Impact of Urbach energy on open-circuit voltage deficit of thin-film solar cells

Correlation between Urbach energy (E-U) and open-circuit voltage deficit (V-OC,V-def) among conventional as well as emerging thin-film solar cells, i.e. Cu(In,Ga)Se-2 (CIGSe), Cu(In,Ga)(S,Se)(2)(CIGSSe), Cu2SnS3 (CTS), Cu2Sn1-xGexS3(CTGS), SnS, Cu2ZnSnSxSe4-x (CZTSSe), perovskite (PVK), Cu(In,Ga)S-2 (CIGS), and CdTe thin-film solar cells, is examined. The E-U is estimated from an exponential tail in a long-wavelength edge of an external quantum efficiency. It is determined that the relation between short-circuit current density (J(SC)) and E-U is not clearly shown because the J(SC) is primarily influenced by the bandgap of the thin-film absorbers. On the other hand, E-U demonstrates a close relation with V-OC,V-def, where the reduction of the Eu leads to the decrease in the V-OC,V-def, implying that the E-U can be used as an indicator of the absorber quality. The E-U of over 20 meV, observed in the CIGS, CTS, CTGS, SnS, and CZTSSe solar cells, gives rise to the high V-OC,V-def and the low conversion efficiency (eta) values. The E-U of below 20 meV, seen in the CIGSe, CIGSSe, PVK, and CdTe solar cells, leads to the low VOC,def and high eta values. Ultimately, the thin-film absorbers with the E-U values of below 20 meV, which is lower than thermal energy under room temperature, is of vital to realize high photovoltaic performances.