Impact of Ag content on device properties of Cu(In,Ga)Se2 solar cells

Abstract

Partial substitution of Cu by Ag in Cu(In,Ga)Se2 (CIGS) solar cells is advantageous as it allows lower temperature growth while maintaining high performance. To understand the role of Ag on device performance, we present a comprehensive analysis of (Ag,Cu)(In,Ga)Se2 (ACIGS) samples with an [Ag]/([Ag]+[Cu]) (AAC) ratio varying from 7% to 22%. The analysis involves a set of material and device characterization techniques as well as numerical simulations. Multiple electrical and material properties show a systematic dependence on the increased Ag content. These include a carrier-density decrease, a grain-size increase, and a flattened [Ga]/([Ga] + [In]) (GGI) profile leading to a higher minimum band gap energy and a reduced back grading. Although the best performing device (PCE = 18.0%) in this set has an AAC = 7%, cells with higher Ag contents have an advantage of a smoother absorber surface which is attractive for tandem applications, despite their slightly inferior conversion efficiencies (PCE = 16.4% for 22% Ag).