Extrinsic doping of ink-based Cu(In,Ga)(S,Se)${}_2$-absorbers for photovoltaic applications

The addition of cesium into Cu(In,Ga)(S,Se)${}_2$-short CIGSSe-absorber layers (fabricated via vacuum deposition methods), has most recently culminated in devices with record power conversion efficiencies up to 23.4%. However, research is increasingly being devoted to the development of ink deposition routes to prepare high-quality CIGSSe thin films while requiring only a fraction of the processing costs. Such non-vacuum deposition routes must compete with efficiencies of incumbent technologies to find adoption on a wide scale. At present, the performance of ink-based devices still fall short of their vacuum counterparts with certified champion cell efficiencies up to 17.7%. The recent performance progression for vacuum-processed CIGSSe exemplifies the importance of controlling the concentration of extrinsic impurities and serves as an inspiration for gains (e.g., morphological, optoelectronic) for devices with ink-based absorber layers. This article reviews extrinsic doping concepts for CIGSSe-type absorbers fabricated by ink-based deposition routes (both nanoparticle dispersions and molecular inks), provides a performance comparison of select high-efficiency ink-based devices, and offers an outlook for future process development in general. It is suggested that the mechanisms by which dopant atoms diffuse, interact, and alter the properties of an ink-based absorber are fundamentally different than those fabricated from vacuum-based processes, and require further investigation. (© 2022 Wiley-VCH GmbH)