3

3

52 Bi E.

Chen H.

Xie F.

Wu Y.

Chen W.

Su Y.

Islam A.

Grätzel M.

Yang X.

Han L. Diffusion engineering of ions and charge carriers for stable efficient perovskite solar cells.

3

2

53 Kim T.W.

Shibayama N.

Cojocaru L.

Uchida S.

Kondo T.

Segawa H. Real-time in situ observation of microstructural change in organometal halide perovskite induced by thermal degradation.

Figure 5 Suppressed Ion Migration Show full caption (A–F) Energy-dispersive X-ray spectra (EDX) mapping of the aged pure PVSK device (A) Ag, (B) I, and (C) Pb, and the aged caffeine-containing PVSK device (D) Ag, (E) I, and (F) Pb. (G and H) EDX line scans of (G) aged pure PVSK device and (H) aged caffeine-containing PVSK device.

Figure 6 Formation of Molecular Lock Show full caption High resolution transmittance electron microscopy (HRTEM) of (A) fresh caffeine containing PVSK, (C) fresh pure PVSK, (E) aged (5 min 30 s) caffeine containing PVSK, and (G) aged (5 min 30 s) pure PVSK. Corresponding fast Fourier transforms (FFTs) of (B) fresh caffeine containing PVSK, (D) fresh pure PVSK, (F) aged (5 min 30 s) caffeine containing PVSK, and (H) aged (5 min 30 s) pure PVSK.

To further investigate the role of caffeine in suppressing ion migration and thermal decomposition, microstructure analysis was carried out. We first conducted cross-sectional scanning transmission electron microscopy (STEM) and energy-dispersive X-ray (EDX) spectroscopy analysis ( Figure 5 ). The samples were directly collected from the devices after the 1,300-h thermal stability test via focused ion beam (FIB). Figure S15 shows the EDX mapping of selective regions on both the control and target devices. The spatial distribution of the Pb and I elements determined the PVSK active layer region. The Ag electrode was above the active layer, separated by the PTAA HTL. For the control sample, there were significant Ag signals (Ag clusters) with a similar intensity as that in the electrode region detected at the interface between the HTL and the active layer region. It is likely that the Ag diffused through the whole PVSK region, as confirmed by the observation that the Ag signals were detected even in the ITO electrode region. More importantly, the iodine signals were clearly detected in the Ag electrode region. Iodine could accumulate at the electrode and interface. It easily reacted with Ag to form AgI, which will negatively impact the device performance. In contrast, there is no obvious indication of such similar ion migration in the caffeine-incorporated PVSK device. To further confirm the result quantitively, line-scanning profiles were also measured. As shown in Figure 5 G, the thickness of the Ag electrode was 50 nm, two sharp peaks at both the electrode and interface were observed, and the Ag signal was detected almost throughout the entire device. However, the thickness of the Ag electrode in the caffeine-incorporated PVSK device maintained its original value of 100 nm. More importantly, a sharp iodine peak was detected in the electrode region of the control device, in agreement with the previous conclusion. Hence, from the STEM results, the suppression of ion migration in the caffeine-containing films ensured the high thermal resistance of the devices. We also conducted real-time high-resolution transmission electron spectroscopy (HRTEM) to study the effect of caffeine on the phase transformation of the PVSK. The electron beam (E-beam) of the HRTEM instrument was utilized as the source of the thermal energy. Figures 6 A–6D show the HRTEM images and the corresponding fast Fourier transforms (FFTs) of the diffraction patterns of both the caffeine-incorporated and pristine PVSKs. MAPbIlayers with various crystallographic orientations were observed in both samples. The representative spot diffractions (yellow circles) with an interplanar spacing of 3.1 Å, which are well matched with the (110) diffraction of MAPbI, are shown in Figures 6 B and 6D. After exposure to the E-beam for 5 min 30 s, the environmental temperature of the samples was elevated to around 135°C. Figures 6 E–6H present the HRTEM images and the corresponding FFTs of the diffraction patterns of the aged caffeine-incorporated and pristine PVSKs. Although the intensity of the (110) diffraction spots of the caffeine-incorporated PVSK became weak, no new diffraction peaks appeared. Notably, a critical alteration of the MAPbIlayer was observed in the control sample with the (110) diffraction spots observed to split (red circle). On the other hand, there was a new broad ring that appeared in the FFT at 3.9 nm, and new diffraction spots were observed at the same place. These morphological characteristics suggest that some crystallized PVSK phase had been transferred to the amorphized phase with precipitated trigonal PbIgrains at this region, which agrees with a previous study that the thermal degradation of PVSK is often considered as the reverse process of the PVSK film growth.We also conducted in situ TEM measurement on the other regions of the films, and the results followed a similar trend as described previously ( Figures S16 and S17 ). From these results, we speculate that the existence of the caffeine additive serves as a molecular lock to interact with the amorphized PVSK phase again to increase the decomposition activation energy of the PVSK, which locks the amorphized phase of the PVSK, thereby preventing the degradation of the PVSK when exposed to high temperatures.