Formamidinium + cesium lead triiodide perovskites: Discrepancies between thin film optical absorption and solar cell efficiency

Optical characterization and simulations have been applied to study formamidinium + cesium lead triiodide (FA1-xCsxPbI3) perovskite based thin films and solar cells. Near infrared to ultraviolet complex dielectric function (ε = ε1 + iε2) spectra of varying cesium-to-formamidinium ratios in solution processed perovskite thin films have been extracted. Analysis of these ε spectra track changes in the positions of critical point (CP) energies, including band gaps and above band gap transitions, with increasing cesium contents. Band gap values are identified as the lowest energy CP, with additional sub-gap features attributed to the presence of defects. Absorption onset values, for which absorption coefficient (α) = 4000 cm−1, are extracted, with x = 0.2 showing the sharpest absorption edge and the least contribution to absorption from defects below the band gap. External quantum efficiency (EQE) simulations of solar cells using ε spectra as input are compared to experimental results and corroborate that the observed sub-gap absorption is due to defects as it does not contribute to EQE.