Exploring the Synthesis of Cu2(Zn,Cd)SnS4 at High Temperatures as a Route for High-Efficiency Solar Cells

Abstract

The present research explores for the first time the intricate relationship between sulfurization temperature at unusual high temperatures (up to 700°C) and the structural/optoelectronic properties of Cu2(Zn,Cd)SnS4 (CZCTS) thin films, synthesized via a two-step sequential process involving the precursor film deposition using aprotic molecular ink followed by thermal treatment in sulfur atmosphere. X-ray diffraction patterns confirms the tetragonal structure. Scanning Electron Micrographs revealed significant grain growth, with grain sizes increasing from ~0.3 μm at 620°C to ~1.5 μm at 680°C, effectively reducing grain boundary recombination. Energy dispersive X-ray spectroscopy demonstrated a Cu-poor and Zn-rich composition, with a consistent Cd incorporation of ~3.7 at%. Raman spectroscopy showcases the homogeneity and purity of the CZCTS crystalline structure. Precise control of the sulfurization temperature plays a crucial role in determining the photovoltaic characteristics of CZCTS-based solar cells. By increasing the grain size and preventing the thermal decomposition of the CZTS phase, the photovoltaic performance peaked at a sulfurization temperature of 680°C, achieving a power conversion efficiency (PCE) of 10.4%, with an open-circuit voltage of 0.701 V, a short-circuit current density of 24.3 mA/cm2 and a fill factor of 60.8%. External quantum efficiency reached a maximum of 83.3% at 580 nm. The bandgap of the CZCTS absorber was determined to be 1.48 eV, optimal for photovoltaic applications. However, further increasing the sulfurization temperature to 700°C resulted in a lower PCE of 8.5%, attributed to interface degradation and secondary phase formation. Temperature-dependent current–voltage measurements revealed a reduction in recombination losses, with an activation energy of 1.24 eV at the CZCTS/CdS interface, indicating effective defect passivation by Cd incorporation. The optimized films, sulfurized at 680°C, displayed an absorber thickness of ~1.2 μm after sulfurization, providing efficient light absorption and charge transport. The findings not only emphasize the critical role of sulfurization temperature in engineering CZCTS film and subsequently their functionality but also provide valuable insights for fine tuning their performance in the field of photovoltaic applications.