Defect Engineering toward Efficient and Stable Perovskite Solar Cells

Abstract

Perovskite solar cells (PSCs) have emerged as highly promising candidates for next-generation photovoltaics due to their high power conversion efficiency (PCE) and compatibility with low-cost, solution-based fabrication. Nevertheless, the presence of bulk, interfacial, and grain boundary defects in their polycrystalline structure induces severe nonradiative recombination, thereby limiting the open- circuit voltage (VOC), PCE, and long-term operational stability. This thesis presents a comprehensive defect engineering framework that systematically addresses these issues by targeting distinct structural levels, including bulk lattice, buried interfaces, and grain boundaries, to achieve both high efficiency and long-term stability in single-junction and perovskite/silicon tandem solar cells. First, bulk defect formation is suppressed by relaxing intrinsic lattice microstrain via A-site cation co- incorporation. The introduction of MDA2+ and Cs+ into FAPbI3 stabilizes the photoactive α-phase while relieving crystallographic stress. X-ray diffraction and Williamson-Hall analyses confirm strain relaxation, which is accompanied by enhanced photoluminescence intensity, prolonged carrier lifetimes, and reduced trap density. This strategy yields single-junction devices with efficiencies exceeding 25% and excellent thermal and operational stability, establishing lattice strain control as a fundamental route to suppress bulk defects. Second, interfacial defect states at buried charge-selective contacts are effectively passivated through a tailored self-assembled monolayer (SAM) approach. The developed ternary SAM, composed of Me- 4PACz, 1-acetylguanidine (AG), and glycerol dimethacrylate (GDMA), incorporates polymerizable moieties that strengthen chemisorption and maintain surface coverage during subsequent solution processing. This molecular design enables robust interface passivation and improved energy-level alignment, leading to a certified PCE of 31.36% (31.72% in-house) in perovskite/silicon tandem solar cells with exceptional durability. This SAM strategy thus bridges interfacial defect control with scalable, ambient-compatible device fabrication. Third, grain boundary (GB) defects, a critical source of nonradiative recombination and instability in wide-bandgap (WBG) perovskites, are suppressed through grain growth engineering. The multifunctional Lewis base additive, AG, facilitates grain boundary migration during thermal annealing, promoting secondary grain growth and reducing GB density. This process produces micrometer-scale grains with enhanced structural coherence and lower trap-state density. The resulting films exhibit improved photophysical properties, suppressed halide segregation, and outstanding resistance to heat and humidity stress. When integrated into perovskite/silicon tandem architectures, AG-treated WBG absorbers deliver enhanced device stability, retaining over 87% of their initial performance after 2,000 hours of thermal aging at 85 ℃. Collectively, this thesis establishes a unified defect engineering framework that rationally integrates lattice strain relaxation, interfacial defect passivation, and grain boundary control to suppress nonradiative recombination at multiple structural levels. Although each approach was independently developed and validated, together they form a coherent strategy that elucidates the defect-performance relationships across single- and multi-junction perovskite photovoltaic devices. The resulting insights not only enhance the efficiency and stability of perovskite absorbers but also align with scalable, ambient-compatible fabrication routes and wide-bandgap requirements. Through this framework, the thesis advances the path toward efficient, stable, and commercially viable perovskite-based photovoltaics spanning from single- to multi-junction architectures.