Buffer-free Cu(In,Ga)Se2-solar cells by near-surface ion implantation

High-efficiency Cu(In,Ga)Se2 thin-film solar cells typically include CdS buffer layers deposited in a chemical bath. In this work, Cu(In,Ga)Se2 devices are presented in which the CdS buffer layer was omitted completely. Instead, low-energy ion implantation of group-II-elements (Cd, Zn, and Mg) is applied in order to establish an n-type surface layer in p-type Cu(In,Ga)Se2 absorber layers. Therefore, thermal annealing procedures were developed which lead to a full recovery of the implantation induced defects and simultaneously minimize the diffusion of the dopants. Such a treatment is shown to provide high-quality p-n junction functionality and buffer-free Cu(In,Ga)Se2 thin-film solar cells with open-circuit voltages close to 600 mV and efficiencies exceeding 10 %.