Fig. 1. (Color online) (a) XRD patterns of ${\mathrm{MA}}_{1\text{-}x}{\mathrm{Cs}}_x{\mathrm{PbI}}_3$ films ($x=0–0.30$). (b) Absorption spectra of ${\mathrm{MA}}_{1\text{-}x}{\mathrm{Cs}}_x{\mathrm{PbI}}_3$ films ($x=0–0.30$).

Fig. 2. (a)–(e) Top-view SEM images of ${\mathrm{MA}}_{1\text{-}x}{\mathrm{Cs}}_x{\mathrm{PbI}}_3$ 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 ${\mathrm{MA}}_{0.85}{\mathrm{Cs}}_{0.15}{\mathrm{PbI}}_3$ and ${\mathrm{MAPbI}}_3$, respectively. (b) EQE spectra of ${\mathrm{MA}}_{0.85}{\mathrm{Cs}}_{0.15}{\mathrm{PbI}}_3$ and ${\mathrm{MAPbI}}_3$-based devices. (c) and (d) PCE distributions of ${\mathrm{MA}}_{0.85}{\mathrm{Cs}}_{0.15}{\mathrm{PbI}}_3$ and ${\mathrm{MAPbI}}_3$-based devices.

Fig. 4. (Color online) XRD patterns and absorption spectra of ${\mathrm{MA}}_{0.85}{\mathrm{Cs}}_{0.15}{\mathrm{PbI}}_3$ and ${\mathrm{MAPbI}}_3$ films before and after thermal treatment. (a) and (b) XRD patterns of ${\mathrm{MA}}_{0.85}{\mathrm{Cs}}_{0.15}{\mathrm{PbI}}_3$ and ${\mathrm{MAPbI}}_3$ films before and after being heated at 150°C for 20 min, respectively. (c) and (d) Absorption spectra of ${\mathrm{MA}}_{0.85}{\mathrm{Cs}}_{0.15}{\mathrm{PbI}}_3$ and ${\mathrm{MAPbI}}_3$ 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 ${\mathrm{MA}}_{0.85}{\mathrm{Cs}}_{0.15}{\mathrm{PbI}}_3$ and ${\mathrm{MAPbI}}_3$ 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 $V_{\mathrm{oc}}$, (b) normalized $J_{\mathrm{sc}}$, (c) normalized FF, and (d) normalized PCE. Insets in (a) are the photographs of devices with ${\mathrm{MA}}_{0.85}{\mathrm{Cs}}_{0.15}{\mathrm{PbI}}_3$ (right) and ${\mathrm{MAPbI}}_3$ (left) after being heated at 140°C for 80 min.