Quasi‐2D perovskites have demonstrated great application potential in light‐emitting diodes (LEDs). Defect passivation with chemicals plays a critical role to achieve high efficiency. However, there are still challenges in comprehensively passivating the defects distributed at surface, bulk, and buried interface of quasi‐2D perovskite emitting films, hindering the further improvement of device performance. Herein, 9,9‐substituted fluorene derivatives with different terminal functional groups are developed tactfully to realize comprehensive passivation, which greatly contributes to reducing nonradiative recombination at surface, suppressing ion migration in bulk, and filling interfacial charge traps at buried interface, respectively. Eventually, quasi‐2D perovskite LEDs have an increased external quantum efficiency from 18.2% to 23.2%, improved operation lifetime by more than six times and lower turn‐on voltage simultaneously. Here the importance of comprehensive passivation is highlighted and guidelines for the design and application of passivators for perovskite optoelectronics are provided.
ST-SCs due to proper band gap and stability. [4] However, the low PCE of FAPbBr 3 solar cells (SCs) seriously hinders their applications. Thus, many efforts have been made to improve the PCE of FAPbBr 3 SCs by additive engineering, [5] interface engineering, [6] and crystallization regulation. [7] For example, Ko et al. added cesium bromide to regulate lattice interactions of FAPbBr 3 with preferred orientation and obtained 8.56% of a PCE due to the highquality crystalline film. [8] Zhang et al. introduced guanidinium bromide to modulate the crystallization process, achieving a PCE of 8.92% with an open-circuit voltage (Voc) of 1.639 V for FAPbBr 3 SCs. [9] Liu et al. constructed 2D/3D perovskite interface by phenethylammonium bromide, which significantly passivated the interface defects and improved the PCE from 7.7% to 9.4%. [10] Although the PCE of FAPbBr 3 SCs has been improved, it is still unclear and rarely investigated on the effect of δ phase (δ-FAPbBr 3 ) on film and device properties. The photoinactive δ-FAPbBr 3 has a large band gap and chain structure, which may affect light absorption and carrier transport. [11] Therefore, the research on δ-FAPbBr 3 is of great significance for FAPbBr 3 SCs performance improvement.In this work, the key factors on the crystallization of δ-FAPbBr 3 and the effect of δ-FAPbBr 3 on device performance were systematically studied for the first time. The hydrophobic poly[bis(4phenyl) (2,4,6-trimethylphenyl) amine] (PTAA) underlayer induces random growth of δ-FAPbBr 3 and its aggregation at the bottom of the perovskite films, which bring more defects and hinder carrier transport. Contrarily, δ-FAPbBr 3 has more regular crystallization and uniform distribution on the hydrophilic (2-(9H-carbazol-9-yl) ethyl) phosphonic acid (2PACz) underlayer, which has less defect states and better carrier transport. By virtue of the management of δ-FAPbBr 3 and the passivation of the upper interface with a phosphonate/phosphine oxide dyad molecule (PE-TPPO), a PCE of 9.12% was obtained, which is the highest efficiency among the inverted FAPbBr 3 SCs. A ST-SC reached a LUE of 3.15% with a PCE of 7.44% and an AVT of 42.31%.
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