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Chalcopyrite solar cells achieve efficiencies above 23%. The latest improvements are due to post‐deposition treatments (PDT) with heavy alkalis. This study provides a comprehensive description of the effect of PDT on the chemical and electronic structure of surface and bulk of Cu(In,Ga)Se2. Chemical changes at the surface appear similar, independent of absorber or alkali. However, the effect on the surface electronic structure differs with absorber or type of treatment, although the improvement of the solar cell efficiency is the same. Thus, changes at the surface cannot be the only effect of the PDT treatment. The main effect of PDT with heavy alkalis concerns bulk recombination. The reduction in bulk recombination goes along with a reduced density of electronic tail states. Improvements in open‐circuit voltage appear together with reduced band bending at grain boundaries. Heavy alkalis accumulate at grain boundaries and are not detected in the grains. This behavior is understood by the energetics of the formation of single‐phase Cu‐alkali compounds. Thus, the efficiency improvement with heavy alkali PDT can be attributed to reduced band bending at grain boundaries, which reduces tail states and nonradiative recombination and is caused by accumulation of heavy alkalis at grain boundaries.

This review summarizes the current status of Cu(In,Ga)(S,Se) 2 (CIGS) thin film solar cell technology with a focus on recent advancements and emerging concepts intended for higher efficiency and novel applications. The recent developments and trends of research in laboratories and industrial achievements communicated within the last years are reviewed, and the major developments linked to alkali post deposition treatment and composition grading in CIGS, surface passivation, buffer, and transparent contact layers are emphasized. Encouraging results have been achieved for CIGS-based tandem solar cells and for improvement in low light device performance. Challenges of technology transfer of lab's record high efficiency cells to average industrial production are obvious from the reported efficiency values. One section is dedicated to development and opportunities offered by flexible and lightweight CIGS modules.

Postdeposition treatments (PDTs) with sodium fluoride (NaF) and potassium fluoride (KF) were introduced as a way to improve the efficiency of Cu­(In,Ga)­Se2 (CIGS) based solar cells. Here, we apply postdeposition treatments with rubidium fluoride (RbF) to low-temperature coevaporated CIGS absorbers after a first PDT with NaF and compare the effects of the addition of Rb and K on the solar cell performance and material properties of the CIGS films. KF and RbF PDTs lead to similar improvements in the open-circuit voltage (V oc) and fill factor (FF), while allowing a reduction of the thickness of the cadmium sulfide (CdS) buffer layer without loss in electronic performance. KF and RbF PDTs lead to comparable modifications of the morphology and composition of the CIGS films. After the PDT, K and Rb accumulate in a nanopatterned copper-poor secondary phase at the CIGS surface, while also diffusing within the CIGS layer and strongly reducing the concentration of lighter alkali element sodium. These findings corroborate theoretical calculations published by another group, which predicted the segregation of potassium indium selenide (KInSe2) and rubidium indium selenide (RbInSe2) at CIGS surfaces under the used PDT conditions.