The Influence of the Cu-rich/Cu-poor sequence on the properties of Cu(In, Ga)Se2 films deposited by in-line co-evaporation process

In this study, CIGS films are grown by the in-line evaporation process with Cu-rich /Cu-poor sequence at a low temperature of ~480°C. The influence of various maximum of Cu/(In +Ga) (ym) during film growth on the structural properties of CIGS films is investigated. It is observed that Cu rich growth at low temperature is also important to improve the crystal quality of CIGS film. Both the In/Ga intermixing and the grain size are enhanced as ym increases. However, higher Cu excess (ym>1.5) yields a rougher CIGS morphology, group III - rich phases near the absorber surface, and deeper crevices extending to the bottom part of CIGS film. It is concluded that a threshold of Cu excess exists for high photovoltaic quality CIGS films, beyond which the inferior structure and undesirable phases of CIGS absorber are obtained, potentially being adverse for the performance of CIGS solar cell.