Grain boundary and Se diffusion investigations in CdSe-CdTe thin-film photovoltaics

Abstract

Cadmium telluride (CdTe) solar cells are a very attractive thin-film photovoltaic technology because of low-cost manufacturing and high light absorption coefficient. Electron hole pairs generated by photons absorbed in the solar cell have a finite lifetime, due to radiative or nonradiative recombination processes. One path towards increasing the efficiency of CdTe solar cells is to minimise non-radiative recombination at grain boundaries. This work aims to combine cathodoluminescence (CL) contrast (optoelectrical properties) and electron backscatter diffraction (structure properties) to investigate the effect of grain boundary misorientation on non-radiative recombination. At the range between 35°-55° misorientation angle, the CL contrast was significantly high, which indicates that those regular grain boundaries are strong non-radiative recombination centres. For coincident site lattices (CSL) such as Σ3, Σ5, Σ7 and Σ9 which have high symmetry, the CL contrast was much smaller, which means those special grain boundaries are not harmful to the device. The reduced recombination velocity was also measured, and the results shown to be consistent with the contrast measurements. Se diffusion from CdSe to CdTe by using standard and bevel cross-sections has also been investigated for two samples with different external quantum efficiency (EQE) and thicknesses (i.e. 100 and 400 nm) of the CdSe layer. The results showed that both devices have voids at the inter-diffusion layer, although for the 400 nm CdSe sample voids were more numerous. Rapid diffusion of Se along the CdTe grain boundaries led to small grains at the interface region, due to Kirkendall voids and solute pinning of the grain boundaries, which resulted in a lower EQE for the 400 nm CdSe device. Transmission electron microscopy (TEM) diffraction patterns confirmed a zinc blende cubic structure throughout the absorber layer, which suggests that the lower EQE is not due to a change in the crystal structure, as has previously been reported in the literature. The Se diffusion coefficient is calculated using scanning TEM energy dispersive X-ray (EDX). The diffusion coefficient measured is 7.2×10−12 cm2/s, which is an order of magnitude smaller than the literature value.