Bulk Carrier Recombination Mechanisms and Photovoltage Deficit in Kesterite Solar Cells

Significant open-circuit voltage deficit (V_OC-def) is regarded as the primary obstacle to achieving efficient kesterite solar cells. By leveraging a synergistic approach that combines photoluminescence, admittance spectroscopy and cathodoluminescence techniques, the theoretical models of radiative recombination in Cu₂ZnSnS₄ kesterite are revisited, allowing for a comprehensive clarification of both radiative and nonradiative recombination loss effects of V_OC-def in the kesterite bulk and at interfaces. This quantitative analysis of V_OC-def reveals that Cu/Zn disorder remains a fundamental limitation for kesterite solar cells, comparable to deep-level defects. Specifically, it is demonstrated that the asymmetric photoluminescence band commonly observed in Cu₂ZnSnS₄ consists of two competing components: tail-impurity recombination (conduction band → Cu_Zn) and quasi-donor-acceptor-pair recombination (Zn_Cu → Cu_Zn). These findings confirm that Cu/Zn antisite defects and related potential fluctuations reduce the effective bandgap. Furthermore, it is confirmed that band tails in kesterite are the result of electrostatic potential fluctuations and bandgap fluctuations. The amplitude of the electrostatic potential fluctuations is estimated to be ≈30 meV. Bandgap fluctuations in kesterite are experimentally distinguished from electrostatic potential fluctuations for the first time, which leads to a bandgap contraction of about 130 meV. These studies provide crucial theoretical support for the advancement of kesterite photovoltaic technology.