The effect of CdTe growth temperature and ZnTe:Cu contacting conditions on CdTe device performance

Abstract

CdTe device performance is strongly dependent on the quality of the back contact and the ability of the back contact to introduce a copper doping profile in the CdTe layer itself. Copper-doped ZnTe (ZnTe:Cu) is a nearly ideal contact material for CdTe solar cells due to its work function and ability to source copper to CdTe. Most of the ZnTe:Cu studies in the past used CdTe grown at relatively low deposition temperatures (550°C and below). Here we investigate the use of ZnTe:Cu as a back contact for CdTe grown at temperatures up to 620°C. We observe a strong interplay between the CdTe absorber deposition conditions and optimized ZnTe:Cu contacting conditions. Device JV characteristics suggest that CdTe solar cells with absorber layers deposited by close-space sublimation (CSS) at high temperature, 600-620°C, are more robust to the back contact Cu doping level and contacting temperature than CdTe grown at lower temperatures. The implication for industrial processes is a ~1% absolute increase in device efficiency for devices in which the CdTe is deposited on PV glass at high temperature. Perhaps more importantly, this increased performance is maintained for a larger window of temperature and doping level of the ZnTe:Cu back contact. For devices with CdTe absorbers deposited at 600°C, device efficiency in excess of 13.5% is maintained for back contacts containing 2-5 wt.% Cu, and for contacting temperatures ranging from 300-360°C. Red-light bias quantum efficiency (QE) and capacitance-voltage (CV) measurements are used to probe the effect of the introduced copper doping profiles and net acceptor density to better understand how ZnTe:Cu sources influences the resulting CdTe device.