Using Time-Resolved Photoconductivity Measurements to Reveal the Urbach Tail and Two Photon Absorption in MHPs

International audienceThe unprecedented rise in power conversion efficiency of solar cells based on metal halide perovskites (MHPs) has led to an enormously growing research effort over the last 6 years. In this paper, we report how the electrodeless transient microwave photoconductivity technique (TRMC) can reveal detailed information, regarding the interesting photo-physical properties of MHPs. First, we recorded two photon absorption (2PA) spectra of MHP thin films over a wavelength regime between 0.5Eg and Eg, from which the 2PA coefficients have been calculated. A two-step upward trend is observed in the 2PA coefficient spectrum for CH3NH3PbI3. Using a parameterized tight binding model, spectra were calculated by integration over all Brillouin zones showing compelling similarity with experimental results. We conclude that the second upward trend in the 2PA spectrum originates from additional optical transitions to the heavy and light electron bands formed by the strong spin-orbit coupling. Next, we performed TRMC measurements using optical excitation above and below the band-gap on pristine MAPbI3, MAPbI3/C60, and MAPbI3/Spiro-OMeTAD thin films. In pristine MAPbI3 we have observed a band-like charge transport for photon energies down to 1.40eV, from which we calculated an Urbach energy of approximately 16 meV, indicative of a relatively low structural disorder. Our TRMC measurements on the bilayers revealed that electrons in defect states close to the VB can be excited by photons with an energy as low as 1.30 eV. Their excitation leads to free electrons in the CB that undergo charge injection into the C60 yielding long-lived charge separation. In contrast, excitation of MAPbI3/Spiro-OMeTAD at 1.3 eV shows a rapidly decaying signal implying that holes are localized and cannot be transferred over the interface to the Spiro-OMeTAD