Defect states in perovskite solar cells associated with hysteresis and performance

External quantum efficiency and transient photocapacitance (TPC) spectra were obtained for perovskite solar cells with methylammonium lead triiodide perovskite absorbers formed by either dip or vapor conversion. These measurements reveal an extended band of sub-gap states in all of the devices studied. The defect band is best fit by a pair of defects, and the appearance of the defect signal in the transient photocapacitance spectra indicates that at least one of the observed defects is in the perovskite absorber. The cells with the largest density of defect states show the lowest short-circuit current density and open-circuit voltage for slow, quasi-steady-state, current density-voltage sweeps and the largest hysteresis in short-circuit current density for fast sweeps. This suggests that defect states in the perovskite absorber limit steady-state device performance, and that these defects or associated mobile charges play a role in the hysteresis observed in current density-voltage measurements.