Surface Modification of Cu(in,Ga)Se2 Film with a Post-Deposition Treatment Using a KI Solution and Its Effect on Solar Cell Performance

Alkali post-deposition treatment (PDT) of Cu(In,Ga)Se-2 (CIGS) film is an effective approach to obtain high-efficiency CIGS solar cells. Herein, a low-cost and scalable surface passivation process is reported when the potassium iodide (KI) alkali source is employed instead of the conventional potassium fluoride (KF). CIGS surface is modified by spin-coating of KI solution and subsequent annealing at 350 degrees C in a Se atmosphere. With the KI PDT, the cell efficiency is enhanced by almost 20% due to the increase of shunt resistance and the lowering of reverse saturation current. To clarify the origin of the efficiency enhancement, the chemical state and electronic structure at the CIGS surface are investigated. The KI PDT yields a valence band lowering by 0.18 eV at the CIGS/CdS interface, leading to the suppression of the interface recombination. This is possible by Cu depletion and Cu cation reduction at the CIGS surface after KI treatment. The developed process may be considered an upcoming alternative alkali PDT for CIGS absorbers. The results provide the knowledge base for further optimization of the interface properties to form high-quality heterojunction in the CIGS solar cells.