Hybrid dielectric light trapping designs for thin-film CdZnTe/Si tandem cells.

Tandem solar cells consisting of high bandgap cadmium telluride alloys atop crystalline silicon have potential for high efficiencies exceeding the Shockley-Queisser limit. However, experimental results have fallen well below this goal significantly because of non-ideal current matching and light trapping. In this work, we simulate cadmium zinc telluride (CZT) and crystalline silicon (c-Si) tandems as an exemplary system to show the role that a hybrid light trapping and bandgap engineering approach can play in improving performance and lowering materials costs for tandem solar cells incorporating crystalline silicon. This work consists of two steps. First, we optimize absorption in the crystalline silicon layer with front pyramidal texturing and asymmetric dielectric back gratings, which results in 121% absorption enhancement from a planar structure. Then, using this pre-optimized light trapping scheme, we model the dispersion of the CdxZn1-xTe alloys, and then adjust the bandgap to realize the best current matching for a range of CZT thicknesses. Using experimental parameters, the corresponding maximum efficiency is predicted to be 16.08 % for a total tandem cell thickness of only 2.2 µm.

Time domain simulation of tandem silicon solar cells with optimal textured light trapping enabled by the quadratic complex rational function.

Amorphous silicon/crystalline silicon (a-Si/c-Si) micromorph tandem cells, with best confirmed efficiency of 12.3%, have yet to fully approach their theoretical performance limits. In this work, we consider a strategy for improving the light trapping and charge collection of a-Si/c-Si micromorph tandem cells using random texturing with adjustable short-range correlations and long-range periodicity. In order to consider the full-spectrum absorption of a-Si and c-Si, a novel dispersion model known as a quadratic complex rational function (QCRF) is applied to photovoltaic materials (e.g., a-Si, c-Si and silver). It has the advantage of accurately modeling experimental semiconductor dielectric values over the entire relevant solar bandwidth from 300-1000 nm in a single simulation. This wide-band dispersion model is then used to model a silicon tandem cell stack (ITO/a-Si:H/c-Si:H/silver), as two parameters are varied: maximum texturing height h and correlation parameter f. Even without any other light trapping methods, our front texturing method demonstrates 12.37% stabilized cell efficiency and 12.79 mA/cm² in a 2 µm-thick active layer.