Origin of Band-Tail and Deep-Donor States in Cu₂ZnSnS₄ Solar Cells and Their Suppression through Sn-Poor Composition

By comparing optical spectral results of both Sn-rich and Sn-poor Cu₂ZnSnS₄ (CZTS) with the previously calculated defect levels, we confirm that the band-tail states in CZTS originate from the high concentration of 2Cu_Zn + Sn_Zn defect clusters, whereas the deep-donor states originate from the high concentration of Sn_Zn. In Sn-rich CZTS, the absorption, reflectance, and photocurrent (PC) spectra show band-tail states that shrink the bandgap to only ∼1.34 eV, while photoluminescence (PL) and PC spectra consistently show that abundant Cu_Zn + Sn_Zn donor states produce a PL peak at ∼1.17 eV and abundant Sn_Zn deep-donor states produce a PL peak near 0.85 eV. In contrast, Sn-poor CZTS shows neither bandgap shrinking nor any deep-donor-defect induced PL and PC signals. These results highlight that a Sn-poor composition is critical for the reduction of band-tailing effects and deep-donor defects and thus the overcoming of the severe open-circuit voltage (V_oc) deficiency problem in CZTS solar cells.