Impact dynamics and deposition of perovskite droplets on PEDOT:PSS and TiO2 coated glass substrates

Perovskite solutions are widely used, to develop next generation photovoltaic solar cells and printed electronics. Droplet-based coating route, comprising inkjet printing, aerosol jet, and spray coating, is a viable deposition approach for high volume manufacturing of such devices. Perovskites are emerging ionic solutions deposited on unconventional substrates, where their droplet impact dynamics and deposition behavior is unexplored. In this work, we studied the impact dynamics of popular perovskite solution droplets, i.e. methylammonium lead halides (CH3NH3PbI3 and CH3NH3PbI3-xClx) on three surfaces, viz. glass coated with thin films of PEDOT:PSS, and compact and mesoporous TiO2, widely used in emerging optoelectronic devices. Droplets of two solutions, with initial diameter similar to 2.2 mm, were impacted onto three substrates, with impinging velocities of 1.40, 1.72, and 1.98 m/s, generating 18 experimental conditions, in the range of 176 < We < 402, where We is the Weber number. Using top- and side-view high-speed imaging, we studied temporal evolution trend in kinematic and spreading stages, and maximum droplet spreading in the wetting stage. We also analyzed the prediction power of existing scaling laws, theoretical and semi-empirical models for maximum spreading diameter with respect to droplet initial diameter, beta(max), against perovskite solution droplets. We found that for the common coated substrates used here, and also in photovoltaic devices, the relative roughness with respect to droplet size was small, and the coated substrates were quite wetting with small contact angles. Thus, the perovskite solution droplet spreading was found to be a function of the We number, according to an existing scaling law and correlation for common liquids, where the coefficient of the correlation was tuned for methylammonium perovskite solution droplets by curve fitting, yielding a modified empirical correlation in the form of beta(max) = 0.8We(0.25).