Perovskite-structured organic–inorganic materials such as CH 3 NH 3 PbI 3 are attracting much interest in the scientific community because of their abilities to function as revolutionary light harvesters and charge transfer materials for solar cells. To achieve high power conversion efficiency (PCE), it is critical to optimize the perovskite film layer. This paper reports the temperature and concentration controls on the two-step solution process. A diffusion-controlled growth mechanism is proposed for this process in tuning the morphology and purity of the perovskite film, which are proven to be important factors contributing to the photovoltaic performance. The highest PCE of 11.92% is achieved with an optimized perovskite crystal size of ∼150 nm and an appropriate amount of residual PbI 2 . This study sheds light on the design and fabrication of highly efficient, low-cost, solution-processed perovskite solar cells.
In the pursuit of high-performance FAPbI 3 -based perovskite solar cells, the two-step sequential deposition method has garnered significant attention due to its superior control over film crystallization dynamics, critical for achieving uniform, and compact perovskite layers. However, the rapid reaction between PbI 2 and FAI often results in residual PbI 2 clusters, which accelerate the degradation of perovskite solar cells (PSCs). Herein, the dual-site molecular additive 1-aminonaphthalene-6-sulfonic acid (ANSA) is introduced to achieve fast nucleation and slow crystallization, thereby enabling the formation of high-quality FAPbI 3 perovskite films. ANSA coordinates with both PbI 2 and DMSO, facilitating the formation of an improved PbI 2 (DMSO) intermediate phase, enabling a more uniform and complete conversion of PbI 2 into the perovskite phase. Additionally, the amino and sulfonic acid functional groups of ANSA contribute to the passivation of undercoordinated Pb 2+ ions, thereby perovskite films with lower trap densities and larger grain sizes are achieved. Consequently, the power conversion efficiency (PCE) increases from 23.27% in the pristine device to 25.79% for the ANSA-treated device. Furthermore, device stability is significantly improved with the modified device retaining 95% of its original PCE after 1500 h of storage.
The role of remnant PbI2 in CH3NH3PbI3 films is still controversial, some investigations have revealed that the remnant PbI2 plays a passivation role, reduces the charge recombination in perovskite solar cells (PSCs), and improves the performance of PSCs, but the opposing views state that remnant PbI2 has no passivation effect and it would deteriorate the stability of the devices. In our investigation, the CH3NH3PbI3 films have been prepared by a two-step spin-coating method and the content of the remnant PbI2 in CH3NH3PbI3 films has been tuned by varying the preparation temperature. It has been found that increasing the heating temperature could increase the coverage of spin-coated PbI2 films, which has led to high coverage CH3NH3PbI3 films and more remnant PbI2 in CH3NH3PbI3 films, and as a result, the performance of PSCs was enhanced obviously and the maximum power conversion efficiency of 14.32 ± 0.28% was achieved by the PSCs prepared at 130/120 °C (PbI2 films were heated at 130 °C and CH3NH3PbI3 films were heated at 120 °C). Furthermore, the dark current, electrochemical impedance spectroscopy and time-resolved fluorescence emission decay measurements revealed that the charge recombination in PSCs has been gradually suppressed and the fluorescence emission lifetime has gradually increased with the content of remnant PbI2 increasing. Thus, the passivation effects of the unreacted and decomposed PbI2 in improving the performance of PSCs have been confirmed unquestionably.
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