CdS/CdTe Solar Cells Containing Directly-Deposited CdSxTe1-x Alloy Layers

A CdS _xTe _1-x layer forms by interdiffusion of CdS and CdTe during the fabrication of thin-film CdTe photovoltaic (PV) devices. The CdS _xTe _1-x layer is thought to be important because it relieves strain at the CdS/CdTe interface that would otherwise exist due to the 10% lattice mismatch between these two materials. Our previous work [1] has indicated that the electrical junction is located in this interdiffused CdS _xTe _1-x region. Further understanding, however, is essential to predict the role of this CdS _xTe _1-x layer in the operation of CdS/CdTe devices. In this study, CdS _xTe _1-x alloy films were deposited by radio-frequency (RF) magnetron sputtering and co-evaporation from CdTe and CdS sources. Both RF-magnetron-sputtered and co-evaporated CdS _xTe _1-x films of lower S content (x<0.3) have a cubic zincblende (ZB) structure akin to CdTe, whereas those of higher S content have a hexagonal wurtzite (WZ) structure like that of CdS. Films become less preferentially oriented as a result of a CdCl ₂ heat treatment (HT) at ∼400°C for 5 min. Films sputtered in a 1% O ₂/Ar ambient are amorphous as deposited, but show CdTe ZB, CdS WZ, and CdTe oxide phases after a CdCl ₂ HT. Films sputtered in O ₂ partial pressure have a much wider bandgap than expected. This may be explained by nanocrystalline size effects seen previously [2] for sputtered oxygenated CdS (CdS:O) films. Initial PV device results show that the introduction of a directly-deposited CdS _xTe _1-x alloy layer into the device structure produces devices of comparable performance to those without the alloy layer when a CdCl ₂ HT is performed. Further investigation is required to determine whether the CdCl ₂ heat treatment step can be altered or eliminated through direct deposition of the alloy layer.