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