  Understanding the type, formation energy and capture cross section of defects
is one of the challenges in the field of organometallic halide perovskite
(OMHP) devices. Currently, such understanding is limited, restricting the power
conversion efficiencies of OMHPs solar cells from reaching their Shockley
Queisser limit. Here, we report on deep level (DL) defects and their effect on
free charge transport properties of single crystalline methylammonium lead
bromide perovskite (MAPB). In order to determine DL activation energy and
capture cross section we used photo-Hall effect spectroscopy (PHES) with
enhanced illumination in both steady-state and dynamic regimes. This method has
shown to be convenient due to the direct DL visualization by sub-bandgap
photo-excitation of trapped carriers. DLs with activation energies of 1.05 eV,
1.5 eV, and 1.9 eV above valence band were detected. The hole capture cross
section was found using photoconductivity relaxation after sub-bandgap
photo-excitation. We found the DL defects responsible for non-radiative
recombination and its impact on band alignment for the first time.
Additionally, the transport properties of MAPB single crystal is measured by
Time of Flight (ToF) at several biases. The analysis of ToF measurement further
confirms increase of Hall mobility and the enhancement of hole transport
produced by sub-bandgap illumination in MAPB devices. Our studies provide a
strong evidence on deep levels in OMHPs and opens a richer picture of the role
and properties of deep levels in MAPB single crystals as a system model for the
first time. The deeper knowledge of the electrical structure of OMHP could open
further opportunities in the development of more feasible technology.
