This PDF file contains the front matter associated with SPIE Proceedings Volume 8823, including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

For the production of high efficiency thin film, Cu(In,Ga)Se2 solar cells, absorber layers with grain sizes of a few hundred nanometers and without detrimental secondary phases are favored. Co-electrodeposition offers a low-cost and material efficient synthesis route, where, in a single step, films containing CuInSe2 are formed. However, the material is nanocrystalline, constitutes multiple phases and has poor photovoltaic properties 1. Therefore a subsequent annealing step is required to produce absorber layers suitable for use in photovoltaic devices. Laser annealing has been demonstrated to improve crystallinity, stimulate atomic diffusion and develop opto-electronic properties when compared to the precursor ². In this work, high resolution X-ray diffraction was used in order to assess the presence of secondary phases in the absorber layer. All diffractograms of laser annealed films exhibited an additional, unknown peak, measurable through the full depth of the material which is independent of precursor composition, annealing time or laser flux. Evaluation of literature on codeposited CuInSe₂, combined with Rietveld refinement suggests a number of possible identities for this peak. The candidates in order of most likely to least likely are structural defects, In₂Se₃, and CuIn₃Se₅. We consider the impact that each of these would have on a device formed via this process and thus its success as a new manufacturing route for CuInSe₂ solar cells.

The reactive co-sputtering was developed as a new way of preparing high quality CuInGaSe₂(CIGS) films from two sets of targets; Cu_0.6Ga _0.4 and Cu_0.4In_0.6 alloy and Cu and (In_0.7Ga_0.3)₂Se₃ compound targets. During sputtering, Cu, In, Ga metallic elements as well as the compound materials were reacted to form CIGS simultaneously in highly reactive elemental Se atmosphere generated by a thermal cracker. CIGS layer had been grown on Mo/soda-lime glass(SLG) at 500°C. For both sets of targets, we controlled the composition of CIGS thin film by changing the RF power for target components. All the films showed a preferential (112) orientation as observed from X-ray diffraction analysis. The composition ratios of CIGS were easily set to 0.71-0.95, 0.10-0.30 for [Cu]/[III] and [Ga]/[III], respectively. The grain size and the surface roughness of a CIGS film increased as the [Cu]/[III] ratios increased. The solar cells were fabricated using a standard base line process in the device structure of grid/ITO/i-ZnO/CdS/CIGS/Mo/ SLG. The best performance was obtained the performance of V_oc = 0.45 V, J_sc =35.6, FF = 0.535, η = 8.6% with a 0.9 μm-CIGS solar cell from alloy targets while V_oc = 0.54 V, J_sc =30.8, FF = 0.509, η = 8.5% with a 0.8 μm-CIGS solar cell from Cu and (In_0.7Ga_0.3)₂Se₃.

CuInSe₂ films for photovoltaic applications grown under Cu-excess have been rarely investigated up to now. In general CuInSe₂ solar cells use an overall Cu-poor absorber. In this work we argue that it is valuable to investigate Cu-rich solar cells, since all the basic material properties are better in Cu-rich absorbers. With less defects in the bulk and better transport properties it is somehow intriguing why devices with Cu-rich absorber perform less. We demonstrate that this can be attributed to the too high doping of these films. Such a high native doping leads to tunneling enhanced recombination and interface recombination, strongly affecting the devices performances. We demonstrate different attempts to overcome the problem of doping: at first a Cu-poor surface was grown on the Cu-rich absorbers which enables to decrease the doping in the space charge region, then to directly decrease the doping in the bulk, the influence of sodium content was investigated. Finally, here we show that different selenium activity during the absorber growth enables to decrease the doping of these films and to open thus a way to fully exploit the favorable properties of the Curich CuInSe₂ films.

Thin–film silicon tandem solar cells consist of an amorphous silicon top cell and a microcrystalline silicon bottom cell stacked in series. In order to match the photocurrents of the top cell and the bottom cell, a proper photon management is essential. In this regard, we present the conceptual design and optical simulations of an intermediate reflector consisting of a stack of microcrystalline silicon oxide layers of different, alternating refractive indices. In contrast to 1–layer intermediate reflectors, the spectral and directional selectivity of these intermediate reflectors result in a gain for the top cell current while simultaneously increasing the charge carrier generation in the bottom cell.

A tandem solar cell comprising two p-i-n solar cells made of amorphous-silicon alloys and backed by a periodically corrugated metallic back-re ector was theoretically investigated. The intrinsic semiconductor layers in both constituent solar cells were taken to be nonhomogeneous with linearly varying bandgap. An AM1.5 solar irradiance spectrum was incorporated in the nite-di erence-time-domain calculations (Lumerical^TM) to obtain the generation rate of electron-hole pairs, which was then used in Synopsys Sentaurus^TM to compute the electrical properties of the solar cell. The short-circuit current increases when the intrinsic layers are nonhomogeneous as compared to homogeneous intrinsic layers. However, electrical simulations showed that a new approach to modeling is needed that can take into account the continuously varying bandgap instead of considering it as piecewise uniform.

Large grained polycrystalline silicon (poly-Si) absorbers were realized by electron beam induced liquid phase crystallization on 2 μm periodically patterned glass substrates and processed into a-Si:H/poly-Si heterojunction thin-film solar cells. The substrates were structured by nanoimprint lithography using a UV curable hybrid polymer sol-gel resist, resulting in a glassy high-temperature stable micro-structured surface. Structural analysis yielded high quality poly-Si material with grain sizes up to several hundred micrometers. An increase of absorption and an enhancement of the external quantum efficiency in the NIR as a consequence of light trapping due to the micro-structured poly-Si/substrate interface were observed. Up to now, only moderate solar cell parameters, a maximum open-circuit voltage of 413 mV and a short-circuit current density of 8 mA cm^-2, were measured being significantly lower to what can be achieved with liquid phase crystallized poly-Si thin-film solar cells on planar glass substrates indicating that the substrate texture has impact on the electrical material quality. By reduction of the SiC interlayer thickness at the micro-structured poly- Si/substrate interface defect-related parasitic absorption was considerably minimized. This encourages the implementation of nanoimprinted tailored substrate textures for light trapping in liquid phase crystallized poly-Si thinfilm solar cells.

Significant progress in fabrication and optimization of organic photovoltaics (OPVs) has been made during the last decade. The main reason for popularity of OPVs is due to their low production cost, large area devices and compatibility with flexible substrates 1-3. Various approaches including optimizing morphology of the active layers 1, 2, introducing new materials as the donor and acceptor 3,4, new device structures such as tandem structure ^{5, 6} have been adapted to improve the efficiency of the organic photovoltaics. However, electrical characteristics of the OPVs do not only depend on the active layer materials or device structure. They can also be defined by the interface properties between active layers and the charge transport layers or the metal contacts. Within this paper, the effect of the thickness variation of the charge transport layer in the electrical properties of the bilayer heterojunction OPVs has been studied. Several devices with CuPc/PTCDI-C8 as the donor/acceptor layers have been fabricated with different thicknesses of electron transport layer. MoO3 and Alq3 have been used respectively as the hole transport layer (HTL) and the electron transport layer (ETL). It has been shown that the S-shape effect in the current–voltage curve is attributed to the accumulation of the charge carriers at the interface between the active layer and the charge transport layer ^{5, 7}.

Understanding the kinetics of dye adsorption on semiconductors is crucial for designing dye-sensitized solar cells (DSSCs) with enhanced efficiency. Harms et al. recently applied the Quartz-Crystal Microbalance with Dissipation Monitoring (QCM-D) to study in situ dye adsorption on flat TiO2 surfaces. QCM-D measures adsorption in real time and therefore allows one to determine the kinetics of the process. In this work, we characterize the adsorption of N3, a commercial RuBipy dye, using the native oxide layer of a titanium sensor to simulate the TiO2 substrate of a DSSC. We report equilibrium constants that are in agreement with previous absorbance studies of N3 adsorption, and therefore demonstrate the native oxide layer of a titanium sensor as a valid and readily available planar TiO2 morphology to study dye adsorption.

Light trapping has been developed to effectively enhance the efficiency of the thin film solar cell by extending the pathlength for light interacting with the active materials. Searching for optimal light trapping design requires a delicate balance among all the competing physical processes, including light refraction, reflection, and absorption. The existing design methods mainly depend on engineers’ intuition to predefine the topology of the light-trapping structure. However, these methods are not capable of handling the topological variation in reaching the optimal design. In this work, a systematic approach based on Genetic Algorithm is introduced to design the scattering pattern for effective light trapping. Inspired by natural evolution, this method can gradually improve the performance of light trapping structure through iterative procedures, producing the most favorable structure with minimized reflection and substantial enhancement in light absorption. Both slot waveguide based solar cell and a more realistic organic solar with a scattering layer consisting of nano-scale patterned front layer is optimized to maximize absorption by strongly coupling incident sun light into the localized photonic modes supported by the multilayer system. Rigorous coupled wave analysis (RCWA) is implemented to evaluate the absorbance. The optimized slot waveguide cell achieves a broadband absorption efficiency of 48.1% and more than 3-fold increase over the Yablonovitch limit and the optimized realistic organic cell exhibits nearly 50% average absorbance over the solar spectrum with short circuit current density five times larger than the control case using planar ITO layer.

Surface texturing in thin-film solar cells provides a promising way of addressing the loss components due to reflection and poor light absorption inside the cells. In this work, we study the reflection suppression performance of different submicron-scale periodic surface texturing morphologies through two dimensional (2D) finite-difference time-domain (FDTD) computations. The broadband reflection response is investigated at two interfaces, air/glass and glass/TCO (transparent conductive oxide), for a spectral range of 300-2500 nm. A Drude-Lorentz model is used to account for material dispersion and absorption within the wavelengths of interest. In order to optimize the light trapping performance, numerical simulations of various surface texture structures are compared with those of flat interfaces. Numerical results show a reduction in reflection at the air/glass interface to values below 0.2% for some of the triangular gratings, compared to up to 4% for the non-textured interface. For the glass/TCO interface, reflection decreases to less than half when compared to the non-textured interface, also for triangular gratings. Further structures that replicate perfect multi-layer anti-reflection coatings are also studied. These structures are tuned to cancel specific wavelengths and can create an arbitrary effective index, overcoming the constraint of the limited number of refractive index values available. The best structures obtained for the air/glass and glass/TCO interfaces are combined in one stack, achieving reflectance values at least one order of magnitude below the non-textured air/glass/TCO stack.

Cu₂ZnSn(S, Se)₄ (CZTSSe) is a promising alternative absorber material for thin-film photovoltaic applications because of its earth-abundant constituents, tunable band gap, and high optical absorption coefficient. Using binary and ternary chalcogenide nanoparticles as precursors we have developed a chemical route to produce high efficiency CZTSSe photovoltaic (PV) devices via solution based methods. The printed CZTSSe films show an interesting microstructure consisting of an upper micrometer-sized polycrystalline layer (large-grain layer) and a bottom fine-grain layer. In this paper, we present our results on characterization of the layers including composition, electronic and optical properties. Based on the observed properties we develop a numerical model for the CZTSSe PV device and present the simulation results. We anticipate that the combination of detailed characterization and device model will help us better understand the limitations of our current devices and indicate potential improvement paths.

The fabrication method of plasmonic nanodots on ITO or nc-ZnO substrate has been developed to improve the efficiency of organic thin film solar cells. Nanoscale metallic nanodots arrays are fabricated by anodic aluminum oxide (AAO) template mask which can have different structural parameters by varying anodization conditions. In this paper, the structural parameters of metallic nanodots, which can be controlled by the diverse structures of AAO template mask, are investigated to enhance the optical properties of organic thin film solar cells. It is found that optical properties of the organic thin film solar cells are improved by finding optimization values of the structural parameters of the metallic nanodot array.

We investigated the structural and sub-structural characteristics of ZnO films obtained by chemical bath deposition from solutions of zinc sulfate, thiourea, and ammonia. The duration of deposition ranged from 20- to 120-minutes. The concentration of thiourea was varied from 0.1- to 3-mol. We detailed the structural and sub-structural characteristics of these films using high-resolution scanning electron microscopy and x-ray diffraction. This research enables us to study features of the films’ structural formation, and to determine their basic characteristics, viz., phase analysis, texture quality, lattice constants, grain size, and size of the coherent scattering domain. Regimes were identified for depositing films with optimal structural characteristics for eventual use in solar-energy applications.

Significant advancements in photovoltaic solar cells are required to support large-scale energy demands with solar power. The first generation of solar cells (SC) available today uses Si. While Si is highly abundant and these types of SC can be easily manufactured, the best power conversion efficiency is only 24%. Developing photovoltaic SC using III-V materials may increase the efficiency while decreasing the manufacturing costs associated with cell fabrication. This paper studies the opportunity to improve two-junctions solar cells made of III-V materials by making the layers very thin and including the antireflective layer in the first junction. In terms of light harvesting, the anti-reflective layer made of a semiconductor is shown to absorb the most part of the incident light.

Large area (72 cm²) doping inversed HIT solar cells (n-a-Si:H/i-a-Si:H/p-c-Si) were investigated by High Resolution Transmission Electron Microscopy (HR-TEM), Spectroscopic Ellipsometry (SE), Fourier Transform Infrared Attenuated Total Reflection spectroscopy (FTIR-ATR) and current-voltage (I-V) measurement. Mixture of microcrystalline and amorphous phase was identified via HR-TEM picture at the interface of i-a-Si:H/p-c-Si heterojunction. Using multilayer and Effective Medium Approximation (EMA) to the SE data, excellent fit was obtained, describing the evolution of microstructure of a-Si:H deposited at 225 °C on p-c-Si. Cody energy gap with combination of FTIR-ATR analyses were consistent with HRTEM and SE results in terms of mixture of microcrystalline and amorphous phase. Presence of such hetero-interface resulted poor open circuit voltage, V_oc, of the fabricated solar cell devices, determined by I-V measurement under 1 sun. Moreover, Voc was also estimated from dark I-V analysis, revealing consistent V_oc values. Efficiencies of fabricated cells over complete c-Si wafer (72 cm2) were calculated as 4.7 and 9.2 %. Improvement in efficiency was interpreted due to the back surface cleaning and selecting aluminum/silver alloy as front contact.

The amorphous/crystalline silicon technology has demonstrated its potentiality leading to high efficiency solar cells. We propose the use of surface photovoltage technique as a contact-less tool for the evaluation of the energetic distribution of the state density at amorphous/crystalline silicon interface. We investigate the effect hydrogen plasma treatments performed on thin amorphous silicon buffer layer deposited over crystalline silicon surface and we compare its effect with that of thermal annealing on the interface. The surface photovoltage technique results to be very sensitive to the different experimental treatments, and therefore it can be considered a precious tool to monitor and improve the interface electronic quality.