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Band tailing and deep defect states in CH3NH3Pb(I1-xBrx)3perovskites as revealed by sub-bandgap photocurrent

  • Author(s): Sutter-Fella, CM
  • Miller, DW
  • Ngo, QP
  • Roe, ET
  • Toma, FM
  • Sharp, ID
  • Lonergan, MC
  • Javey, A
  • et al.
Abstract

© 2017 American Chemical Society. Organometal halide perovskite semiconductors have emerged as promising candidates for optoelectronic applications because of the outstanding charge carrier transport properties, achieved with low-temperature synthesis. Here, we present highly sensitive sub-bandgap external quantum efficiency (EQE) measurements of Au/spiro-OMeTAD/CH3NH3Pb(I1-xBrx)3/TiO2/FTO/glass photovoltaic devices. The room-temperature spectra show exponential band tails with a sharp onset characterized by low Urbach energies (Eu) over the full halide composition space. The Urbach energies are 15-23 meV, lower than those for most semiconductors with similar bandgaps (especially with Eg > 1.9 eV). Intentional aging of CH3NH3Pb(I1-xBrx)3for up to 2300 h, reveals no change in Eu, despite the appearance of the PbI2phase due to decomposition, and confirms a high degree of crystal ordering. Moreover, sub-bandgap EQE measurements reveal an extended band of sub-bandgap electronic states that can be fit with one or two point defects for pure CH3NH3PbI3or mixed CH3NH3Pb(I1-xBrx)3compositions, respectively. The study provides experimental evidence of defect states close to the midgap that could impact photocarrier recombination and energy conversion efficiency in higher bandgap CH3NH3Pb(I1-xBrx)3alloys.

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