Bromine-based hybrid perovskites exhibit excellent performance as the top layer of tandem cells, but their overall efficiency as single-junction devices is still very limited. In this work, trimethylolpropane ethoxylated triacrylate (TET), which can form a cross-linking polymer at the grain boundary after heating at 150 °C, is introduced as an additive in FAPbBr3-based perovskite solar cells. The FAPbBr3 film with TET exhibits pinhole-free crystal grains with a large grain size. With a concentration of 5 mg/mL TET, the average grain size increases from 313 to 505 nm. What is more, Raman spectroscopy and X-ray photoelectron spectroscopy demonstrate that the cross-linked TET has a strong interaction with FAPbBr3, which can passivate the defects of grain boundaries. Therefore, the FAPbBr3-based perovskite solar cells with TET achieve a power conversion efficiency of 8.93%. As we know, this is the highest value in FAPbBr3-based hybrid perovskites with an inverted planar structure. This work provides a new insight into the in situ cross-linking additive TET, which only exists at the grain boundary, does not enter the lattice, and has a crystallization control and defect passivation effect on FAPbBr3 thin films, which will provide a new direction for further improving the performance of FAPbBr3 perovskite solar cells.

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