Carolin Sutter-Fella1 D. Westley Miller2 Quynh Ngo5 Ellis Roe2 Francesca Maria Toma1 Ian Sharp3 Mark Lonergan4 Ali Javey5

1, Lawrence Berkeley National Laboratory, Berkeley, California, United States
2, University of Oregon, Eugene, Oregon, United States
5, University of California, Berkeley, Berkeley, California, United States
3, Technische Universitaet Muenchen, Muenchen, , Germany
4, University of Oregon, Eugene, Oregon, United States

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. All CH3NH3Pb(I1−xBrx)3 compositions, with
corresponding bandgaps of 1.6 ≤ Eg ≤ 2.3 eV, exhibit Urbach energies in the range of 15 -23
meV, lower than those for most semiconductors with similar bandgaps (especially with Eg >
1.9 eV). Intentional aging of CH3NH3Pb(I1−xBrx)3 for up to 2300 h, reveals no change in Eu,
despite the appearance of the PbI2 phase due to decomposition, and confirms a high degree of
crystal ordering. Moreover, sub-bandgap EQE measurements reveal an extended band of subbandgap electronic states that can be fit with one or two point defects for pure CH3NH3PbI3 or
mixed CH3NH3Pb(I1−xBrx)3 compositions, respectively. The study presents experimental
evidence of defect states close to midgap that could impact photocarrier recombination and
energy conversion efficiency in higher bandgap CH3NH3Pb(I1−xBrx)3 alloys. The combination
of sub-bandgap EQE and photoluminescence-based studies provides a future path toward
assessing the roles of synthesis and processing on efficiency-limiting recombination centers in
the material.