than the control devices, which may be related to the suppressed ion migration by the formation of a low dimensional capping layer on the surface and GBs of the perovskite lm. Fig. 3e shows the comparison of stabilized current density of the reference and PPEAI-treated (2 mg mL1) devices with a test for 500 s, and the modied device shows a more stable output. The incident photon-to-current conversion efficiency (IPCE) spectra of the reference and PPEAI-modied (2 mg mL1) devices are shown in Fig. 3f. Here a stronger external quantum efficiency of the modied device can be observed compared to that of the reference one. The integration of the IPCE spectrum with the AM 1.5 solar emission spectrum produced a photocurrent that is consistent with the Jsc derived from the J–V curve.