Band Tailing and Deep Defect States in CH₃NH₃Pb(I_1–xBr_x)₃ Perovskites As Revealed by Sub-Bandgap Photocurrent

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/CH 3 NH 3 Pb(I 1– x Br x ) 3 /TiO 2 /FTO/glass photovoltaic devices. The room-temperature spectra show exponential band tails with a sharp onset characterized by low Urbach energies ( E u ) over the full halide composition space. The Urbach energies are 15–23 meV, lower than those for most semiconductors with similar bandgaps (especially with E g > 1.9 eV). Intentional aging of CH 3 NH 3 Pb(I 1– x Br x ) 3 for up to 2300 h, reveals no change in E u, despite the appearance of the PbI 2 phase 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 CH 3 NH 3 PbI 3 or mixed CH 3 NH 3 Pb(I 1– x Br x ) 3 compositions, 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 CH 3 NH 3 Pb(I 1– x Br x ) 3 alloys.