Tuning the Crystallization and Photothermal Aging Chemistry of CsPb0.4Sn0.6I3 for Inorganic Perovskite Tandem Photovoltaics

The poor performance of inorganic narrow bandgap perovskite solar cells (PSCs) hinders the development of inorganic perovskite tandem solar cells (IPTSCs). We modulate the crystallization and photothermal aging chemistry for CsPb0.4Sn0.6I3 (1.31 eV) with guanidinoacetic acid (GCA) to develop IPTSC. The CsPb0.4Sn0.6I3:GCA PSC reaches an efficiency of 16.93% and maintains an initial efficiency of ~80% (T80) for 1300 h under maximum power point tracking (MPPT) at 65 °C. We identify that there are not only ionic migration species (I‐, I3‐), but also molecular migration species (SnI4, I2) for CsPb0.4Sn0.6I3 correlated to the photothermal dynamics. For CsPb0.4Sn0.6I3 film, the intractable pinholes accelerate the iodine migration to the electrode and photothermal degradation. The photodegradation of PbI2 produces I2 and then promotes the Sn2+ oxidation to Sn4+, causing tin migration in the form of SnI4 to accumulate at the ETL/perovskite interface, and in turn generating more and more pinholes and Sn‐Pb segregation. In CsPb0.4Sn0.6I3:GCA film, due to the coordination bonds with Pb/Sn cations and hydrogen bonds with I‐ ions, GCA incorporation‐induced pinhole‐free morphology can significantly suppress ion/molecule migration. Hence, combined with CsPbI2Br subcell, 2T IPTSC delivers an efficiency of 22.18%, accompanied by T80 = 850 h under MPPT at 65 °C.