Molecular passivation is a prominent approach for improving the performance and operation stability of halide perovskite solar cells (HPSCs). Herein, we reveal discernible effects of diammonium molecules with either an aryl or alkyl core onto Methylammonium-free perovskites. Piperazine dihydriodide (PZDI), characterized by an alkyl core-electron cloud-rich-NH terminal, proves effective in mitigating surface and bulk defects and modifying surface chemistry or interfacial energy band, ultimately leading to improved carrier extraction. Benefiting from superior PZDI passivation, the device achieves an impressive efficiency of 23.17% (area ~1 cm²) (low open circuit voltage deficit ~0.327 V) along with superior operational stability. We achieve a certified efficiency of ~21.47% (area ~1.024 cm²) for inverted HPSC. PZDI strengthens adhesion to the perovskite via -NH₂I and Mulliken charge distribution. Device analysis corroborates that stronger bonding interaction attenuates the defect densities and suppresses ion migration. This work underscores the crucial role of bifunctional molecules with stronger surface adsorption in defect mitigation, setting the stage for the design of charge-regulated molecular passivation to enhance the performance and stability of HPSC.

The performance of perovskite device was found to be influenced by the interface quality and bulk defect activities induced in perovskite grown on HTL during device fabrication.

The degradation of perovskite device performance is found to be driven by interfacial deterioration in the device and induced deep trap assisted recombination in the perovskite with aging.

Bismuth-based halide perovskites (Bi-HaP) are low toxicity and air-stable materials with promising photo-absorber properties. In this study, we fabricated Bi-HaP (Cs₃Bi₂I₉ and CsBi₃I₁₀) films by a solution process followed by solvent annealing and investigated the crystal growth and optoelectronic properties of these materials. A compact and large grain morphology of the Bi-HaP films was realized by annealing under ambient solvent vapor conditions. Collective analysis of XRD patterns, and Raman spectra, absorption and PL spectra of the fabricated films corroborates that the Cs₃Bi₂I₉ film (E_g ∼ 2.08 eV) with a hexagonal crystal phase is more stable under annealing conditions in a wide temperature range and ambient solvent vapor annealing conditions as compared to the other CsBi₃I₁₀ thin film, having the narrower E_g ∼ 1.8 eV, of the Bi-HaP family. We have achieved the best power conversion efficiency as high as ∼1.26% with the open circuit voltage of 0.74 V for the device fabricated with Cs₃Bi₂I₉. The analysis of material properties and device characteristics indicates that morphology tailoring, surface chemistry control, and interface band offset engineering are important for the further improvement of Bi-HaP-based devices.