphene) are seen to meet these requirements and be an ideal alternative to Pt CEs. Although the carbon nanomaterials have been successfully applied as CEs in DSSCs, these materials still suffer from high charge transfer resistance (R_ct), and less catalytic surface area leading to inferior photo-conversion efficiency. The high catalytic activities of carbon nanomaterials are closely related to their defective sites and the electronic structures within the carbon matrix. An attempt to increase these defective sites however leads to a decrease in electrical conductivity and thus lower performance. As such, it still remains a challenge to seek for new carbon nanomaterials with abundant catalytic surface areas whilst also maintaining the high electrical conductivity. Here, we find that the heteroatom (sulphur and nitrogen) doped hybrid carbon materials with CNT and graphene fulfils these requirements and presents a viable alternative to Pt CE. We also selectively control the defective sites (doping levels) in these hybrid carbon nanomaterials while still maintaining the high conductivity of pristine graphene and CNTs. Consequently, we find that higher PV efficiencies is achieved using these hybrid carbon nanomaterials.

Vertically oriented Ta-W-O nanotube array films were fabricated via the anodization of Ta-W alloy foils in HF-containing electrolytes. HF concentration is a key parameter in achieving well-adhered nanotube array structure. X-ray photoelectron spectroscopy (XPS) and diffuse reflectance measurements confirm the staggered band-alignment between Ta₂O₅ and WO₃, which facilitates the separation of charge carriers. The nanotubes made of Ta-W films containing 10% W showed 100-fold improvement in the measured photocurrent compared to pristine Ta₂O₅ upon their use to split water photoelectrochemically. This enhancement was related to the efficient charge transport and the red shift in absorption spectrum with increase of the W content, which was asserted by ultrafast transient absorption (TA) spectroscopy measurements. The TA measurements showed the elimination of trap states upon annealing Ta-W-O nanotubes and, hence, minimizing the charge carrier trapping, whereas the trap states remain in pristine Ta₂O₅ nanotubes even after annealing.

The excited-state interactions of bimolecular non-covalent systems consisting of cationic poly[(9,9-di(3,3&’-N,N&’-trimethyl-ammonium) propyl fluorenyl-2,7-diyl)-alt-co-(9,9-dioctyl-fluorenyl-2,7-diyl)] diiodide salt (PFN) and 1,4-dicyanobenzene (DCB) have been explored by steady-state and time-resolved techniques including femto- and nanosecond transient absorption (fs-ns-TA) and fs-Infrared (IR) spectroscopies with broadband capabilities. The experimental results demonstrate that the photo-induced electron transfer (PET) from PFN to DCB occurs in picosecond time scale, leading to the formation of PFN^+bull; and DCB^-bull; radical ions. Interestingly, following vibrational marker modes on the acceptor side in real time provides direct evidence and insight into the electron transfer process concluded indirectly from UV-Vis experiments. A band narrowing on a time scale of picoseconds observed on the antisymmetric CN stretching vibration of the dicyanobenzene radical anion provide a clear experimental evidence for a substantial part of the excess energy is channeled into vibrational modes of the electron transfer product, and the dissociation of the geminate ion pairs. More importantly, our nanosecond time-resolved data indicate long-lived charge separated state (~ 30 ns) due to the dissociation of the contact radical ion pair into free ions which is very promising and unique feature for organic solar applications.

Ultrafast charge transfer, separation and recombination at the donor-acceptor interface, has been demonstrated to be key in solar cell performance. How to control these ultrafast processes remains a topic for debate. Here, we use porphyrin (TMPyP)-graphene carboxylate (GC) as a unique model system to understand these processes at the molecular level.^1,2 The steady state interaction between GC and porphyrin results in a red-shifted absorption spectrum providing a clear indication for the binding affinity between porphyrin and GC via electrostatic and π-π interaction. Our steady-state and femtosecond (fs) time-resolved data clearly demonstrate that the charge transfer process at GC interfaces can be tuned by changing the electronic structure of the meso unit and the redox properties of the porphyrin cavity. More specifically, only positively-charged porphyrins can be strongly attracted on the surface of GC, which allows ultrafast interfacial electron injection upon laser pulse excitation. In addition, our results indicate that with careful control of the charge localization of the TMPyP cavity using Cyclodextrin (β-CD) as an external cage, we successfully improved the interfacial-electron injection efficiency from cationic TMPyP to GC by 120% compared to TMPyP alone. In this case, β-CD not only restricts free pyridinium rotation, but also blocks ICT between the macrocycle and the meso substituents of TMPyP; subsequently, electron density is localized on the cavity of TMPyP and its oxidation potential is reduced, increasing the ability to donate electrons to GC. Interestingly, unlike the interaction between TMPyP and GC, which is static and dynamic in nature, the interaction of TMPyP- β-CD with GC takes places via an efficient static mechanism. Finally, the novel new insights in this study holds promise for a variety of potential application that principally rely on interfacial charge transfer, such as photocatalysis.
Reference :
1.Ultrafast Electron Injection at the Cationic Porphyrin- Graphene Interface Assisted by Molecular Flattening shawkat Aly, Manas Parida, Erkki Alarousu and Omar AbdelsaboorChem. Commun., 50, 10452-10455. (2014)
2. To what extent can charge localization influence electron injection efficiency at graphene-porphyrin interfaces? Manas R. Parida, Shawkat M. Aly, Erkki Alarousu, A. Sridharan, D.H. Nagaraju, Husam N. Alshareef, and Omar F. MohammedPhys. Chem. Chem. Phys.,17, 14513-14517 (2015)

We developed a postgrowth modification method of two-dimensional WO₃ nanoflakes by a simultaneous solution etching and reducing process in a weakly acidic condition. The obtained dual etched and reduced WO₃ nanoflakes have a much rougher surface, in which oxygen vacancies are created during the simultaneous etching/reducing process for optimized photoelectrochemical performance. The obtained photoanodes show an enhanced photocurrent density ofsim;1.10 mA/cm² at 1.0 V vs Ag/AgCl (sim;1.23 V vs reversible hydrogen electrode), compared to 0.62 mA/cm² of pristine WO₃ nanoflakes. The electrochemical impedance spectroscopy measurement and the density functional theory calculation demonstrate that this improved performance of dual etched and reduced WO₃ nanoflakes is attributed to the increase of charge carrier density as a result of the synergetic effect of etching and reducing. This simultaneous etching and reducing method is convenient and facile and requires only mild solution conditions, which may provide a general approach to tune the surface morphology of other transition metal oxide materials.

Highly effective light-trapping and optimal in-coupling of light is crucial to enhance the overall device performance in thin-film photovoltaics, such as organic and hybrid solar cells. Here we present two ways how the transparent electrode architecture influences the device performance.
We prepared an electrode structured with a TiO₂/Ag/TiO₂ (TAT) multilayer as ITO replacement. This electrode allows to directly tune the optical cavity mode towards maximized photocurrent generation by varying the thickness of the layers in the sandwich structure. This enables tailored optimization of the transparent electrode for different organic thin film photovoltaics without alteration of their electro-optical properties. OPV featuring our TAT multilayer show an average improvement of ~15% over the ITO reference and allow power conversion efficiencies up to 8.7% in PTB7:PC71BM devices.
We also investigated wavelength-scale structured ITO/TiO₂ electrode prepared via direct laser interference patterning (DLIP) the TiO₂ layer. Two representative thin-film solar cell architectures are deposited on top: an organic solar cell featuring blended P3HT:PCBM as active material, and a hybrid solar cell with Sb₂S₃ as inorganic active material. A direct correlation in the asymmetry in total absorption enhancement and in structure-induced light in-coupling is spectroscopically observed for the two systems.

Dye-sensitized solar cells (DSSCs) have emerged as a promising solar energy harvesting technology cell due to efficiencies so far measured up to 11.9% and straightforward processes for solar cell manufacturing. Platinum is traditionally used in DSSCs as the counter electrode due to its corrosion resistance to iodide species and its excellent electrocatalytic activity toward the iodide/triiodide redox couple. However the use of this precious metal ($50/gram) remains a barrier to scalability and commercialization of DSSCs providing an incentive to develop DSSC counter electrodes using cheaper, sustainable materials. In this work we demonstrate the passivation of highly reactive porous silicon with graphenic carbon (C-Psi) and their use as a counter electrode in DSSCs exhibiting an efficiency of 5.38%, which is comparable to the 5.80% efficiency of a DSSC made with the conventional platinized counter electrode. We have studied the effect of chemical vapor deposition carbonization parameters on coverage and quality of graphene coating, and shown how this affects overall solar cell performance. An atomically thin coating of few layer graphene was found to be optimal for passivation of porous silicon as well as activation of the surface for triiodide reduction.
The design of next generation of energy systems requires the co-existence of both energy conversion and energy storage modalities within a single component. The versatility of materials such as silicon to be utilized simultaneously for energy storage and energy harvesting opens unique opportunities for the integration of these systems on a single platform. By using the C-PSi counter electrode as a multifunctional electrochemical platform, we designed a single device that captures solar energy and converts it to chemical energy via charging of an electrochemical supercapacitor. This architecture enables constant power supply on the timescale of tens of seconds, even when the light source is blocked, achieving an overall efficiency of 2.1%.

To meet rapidly rising energy demand without devastating the environment, we need a new energy scheme where renewable sources will play a key role. Of many chemical candidates for this new energy infrastructure, H₂ produced by H₂O splitting holds great promise owing to its high energy density per kilogram and potential ease of transportation and utilization. More importantly, the energetics required match those of the solar spectrum well. To date, the most efficient approaches reported in the literature either involve or are entirely powered by photovoltaic modules. The comparatively simpler, lower cost approach of direct water photolysis (also known as photoelectrochemistry, PEC) suffers from low efficiency, poor durability or both. Poor integration of catalysts with photoelectrodes is one important reason for this slow development. On a system level, its operation needs to be matched with a photocathode. Because the only commercially successful membranes to separate the photoanode and photocathode are proton exchange ones, PEC H₂O oxidation in acidic solutions deserves particular attention, but has not been demonstrated to date in a dual absorber photoelectrochemical cell. The lack of functionally stable water-oxidation catalysts (WOCs) in acids is a key reason for this slow development. The only WOCs that are stable at low pH are Ir-based ones, which are typically too expensive to be implemented broadly. Herein, we show that this deficiency may be corrected by applying an ultra-thin monolayer of a molecular Ir WOC to hematite for solar water splitting in acidic solutions. Hematite in this case is prepared by solution synthesized method, the unique nano-structure developed by re-growth strategy helps heal the surface deficiencies to provide good PEC performance. The turn-on voltage is observed to shift cathodically by 250 mV upon the application of a monolayer of the molecular Ir WOC. X-ray photoelectron spectroscopy (XPS) proved the observed performance enhancement was indeed a result of Ir het-WOC decoration, and evolved O₂ detection (the Faradaic efficiency is almost 100 %) experimentally verified the measured photocurrents were indeed water oxidation. When the molecular WOC is replaced by a heterogeneous multi-layer derivative, which yielded comparable results on the surface of hematite as the Ir het-WOC, stable solar water splitting for over 5 h is achieved with near unity Faradaic efficiency which promises the integration with amorphous silicon photocathode as un-assisted dual absorber PEC cell for water splitting in acidic solutions.

Surface passivation of semiconductor nanocrystal quantum dots (NQDs) relates to the properties of NQDs as the dangling bonds of the surface atoms dictate the dynamics of photo-generated carriers. Long-chain alkyl ligands, typical capping ligands for colloidal NQDs, fall short of covering nanocrystal surface due to steric hindrance. The incomplete passivation plays a major role in nanocrystal stability, or lack ther of. In addition, optical properties of nanocrystals alter significantly due to dangling bonds and trap states.
In this presentation, we will discuss the effect of chlorine (Cl) ion on the surface of CdSe NQDs (Cl-CdSe NQDs). Transient absorption spectroscopy reveals that passivated Cl diminishes electron trap states in CdSe NCs, resulting in an increase of photoluminescence quantum yield, e.g., from 4.6 % to 7.6 % after Cl treatment. In order to elucidate the effect of Cl treatment on electron trap states, we compared the photocatalytic hydrogen evolution rate by bare CdSe and Cl-CdSe NQDs. In the case of large NQDs, Cl-CdSe NQDs show high hydrogen evolution rate, as the passivation of trap states with energy levels below the reduction potential necessary for H₂O reduction, while bare CdSe NQDs exhibit slower reduction. By contrast, in the case of small NQDs with higher degree of quantum confinement effect, trap states do not come into play as the conduction band-edge and trap states are above the water reduction potential. In this case, the lack of trap states via Cl passivation reduces electron-hole separation, resulting in reduced photocatalytic activity.

We have demonstrated over 18% power conversion efficiency (PCE) of inverted perovskite solar cells consisting of graphene/MoO₃/PEDOT:PSS/CH₃NH₃PbI₃/C₆₀/BCP/LiF/Al, which is comparable to that of perovskite solar cells (~19%) with indium tin oxide (ITO) electrodes. A single-layer graphene was synthesized using chemical vapor deposition on thin copper layer and transferred to a glass substrate. The PEDOT:PSS and CH₃NH₃PbI₃ layers were prepared by spin-coating and the other layers were deposited by vacuum thermal evaporation. A MoO₃/PEDOT:PSS bilayer was used as a hole transporting layer on the graphene electrodes because i) a MoO₃ layer can provide hydrophilicity to the graphene surface, allowing for PEDOT:PSS to wet and form a continuous layer on graphene, and ii) a MoO₃ layer can dope holes into graphene and increase its low work function (-4.3 eV) to the higher level, resulting in nearly ohmic contacts at graphene and PEDOT:PSS interfaces. As the thickness of the thermally evaporated MoO₃ layer was varied from 0.5 to 2 nm, sheet resistance of the pristine single-layer graphene (2.2 kOmega;/cm²) was decreased up to ~500Omega;/cm². Although the resistance of the doped graphene is still much higher than that of ITO (9.2 Omega;/cm²), graphene is optically more transparent (~98% transmittance) than ITO (~90%) and its energy level is more favorably aligned with the hole-transporting layer. As a result, the overall PCEs of the cells with graphene and ITO electrodes were observed to be similar. Both types of cells were nearly hysteresis-free, likely due to well-formed interfaces between the smooth perovskite film and the organic layers. Given that graphene is mechanically robust and flexible than common transparent electrodes such as ITO and FTO (Fluorine-doped thin oxide), this work demonstrates great potential of graphene for developing flexible solar cells with high efficiency and stable performance.

Colloidal, catalyst decorated semiconductor nanocrystal are receiving increasing attention as an possibly cheap and scalable means for the photocatalytic generation of hydrogen from water.^1,2 A major shortcoming in many colloidal nanocrystal systems, however, is the short lifetime due to ligand or nanocrystal oxidation leading to aggregation and precipitation. Here we show, that the colloidal stability as well as hydrogen generation efficiency of cysteine coated, Pt-decorated CdS nanocrystals (using Na₂SO₃ as hole scavengers) can be significantly enhanced by replacing the original ligands in situ with triethanolamine (TEOA).³ The presence of TEOA extends the highly efficient and steady photocatalytic H₂ generation period by an order of magnitude. External quantum efficiencies up to 31.5% and turnover frequencies up to 0.11 H₂/Pt/s are achieved. The short, dendritic structure and rather weak adsorption of TEOA to the nanocrystal as well as sufficient free TEOA in solution are the keys to obtaining enhanced stability and efficient H₂ evolution simultaneously.
1. M. Berr, A. Vaneski, A. S. Susha, J. Rodri#769;guez-Ferna#769;ndez, M. Do#776;blinger, F. Ja#776;ckel, A. L. Rogach, and J. Feldmann, Appl. Phys. Lett., 2010, 97, 093108.
2. T. Simon, N. Bouchonville, M. J. Berr, A. Vaneski, A. Adrovic, D. Volbers, R. Wyrwich, M. Döblinger, A. S. Susha, A. L. Rogach, F. Jäckel, J. K. Stolarczyk, J. Feldmann, Nature Materials 2014, 13, 1013.
3. W. Li et al. submitted

Quantum dots (QDs) are very attractive materials for 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 Shockleyminus;Queisser limit in solar cells.² QD based solar cells are widely considered as a viable alternative to silicon and inorganic thin film based cells.³ 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 p-type electrodes (NiO or CuScN nanowires) for solar cell fabrication.
In the classical QD sensitized solar cells the light is absorbed by the QDs, which then inject electrons into an n-type material (TiO2), while the hole is regenerated by the liquid electrolyte. 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. We investigated the hole transfer rate from QDs to p-type semiconductors using steady state and time-resolved photoluminescence. We found hole transfer rate of 2- 5 x 10⁷/s from the CuInS₂ to CuScN nanowires and 3-4 x 10⁷/s to NiO. However, this transfer rate becomes an order of magnitude higher (i.e. 4-6 x 10⁸/s) for the case of CuInS_xSe_2-x when we used them with NiO. Moreover, the resulting hole transfer rates also follow solar efficiencies.
1 A. J. Nozik, M. C. Beard, J. M. Luther, M. Law, R. J. Ellingson and J. C. Johnson, Chem. Rev., 2010, 110, 6873-90.
2 J. B. Sambur, T. Novet and B. A. Parkinson, Science, 2010, 330, 63-6.
3 P. V Kamat, J. Phys. Chem. Lett., 2013, 4, 908-918.

Zinc doped iron oxide nanotubes (ZnFe-ox-NTs) were prepared by the polycondensation of a mixture of zinc(II)-nitrate-hexahydrate and iron(III)-nitrate-nonahydrate taken on the surface of self-organized block copolymer (Pluronic F-127) in a 1-propanol. This reaction was conducted at 318 K for 5 days to make gel slowly. The gel prepared was dried at 393 K in open air oven, and then the temperature was increased to 543 K in order to complete the polycondensation reaction and to burn out Pluronic F-127. The product obtained was well ground by using an agate mortar and rinsed several time by EtOH. Next the structural annealing of crude nanotubes was carried at 543 K for 6 hours. Nanotube structure was confirmed by TEM. Compositional formulae for the products were determined by the combination study of XRD and XPS, formulated by Zn_xFe_3-xO₄ where x can be controlled in the range between 0 and 0.66 as function of the mixing ratio of zinc-nitrate.
Band gap was determined by a Tauc plot method using optical absorption spectrum in the range between 200 and 1000 nm. For Zn undoped iron oxide nanotube sample (Zn₀Fe₃O₄-NTs), band gap was estimated to be ~2.3 eV, and its value decreased by ~2.0 eV for Zn_0.27Fe_2.73O₄-NTs. Photoelectrode was made by dropping viscous aqueous solution of ZnFe-ox-NTs/acetylacetone onto FTO glass spun at 3000 rpm, then heat-treated at 270 °C in open air. Pt was used as counter electrode. Spectral sensitivity was measured by irradiating band-pass filtered light using the electrolyte of LiClO₄/LiI/I₂/acetonitrile. Resulting that maximum sensitivity was located at the wavelength of ~510-540 nm, which is longer than that of Fe₂O₃ film (~390 nm). In addition, the sensitivity in the long wavelength region (< 540 nm) was improved by Zn doping. This can be explained by narrowing the band gap for nanotube samples and also by widening of the LUMO level band. Detailed thickness dependence of nanotube layer on IPCE will be reported at the meeting.

Key biochemical reactions in nature like photosynthesis and respiration, are multiple electron reactions. In nature these reactions are highly efficient. Artificial hydrogen production by photochemical cells is an example of performing a similar reaction in an artificial setting. However, the practical production of hydrogen is hampered by the need to apply additional voltage to initiate the reaction; the over-potential, which results in low efficiency and lack of selectivity in the oxidation process, as other molecules besides water tend to be oxidized. Hence, although hydrogen is considered to be the ultimate fuel of the future its efficient production remains an important challenge.
We found that in a cell in which the anode is coated with chiral molecules, the over-potential required for hydrogen production drops remarkably, as compared with cells containing achiral molecules. We propose that the spin specificity of electron transfer through chiral molecules is the origin of a more efficient oxidation process in which a molecule (either oxygen or disulfide) is formed in its triplet ground state. The lower over-potential is therefore due to a correlation that exists in the spins&’ alignment in the atoms composing these molecules. This finding opens the way for efficient production of hydrogen.
In the present study, we investigated a cell based on a TiO₂ electrode to which CdSe nanoparticles (NPs) are attached by molecules. The NPs serve as photocatalysts for hydrogen evolution from an aqueous Na₂S/Na₂SO₃ (sacrificial electrolyte) solution when irradiated with visible light. We investigated the evolution of hydrogen when the nanoparticles are tethered to TiO₂ via achiral and chiral molecules. Different chiral molecules were investigated, i.e. two oligopeptides having different lengths and double-stranded DNA. Correlation was found between the efficiency of these molecules as spin filters and the reduction in the over-potential.

Bottom-up control of the structure of nanoscale materials provides the opportunity to tune the absorption and scattering through the introduction of photonic crystals and photonic modes. The structural parameters of Si nanowires (NWs) grown by the vapor-liquid-solid mechanism can be manipulated axially through a selective doping and etching process, or axially through shell deposition, affording a controllable platform for developing structurally determined light interactions in nanoscale materials. The axial doping and etching process is used to construct photonic crystals along the axis of the NW, and the photonic properties of these structures are observed experimentally through spectroscopy along with a novel optical waveguide microscopy technique. The waveguide-based microscopy we have developed enhances control of the illumination polarization and wavevector in these experiments while simultaneously reducing background scattering. Improved photovoltaic properties from the shell deposition process are experimentally realized through the construction of radial P-I-N devices with dielectric shells deposited by plasma-enhanced chemical vapor deposition, which exhibit radial photonic modes based on the size and composition of the shell. Through theoretical studies and controlled synthesis, we are able to harness the unique interactions of nanoscale materials with light to produce enhanced photonic and photovoltaic properties in silicon nanowires.

Solar panels have become one of the most promising answers to green and renewable energy issues. Commercial mono c-Si solar cells have actually an efficiency of 22-25% which is mainly limited by the thermalization of high energy photogenerated carriers due to the mismatch between their energy and the c-Si band gap energy.
The quantum cutting (QC) is one of the interesting ways to improve the solar cells efficiency by overcoming the thermalization of those hot carriers. A conventional solar cell is then topped by a QC layer where a down converting (DC) effect takes place which consists in converting the incident UV-Visible photon energy into two IR photons which are more efficiently absorbed by Si solar cell. To create such a QC effect, many systems were studied such as Pr³⁺-Yb³⁺⁽¹⁾ or Tb³⁺-Yb^{3+ (2)} co-doped matrices. Unfortunately, up to now, the proposed host matrices were hardly compatible with the silicon technology. Thus, the last five years researches have been focusing on silicon integration: Zn₂SiO₄ :Tb³⁺ minus;Yb³⁺⁽³⁾, Er³⁺ in SiO₂⁽⁴⁾, or recently Ce³⁺-Yb³⁺ in SiO_x⁽⁵⁾ and in SiO_xN_y with Tb³⁺-Yb^{3+ (6)} co-doped systems by our group, revealing a semi-empirical additional external quantum efficiency of about 2%.
In this study, we propose to investigate the Ce³⁺-Yb³⁺ co-doped system in a SiO_xN_y matrix. Indeed, one of the advantages of this matrix is the reduction of the rare earth clustering observed in other matrices such as SiO_x. The UV energy is absorbed by Ce³⁺ ions between the 4f level and the 5d band. This energy is transferred by a cooperative energy transfer mechanism to two Yb³⁺ ions which emit at a 980 nm wavelength (1.2 eV), slightly above the Si gap of the solar cell.
Our layers are deposited by using a radio frequency co-sputtering technique, with help of CeO₂, Yb and Si targets under a nitrogen flow. The results reveal an intense photoluminescence of Ce³⁺ ions. Such intensity is optimized by means of several deposition parameters: target power, the nitrogen flow, deposition and annealing temperature. Structural investigations are performed with help of RBS and HRTEM. The ellipsometry experiment shows a refractive index between 1.9 and 3.8 at 1.95 eV with FTIR measurements revealing low oxygen content.
For the sample presenting the most intense Ce³⁺ emission, a co-doping with Yb³⁺ ions is carried out to test the cooperative energy transfer mechanism with the Ce³⁺ ions. Quantum efficiency of such mechanism is also investigated by using decay time measurements to estimate the quantum efficiency.
1. Li, D. H. et al.Phys. B Condens. Matter446, 12-16 (2014).
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The generation of renewable hydrogen through the photoelectrochemical water splitting route requires transfer of photogenerated electrons from the light harvester semiconductor to an efficient electrocatalyst [1]. Durability, stability and economic viability are also critical requirements those photocathodes need to face [2]. State-of-the-art inorganic semiconductors-based photocathodes are very limited and prone to photocorrosion [3] whereas organic ones are many more, offer greater flexibility but have been much less studied [4]. Recently, some organic semiconductors have been proved to be active towards hydrogen evolution [5] as well as stable in aqueous environment [6]. The Photogenerated hydrogen by organic catalytic systems (PHOCS) European project is devoted to such state of the art problem, based on the role of organic electronics for energy applications [7]. In this work we report our latest findings in producing and characterizing hydrogen evolving photocathodes based on hybrid organic/inorganic interfaces. The active layer concerns a conventional donor-acceptor bulk heterojunction blend while the charge selective layers were introduced to maximize charge collection. In this regard, nanostructured titanium oxide grown from the gas phase was used as an electron selective layer being critical for greater performance. Photocurrents densities as high as 8 mA/cm² at 0 V have been achieved exhibiting 100% faradic efficiency for hydrogen generation as well as positive and stable onset potentials, hence desirable towards a further tandem configuration. However, device performance is compromised by the porous nature of the inorganic nanostructured top layer, therefore some protective strategies and alternatives were assessed. These results pave the way towards the forthcoming use of organic semiconductors interfaced with inorganic nanostructures for hydrogen generation, thus strengthening a wider and more flexible exploitation of organic electronics for energy applications.
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7. www.phocs.eu

One of the most remarkable aspects of dye-sensitized solar cells is the possibility of manufacturing on flexible substrates. Therefore, it is vital to optimize a production technique (sequence) in order to manufacture large area cells and modules for mobile applications. Therefore, in this study we aimed to develop a highly versatile, cost and time effective fabrication process for flexible photoanodes for dye sensitized solar cells (DSSC). Considering the impossibility of heat treatment process in order to obtain a nanoporous TiO₂ layer for flexible photoanodes as in the commercial ones, electrophoretic deposition (EPD) and subsequent cold isostatic pressing (CIP) were optimized. However, upon large-scale production of TiO₂ nanoparticles, the batches inevitably contain both anatase and rutile phases. Therefore, using isoelectric point and zeta potential concepts of EPD processes, the anatase and rutile phases are separated during deposition of nanoparticles, eliminating the rutile phase. In order to achieve this property, several binder-free, low additive alcohol based colloidal solutions were prepared and their coating uniformity, morphology and phase separation performances are investigated. Comparing the EPD solutions using SEM, XRD, Zeta potential analysis methods, a low additive ethanol based solution has exhibited excellent coating uniformity performances eliminating almost all rutile phase from prepared photoanode. The cells were fabricated having 0.5x0.5 cm² , 1.0x1.0 cm² and 1.0x1.0 in² were investigated under AM1.5 conditions and efficiencies were obtained as 4.22%, 4.13% and 4.01% respectively.

We demonstrate a simple wafer-scale process by which an individual silicon wafer can be processed into a multifunctional platform where one side is adapted to replace platinum and enable triiodide reduction in a dye-sensitized solar cell (DSSC), and the other side provides on-board charge storage as an electrochemical supercapacitor. This builds upon electrochemical fabrication of dual-sided porous silicon, and subsequent carbon surface passivation for silicon electrochemical stability. The utilization of this silicon multifunctional platform as a combined energy storage and conversion system yields a total device efficiency of 2.1%, where the high frequency discharge capability of the integrated supercapacitor gives promise for dynamic load-leveling operations to overcome current and voltage fluctuations during solar energy harvesting. On a large scale, our advances in integrated energy storage and conversion technology may prove to be key to developing decentralized energy grids, which currently have low tolerance to overcome the variability introduced by intermittent generation, and allow for greater penetration of solar resources.

Semiconductor quantum dots (QDs) are intriguing light-harvesters and excited-state charge donors for solar energy conversion, due to their size-dependent optical properties and band-edge potentials, high oscillator strengths, and the possibilities of hot-carrier extraction and multiexciton generation. However, excited-state relaxation mechanisms of QDs are complex and multiexponential, which in turn complicates the mechanisms of photoinduced charge-transfer processes at QD-containing interfaces. In this regard, excited-state charge-transfer process of QDs differ fundamentally from than those of molecular chromophores.
This presentation will focus on the mechanism and dynamics of excited-state electron transfer from CdSe QDs to molecularly-linked TiO₂ nanoparticles with particular emphasis on the relative rate constants and quantum yields of electron transfer from band-edge vs. trap states of CdSe QDs. CdSe QDs were covalently linked to TiO₂ by using 3-mercaptopropionic acid (MPA) linker. Steady-state and time-resolved emission spectroscopy were used to study dynamics of electron transfer from CdSe QDs to TiO₂. The time-resolved photoluminescence decays were acquired using time-correlated single photon counting (TCSPC). Electrons were injected from both band-edge and trap states of CdSe with rate constants of (1.6±0.8)×10⁸ sec^-1 and (8±6)×10⁷ sec^-1, respectively. Rate constants of electron injection from trap states were measurably slower than from the band edge. Control experiments were done by using QD-MPA-ZrO₂ assemblies for which, due to the interfacial energetic offsets, electron transfer was assumed to be absent.
Ongoing experiments on charge-transfer dynamics of CdSe/ZnS core/shell QDs tethered to TiO₂ via MPA will be discussed as well. Our goals are to study the charge transfer in absence of trap states and to establish conditions in which electrons can be extracted efficiently from band-edge states without the deleterious decrease of the electron&’s potential energy upon trapping. This research provides new insight into the contributions of both bulk-like conduction band states and surface-localized trap states of QDs to charge-transfer processes relevant to solar energy conversion.
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Porphyrin molecules are common chromophores in both natural photosynthesis and in synthetic solar harvesting, such as the dye-sensitized solar cell. In order to harvest the energy of the photon the system has to i) absorb the photon, ii) facilitate charge separation and iii) inhibit charge recombination. Using non-adiabatic excited states molecular dynamics through real-time propagation TD-DFT we investigate how light interacts with the electronic structure of the porphyrin chromophore. The calculated absorption spectra reproduces the Soret band and Q band of the porphyrin molecule very well, allowing us to investigate the influence of the central metal-ion on the absorption. Further, we investigate electron transport properties of porphyrin-fullerene dyads bridged with pi-conjugated molecular wires. The photo-excited state in the chromophore is described using delta-SCF. The state is further propagated in time to investigate how the electron is transported from the chromophore to the C60 electron acceptor. This provides a microscopic insight to vibrational and steric contributions to charge transport properties in liked porphyrin-fullerene dyads.
Support from the Swedish Research Council (VR) is thankfully acknowledged. Simulations are performed on allocations from XSEDE, NERSC and Harvard RC.

Solar cell devices have attracted much interest from the research community because they are a sustainable and cost-effective technology. Dye-sensitized solar cells (DSSCs), in particular, have gained tremendous attention due to its ease of fabrication and low production cost. DSSCs also have a wide range of applications, such as building integrated photovoltaic (BIPV) technologies and flexible/wearable solar cells.
In an effort to improve DSSC photovoltaic characteristics, a fluorescence-assisted approach was considered, by using a fluorescent photoelectrode material made out of zinc oxide (ZnO) nanostructures. The fluorescence emissions from the photoelectrode layer can be used as a secondary light source to enhance photovoltaic characteristics. Furthermore, the transparency of ZnO allows it to be used as a photoelectrode for DSSCs.
Since both fluorescence and transparency are essential characteristics for developing a fluorescence-assisted DSSC photoelectrodes, transparent ZnO with fluorescence emission have been synthesized by adjusting synthesis conditions. By adjusting the length and diameter of 1-dimensional ZnO nanostructures, optical and electrical characteristics were optimized. Here we demonstrate the successful synthesis of transparent and fluorescent 1-dimensional semiconducting ZnO nanostructures for DSSC photoelectrode application.

Recently, Dielectric/Metal/Dielectric (DMD) electrodes as ITO alternatives have attracted much interest because of the unique optical and electronic properties as well as mechanical flexibility. However, there were still many problems in application of DMD electrodes to highly efficient top-illuminated organic solar cells (OSCs). First, there is Fresnel reflection loss at the air-DMD interface due to the highly complex refractive index (N = n - ik) of the top dielectrics, such as MoO₃, WO₃, V₂O₅, and ZnS. Second, DMD electrodes have peak transparency at a specific dielectric thickness for a given wavelength. Therefore, if the thickness of the dielectric layer deviates from the optimal thickness in the DMD structure, the electrode suffers from drastically reduced transmission. Third, nano-structural patterns have not yet been implemented on inorganic dielectric materials in the DMD electrodes, which are only produced by physical deposition. Even though the inductively coupled plasma (ICP) etching process, laser interference lithography, electron beam, and ion beam lithography can be used to produce nanostructures on inorganic dielectric materials, many of these techniques involve harsh processing conditions, which readily cause damages to organic active materials. This is why conventional DMDs have a planar geometry structure.
In this work, we report the novel design of nanostructured Dielectric/Metal/Polymer (DMP) with high transmittance and excellent haze in top-illuminated OSCs, resulting in increase of efficiency by 28% compared to bottom cells. This improvement was achieved by overcoming critical problems in planar Dielectric/Metal/Dielectric (DMD) based solar cells. To design advanced structure, PDMS, known as a hybrid polymer with dielectric property, was employed instead of an inorganic dielectric layer. With the low refractive index (n = 1.45, k = 0.0001) of PDMS, a transparency insensitive to the polymer thickness up to 300 mm could be achieved even in on nanostructured electrode. Consequently, incident light could pass through the PDMS layer, without reflection loss, resulting in the enhanced transmittance. Additionally, we integrated inverted hexagonal pyramid patterned PDMS with on front side of MoO₃/Ag by a simple lamination technique without damages to organic active layer. The nanostructured MAP allows incident lights to be diffracted to enhance light path length in the active layer, resulting in enhanced omnidirectional optical characteristics and power conversion efficiency of 6.75 %. From the device simulation using rigorous coupled wave analysis software, we inferred that the lateral distribution of light propagation due to nanostructures could increase light path length in the active PTB7:PC70BM layer. Finally, the results offer new opportunities in development of top-illuminated OSCs based on flexible metal foil or stainless steel substrates, which assist industrialization of very cost-efficient solar modules.

Lead halide perovskites have achieved phenomenal successes in photovoltaics due to their suitable bandgaps, long diffusion lengths, and balanced charge transport. However, the extreme susceptibility of perovskites to water or air has imposed a seemingly insurmountable barrier for leveraging these unique materials into solar-to-fuel applications such as photoelectrochemical conversion. Here we developed a CH₃NH₃PbI₃-based photoanode with an ultrathin Ni (8 nm) surface layer, which functions as both a physical passivation barrier and a hole-transferring catalyst. Remarkably, a much enhanced photocurrent density, an unassisted photoelectrochemical conversion capability, and a substantially better stability against water have been achieved, which are exceeding most of the previously reported photoanodes as well as a similar CH3NH3PbI3-based device structure but without the Ni surface layer. The photocurrent density is measured over 10 mA/cm2 in 0.1 M Na₂S at 0 V vs Ag/AgCl, this PEC photocurrent density can be maintained over 2 mA/cm² after 15minus;20 min of continuous PEC tests, which is strikingly phenomenal for the CH₃NH₃PbI₃ electrode and also substantially higher than the similar device structure but without Ni coating. Ni 2p spectra for the same sample taken after three different Ar ion milling times into the surface reveal the gradual change from oxidized Ni to metallic nickel. This leads to a fast transfer of the photogenerated holes from the inner CH₃NH₃PbI₃ layer toward the external Au and Ni surface and subsequently to the electrolyte for oxidation, as confirmed by the significantly increased photocurrent, which in the meantime suppresses the photocorrosion of the electrode. In comparison, additional Au layers with equal or even much larger thickness only yield a negligible effect on both the PEC activity and stability enhancement, indicating the imperative role of the Ni surface layer. Our study indicates that the recent research breakthroughs in perovskite can be well inherited into the PEC field for hydrogen production and artificial photosynthesis, and the development on water-resistive perovskite devices can also be leveraged to improve long-term stability of perovskite solar cells under ambient conditions. The perovskite solar cell with 8 nm Ni coating maintains much higher power conversion efficiency than bare perovskite solar cell after being stored in ambient environment for 12 days. Further improvement on the Au and Ni layer fabrication quality such as using molecular beam epitaxy (MBE) may lead to higher stability of the perovskite photoelectrodes.

We present the optical constants determined from spectroscopic ellipsometry of single crystals of CH₃NH₃PbX₃ (where X = I, Br, Cl). These results are interpreted by comparison with optical constants and band structure derived from the highest level of ab initio calculations using the relativistic the quasiparticle self-consistent GW approximation (QS-GW). The calculations reproduce the majority of observed absorption features. However the increasingly strong absorption peak observed at the band edge moving from X = I to Br to Cl attributed to excitonic absorption is not predicted by QS-GW since electron-hole interactions are not accounted for by the theory.
Strikingly, the analysis shows that transitions from the highest valence band (VB1) to the lowest conduction band (CB1) at different momenta are responsible for all three of the main absorption peaks at (1.6, 2.5 and 3.1 eV) in CH₃NH₃PbI₃ (only minor contributions from the second highest VB and to the second lowest CB are involved).
This observation of is particular relevance to the interpretation of transient absorption spectroscopy (TAS) measurements, the nature of which are the subject of considerable debate. We use our optical constants and energy band diagram of CH₃NH₃PbI₃ to simulate its TAS and find good agreement with measurement. The results suggest that the bleaching states observed at 480 and 760 nm can be assigned to a reduction in the VB1 → CB1 transition at two different regions of momentum space. It is not necessary to invoke additional transitions from VB2 to explain the spectra.
Our calculations also indicate that the orientation of CH₃NH₃ cations can have a significant influence (up to ~0.14 eV) on the position of the bandgap suggesting that collective orientation of the organic moieties could result in significant local variations of the optical properties.

We report here normal planar structured perovskite solar cell without I-V hysteresis via interfacial engineering. In normal planar structure with cell configuration of FTO/thin film TiO₂/perovskite/hole transport material/Ag, considerable I-V hysteresis has been unavoidably observed. To solve this problem, we analyzed potential factors affecting I-V hysteresis, such as ferroelectricity, ion migration and interface using a simple equivalent circuit. Since little hysteresis has been reported from the cell configuration being analogue to organic photovoltaics, bulk property of perovskite layer may not be mainly involved in I-V hysteresis. Interface between perovskite and thin film TiO₂ or FTO was modified to reduce interfacial capacitance and by doing so we realized no I-V hysteresis in the normal planar structure. Charge mobility and extraction was analyzed by photo induced charge extraction by linearly increasing voltage method (photo-CELIV). A capacitance component of perovskite-substrate interface was analyzed by impedance spectroscopy. Interfacial modification led to changes in charge extraction property and capacitance, which was found to be responsible for no I-V hysteresis. Theoretical simulation further supported the observed results. Among the interfaces contacting perovskite layer, we extracted the interface contributing to mainly I-V hysteresis by investigating different selective contacts and hole transport materials.

Organo-lead-halide perovskite based solar cells are the new solar cells that hold promises for large-scale solar-to-electricity conversion at low-cost. Since the first few reports, optimization in perovskite synthesis and device architecture has led to record power conversion efficiencies (PCEs) as high as 20.1% in just a few years. On the other hand, there are few studies on the lifetime and degradation mechanisms of fabricated OHP-based solar cells under operation conditions. In this work, we have studied the stability of FTO/compact-TiO₂/mesoporous-Al₂O₃/MAPbBr₃/spiro-MeOTAD/Au cells under continuous light illumination (AM1.5G) and applied bias tracking the maximum power point (MPP) for 100 hours. The cells were kept in controlled environment of dry-N₂ and air with controlled relative humidity (~30%). We have taken the caution to determine the steady-state solar cell parameters (PCE, Jsc, Voc, and FF) because of large hysteresis effects observed on MAPbBr₃ based solar cells. Time-evolution of stability profile operated in N₂ and air showed the complexity in behavior. Post-mortem analysis of cells and systematic studies of individual layers by multiple techniques (X-ray photoelectron spectroscopy X-ray diffraction, X-ray fluorescence, UV-vis) reveals the mechanisms for the solar cell behaviors.

Ammonium metal halide perovskite solar cells (PSCs) have attracted great attention recently as a new research hotspot in the fields of photovoltaic devices. Solar cells with over 20% power conversion efficiency (PCE) have been reported using ammonium lead iodide perovskite as an absorbing layer¹. 2,2prime;,7,7prime;-tetrakis(N,N-di-p-methoxyphenylamine)-9,9prime;-spirobifluorene (Spiro-MeOTAD) is one of the most commonly used hole transport materials in PSCs. However, the intrinsic conductivity of Spiro-MeOTAD is relatively low unless it is doped with bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) to oxidize the Spiro-MeOTAD. The mechanism for the generation of oxidized Spiro-MeOTAD in PSCs with LiTFSI is still under debate. Here we report a spectrum-dependent mechanism for the formation of oxidized Spiro-MeOTAD with LiTFSI. In our research, optical long pass filters are integrated with a solar simulator to control the spectral range and subsequently illuminate our samples. According to ultraviolet-visible spectroscopy (UV-Vis) and four-point probe conductivity measurements, two different Spiro-MeOTAD oxidization processes are observed depending on the spectral range of illumination and the participation of ammonium metal halide perovskite. The influence of oxidized Spiro-MeOTAD on device performance is investigated with J-V test and electrochemical impedance spectroscopy (EIS) as well. This is further confirmation of the spectrum-dependent mechanism.
1 W.S. Yang, J.H. Noh, N.J. Jeon, Y.C. Kim, S. Ryu, J. Seo, S.I. Seok, Science, 348, 1234 (2015)

We report highly reproducible perovskite solar cell with power conversion efficiency (PCE) of 19.7% via acid-base adduct approach. Adduct of PbI₂ formed with Lewis base N,N-dimethylsulfoxide (DMSO) and iodide (I^-) in methylammonium iodide (CH₃NH₃I). Contrary to conventional perovskite precursors including PbI₂ and equimolar CH₃NH₃I in N,N-dimethylformamide (DMF) solvent, we prepared a perovskite solutions with additionally equimolar DMSO (1:1:1 mol%) in order to form a well-defined CH₃NH₃Ibull;PbI₂bull;DMSO adduct film, where the solvent DMF was selectively dissolved. The transparent adduct film was converted to a dark brown perovskite film by removing the DMSO in the adduct film at mild temperature by taking advantage of volatile characteristics of DMSO in adduct. Infrared (IR) spectroscopic studies showed that the S=O stretching frequency of DMSO at 1045 cm^-1 shifted to lower frequency of 1015 cm^-1 upon reacting DMSO with PbI₂ and CH₃NH₃I, which is indicative of adduct formation. Charge extraction and mobility characteristics of the perovskite fabricated by the adduct approach were improved, compared with the conventional perovskite fabricated without DMSO-induced adduct. In addition, pinhole free large grains were obtained by adduct approach. As a result, a best PCE as high as 19.7% was achieved along with average PCE of 18.3%.

The rapid advancement of thin-#64257;lm photovoltaic (PV) technology increases the real possibility of large-area Si-based tandems reaching 30% ef#64257;ciency, although light in these devices must be managed carefully. We identify the optical requirements to reach high ef#64257;ciencies. Strict conditions are placed on material parasitic absorption and transmission of contacts: Absorption of 20% of sub-bandgap light leads to the required top-cell ef#64257;ciencies of 18% at a bandgap of 1.5 eV to break even and 23% to reach tandem ef#64257;ciencies of 30%. Perovskite-silicon tandem cells present the #64257;rst low-cost devices capable of improving standalone 25% ef#64257;ciencies and we quantify the ef#64257;ciency gains and reduced thickness afforded by wavelength-selective light trapping. Applying these principles to a perovskite-based top cell characterized by strong absorption and high luminescence ef#64257;ciency, our modeling shows that tandem ef#64257;ciencies greater than 30% are possible with a bandgap of E g = 1.55 eV and carrier diffusion lengths less than 100 nm. At an optimal top-cell bandgap of 1.7 eV, with diffusion lengths of current vapor-deposited CH₃NH₃PbI_xCl_{1 minus; x} perovskites, we show numerically that tandem ef#64257;ciencies beyond 35% are achievable with careful light management.
We also use spectrally-resolved photoluminescence to experimentally determine the absolute value of the band-to-band absorption coefficient of perovskite methylammonium lead iodide CH₃NH₃PbI₃ film from 675nm to 1400nm. Unlike other methods that are used to extract the absorption coefficient, photoluminescence is only affected by the band-to-band absorption and is capable of detecting absorption events at very low energy level. Absorption coefficient values as low as 10^-14cm^-1 are detected at room temperature, which extends the range of absorption data for this material by 14 orders of magnitude.
Finally, we study the band-to-band absorption enhancement due to various types of light trapping structures with photoluminescence (PL) on monocrystalline silicon wafers. Four basic light trapping structures are examined: reactive ion etched texture (RIE), metal-assisted etched texture (MET), random pyramid texture (RAN) and plasmonic Ag nanoparticles with a diffusive reflector (Ag/DR). We also compare two novel combined structures of front side RIE/rear side RAN and front side RIE/rear side Ag/DR. The results show that by combining RIE with RAN and Ag/DR, we can fabricate two structures with excellent light trapping efficiencies of 55% and 52% respectively, which is well above previously reported values for similar wafer thicknesses. A comparison of the measured band-band absorption and the EQE of back-contact silicon solar cells demonstrates that PL extracted absorption provides a very good indication of long wavelength performance for high efficiency silicon solar cells.

Organometal halide perovskites have raced to the forefront of photovoltaic and light emission research due to their unique optical and electrical properties. While most of the current research has focused on thin films, the use of nanoparticles could further enhance the potential of this new material. To this end we synthesize colloidal perovskite nanocrystals with different solution-based fabrication strategies. By specifically selecting ligands and the precursor materials used we are able to not only control the morphology and dimensionality of the resulting nanocrystals, but can tune their optical and electronic properties as well. We look into quantum confinement in perovskite nanoplatelets, especially focusing on the properties of the exciton in these highly confined systems. Additionally, we vary the precursors to further tune the bandgap of such structures and we investigate energy transfer between such nanostructures as well as exciton diffusion.

The reported efficiencies of devices based on organometal halide perovskites (OMHPs) are rapidly increasing, however a solid understanding of the photophysical properties is still lacking. In this work, we study the kinetics of charge carriers in 200 nm thin films of CH₃NH₃PbI₃ prepared by sequential physical vapour deposition (PVD) of the precursors CH₃NH₃I and PbI₂. SEM and AFM microscopy show that this preparation method yields smooth films with a thickness variation less than 2 nm. The X-ray diffraction pattern shows only the (110) and (220) directions. Terahertz spectroscopy and time-resolved microwave conductivity (TRMC) are used to examine the opto-electronic properties of the perovskite layers. Mobilities are > 50 cm²/Vs, which are related to large crystalline domains within the film. In addition, for low light intensities (<10¹² photons/cm²) we find no relaxation of the charge carrier mobility on time scales up to 1 ns. We apply a kinetic model to describe the TRMC traces recorded using laser intensities varying over 3 orders of magnitude with one set of kinetic parameters, yielding a trap density of less than 2 × 10¹⁵ cm^-3 and a dark carrier concentration of 4 × 10¹⁵ cm^-3. Altogether, we show that stoichiometric PVD results in perfectly flat nearly intrinsic films in which the charge carrier mobilities are substantially higher than in solution-processed OMHPs.

Developing a strategy that enables perovskite solar cells to show high performance and simultaneously exhibit long term stability is one of the main challenges to exploit the true potential of perovskites for photovoltaic applications. Herein, we demonstrate that using judiciously designed aminoalkylphosphonates to modify the surface of methylammonium lead triiodide (MAPbI₃) perovskite allows controlling the crystal nucleation and growth leading to dramatic enhancement in device performance and durability. We apply a simple one-step method, which involves spin-coating MAPbI₃ precursor solution containing 4-aminobutylphosphonic acid chloride (4-ABPACl). Using STEM-EDS, XRD and ¹H NMR analyses, we establish that 4-ABPACl surround the MAPbI₃ structures via strong hydrogen bonding of the -PO(OH)₂ and -NH₃⁺ terminal groups to the perovskite surface affording crosslinking of MAPbI₃ structures. Such architecture not only achieves a dramatic increase of the photovoltaic performance from 8.8% to 16.7% but also enhances greatly the cell stability in ambient condition or at 85 ^oC.

Semiconductor nanowires are among the most promising candidates for next generation high-efficiency photovoltaics. This is primarily due to their outstanding optical and electrical properties which provide large optical cross sections while simultaneously decoupling the photon absorption and charge carrier extraction length scales. These effects relax the requirements for both the minority carrier diffusion length and the amount of semiconductor necessary for the optimal utilization of the incident light power. This talk will detail recent efforts in our group focused on synthesis and characterization of nanowires for solar energy conversion. Synthetic descriptions will focus on novel metal-semiconductor epitaxial core-shell nanowires grown in solution as well as novel nanowire materials (e.g. Lenaite AgFeS₂) and traditional growth mechanisms applied to new materials (VLS growth of CH₃NH₃PbI₃ hybrid perovskite). Next a novel method for 3D mapping of the internal structure of core-shell nanomaterials, using only a standard SEM operated at a variety of voltages will be presented. Finally, a unique laser spectroscopy setup that allows for diffraction limited mapping of single nanowire solar cell absorption, photocurrent, external and internal quantum efficiency, and photoluminescence will be described and applied to Si, Cu₂O, InP and CH₃NH₃PbBr₃. By combining all these different characterization methods on the same nanowire solar cell, we hope to gain new insights into the working mechanisms and limiting factors in conversion efficiency.

Thin film solar cells have advantages of being low-cost and flexible. However, materials such as amorphous silicon (a-Si), Cadmium telluride (CdTe), etc., have relatively lower power conversion efficiency as compared with the conventional crystalline silicon solar cells. Meanwhile, bendability and reliability of the devices also depend on adhesion of the thin film materials on the flexible substrate, especially when plastic substrates are used. Introducing substrate nano/micro-texture is one of the promising methods to improve the performance of the thin film solar devices by enhanced optical absorption. Recently, we have developed a set of low cost and scalable approaches to fabricate various three-dimensional (3-D) nanostructures on metallic and plastic flexible substrates. By controlling the periodicity and the roughness of the nano-textures, it is discovered that optical absorption and the energy conversion efficiency of amorphous Si and organometallic perovskite materials based thin film solar cells can be largely improved over the planar counterparts. In addition, experiments suggest that thin film solar cells fabricated on nanostructured substrates also have significantly improved reliability. These results can shed light on design optimization of high performance flexible thin film solar cells.

Light trapping in sub-wavelength semiconductor nanowires offers a promising approach to simultaneously reducing material consumption and enhancing photovoltaic performance. Nevertheless, the absorption efficiency of a nanowire, defined by the ratio of optical absorption cross section to the nanowire diameter, lingers around 1 in existing nanowire photonic devices, and the absorption enhancement suffers from a narrow spectral width. Here, we show that the absorption efficiency can be significantly improved in nanowires with higher refractive indices, by an experimental observation of up to 350% apparent external quantum efficiency (EQE) in lead sulfide (PbS) nanowire resonators, a 3-fold increase compared to Si nanowires. Furthermore, broadband absorption enhancement is achieved in single tapered NWs, where light of various wavelengths is absorbed at segments with different diameters. Overall, we fabricate and demonstrate single PbS nanowire Schottky solar cells with a power conversion efficiency (PCE) of 3.9%, which is comparable to single Si nanowire coaxial p-n junction cells, but with much simpler fabrication processes. The high efficiency is enabled by a combination of optical resonance, near bandgap open circuit voltage, and long minority carrier diffusion length. This work provides valuable insights for optimizing light absorption and solar conversion efficiency by harnessing the resonance absorption particularly of high refractive index materials and engineering the doping concentration of semiconductor nanowires.

Semiconducting nanowires have been recognized as promising materials for high-performance electronics and optics where optical and electrical properties can be tuned individually. The feasibility of III-V nanowire integration with existing silicon processing technology due to the small footprint between the silicon substrate and the nanowire material has further sparked that interest. For NWs to provide the new architecture for next generation photovoltaics there is a strong need to take complete control over synthesis. By optimizing growth conditions with respect to tapering we created nanowire-InP nanowire based solar cells using Au seed particles for growth.
We will report on the growth, processing and characterization of nanowire array-based solar cells with 13.8 % efficiency [1]. First, gold particles were patterned on InP substrates with a 500 nm pitch, using nanoimprint lithography. Then, about 1.5 mu;m long InP nanowires were grown using DEZn and TESn as doping precursors, to create an axially defined p-i-n junction. HCl was used to prevent radial overgrowth. The nanowires were processed as-grown with a transparent conductive oxide as top contact to create 1x1 mm2 solar cells, with 4 million nanowires per cell.
The solar cells were investigated using a sun simulator at Fraunhofer ISE CalLab reference setup. Although the 180 nm-diameter NWs only covered 12 % of the surface, the photocurrents were 71 % of the theoretical maximum for an InP solar cell. This is six times the limit in a simple ray optics description, and comparable to the record planar InP cell. To understand the absorption, we used three-dimensional electromagnetic optical modeling [2, 3]. We find excellent agreement between the spectra of modeled absorption and the experimentally measured external quantum efficiency.
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)

InP nanowire solar cells have been proven to be among the most promising candidates for next generation photovoltaics by surpassing the energy conversion efficiency of their traditional counterpart, InP thin film solar cells. Their low surface recombination velocity and close to ideal band gap of ~ 1.4 eV make them an ideal platform to study the limits of semiconductor nanowire based solar cells. Here we present for the first time spatially resolved quantitative absorption and internal quantum efficiency (IQE) measurements of single InP p-i-n nanowire solar cells. We show how those measurements can be used to gain more insight into the light absorption, charge carrier separation and extraction mechanisms present in a device. Finally we use those insights to study the effects of surface functionalization on the nanowire solar cell performance.

Achieving directional charge transfer across semiconductor interfaces requires careful consideration of relative band alignments. Here, we demonstrate a promising tunable platform for light harvesting and excited-state charge transfer based on interfacing β-Pb_xV₂O₅ nanowires with CdSe quantum dots. Two distinct routes are developed for assembling the heterostructures: linker-assisted assembly mediated by a bifunctional ligand and successive ionic layer adsorption and reaction (SILAR). In the former case, the thiol end of a molecular linker is found to bind to the quantum dot surfaces, whereas a protonated amine moiety interacts electrostatically with the negatively charged nanowire surfaces. In the alternative SILAR route, the surface coverage of CdSe nanostructures on the β-Pb_xV₂O₅ nanowires is tuned by varying the number of successive precipitation cycles. High-energy valence band X-ray photoelectron spectroscopy measurements indicate that “mid-gap” states of the β-Pb_xV₂O₅ nanowires derived from the stereoactive lone pairs on the intercalated lead cations are closely overlapped in energy with the valence band edges of CdSe quantum dots that are primarily Se 4p in origin. Both the midgap states and the valence-band levels are in principle tunable by variation of cation stoichiometry and particle size, respectively, providing a means to modulate the thermodynamic driving force for charge transfer. Steady-state and time-resolved photoluminescence measurements reveal dynamic quenching of the trapstate emission of CdSe quantum dots upon exposure to β Pb_xV₂O₅ nanowires. This result is consistent with a mechanism involving the transfer of photogenerated holes from CdSe quantum dots to the midgap states of β-Pb_xV₂O₅ nanowires.

Due to their cost effective and flexible nature, organic photovoltaics are considered as a new and viable alternative for the silicon-based solar cells. However, power conversion efficiency (PCE), which depends on different factors (such as the morphology of the photoactive materials used, structure of the cell, and fabrication technique), is low due to the low conductivity and narrow absorption spectra. Since highly ordered nanoscale morphologies of the photoactive materials are ideal for the development of high efficient organic photovoltaics, self-assembly is an attractive pathway to introduce order from the nanoscale to the macroscale in order to enhance transport properties and, as a result, efficiency. In this study, self-assembled rosette nanotubes (RNTs) functionalized with porphyrin and/or oligothiophene with thermodynamically stable structures have been used as photoactive layer. Our results have shown that organic solar cells made of RNTs have significantly higher PCE than the corresponding non-assembled (disordered) devices.

Nano-structuring the photoactive semiconducting materials have become an attractive alternative path for increasing the efficiency of photovoltaic devices. However, maintaining the monodispersity over the entire nanostructured pattern still presents a significant challenge to overcome since the overall functionality of nanodevice is highly dependent on the morphology of the individual nanofeatures. In this report, we describe a method for fabrication of vertically aligned and highly ordered copper indium selenide (CISe) nanotube and nanorod arrays for solar energy conversion. These ordered nanostructures were grown by confined electrodeposition on nanoelectrode patterns defined by electron beam lithography. The simplicity and the ability to manipulate morphological parameters such as nanotube/nanorod diameter, length, tube wall thickness, and the gap between nanotubes/nanorods with a high degree of precision and reproducibility throughout the entire pattern make this protocol especially attractive for the study of effect of physical dimensions of the photoabsorber on the solar energy conversion efficiency. Photoelectrochemical characterization of the as prepared nanotube/nanorod arrays show that a photocurrent comparable to that from a thin film device fabricated under similar conditions could be obtained from the nanodevice with the use of only a fraction (~10%) of the photoactive material compared to the thin film device. These nanotube/nanorod arrays have been characterized through powder X-ray diffraction, SEM, EDS, and UV-Vis studies for elucidating their structure, morphology, elemental composition and electronic nature. Photoconversion efficiency has been studied through photoelectrochemical studies in a liquid junction cell.

Plasmonic enhancement of mesoscopic solar cells, including dye-sensitized solar cells (DSSCs) and perovskite-sensitized solar cells (PSSCs), has gained tremendous attention due to enhanced light harvesting characteristics generating 30-60% improvement in power conversion efficiencies as well as straightforward incorporation into standard processing techniques. Despite the progress, however, the fundamental mechanism that contributes to enhanced light harvesting and improved charge injection and charge separation remains poorly understood. Here we have investigated the impact of incorporating Au/Ag core/shell bimetallic metal nanostructures in three different mesoscopic solar cell systems: N719 sensitized DSSCs, methylammonium lead triiodide (MAPbI₃) PSSCs, and methylammonium lead tribromide (MAPbBr₃) PSSCs. We have utilized transient absorption spectroscopy to examine exciton generation, electron injection, and charge recombination within the various systems over a range of nanoparticle loadings and compared these parameters to non-enhanced devices. Our results demonstrate plasmon-enhanced light trapping and corresponding charge-carrier generation occurs at time scales of several hundred picoseconds, suggesting metal nanostructures increase light absorption in the active layer of solar cells, enhancing the number of excitons generated. Therefore, a larger number of electrons is available to rapidly transfer to the TiO₂ conduction band when plasmonic nanostructures are present, which results in faster electron injection into the TiO₂ conduction band before recombination can occur in the bulk. We have provided a detailed mechanistic understanding of our results. This fundamental insight will ultimately result in implementation of plasmonic enhancement to other mesoscopic and planar solar devices to achieve unparalleled light harvesting characteristics.

In this work, we demonstrate that woodpile (WP) and inverse woodpile (IWP) photonic crystal nanocrystalline silicon (nano-Si) structures may be engineered for light trapping in solar cells. We use finite difference time domain simulations to show that the geometry of these photonic crystals may be varied such that there is an increased photonic density of states above the band gap. We additionally show that the utilization of a silver back contact as opposed to a perfectly electric conductor only decreases the solar absorption by a small amount. Finally, we show that these photonic crystals exhibit enhanced absorption over a wide range of incidence angles.
For a fixed equivalent thickness of 200 nm, we found a 104% and 77% absorption enhancement in the optimized WP and IWP structures respectively compared to nano-Si thin film. The WP and IWP structures have significantly superior absorption than the nano-Si thin film in almost the whole range of the solar spectrum. These structures have several additional resonance peaks in the infrared and visible parts of the spectrum. There is an increased photonic density of states in these parts of the spectrum, such that the absorption is enhanced. We also investigate the dispersive band structures for the IWP and WP structures. This study demonstrates a new structure for light trapping over a broad range of photon energies and incidence angles. Other equivalent thicknesses are additionally studied.

Nitrogen doping was found to extend lanthanum dititanate&’s (LTO), La₂Ti₂O₇, absorption from 380 nm to 550 nm without decreasing ultraviolet photoactivity, giving a promising 2.3 eV bandgap for solar water splitting. In this presentation, transient absorption spectroscopy with a supercontinuum and THz probe confirm N-doping creates a continuum of states above the intrinsic LTO&’s valence band (VB), shrinking the band gap without diminishing excited carrier lifetimes. The supercontinuum probe reveals excited state absorption and sub-band gap state densities match density functional theory (DFT) predictions. The lifetime is measured for excitation at the VB-CB and dopant continuum-CB transitions, showing that the added states increase initial thermalization of excited carriers but maintain lifetimes comparable to intrinsic LTO. The THz probe confirms the visible light excited carriers are mobile and not trapped, allowing a large increase in photoactivity. Further, by adding reduced graphene oxide (RGO) and plasmonic gold nanoparticles to the N-doped LTO, charge extraction and light absorption is improved, tripling the hydrogen generation rate.

The oxidation of cermet solar absorbers has become one of the major concerns in high-temperature concentrated solar power (CSP) systems. The prevention of metal nanostructure oxidation in cermet absorbers and the development of atmospherically stable CSP systems at high working temperatures are thus very important. Recently, we have demonstrated a high-optical-performance, solution processed Ni nanochain/SiO_x with promising anti-oxidizing behavior at temperatures up to 600 °C for several hours in air, while the mechanism and interfacial transformation are not completely understood [1]. In this paper, we focus on the interfacial analysis of these anti-oxidation high-performance Ni nanochain-SiO_x (x<2) selective solar thermal absorbers, studying interfacial formation of nickel silicides at the Ni/SiO_x interface and the corresponding mechanisms of the anti-oxidation behavior. To further enhance the anti-oxidation effect, we investigate high-temperature (>900 ^oC), pre-operation annealing in N₂ ambient to form desirable nickel silicide phases at the Ni/SiO_x interface, as confirmed by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). A remarkable saturation of the oxidation process shows up after annealing at 550 °C in air for 12 h from X-ray diffraction (XRD) and optical reflectance spectra analyses. The oxidation vs. time curve derived from XRD shows clear saturation behavior deviating from the kinetic Deal-Grove oxidation model. In addition, air annealing at > 550 °C for up to 32 h doesn&’t induce any further impact on the optical or structural properties of the solar thermal coatings. Note that the surface plasma polariton (SPP) response in metal nanostructures can be used as a very sensitive technique to detect any interfacial changes. Therefore, the optical reflectance spectra evolution of these plasmonic structures can be associated with the interfacial formation of Ni oxide and silicide, as well as self-terminated oxidation upon 12h annealing at 550 °C. These results are unusual considering that the average diameter of the Ni nanoparticles is only 80 nm. They strongly suggest the possibility of anti-oxidizing Ni/SiO_x cermet solar selective absorbers towards a thermodynamically stable state via interfacial engineering. More promisingly, the principles of self-terminated oxidation could apply to many other metal nanomaterials to achieve desirable performance.
Reference
[1] X. B. Yu, X. X. Wang, Q. L. Zhang, J. C. Li, and J. F. Liu, J. Appl. Phys. 116, 073508 (2014).

Complex three-dimensional (3-D) super structural titanium dioxide (TiO₂) architectures with controlled shape and size were synthesised by facile hydrothermal synthesis route using titanium tetrachloride (TiCl₄) as the metal precursor and hydrochloric acid (HCl)/ammonium chloride (NH₄Cl) as the mineraliser/structure directing agent. Interesting hierarchical architectures like nanoparticles assembled spheres, nanowires assembled spheres, nanorods assembled spheres, and nanosheets assembled spheres were achieved by controlled variation of crystal structure modifier. The synthesised materials were characterised in detail by different analytical techniques and the findings are consistent with each other. A detailed mechanism of formation for these hierarchical architectures is postulated based on detailed analysis. The as-prepared TiO₂ architectures exhibit superior performance as visible light photocatalyst for the degradation of Rhodamine-B (RhB) and photoanode for dye sensitized solar cells.
Owing to its unique physico-chemical and opto-electronic properties TiO₂ is considered as, one of the most important metal oxide semiconductor and its potential importance have been demonstrated in many areas of science and technology.¹ Over the years several forms of TiO₂ hierarchical architectures have been reported.² However, the development of facile and effective methods for creating novel super structural architectures which can perform better than the existing materials is a topic of research and remains a great challenge for materials society.³ Herein, we demonstrate in detail the preparation of 3-D hierarchical rutile TiO₂ architectures with controlled morphologies through facile hydrothermal synthesis method notably without use of any surfactant or template. The synthesized materials were characterized extensively by XRD, TG-DTA, micro-Raman, and UV-DRS. Surface area and pore size distribution were determined from N₂ adsorption and desorption isotherms. Morphological evaluations of the materials were carried out by electron microscopy in scanning and transmission mode. The TiO₂ architectures were used as photocatalyst for the degradation of Rhodamine-B (RhB) and photo-anode for DSSCs. The results demonstrate that the synthesised material exhibits ~10 times better enhanced photocatalytic activity towards the degradation of RhB compared to commercial TiO₂. The DSSCs thus fabricated exhibited ~40% much higher energy conversion efficiency compared to commercial P25. This could be attributed to the unique hierarchical structure, high surface area, higher light scattering ability, and lower electron recombination rates.
References
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We present a new approach to designing highly-antireflecting and broadband surface nanotextures with densely packed feature sizes smaller than 50 nanometers using block copolymer self assembly. Block copolymer thin films provide a robust method for generating regular, uniform patterns at length scales in the range of ten nanometers, over arbitrarily large areas. A significant advantage of this approach is its ease of integration with all other aspects of traditional thin-film processing, including plasma-based etching and metallization.
Here, we precisely tailor the dimensions of the nanotexture design in the range of ~10 to 70 nanometers, reducing the average reflectance from a polished silicon wafer to less than 1 percent across a broad wavelength range from 350 nm to 1 micron using textures less than 200 nm tall. The surface nanotextures maintain less than 5 percent broadband reflectance even for incident light angles as high as 60 degrees. Implementing this efficient antireflection approach in crystalline silicon solar cells significantly betters the performance gain compared to an optimized, planar antireflection coating.
We will further highlight the richness of this self-assembly based fabrication approach through examples of its implementation in silicon, silicon nitride, glass, and plastic thin films, and its ability to render solar cell surfaces highly water repellent.

Solar cells can employ semiconductor-sensitized nanostructured architectures to decouple the functions of light harvesting and charge transport into different materials. This allows use of inexpensive materials and processing routes that would not be possible in planar solar cells. We report on extremely thin absorber (ETA) solar cells that use ZnO nanowire arrays coated with an ultrathin (<5nm) CdS buffer layer and a thin (10-40 nm) CdSe absorber layer. CdSe absorbs visible light, and photoexcited electrons are injected into ZnO while photoexcited holes oxidize the redox species in an electrolyte. After charge separation, transport of electrons through ZnO and holes through liquid electrolyte must occur faster than interfacial recombination. Understanding the fundamental carrier dynamics of charge transport and interfacial recombination in ETA cell is vital to cell design.
Our approach correlates I-V characterization with transient photocurrent (TPC), transient photovoltage (TPV), and electrochemical impedance spectroscopy (EIS) to study electron transport and interfacial recombination in ETA cells. I-V characteristics of our ETA cells reveal that Jsc is maximized with a CdSe thickness of ~30 nm for a given nanowire array, which balances light harvesting and charge collection effects. TPC at short circuit showed electron transport times of 70 mu;s for ETA cells made from 500 nm long nanowires coated with 10 nm of CdSe. Transport times increased with nanowire length but were independent of CdSe thickness for thin absorbers, indicating that transport through the nanowires dominates the collection time. Recombination times were on the millisecond scale, as measured by TPV and EIS at open circuit. Transport times are much faster than recombination times, indicating efficient charge collection. Interestingly, transport times decreased 10-fold for CdSe coatings above a critical thickness of 40 nm, which is estimated by Mott-Schottky analysis to be the depletion width. Additionally, cells with thin CdSe coatings showed reasonably good fill factors, but thicker CdSe coatings exhibited significant bias-dependent collection. Bias-dependent EIS revealed that cells with the thinnest CdSe shells had the highest recombination resistance. Though counterintuitive in the context of pinholes, this result indicates that the depletion region formed at the electrolyte-CdSe interface extends into the ZnO for thin CdSe, thus suppressing interfacial recombination but also slowing transport within the nanowires by confining electrons to a very narrow core. In contrast, when the CdSe thickness exceeds the depletion width, photocurrent is limited by charge separation across the CdSe coating; recombination resistance is low but independent of bias. These results reveal the importance of managing relative length scales of absorber coating thickness and depletion width to control charge separation and transport in semiconductor-sensitized nanostructured solar cells.

Methylammonium lead halides (MAPbX₃) are promising materials for photovoltaic applications, but the physical processes behind their performance are not yet fully understood. Unusual phenomena such as the photocurrent-voltage hysteresis continue to puzzle the scientific community. This necessitates the ongoing research on the material properties of the perovskite compound.
In our piezoresponse force microscopy (PFM) study on µm-sized MAPbI_3-xCl_x grains we observed a periodically alternating structure reminiscent of ferroelastic domain patterns. We propose a domain orientation and suggest that the ferroelastic behavior is induced by internal strain during the crystallization. Although ferroelasticity is often connected to ferrelectricity we did not find any evidence of ferroelectricty in PFM switching experiments. However, this result could be explained by the domain orientation in respect to the sample surface.
This first observation of a ferroelastic domain pattern will provide a better understanding of the ferroic behavior of MAPbX₃.

Colloidal Quantum Dots (CQDs) are nanoengineered solution-processable materials that offer size-bandgap tunability making them attractive materials for tunable optoelectronic devices. In order to make solid state films, these materials must undergo surface manipulation often involving replacing long insulating ligand molecules with shorter ones, thus enabling electrical connectivity. Recently, there have been wide reports of ligands influencing several other properties, such as Stokes shift, optical extinction coefficients, band alignment, and electronic doping. Hence, CQDs offer the unique opportunity of simultaneously engineering materials properties through a combinatorial ligand approach.
We took the view that understanding the impact of different types of ligands would be best realized through a combined experimental and theoretical approach. We combined high-resolution transmission electron microscopy (HR-TEM) imaging with density functional theory (DFT) in order to reveal and compare the impact of different types of connectivity between PbS CQDs, spanning organic, atomic, and fused linking motifs. We carried out the studies on CQDs with different bandgaps, in order to understand the size-dependence of making connected films. Our findings suggest that there while large 8 nm nanoparticles may preserve order and packing within the film, and offer a more controlled way of connecting CQDs, small 3 nm photovoltaics-relevant nanocrystals are highly sensitive to the ligand treatment conditions, often resulting in completely random packing and clustering.
We then focus on the specific application of photovoltaic energy conversion using these materials. We discuss the tradeoffs between attaining high mobility and maintaining a trap-free energy gap, which may be realized by combining different types of ligands that offer the best control for connectivity and passivation, respectively. Our findings suggest that optimal CQD films may be realized through a highly selective, facet-dependent, ligand-binding strategy that will involve several different types of ligands to serve as levers for critical CQD materials properties. Such advancement in surface engineering would result in unprecedented control over materials properties and pave the way for novel applications for CQD solid-state materials.

Recently, solar energy has a lot of attention in the fields of renewable energy due to cleanness, safety, and unlimitness. Photoelectrochemical water splitting is one of the most promising solar conversion technology, which solar energy can be converted as chemical energy in form of H₂. In order to efficient solar-to-hydrogen conversion, controlled semiconductors are needed to absorb sunlight and make some photoelectrochemical reactions. Several semiconductor metal oxide, including TiO₂, Fe₂O₃, WO₃, and BiVO₄, have suitable electronic properties. Especially, TiO₂ has been considered one of the most promising materials for photoelectrochemical water splitting due to its photostability, relatively low cost, and non-toxicity. However, the photoelectrochemical water splitting of TiO₂ has some limitation for efficiency because TiO₂ can only absorb ultra-violet light according to their electronic band gap, which accounts for only 4% of the incoming solar energy. In order to overcome this low efficiency, heterostructure of two different metal oxides has been recently considered as a new photoanode materials.
In this study, we fabricated hierarchical TiO₂/WO₃ heterostructure photoanode using nanoimprinting of TiO₂/WO₃ nanoaprticle (NP) dispersed resin and subsequent hydrothermal growth. First, WO₃/TiO₂ NP dispersed resin is synthesized using TiO₂ NP solution, WO₃ NP solution, monomer, and photo-initiator. Then, WO₃/TiO₃ microstructure is formed on transparent conducting layer using nanoimprinting of WO₃/TiO₂ NP dispersed resin. Subsequently, WO₃/TiO₂ microstructure is sintered at high temperature over 500 °C. Finally, WiO₃ nanowires (NWs) are grown on WO₃/TiO₂ microstructure in order to fabricate hierarchical WO₃/TiO₂ heterostructure and then, Au nanoparticles(NPs) are decorated for enhance the light absorption. Optical properties of Au NPs decorated hierarchical WO₃/TiO₂ heterostructure are characterized using UV-Visible spectroscopy. In addition, photocurrent and external quantum efficiency of Au NPs decorated hierarchical WO₃/TiO₂ heterostructure photoanode are measured using solar simulator and incident photon-to-current efficiency measurement. Compared with hierarchical TiO₂ structure, Au NPs decorated hierarchical WO₃/TiO₂ heterostructure exhibit the high efficiency over 2 times and photocurrent in the range of visible light.

We identify the optical modes existing in a sphere-on-plane system consisting of an ultra-thin (16 - 50 nm) conjugated polymer spacer layer sandwiched between a plasmonic Au nanoparticle and a thin metal film, and successfully manipulate these modes to enhance the absorptive and emissive function of the polymer[1]. The scattering, absorption and photoluminescence spectral responses in this complex system are mediated by one or more of the following modes: a horizontal image dipole coupling mode at a wavelength of ~550 nm; a vertical image dipole coupling mode at ~620 nm; and horizontal and vertical coupling modes between localized surface plasmon resonances (LSPRs) supported by the nanoparticle and surface plasmon polaritons (SPPs) supported by the metal film at wavelengths above 630 nm. Additionally, single-particle dark-field spectroscopy indicates the existence of a sub-set of particles for which the signal of the horizontal image dipole coupling mode is quenched. This is attributed to the partial-embedding of Au nanoparticles into the polymer spacer and leads to higher scattered light intensities at longer wavelengths. With the aid of electromagnetic simulations, we succeeded in developing an interpretation of the experimentally observed dark-field spectral variations with changes in the spacer thickness and degree of nanoparticle embedding. Furthermore, absorption enhancement in the polymer spacer increases with decreasing thickness, demonstrating the increased light trapping ability of the Au nanoparticles for ultra-thin optically absorbing layers. In addition, since this sphere-on-plane system provides large electric field enhancement in the conjugated polymer spacer at controlled resonance wavelengths[2], whereas the underlying metal film typically results in rapid quenching of the fluorescence of the conjugated polymer, photoluminescence of different polymer spacers in such sphere-on-plane systems is studied to fully characterize the competing plasmonic mechanisms. This work shows how electromagnetic coupling modes can be manipulated in a sphere-on-plane system with an ultra-thin conjugated polymer spacer and is a foundation for studies and applications of plasmonic structures for light management in optoelectronic devices with active layer thickness well below that used in more conventional devices.
B. Yu, J. Woo, M. Kong, D. M. O&’Carroll, Nanoscale, in-press.
B. Yu, S. Goodman, A. Abdelaziz, D. M. O&’Carroll, Appl. Phys. Lett., 2012, 101, 151106.

Photoelectrochemical splitting of water to produce hydrogen is a viable approach to transform sunlight into chemical energy, which has triggered a quest for suitable photocatalysts. Transition metal oxides such as α-Fe₂O₃ and TiO₂ are favorable candidates and provide intrinsic advantages in terms of high photo-stability and sufficient mobility of charge carriers, besides their earth-abundance. Despite these advantages, no commercially viable material exists that is able to maintain the proposed minimum 10 % requirement for solar energy to hydrogen fuel efficiency (STH). In the search for strategies enabling an enhancement of the photo electrochemical properties of metal oxide, various methods such as doping of metal oxide, co-catalyst or mutlilayering were conducted.
In this study, we have focused on Interfacial modification of α-metal oxide multilayer photoanodes deposited by plasma enhanced chemical vapor deposition (PE-CVD). Different mechanisms such as patterning of multilayering with different structure (bar structure or line structure) or graphene supporting were examined in this study. The α- bilayer electrode exhibited enhanced PEC responses in terms of a lower onset potential and a higher photocurrent density when compared to the single layer α-Fe₂O₃ electrode. This enhancement was observed to be due to synergistic light absorption with the bilayer electrode, although charge carrier recombination occurred faster due to interfacial defect states. The incorporation of a graphene layer between the α-Fe₂O₃/TiO₂ double layer and the FTO substrate resulted in a doubling of the photocurrent, but lead to a loss of the synergistic effect between the two active metal oxide layers. However, depositing the graphene between the two metal oxide layers, α-Fe₂O₃/graphene/TiO₂ resulted in an even higher photocurrent, while retaining the enhanced onset potential of the double layer electrode. This enhancement was observed to be due to either the passivation of the oxide defect states or enhancement of the charge transfer between the two oxide layers.

Perovskite solar cells (PSC) have emerged as a breakthrough photovoltaic technology, with solar power conversion efficiencies already exceeding 20%. Yet the records carry a heavy asterisk, as the stability of the devices remains poor. Shaping the PSC into a commercially viable technology for low-cost, high-efficiency solar energy requires ensuring that the devices can provide a reasonable service life. This requires greatly enhancing the stability of the materials over present performance, and the problem appears to lie mainly with the perovskite material itself, which undergoes decomposition and/or phase changes, especially upon exposure to moisture. Improving device stability, therefore, requires an understanding of the mechanisms and circumstances that result in degradation of the perovskite layer.
In this work, organolead-halide perovskites, fabricated using different techniques and compositions, are coated on glass substrates and subjected to a controlled atmosphere where temperature, moisture, and oxygen content are varied. Gaseous products evolved from the perovskite material are captured and analyzed by an online GC-MS, indicative of the chemical reactions responsible for degradation. XRD spectra of the films over time report on changes in crystal structure resulting from these and other reactions. Whole PSC devices subjected to the same treatments, or made from perovskite films subjected to such treatments and then finished with a hole-transport layer and contacts, are measured to correlate the chemical and materials shifts with device performance. Together, the characterizations give insight as to the mechanisms and effects of degradation as an effect of key parameters of the environment.

The effect of size-dispersion in colloidal quantum dot (CQDs) solids on the charge carrier mobility was computed using charge hopping transport models. The experimental film deposition processes were replicated using a molecular dynamics simulation where the equilibrium positions of CQDs with a certain radii distribution were determined under the granular potential. Those radii and positions of CQDs were then used in the charge hopping transport simulator where the carrier mobility was estimated. We observed large decreases (up to 70%) in electron mobility for typical experimental polydispersity (about 10%) in CQD films. These large degradations in hopping charge transport were investigated using transport vector analysis with which suggested that the site energy differences raised the portion of the off-axis rate of charge transport to the electric field direction. Furthermore, we have shown that controlling the size distribution remarkably impacted the charge carrier mobility and suggested that tailored and potentially experimentally achievable re-arrangement of the CQD size ensemble can mediate the mobility drops even in highly dispersive cases, and presents an avenue towards improved charge transport. In addition, we studied the degradation in CQD solar cells with respect to the polydispersity and how these enhanced charge transport can boost the photovoltaic performances.

The utilization of noble metal nanoparticles (NPs) (e.g. Au and Ag) as photo antennas in metal oxides having a large band gap is of particular interest for surface plasmon enhanced photochemical water splitting. Current synthetic approaches of metal oxide/noble metal hybrids, however, lack controllability in engineering their nanostructures. We herein present a new bottom-up synthetic approach to preparing mesoporous transition metal oxide/noble metal hybrid catalysts via a ligand-assisted co-assembly of amphiphilic block polymer micelles and polymer-tethered noble metal NPs. The use of polymer ligands that can improve the stability of metal NPs under the extreme conditions used for the sol-gel chemistry is a key to the controllable synthesis of hybrid catalysts. It is found that the hybrid catalyst possesses long-range, periodically ordered mesoporous nanostructures with an average pore size of 25 nm and a wall thickness of 18 nm. Noble metal NPs are homogenously embed into the mesostructural channels of the hybrid catalysts without NP fusion or migration. Our synthetic approach offers a general and straightforward method to precisely tune the sizes and loading amounts of noble metal NPs in metal oxides; and this system, thus, provides a solid platform to clearly understand the role of noble metal NPs in photochemical water splitting. We further demonstrate that the presence of trace amounts of metal NPs (~0.1 wt%) can largely enhance the photocatalytic activity for water oxidation up to 4 fold. Our findings can conceivably be applied to other semiconductors/noble metal catalysts which may stand out as a new methodology to build up highly efficient solar conversion systems.

TiO₂ is a well-studied photocatalyst due to its several advantages such as a high chemical stability, low cost, non-toxicity and chemical inertness. Among the various polymorphs of TiO_2, anatase is known to be photocatalytically active and technologically important. An efficient photocatalyst should have a high surface area and a narrow bandgap for efficient solar energy conversion. Surfactant-assisted synthesis regulates the shape and size of the nanocrystals, whereas the doping with either anions or cations allows for the engineering of the band gap. In this work, we have used hydrothermal method to synthesize uniform monodisperse nanocrystals of both pristine and Fe-doped TiO₂ and tested their photocatalytic performance in dye degradation reactions. X-Ray diffraction studies performed were in good agreement with Raman spectroscopy experiments showing the formation of phase pure anatase TiO₂ under solvothermal conditions. Electron microscopy showed that the nanocrystals retain diamond-type morphology, they are well-separated from each other as a result of the retention of oleic acid on their surfaces and have the tendency to self-assemble onto planar surfaces. Diffuse Reflectance Spectroscopy (DRS) was employed to understand the effect of dopant ion on the band structure of TiO₂ and showed a progressive decrease of the band gap value with increasing the content of transition metal. BET studies will be used for surface area analysis, being therefore suitable as catalysts in various types of photocatalytic processes.

The strong light absorption of single-walled carbon nanotubes (SWNTs) in the visible and near infrared (NIR) regions, alongside their outstanding transport characteristics, have generated considerable interest in using them as active components in photovoltaic (PV) applications. Harvesting such potential typically utilizes heterojunctions of semiconducting (sem)-SWNTs with fullerenes, π-conjugated or combination of both. These heterojunctions however, exhibit considerable inhomogeneity in terms of the localized environment around SWNTs. In order to improve on such inhomogeneity, our group has relied on the highly organized flavin mononucleotide (FMN) assembly around various SWNTs.¹ By appropriate substitution of the flexible side group, C₆₀ can be linked to an organic soluble flavin derivative (FC12)^1a to afford an intramolecular linked structure, termed FC₆₀, which can then be organized around SWNTs. In this contribution, we describe the synthesis and characterization of FC60 and discuss methods on how to successfully incorporate it within the FC12 helix. Moreover, we show that the incorporation of as much as 1% FC₆₀, causes effective photo-induced charge transfer quenching of the NIR emission of SWNTs, as indicated by steady state and transient spectroscopy. The orderly insertion of FC₆₀ within the FC12 helix induces minimal dispersion (in terms of line broadening of the E^S_ii transitions of SWNTs), which can ultimately enable a new class of highly organized supramolecular architectures with spectrally sharp response in the visible and NIR regions.
1. (a) S.-Y. Ju, W. Kopcha, F. Papadimitrakopoulos, Science, 2009, 323, 1319-1323; (b) R. Sharifi, M. Samaraweera, J. A. Gascoacute;n, F. Papadimitrakopoulos, J. Am. Chem. Soc., 2014, 136(20), 7452-7463.

Understanding interfacial charge transfer dynamics in thin films of semiconducting materials is a critical factor to achieve high performance solar energy conversion and storage devices. In this work we propose a multiscale characterization of interfacial processes via transient spectroelectrochemistry and polarization spectroscopy techniques. This approach enables the investigation of charge transfer between different phases but also lateral charge transfer phenomena at functionalized interfaces.
Intermolecular charge transport between molecules chemically anchored to the surface of oxide nanocrystals has been applied to high performance lithium ion batteries,¹ electrochromic devices² and solar fuel production.³ The role of this process in solar energy photo-electrochemical devices has been proposed⁴ but not demonstrated before now. We show evidence that charge diffusion on the surface of sensitized nanostructured is of fundamental importance to the functioning of different optoelectronic systems.
We address the optimization of the dye regeneration process occurring during photoconversion in solid state dye sensitized solar cells. We find that the fraction of dye regeneration attributable to inter-dye hole percolation can be >50% of the overall regenerated dyes under low pore filling conditions by the hole transporting material spiro OMeTAD. This fraction is reduced to about 5% when the infiltration of the HTM in the pores is optimized. We have therefore clarified the process of charge collection in this class of devices and demonstrated that hole percolation in the dye monolayer is necessary to achieve 100% regeneration yield.
Secondly, we show transient absorption and transient absorption anisotropy spectroscopy data suggesting correlation between the electron-hole recombination kinetics in dye sensitized mesoporous TiO₂ on the mobility of holes on the surface of the nanocrystals. Strikingly, our study shows that control on “surface charges” can delay photo-generated carriers recombination by one order of magnitude. The result is supported by quantum chemical calculations and Monte Carlo simulations of charge transport and recombination which show agreement with our multiscale experimental characterization. Our finding has deep relevance for the community investigating interfacial processes for efficient solar fuel and photovoltaic devices using functionalized nanostructures.
Finally, we present a transient spectroelectrochemical approach to investigate the optoelectronic properties, charge transport and electron transfer dynamics in thin film semiconductors and molecular functionalized nanostructured materials for solar energy conversion and storage.
References:
(1) Wang, Q.; Evans, N.; Zakeeruddin, S. M.; Exnar, I.; Grätzel, M. Molecular Wiring of Insulators: Charging and Discharging Electrode Materials for High-Energy Lithium-Ion Batteries by Molecular Charge Transport Layers. J. Am. Chem. Soc. 2007, 129, 3163-3167.
(2) Wang, Q.; Zakeeruddin, S. M.; Cremer, J.; Bäuerle, P.; Humphry-Baker, R.; Grätzel, M. Cross Surface Ambipolar Charge Percolation in Molecular Triads on Mesoscopic Oxide Films. J. Am. Chem. Soc. 2005, 127, 5706-5713.
(3) Ardo, S.; Meyer, G. J. Characterization of Photoinduced Self-Exchange Reactions at Molecule-Semiconductor Interfaces by Transient Polarization Spectroscopy: Lateral Intermolecular Energy and Hole Transfer across Sensitized TiO2 Thin Films. J. Am. Chem. Soc. 2011, 133, 15384-15396.
(4) Weisspfennig, C. T.; Hollman, D. J.; Menelaou, C.; Stranks, S. D.; Joyce, H. J.; Johnston, M. B.; Snaith, H. J.; Herz, L. M. Dependence of Dye Regeneration and Charge Collection on the Pore-Filling Fraction in Solid-State Dye-Sensitized Solar Cells. Adv. Funct. Mater. 2013, 24, 668-677.

Lead-halide perovskites have emerged as a new class of materials for electro-optical, especially photovoltaic applications with power conversion efficiencies up to 24 %. ^[1] Beyond solar cell efficiencies, methylammonium lead iodide perovskites are investigated extensively to understand the unique properties of the material. In addition to 3-dimensional perovskites, 2-dimensional perovskites are also investigated, especially since they allow better tuning of the optoelectronic properties.
Time-resolved transient absorption and photoluminescence spectroscopy in the visible spectral range allow precise characterization of the charge generation and recombination mechanisms in lead-halide perovskite films. In this work, we use these techniques to investigate the effects of light-induced changes of solvent-annealed methylammonium lead iodide films under nitrogen atmosphere. To gain further insight, we use complementary characterization techniques, namely scanning-probe and electron microscopy, to study the morphology of the film. Transient absorption measurements of solvent-annealed perovskite films under vacuum resemble a large share of the published photophysical properties. ^[2,3] Here we show light-induced morphology changes - formation of quantum-well like nano flakes - and corresponding transient absorption and photoluminescence measurements. Our findings indicate that the formation of nano flakes on top of the perovskite film dramatically changes the photophysics. The photophysical changes are visible in the quenching of the spectral signature of a depopulated ground state and the appearance of a derivative-like spectral feature with decay time similar to the photoluminescence of the film. Similar derivative-like features have been observed for solution-grown perovskite quantum wells. ^[4]
[1] WCPEC-6 Conference Highlights (18.06.2015).
[2] Christians et al.: J. Phys. Chem. Lett., 2015, 6 (11), pp 2086-2095
[3] Chen et al.: J. Phys. Chem. Lett., 2015, 6 (1), pp 153-158
[4] Wu et al.: J. Phys. Chem. C, 10.1021/acs.jpcc.5b00148

Desert environments are characterized simultaneously by high solar irradiance and by windy and sandy conditions. In order to fully exploit the solar resource, fouling of solar panel surfaces by dust accumulation must be avoided. Furthermore, in such environments, where water is a scarce resource, cleaning requirements should also be minimized. This is a problem that applies not only to solar panels but also glass building facades and vehicle windshields in desert societies such as the Gulf states. We present a study of the fouling behavior of glass treated with commercial nanoparticle-based coatings, and an analysis on the long-term impact of such coating on solar cell performance under real conditions in Abu Dhabi, UAE. Studies of surface wettability will also be included as part of an investigation of the self-cleaning properties of the coatings.

Manipulating the charge carrier dynamics of semiconductor nanocrystals is the key element to utilize them for photoactive applications. The presence of intra bandgap defect states can trap the excited carriers shortening their lifetime significantly, and consequently limit their applicability. To remove these undesirable defect states, a profound knowledge about their origin and nature is required. In this report, we monitor the ultrafast charge carrier dynamics of ternary CuInS₂, CuInSSe, and CuInSe₂ nanocrystals before and after ZnS shelling using state-of-the-art time-resolved laser techniques including femtosecond broadband transient absorption spectroscopy.¹ The experimental results demonstrate that ZnS shelling efficiently removes a significant fraction of carrier trapping sites and the carrier lifetime can be elongated upto ~1.8 times. Further, we extended this study for quaternary copper indium gallium selenide (CIGSe) nanocrystals which have many desirable attributes to be used as a photovoltaic material, but relative abundance of intra bandgap trap states and lack of their knowledge limit their applicability. We synthesized sub-10 nm CIGSe nanocrystals for the first time by proper manipulation of reaction parameters and monitored their carrier dynamics precisely in real time as a function of trap states. Here also, ZnS shelling is found to be efficient to remove carrier trapping sites and can elongate the carrier lifetime significantly upto ~120 ns. As a result, the photon conversion efficiency of solution processed photovoltaic device incorporating the nanocrystals can be increased to a factor of ~2.2 and the clear signature of trap state removal is evident by the transient photocurrent experiments.² We report a clear quantitative insight of the charge carrier dynamics and effect of shelling in ternary/quaternary nanocrystals, and also their immediate effect to enhance the photoresponse of the nanocrystals, providing a better understanding to unlock their full potential for photovoltaics and light emitting applications.
References:
Bose, R.; Ahmed, G. H.; Alarousu, E.; Parida, M. R.; Abdelhady, A. L.; Bakr, O. M.; Mohammed, O. F., Direct Femtosecond Observation of Charge Carrier Recombination in Ternary Semiconductor Nanocrystals: The Effect of Composition and Shelling. J. Phys. Chem. C,2015, 119, 3439-3446.
Bose, R.; Bera, A.; Parida, M.; Abdelhady, A. L.; Alarousu, E.; Bakr, O. M.; Mohammed, O. F., Enhanced Photoresponse of CIGSe nanocrystals by Trap States Engineering- An Ultrafast Study. Chem. Mater., To be submitted.

Although being considered a scarce element, huge amounts of depleted uranium sources (e.g. alone 700,000 tons UF₆ in the USA), originally produced as waste streams in the enrichment process of nuclear fuels, are currently stored without any prospect for further applications which poses a perpetual environmental hazard, due to accidental release of volatile, corrosive and toxic compounds. In order to reduce this amount of uranium waste, its reuse is discussed in the context of semiconductor materials as well as catalysts for thermochemical water splitting, destruction of air pollutants and CO₂ activation.
Herein we present the use of air-stable and volatile uranium metal-organic precursors for the gas phase deposition (thermal and plasma-assisted chemical vapor deposition) of uranium oxide thin-films and their use as photoelectrodes in photoelectrochemical water splitting devices. Uranium oxides with band-gap energies in the range of 2.0 - 2.6 eV coupled with their good electrical and catalytic properties mostly driven by facile valence dynamics of the uranium cations are promising electrode materials in photoelectrochemical water splitting reactions. However, when tested in a standard PEC setup using different acidic and basic electrolytes, they were found to decompose, when electric potential was applied. In order to prevent dissolution of the uranium oxide layers, we prepared nanostructured bilayered thin films with a corrosion-resistant metal-oxide layer for protection against dissolution at the semiconductor/electrolyte interface.

The search for the next generation of low-cost high-performance solar cells is underway. Several classes of potential generation III solar energy devices, including dye-sensitized solar cells (DSSCs) and organo-halide perovskite devices, make use of high surface area, mesoporous p-type cathodes for charge separation and transport of charge carriers. However, the performance of these p-type cathode materials typically lags behind n-type anode materials such as TiO₂. In this work, the spinel, Zn_xCo_3-xO₄, is presented as a candidate p-type solar photocathode. A solid-state mechanochemical synthesis was developed, producing nanospheres with an average diameter of 24 ± 5 nm. The solid state preparation allows for highly tunable Zn concentration in the spinel lattice. The mesoscale photocathodes&’ performance was evaluated in a p-DSSC configuration with the molecular chromophore P1 and an I^-/I^3- electrolyte. DSSCs were tested with variable Zn concentration in the particles, and showed that increased Zn substitution resulted in increased performance. Poor surface dye-loading was identified as a preliminary performance limiter, and various surface modifications on the material were investigated to maximize dye-loading and increase DSSC performance. Surface modifications included controlling both the temperature and atmosphere while annealing the thin film photocathode, yielding increases in dye-loading, photocurrent, and overall DSSC performance. The results suggest that Zn_xCo_3-xO₄ could be developed into a high performance material for a variety of solar energy applications.

Heating, cooling, and lighting in buildings account for 24% of the total energy consumption in the US. If near infrared (NIR, heat) and visible (VIS, light) solar radiation through windows can be actively and independently controlled in response to exterior conditions, this enormous energy demand can be significantly reduced. We present the design and fabrication of a nanocomposite electrochromic material that performs near-ideal solar modulation.
Two active electrochromic materials, NIR-plasmonic tungsten oxide nanocrystals (WO_3-x) and VIS-polaronic niobia glass (NbO_x), are organized into a mesostructured architecture. The inherently different switching potentials of WO_3-x and NbO_x is a prerequisite to NIR-VIS dual-band switching^[1]. However, a judicious arrangement of the two components was essential to avoid deleterious interactions such as blocking of charge transport pathways. Our nanocomposite architecture comprises a mesoscale WO_3-x framework, interpenetrating NbO_x domains, and small mesopore channels introduced at the WO_3-x-NbO_x heterointerface. The high density of the heterointerface and the open pore channels allow efficient electrolyte percolation promoting exceptionally rapid and independent charging and discharging of each component. Moreover, enhancements in charge capacity, optical contrast, and cycling durability (>2000 cycles) were also observed. In result, we achieved unprecedented modulation of thermal loads at the ‘Cool mode&’ (36%T_NIR : 73%T_Vis), while the neutral color ‘Dark mode&’ (7%T_NIR : 22%T_Vis) rivals the best tinting performance of VIS-switching windows commercialized today^[2].
[1] Llordes, A., Garcia, G., Gazquez, J. & Milliron, D. J., Nature500, 323-326
[2] Kim, J., Ong, G. K., Wang, Y., LeBlanc, G., Williams, T. E., Mattox. T. M., Helms, B. A., Milliron, D. J., Submitted

Scanning force microscopy (SFM) is a versatile tool for studying nano- and sub-nanoscale surface structures. Using advanced SFM methods, morphological features can be correlated with a vast number of material properties, such as mechanical or electrical properties. This information is of particular interest in the field of novel photovoltaic materials such as perovskites, where the nanoscale structure of the materials has a huge impact on the device performance. Additional to topographic information, electrical SFM methods can map various surface properties on a nanometer length scale. The accessible surface properties include electrical conductivity (conductive SFM), surface potential in Kelvin probe force microscopy, (KPFM) or electromechanical coupling in Piezoresponse force microscopy (PFM).
In order to study light induced electrical effects on the level of single nanostructures, a new SFM setup in inert atmosphere with a sample illumination was built. With this photoelectric SFM, we are studying the electrical properties of perovskite films and solar cells [1]. In order to investigate operating devices across the internal interfaces, we developed a procedure to fine polish a device cross section in order to get a smooth topography. Subsequently, we used high-resolution frequency modulation KPFM to map the potential distribution in dark and under illumination with white light. Our measurements showed that SFM can reveal potential distributions inside a working device with nanometer resolution [2]. With piezoresponse force microscopy (PFM) on micron-sized perovskite grains we observed a periodically alternating structure reminiscent of ferroelastic domain patterns. From vertical and lateral PFM, a domain orientation with respect to the sample surface is proposed. Structural changes due to degradation by water and air exposure in perovskite films were investigated in-situ. Finally, conductive SFM experiments provide information about conducting pathways and photocurrent generation in the perovskite layers.
Such experiments provide valuable information for the optimization of the light harvesting abilities and lifetime in perovskite materials.
[1] R. Berger, A.L. Domanski and S.A.L. Weber Electrical Characterization of Organic Solar Cell Materials based on Scanning Force Microscope Methods, Eur. Polym. J., 49, 1907 (2013).
[2] Bergmann, V.W.; Weber, S.A.L.; Ramos, J.A.; Nazeeruddin, M.K.; Grätzel, M.; Li, D; Domanski, A.L.; Lieberwirth, I.; Ahmad, S. and Berger, R. Real-space observation of unbalanced charge distribution inside a perovskite-sensitized solar cell, Nature Communications, 5, 5001 (2014).

Recently, there has been considerable interest in using single-walled carbon nanotubes (SWNTs) as active components in photovoltaic (PV) applications. To harvest such potential, heterojunctions were made from: (i) semiconducting (sem)-SWNTs and fullerenes, (ii)π-conjugated polymers and sem-SWNTs, and (iii) hybrid approaches, where both C₆₀ and SWNTs were used in combination with conjugated polymers. These heterojunctions however display considerable dispersion in terms of the localized environment around SWNTs. In order to improve on such dispersion, our group has relied on the highly organized flavin mononucleotide (FMN) assembly around various SWNTs.¹ By appropriate substitution of the flexible side group, C₆₀ can be linked to an organic soluble flavin (FC₁₂)^1a to afford an intramolecular linked structure termed FC₆₀, which can then be organized around SWNTs. In this contribution, we present the molecular dynamics (MD) results of FC₁₂-wrapped SWNTs, where one FC₁₂ molecule of the helix has been exchanged with an FC₆₀. Our results indicate that the flexible side chain of FC₆₀ linking the C₆₀ with the isoalloxazine ring bends in such fashion to form a ground state complex that affords direct π-π stacking overlap with the flavin helix. Different N₁₀ nitrogen hybridizations (i.e.sp² and sp³) of the isoalloxazine ring lead to different bended conformations in both free (in solution) and helix-bound FC₆₀.
1. (a) S.-Y. Ju, W. Kopcha, F. Papadimitrakopoulos, Science, 2009, 323, 1319-1323; (b) R. Sharifi, M. Samaraweera, J. A. Gascoacute;n, F. Papadimitrakopoulos, J. Am. Chem. Soc., 2014, 136(20), 7452-7463.

The potential roll-to-roll, low-cost fabrication of organic photovoltaics has become increasingly attractive for power generation to even the most remote locations. P3HT (poly(3-hexylthiophene)) and PCBM ([6,6]-phenyl-C-61-butyric acid methyl ester) are the most widely used donor/acceptor materials in organic photovoltaics (OPVs). One of the biggest challenges of organic semiconductors is their low charge carrier mobility. The four to five orders of magnitude higher charge carrier mobility of semiconducting (sem-) single walled carbon nanotubes (SWNTs) can potentially address this issue, if one manages to appropriately configure them in heterojunction structure with the proper carrier extraction architecture. To this effect, the supramolecular organizations of flavin mononucleotide (FMN) around single walled carbon nanotubes (SWNTs) was shown to provide effective nanotube dispersions and the ability to impart selective enrichment of sem-SWNTs by recognizing the underlying nanotube helical pattern. Utilizing this approach, we herein present our recent work in outfitting the organic analogue of FMN (FC12) with a terminal PCBM functionality. The presence of PCBM moiety provides an effective photoluminescence (PL) quenching for both FC₆₀ and FC12-dispersed SWNTs with ca. 1% incorporated FC₆₀. Spectroscopic characterization enables to verify the incorporation of FC₆₀ within the FC12 helix. The suggested system here, helps to improve homogeneous environment around the SWNTs and SWNT individualization which can promote exciton dissociation and then single layer processability of organic photovoltaics.

We investigated the energy-down-shift having energy-tuning effect of Mn doped Cd_0.5Zn_0.5S-ZnS core-shell quantum dots on power-conversion efficiency enhancement in silicon solar cells for the first time. We found that Mn doped Cd_0.5Zn_0.5S-ZnS core (8.0 nm in diameter)-shell (4.8 nm in thickness) quantum dots (QDs) demonstrated a typical energy-down-shift (2.76-4.96 → 2.05 eV), which absorb ultra-violet(UV) light (250-450 nm in wavelength) and emit orange visible light (~602 nm in wavelength). They showed the quantum yield of ~59 % and their coating on the SiN_X film textured p-type silicon solar-cells enhanced the external-quantum-efficiency(EQE) of ~20 % at 300-450 nm in wavelength, thereby enhancing the short-circuit-current-density(JSC) of ~1.42 mA/cm² and the power-conversion-efficiency(PCE) of ~0.55 % (relatively ~3.07 % increase compared with the reference without QDs for p-type silicon solar cells). In particular, the PCE peaked at a specific coating thickness of the Cd_0.5Zn_0.5S-ZnS core-shell QD layer; i.e., the PCE enhancement of 0.55 % at an 8.8 nm thick QD layer.
*This work was supported by Basic Science Research Program through the Ministry of Higher Education, Kingdom of Saudi Arabia for supporting this research through a grant (PCSED-008-14) under the Promising Center for Sensors and Electronic Devices (PCSED) at Najran University, Kingdom of Saudi Arabia and Brain Korea 21 PLUS Program in 2014.

High quality, thickness-controlled, CdS nanosheets (NSs) have been obtained through the thermal decomposition of cadmium diethyldithiocarbamate in octadecene. Ensembles with discrete thicknesses of 1.50 nm, 1.80 nm, and 2.16 nm have been made with corresponding lateral dimensions on the order of 90 nm x 20 nm. In tandem, associated Ni nanoparticle (NP) decorated counterparts have been made through the photodeposition of Ni onto NS basal planes.
Subsequent photocatalytic hydrogen generation measurements have compared the performance of CdS NSs with their Ni NP decorated counterparts in water/ethanol mixtures. Of interest is how the large absorption efficiencies, room temperature excitonic resonances, long lifetimes and corresponding suppressed Auger responses of 2D NSs impact their photocatalytic response. Apparent quantum yields as large as 25% have been seen for Ni NP decorated NSs with transient yields as large as 64% within the first 2 hours of irradiation. Ensemble femtosecond transient differential absorption spectroscopy show that the high efficiency of CdS/Ni NS photocatalysts ultimately stems from efficient electron transfer from CdS to Ni. In this regard, the CdS/Ni metal/semiconductor heterojunction acts to dissociate strongly bound excitons in CdS NSs, creating free carriers needed to carry out relevant reduction chemistries. This, in turn, suggests that the combination of several intrinsic features of 2D CdS NSs namely, their large absorption efficiencies, their strong exciton binding energies, and, more importantly, their long exciton lifetimes -all coupled with the presence of a metal/semiconductor heterojunction- results in efficient hydrogen generation.

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.

Metal oxides semiconductors are promising for photoelectrodes in a photoelectrochemical (PEC) solar water-splitting device due to their abundance, light absorption properties and stability in aqueous media. Identification of optimized photoelectrode materials can be achieved by combinatorial fabrication and high-throughput characterization methods. By systematically mixing materials of interest, in a reactive sputter deposition process, thin film materials libraries, exhibiting combined thickness and compositional gradients as well as structural variations can be synthesized. Additionally, glancing angle deposition, varying sputter deposition conditions and selective dissolution methods present facile routes to synthesis of highly porous nanostructured thin films. Nanostructuring results in effectively increasing the surface area to allow larger solid/electrolyte interface for efficient charge transfer process. With this approach, Ti-W-O, Fe-W-O and Fe-Cr-Al-O materials libraries were thoroughly screened for potential photoelectrode materials. High-throughput investigation of compositional, structural and functional data on the materials libraries were performed by automated EDX, XRD, thickness and PEC measurements. The analysis of the data enabled to establish correlations between composition, morphology, thickness, crystallinity and PEC properties. For the Ti-W-O system exhibiting a compositional range between (Ti₈₀W₂₀)O_x and (Ti₉₇W₃)O_x, the PEC measurements showed the maximum quantum efficiency of 37.5 % (at lambda; = 350 nm) and a maximum peak photocurrent density of 70.3 µA/cm² at 94.4 at.% Ti. Extensive phase analysis in the case of Fe-W-O indicated the presence of a ternary phase Fe₂O₆W at the local photocurrent maxima, while the sub-stoichiometric W₅O₁₄ phase (with 15 at.% Fe) demonstratedthe maximum quantum efficiency of 45 % at lambda; = 300 nm and a highest photocurrent density values of 65 µA/cmsup2;. In the Fe-Cr-Al-O system, two distinct semiconductor compositions p-type Fe_36.5Cr_55.5Al₈O_x and n-type Fe₅₁Cr₄₇Al₂O_x were identified. The p-type material exhibited a bandgap of 1.55 eV and an open circuit potential of 0.85 V vs RHE. On the other hand, the n-type material demonstrated a bandgap of 1.97 eV and an open circuit potential of 0.6 V. The detailed structural analysis suggested presence of spinel-type structures.

The production of hydrogen by photocatalytic water splitting is a potentially clean and renewable source for hydrogen fuel. Cadmium chalcogenides are attractive photocatalysts because they have the potential to convert water into hydrogen and oxygen using photons in the visible spectrum. Cadmium sulfide rods with embedded cadmium selenide quantum dots (CdSe@CdS) are particularly attractive because their energy bands can be ideally tuned for this task; however, two crucial drawbacks hinder the implementation of these materials in wide spread use: poor charge transfer and photochemical instability. Utilizing photochemical deposition of co-catalysts onto CdSe@CdS substrates we can address each of these weaknesses. We report conditions under which metal capped CdSe@CdS can achieve record efficiency for the water reduction half-reaction. We also report long term photostability for CdSe@CdS under high intensity 455nm light (a viable wavelength for photocatalytic water reduction by CdSe@CdS) by growing metal oxide co-catalysts on the surface of our rods.

We report a nitrogen-doped carbon nanodot (N-Cdot)/TiO₂ nanowire photoanode for solar-driven, real-time, and sensitive photoelectrochemical probing of the cellular generation of H₂S, an important endogenous gasotransmitter based on a tunable interfacial charge carrier transfer mechanism. Synthesized by a microwave-assisted solvothermal method and subsequent surface chemical conjugation, the obtained N-Cdot/TiO₂ nanowire photoanode shows much enhanced photoelectrochemical photocurrent compared with pristine TiO₂ nanowires. This photocurrent increase is attributed to the injection of photogenerated electrons from N-Cdots to TiO₂ nanowires, confirmed by density functional theory simulation. In addition, the charge transfer efficiency is quenched by Cu²⁺, whereas the introduction of H₂S or S^2- ions resets the charge transfer and subsequently the photocurrent, thus leading to sensitive photoelectrochemical recording of the H₂S level in buffer and cellular environments. Moreover, this N-Cdot-TiO₂ nanowire photoanode has been demonstrated for direct growth and interfacing of H9c2 cardiac myoblasts, with the capability of interrogating H₂S cellular generation pathways by vascular endothelial growth factor stimulation as well as inhibition.

Optical upconversion converts two or more low-energy photons into a higher-energy photon. Upconversion that converts infrared light into the visible is particularly useful for photovoltaic applications, and can help surpass the Shockley-Queisser limit by capturing the sub-bandgap photons. To date, two main processes have been studied for upconversion under incoherent excitation: luminescence from lanthanide ions, and sensitized triplet-triplet-annihilation (TTA). While the latter typically works efficiently at much lower excitation intensities than the lanthanides, demonstrations have been from pump wavelength lambda; < 790 nm, and mostly in solution. Here, we report TTA-based upconversion in solid-state thin film from excitation beyond lambda; = 1 mu;m to emission at lambda; = 612 nm, utilizing lead sulfide colloidal nanocrystals as the triplet sensitizer. We show that upconversion becomes efficient at an absorbed power density of 12 mW cm^-2 under lambda; = 808 nm monochromatic excitation, which is equivalent to less than one sun of AM 1.5 broad-spectrum excitation in photon flux. Given the wide bandgap tunability (up to lambda; = 2 mu;m) of the nanocrystals and their broadband absorption, upconversion via nanocrystal-sensitized TTA promises to harness the infrared for conventional solar cell technologies.

Perovskites have emerged as a highly flexible materials platform for the realization of solution processable solar cells with efficiencies rapidly approaching those of established commercial photovoltaic technologies. Recently, advances in colloidal synthesis have permitted the preparation of quantum dots of perovskite materials with good size distributions and high photoluminescence (PL) quantum yields. Here we explore the size and composition dependent optical properties of perovskite quantum dots at both the single-particle and ensemble levels. Transient absorption and time resolved PL experiments allow us to quantify the degeneracy of the band edge exciton states, characteristic time constants of radiative and nonradiative Auger recombination, the rates of intraband relaxation, and the strength of exciton-exciton coupling. Single-dot experiments reveal fluorescence intermittency (“blinking”) in both PL intensity and time domains, with statistical properties similar to those of conventional quantum dots. Further, the analysis of pump-flux dependent PL intensity and lifetime trajectories allows us to identify the mechanism for intermittency. We will also present the results of ongoing studies of carrier multiplication in composition- and shaped-controlled perovskite quantum dots and discuss the issues of their photodegradation. Our studies indicate that while some of the optical properties of perovskite quantum dots are similar to those of well studied II-VI quantum dots there are also certain distinctions especially with regards to the properties of multicarrier states.

Photon upconversion has many optoelectronic applications (for example, in solar cells, photodetectors or data storage optical devices). However, the conversion of infrared light to visible light has been limited by the low absorption cross-sections and tunability of lanthanide-doped materials. We demonstrate efficient upconversion with semiconductor nanocrystals and molecular emitters using continuous wave light. The absorption of low energy photons by the nanocrystal results in the sensitization of the triplet state of the molecule, which then undergoes triplet-triplet annihilation to emit upconverted light. The ligands bound to the nanoparticles during the upconversion process mediate this energy transfer. The use of different combinations of nanocrystals and emitters shows that this is a general platform with great flexibility, both in the choice of excitation and emission wavelengths for upconversion.

Layered metal dichalcogenides (LMDs) have emerged as exciting 2D semiconductors for energy device applications, partly due to the strong dependence of their bandgaps on the number of LMD layers. MoS2, WS2 and WSe2 show a transition from indirect bandgap in multilayer to direct bandgap in monolayer, and this bandgap inversion provides enhanced photoluminescence (PL) for monolayer LMDs, making them appealing light harvesting materials for photovoltaics (PVs). The large surface area of these light emitting LMDs is also suitable for catalysis when doped with appropriate nanoparticles or it can provide support for the development of heterostructures with enhanced light absorbing capabilities such as colloidal quantum dots. Recently we introduced SnS₂ as a 2D LMD with great potential for the development of photovoltaic devices and photodetectors [1] Here we combined SnS₂ with colloidal (0-D) nanomaterials in the form of CdSe/ZnS quantum dots to obtain heterostructured nanomaterials with improved light harvesting properties. Specifically, we demonstrate efficient energy transfer from isolated Qdots to single and few layers of SnS₂ with time-resolved single nanoparticle microscopy and find that the rate of energy transfer between such Qdots and SnS₂ nanosheets increases with the number of SnS₂ layers. We then move to device to demonstrate incorporation of such heterostructures in a field effect transistor architecture and to find that such devices exhibit improved photocurrent generation due to the addition of colloidal Qdots.
[1]. Yuan Huang , Eli Sutter , Jerzy T. Sadowski , Mircea Cotlet , Oliver L.A. Monti , David A Racke , Mahesh R Neupane , Darshana Wickramaratne , Roger K. Lake , Bruce A Parkinson , and Peter SutterTin Disulfide - An Emerging Layered Metal Dichalcogenide Semiconductor: Materials Properties and Device Characteristics. ACS Nano, 2014, 8 (10), pp 10743-10755.

Photoelectrochemical energy conversion involves light-induced charge transfer excitations prior to chemical bond breaking and making. Quantum mechanical simulations usually employ density functional theory (DFT), but conventional DFT fails to treat these types of excitations correctly due to exchange-correlation functional limitations. By contrast, embedded correlated wavefunction (ECW) theory treats charge transfer accurately by properly including exact electron exchange and correlation in a region of interest while the extended background is described via periodic DFT, encapsulated in a so-called embedding potential. We will describe how an unusual form of photoelectrocatalysis can also be captured by this theory, namely plasmon-induced hot electron dissociation of molecules on gold and aluminum nanoparticles.

The solar-driven photocatalytic splitting of water into hydrogen and oxygen is a potential source of clean and renewable fuels. However, four decades of global research have proven this multi-step reaction to be highly challenging. The design of effective artificial photo-catalytic systems will depend on our ability to correlate the photocatalyst structure, composition, and morphology with its activity.
I will present our strategies, and most recent results, in taking photocatalyst production to new and unexplored frontiers. I will focus on unique design of innovative nano scale particles, which harness nano phenomena for improved activity, and methodologies for the construction of sophisticated heterostructures. I will demonstrate how vital is the ability to characterize our hybrid nanostructures on the atomic level, and how we can benefit from information on the structure-properties relationship for the future design of an efficient photocatalyst for solar-to-fuel energy conversion.

Metal oxide protection layers for photoanodes may enable the development of large-scale solar fuel and solar chemical synthesis. ALD-TiO₂ remains the most widely used material because of its excellent stability under water oxidation conditions and potential for high electrical conductivity both as an ultrathin film and with thicknesses exceeding 100 nm [1-3]. However, the devices fabricated with the most conductive ALD-TiO₂ films exhibit poor photovoltages of ~ 400 mV and less [3] while photoanodes reported to have higher voltages required ultrathin protection layers for efficient operation [1]. To achieve overall water splitting efficiencies over 20%, photovoltages over 600 mV with sufficiently thick TiO₂ for long-term protection are required for silicon to be a viable bottom cell in a tandem architecture.
Here we investigate the trade-off between conductivity of the ALD-grown TiO₂ protection layer on silicon anodes and the photovoltage. We report a novel observation of photovoltage loss associated with charge transfer in these metal oxide protected devices. By eliminating the loss, we are able to achieve photovoltages as high as 630 mV with thicker (> 5 nm) protection layers, the maximum reported to date for single-junction water-splitting silicon cells. The loss mechanism is systematically probed in MOS Schottky junction cells compared to buried junction p⁺n cells, revealing the need to maintain a characteristic hole density at the semiconductor/insulator interface. A capacitor model that predicts this loss is developed, and is related to the dielectric properties of the protective oxide, achieving excellent agreement with the data. From these findings, we extract design principles for simultaneous optimization of charge transfer resistance and interface quality to maximize the photovoltage of insulator-protected water splitting devices.
Building on this work, we investigate strategies of controlling conductivity in ALD-TiO₂ to optimize fill factor without sacrificing voltage. Photovoltages exceeding 600 mV for both nSi MIS and p+n Si buried junction anodes in water splitting cells are demonstrated, uncovering new and reliable pathways to high conductivity and overall high efficiency.
[1] Y.W. Chen, et al. Nature Mat. 2011, 10, 539-544.
[2] A. G. Scheuermann, et al. Energy Environ. Sci. 2013, 6, 2487-2496.
[3] S. Hu, et al. Science 2014, 344, 1005minus;1009.

A fully integrated, efficient, stable and scalable solar-driven water-splitting cell requires synergistic incorporation of multiple material assemblies, including light absorber, protection layer, catalysts, gas separation membrane and electrolye, into a single device. Significant advancements in integrating and prototyping development of solar-hydrogen devices have been made at Joint Center for Artificial Photosynthesis (JCAP).
Herein, two strategies that achieved stable and efficient prototype cell are presented. One strategy leverages the uniform coating of “leaky” TiO₂ for stable operation in alkaline conditions. A monolithically integrated device consisting of a tandem-junction GaAs/InGaP photoanode exhibited a solar-to-hydrogen conversion efficiency of 10.5% under 1 sun illumination, with stable performance for > 40 h of continuous operation. The monolithically integrated, membrane-base, wireless solar-hydrogen prototype system (~1 cm²) comprised of NiMo/GaAs/InGaP/TiO₂/Ni was constructed and a hydrogen production rate of 0.81 mu;L s^-1 and a solar-to-hydrogen conversion efficiency of 8.6% was observed at 1 sun illumination in 1.0 M KOH in a full cell configuration with minimal product crossover.
Alternatively, photoelectrodes patterned with transparent dielectrics and dark/metallic catalyst islands with a low filling fraction have the potential to achieve stable and efficient prototype with minimal parasitic absorption of electrocatalysts. A redox inactive underlying metal in conjunction with Ni-based catalyst for oxygen evolution reaction was employed to prevent (photo)electrochemical corrosion under anodic conditions. This general strategy was applied to a range of semiconductor substrates and achieved long-term stability in large-scale prototypes. Specifically, in a triple junction III-V photoabsober, in which excess photovoltage was readily available, prototypes with patterned catalysts exhibited higher energy conversion efficiency in relative to a device incorporating uniformly coated catalysts. In addition, preliminary results on photoelectrochemical generation and compression of hydrogen in a solar water-splitting prototype will also be presented.

Cu₂O is a p-type semiconductor with a band gap of ~2.2 eV which attracts much attention for application in photovoltaics, photocatalysis and solar water splitting, but is not intrinsically stable under illumination in aqueous solutions. Cu_xS compounds have a much higher intrinsic stability, and fascinating, though not completely understood, optoelectronic properties. Building on methods from the field of catalysis, we explore new strategies to assemble Cu-based semiconductor nanoparticles with controlled size, amonst others using mesoporous silica supports. For instance the size of pure-phase Cu₂O nanoparticles was tuned from 2 nm to 16 nm by varying either the concentration of the Cu-precursor or the pore diameter of the supporting silica, followed by subsequent conversion to Cu₂O by gas-phase reduction with carbon monoxide. The activity, stability and selectivity of the as-obtained nanoparticles, for different sizes and compositions, were examined for solar-driven H₂ evolution and in photocatalytic oxidation. A few examples will be highlighted, such as the relatively high stability of silica-supported Cu₂O nanoparticles, and the possibility to influence the selectivity of the photocatalyzed oxidation reactions.

Semiconductor nanowires represent an unique form of materials for solar energy harvesting and chemical transformation. The one-dimenstional electrical transport allows for efficient solar energy harvesting, and its large surface areas are beneficial for subsequent charge transfer and catalytic reactions. Here several examples are presented in the field of solar-to-chemical conversion. Fundamental study at single-nanowire level has provided useful insight for these claimed benefits for semiconductor nanowires. This understanding allows us to design integrated nanosystems for spontaneous solar-driven water-splitting and CO₂ reduction. With biocatalysts powered by synthetic biology, a variety of value-added chemicals can be produced from CO₂, H₂O, and sunlight. The nanowire morphology not only increases the charge transfer rate for bio-catalyzed CO₂ reduction, but also acts as a "shield" to protect the oxygen-senstive biocatalysts in O₂/CO₂ mixed gas environment.

Hematite is an attractive material for photoelectrochemical (PEC) water splitting, owing to its earth abundance, stability, and favorable band gap. However, the slow kinetics for water oxidation on the hematite surface demands that a catalytic layer be applied in order to increase the rate of water oxidation. Recently, there has been debate about the role of these catalytic layers and their role in increasing the performance of hematite. It is unclear whether the layer is acting to reduce the activation energy of the water oxidation reaction, thus increasing the kinetics, or if the layer is making the reaction more thermodynamically favorable by decreasing the recombination of photoexcited charges, thus increasing the charge separation by the photoelectrode. Our previous work has indicated that a catalyst layer, such as NiFeO_x, works in the latter case. Our current work focuses on studying the rate constants of charges on the surface of hematite. Using intensity modulated photocurrent spectroscopy (IMPS) we are able to determine the rate of water oxidation and recombination processes that occur at the surface of hematite. Using IMPS we found NiFeO_x indeed decreases the rate of recombination at the hematite surface, but also that NiFeO_x actually decreases the rate of water oxidation. These surprising results are supported by intensity modulated photovoltage spectroscopy (IMVS) and also transient studies. Together these results present a new understanding of catalytic layers for water oxidation and help pave the way for creating more effective passivation layers that lead to efficient PEC water splitting electrodes.

Photocatalytic water splitting is of great importance, due to its efficient utilization of solar energy to chemical energy. Generally, hydrogen production from water reduction requires two electrons, while oxygen evolution from water oxidation is more challenging since it requires four holes to generate two oxygen-oxygen bonds for the production of per oxygen molecule. That is the reason why the reported active photocatalysts for oxygen evolution from water are scarce, such as WO3 and BiVO4. Two key requirements should be considered for the fabrication of photocatalysts with efficient solar oxygen generation from water: one is excellent photo-adsorption ability within the solar spectrum, and the other is low recombination rate of photogenerated electron-hole pairs. In terms of the first issue, some semiconductors themselves possess efficient absorption in the visible-light solar spectrum, which occupies ~43% the solar radiation energy, such as Bi2WO6 and Bi2MoO6 with aurivillius structure, and BiVO4 with a monoclinic scheelite structure. However, most of these semiconductors are not ideal photocatalytic materials, because of their low quantum efficiency. Thus, graphene (GR)-based semiconductor photocatalysts have attracted extensive attention because of their usefulness in environmental and energy applications. Here, we will summarize the recent progress in the fabrication of GR-based semiconductor photocatalysts for oxygen evolution from water under visible-light irradiation. Morphology control and composition design of the semiconductors on GR sheets are reviewed in relation to the properties of photocatalytic oxygen generation. The importance of the interface between semiconductors and GR are highlighted.

Hematite (α-Fe₂O₃) photoanodes are widely studied as candidates for solar water splitting. While hematite is one of very few materials that possess the essential properties for solar water splitting, to this day only very modest solar to hydrogen conversion efficiencies were reported. Motivated by research in photovoltaic cells wherein the efficiency is known to increase with increasing flux concentration, this work explores water photo-oxidation with thin film hematite photoanodes under high flux concentration. Although this effect has been studied extensively in photovoltaic cells, there is very little known on the performance of water splitting photoanodes and photoelectrochemical cells under high flux concentration. In this work we report the performance of thin film hematite photoanodes at flux concentrations ranging from 1 (1 Sun) to 27. As in photovoltaics, the maximum photocurrent was found to scale linearly with the incident flux concentration. Notably, the onset potential of water photo-oxidation was found to shift to lower potentials with increasing flux concentration at a rate of 210 mV per decade, about three times faster than the open circuit voltage of concentrated photovoltaic cells. This rapid increase is highly beneficial for water splitting tandem cells, and it may shed new light on the water photo-oxidation mechanism.

WO3/TiO2 was successfully synthesized through one step hydrothermal environmentally-friendly process. It is well known that the heterostructure (HE) formation by one step synthesis is a difficult procedure to control as different oxides have unique synthesis conditions, (pH, temperature, etc.). In this work we report a facile and green process in order to obtain HE by two distinct routes: 1) by crystallizing WO₃ in the presence of pre-formed TiO₂ nanoparticles and 2) both pre formed WO₃ and TiO₂ as building blocks to achieve high contact between the oxides. The formation of HE was verified by XRD measurements. Also, the interface identification was provided by HRTEM. Interestingly the distinct routes have shown differences of structural and electronic properties of the HE systems. The surface area observed after BET analysis was 4 fold higher for the route 2 when compared to the route 1 synthesis. Also, the bandgap energy (E_g) - estimated from DRS data - was verified an increase in E_g when compared to the isolated oxides, for route 1. The opposite have been observed for the route 2. From these results it is expected different photoelectrochemical responses for the different studied routes to produce the HE. Photo-electrochemical techniques (PEC) were applied to evaluate the effect of HE formation. The advantage is the possibility of directly evaluate the electronic transfer during the photocatalytical process, as well as describe important features to this property, such as the Fermi level of the oxides. A key aspect in HE formation is the possibility of Eg engineering that may provide a more active system by offering effective charge separation. This fact is explained by the electron transfer from the conduction bands and the hole transfer between the valence bands under the driving force of Fermi energy difference between the two semiconductors, decreasing the recombination rate and increasing the lifetime of the electron-hole pair. PEC tests were evaluated using current-potential in darkness and under UVC radiation. Carbon paste electrode was utilized with and without the HE in the photoelectrochemical cell. Under UVC light, the maximum photocurrent increased in both cases, which may be due to the photoelectric effect of the HE systems. However, this increase is clearly more important in route 1, when compared to the route 2 photoelectrode, indicating the efficiency of charge separation (flat band potential value, V_fb). This shift can be attributed to the electronic differences for the synthetic routes and further studies regarding the electron-hole pair recombination will be investigated. It is possible to obtain the pseudo first order velocity reaction (life time inverse) to charge recombination and transference process. This method is based in cyclic voltammetry without illumination and open circuit voltage decay after illumination. This approach will be a good tool to understand the photocatalytical behavior of these heterostructures.

The quest for abundant, renewable energy is currently one of the world&’s greatest technological challenges. One solution to this problem is the conversion of solar energy to storable chemical fuels, such as H₂. Hydrogen generated from photoelectrochemical (PEC) conversion of water has the potential to provide clean, sustainable, abundant and transportable energy. A PEC cell requires a semiconductor photoelectrode that fulfills several essential prerequisites: a small semiconductor bandgap for efficiently harvesting a large proportion of the solar spectrum, appropriate band edges for water oxidation and reduction, high conversion efficiency of photogenerated carriers to hydrogen, durability in aqueous environments, and low cost. Though vigorous efforts have been made, no single semiconductor has been found to meet all these requirements. Three-dimensional inverse opal (IO) nanostructures are potential candidate materials for nanostructured photoanode to realize highly efficient PEC solar energy harvesting. We report detailed studies performed to evaluate the optical and PEC performances of the TiO₂ inverse opal based nanostructures and establish key parameters which enable efficient PEC solar energy harvesting.
TiO₂ IO nanostructures were developed by controlled infiltrations of self-assembled polystyrene opal templates using ALD method.¹ As fabricated TiO₂ IO samples using stop-flow ALD are photosensitized with CdS quantum dots and applied as a photoanode for visible light driven PEC hydrogen generation.² Bilayer photoanodes based on optically and electrically coupled ZnO nanowires with TiO₂ IO were developed using ALD to improve photon absorptions and light harvesting efficiency through photonic crystal light manipulations.³ In order to address the high surface area requirement, we designed 3D ordered nanobushes by hierarchically integrating dense networks of ZnO nanowires with TiO₂ IO for efficient solar to hydrogen conversions.⁴
A novel method for photosensitization based on atomic layer deposition and ion exchange reaction (ALDIER) was demonstrated to achieve a highly controllable and homogeneous coating of photosensitizer particles on nanostructured TiO₂ substrates.⁵ Finally, we report a novel photoanode for water splitting developed by spatially coating up-conversion nanoparticles which allows spectral conversions and broadband solar light absorptions by the photoanode for enhanced charge separation.⁶
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[2]. C. W Cheng, & S K Karuturi et al., Small 2011, 8 (1), 37-42.
[3]. S K Karuturi et al., Nano Energy 2012, 1(2), 322.
[4]. S K Karuturi et al., Adv. Mater. 2012, 24(30), 4157-62.
[5]. J. L Luo, & S K Karuturi et al., Sci. Rep. 2012, 2, 451.
[6]. L. T Su & S K Karuturi et al., Adv. Mater. 2013, 25, 1603-1607.

The net growth in population in developing countries predicted for the next decades highlights the need to find alternative energy sources in order to sustain the energy demand. In this context solar energy is a perfect candidate as the amount of energy reaching the earth in a year significantly exceeds our energy demands.¹ The unprecedented synthetic effort for the development of systems for solar H₂ production requires an equivalent development in the understanding of these systems. Efficient and robust solar fuels systems have been recently developed using new state-of-the-art synthetic methods capable of producing extremely efficient but complicated structures. One example of this complex architecture is the Cu₂O /AZO/TiO₂/RuO_x multi-junction photocatalyst developed by Grätzel and co-workers.² This system is formed by a good light absorber protected against photodegradation with AZO/TiO₂ and activated for water reduction with RuO_x. We have studied the kinetics of charge carriers in this system using transient absorption spectroscopy (TAS) to gain insight on how the formation of the junctions affects charge separation and ultimately water reduction. TAS is a powerful and versatile technique for the study of solar fuels systems as it allows the direct monitoring of charges and excited states from the timescale of generation to the timescale of the catalysis.³ In this talk I will explain how we have modified the classical TAS experiment to be able to study the proton reduction reaction in the Cu₂O photocathode under operation conditions. I will show that despite the multiple junctions which aid charge separation and prevent photodegradation this system suffers from fast electron/hole recombination that competes with water reduction. I will also explain the role of the catalyst layer not only in enabling the catalysis but also in achieving efficient charge separation across the multi-layer structure. The conclusions from this work can help guide the design of multi-layer based photoelectrodes for water splitting devices.
References
1. N. S. Lewis; D. G. Nocera, Proc. Natl. Acad. Sci. U.S.A., 2006, 103, 15729-15735.
2. Tilley, S. D.; Schreier, M.; Azevedo, J.; Stefik, M.; Graetzel, M., Advanced Functional Materials 2014,24, 303-311.
3. Pastor, E.; Pesci, F. M.; Reynal, A.; Handoko, A. D.; Guo, M.; An, X.; Cowan, A. J.; Klug, D. R.; Durrant, J. R.; Tang, J., Phys. Chem. Chem. Phys. 2014,16, 5922-5926.

The efficiency of solar cells is limited by the so-called spectral mismatch between the solar spectrum and the solar cell. A possible way to reduce these losses is to convert ultraviolet photons into visible photons that can be converted into a photocurrent by the solar cell. This photon conversion process is called downshifting. Here we propose to implement the downshifting functionality into the encapsulant of solar cells. Carefully selected and synthesized coordination complexes enable the downshifting property. We show that the spectral response of silicon solar cells can be increased in the ultraviolet by 27 absolute % and the overall efficiency by almost 1 absolute % thanks to downshifting.