Optimisation of SnO2 electron transport layer annealing temperature for flexible perovskite solar cells

Organic–inorganic metal halide perovskites (OIHPs) have emerged as promising photovoltaic materials for next-generation solar energy conversion due to their excellent optoelectronic properties and low-cost fabrication. However, the development of flexible perovskite solar cells (FPSCs) is limited by the poor thermal stability of polyethylene terephthalate (PET) substrates, which can deform or degrade during high-temperature processing. Conventional electron transport layers (ETLs), such as titanium dioxide (TiO2), require elevated annealing temperatures that are unsuitable for flexible devices. Therefore, this study investigated the optimisation of the annealing temperature of the SnO2 ETL for FPSCs fabricated on PET/ITO substrates with the architecture PET/ITO/SnO2/MAPbI3/P3HT/Cu. The SnO2 films were annealed at 110–160 ^∘ C ∘ C and characterised using ultraviolet–visible (UV–Vis) spectroscopy, photoluminescence (PL) spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), and Hall effect measurements. Photovoltaic (PV) performance was evaluated using current density–voltage (J–V) measurements. The results identified 150 ^∘ C ∘ C as the optimum annealing temperature, producing improved film uniformity, enhanced optical transmittance, reduced charge recombination, and lower electrical resistance without compromising the structural integrity of the PET substrate. The optimised device achieved a power conversion efficiency (PCE) of 5.29%, retained 80.3% of its initial PCE after 1000 bending cycles at a 5 mm radius, and maintained 80% of its initial PCE after 1000 h of storage in dry air ( ~ ∼ 30% RH, 25 ^∘ C ∘ C ) without encapsulation. These findings demonstrate that optimised low-temperature SnO₂ processing is an effective strategy for enhancing the photovoltaic performance, mechanical flexibility, and long-term stability of FPSCs.