Sub-Bandgap States in Lead-Halide Perovskites Revealed by two-Dimensional Electronic Spectroscopy

Methylammonium lead halide perovskites possess excellent properties for photovoltaic applications, with the power conversion efficiency of solar cell devices based on them exceeding 22%. [1] Being solution processed semiconductors, it is widely expected that the crystal structure contains many more defects compared to well-grown semiconductors. Surprisingly however, typical effects from traps (low fill factor, low open circuit voltage, and strong non-radiative recombination) are not observed in their photophysical properties [2,3]. In this work, we study iodide and bromide perovskites with two-dimensional electronic spectroscopy (2DES) with -10 fs pulses spectrally covering the bandgap and sub-bandgap regions. Even though in linear absorption the onset of the excitonic peak and the band edge are very sharp, in 2DES we observe an instantaneous bandgap signal following sub-bandgap excitation extending down to the limit of our spectral coverage. The superior sensitivity of 2DES to detect these sub-bandgap states is due to the fact that absorption to a sub-bandgap electron trap state creates a hole in the valence band, which in turn generates a bleach signal of the very bright bandgap transition. [4] This observation was made for methylammonium lead iodide and bromide perovkites, showing that not only sub-bandgap states do exist, but that they can be populated by direct absorption. We also use complementary transient absorption experiments with narrowband and broadband pulses to demonstrate that excitation above the bandgap creates an instantaneous transmission increase extending well below the bandgap. However, the dynamics of these signals do not show capture of conduction band electrons by the traps within the investigated timescales, which is consistent with other studies on high quality perovskites. [3,5].