Optimization of PCBM electron transport layer concentration for the fabrication of lead-free flexible perovskite solar cells

The rapid development of flexible perovskite solar cells (FPSCs) has attracted significant interest as lead-free alternatives to address environmental and health concerns. However, challenges persist in optimizing the electron transport layers (ETLs) to enhance their efficiency and stability, particularly in flexible devices. This study aims to address these challenges by investigating the effects of varying the [6-6]-phenyl-C61-butyric acid methyl ester (PCBM) concentration (20, 30, and 40 mg/ml) on the performance of lead-free FPSCs with cesium bismuth iodide (Cs3Bi2I9) as the active layer. Utilizing an inverted device structure polyethylene naphthalate/indium tin oxide/poly(3,4-ethylenedioxythiophene):polystyrene sulfonate/Cs3Bi2I9/PCBM/silver, a systematic experimental methodology was employed to precisely control spin-coating parameters, thermal annealing, and solvent evaporation. The findings revealed that a PCBM concentration of 30 mg/ml yielded the optimal balance between film uniformity, surface morphology, and device performance, achieving a Jsc of 0.23 mA/cm2 and a power conversion efficiency of 0.42%. Conversely, lower and higher concentrations resulted in suboptimal performance, owing to insufficient film coverage and increased surface roughness, respectively. This study contributes to the emerging field of sustainable photovoltaics by demonstrating the critical role of ETL optimization in lead-free FPSCs. The results provide insight into the interplay between the PCBM concentration, charge transport dynamics, and device performance, offering a pathway for scalable and eco-friendly solar cell technologies. While policy implications have not been directly addressed, advancements highlight the potential for reducing reliance on toxic lead-based materials in solar energy applications, aligning with global sustainability goals.