Interplay of Mobile Ions and Injected Carriers Creates Recombination Centers in Metal Halide Perovskites under Bias

Abstract

We study the interaction of mobile ions and electronic charges to form nonradiative defects during electric biasing of methylammonium lead triiodide (MAPbI 3 ) and formamidinium lead triiodide (FAPbI 3 ) thin films. Using multimodal microscopy that combines in situ photoluminescence and scanning Kelvin probe microscopy in a lateral electrode geometry, we correlate temporal changes in radiative recombination with the spatial movement of ionic and electronic charge carriers. Importantly, we compare trap formation with both charge injecting and blocking contacts. Even though ion migration takes place in both cases, we observe the formation of new nonradiative defects in MAPbI 3 only in the presence of injected electrons, suggesting that redox processes play a key role. On the basis of density functional theory (DFT) simulations, we propose that reduction of Pb 2+ to Pb 0 is responsible for the new defects formed in our films. These results underscore that defect properties in metal halide perovskites are not only determined by the migration of mobile ions but are also highly sensitive to their interaction with injected electronic charge.

Trap States, Electric Fields, and Phase Segregation in Mixed‐Halide Perovskite Photovoltaic Devices

“…The timescale and nature of the changes that occur in the PL presented in Figure 1b are similar to those that have been observed to occur in the single‐halide perovskite MAPbI 3 under similar bias conditions . Therefore, as has been done in the literature for MAPbI 3 , we attribute the decrease in PL intensity (Figure 1b) and charge‐carrier lifetime (Figure 1c) that occurs with an increased applied voltage to the movement of charged, mobile trap states in the perovskite.…”

“…We note that we did not find such behavior under similar conditions for a solution processed MAPbI 3 device (see Figure S6 in the Supporting Information), likely due to intrinsic material differences affording different amounts of ion migration between the two materials, and differences in device performance altering how much charge‐carrier extraction affected the emitted PL. Correlations between applied voltage and loss of radiative efficiency have previously been reported in MAPbI 3 and other metal halide perovskites …”

“…Particularly important for photovoltaic devices is the interaction of halide ions and trap states with electric fields, since efficient power extraction depends on the combination of external and built‐in electric fields within a solar device. Additionally, it has been observed that electric fields can influence the PL from MAPbI 3 films through trap state movement, a process which—similarly to halide segregation—is attributed to the movement of mobile, charged species in the perovskite material. Therefore, in order to fully understand the limiting aspects of mixed‐halide perovskite devices under normal working conditions it is necessary to consider the complex interaction between electric fields, different electronic trap states and halide segregation.…”

Creation and Annihilation of Nonradiative Recombination Centers in Polycrystalline Metal Halide Perovskites by Alternating Electric Field and Light

“…However, when the sample is already prepared, ion migration is probably one of the most important phenomenon responsible for further sample evolution and its eventual degradation . The dominant species for ion migration are most likely the halogen ions (e.g., I − in MAPbI 3 ), however MA + and others can also migrate but with higher activation energy . It is known that light illumination can induce and enhance ion migration in MHP materials by yet not fully understood mechanisms .…”

“…When an MHP material is placed in an external EF, both free charges and ions are able to drift along the field direction . As a first approximation, the averaged EF inside the material is lower than that outside by an effective dielectric constant ε. MHP and in particular MAPbI 3 and MAPbBr 3 possess an apparent giant ε reaching 1000 or more for a static EF.…”