Understanding the Relationship Between Grain-Size, Stoichiometry and Conductivity in Evaporated CsPbBr₃ Perovskite Thin-films via Non-Contact Characterization Techniques

The best-solar cells are continuous, defect free films. Films with large grains, at least one micron in diameter, are preferred. Grain-sizes of CsPbBr₃ perovskite thin-films change as a function of substrate temperature during vapor deposition and post-deposition annealing. Interestingly, the stoichiometry of the perovskite changes with extended annealing times post-deposition. We hypothesize PbBr₂ evaporates during annealing. By heating the substrate during evaporation, we achieve similar grain sizes without the stoichiometric shift. Optimizing the vapor deposition to produce the optimal stoichiometries and grain-sizes without any post processing will keep the production cost lower and material-to-device development faster. A quality solar device also requires high conductivity. Larger grains tend to provide higher conductivities. Using two non-contact scanned probe techniques, broadband local dielectric spectroscopy (BLDS) and phase-kick Electric Force Microscopy (pk-EFM) we are quantifying conductivity and photoconductivity lifetimes as a function of grain size. BLDS measurements reveal a wide range of dark conductivity and photoconductivity in lead-halide perovskites as a function of perovskite composition, substrate composition, perovskite dimensionality, temperature, and light intensity. We interpret measurements of non-contact friction and BLDS spectra versus height using microscopic theory to quantify conductivity, charge density, and dielectric constant [1,2]. Pk-EFM, developed in the Marohn Lab at Cornell University, enables nanosecond temporal resolution of charge-recombination dynamics, with the high spatial resolution of a scanning-probe microscope measurement [3]. It is a technique complementary to time-resolved microwave conductivity (TRMC) but does not require an insulating substrate. We use TRMC to validate the pk-EFM results. Both pk-EFM and TRMC are being used to track the effect of sample degradation and aging on conductivity. We can gain insight on sample stability by quantifying the steady-state and transient ionic and electronic conductivity.
[1] Tirmzi, A. M.et al. J. Phys. Chem. C 2019, 123 (6), 3402–3415. doi:10.1021/acs.jpcc.8b11783
[2] Tirmzi, A. M. et al. J. Phys. Chem. C, 2020, 124, 13639 - 13648. doi:10.1021/acs.jpcc.0c04467
[3] Dwyer, R. P. et al. Sci. Adv. 2017, 3 (6), e1602951. doi:10.1126/sciadv.1602951.