This PDF file contains the front matter associated with SPIE Proceedings Volume XXXX, including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

Hot carrier solar cell (HCSC) requires a slow cooling rate of carriers in the absorber, which can potentially be fullled by semiconductor superlattices. In this paper the energy relaxation time of electrons in InN InxGa1-xN superlattices are computed with Monte Carlo simulations considering the multi-stage energy loss of electrons. As a result the effect of each stage in the relaxation process is revealed for superlattice absorbers. The energy relaxation rate figures are obtained for different material systems of the absorber, i.e. for different combinations of Indium compositions and the thicknesses of well and barrier layers in the superlattices. The optimum material system for the absorber has been suggested, with the potential to realize HCSCs with high efficiency.

Tailoring the solar absorptivity (αs) and thermal emissivity (ƐT) of materials constitutes an innovative approach to solar energy control and energy conversion. Numerous ceramic and metallic materials are currently available for solar absorbance/thermal emittance control. However, conventional metal oxides and dielectric/metal/dielectric multi-coatings have limited utility due to residual shear stresses resulting from the different coefficient of thermal expansion of the layered materials. This research presents an alternate approach based on nanoparticle-filled polymers to afford mechanically durable solar-absorptive and thermally-emissive polymer nanocomposites. The αs and ƐT were measured with various nano inclusions, such as carbon nanophase particles (CNPs), at different concentrations. Research has shown that adding only 5 wt% CNPs increased the αs and T by a factor of about 47 and 2, respectively, compared to the pristine polymer. The effect of solar irradiation control of the nanocomposite on solar energy conversion was studied. The solar irradiation control coatings increased the power generation of solar thermoelectric cells by more than 380% compared to that of a control power cell without solar irradiation control coatings.

Semiconductor quantum wells and superlattices have found numerous applications in optoelectronic devices, such as lasers, LEDs and SOAs, and are an increasingly common feature of high efficiency solar cells and photodetectors. In this paper we will highlight some of the recent developments in the use of low-dimensional III-V semiconductors to improve the performance of photovoltaics by tailoring the bandgap of the junction. We also discuss novel structures designed to maximize photo-generated carrier escape and the application of quantum confinement to other components of the solar cell, such as tunnel junctions. Recent developments in type-II superlattices for photodetectors will also be discussed, including the graded-gap LWIR device based on the W-structured superlattices demonstrated at the Naval Research Laboratory. Modeled results will be presented using the NRL BANDS^TM integrated 8-band kp and Poisson solver, which was developed for computing the bandstructures of superlattice and multi-quantum well photodiodes

Silicon rich silicon oxynitride layers were deposited by ECR-PECVD in order to form silicon nanoparticles upon high thermal annealing at 1100°C. The effect of the gas precursor type and flows on the atomic composition and the structural properties was assessed by RBS and ERDA analysis as well as by Raman spectroscopy. The morphological and crystalline properties of the resulting nanoparticles were investigated by TEM analysis. We have found that the silicon nanoparticules average size and the crystalline fraction depend strongly on the silicon excess in the SiN and SiON layer.

Organic bulk-heterojunction solar cells have several good characteristics, such as ease of fabrication, and low-cost materials. However, the bottleneck in their adoption is their much lower efficiency as compared with their silicon counterparts. In our previous work, we demonstrated that by appropriately inserting AuNPs in the OPV device, the efficiency can be increased by 30% and that silanization of ITO positively impacts device performance, where we identified the field enhancement due to AuNPs as the main reason for the increase in the efficiency of the device. In this work, we further investigate the impact of self-assembly of the gold nanoparticles on the efficiency by also considering two other factors which can possibly contribute to the improvement of our structure’s performance. One is the change in the substrate’s workfunction after silanization, and the other factor is the variations in PEDOT: PSS characteristics due to the AuNPs’ plasmonic resonance. We conclude that the AuNPs not only increase the photon absorption efficiency but also increase the conductivity of the surrounding medium (PEDOT: PSS) thereby facilitating charge transport through PEDOT: PSS.

The specific application of photon upconversion (UC) in photovoltaics is only possible when the following requirements are fulfilled: First, the excitation intensity necessary for effective UC needs to be small (as low as 1Wcm^-2) − comparable with light intensities obtainable from the moderate concentrated sunlight. Second, the excitation spectral power density required for effective UC needs to be comparable with those of the terrestrial sun irradiation (in order of 100μWnm-1). Third, efficient UC must be obtained by non-coherent light excitation (sunlight). And last but not least – compatibility between the UC device and the photovoltaic device, including good optical coupling has to be realized. Up to now the triplet-triplet annihilation-supported upconversion (TTA – UC) is the only upconversion process excited with moderate concentrated sunlight. Our group demonstrates UCd based on various UCmolecular systems efficiently transforming the NIR and IR-A part of the sun spectrum into the VIS-range, operating at moderate sunlight concentrations (10-50 suns, AM1.5). The next important accomplished requirement is the transfer of the hydrophobic UC-molecular system from an organic solvent to the aqueous environment. These new aqueous UC systems with high efficiencies ensure good sealing of the UC device and in this way its compatibility with different solar cells.

This study demonstrates the feasibility of improving the optical properties of a vertically aligned quantum dot (QD) structure and the performance of a quantum dot intermediate band solar cell (QD-IBSC) by capping a GaAsSb layer on the InAs QD. Experimental results indicate that this capping process significantly improves dot-size uniformity because of the strain modification in vertically aligned dot layer growth. A solar cell device with an InAs/GaAsSb columnar dot structure increases the short-circuit current density (J_sc) by 8.8%, compared to a GaAs reference cell. This dot structure also increases quantum efficiency by up to 1200 nm through the absorption of lower-energy photons. The InAs/GaAsSb QD-IBSC also improves the open-circuit voltage (V_oc), indicating a reduction in misfit defect density and recombination current density. Power dependent photoluminescence (PL) and time resolved photoluminescence (TRPL) measurement are employed to characterize the optical properties of typical InAs/GaAs type-I and InAs/GaAsSb type-II vertically aligned quantum dot structure. Extended carrier lifetime is demonstrated in the columnar InAs/GaAsSb type-II band structure. The results of this study confirm the ability and thermal stability of a columnar InAs/GaAsSb QD structure to enhance device performance.

Silicon nanowire (SiNW) arrays are widespread applied on hybrid photovoltaic devices because SiNW arrays can substitute the pyramid texture and anti-reflection coating due to its strong light trapping. Also, SiNWs can be prepared through a cost-efficient process of metal-assisted chemical etching. However, though longer SiNW arrays have lower reflectance, the top of long SiNWs aggregate together to make junction synthesis difficult for SiNW/organic hybrid solar cell. To control and analyze the effect of SiNW array morphology on hybrid solar cells, here we change the metal deposition condition for metal-assisted chemical etching to obtain different SiNW array morphologies. The experiment was separated to two groups, by depositing metal, say, Ag, before etching (BE) or during etching (DE). For group BE, Ag was deposited on n-type Si (n-Si) wafers by thermal evaporation; then etched by H₂O₂ and HF. For group DE, n-Si was etched by Ag⁺ and HF directly. Ag was deposited on n-Si during etching process. Afterwards, residual Ag and SiO₂ were removed by HNO₃ and buffered HF, successively; then Ti and Ag were evaporated on the bottom of Si to be a cathode. Finally, SiNWs were stuck on the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) that was spincoated on the ITO coated glass to form SiNW/organic heterojunction. The results show that group BE has reflectance lower than that in group DE in solar spectrum. However, group BE has smaller power conversion efficiency (PCE) of 8.65% and short-circuit current density (J_sc) of 24.94 mA/cm² than group DE of PCE of 9.47% and J_sc of 26.81 mA/cm².

As wafer-based solar cells become thinner, light-trapping textures for absorption enhancement will gain in importance. In this work, crystalline silicon wafers were textured with wavelength-scale diffraction grating surface textures by nanoimprint lithography using interference lithography as a mastering technology. This technique allows fine-tailored nanostructures to be realized on large areas with high throughput. Solar cell precursors were fabricated, with the surface textures on the rear side, for optical absorption measurements. Large absorption enhancements are observed in the wavelength range in which the silicon wafer absorbs weakly. It is shown experimentally that bi-periodic crossed gratings perform better than uni-periodic linear gratings. Optical simulations have been made of the fabricated structures, allowing the total absorption to be decomposed into useful absorption in the silicon and parasitic absorption in the rear reflector. Using the calculated silicon absorption, promising absorbed photocurrent density enhancements have been calculated for solar cells employing the nano-textures. Finally, first results are presented of a passivation layer deposition technique that planarizes the rear reflector for the purpose of reducing the parasitic absorption.

The defect absorption in hydrogenated amorphous silicon (a-Si:H) photosensitive devices in the presence of resonant absorbing silver nanoparticles (Ag NPs) is investigated. Defect states are created in the a-Si:H network by the incorporated Ag NPs in their direct environment. The strong electromagnetic fields accompanied by the localized surface plasmon (LSP) resonance of the Ag NPs enable high transition rates between the defect states and the conduction band. This results in an observable signal for near infrared (NIR) photon energies in external quantum efficiency (EQE) measurements. By applying different Ag NP size distributions to the devices the LSP resonance is shifted together with a shift of the EQE peak observed for NIR energies. This indicates that the available defect states in the a-Si:H bandgap are addressed by the LSP resonance. Dominant transitions take place for electrons occupying defect levels having an energetic position equal to the LSP resonance energy. Boron doping of the a-Si:H environment shifts the Fermi level towards or below the introduced defect states. These are depleted and cannot contribute to sub bandgap photon absorption. This is an indication for the consistence of the proposed model for resonant defect absorption of the bandgap defect states.

Amorphous silicon (a-Si) is a promising material to serve as the top absorbing layers in tandem solar cells. However, due to its intrinsic high defect density, the thicknesses of a-Si layers are limited to a few hundred nanometers. This severely restricts the top cell current density, and therefore the total current density. In this research, we propose a novel structure that utilizing a-Si and crystalline silicon (c-Si) nanowire arrays to serve as top and bottom cells in a tandem solar cell. Two configurations of nanowire tandem solar cells (NWTCs) - a-Si nanowires (NWs) are aligned with c-Si ones (NWTC B) and are horizontally shifted relative to c-Si ones (NWTC A) - are respectively investigated. Numerical analyses present that NWTC A leads to a remarkable photocurrent enhancement of 15.7% and 21.3% when respectively compared with the reference cell (planar tandem cell with c-Si top cell and a-Si bottom cell) with and without optimal IRL, while the performance of NWTC B is similar to that of the reference cell with optimal IRL. Additionally, the usage of c-Si in NWTC A is reduced by 44% compared with the reference cell with IRL. The priorities of NWTC A are mainly attributed to the dielectric (air) attached to the back side of its top cell, causing light to reflect back. In contrast, the back side of NWTC B is mainly attached to c-Si, causing little reflection. The compatibility of intermediate reflecting layers (IRLs) and our proposed structure is studied.

The well-known ability of metallic grooves and nanorods to induce the formation of plasmonic resonant features has stimulated the interest in these structures for light harvesting applications. We show that the presence of semi-circular shaped nanorods improves photon absorption in a thin semiconductor film in contact with the plasmonic nanostructure. Geometric parameters like radius of nanoparticle and semiconductor thickness are investigated to tailor the dispersion of resonant modes of the metal/semiconductor system: both localized surface plasmon and guided mode resonances are found to improve absorption features and extend absorption enhancement across the solar spectrum. We demonstrate that properly designed structures are able to trap wideband light sources (sunlight for photovoltaic devices) in thin semiconductor films, and have the potential to enhance photon absorption in spectral regions where the semiconductor is a poor absorber, i.e. near the electronic band edge.

Semiconductor quantum dots (QDs) are characterized by high extinction coefficients adjustable by varying the nanoparticle size and a high quantum yield of charge generation. They have the advantage of efficient charge transfer from QDs to organic semiconductors. An advanced photovoltaic cell where a SnO₂/ITO electrode is covered with layers of CdSe QDs integrated in a polyimide (PI) organic semiconductor (about 100 nm thick) and Cu–phthalocyanine (20–40 nm thick) has been developed.Laser-induced photoluminescence analysis has permitted the optimization of the QD concentration in the PI matrix. Special attention has been paid to the electrode surface quality, including the effect of oxygen-plasma treatment of the transparent SnO₂/ITO electrode surface on the heterostructure photoconductivity. The mechanisms of excitation and charge transfers from QDs to the organic semiconductor and their effects on the efficiency of solar radiation conversion to electricity are discussed. Photovoltaic study of the structures developed has been performed, and the effect of the Cu–phthalocyanine layer on their photoconductivity has been estimated. The photovoltaic efficiency of optimized PI–CdSe hybrid structures approaches that of the best performing systems based on the MEH–PPV organic semiconductor. Incorporation of CdSe QDs in MEH–PPV has been demonstrated to increase the photovoltaic efficiency of the system by 50%, thus allowing the development of novel QD-based inorganic/organic hybrid materials with considerably improved photovoltaic properties.

In this work we present the synthesis and photovoltaic application of four different vertically-aligned ZnO nanostructured electrodes: ZnO nanorods prepared by the a) low-temperature hydrothermal method (LT-HM) and the b) autoclave method (A-HM), c) ZnO nanotrees (NTs) and d) ZnO core-shell NRs with an indium sulfide layer as the shell (CS). The electrodes have been applied in Dye sensitized solar cells (DSCs) and Polymer solar cells (PSCs). The photovoltaic properties of each type of nanostructured electrode were optimized separately. Our results show that the optimal power conversion efficiency depends in great extent on NR dimensions (length and diameter) and the final ZnO nanostructure. In this respect, we have observed an increase in power conversion efficiency when the NR nanostructure is modified as follows: ZnO NRs LT-HM < A-HM < NT< CS for Dye semnsitized solar cells. In the case of PSCs the best power conversion efficiency was obtained for the CS sample.

A reduction of material consumption in thin-film photovoltaic devices can make solar energy economically more viable. However, since thin films essentially absorb less light, there is an imminent need for existing technology to improve light harvesting. We present an effective approach of better light absorption, enhanced photocurrent generation and therefore higher quantum efficiency of poly (3-hexylthiophene): 1-(3-methoxycarbonyl) propyl-1-phenyl-[6, 6]-methanofullerene (P3HT:PCBM) bulk heterojunction photovoltaic/photodetector devices. We have integrated a thin semi-continuous gold film (SCGF) (~20nm) sputtered at percolation threshold between the active P3HT:PCBM layer and the indium-tin-oxide (ITO) electrode. At critical metal concentrations, i.e. percolation threshold, the light reaching the SCGF undergoes a broadband trapping with characteristic time of 200 fs, through complex interactions with fractal gold clusters. This thin SCGF together with the ITO serves as an anode. The interface between SCGF and the polymer represents the metaldielectric composite (MDC) that supports broad-band surface plasmon resonances that store electromagnetic radiation at the nanoscale and acts as an effective bulk type of concentrator without the need of increasing the photovoltaic device physical collection area. Here we report a six-fold enhancement in the integral quantum efficiency over the solar spectrum for device employing plasmon-active gold layer. Such enhancement is an important contribution for the future design of more efficient photodetecting/photovoltaic devices. The experimental results are supported by the theoretical modeling of metal-dielectric composites by block elimination method in 3D. The AC and DC responses of MDC, local field distribution, broad optical response and photon trapping in the percolating MDC were numerically calculated.