Unveiling the ambipolar carrier transport property of SnO_2−X for multiple-functional interlayers in perovskite solar cells

Tin oxides are the most promising electron transport layers in perovskite solar cells. An ambipolar carrier transport property has been recently demonstrated which enables a simple interconnection structure for all-perovskite tandem solar cells. However, the underlying mechanism for its ambipolar behavior is unclear, which cannot be explained by the intrinsic defects in SnO_2−x. Here, by using density functional theory calculations, we unveil the origin of the ambipolar carrier transport of non-stoichiometry SnO_2−x with a structure of SnO embedded in the SnO₂ matrix. The hybridization of O 2p and Sn 5s orbitals of SnO introduces mid-gap states in the bandgap of SnO₂, enabling hole transport property for SnO_2−x when x is > 0.2. Increasing the percentage of SnO in SnO_2−x significantly enhances the hole transport capability of SnO_2−x due to the enlarged Sn–O–Sn angles that increase orbital overlapping between O and Sn atoms, thus providing strategies for the further tuning of the carrier transport properties of SnO_2−x by compositional and structural designs.