Center for Functional Nanomaterials Colloquium

There are two great scientific puzzles haunting next-generation solar cell materials. In the best organic donor-acceptor heterojunctions, why do charges take hundreds to thousands of times longer to recombine than Langevin theory predicts? In lead-halide perovskite materials, why are the materials so operationally unstable and is this instability fundamental to the material or can it be mitigated through synthesis or processing? To solve these puzzles, we have invented new ways to characterize the electronic properties of materials at the nanoscale. One such invention is "phase-kick" electric force microscopy, pk-EFM, a linear-response method for detecting and imaging nanosecond conductivity transients. This experiment records changes in cantilever phase as a function of the time delay between sub-nanosecond laser pulses and nanosecond voltage pulses. This experiment has uncovered, in 2D lead-halide perovskites, surprisingly large variations in conductivity dynamics in samples having nominally identical photoluminescence transients. We propose that pk-EFM will enable definitive new tests of theories of non-Langevin recombination in organic photovoltaic materials. A second suite of measurements quantifies steady-state electronic properties. Here we observe non-contact friction and the local "dielectric spectrum", the cantilever frequency shift versus tip voltage-modulation frequency, as a function of tip-sample separation and illumination intensity. A rigorous theory of these measurements, built on Maxwell's equations and the fluctuation-dissipation theorem, connects observables to the sample's dielectric constant, charge density, and conductivity. Measurements of lead-halide perovskites, in the dark and under illumination, reveals that (1) conductivity is substrate dependent and (2) in a wide variety of perovskites, light creates persistent ionic conductivity lasting tens of seconds at room temperature. The conductivity recovery has an activation energy of 0.5 eV, consistent with the recombination of photogenerated halide vacancies and interstitials. We conclude that light creates atomic defects in lead-halide perovskites.