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 last decade, in part due to <i>V_oc</i>-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 favourable energy band alignment at the absorber/buffer interface. However, any associated gain in <i>V_oc</i> was negated by the introduction of photo-active defects at the interface (Campbell <i>et al.</i>, J. Appl. Phys 127, 205305, 2020). This present study incorporates a hybrid Cd/In dual buffer in CZTSSe devices which demonstrate an average relative increase of 11.5% in PCE compared to CZTSSe devices with standard CdS buffer, with the best performing dual buffer device (CZTSSe/CdS/In₂S₃) achieving 7.8% PCE (compared to 5.9% PCE in the CZTSSe/In₂S₃/CdS hybrid buffer device).<br/>Spectral response measurements of hybrid buffer devices confirmed the presence of photo-active interface defects when the In₂S₃ buffer is adjacent to the CZTSSe absorber. 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 the notable 9.0% average increase in <i>J_sc</i> observed in the CZTSSe/CdS/In₂S₃ in comparison to the CdS-only CZTSSe solar cells.<br/>Energy dispersive X-ray (EDX), secondary ion mass spectroscopy (SIMS) and grazing incidence X-ray diffraction (GIXRD) compositional analysis of the CZTSSe layer in the three types of kesterite solar cells suggests there is diffusion of elemental In into the absorbers with a hybrid buffer. In incorporation into the CZTSSe absorber increases the doping density of the SCR in the absorber. In diffusion arises from the post-deposition heat treatment (PDHT) of the hybrid buffer layers following their successive chemical bath depositions. When the In₂S₃ buffer is adjacent to the absorber of the hybrid buffer devices it is subjected to a double PDHT which greatly increases In incorporation into the absorber. As a result, the doping density in this absorber reaches 10¹⁷ cm^-3 which negatively affects device performance. However, in the CZTSSe/CdS/In₂S₃ device, the single PDHT of the In₂S₃ layer is sufficient to increase the doping density of the SCR region of the CZTSSe without adversely affecting overall device performance.<br/>It is expected that optimisation of the thickness of the hybrid buffer layers will lead to further improvements in device performance. These hybrid In₂S₃/CdS buffers provide a promising way to boost the efficiency of kesterite CZTSSe solar cells.