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

The microstructure of 4?13 ?m 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 (?m) = 0.050x ? 0.10 (4 < x < 12 ?m), 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.