Postdeposition Sulfurization of CuInSe2 Solar Absorbers by Employing Sacrificial CuInS2 Precursor Layers

Abstract

Herein, a new route of sulfur grading in CuInSe2 (CISe) thin‐film solar absorbers by introducing an ultrathin (<50 nm) sacrificial sputtered CuInS2 (CIS) layer on top of the CISe. Different CIS top layer compositions (Cu‐poor to Cu‐rich) are analyzed, before and after a high‐temperature treatment in selenium (Se)‐ or selenium+sulfur (SeS)‐rich atmospheres. An [S]/([S] + [Se]) grading from the surface into the bulk of the Se‐ and SeS‐treated samples is observed, and evidence of the formation of a mixed CuIn(S,Se)2 phase by Raman analysis and X‐ray diffraction is provided. The optical bandgap from quantum efficiency measurements of solar cells is increased from 1.00 eV for the CISe reference to 1.14 and 1.30 eV for the Se‐ and SeS‐treated bilayer samples, respectively. A ≈150 mV higher VOC is observed for the SeS‐treated bilayer sample, but the cell exhibits blocking characteristics resulting in lower efficiency as compared with the CISe reference. This blocking is attributed to an internal electron barrier at the interface to the sulfur‐rich surface layer. The difference in reaction routes and possible ways to improve the developed sulfurization process are discussed.