Design and optimization of next generation high aspect ratio periodic thin film photovoltaic cells

Abstract

The typical thin-film photovoltaic cells incorporate a textured transparent conductive oxide film to efficiently harvest solar energy [1]. It is well known that the light trapping performance provided by the random texturing has largely fallen short as compared to the initial expectations based on the performance achieved from similar features in thick wafer based cells [1-3]. Several reasons have been accounted for this including interference and coherent effects, smaller photonic density of states in thin film layers, and inability to construct Lambertian scattering surfaces with small textures [1-3]. Coherent periodic structures that can better confine the incident solar energy in the volume occupied by the efficient photoactive materials have been proposed [2,3]. Several research groups have concluded that the high aspect ratio structures such as a periodic array of very thin but tall pillars or cones will be needed for high performance cells. Similarly, high aspect ration holes have also been proposed [2,3]. In this work we performed rigorous opto-electronic analysis of several different device configurations for the next generation high aspect ratio periodic thin film photovoltaic cells. Each device configuration is exhaustively optimized for all geometrical variables to achieve maximum short circuit current under 1SUN AM1.5g radiation. This enables us to objectively compare the potential of different proposed cell designs. We show results from our studies of high aspect ratio pillars versus high aspect ratio perforations (holes); constant cross-section cylindrical pillars versus tapered cross-section conical pillars; square cross-section versus circular cross-section pillars and holes and square periodic lattice versus hexagonal periodic lattice. We also studied differences between the cases when a-Si p-i-n stack is conformally deposited on the high aspect ratio periodic features and when a thick layer of TCO is used to planarize the features before a p-i-n stack is grown. Results will be shown for both the substrate and superstrate configuration of devices. We developed a fully rigorous and coherent opto-electronic simulation platform that is used for this work. In the past we developed a Fourier domain based rigorous coherent optical simulator REMS that was used for studying the random textured devices. We showed that this simulator predicted both the optical scattering properties such as spectral haze and reflectivity of textured TCO surfaces as well as the QE of single junction and tandem junction devices grown on these surfaces with very good accuracy [1-3]. In the present work we extend this simulation capability to analyze and optimize next generation patterned periodic solar cells. In addition to the optical light trapping, these high aspect ratio structures may greatly alter the charge carriers transport pattern and collection efficiency. In a p-i-n stack conformally deposited on a columnar structure, the charge carriers are expected to be collected in the direction perpendicular to the axis of the columns while photons are expected to be absorbed while waveguiding in the direction parallel to the axis. Hence, it is very important to rigorously study the charge carrier dynamics coupled with a full 3D Maxwell solver. Here, we integrated the very fast REMS 3D Maxwell solver with the established device simulator Sentaurus™ from Synpsys <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</sup> Inc. REMS is Fourier domain based method and is very fast compared to the FDTD or FEM based methods. This allows us to exhaustively optimize the several geometrical features of the device configuration in reasonable amount of time [1-3].