Time-Resolved Electrostatic Force Microscopy Measures Effect of Phosphonic Acid Hole Transport Layers on Lead Halide Perovskite Carrier Dynamics on the Nanoscale

Phosphonic acids hole transport layers (HTLs) demonstrate great promise in improving lead halide perovskite device performance metrics such as open circuit voltage and radiative recombination lifetime. As perovskite thin films exhibit morphological features on the scale of hundreds of nanometers, and carrier dynamic processes on the order of microseconds and below, it is important to investigate the effects of these HTLs at sub-diffraction limited lengthscales with high temporal resolution. Time-resolved electrostatic force microscopy (trEFM) is an atomic force microscopy method that measures photoinduced charging in samples under realistic intensities with sub-microsecond time resolution and nanometer-scale spatial resolution. Here, we use trEFM to investigate the effect of two phosphonic acid HTLs, MeO2PACz and Me4PACz, on carrier dynamics at grains and grain boundaries. We find that phosphonic acid HTLs promote slower dynamics as captured by trEFM, which agrees with bulk carrier lifetime measurements and provides a nanoscale view of charging dynamics. Furthermore, we observe variation in those dynamics across grain boundaries, with grain boundaries exhibiting slower charging compared to grain centers. The microsecond timescales suggest that trEFM measures fast capacitive charging, which may be convolved with carrier trapping, non-radiative recombination, and ion motion. These experiments indicate that trEFM is a powerful tool for probing dynamic processes in perovskites at the nanoscale.