Defect induced absorption in mixed composition perovskite thin films and solar cells*

 Device performance, namely, current density (Jsc), open circuit voltage (Voc), fill factor (FF), and efficiency of mixed perovskite (ABX3:  A=MA, FA, Cs; B=Pb, Sn; X=I, Br, Cl) thin film solar cells vary with the composition. The major parameters affecting the device performance with varying compositions are crystal structure, phase stability, optical properties, and defect states of the absorber material. Here, we use spectroscopic ellipsometry and photothermal deflection spectroscopy to study to the near-bandgap optical response in terms of the complex refractive index (N = n + ik) of mixed composition perovskite thin films. Sub-bandgap absorption manifests as somewhat well-defined features and Urbach tails below the band gap.  Solar cell device performance parameters including Voc, FF, and efficiency as well as external quantum efficiency (EQE) are correlated with the Urbach energies and presence of defect states.