Enhanced photostability of CsPbI2Br-based perovskite solar cells through suppression of phase segregation using a zwitterionic additive

All-inorganic perovskite solar cells have attracted the interest of researchers all over the world owing to their high thermal stability in contrast to their organic–inorganic hybrid counterparts. Among the family of inorganic light absorbers, mixed halide perovskites CsPbI3−xBrx play an important role due to the tunability of their optoelectronic properties, e.g., for application in tandem solar cells. However, such light-absorbing materials are unstable under illumination due to their light-induced segregation to Br-rich and I-rich phases, which negatively affects the long-term stability of solar cells and represents a major challenge for the practical application of these materials. Herein, we show that photoinduced aging of a widely used perovskite light absorber such as CsPbI2Br can be suppressed significantly by using D,L-asparagine (Asn) as a stabilizing agent. By applying a few complementary methods including s-SNOM microscopy, we reveal that the Asn additive distributed along the grain boundaries or on the surface leads to the passivation of defects in the absorber material. The introduction of 0.5% Asn into the CsPbI2Br film leads to improved crystallinity of the CsPbI2Br film and results in a light power conversion efficiency of 12.1%, whereas the reference devices showed 11.6% efficiency. Increasing the concentration to 2% resulted in a slight decrease in the PCE. However, CsPbI2Br films loaded with 2% Asn do not undergo any phase segregation and no degradation occurs upon continuous exposure to light with a power of 100 mW cm−2 for 2000 h. In addition, the perovskite solar cells based on 2% Asn–CsPbI2Br delivered a light power conversion efficiency of 9.1% and retained ∼70% of the initial efficiency after 1500 h of continuous light soaking, whereas the reference devices lost more than 60% of their performance already after 150 h under the same conditions.