ZnTe-CdS thin film photovoltaic cells

ZnTe-CdS heterojunctions, developed for photovoltaic tandem systems, have been fabricated by evaporating ZnTe on polycrystalline CdS films. The CdS films are vacuum deposited onto glass substrates with silver metallization. The deposition of p-type ZnTe is performed by coevaporation of ZnTe and dopants (Cu, Ag, Te) from two individually controlled evaporation boats. The properties of the ZnTe films have been investigated by means of optical transmission, cathodoluminescence, and resistivity measurements. For the characterization of the heterojunction measurements of the I–V characteristics, the spectral response, the electron beam induced current (EBIC), and the photocapacitance have been carried out. The measurements indicate that the largest part of the photocurrent is generated in the CdS layer. The open circuit voltage ranges from 0.5 to 0.93 V with most of the cells near 0.75 V. Short circuit current densities of about 3 mA/cm2 have been obtained. The I-V characteristics commonly show a “flat-spot” or “broken-knee” effect. The theoretical model assumes multistep tunnelling and recombination processes for carrier transport and, probably in grain boundaries, the formation of Cu2S(low barrier, low open circuit voltage) and ZnCdTe (high barrier, high open circuit voltage) in some parts of the cells.