Grain and crystal texture properties of absorber layers in MOCVD-grown CdTe/CdS solar cells

Abstract

The microstructure of 4–13 µm thick CdTe absorber layers in CdTe/CdS/ITO/glass solar cell structures grown by metal-organic chemical vapour deposition (MOCVD) at 350 °C has been studied. The crystalline texture, lattice parameter and grain size were measured as a function of thickness for the as-grown layers, and as a function of annealing temperature and time for annealing in both nitrogen (N2) and cadmium chloride (CdCl2) environments. The average grain sizes developed with thickness as r (µm) = 0.050x − 0.10 (4 < x < 12 µm), and this behaviour is contrasted with that for close-spaced sublimation material grown at 500 °C. Annealing in both ambients promoted grain growth (with Rayleigh grain size distribution functions and Burke–Turnbull exponents being n = 7 at 440 °C and ∼4 at 400 °C), a development of the grown-in preferred orientation from [1 1 1] to [2 1 1], and relief of the grown-in compressive stress. A growth mechanism by which development of the [2 1 1] preferred orientation may accompany grain growth is described. It is concluded that MOCVD growth at temperatures higher than 350 °C used here will be required to produce the larger grain sizes required for photovoltaic applications.