Symposium NT1 : Functional Nanostructures and Metamaterials for Solar Energy and Novel Optical Phenomena

In order to utilize solar power for the production of solar electricity and solar fuels on a global scale, it will be necessary to develop solar photon conversion systems that have an appropriate combination of high efficiency (delivered watts/m²) and low capital cost ($/m²). One potential, long-term approach to attain high conversion efficiencies above the well-known Shockley-Queisser thermodynamic limit of 32% is to utilize the unique properties of quantum dot/rod (QD/QR) nanostructures to control the relaxation dynamics of photogenerated carriers to produce either enhanced photocurrent through efficient photogenerated electron-hole pair multiplication or enhanced photopotential through hot electron transport and transfer processes. To achieve these desirable effects it is necessary to understand and control the dynamics of hot electron and hole relaxation, cooling, charge transport, and interfacial charge transfer of the photogenerated carriers. These fundamental dynamics in various bulk and nanoscale semiconductors have been studied for many years using transient absorption, photoluminescence, photocurrent, and THz spectroscopy with fs to ns time resolution The prediction that the generation of more than one electron-hole pair (which exist as excitons in size-quantized nanostructures) per absorbed photon would be an efficient process in QDs and QRs has been confirmed over the past several years in different classes of materials and their architectures. Very efficient and ultrafast multiple exciton generation (MEG), also called Carrier Multiplication (CM), from absorbed single high energy photons has been reported in Group IV-VI and Group IV semiconductors and associated solar photon conversion devices for solar electricity and solar fuels (e.g. H₂) production. Selected aspects of this work will be summarized and recent advances will be discussed. Finally, the analogous MEG effect in molecules (called singlet fission) and its use in molecular-based solar cells will also be discussed

Among various photovoltaics, some require electrolyte to serves as “shuttles” for charge transmission, e.g. dye-sensitized solar cells (DSCs). As one key component in DSCs, counter electrode is responsible for collection of electrons from external circuit and reduction of the redox couple, e. g. triiodide (I₃^–) and iodide (I^–). Generally, CE can be prepared by sputtering of platinum (Pt) on conductive glass. However, there are many limitations in terms of using Pt as CEs, like resource scarcity. Consequently, minimizing the use of Pt or replacing Pt with inexpensive but efficient catalyst is meaningful and has become one of the priorities in this field. Recent years, we have developed dozens of inorganic materials that proved very efficient and nearly approached Pt when fabricated into CE for DSCs. These non-Pt materials include oxides, sulfides, selenides, and tellurides. In our work, the structure-effect relationships at multiscale were revealed systematically, for example, the dependence of catalytic activity and selectivity on crystallinity, microscopic morphology, surface structure, and so on.
Single-atom catalysts (SACs) currently is a new frontier in the fields of catalytic science. SACs can be regarded as one extreme case because the particle size reaches the limit. In our work, we for the first time demonstrated the use of SAC of FeO_x-supported Pt single atoms (Pt/FeO_x-SAC) as counter electrode (CE) in dye-sensitized solar cells (DSCs). The electrocatalytic and photovoltaic behaviors of such SACs in mesoporous photovoltaic device, and the remarkable atom utility were well illustrated. Compared with conventional Pt CE, the SAC-based CE exhibited a much better activity and reversibility for catalytic reduction of triiodide into iodide. Pt/FeO_x-SAC with an extremely Pt loading (only 0.08%) exhibited the most effective atom utilitzation and doubled the power conversion efficiency (PCE) of the DSCs based on bare FeO_x. Using such Pt/FeO_x-SACs, high PCEs were achieved while the consumption of Pt in DSCs would be reduced significantly. We ascribed the superior catalytic performance of Pt/FeO_x-SAC to the unsaturated coordination environment of Pt single atom. Besides, the Pt/FeO_x-SAC also exhibited good stability in the electrolyte solution because Pt single atoms were chemically and tightly bonded on the support. Finally, we proposed some effective strategies to further improve the performance of SAC in DSCs.

Electrospun titanium dioxide nanofibers (TiO2-NFs) and single-walled carbon nanotubes (SWCNTs) were combined to form a nanocomposite light-scattering layer for dye-sensitized solar cells (DSSCs). No nanoparticulates were present in the light-scattering layer and the combination of TiO2-NFs and SWCNTs and formed a fibrous lawn-like nanoarchitecture. The integration of SWCNTs to the TiO2-NFs DSSCs increased the power conversion efficiency (PCE) from 2.90 % to 4.83 % under 1 sun illumination. Furthermore, TiO2-NF and TiO2- NF/SWCNT-based DSSCs reached PCE values of up to 3.7 % and 6.6 % under reduced light intensities (0.25 sun). We hypothesize that the addition of SWCNTs provides additional charge-transport channels, which contributes to an increase in the diffusion coefficient and significant improvement in device performance.¹


References

(1) Macdonald, T. J.; Tune, D. D.; Dewi, M. R.; Gibson, C. T.; Shapter, J. G.; Nann, T. ChemSusChem 2015, n/a – n/a

The use of plasmonic structures have been a great deal of interest in fabricating electric-field enhanced organic device applications such as biosensors and photovoltaic devices. Especially, the plasmonic solar cell is a promissing approach to create additional light traping layer facilitating the improvement light absorption capability and efficiency of the solar cells without increase of the active layer thickness. In this work, we studied the effect of grating-coupling surface plasmon excitation on organic thin film solar cells. The solar cell Al/P3HT:PCBM/PEDOT:PSS/ITO/glass was fabricated. The subwavelength diffraction grating pattern was transferred form a master template to the active layer of the solar cell using PDMS mold. The formation of the grating structure in the surface of the blended polymer was studied. The effect of two different diffraction gratings, in particular periodic (BD-R) and non-periodic (BD) were studied. The use of used-BD-R and BD can be an alternative way to reduce electronic wastes yet benefit the energy industrial. The SPR reflectivity results reveal the increase of trapped light as a broad band absorption from 400 nm to 650 nm in the device with grating structure which is originated from the effect of light scattering. The increase of the absorption at the wavelength long than 650 nm can be observed and assigned to the GCSPR of Al grating as the present of red-shifted dip when increase the incident angle. Introducing BD and BDR diffraction grating patterns as the additional light trapping layer could increase the short-circuit photocurrent and the efficiency of the solar cells. Furthermore, gold nanoparticles were added in the devices to study the mutiple plasmonic effext, i.e. both garting-coupled propagating plasmon and gold nanoparticles localized surface plasmon., on the perfoemance of organic thin film solar cells.

Polymer:fullerene-based photovoltaics have shown great promise as third-generation solar cells due to their solution-processability, low toxicity, earth-abundant elemental constituents, and short energy payback times. Although polymer:fullerene-based photovoltaic efficiency has recently exceeded 10 %, the efficiency is still limited for two primary reasons: 1) the competing constraints on the thickness of the polymer:fullerene active layer, in that it must be kept thin enough to minimize charge recombination while simultaneously being thick enough to absorb all of the incident light; and 2) conjugated polymers typically have narrow absorption bandwidths, preventing absorption of the full solar spectrum. Here, we simultaneously offer solutions to both efficiency limitations by employing plasmonic metasurfaces modified by MoS₂ thin-film coatings. We demonstrate through integrating sphere reflectance measurements that the plasmonic metasurface improves absorption within thin-films of P3HT:PCBM, one of the most widely-studied polymer:fullerene active layers. Using electromagnetic simulations, we show that the absorption enhancement arises from excitation of both localized and propagating surface plasmon modes, in addition to electromagnetic coupling between surface plasmons from the metasurface and the optical transitions of the P3HT:PCBM. To address the narrow absorption bandwidth of typical polymer:fullerene active layers, we employed MoS₂-metasurface heterostructures, and we show that the MoS₂ plays an active role in the charge carrier photogeneration in P3HT:PCBM, resulting in a 6-fold increase in the P3HT polaron population. Using microscope-coupled reflectance measurements and electromagnetic simulations, we show that active layer absorption is broadened by 170 nm for P3HT:PCBM using MoS₂-metasurface heterostructures as substrates. We show that the broadened and enhanced absorption spectrum and enhanced polaron population can only be achieved through the combined synergistic effect of the MoS₂ and the plasmonic metasurface. Future work includes employing MoS₂-metasurface heterostructures as electrodes in polymer:fullerene photovoltaics.

Conventional light trapping in solar cells is based on incoherent light scattering achieved with rough interfaces. However, this approach is not suitable for thin-film solar cells with thicknesses below 1µm.

We propose a new paradigm for light trapping. It is based on multi-resonant absorption in periodically nanostructured absorbers. Broadband absorption is achieved with a series of overlapping resonant modes in the critical coupling regime. We have developed a simple theoretical model and derived a closed-form expression for the absorption limit. We demonstrate that this multi-resonant critical coupling limit exceeds the conventional lambertian limit (4n²) for any absorber thickness (Jsc).

These results could allow a drastic reduction of the absorber thickness in thin-film photovoltaics. We provide numerical and experimental examples of multi-resonant absorption in ultra-thin semiconductor layers. We also present our latest experimental results of ultra-thin solar cells made of various semiconductor materials (GaAs, CIGS, c-Si). We will discuss the record short-circuit currents achieved in ultra-thin photovoltaic devices, and the technological challenges that should be addressed to reach the theoretical limits.

Today, there is a clear worldwide consensus regarding the need for the long-term replacement of fossil fuels by low-cost, high-efficient, renewable and environmentally friendly sources of energy. The solar cell technology becomes tremendously developing field of research which can meet the target. While the single crystal silicon solar cells possess high efficiency ~25 % [1], the initial cost and their fragility motivated researchers to focus on the new type thin film-based devices, such as amorphous silicon [2], CdTe [3] or CuGa_1-xIn_xSe₂ (CIGS) [4], dye-sensitized cells (DSSC) [5], organic cells [6] and perovskites [7] which can be deposited on any substrates. Still, there is a reason why thin film solar panels have not replaced older types yet. They are just not as efficient. Additionally, some thin film materials have shown degradation of performance over time and can be toxic (CdTe). However, the hybrid photocells employing TiO₂ oxide/chromophore interface have potential to improve the functionality of the new solar cells [8].
For instance, tailoring the TiO₂ anode chromophore interface can increase the efficiency of the cells, such as DSSC and perovskite-based solar cells. The enhancement can be achieved by increasing the interfacial surface area between the chromophore and the TiO₂ oxide in order to facilitate charge separation. Unlike randomly ordered mesoporous TiO₂ support, ordered nanostructures, such as self-organized TiO₂ nanotubes with high aspect ratio or TiO₂ nanowires, offer the advantage of directed charge transport and controlled phase separation between donor and acceptor materials and thus they seem to be one of the most promising nanoscale solar hybrid technologies [9].
Here, we will present initial photo-electrochemical results for anodic TiO₂ nanotubes grown with different diameter sizes utilized as highly ordered n-type conductive scaffold for inorganic chromophores [10, 11]. Downscaling the nanotube diameter significantly increases the number of nanotubes per square unit and thus the active surface area increases as well. The photo-chemical cells were fabricated using the solvent solution method, when TiO₂ nanotubes were infilled with inorganic chromophores dissolved in appropriate solvents by soaking or spin-coating. The possible improvement of such hybrid solar cells will be discussed.

[1] M. A. Green et al. Progress in Photovoltaics: Research and Applications 23 (2015) 1.
[2] D. E. Carlson and C. R. Wronski, Apllied Phis. Lett. 28 (1976) 671.
[3] J. Britt and C. Ferekides Apllied Phisi. Lett. 62 (1993) 2851.
[4] P. Jackson et al. Photovoltaics: Research and Applications 19 (2011) 894.
[5] B. ORegan and M. Grätzel, Nature 353 (1991) 737.
[6] D. Wohrle and D. Meissner Advanced Materials 3 (1991) 129.
[7] M. Liu et al. Nature 501 (2013) 395.
[8] J. M. Macak et al., Curr. Opin. Solid State Mater. Sci. 1-2 (2007) 3.
[9] E. C. Garnet et al. Annual Review of Materials Research 43 (2011) 269.
[10] MS in preparation
[11] MS in preparation.

For plasmonic metal-semiconductor heterojunctions, plasmon can induce the charge carries in semiconductors via two near-field mechanisms, that is, the plasmon-induced resonance energy transfer (PIRET) and the hot electron injection. This presentation will discuss the difference between the two mechanisms. PIRET proceeds via the dipole-dipole interaction between the metal and the semiconductor. It depends on the gap and the spectra overlap between the metal and the semiconductor. The hot electron injection from the metal to the semiconductor depends on the Schottky barrier at the metal-semiconductor interface. The occurrence of two mechanisms in a metal-semiconductor heterojunction is controlled by some factors such as the physical contact and the spectral overlap.

Graphene-like two-dimensional (2D) crystals with non-zero band gaps exhibit highly interesting anisotropic dimensionality-dependent properties and bear high potential for ultrathin electronics. Amongst the most investigated 2D semiconductors, one material recently moved into focus: Exfoliated InSe, a van-der-Waals layered III-VI metal chalcogenide with an atomically smooth surface. It has proven its highly promising (opto-)electronic properties as high mobility field-effect transistor [1] and even outperformed other 2D crystal-based compounds like MoS₂ and GaSe as high responsivity (visible to NIR) photodetector [2, 3].
We have synthesized ultrathin 2D InSe layers (<6 nm with organic ligands) by a ligand templated colloidal method and have fully characterized the 2D structures with electron and atomic force microscopy, X-ray photoelectron spectroscopy and scattering methods [4]. By applying ultrafast transient absorption spectroscopy, we evaluate the charge carrier and recombination dynamics in atomically thin InSe and extract (intrinsic) mobilities of ~30 cm²/Vs by using time-resolved terahertz spectroscopy. The combination of colloidal synthesis and pump-probe spectroscopy allows us to assess the potential of solution-processed atomic InSe layers for next generation electronics.

[1] Sucharitakul, S.; Goble, N. J.; Kumar, U. R.; Sankar, R.; Bogorad, Z. A.; Chou, F.-C.; Chen, Y.-T.; Gao, X. P. A., Nano Lett. 2015, DOI:10.1021/acs.nanolett.5b00493.
[2] Tamalampudi, S. R.; Lu, Y.-Y.; Kumar, R. U.; Sankar, R.; Liao, C.-D.; Moorthy, K. B., Karukanara; Cheng, C.-H.; Chou, F. C.; Chen, Y.-T., Nano Lett. 2014, 14, 2800-2806.
[3] Lei, S.; Ge, L.; Najmaei, S.; George, A.; Kappera, R.; and Lou, J.; Chhowalla, M.; Yamaguchi, H.; Gupta, G.; Vajtai, R.; Mohite, A. D.; Ajayan, P. M.; ACS Nano 2014, 8, 1263-1272.
[4] ] Lauth, J.; Gorris, F. E. S.; Samadi Khoshkhoo, M.; Chassé, T.; Wiebke Friedrich, W.; Lebedeva, V.; Meyer, A.; Klinke, C.; Kornowski, A.; Scheele, M.; Weller, H., submitted.

A new scientific principle[i] has produced record-breaking solar cells. This is exemplified by the mantra: “A great solar cell also needs to be a great LED”. It is essential to remove the original semiconductor substrate, which absorbs luminescence, and to replace it with a high reflectivity mirror.
In thermo-photovoltaics, high energy photons from a thermal source are converted to electricity. The question is what to do about the majority of low energy infrared photons? It was recognized that the semiconductor band-edge itself can provide excellent spectral filtering for thermophotovoltaics, efficiently reflecting the unused infrared radiation back to the heat source. Exactly those low energy photons that fail to produce an electron-hole pair, are the photons that need to be recycled.
Thus the effort to reflect band-edge luminescence in solar cells has serendipitously created the technology to reflect all infrared wavelengths, which can revolutionize thermo-photovoltaics. We have never before had such high rear reflectivity for sub-bandgap radiation, permitting step-function spectral control of the unused infrared photons for the first time. This enables conversion from heat[ii] to electricity with >50% efficiency. Such a lightweight “engine” can provide power to electric cars, aerial vehicles, spacecraft, homes, and stationary power plants.


[i] O. D. Miller, Eli Yablonovitch, and S. R. Kurtz, “Strong Internal and External Luminescence as Solar Cells Approach the Shockley–Queisser Limit”, IEEE J. Photovoltaics, vol. 2, pp. 303-311 (2012). DOI: 10.1109/JPHOTOV.2012.2198434

[ii] The heat source can be combustion, radio-activity, or solar thermal.

We show that the use of photonic structures, which allows control of both solar and thermal radiation spectra, has important implications for various aspects of solar energy conversion, including voltage, current and cooling considerations.

Silicon heterojunction (SHJ) solar cells, where the crystalline Si is passivated by thin layers of a-Si:H, have attracted significant interest due to their record voltages. However, reflection from macroscopic metallic ‘fingers’ has constrained current generation – and efficiency – in front contacted cells. These fingers are essential for harvesting electrons due to poor lateral transport in indium tin oxide (ITO), which is both the most common transparent conductive electrode (TCE) and limited by a fundamental tradeoff between transmission and conductance. Since the optimal finger spacing depends on TCE conductivity, the efficiency of SHJ cells is married to the quality of the TCE layer.

In this presentation we introduce a hybrid TCE which decouples the optical and electrical functionalities, and enables the independent optimization of each function. The geometry consists of Ag nanowires (80 nm wide, 120 nm tall) arranged in a sparse square grid, fabricated on top of an ultrathin ITO layer to transport charge in the interstitial regions. The nanowires are conformally coated with Si₃N₄, providing both environmental stability and an antireflection coating free from the interband and free carrier losses in ITO. For cells with fingers spaced by the standard 2 mm, replacing ITO with Si₃N₄ reduces optical loss by 1.1 mA cm^-2_.

We apply this nanostructured hybrid electrode to large-area (4 cm²) untextured SHJ solar cells using substrate-conformal imprint lithography (SCIL), a fabrication method that enables rapid, wafer-scale fabrication of nanowire (NW) arrays with full geometric control. The SCIL fabrication process does not damage the passivating a-Si:H layer, with consistent values for V_oc measured on cells with and without nanowire modification. Optically, the nanowire networks exhibit broadband anomalous transmission due to detuning of the plasmonic nanowire resonances from the solar spectrum, shading 30% less light than the NW area coverage. Electrically, the square grids of nanowires have measured sheet resistances of 4.0, 7.2, and 15.0 Ω/sq. for pitches of 1, 2, and 4 µm, respectively. This is a significant (10x) improvement relative to an industrially processed ITO electrode with a measured 150 Ω/sq. sheet resistance. Due to the improved transmission and conductivity, the hybrid electrode enables an increase in finger pitch from 2 to 5 mm, reducing shading. For the champion hybrid electrode SHJ cell we measure a J_sc enhancement of 1.4 mA cm^-2 (32.7 to 34.1 mA cm^-2) with a simultaneous increase in FF (0.622 to 0.670), yielding an absolute efficiency enhancement of 2.2% (13.8% to 16.0%).

This demonstration of an engineered ‘meta-electrode’ within the electrical and optical environment of high performance SHJ solar cells shows that large-area nanostructuring provides practical pathway to increased performance and a reduced dependence on rare metals, especially indium.

Gels and aerogels manufactured from a variety of nanoparticles available in colloidal solutions have recently proven to provide an opportunity to marry the nanoscale world with that of materials of macro dimensions which can be easily manipulated and processed, whilst maintaining most of the nanoscale properties [1]. The materials carry an enormous potential for applications. This is largely related to their extremely low density and high porosity providing access to the capacious inner surface of the interconnected nanoobjects they consist of. The aerogel materials may be further processed in order to achieve improvements in their properties relevant to applications in optical sensing and catalysis.
The commercialization of polymer electrolyte fuel cells (PEFC) is still hindered by the cathode electrocatalyst for the oxygen reduction reaction (ORR) not fulfilling the criteria of low cost, high performance, and high durability. We recently developed a facile strategy for the controllable synthesis of nanoparticle-based bimetallic Pt_xPd_y aerogels with high surface area and large porosity, which act as highly active and stable catalysts for the ORR in PEFC cathodes. In addition to excellent durability the Pt_xPd_y aerogels show superior electrocatalytic activity towards the ORR with the Pt₈₀Pd₂₀ aerogel exhibiting a five times mass activity enhancement compared to commercial Pt/C catalysts [2].
The reminder of the presentation will be devoted to a) sensors derived from aerogels [3], and b) nanocrystals incorporated into macrocrystals of varying compositions [4]. In the latter superstructures the nanocrystals exhibit remarkable photostabilities, enhanced emission quantum yields and may act as color conversion materials.
References
[1] a) Mohanan JL, Arachchige IU, Brock SL (2005). Science 307, 397-400, b) Herrmann AK, Formanek P, Borchardt L, Klose M, Giebeler L, Eckert J, Kaskel S, Gaponik N, Eychmüller A (2014). Chem. Mater.Acc. Chem. Res. 48, 154-162.

[2] Liu W, Rodriguez P, Borchardt L, Foelske A, Yuan J, Herrmann AK, Geiger D, Zheng Z, Kaskel S, Gaponik N, Kötz R, Schmidt TJ, Eychmüller A (2013). Angew. Chem. Int. Ed. 52, 9849-9852.

[3] Yuan J, Gaponik N, Eychmüller A (2012). Anal. Chem. 84, 5047-5052, (2013) Angew. Chemie Int. Ed. 52, 976-979.

[4] a) Otto T, Müller M, Mundra P, Lesnyak V, Demir HV, Gaponik N, Eychmüller A (2012). Nano Lett. 12, 5348-5354, b) Müller M, Kaiser M, Stachowski GM, Resch-Genger U, Gaponik N, Eychmüller A (2014). Chem. Mater. 26, 3231-3237, c) Adam M, Tietze R, Gaponik N, Eychmüller A (2015). Z. Phys. Chem. 229, 109-118. d) Adam M, Wang Z, Dubavik A, Stachowski GM, Meerbach C, Soran-Erdem Z, Rengers C, Demir HV, Gaponik N, Eychmüller A (2015). Adv. Funct. Mat. 25, 2638-2645.

Photon driven chemical reactions at metal surfaces have been studied for over forty years with a majority of research focused on non-thermal, electron driven chemical processes in the context of controlling reaction selectivity and understanding fundamentals of non-adiabatic surface reactivity. A majority of these studies have been executed on metal single crystals, where surface electric fields have relatively small magnitudes and chemistry is typically driven through substrate-mediated excitation of adsorbate-metal bonds that competes with hot charge carrier diffusion into the bulk. These characteristics of photon driven reactions on metal single crystals result in processes with characteristically low yields and minimal, if any, ability to control the outcome of probed chemistry as the processes of photon absorption and induced chemical reactions are delocalized from each other.

Metal nanoparticles offer opportunities to overcome these limitations through the enhanced electric fields created in response to excitation of localized surface plasmon resonance and exploiting huge surface area to volume ratios that allows chemistry by direct photoexcitation of adsorbate-metal bonds. In this talk I will discuss recent results where we show that decaying plasmonic excitations at junctions of Ag nanocrystals can be efficiently funneled into catalyzing O₂ dissociation on the Ag surface. Associated with plasmon driven chemical reactivity are unique wavelength, intensity and isotope dependencies, which shed light on fundamental mechanistic insights. The critical role of “hot spots” at nanoparticle clusters will be addressed. In a second example, I will describe how the large surface area to volume ratio characteristic of sub 5 nanometer diameter metal crystals can be exploited to control outcome of catalytic processes through direct photoexcitation of targeted adsorbate metal-bonds. The implications of these results and unresolved questions will be discussed.

Manipulating light in a controllable manner is always highly desired in photonics research. In a long time, the phase control plays the vital role. Recently, electromagnetic meta-surfaces, made by carefully designed inhomogeneous microstructures to supply abrupt phase changes for impinging light, were found to exhibit strong abilities to control light propagations, leading to extraordinary physical effects such as generalized Snell’s law [1-4], a new bridge to link propagating-wave and surface-plasmon-polariton (SPP) [3,5], flat-lens focusing [6], meta-hologram [7].

In this talk, I will introduce our recent works on gradient metasurfaces. The first one is about the experimental realization of photonic spin Hall effect (PSHE) with nearly 100% efficiency [8], where a general criterion to achieve this goal is theoretically derived based on the Jones Matrix analysis. The second one is about a newly proposed concept to design the high efficiency surface plasmon coupler. The new coupler can solve the inherent problems in conventional ones (i.e., the reflection and the decoupling effect) and therefore exhibits the high-efficiency performance [9].

1. N. Yu, P. Genevet, M. A. Kats, et al.. Science 334, 333 (2011).
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4. S. L. Sun, K.-Y. Yang, C.-M. Wang, et al. Nano Letters 12, 6223 (2012).
5. C. Qu, S. Y. Xiao, S. L. Sun, EPL 101, 54302 (2013).
6. X. Li, S. Y. Xiao, B. Cai, et al. Optics Letters 37, 4940 (2012).
7. W. T. Chen, K.-Y. Yang, C.-M. Wang, et al. Nano Letters 14，225-230 (2014).
8. W. Luo, S. Xiao, Q. He, et al. Advanced Optical Materials 3, 1102 (2015).
9. W. Sun, Q. He, S. L. Sun, et al. Light: Science & Applications 5, e16003 (2016).

Nanostructuring organic polymers and organic/inorganic hybrid materials and control of blend morphologies at the molecular level have become the prerequisites for organic photovoltaics (OPVs) that are widely perceived as low-cost alternative energy sources. To achieve all-around high performance, multiple organic and inorganic entities, each designed for specific functions, are commonly incorporated into a single device. Current state-of-the-art approaches to morphology control in these multi-component systems typically involve physical blending and optimization via thermal/solvent annealing. Such trial-and-error approaches are however highly system dependent, lack controllability on the molecular level and generally lead to morphologies at only thermodynamically meta-stable states. We present our efforts in developing a versatile toolbox employing supramolecular chemistry that is capable of precisely nanostructuring multi-component organic/inorganic hybrid materials through self-assembly processes. Specifically, we show that well-defined core-shell composite nanofibers (NFs) containing precisely placed conjugated polymers, inorganic quantum dots and fullerene derivatives, can be obtained through cooperation of orthogonal non-covalent interactions including conjugated polymer crystallization, fullerene aggregation, hydrogen bonding interactions and metal-ligand coordination.^1-4 OPV devices applying these NFs display much improved efficiencies and stability over their conventional bulk heterojunction (BHJ) counterparts.

1. Li, F.; Yager, K. G.; Dawson, N. M.; Yang, J.; Malloy, K. J.; Qin, Y.* Macromolecules 2013, 46, 9021.
2. Li, F.; Yager, K. G.; Dawson, N. M.; Jiang, Y.-B.; Malloy, K. J.; Qin, Y.* Chem. Mater. 2014, 26, 3747.
3. Li, F.; Yager, K. G.; Dawson, N. M.; Jiang, Y.-B.; Malloy, K. J.; Qin, Y.* Polym. Chem. 2015, 6, 721.
4. Li, F.; Dawson, N.; Jiang, Y.-B.; Malloy, K. and Qin Y.* Polymer 2015, 76, 220.

As interfacial layers play a crucial role in determining device efficiency and lifetime, the introduction of appropriate interfacial layers has become a significant approach in improving the performance of hybrid solar cells. Recently, the addition of fullerenes as modifiers of cathode buffer layers (CBLs) such as zinc oxide has shown promising results. Aside from improving the interfacial contact between the inorganic CBL and the organic active layer, fullerenes as interfacial layers have also been reported to induce better crystallinity in the active layer. For ZnO CBLs, modification with fullerenes exhibited the passivation of surface defect states and increase in work function. The solubility of fullerenes and its derivatives, however, is challenging, thus limiting their application in solution processes. Herein, using a water-soluble fullerene derivative, C₆₀ pyrrolidine tris-acid (CPTA), we demonstrate the growth of fullerene-modified ZnO thin films by electrochemical deposition. These films were used as CBLs for inverted hybrid solar cells with the structure Au/MoO₃/P3HT:PCBM/ZnO-CPTA/ITO. Our films had a nanorod array morphology and exhibited high optical transmittance. The devices showed improved charge current density compared with devices using unmodified ZnO.

Semi-transparent organic solar cells (st-OSCs) not only inherit the unique properties of organic solar cells (OSCs), but also bring innovation to both anode and cathode to preserve high transparency. The high recovered efficiency of st-OSCs with sunlight illumination from both electrodes is of great interest in different applications. However, the performance of st-OSCs is still low compared to opaque OSCs (o-OSCs) due to single pass of the light through devices and thus reduction of light absorption. In addition, the severe imbalance of device efficiency can exist between top- and bottom-illumination cases. Here, we propose a hybrid light incoupling design with structure of metal/nanoparticle/dielectric (M/NP/D) to optically improve the recovered efficiency of st-OSCs and achieve a more balance recovered efficiency between top- and bottom-illumination cases.[1] The M/NP/D nanostructure consists of high index and low-loss nanoparticles (NPs) (here we use Si NPs scatter instead of metal NPs) and index matching material (here we use Tris(8-hydroxyquinolinato) aluminium-Alq₃) on ultra-thin Ag film, i.e. Ag/Si NPs/Alq₃ as the top hybrid electrode of st-OSCs. It should be noted that the usage of Alq₃ and Si NP in the proposed hybrid M/NP/D nanostructure has less intrinsic loss as compared the previous reported metal nanomaterials.
With the excitation of magnetic resonance of Si NP, wave destructive interference from Alq₃, and total internal reflection existed in the hybrid structure, the transmission of the electrode is improved complementary in long (due to Si NPs) as well as short wavelength regions (due to Alq₃ layer) with additional synergetic improvement (due to the hybrid M/NP/D nanostructure), resulting in a broadband enhancement that almost cover the whole visible spectrum. After the fabrication of the st-OSC with the hybrid M/NP/D structure as top electrode, the overall transparency of the st-OSCs is improved by 61%. Simultaneously, st-OSCs with hybrid electrode achieved PCE enhancement of about 34% compared to the optimized st-OSCs without light incoupling structure. The high recovered efficiency up to 68% and 87% are demonstrated in top and bottom illuminated devices, respectively. Notably, besides the large enhancement of light incoupling from hybrid Ag/Si NPs/Alq₃ nanostructure, the theoretical and experimental results show that the utilization of Si NPs can not only promote active layer absorption at long wavelength region but also favor an alleviated angular dependence of device performance (J_sc and PCE) in top-illumination.
Consequently, the hybrid transparent electrode with M/NP/D nanostructure can be beneficial to the future transparent photovoltaic applications.

[1] X. G. Ren, X. C. Li, W. C. H. Choy, Optically enhanced semi-transparent organic solar cells through hybrid metal/nanoparticle/dielectric nanostructure, Nano Energy, 2015. 17, 187.

Silver nanowires (AgNWs) are promising materials for use in flexible, transparent electrodes for optoelectronic devices. However, random networks of AgNWs in traditional solution process have a large surface roughness and junction contact resistances, limiting the optoelectronic device performances. Controlling the NW alignment can significantly affect the performance of transparent electrodes. Here, we introduce a capillary printing technique to fabricate uniformly aligned AgNW networks, which greatly lower the percolation threshold and surface roughness of AgNW networks, resulting in a sheet resistance of 19.5 Ω sq^-1 and an optical transmittance of 96.7%, which can be favorably compared to random AgNWs (92.9%, 20 Ω sq^-1). Polymer light-emitting diodes (PLEDs) and polymer solar cells (PSCs) using aligned AgNW electrodes show superior optoelectronic performances, including luminance and power efficiencies of 14.25 cd A^-1 and 10.62 lm W^-1, respectively, in PLEDs, and power conversion efficiencies of 8.57% in PSCs, which is the highest value reported so far using AgNW-based transparent electrodes for fluorescent PLEDs and PSCs.

Transparent electrode as a front contact of flexible organic thin film solar cells requires three properties simultaneously, which are high electrical conductivity, high optical transparency, and high mechanical flexibility. To achieve these characteristics, new-generation transparent electrodes based on metal nanowire networks, graphene, carbon nanotube, and metal mesh are recently investigated. In particular, transparent electrodes based on regular metal mesh show promising performance. However, widespread adoption of metal-mesh based transparent electrodes has been hindered by several challenges. First, fabricating metal-mesh transparent electrodes often involves the physical deposition of metal materials from the vapor phase, which requires expensive and time-consuming vacuum-based processing. Second, a thick layer of metal mesh on the substrate, as required to achieve sufficiently high conductivity in many applications, may easily cause electrical short circuiting. Third, the weak adhesion between the metal mesh and substrate surface results in poor flexibility. The aforementioned limitations call for new metal-mesh electrode structures as well as improved fabrication methods for its production and applications.

In this study, we propose a new structure for flexible transparent electrodes with fully embedded metal mesh, develop a vacuum-free solution-processed fabrication for the metal-mesh electrode, and present its applications in dye sensitized solar cells (DSSCs) and perovskite solar cells. The fabrication process consists of simple solution-processed steps involving lithography, electroplating and imprint transfer. The embedded metal-mesh electrodes exhibited an optical transmittance higher than 90 % (over a wide spectrum of wavelength from 300 nm to 1200 nm), sheet resistance lower than 1 Ω/sq, and a ratio of electrical conductivity to optical conductivity (σ_dc/σ_opt) of more than 10⁴, among the highest of all reported transparent electrodes. Furthermore, it can be bent to 3 mm radius of curvature with negligible degradation of conductance. Based on the proposed electrodes, prototype DSSC and perovskite solar cells are fabricated. In the preliminary experiments, using it as a cathode in the DSSCs, the prototype samples demonstrated power conversion efficiency of 6.3 % for front-side illumination and 4.6 % (higher than FTO glass and ITO-PEN based DSSCs) for back-side illumination. The improvement in the back-side illumination configuration is attributed to higher short-circuit current density (J_sc = 9.04 mA/cm², 40% higher as compared to standard ITO-PEN based DSSCs). The promising optical and electrical properties of our transparent electrode offer the potential use in other type of flexible solar cells such as perovskite solar cells and other flexible optoelectronic devices that we are currently investigating.

We demonstrate novel all-back-contact Si nanohole solar cells via the simple direct deposition of molybdenum oxide (MoO_x) and lithium fluoride (LiF) thin films as dopant-free and selective carrier contacts (SCCs). This is in contrast to conventionally used high-temperature thermal doping processes, which require multistep patterning processes to produce diffusion masks. Both MoO_x and LiF thin films are inserted between the Si absorber and Al electrodes interdigitatedly at the rear cell surfaces, facilitating effective carrier collection at the MoO_x/Si interface and suppressed recombination at the Si and LiF/Al electrode interface. With optimized MoOx and LiF film thickness as well as the all-back-contact design, our 1-cm² Si nanohole solar cells exhibit a conversion efficiency of up to 15.4 %, with an open-circuit voltage of 561 mV and a fill factor of 74.6 %. In particular, because of the significant reduction in Auger/surface recombination as well as the excellent Si-nanohole light absorption, our solar cells exhibit an external quantum efficiency of 83.4 % for short-wavelength light (~400 nm), resulting in a dramatic improvement (54.6%) in the short-circuit current density (36.8 mA/cm²) compared to that of a planar cell (23.8 mA/cm²). Hence, our all-back-contact design using MoO_x and LiF films formed by a simple deposition process presents a unique opportunity to develop highly efficient and low-cost Si nanostructured solar cells.

Our investigation demonstrates that a solution-processed CuI hole-transport layer is a viable and promising alternative to the acidic and more expensive PEDOT:PSS material. This approach incorporates a p-type CuI hole-transport layer that replaces the conventional accepted standard, PEDOT:PSS layer, during the fabrication of high-efficiency poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester solar cells. By optimizing the concentration (0.8 M) of CuI in the precursor solution, the performance of the solar cell is comparable to that of the solar cell using PEDOT. Given the advantageously low processing temperature of CuI, this novel approach leads itself available for the enabling the next generation of flexible solar cells.

An intermediate band in a semiconductor opens new paths for light absorption and increases the conversion efficiency of light into electricity. Closely positioned quantum dots are being considered as an approach to realize efficient and tunable intermediate bands.[1] We have studied the structural and electronic properties of InAs quantum dots (QDs) embedded in Al_0.3Ga_0.7As, (bandgap of 1.99 eV) designed for optimized intermediate band solar cells.[2]

The InAs/AlGaAs QD structures were grown by metalorganic chemical vapor deposition. InAs dots formed as a result of Stranski-Krastanov growth. Each layer of QDs was capped by a 20 nm and 5 nm GaAs layer in samples A and B, respectively. The temperature was then raised during deposition of the capping layer. Once the capping layer was completed, not necessarily fully covering the dots, growth was halted and the temperature was raised to 630 ^oC which is known as indium flushing.[3] The structure consisted of ten layers of InAs dots separated by 90-nm-thick Al_0.3Ga_0.7As barriers. Two-beam diffraction contrast images of the QDs in each sample were recorded in a Philips CM200-FEG electron microscope. The InAs dots were also studied from high-angle annular dark field images which were taken in a JEOL ARM200F electron microscope.

The large lattice mismatch between the QDs and the matrix results in misfit dislocations. Fully-capped QDs (in sample A) exhibit moiré fringes in transmission electron microscope images, indicating significant misfit strain relaxation. Partially-capped QDs (in sample B) are found to be fully strained. The critical thickness for misfit strain relaxation has been determined by experiments. A model based on the equilibrium of the lattice misfit force and the dislocation line tension acting on misfit dislocation loops, is used to understand the experimental observations.

Strong photoluminescence peaks were observed at around 1.3 eV for the strained QDs (Sample B), a value that is close to optimum for intermediate band solar cells. No such luminescence is observed from relaxed QDs. The suppression of plastic strain relaxation improves the photoluminescence of the system indicating that the intermediate band solar cells are promising devices to overcome the standard solar cell efficiency limit leading to better harvesting of light and energy conversion.

References:
[1] A. Martí et al, in Conference Record of the 28th IEEE Photovoltaics Specialists Conference, (IEEE, New York, USA, 2000), pp.940–943.
[2] R. Jakomin et al, J. Appl. Phys. 116, 093511 (2014).
[3] Z. Wasilewski et al, Cryst. Growth 202, 1131(1999).

Semiconductor nanostructures are the most attractive class of materials for current and future functional nanodevices in many applications. Zinc oxide (ZnO) is one such promising semiconductor, which has recently attracted significant attention due to its wide direct band gap, high exciton binding energy and its ability to form a variety of nanostructured configurations. One interesting feature of ZnO is the possibility of tuning its band gap by incorporating magnesium in the structure via alloying. The ZnMgO alloy system raises the possibility of a new optically tunable family of wide band gap materials, which can be used for UV optoelectronic devices. Previous attempts have been made to grown ZnMgO nanostructures with a high Mg content using a variety of techniques, but have not been very fruitful, with results being mainly restricted to a low Mg content and thus a wurtzite structure. Here we report a large tuning in the Mg content of ZnMgO nanostructures grown via vapor transport method, with varying Mg content under different growth conditions. The grown nanostructures, accordingly, varied from wurtzite structure with a low Mg content, to cubic structure with a large Mg content. The vapor transport method was used as it is a simple and cost effective technique for the growth of nanostructures and allows for readily varying the composition of the nanostructures by varying the input content of source materials and the operating conditions.
ZnMgO nanostructures were synthesized using ZnO and magnesium boride (MgB₂) powders as source materials on ZnO coated Si substrates. Two different types of nanostructures were grown with two different growth conditions. The structural, compositional and optical properties of the grown nanostructures were investigated by field emission scanning electron microscopy, energy dispersive x-ray spectroscopy and Raman scattering. In the first type, cubic nanostructures were observed while in the second type, branched nanowires along with wurtzite nanoclusters were observed. EDX analysis of the grown nanostructures revealed a non-uniform concentration of Mg in both types. The cubic nature of the first type points to a Mg concentration of above 67% while the wurtzite nature of the nanoclusters in the second type points to a Mg concentration below 36%, as per literature. The Raman spectra of both types exhibited peaks corresponding to the host phonons of ZnO and peaks in the 600-650 cm^-1 region corresponding to ZnMgO alloy system. The peaks in 600-650 cm^-1 region are indicative of a blue shift brought about by Mg substitution. The first type exhibited a single peak in the 600-650 cm^-1 region while the second type exhibited two peaks in the region possibly indicating the occurrence of phase separation in the second type.
The fabrication of the above mentioned nanostructures along with the results of the various characterization techniques performed, including optical properties in UV range, will be presented in detail.

Working gas pressure during sputter deposition can significantly affect the conformality of a thin film when it is grown on a nanostructured surface. In this study, we fabricated core-shell nanostructured photodetectors, where n-type In₂S₃ nanorod arrays (core) were coated with p-type CuInS₂ (CIS) films (shell) at relatively low and high Ar gas pressures. In₂S₃ nanorods were prepared by glancing angle deposition (GLAD) technique using a thermal evaporator unit. CIS films were deposited by RF sputtering at Ar pressures of 2.75x10^-2 mbar (high pressure) and 7.5x10^-3 mbar (low pressure). The morphological characterization was carried out by means of the SEM. The photocurrent measurement was conducted under AM 1.5 irradiance. Nanostructured photodetectors of high pressure CIS films were shown to demonstrate enhanced photoresponse with 0.099 mA photocurrent value about ~2.4 times higher than low pressure CIS thin film devices. The enhancement originates from the improved core-shell structure achieved by more conformal coating of the CIS shell. In addition, the results were compared to their counterpart thin-film devices for both high and low pressure growth conditions. Nanorod devices with high and low pressure CIS films showed photocurrent values ~20-fold and
~18-fold increase compared to those of high and low pressure film devices, respectively. This finding can be explained by the higher light absorption property of nanorods, and the reduced inter-electrode distance as a result of core-shell structure, which allows the effective capture of the photogenerated carriers. Therefore, the results of this work can pave way to the development of high photoresponce core-shell semiconductor devices fabricated by physical vapor deposition.

Anti-reflection coatings (ARCs) in typical solar cells (SCs) adopt single- or multi-layered thin films with intermediate refractive indices between air and SC materials with a thickness corresponding to quarter wavelength. But they are effective only for a limited range of wavelength and incident angle. In this work, we fabricated a variety of ITO nanostructures such as thin films, nanorods, nanotrees etc. through an oblique angle deposition technique using electron-beam evaporator and applied these nanostructures into ARCs for planar p-Si SCs. From SEM and TEM analyses, the ITO nanotree was found to be grown via a self-catalytic mechanism above ~ 200 °C and the branches have an epitaxial relationship with the trunks. We prepared four Si solar cells with different ARCs; a thin film of TiO₂ with quarter-wavelength thickness (thin film ARC), dense ITO nanorods (dense ARC), dense ITO nanorods/porous ITO nanotrees (hybrid ARC) along with a reference solar cell without any ARC (no ARC). The reflectance is significantly decreased from 35% (no ARC) to 12% (thin film ARC), 9% (dense ARC) and finally 7% (hybrid ARC), which is responsible for the enhancement of solar cell efficiency to 25% (thin film ARC), 38% (dense ARC), and 47% (hybrid ARC) compared with the reference cell. Also in the angular reflectance measurements from 20° to 70°, the hybrid ARC has a very low reflectance of 7% and excellent omni-directionally reflectance, which is significantly low compared with 11% (dense ARC), 18% (thin film ARC), and 45% (no ARC). By considering the angular dependence of solar cells, the annual power density was calculated based on the meridian transitaltitude of Seoul, Republic of Korea. The power enhancement by omnidirectional light absorption is as high as 33.5% in the hybrid ARC solar cell, compared with 16.3% (dense ARC) and 1.4% (thin film ARC). Our results suggest that onmi-directional ARC can be a breakthough technology for the solar power generation.

Copper-indium-selenide (CISe) quantum dots (QDs) are promising candidates which can replace the toxic cadmium- and lead-chalcogenide QDs typically employed in QD solar cell (QDSC) applications. However, there is still a huge gap between the performance of CISe QDSCs and that of conventional toxic QD based devices. Here, we present highly efficient heavy-metal-free CISe QD sensitized solar cells exceeding 8% photoconversion efficiency.¹ By changing the size of QDs, electronic band alignment of QDs can be tuned to optimize the energetics for the effective light absorption and injection of electrons into the TiO2.² In addition, we find that much thicker ZnS protecting layers compared to the conventional one are required to suppress the recombination losses. Both interfacial electron recombination with the electrolyte and nonradiative recombination associated with QDs are graetly reduced, while energetic characteristics of the photoanode are maintained. Finally, our best cell yields a photoconversion efficiency of 8.10% under AM 1.5 G one sun illumination, a record high value for heavy-metal-free QDSCs.

Reference
1) ACS Nano, 2015, 9, 11286–11295.
2) Phys. Chem. Chem. Phys. 2013, 15, 20517-20525.

Most devices used for solar energy harvesting consist of thin, stacked films of metals and semiconductors. In this presentation, I will demonstrate how one can improve the performance of such devices by nano-structuring the constituent layers at length scales below the wavelength of light. The resulting metafilms and metasurfaces offer opportunities to dramatically modify the optical absorption and reflection properties of device layers. This is accomplished by encoding the optical response of nanoscale resonant building blocks into the effective properties of the films and surfaces. To illustrate these points, I will show how nanopatterned metal and semiconductor layers may be used to enhance the performance of solar energy harvesting devices.

Plasmonic metasurfaces enhance light-matter interaction by focusing light into extremely subwavelength dimensions. These carefully designed structures have been used in extremely thin optical component which can mold the wavefront, with exciting applications in optical lenses, beam steering, and biosensing applications. Adding dynamic tunability to these devices opens up the possibility for new application in single pixel detection and 3D imaging as well as optical modulators and switches. However the existing approaches for designing active optical devices in infrared, are either slow or have small refractive index change. Integrating plasmonic metasurfaces with single-layer graphene (SLG) opens exciting opportunities for developing active plasmonic devices because the amplitude and phase of the transmitted and reflected light can be rapidly modulated by injecting charge carriers into graphene [1]. I will describe our recent experimental results demonstrating strong phase modulation in reflection. The phase change due to electric gating of the SLG in the mid-infrared frequency band was measured using a Michelson interferometer. It is shown that the phase of the reflected light can be modulated while keeping its amplitude constant. The phase change is used to demonstrate an electrically controlled (i.e. no moving parts) interferometry capable of measuring distances with sub-micron accuracy. Because of the potentially nanosecond-scale measurement time, active metasurfaces represent a promising platform for ultra-fast standoff detection. [1] Nima Dabidian, Iskandar Kholmanov, Alexander B. Khanikaev, Kaya Tatar, Simeon Trendafilov, S. Hossein Mousavi, Carl Magnuson, Rodney S. Ruoff, and Gennady Shvets, “Electrical Switching of Infrared Light Using Graphene Integration with Plasmonic Fano Resonant Metasurfaces”, ACS Nano 2, 216 (2015).

We report the growth of compositionally graded ZnCdSSe nanosheet heterostructures realized by a novel growth method using a simple CVD route. The resulting products show great promise for lasers and LEDs emitting the white light or any color in the CIE chromatic coordinates. The nanosheets have thicknesses in the range of 60-350 nm and lateral dimensions of up to 60 μm by 40 μm. Compositionally graded heterostructures have attracted a great deal of attentions and various applications were demonstrated using such structures [1-3]. It was shown that nanosheets are the preferred morphology for photonic applications especially for lasers [2]. However, the realization of such nanosheets with high ZnS (Se)-contents for blue emission represent a significant challenge due to the low vapor pressure of ZnS (Se). To overcome such low pressure restriction and to realize multisegment ZnxCd1-xSySe1-y quaternary alloy nanosheets in a side-by-side cavity to emit in the full visible spectrum, we developed a two-step strategy [1]. The novel growth method takes full advantage of the interplay between VLS and VS growth mechanisms, followed by a dual-ion (simultaneous anion and cation) exchange process. The latter one is the most important aspect of our growth method and the evidence of this process was studied in detail through EDS, XRD and PL analyses.Specifically, ZnS and CdSe powders were used as precursors and temperature-dependent composition control was utilized in the growth. As the growth temperature decreases composition of ZnCdSSe structures shifts from ZnS-rich to CdSe-rich side and the morphology changes from nanowire to nanosheet, respectively. First, Cd- and Se-rich ZnCdSSe nanosheet structures are grown at 640°C. The substrate was then moved to 780°C region using a magnet driven rod to further promote diffusion processes. This causes the structures to transform uniformly into a Zn- and Se-rich ZnCdSSe alloy without changing morphology or crystal structure, allowing blue emission. The growth process was then followed by growth at 740°C and the second segment was synthesized by incorporating more Cd and Se ions for green emission. The final growth step at 640°C, added the CdSe-rich segment for red emission, resulting finally in multisegment heterostructure capable of simultaneous RGB (white) light emission. The lower substrate temperatures during third and fourth steps of the growth favor the VS mechanism for 2D nanosheet growth unlike the second case where the exchange process is dominant over the ion transport. By manipulating the position of the substrate and the growth sequence we can grow multiple parallel segments consisting of desired alloy compositions in an ordered or any desired manner for a specific sequence of segments with different bandgaps. [1] F. Fan et al. Nature nanotechnology 10, 796-803 (2015) [2] F. Fan et al. Semicond. Sci. Technol. 28, 065005 (2013)[3] Z. Liu et al. Nano letters 13.10 4945-4950 (2013)

Building-integrated sunlight harvesting could revolutionize urban architecture by enabling buildings to generate their own electricity. Luminescent solar concentrators (LSCs) are one promising approach wherein windows are turned into daytime power supplies. LSCs work by tinting windows with a fluorophore that down converts direct or diffuse light into a wave guided luminescence for collection by an edge-coupled photovoltaic. The fluorophores typically investigated are highly emissive dyes or quantum dots that are then embedded in or coated upon a glass or plastic waveguide. The luminescence propagates to the waveguide edges by total internal reflection and is converted into electricity by high-efficiency solar cells installed along the slab perimeter. Since the surface of the slab exposed to sunlight can be much larger than the surface of its edges, the LSC effectively increases the photon density incident onto the solar cell, which boosts both the photocurrent and the photovoltage. Moreover, by matching the emission wavelength of active chromophores to the spectral peak of the external quantum efficiency, one can achieve a further increase in the power output of the devices. It is also noteworthy that the color and the degree of transparency of an LSC, that are defined by the type and the concentration of the fluorophore, can be chosen according to specific building requirements and/or aesthetic criteria.

Wide use of LSCs has so far been hindered by the lack of suitable emitters. Typically used conjugated organic fluorophores provide a limited coverage of the solar spectrum and suffer from significant optical losses associated with re-absorption of guided luminescence. These deficiencies reduce the light harvesting efficiency of LSCs and also lead to strong coloring of devices, which imposes certain constrains on their usage in architecture. Colloidal quantum dots can help overcome these limitations. They can feature near unity photoluminescence quantum yields (QY) and narrow, widely tunable emission spectra that can be readily matched to various low-cost, high-efficiency solar cells.

An interesting class of low-toxic materials that provide a large, hundreds of meV Stokes shift are QDs of ternary I-III-VI₂ semiconductors such as CuInS₂, CuInSe₂, and their alloys (CuInSeS). An attractive feature of these QDs is that they do not contain toxic 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. In this presentation, McDaniel will discuss recent colorless LSC results published in Nature Nanotechnology (2015, 10, 878–885) demonstrating a power conversion efficiency of 3.2% at 80% transmittance using CuInSeS quantum dots.

Metamaterial infrared absorber was applied for a background-suppressed surface-enhanced molecular detection technique. By utilizing the resonant coupling of plasmonic modes of a metamaterial absorber and infrared (IR) vibrational modes of a self-assembled monolayer (SAM), attomole level sensitivity was experimentally demonstrated. IR absorption spectroscopy of molecular vibrations is of importance in chemical, material, medical science and so on, since it provides essential information of the molecular structure, composition, and orientation. In the vibrational spectroscopic techniques, in addition to the weak signals from the molecules, strong background degrades the signal-to-noise ratio, and suppression of the background is crucial for the further improvement of the sensitivity. Here, we demonstrate low-background resonant Surface Enhanced IR Absorption (SEIRA) by using the metamaterial IR absorber that offers significant background suppression as well as plasmonic enhancement. The fabricated metamaterial consisted of 1D array of Au micro-ribbons on a thick Au film separated by a transparent gap layer made of MgF₂. The surface structures were designed to exhibit an anomalous IR absorption at ~ 3000 cm^-1, which spectrally overlapped with C-H stretching vibrational modes. 16-Mercaptohexadecanoic acid (16-MHDA) was used as a test molecule, which formed a 2-nm thick SAM with their thiol head-group chemisorbed on the Au surface. In the FTIR measurements, the symmetric and asymmetric C-H stretching modes were clearly observed as reflection peaks within a broad plasmonic absorption of the metamaterial. Our metamaterial approach may open up new doors for further lowering the detection limit of far-field IR vibrational spectroscopy.
[1] A. Ishikawa and T. Tanaka, Scientific Reports 5, 12570 (2015).

Circularly polarized light is utilized in various optical techniques and devices. However, using conventional optical systems to generate, analyse and detect circularly polarized light involves multiple optical elements, making it challenging to realize miniature and integrated devices. While a number of ultracompact optical elements for manipulating circularly polarized light have recently been demonstrated, the development of an efficient and highly selective circularly polarized light photodetector remains challenging. Here we report on an ultracompact circularly polarized light detector that combines large engineered chirality, realized using chiral plasmonic metamaterials, with hot electron injection. We demonstrate the detector’s ability to distinguish between left and right hand circularly polarized light without the use of additional optical elements. Implementation of this photodetector could lead to enhanced security in fibre and free-space communication, as well as emission, imaging and sensing applications for circularly polarized light using a highly integrated photonic platform.

Split-ring resonators (SRRs) is a promising reserach theme that have been attracted a lot of interest in the field of plasmonics sensor. SRRs be implament by sensing plasmon resonance shift (∂λ) when exposed to a medium with a refractive index change ∂n. In general, the plasmon field of planar SRRs leak into the substrates, which decrease volume of response and performance. These limitations can be solved by utilizing vertical SRRs. The plasmon fields localized in vertical SRR gaps keep away the substrate, allowing for significantly enhanced sensitivity. Here, we demonstrated the highest sensitivity among reported SRR-based sensors in optical region.

We demonstrate a new strategy for fabricating next-generation light management film using molecular assembling-based chirality. This is based on nano-fibrillar aggregation of the fluorescent glutamide derivatives in polymer and induction of enhanced circular dichroism (CD) and circularly polarized luminescence (CPL) through highly ordered chiral arrangement. Glutamide is a generic term to indicate a glutamic acid derivative, which is characterized by the fact that a functional head group such as a fluorescent group and double alkyl chains are combined into a glutamic acid through amide bonds.
When the glutamides functionalized with fluorophores such as pyrene and anthracene were dissolved in proper organic solvents and then kept at 20 C, not only high Stokes shifts were observed in their fluorescent spectra, but also strong CD signals were detected around their absorption bands. These phenomena can be explained by their excimer emissions through chirally ordered stacking among the fluorophores due to self-assembling driven with the glutamide units. TEM observations indicated that nanofibillar aggregates were mainly produced in their cast films.
The polymer film was prepared from a mixture of the fluorophoric glutamide (1 wt%) with polymer (99 wt%) such as polystyrene and poly(methyl methacrylate) by a usual casting method. The resultant films were transparent and colorless, but exhibited excimer emission with large CD signals similarly to those in the solution states. In addition, the TEM observation indicated nanofibrillar aggregates in the polymer film. These results indicate that the chirally ordered stacking states of the fluorophores can be reproduced even in polymer. We detected circularly polarized luminescence (CPL) signals around their emission bands in both a solution and a polymer film. In this paper, we also describe that the intensity and emission bands can be tuned by adjusting the secondary chirality based on the glutamide aggregates as chiral templates.

Recently, localized surface plasmon resonances (LSPRs) arising in copper chalcogenide nanocrystals (NCs), also called “self-doped” semiconductor NCs, are under intense investigation and expected to facilitate the light harvesting, nonlinear optics, and quantum information processing. However, systematic studies of the influence factors on the plasmonic properties are rarely achieved due to the poorly-controlled synthesis or the mutable light-mater interactions. Therefore, controllable synthesis of copper chalcogenide NCs, including desired geometry, composition and surrounding environment, is of high significance for the modulation of their optoelectronic response and the corresponding applications.
In this study, highly monodisperse copper chalcogenide (Cu_2-xS) NCs are synthesized through a one-pot strategy by using copper nitride as “uncontaminated” copper precursors. Following this method, the size of Cu_2-xS NCs and composition of Cu_2-xS_ySe_1-y NCs can be readily controlled by varying the ratio of the precursors. The plasmonic properties of the copper chalcogenide NCs are investigated under a steady state by tuning the plasmonic resonance absorption band to a limiting condition (denoted “pinning” phenomena). It is observed that the pinning frequency increases (from 1.09 to 1.23 eV) with the increment of the NC size (from 5.4 to 11.0 nm) and blue-shifts (from 0.90 to 1.00 eV) with the increase of selenium content (from 11% to 66%), which can be attributed to the influence of surface scattering and effective mass of free carriers, respectively. Additionally, the plasmonic absorption bands of Cu_2-xS NCs encapsulated by two single-layer graphene pin at 1525−1550 nm during the oxidation process, which is influenced by both the dielectric constant and redox potential of the surrounding environment. This study demonstrates the controllable synthesis and precise fundamental plasmonic properties of the copper chalcogenide NCs, ensuring the potential plasmonic-related techniques (photothermal therapy, photoacoustic tomography, photothermal catalysis, etc.) with high efficiency, accuracy and excellent spatial resolution.

Plexcitons are hybrid plasmon-exciton states with properties that interpolate between photonic and material excitations. They serve as an interesting laboratory to explore the interplay between coherent and incoherent excitation energy transfer pathways between molecular assemblies. Here, I present a theoretical study of a model system composed of two weakly coupled plexciton films and the rich phase diagram that ensues as a function of angle of excitation of the donor plexciton, the orientation of the acceptor plexciton, and the distance separating these moieties. Coherence is imprinted into the system by default due to the inherent spatial coherence of the plasmons. An exquisite experimental control of the different channels of energy transfer can be obtained. This work is in collaboration with experimentalist Prof. Vinod Menon at CCNY. Our studies suggest interesting opportunities in molecular polaritonics, from the design of exotic states of matter (topological phases, analogues of topological insulators) to a more refined control of energy transfer in the nanoscale. This model system serves as a platform to understand the opportunities that plexcitons offer for solar light-harvesting.

The stability of AuNRs is critical for various optoelectronic and sensing systems since their optical properties depend on their shape and volume. How reshaping depends on the structure, diffusivity and chemistry of the local environment though, has not been systematically studied. To address this challenge, we discuss the reshaping of AuNRs embedded in poly(vinyl alcohol) (PVA) upon exposure to UV-Vis light. The optical stability depends significantly on the initial aspect ratio and volume of the AuNR, as well as the chemistry and composition of the surface stabilizers. Fundamentally, optical reshaping is accelerated by increasing hexadecyltrimethylammonium bromide(CTAB) concentration at high convex curvature. These parameters can be rationally introduced to fabricate colorimetric sensing nanocomposites which reflect the history of UV-Vis light exposure. Furthermore, combining patterned intensity with deformation induced rod alignment creates a mechanically compliant nanocomposite with polarization and spectral responsivity for the fabrication of optoelectronic devices, sensors, and optical filters.

Palladium-hydrogen is a prototypical metal-hydrogen system, which is well suited for active plasmonics. Upon hydrogenation of Pd to PdH_x, it is possible to change the dielectric function and hence the plasmonic resonance^1–3. Recently, also materials such as Yttrium and Magnesium have shown such properties^4,5. It is therefore not at all surprising that a lot of attention has been devoted to the ab- and desorption of hydrogen in nanosized plasmonic palladium particles. Here, we show that the large body of data on the thermodynamics of palladium nanostructures available so far in literature^6–10 exhibits general patterns that lead to unambiguous conclusions about the detailed microscopic processes involved in H absorption and desorption in Pd nanoparticles that can be used as prototypes for active plasmonic elements. On the basis of a remarkably robust scaling law for the hysteresis in absorption-desorption isotherms, we show that hydrogen absorption in palladium nanoparticles is consistent with a coherent interface model and is thus clearly different from bulk Pd behavior. However, H desorption occurs fully coherently only for small nanoparticles (typically smaller than 50 nm) at temperatures sufficiently close to the critical temperature. For larger particles it is partially incoherent as in bulk, where dilute α-PdH_x and high concentration β-PdH_x phases coexist. In support of these conclusions, we also developed a semi-analytical model that is able to reproduce all the essential features of the thermodynamics of H in Pd nanoparticles over a wide range of sizes (from a few nm up to hundreds of nm) as long as no dislocations are generated in the core of the particles.

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