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.
Highlights
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
Unlike inorganic solar cell materials such as III-V semiconductors, in which sharp exciton resonances greatly simplify extraction of Eb, the solution-processed organometal halide perovskites possess substantial broadening due to defects[25] and intrinsic dynamic disorder associated with the freedom of orientation of methylammonium ions[26,27]
Excitons, being charge-neutral spatially-localized excitations, have much longer T2 times than delocalized free carrier and band tail transitions[29,30]. This sensitivity to excitonic effects has been exploited to observe the fundamental exciton in LT-GaAs31, in which the optical band edge is strongly broadened by AsGa antisite defects preventing the observation of any signature of the exciton in linear absorption, as well as the exciton resonance at the spin-orbit split-off band gap in InP32, which is masked in linear spectroscopy by strong degenerate interband transitions associated with the lower-energy band gaps
Summary
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. Extraction of Eb from magneto absorption techniques is complicated by the strong broadening in the organometal halide perovskites, resulting in the need to use large magnetic fields (~20 Tesla)[9,10] Due to these challenges, even for the most widely studied material CH3NH3PbI3, the value of Eb has been quite controversial, with reports ranging from 2 meV to 55 meV2–4,7–19. Excitons, being charge-neutral spatially-localized excitations, have much longer T2 times than delocalized free carrier and band tail transitions[29,30] This sensitivity to excitonic effects has been exploited to observe the fundamental exciton in LT-GaAs31, in which the optical band edge is strongly broadened by AsGa antisite defects preventing the observation of any signature of the exciton in linear absorption, as well as the exciton resonance at the spin-orbit split-off band gap in InP32, which is masked in linear spectroscopy by strong degenerate interband transitions associated with the lower-energy band gaps. Together with the difficulty in fully characterizing the dielectric constant including the potential contributions of phonons[13,17] and related dispersion[19], the close proximity of the free and bound exciton states we observe has likely contributed to the broad range of exciton binding energies reported in the literature[2,3,4,7,8,9,10,11,12,13,14,15,16,17,18,19,22]
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