PMMA interlayers prepared with various solvents and their effects on the thermal stability of ambient air-processed MAPbI3 perovskite films

Poly(methyl methacrylate) (PMMA) can be used as a charge tunneling interlayer or as an encapsulating layer in perovskite solar cells (PSCs). Its incorporation into the PSCs can enhance the hysteresis behavior, performance, and operational stability of the device. This work shows that the choice of solvents to prepare PMMA interlayers on the ambient air-prepared methylammonium lead iodide (MAPbI3) films can significantly affect the thermal stability of MAPbI3 films. The solvents used were chlorobenzene (CB), ethyl acetate (EA), (1-methoxy-2-propyl) acetate (PGMEA), and CB with 6 vol% isopropanol (IPA) (CB/IPA). Their corresponding PMMA interlayers are designated as CB-PMMA, EA-PMMA, PGMEA-PMMA, and CB/IPA-PMMA, respectively. The CB-PMMA interlayers had the least thermal stability enhancement effect on the MAPbI3 films, as the films suffered severe thermal degradation after annealing at 150 °C for 9 h. In contrast, the EA-PMMA, PGMEA-PMMA, and CB/IPA-PMMA interlayers effectively enhanced the thermal stability of the MAPbI3 films, with only mild thermal degradation observed after the same annealing process. This significant thermal stability enhancement behavior is attributed to the ability of the solvents to extract moisture from the MAPbI3 films during the PMMA spin coating process, thus preventing the entrapped moisture from accelerating the thermal degradation of MAPbI3 films. Therefore, the choice of solvents for PMMA plays a crucial role in enhancing the thermal stability of the MAPbI3 films.