Concentration-dependent back interface passivation with SiO2 nanoparticles for efficient kesterite thin-film solar cells

Back-interface carrier recombination critically limits the open-circuit voltage ( V OC ) of CZTSSe thin-film solar cells. In this work, we systematically tune the back interface between the CZTSSe absorber and the Mo-coated glass substrate by spin-coating a dispersion of SiO 2 nanoparticles at varying concentrations (0%-3.00%). The nanoparticle coverage exhibits a clear concentration dependence, where low concentrations (0.15%-1.50%) improve the absorber crystallinity and back-interface contact, while an excessive concentration (3.00%) forms an insulating layer that severely degrades device performance. At the optimal concentration of 0.30%, the CZTSSe absorber layer shows markedly improved the crystallinity, with a denser bottom small-grain layer, fewer voids, and effective suppression of deep-level [2Cu Zn + Sn Zn ] defect clusters, as revealed by Raman spectroscopy. Consequently, the total area power conversion efficiency increases from 9.03% to 10.24%, accompanied by substantial gains in V OC and fill factor. Capacitance-voltage and drive-level capacitance profiling reveal a reduced interface defect density (from 8.02 × 10 15 to 4.35 × 10 15 cm −3 ) and a widened depletion region (from 0.114 to 0.174 μm). Electrochemical impedance spectroscopy further confirms significantly suppressed interfacial recombination, with the recombination resistance increasing from 7.10 to 20.50 kΩ. These results demonstrate that concentration-dependent back interface passivation with SiO 2 nanoparticles provides a simple and effective strategy for efficient CZTSSe solar cells, achieving enhanced crystallinity, effective defect passivation, and reduced interfacial recombination losses.