Simultaneous observation of free and defect-bound excitons in CH3NH3PbI3 using four-wave mixing spectroscopy

摘要

Solar cells incorporating organic-inorganic perovskite, which may befabricated using low-cost solution-based processing, have witnessed a dramaticrise in efficiencies yet their fundamental photophysical properties are notwell understood. The exciton binding energy, central to the charge collectionprocess, has been the subject of considerable controversy due to subtleties inextracting it from conventional linear spectroscopy techniques due to strongbroadening tied to disorder. Here we report the simultaneous observation offree and defect-bound excitons in CH3NH3PbI3 films using four-wave mixing (FWM)spectroscopy. Due to the high sensitivity of FWM to excitons, tied to theirlonger coherence decay times than unbound electron-hole pairs, we show that theexciton resonance energies can be directly observed from the nonlinear opticalspectra. Our results indicate low-temperature binding energies of 13 meV (29meV) for the free (defect-bound) exciton, with the 16 meV localization energyfor excitons attributed to binding to point defects. Our findings shed light onthe wide range of binding energies (2-55 meV) reported in recent years.