Ali has published a new paper with two researchers from the Department of Energy’s National Renewable Energy Laboratory — postdoctoral fellow Dr. Jeffrey Christians and staff scientist Dr. David Moore.
Tirmzi, A. M.; Christians, J. A.; Dwyer, R. P.; Moore, D. T. & Marohn, J. A. “Substrate-dependent photoconductivity dynamics in a high-efficiency hybrid perovskite alloy”, Journal of Physical Chemistry C, 2019, 123, 3402 - 3415, DOI:10.1021/acs.jpcc.8b11783.
In Ali’s previous paper he reported that illuminating CsPbBr3 led to an increase in conductivity, likely ionic conductivity, that lasted 10s to 100s after the light was removed. We chose CsPbBr3 for this first study because of its stability and chemical simplicity. In this paper Ali investigates a state-of-the-art perovskite alloy prepared by Christians and Moore at DOE NREL. Despite having a considerably more complicated composition, this alloy shows the same persistent photoinduced conductivity phenomenon as the simpler CsPbBr3. Moreover, the alloy conductivity is now substrate dependent.
The abstract of the paper reads
Films of (FA0.79 MA0.16 Cs0.05)0.97 Pb(I0.84 Br0.16)2.97 were grown over TiO2, SnO2, indium tin oxide (ITO), and NiO. Film conductivity was interrogated by measuring the in-phase and out-of-phase forces acting between the film and a charged microcantilever. We followed the films’ conductivity versus time, frequency, light intensity, and temperature (233 – 312 K). Perovskite conductivity was high and light-independent over ITO and NiO. Over TiO2 and SnO2, the conductivity was low in the dark, increased with light intensity, and persisted for 10’s of seconds after the light was removed. At an elevated temperature over TiO2, the rate of conductivity recovery in the dark showed an activated temperature dependence (Ea = 0.58 eV). Surprisingly, the light-induced conductivity over TiO2 and SnO2 relaxed essentially instantaneously at a low temperature. We use a transmission-line model for mixed ionic–electronic conductors to show that the measurements presented are sensitive to the sum of electronic and ionic conductivities. We rationalize the seemingly incongruous observations using the idea that holes, introduced either by equilibration with the substrate or via optical irradiation, create iodide vacancies.
This work was funded by the U.S. National Science Foundation and the U.S. Department of Energy.