Fig. 1. (Color online) (a) XRD patterns of MA1-xCsxPbI3 films (x=0–0.30). (b) Absorption spectra of MA1-xCsxPbI3 films (x=0–0.30).

Fig. 2. (a)–(e) Top-view SEM images of MA1-xCsxPbI3 films for x=0, 0.05, 0.10, 0.15, 0.20, and 0.30. Scale bars are all 250 nm. (g) Cross-section SEM images of a solar cell. The scale bar is 1 μm.

Fig. 3. (Color online) (a) Best performances of PV devices incorporating MA0.85Cs0.15PbI3 and MAPbI3, respectively. (b) EQE spectra of MA0.85Cs0.15PbI3 and MAPbI3-based devices. (c) and (d) PCE distributions of MA0.85Cs0.15PbI3 and MAPbI3-based devices.

Fig. 4. (Color online) XRD patterns and absorption spectra of MA0.85Cs0.15PbI3 and MAPbI3 films before and after thermal treatment. (a) and (b) XRD patterns of MA0.85Cs0.15PbI3 and MAPbI3 films before and after being heated at 150°C for 20 min, respectively. (c) and (d) Absorption spectra of MA0.85Cs0.15PbI3 and MAPbI3 films before and after being heated at 150°C for 3 h, respectively. For comparison, the insets in (c) and (d) are the photographs of MA0.85Cs0.15PbI3 and MAPbI3 films before (the left) and after (the right) being heated at 150°C for 3 h.

Fig. 5. (Color online) Thermal stability of devices under a heat stress test at 140°C. (a) Normalized Voc, (b) normalized Jsc, (c) normalized FF, and (d) normalized PCE. Insets in (a) are the photographs of devices with MA0.85Cs0.15PbI3 (right) and MAPbI3 (left) after being heated at 140°C for 80 min.