Atomic Layer Deposition of Zinc Tin Oxide Buffer Layers, Investigation of Conduction Band Minimum Through Analysis of CISe and CIGSe Solar Cells

In recent years, atomic layer deposited (ALD) zinc tin oxide buffer layers have emerged as a nontoxic and wide band gap alternative to the popular chemical bath deposited (CBD) cadmium sulfide (CdS). An important parameter that predicts the performance of a buffer is its conduction band minimum (CBM). According to the literature, the CBM of ZnSnO depends on the tin/(tin+zinc) (TTZ) atomic ratio, the CBM is predicted increases as the TTZ increases, although agreement on the precise dependence has not yet been reached [1], [2], [3]. As we alter the composition of the ZnSnO, varying TTZ from 0 to 36.5%, we predict a change of CBM on the scale of a few hundred meV, with a maximal value at 30-35%TTZ [3]. We produce CIGSe and CISe solar cells with various ZnSnO and CdS buffers, and measure their performances. As we change the buffer composition, we expect the following possible issues [4]: 1. In the absorber-buffer interface, a decrease in the CBM (cliff) would result in a reduction in open circuit voltage. 2. A large Increase in CBM between the absorber and the buffer would result in decreases in fill factor and short circuit current. In the analysis of the cells, we notice that in the range of ~10-25% TTZ the composition has little to no effect on cell performance, and is mostly similar to the CdS reference. However, at higher tin concentrations, we see considerably worse performance, particularly in the fill factor. We additionally measure two CISe cells with buffer TTZ compositions of 31 and 35% which block current almost entirely. These observations are compatible with the predicted transport barriers caused by increased buffer CBM: extreme blocking of the lower bandgap absorbers, and more mild effects on the higher bandgap ones.