Enhanced Carrier Collection in Cd/In-Based Dual Buffers in Kesterite Thin-Film Solar Cells from Nanoparticle Inks

Increasing the power conversion efficiency (PCE) of kesterite Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells has remained challenging over the past decade, in part due to open-circuit voltage (V_OC)-limiting defect states at the absorber/buffer interface. Previously, we found that substituting the conventional CdS buffer layer with In₂S₃ in CZTSSe devices fabricated from nanoparticle inks produced an increase in the apparent doping density of the CZTSSe film and a higher built-in voltage arising from a more favorable energy-band alignment at the absorber/buffer interface. However, any associated gain in V_OC was negated by the introduction of photoactive defects at the interface. This present study incorporates a hybrid Cd/In dual buffer in CZTSSe devices that demonstrate an average relative increase of 11.5% in PCE compared to CZTSSe devices with a standard CdS buffer. Current density-voltage analysis using a double-diode model revealed the presence of (i) a large recombination current in the quasi-neutral region (QNR) of the CZTSSe absorber in the standard CdS-based device, (ii) a large recombination current in the space-charge region (SCR) of the hybrid buffer CZTSSe-In₂S₃-CdS device, and (iii) reduced recombination currents in both the QNR and SCR of the CZTSSe-CdS-In₂S₃ device. This accounts for a notable 9.0% average increase in the short-circuit current density (J_SC) observed in CZTSSe-CdS-In₂S₃ in comparison to the CdS-only CZTSSe solar cells. Energy-dispersive X-ray, secondary-ion mass spectroscopy, and grazing-incidence X-ray diffraction compositional analysis of the CZTSSe layer in the three types of kesterite solar cells suggest that there is diffusion of elemental In and Cd into the absorbers with a hybrid buffer. Enhanced Cd diffusion concomitant with a double postdeposition heat treatment of the hybrid buffer layers in the CZTSSe-CdS-In₂S₃ device increases carrier collection and extraction and boosts J_SC. This is evidenced by electron-beam-induced current measurements, where higher current generation and collection near to the p-n junction is observed, accounting for the increase in J_SC in this device. It is expected that optimization of the heat treatment of the hybrid buffer layers will lead to further improvements in the device performance.