Lead-tin (Pb-Sn) mixed perovskite solar cells (PSCs) enable bandgap tuning (1.1–1.4 eV), reduce toxicity, and offer great potential for efficient all-perovskite tandem devices. Although they exhibit promising optoelectronic properties, Pb–Sn mixed perovskites still face significant challenges that limit their overall device performance. One critical issue is the loss of open-circuit voltage (VOC) due to energy-level misalignment and defects arising from the coexistence of Pb and Sn within the perovskite structure. This energy-level mismatch hinders efficient charge extraction at the interfaces. At the same time, the mixed-metal-induced defects create local variations in the conduction and valence bands, leading to band tailing, bandgap fluctuations, and enhanced non-radiative recombination. Another issue is the rapid formation of the perovskite film, especially the Sn-rich phase, leading to tiny gaps where Pb and Sn atoms should be, thereby creating vacancies. Consequently, the resulting films exhibit poor interface between the perovskite and adjacent transport layers, further promoting recombination. Moreover, uncontrolled and poor crystallization of the perovskite film can create tiny gaps, pinholes, or grain boundaries, which act as shunt pathways. These key factors lead to leakage currents and a reduction in VOC, consequently decreasing overall device efficiency. This thesis aims to address these limitations by using interfacial engineering and controlling crystallization kinetics. In the first project, we improved the charge transport dynamics of the Pb-Sn perovskite (FASnI₃)₀.₆(MAPbI₃)₀.₄ using an interface engineering strategy by inserting an ultra-thin layer of poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) between the commonly used hole transport layer (HTL) poly(3,4 ethylenedioxythiophene) polystyrene sulfonic acid (PEDOT: PSS) and the Pb-Sn mixed perovskite layer. This selective interfacial material effectively passivates surface defects, aligns more closely with the perovskite’s valence band, and thereby enhances charge extraction. As a result, the VOC was substantially improved, reaching 0.82 V, thereby increasing the power conversion efficiency (PCE) to 20.3%. ix In the second project, the HTL–perovskite interface was improved by optimizing the PTAA interlayer concentration in single-junction narrow-bandgap (NBG) perovskite solar cells. This work built upon the advancements of the first project to enhance the compatibility of NBG cells within all-perovskite tandem architectures. In these configurations, the NBG sub-cell functions as the bottom cell, where efficient charge extraction and suppressed interfacial recombination are critical for maximizing device performance. By integrating this high-performance NBG cell with a wide-bandgap (WBG) top cell, a monolithic all-perovskite tandem solar cell was realized. Specifically, Cs₀.₃FA₀.₆MA₀.₁Pb(I₀.₇Br₀.₃)₃, which has a bandgap of approximately 1.70 eV—the system was successfully translated into an all-perovskite tandem structure, achieving a VOC exceeding 2.0 V and a PCE of 25.1%. In the third project, a gas quenching (GQ) approach is introduced as an alternative to conventional anti-solvent (AS) quenching to achieve slower crystallization, smoother perovskite films with fewer pinholes, enhanced VOC, and improved PSC efficiency. The GQ approach resulted in slower crystallization and modified film quality. Devices fabricated using the GQ approach also demonstrated better performance with higher VOC values. Specifically, devices prepared with the AS method achieved a champion PCE of 18.3%, whereas those prepared with the GQ method achieved a higher PCE of 19.1%. This improvement in efficiency, combined with enhanced reproducibility and the simplicity and environmental friendliness of the GQ approach, underscores its advantages for processing NBG Pb-Sn mixed perovskite films. Therefore, the GQ method holds significant potential for application in the fabrication of NBG Pb-Sn PSCs.