In the two-step method for perovskite film fabrication, insufficient reaction between organic ammonium salts and PbI 2 affects the active layer quality, resulting in poor power conversion efficiency (PCE). In this work, we introduce pentanamine acetic acid (PenAAc) and 5-aminovaleric acid iodine (5-AVAI) as two efficient additives to modify PbI 2 layer during perovskite films fabrication via the two-step method in an air environment. After additive modifications, the quality of both PbI 2 layers is improved: PenAAc enhances crystallinity and increases PbI 2 grain size, while 5-AVAI induces a uniform and smooth PbI 2 layer. The optimized PbI 2 films facilitate the reaction between PbI 2 and organic salts, yielding a high-quality perovskite layer with enhanced crystallinity and reduced defect density. In contrast, the macromolecule structure of 5-AVAI does not diffuse after annealing, remaining in the PbI 2 layer and exerting minimal influence on the perovskite surface. However, dissociative CH 3 COO − groups in PenAAc molecules diffuse from the PbI 2 layer to the perovskite surface during annealing, forming strong interactions between I - and Pb 2+ with CH 3 COO − groups. This further improves perovskite film quality and accelerates charge carrier extraction/transport in devices. Consequently, additive-modified perovskite solar cells achieve excellent PCEs of 24.50 % (5-AVAI) and 24.65 % (PenAAc), significantly surpassing the control device (22.84 %). Moreover, hysteresis index is reduced from 4.20 % (control) to 0.33 % (5-AVAI) and 0.08 % (PenAAc). Additionally, the modified devices also demonstrated enhanced environmental stability across humidity, light, and thermal stress conditions.
Black-phase formamidinium lead iodide (FAPbI 3 ), with a narrow bandgap and high thermal stability, has emerged as an in-demand material for highly efficient perovskite solar cells (PSCs). In a two-step sequential deposition, the PbI 2 film plays an important role in the formation of a perovskite film with desirable qualities. This paper explores using N-methyl-2-pyrrolidone (NMP), a strong Lewis base, and N,N-dimethylformamide (DMF) as a mixed precursor solvent (DMF/NMP) of PbI 2 and reports on preparing PbI 2 films with a porous morphology by thermal treatment. Porous PbI 2 films ensure the diffusion and sufficient reaction of the formamidinium iodide solution to form a smooth perovskite film. In addition, a dynamic spin coating method is also introduced to improve the uniformity of the perovskite film. Both methods yield a pure α-phase FAPbI 3 film immediately in the unannealed state, which is necessary for the perovskite film to maintain phase stability. Finally, PSCs with a power conversion efficiency of 21.20% (0.13 cm 2 ) are fabricated and optimized. The unencapsulated PSCs retain 90% of the initial efficiency for 1000 hours in dry air and exhibit a good thermal stability when heated to 85 °C.
The two-step deposited method with the advantages of high reproducibility and controllability is widely used in fabricating highly efficient perovskite solar cells (PSCs). However, it is still a challenge to fabricate air-processed PSCs through this method due to the existing techniques with glove box circumstance that are not well extended to the ambient air-condition fabrication process. Here, hexamethylphosphoramide (HMPA) used as a solvent additive is incorporated into the first-step PbI2 precursor solution, which can effectively modify the morphology and crystallinity of the PbI2 layer, thereby creating more opportunities for the infiltration of the second-step organic salt. This two-step coating leads to the full conversion from PbI2 to the perovskite crystal and large grains, reduced defects, and enhanced uniformity for the final perovskite film. Consequently, a champion efficiency of 20.84% of the HMPA-based device fabricated using this procedure is achieved. The HMPA-based device shows long-term stability of the remaining 85.7% of the initial efficiency after 1000 h of storage in an ambient air environment. This method demonstrates the great potential toward air-processed efficient PSCs.
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