Flexible perovskite solar cells (PSCs) have attracted tremendous attention due to their potential application in portable and wearable electronics. However, the photoelectric conversion efficiency (PCE) of flexible PSCs is still far lower than that of usual rigid PSCs. Moreover, the mechanical stability of flexible PSCs cannot meet the needs of commercial applications because of the cracking of perovskite grains caused by bending stress. Here, we introduced a spacer cation additive (2-(chloromethyl) pyridine hydrochloride, CPHC) within the perovskite organic precursor to improve the device PCE and its mechanical stability. We observed that the CPHC spacer cation additive could simultaneously facilitate the crystallization of perovskite and stitch the grain boundaries to improve the flexibility. Compared to the 17.64% PCE of the control devices, the target flexible PSCs achieved a more highly efficiency over 19% with an improved mechanical stability (87.2% of the initial PCE after the 1000 cycles with the bending radius R = 6 mm). In addition, compared to methylammonium or formamidinium cation, due to the stronger hydrophobic and larger activation energy barrier for the ion migration of the CPHC spacer cation, the device retained over 80% of the initial PCE after 30 days storage in an ambient environment.
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