Investigation of Interfacial Charge Transfer in Solution Processed Cs<sub>2</sub>SnI<sub>6</sub> Thin Films

Cesium tin halide based perovskite Cs<sub>2</sub>SnI<sub>6</sub> has been subjected to in-depth investigations owing to its potentiality toward the realization of environment benign Pb free and stable solar cells. In spite of the fact that Cs<sub>2</sub>SnI<sub>6</sub> has been successfully utilized as an efficient hole transport material owing to its p-type semiconducting nature, however, the nature of the majority carrier is still under debate. Therefore, intrinsic properties of Cs<sub>2</sub>SnI<sub>6</sub> have been investigated in detail to explore its potentiality as light absorber along with facile electron and hole transport. A high absorption coefficient (5 × 10<sup>4</sup> cm<sup>–1</sup>) at 700 nm indicates the penetration depth of 700 nm light to be 0.2 μm, which is comparable to conventional Pb based solar cells. Preparation of pure and CsI impurity free dense thin films with controllable thicknesses of Cs<sub>2</sub>SnI<sub>6</sub> by the solution processable method has been reported to be difficult owing to its poor solubility. An amicable solution to circumvent such problems of Cs<sub>2</sub>SnI<sub>6</sub> has been provided utilizing spray-coating in combination with spin-coating. The presence of two emission peaks at 710 and 885 nm in the prepared Cs<sub>2</sub>SnI<sub>6</sub> thin films indicated coexistence of quantum dot and bulk parts which were further supported by transmission electron microscopy (TEM) investigations. Time-resolved photoluminescence (PL) and transient absorption spectroscopy (TAS) were employed to investigate the excitation carrier lifetime, which revealed fast decay kinetics in the picoseconds (ps) to nanoseconds (ns) time domains. Time-resolved microwave photoconductivity decay (MPCD) measurement provided the mobile charge carrier lifetime exceeding 300 ns, which was also in agreement with the nanosecond transient absorption spectroscopy (ns-TAS) indicating slow charge decay lasting up to 20 μs. TA assisted interfacial charge transfer investigations utilizing Cs<sub>2</sub>SnI<sub>6</sub> in combination with n-type PCBM and p-type P3HT exhibited both intrinsic electron and hole transport.