Materials to Module: Engineering Strategies for Tin-Based Perovskite Solar Cells via Interface Optimisation, Crystal Growth, Composition Tuning, and Laser Processing

Abstract

Tin-based perovskites (TBPS) have emerged as a promising non-toxic alternative to lead-based perovskites for photovoltaic and optoelectronic applications, owing to their suitable bandgap (~1.2–1.4 eV), strong absorption, and efficient charge transport. However, their practical implementation is hindered by intrinsic instability, primarily due to the oxidation of Sn2+ to Sn4+, leading to self-doping, high defect densities, and increased non-radiative recombination. Additionally, the strong Lewis acidity of Sn2+ induces rapid crystallisation and poor film morphology, further limiting device performance. This thesis systematically investigates interface engineering strategies to overcome these limitations. Comparative analysis of FASnI3-based solar cells using PEDOT: PSS and NiOx as hole transport layers (HTLs) reveals a trade-off between efficiency and stability. NiOx based devices achieve higher initial efficiencies due to favourable energy level alignment, while PEDOT: PSS offers better long-term operational stability. On the electron transport side, thermally evaporated ICBA layers produce compact films with improved JSC, although a slight drop in VOC is observed due to deeper LUMO levels and energy disorder, as validated by UPS. Thermal processing of PEAFASnI3 films is further optimised, with 100 °C annealing yielding optimal crystallinity, larger grain size, and reduced defects, translating to improved VOC, JSC, and fill factor. Excessive annealing at 120 °C degrades performance due to defect generation. These experimental findings are corroborated by DFT and MD simulations, showing reduced trap densities and favourable valence band shifts. Additional passivation using PVDF and a three-step annealing process significantly improves film quality and suppresses non-radiative losses. Compositional tuning via partial substitution of formamidinium iodide (FAI) with methylammonium halides (MAX; X = Cl, Br, I) effectively modulates film morphology and optoelectronic properties. MABr incorporation yields the best device performance, enhancing VOC, FF, and PCE, though long-term stability under ambient conditions remains a concern. The degradation is linked to reduced SnF2 surface passivation. Interestingly, MACl-based films demonstrate superior phase stability, offering a direction for improving ambient durability. Finally, a simplified laser scribing process is developed for monolithic module integration using a single 532 nm nanosecond pulsed laser. This technique successfully performs all scribing steps (P1, P2 and P3) on ITO/glass substrates, achieving clean patterning, layer-specific ablation, and reliable interconnects. The streamlined process reduces equipment costs and fabrication complexity, enabling scalable production of lead-free perovskite mini modules. In summary, this work advances tin-based perovskite solar cell development by integrating interface design, thermal and compositional engineering, and scalable module fabrication. The findings lay a robust foundation for high-performance, stable, and eco-friendly perovskite photovoltaics.