Textured Transparent Conductive Oxide Research Articles

Optical Modeling of Thin-Film Amorphous Silicon Solar Cells Deposited on Nano-Textured Glass Substrates

We have introduced an approach to establish a methodology for 3D optical simulation that allows analyzing optical losses in the individual layers of a thin-film solar cell structure. Using commercial Finite-Difference Time-Domain (FDTD) tool, where Maxwell’s Curl equations were rigorously solved for optimizing such cells, a computer modeling has been performed. We have reported the ways to investigate efficient light-trapping schemes by using periodically textured transparent conductive oxide (TCO) in thin-film amorphous silicon solar cells. The optical effects in small area thin film silicon p-i-n solar cells deposited on glass substrates coated with aluminum doped zinc oxide (ZnO:Al) have been addressed. In order to enhance the efficiency, TCO surface morphology has been analyzed, where pyramidal and parabolic textured surfaces have been used. For these cells, the quantum efficiency, short-circuit current, total reflectance, and all absorption losses have been successfully computed and analyzed. The investigation was carried out based on our proposed model that exhibits maximum current density of 17.32 mA/cm2 for the absorbing layer thickness of 300 nm.

Numerical simulations for high efficiency HIT solar cells using microcrystalline silicon as emitter and back surface field (BSF) layers

In present article the influence of thickness and band gap of microcrystalline silicon emitter layer, amorphous silicon front and back intrinsic layers and p-type crystalline silicon (c-Si) wafer thickness on the performance of TCO/μc-Si:H(n)/a-Si:H(i)/c-Si(p)/a-Si:H(i)/μc-Si:H(p+)/Ag Heterojunction with thin intrinsic layer (HIT) solar cell along with other structural possibilities were investigated through computer simulations using AFORS-HET software. These simulations revealed the importance of inclusion of intrinsic a-Si:H thin layer in improving the performance of solar cell with the help of interface passivation. Also microcrystalline BSF can raise the conversion efficiency more than 4% compared to HIT solar cell having no BSF layer. Highest stable efficiency of 24.12% for p-type substrate based HITBSF (HIT with back surface field) solar cells was observed. Furthermore the effect of textured transparent conductive oxide (TCO) on solar cells was investigated where the enhanced light trapping was observed with the use of textured TCO surface which raised the performance of solar cells. These optimizations may help in fabricating μc-Si emitter and BSF based HIT solar cells with stable efficiencies compared to possibly degraded efficiencies as in case of a-Si:H based HIT solar cell structures studied so far.

Flexible n-i-p thin film silicon solar cells on polyimide foils with textured ZnO:Ga back reflector

In thin film silicon solar cells on opaque substrates in n-i-p deposition sequence where the textured transparent conductive oxide (TCO) layer serves as a back reflector, one can independently optimize the morphology of the TCO layer without compromise on transparency and conductivity of this layer and further adjust the electro-optical properties of the back contact by using additional layers on top of the textured TCO. In the present work, we use this strategy to obtain textured back reflectors for solar cells in n-i-p deposition sequence on non-transparent flexible plastic foils. Gallium doped ZnO (ZnO:Ga) films were deposited on polyimide substrates by DC magnetron sputtering at a temperature of 200°C. A wet-chemical etching step was performed by dipping the ZnO:Ga covered foil into a diluted HCl solution. The textured ZnO:Ga is then coated with a highly reflective Ag/ZnO double layer. On this back reflector, we develop thin film silicon solar cells with a microcrystalline silicon absorber layer. The current density for the cell with the textured ZnO:Ga layer is ~23mA/cm2, 4mA/cm2 higher than the one without such layer, and a maximum efficiency of 7.5% is obtained for a 1cm2 cell.

Periodic nano-textures enhance efficiency in multi-junction silicon thin-film solar cells

In this work, we report on the prototyping of thin-film silicon tandem solar cells with periodic light-trapping textures. Our approach combines an industrial applicable nano-imprint process, for the fabrication of advanced light management concepts, with state-of-the-art thin film silicon solar cell technology. In a joint experimental effort between project partners of the European thin-film silicon project “Fast-Track”, we demonstrate that optimal periodic light-trapping nano-textures at the front-side of a multi-junction solar cell enhance the power conversion efficiency of the device in comparison to solar cells with conventional textured TCO (transparent conductive oxide) front contacts from 11.7% to 12.1%. This improved power conversion efficiency is mainly driven by an enhancement of the short-circuit current density through the use of periodic light-trapping nano-textures.

Advanced light trapping designs for high efficiency thin film silicon solar cells

Abstract State-of-the-art optical trapping designs for tandem thin film silicon solar cells in the superstrate configuration commonly feature two elements: a textured transparent conductive oxide front contact and a low refractive index interlayer between top and bottom cell. We investigate more advanced super light trapping schemes that have been designed and implemented in thin film tandem junctions at different levels in the solar cells to further enhance light trapping capabilities. Regularly patterned nano-imprinted substrates offer an additional capability to tune the substrate morphology and optimize it for enhanced solar cell performance. We show that the onset of defect formation in thin film layers can be controlled and that the spectral response in the infra-red part of the spectrum is increased. Intermediate reflectors with ultra-low refractive indeces and plasmonic properties lead to increased light confinement in top cells. Results show that reducing the refractive index below 1.5 still leads to a substantial current increase in the top cell. These innovative designs improve the output current in amorphous/microcrystalline tandem devices with thin photo-active layers. In addition, they surprisingly enhance the electrical parameters of the tandem cells with a record open circuit voltage of 1.47 V. Plasmonic effects of metal nano-particles embedded in interlayers do not compromise the electrical solar cell parameters and similarly strengthen light confinement in the top cell, however detrimental effects are observed in the bottom cell current output.

Rigorous optical modeling and optimization of thin‐film photovoltaic cells with textured transparent conductive oxides

ABSTRACTTypical thin‐film photovoltaic (PV) cells incorporate a textured transparent conductive oxide to enhance light trapping and efficiently harvest solar energy. Rigorous coherent optical simulations of these devices and a complete characterization of these textured films are a challenging problem because of the several orders of magnitude difference between the wavelengths of interest and the spatial dimension of the sample that needs to be evaluated. In this paper, a practical approach for rigorous and predictive modeling of optical properties of thin‐film PV cells incorporating a vast variety of light‐trapping structures including semi‐coherent textured films and patterned coherent structures is presented. In contrast to the existing semi‐empirical device models, it is demonstrated that the presented methodology can accurately predict the scattering properties of textured fluorine‐doped tin oxide and aluminum‐doped zinc oxide conductive transparent films. It is further shown that the optical response of single‐junction and tandem‐junction PV devices incorporating such films can also be predicted with good accuracy as compared with the measured results. Next, a methodology to identify the sufficient statistical fingerprints of semi‐coherent textured films that are needed to unambiguously predict the light propagation in thin‐film cells is presented. This comprehensive approach then lends itself to identifying the optimal surface morphology needed for strong light trapping. This rigorous approach automatically includes the effects of important loss mechanisms such as the surface plasmon‐enhanced absorption in textured metal surfaces that are otherwise very difficult to account for semi‐coherent approaches based on scalar scattering theory. Copyright © 2011 John Wiley & Sons, Ltd.