An investigation of inorganic ETL materials for carbon-based HTL-free perovskite solar cell

Nanomaterials and nanotechnology have a pivotal role in reducing the footprint of target devices in many fields, in particular electronics and optics. The design of solar cells for meeting energy needs has also been taking advantage of such advancements in nanotechnology. One of the emerging types of solar cells is perovskite solar cells, which are not only cheap but efficient as well. There has been a lot of progress made in their design based on materials and structure. The continuous endeavors for further reduction of the device cost for large-scale deployment have led to a Hole Transparent Layer (HTL)-free structure with carbon electrodes instead of noble metals, as the work function of carbon is close to that of gold. The device layers, including the absorber and Electron Transport Layer (ETL), hold fundamental importance for an HTL-free design where TiO2 has been a very common material for ETL in perovskite solar cells, but it offers low conductivity and is susceptible to photo-catalysis on exposure to UV light. Therefore, we have investigated different materials, including WS₂ (BG: 1.8 eV), WO3 (BG: 2.6 eV), ZnSe (BG: 2.82 eV), ZnO (BG: 3.3 eV) for being suitable alternatives to TiO₂ (BG: 3.2 eV) as an ETL material. The device structure follows the configuration FTO/ETL/IDL/CH₃NH₃PbI₃/carbon. The optimization of different design parameters, including layer thicknesses, doping concertation, carrier mobility, diffusion length, and operating temperature, has been carried out. The device with ETL made of ZnSe yielded the best results as J_sc of 24.77 mA/cm², V_oc of 1.25 V, FF of 86.29%, and a PCE of 26.76% were obtained.