To further understand the optoelectronic properties of metal halide perovskites, we investigate sub-bandgap absorption in methylammonium lead iodide (MAPbI(3)) films. Charge carrier dynamics are studied using time-resolved microwave conductivity measurements using sub-bandgap excitation. From changes in the decay dynamics as a function of excitation energy and intensity, we have identified three regimes: (i) Band-like charge transport at photon energies above 1.48 eV; (ii) a transitional regime between 1.48 and 1.40 eV; and (iii) below 1.40 eV localized optically active defects (8 X 10(13) cm(-3)) dominate the absorption at low intensities, while two-photon absorption is observed at high intensities. We determined an Urbach energy of approximately 11.3 meV, indicative of a low structural and/or thermal disorder. Surprisingly, even excitation 120 meV below the bandgap leads to efficient charge transfer into electron (C60) or hole (spiro-OMeTAD) transport layers. Therefore, we conclude that for MAPbI(3), the band tail states do not lead to nonradiative losses.

American Chemical Society (ACS)
ACS Energy Lett.
Hybrid Solar Cells

Caselli, V.M, Wei, Z, Ackermans, M.M, Hutter, E.M, Ehrler, B, & Savenije, T.J. (2020). Charge Carrier Dynamics upon Sub-bandgap Excitation in Methylammonium Lead Iodide Thin Films: Effects of Urbach Tail, Deep Defects, and Two-Photon Absorption. ACS Energy Lett., 5(12), 3821–3827. doi:10.1021/acsenergylett.0c02067