Efficiency Improvement in Lead-Free Perovskite Solar Cells Using All-Inorganic Electron Transfer Layer (ETL) and Hole Transfer Layer (HTL)

In this study, various factors such as individual layer thickness, acceptor density (NA), defect density, interface defect density, and the metal electrode work function affecting the efficiency of the lead-free perovskite solar cell (PSC) were investigated using the SCAPS-1D (Solar Cell Capacitance Simulator in 1 Dimension) simulation. The analyzed device had a structure of FTO/ZnO/CsSnI₃/NiO_x/Au. ZnO served as the electron transport layer (ETL), CsSnI₃ as the perovskite absorption layer (PAL), and NiO_x as the hole transport layer (HTL), all contributing to optimizing device performance.
Organic materials are highly affected by moisture and oxygen, whereas inorganic materials are less sensitive to these factors, thus simplifying the overall photovoltaic solar cell structure. The inorganic ETL materials selected in this study were chosen from those exhibiting n-type semiconductor characteristics, such as ZnO and TiO₂, known for their wide bandgap energy characteristics. Among them, ZnO, which is plentiful and non-toxic, offers a broad bandgap (Eg = 3.26 eV) and high electron mobility, ranking it among the most promising ETL materials. Materials such as NiO_x and Cu₂O, which exhibit p-type semiconductor characteristics, were chosen as inorganic HTL materials. Especially, NiO_x is noted for its excellent electron-blocking ability and chemical stability.
Tin-based perovskite materials were applied as the absorption layer. This Tin-based inorganic perovskite (CsSnX₃) offers advantages like a band gap near 1.4 eV and high mobility. Specifically, CsSnI₃ has a band gap ranging from 1.3 to 1.4 eV and demonstrates a high absorption coefficient of 10⁴ cm^-1 in the visible spectrum. Additionally, it boasts superior thermal stability with a melting point of 451°C, significantly higher than conventional perovskites such as MASnI₃ and FASnI₃, which melt at 200°C. For this device optimization, SCAPS-1D software was employed. To optimize the power conversion efficiency (PCE) of the device, the thickness of each layer (ETL, HTL, and PAL) was first considered. The optimal thicknesses were determined to be 20nm for the ETL (ZnO), 700nm for the PAL (CsSnI₃), and 10nm for the HTL (NiO_x), with Au as the metal electrode. Following the optimization of thickness, the ideal PAL acceptor density (NA) was determined to be 2×10¹⁹ cm^-3 to achieve optimal PCE.
As a result of this optimization process, efficiency increased from 11% to maximum around 23%. These findings are expected to enhance the performance of eco-friendly, lead-free inorganic solar cells utilizing Sn-based perovskite as the PAL.