Short-range versus long-range structure in Cu(In,Ga)Se2, Cu(In,Ga)3Se5, and Cu(In,Ga)5Se8

Abstract

The Cu-poor phases Cu(In,Ga)3Se5 and Cu(In,Ga)5Se8 play an important role both for understanding the Cu-(In,Ga)-Se material system and for growing high-efficiency Cu(In,Ga)Se2 thin film solar cells. Using extended X-ray absorption fine structure spectroscopy, we have studied the element-specific short-range structure of Cu(In,Ga)Se2, Cu(In,Ga)3Se5, and Cu(In,Ga)5Se8 alloys spanning the entire compositional range. The materials feature different local atomic arrangements and the element-specific average bond lengths remain nearly constant despite significant changes of the lattice constants with increasing In to Ga ratio and decreasing Cu content. In particular, the average bond lengths of Cu-Se and Ga-Se are almost identical while the average In-Se bond length is significantly longer in all three phases. The distance between lattice sites with mixed site occupation therefore corresponds to the weighted average of different element-specific bond lengths rather than to the individual bond lengths themselves. Furthermore, the increasing number of vacancies with decreasing Cu content lead to both a significant unit cell contraction and a slight bond length expansion. The crystallographic long-range structure and the element-specific short-range structure thus describe different structural aspects that are certainly interrelated but obviously not identical.