G. Jost

Fabrication of Light-Scattering Multiscale Textures by Nanoimprinting for the Application to Thin-Film Silicon Solar Cells

In this study, nanoimprint processing was used to realize various multiscale textures on glass substrates for application in thin-film photovoltaic devices. The multiscale textures are formed by a combination of large and small features, which proofed to be beneficial for light trapping in silicon thin-film solar cells. Two approaches for the fabrication of multiscale textures are presented in this study. In the first approach, the multiscale texture is realized at the lacquer/transparent conductive oxide (TCO) interface, and in the second approach, the multiscale texture is realized at the TCO/Si interface. Various types of multiscale textures were fabricated and tested in microcrystalline thin-film silicon solar cells in p-i-n configuration to identify the optimal texture for the light management. It was found that the best light-scattering multiscale texture was realized using an imprint-textured glass substrate, which contains large craters, in combination with HF-etched TCO (ZnO:Al), which contains small features, on top of the imprint. With this structure (of the second approach), the short-circuit current density of the solar cell devices was improved by 0.6 mA/cm-2 using multiscale textures realized by nanoimprint processing.

Influence of Interface Textures on Light Management in Thin-Film Silicon Solar Cells With Intermediate Reflector

High-efficiency thin-film silicon solar cells require advanced textures at the front contacts for light management. In this contribution, the influence of the texture of various transparent conductive oxides (TCO) on the effectiveness of an intermediate reflector layer (IRL) in a-Si:H/μc-Si:H tandem solar cells is investigated. The employed front side TCOs include several types of sputter-etched ZnO:Al, LPCVD ZnO:B and APCVD SnO2:F. The topographies after different stages of the deposition process of the tandem solar cell, at the front TCO, after deposition of the amorphous top cell and after the deposition of the microcrystalline bottom cell, were characterized by atomic force microscopy at precisely the same spot. The external quantum efficiency of the fabricated solar cells were measured and successfully reproduced by a finite-difference time-domain method applying the measured topographies at each interface of the solar cell. With these simulations, the impact of structure type and feature size on the effectiveness of the IRL is investigated. The highest IRL effectiveness in a tandem solar cell was found for double-textured ZnO:Al. In this contribution, we study the interplay between interface textures and parasitic losses. Our findings are relevant for the design of topography for optimized IRL performance.

Correlation between surface topography and short-circuit current density for thin-film silicon solar cells

The scattering of light by the textured transparent conductive oxide (TCO) in thin-film silicon solar cells is frequently described by transmission haze and angular intensity distribution (AID) at the interface between the TCO and air. The scattering is expected to improve the light trapping and, therefore, the absorption of the solar cell. Using these scattering properties as input parameters for the electrical modeling of thin-film solar cells leads to significant deviations from the measurements for short circuit current densities. The major disadvantage of the AID measurement at the TCO/air interface is that in real thin-film silicon solar cells the TCO/Si interface is relevant. We use a model that is based on scalar scattering theory to calculate the scattering properties at the transition into air and into silicon. The model takes into account the measured surface topography and the optical constants of the adjacent media. For a series of μc-Si:H cells on ZnO:Al with different surface topographies, AID and the transmission haze into a μc-Si:H half space are calculated. From these results, a quantity is derived that describes the scattering efficiency. This quantity is compared to the short circuit current densities of μc-Si:H solar cells showing good agreement. It will be shown that for artificially modified textures an increase in the short-circuit current density and thus, the efficiency of thin-film silicon solar cells can be achieved.