Path toward the Performance Upgrade of Lead-Free Perovskite Solar Cells Using Cu2ZnSn1–xGexS4 as a Hole Transport Layer: A Theoretical Simulation Approach

Abstract

Recently, formamidinium tin iodide (CH(NH2)2SnI3, FASnI3) perovskite has emerged as a promising candidate for lead-free perovskite solar cells. However, a limited power conversion efficiency (PCE) was achieved when the conventional TiO2 and Spiro-OMeTAD were selected as the electron transport layer and hole transport layer (ETL/HTL), respectively. By employing numerical modeling of the FTO/TiO2/FASnI3/Spiro-OMeTAD/Au heterostructure, we noticed that the conduction-band offset (ΔEC) between TiO2/perovskite and valence-band offset (ΔEV) between perovskite/Spiro-OMeTAD layers are suboptimal, resulting in limited PCE. To overcome this shortcoming, we replace WO3 and inorganic Cu2ZnSn1–xGexS4 (CZTGS) as the ETL and HTL, respectively, which dramatically improves the PCE by creating a suitable ΔEC and ΔEV. This behavior has been investigated by the simulation using impedance spectroscopy, revealing that the high VOC and PCE are attributed to the relatively large recombination resistance (Rrec) at the CZTGS/perovskite interface. Further enhancement in PCE has been attained by replacing the MoOx:Au composite for the Au back contact. Considering the paramount importance of electronic levels of the active layer in device physics, we further optimize the band gap and electron affinity of the FASnI3 layer and study the corresponding changes in solar cell parameters. The combined effect of material simulation on the modified device exhibited an inspiring PCE of 17.1% and VOC of 0.75 V, which are mainly attributed to the correct energy level alignment at different heterojunctions and suitable work function of the alternative back contact.