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

TL;DR

This study uses four-wave mixing spectroscopy to directly observe free and defect-bound excitons in CH3NH3PbI3, clarifying their binding energies and shedding light on previous discrepancies in reported values.

Contribution

It demonstrates the use of four-wave mixing spectroscopy to simultaneously observe and measure free and defect-bound excitons in perovskite films, providing direct insight into their binding energies.

Findings

Observed free and defect-bound excitons using FWM spectroscopy

Determined exciton binding energies of 13 meV and 29 meV

Identified localization energy of 16 meV for defect-bound excitons

Abstract

Solar cells incorporating organic-inorganic perovskite, which may be fabricated using low-cost solution-based processing, have witnessed a dramatic rise in efficiencies yet their fundamental photophysical properties are not well understood. The exciton binding energy, central to the charge collection process, has been the subject of considerable controversy due to subtleties in extracting it from conventional linear spectroscopy techniques due to strong broadening tied to disorder. Here we report the simultaneous observation of free and defect-bound excitons in CH3NH3PbI3 films using four-wave mixing (FWM) spectroscopy. Due to the high sensitivity of FWM to excitons, tied to their longer coherence decay times than unbound electron-hole pairs, we show that the exciton resonance energies can be directly observed from the nonlinear optical spectra. Our results indicate low-temperature binding energies of 13 meV (29 meV) for the free (defect-bound) exciton, with the 16 meV localization energy for excitons attributed to binding to point defects. Our findings shed light on the wide range of binding energies (2-55 meV) reported in recent years.