Optimization of transport layers and interfaces in FAPbI3 based perovskite solar cells

Perovskite solar cells (PSCs) are a promising photovoltaic technology with the potential to achieve high power conversion efficiencies while maintaining a low-cost fabrication process. However, a critical performance bottleneck in PSCs is the high defect density at the interface between the perovskite absorber and the charge transport layers, leading to significant non-radiative recombination and thus limiting the overall efficiency. In this research, formamidinium lead triiodide (FAPbI3) has been investigated as a perovskite composition with a suitable bandgap for high-efficiency single-junction devices. This work explored strategies to mitigate interfacial recombination losses to enhance efficiency. The quality of the interfaces was analyzed through photoluminescence measurements. While the bare perovskite on glass exhibited a strong photoluminescence signal, the addition of the hole transport layer (HTL) 2,2',7,7'-tetrakis(N,N-di-pmethoxyphenyl- amine)9,9'-spirobifluorene (Spiro-MeOTAD) increases the non-radiative recombination. To address this issue, three different passivation layers were implemented: cyclohexylethylammonium iodide (CEAI), phenethylammonium iodide (PEAI), and 4-fluorophenethylammonium iodide (FPEAI). The introduction of CEAI and PEAI in complete devices improved power conversion efficiencies from 17% to 22%. To reduce non-radiative recombination losses on the electron transport layer (ETL) side, both the transparent conductive oxide (TCO) and its combination with different ETL materials were systematically investigated. A comparison between the two most used TCOs, fluorine-doped tin oxide (FTO) and indium tin oxide (ITO), revealed that both offer comparable optical transparency and conductivity, making them viable candidates for efficient PSCs. To further improve the interface between the electrode and the perovskite, tin oxide (SnOx) and titanium oxide (TiO2) were optimized as electron transport materials for FTO substrates. The best performance was achieved using a compact/mesoporous TiO2 double layer, which enhanced both cell efficiency and reproducibility. Despite these optimizations, reproducibility remained a major challenge due to the sensitivity of perovskite crystallization to processing conditions. Therefore, this work investigated key factors influencing FAPbI3 film formation, identifying glass surface properties and the delay between wet film deposition and temperatureinduced crystallization as critical parameters. In addition, around 1 sun equivalent of illumination intensity during crystallization was found to impact the final film morphology.