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Substrate-Dependent Photoconductivity Dynamics in a High-Efficiency Hybrid Perovskite Alloy

Tirmzi, Ali Moeed, Christians, Jeffrey A., Dwyer, Ryan P., Moore, David T., Marohn, John A.
Journal of physical chemistry 2019 v.123 no.6 pp. 3402-3415
alloys, indium tin oxide, irradiation, light intensity, models, nickel oxide, physical chemistry, temperature, tin dioxide, titanium dioxide
Films of (FA₀.₇₉MA₀.₁₆Cs₀.₀₅)₀.₉₇Pb(I₀.₈₄Br₀.₁₆)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.