Possibilities to decrease the absorber thickness reducing optical and recombination losses in CdS/CdTe solar cells

Abstract

Based on the optical constants, calculations of optical losses in CdS/CdTe solar cells have been carried out taking into account reflections at the interfaces and absorption in the TCO and CdS layers. It has been shown that the losses caused by reflections at the interfaces result in lowering the short-circuit current by ~9 %, whereas absorption in the TCO and CdS layers with the typical thicknesses lead to losses of 15–24 %. Calculations of the integrated absorptive capacity of CdTe layer taking into account the spectral distributions of the standard AM1.5 solar radiation and the absorption coefficient of CdTe have been carried out. It is shown that in CdTe, the almost complete absorption of photons (≥99.9 %) in the hv > Eg range is observed at a layer thickness of more than 20–30 μm, and the absorptive capacity of photons in a CdTe layer of thickness 1 μm is about 93 %. Based on the continuity equation and taking into account the recombination at the front and rear surfaces of the CdTe layer as well as in the space charge region, the restrictions imposed on the thickness of the absorber layer in CdS/CdTe heterojunction are studied. In all cases, along with the mobilities and lifetimes of charge carriers, the concentration of uncompensated impurities in CdTe plays a key role in the generation of photocurrent. When the CdTe absorber layer is thin (<1 μm), it is impossible to avoid a noticeable decrease of the short-circuit current density. The losses of the short-circuit current are equal to 19–20 % when the thickness of the CdTe layer is 0.5 μm whereas only 5 % for a typical thickness of 2–3 μm.