Sputtered Cd1−xZnxTe films for top junctions in tandem solar cells

Cd1−xZnxTe alloy films with 1.6 and 1.7 eV band gaps were deposited by RF magnetron sputtering from targets made either of mixed powders or alloys of CdTe and ZnTe (25% and 40%). High-quality polycrystalline films with the (1 1 1) preferred orientation were obtained. The films were characterized using X-ray diffraction (XRD), scanning electron microscopy, resistivity, optical absorption, Raman, and photoluminescence. The EDS, XRD, and optical absorption analysis indicated that the x-value of the as-grown films were typically ∼0.20 and 0.30 for films sputtered from 25% and 40% ZnTe containing targets, respectively. The as-deposited alloy films exhibit quite low photovoltaic performance when used to make cells with CdS as the hetero-junction partner. Therefore, we have studied various post-deposition treatments with vapors of chlorine-containing materials, CdCl2 and ZnCl2, in dry air or H2/Ar ambient at ∼390 °C. The best performance of a Cd1−xZnxTe cell (VOC=737mV, JSC=19mA/cm2) was found for treatment with vapors of the mixed CdCl2+0.5%ZnCl2 in an H2/Ar ambient after pre-annealing at ∼520 °C in pure H2/Ar.