Short-Range Migration of A-Site Cations Inhibit Photoinduced Phase Segregation in FAxMAyCs1–x–yPbI3–zBrz Single Crystals

Abstract

Mixed halide perovskites, particularly for mixed iodine (I) and bromine (Br), are promising semiconductor materials for multi-junction solar cells due to their band gap modulations; however numerous research studies have shown that halides undergo ion migration under light excitation, resulting in undesired phase segregation, which severely affects the device long-term stability. In high-Br-content (40%) mixed halide perovskites, I-rich and Br-rich regions simultaneously come into being under illuminations, whereas in a low Br content (<10%), the photoinduced phase segregation process is still unclear until now. On the other hand, the severity of phase segregation relies on defect density, especially prone to occur at grain boundaries in thin films. Here, we investigated the regulation of the phase segregation process by A-site cations in FAxMAyCs1–x–yPbI3–zBrz single crystals (SCs) under illuminations that evade disturbance of grain boundaries. It reveals that the introduction of MA, Cs, and Br into FAxMAyCs1–x–yPbI3–zBrz SCs leads to crystal lattice distortion, which confins the long-range migration of ions, and only the short-range migration at a local range occurs under illuminations. This short-range migration further affects the directional polarization of MA and brings lattice relaxation, which is helpful for eliminating local stress imbalance and shallow level traps, ultimately enhancing the stability under illumination.