Tuning the Crystallization and Photothermal Aging Chemistry of CsPb _0.4 Sn _0.6 I ₃ for Inorganic Perovskite Tandem Photovoltaics

Abstract 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 CsPb 0.4 Sn 0.6 I 3 (1.31 eV) with guanidinoacetic acid (GCA) to develop IPTSC. The CsPb 0.4 Sn 0.6 I 3 :GCA PSC reaches an efficiency of 16.93% and maintains an initial efficiency of ∼80% ( T 80 ) for 1300 h under maximum power point tracking (MPPT) at 65 °C. We identify that there are not only ionic migration species (I − , I 3 − ) but also molecular migration species (SnI 4 , I 2 ) for CsPb 0.4 Sn 0.6 I 3 correlated to the photothermal dynamics. For CsPb 0.4 Sn 0.6 I 3 film, the intractable pinholes accelerate the iodine migration to the electrode and photothermal degradation. The photodegradation of PbI 2 produces I 2 and then promotes the Sn 2+ oxidation to Sn 4+ , causing tin migration in the form of SnI 4 to accumulate at the electron transport layer/perovskite interface, and in turn generating more pinholes and Sn‐Pb segregation. In CsPb 0.4 Sn 0.6 I 3 :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. Combined with CsPbI 2 Br subcell, two‐terminal IPTSC delivers an efficiency of 22.18%, accompanied by T 80 = 850 h under MPPT at 65 °C.