Characterization of Cu(In,Ga)Se 2 materials used in record performance solar cells

Abstract

Solar cells based on polycrystalline thin-film Cu(In,Ga)Se 2 materials have recently achieved a new level of performance with a certified efficiency of 19.5%. In this contribution, some physical characteristics of the absorber materials (and devices) leading to such performance are presented. From the absorber composition and the device quantum efficiency data, we found that these materials have an atomic bulk composition of 0.88 < Cu / (In + Ga) < 0.95 and Ga / (In + Ga) ∼ 0.3 leading to an empirical effective band gap of 1.14 eV for which maximum performance is attained. These chalcopyrite absorber materials are also characterized by a strong <220/204> preferred orientation. Because of this key structural aspect found in our high-efficiency absorbers, we present a comparison for some physical characteristics of the absorber as related to typical preferred orientations observed in this material system, namely <112> and <220/204>. We find that <220/204>-oriented thin films are in general more homogeneous than <112>-oriented films in terms of their optoelectronic properties, and they lead to materials with a lower density of nonradiative recombination centers.