Effects of CdTe selenization on the electrical properties of the absorber for the fabrication of CdSexTe1-x/CdTe based solar cells

CdTe thin film solar cells are the most successful thin film photovoltaic devices in terms of production yield. In the last few years, by changing the original solar cell structure, the efficiency was increased from around 17 % up to 22 %. In particular two main innovations have been introduced: a CdSexTe1-x layer, that narrows the band gap of the absorber near the junction, and a large band gap buffer layer (i.e. MgxZn1-xO). These allow to improve the absorption in the long and in the short wavelength region, respectively. Generally, CdSexTe1-x is formed either by the deposition of a CdSe layer at the junction subsequently mixed with CdTe or by directly depositing a CdSexTe1-x compound, we have instead introduced a different method where CdSexTe1-x is formed by treating a thin CdTe layer at high temperature in selenium atmosphere. The finished device has been fabricated by substituting CdS with SnO2 and efficiencies exceeding 14 %, with current densities higher than 26 mA/cm2, are obtained by a low substrate temperature process. In this work we analyze and compare our older standard FTO/SnO2/CdS/CdTe with the novel FTO/SnO2/CdSexTe1-x/CdTe devices by capacitance voltage, drive level capacitance profiling, and admittance spectroscopy. The results highlight the effect of the selenization process in terms of carrier concentration, carrier profile, and formation of deep and shallow defects. The overall performance of the completed devices is also presented and compared.