Urbach energy of Cu(In,Ga)Se2 and Cu(In,Ga)(S,Se)2 absorbers prepared by various methods: Indicator of their quality

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Correlation between Urbach energy (EU) and open-circuit voltage deficit (VOC,def) of Cu(In,Ga)Se2 (CIGSe) and Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells is examined, where the CIGSe and CIGSSe absorbers are deposited by different methods. The EU is directly defined from an exponential tail in a long-wavelength edge of an external quantum efficiency. It is shown that the EU is decreased with increased carrier lifetimes corelating to the VOC,def of the solar cells. Consequently, the EU demonstrates a close relation with the VOC,def, where the reduced EU decreases the VOC,def. In addition, conversion efficiency (η) values of the CIGSe solar cells grown by 3-stage and multi-layer precursor methods are significantly enhanced close to 20% with a reduction of the VOC,def, when the EU values of their CIGSe absorbers are reduced to approximately 20 meV. The η values are over 20% with the further decrease in the VOC,def, when the EU values are below 20 meV, mainly estimated from CIGSSe absorbers grown by selenization and sulfurization process. According to the result, the EU can be utilized as one of the intriguing indicators to evaluate the CIGSe absorber quality for the development of the solar cells and should be low (<20 meV), thereby leading to the improved photovoltaic performances.