Effect of Marangoni Convection on the Perovskite Thin Liquid Film Deposition

Abstract

Motivated by recent advances in the development of thin-film perovskite solar cells, the evaporation and deposition of a perovskite thin liquid film on a hydrophilic substrate, also in some cases subjected to ultrasonic vibrations, are studied in this paper. In practice, in the literature, the complexity of the underlying phenomenon has led to the study of a thick (macroscale) liquid film in the absence of solidification. Here, we investigate evaporation mechanics of a thin (microscale) liquid film of perovskite solution. We demonstrate flow fields within the film and study the reason behind the formation of such flow patterns within a thin liquid film and attribute such flows to surface-tension-induced Marangoni flows and rule out the possibility of the presence of buoyancy-induced Rayleigh−Benard convection. We show that perovskite deposition starts at the film contact lines and propagates toward the film center, creating two regions of the perovskite film with distinct characteristics. Our thermography results (temperature mapping) of the film surface show agreement between the temperature map and the flow patterns on the film surface. Moreover, we show that imposing ultrasonic vibration on an evaporating liquid film results in a more uniformly distributed flow pattern across the film due to micromixing enhancement achieved by ultrasonic vibrations.

Quasi-2D Model to Predict Solid Microstructure in Drying Thin Films

“…We also neglect convective effects due to gradients that may develop in height, surface tension (Marangoni and “coffee ring”), or temperature (Rayleigh–Benard). , These assumptions are reasonable for sufficiently thin films where the evaporation rate is such that instabilities cannot develop appreciably and for steady-state coatings (such as roll-to-roll) where dewetting is avoided. This assumption is justified experimentally by Ahmadian-Yazdi and Eslamian, who have shown that in the context of perovskite films thinner than 80 μm Marangoni type instabilities can be ignored . Thus, the model and discussion in this work are restricted to thin films that remain stable during the drying process.…”

“…However, it is easy to think of scenarios where the dominant transport mechanism is rotational convection cells, such as those induced by Rayleigh–Bernard or Marangoni effects. Such convective flows could significantly enhance the transport of solute to the interface, which would largely decrease the effective Pe and Bi *. ,, Therefore, it is not clear to what degree Pe may be controlled in practice. For example, by heating the substrate from below (Rayleigh–Bernard) and/or from above (Marangoni), it is possible that significant convection within the thin films occurs, which would increase the diffusion velocity and thus decrease Pe .…”

“…This assumption is justified experimentally by Ahmadian-Yazdi and Eslamian, who have shown that in the context of perovskite films thinner than 80 μm Marangoni type instabilities can be ignored. 18 Thus, the model and discussion in this work are restricted to thin films that remain stable during the drying process.…”

Boosted Charge‐Carrier Transport in Triple‐Cation Perovskites by Ultrasonic Vibration Post Treatment

“…The temperature of perovskite thin‐film surface increased up to 40 °C after 3 min vibration; [ 42 ] while for the annealing process at 100 °C the surface temperature was found to be up to 78 °C. [ 67 ] The heat generated during the annealing process is much higher in comparison with annealing‐free ultrasonic vibration. Our results indicate that ultrasonic vibration post treatment can prevent the defect formation in the perovskite films during the high‐temperature annealing process.…”

In Situ Process Monitoring and Multichannel Imaging for Vacuum‐Assisted Growth Control of Inkjet‐Printed and Blade‐Coated Perovskite Thin‐Films

“…In contrast, optical characterization techniques based on photons in the wavelengths range from UV to near infrared have been applied in production so far, for example photoluminescence (PL) imaging in Si-and CuInGaSe 2 -photovoltaics, [40,41] and can be operated in ambient environments. Common characterization techniques used for perovskites so far encompass Raman spectroscopy, [42,43] thermography, [44] absorption and reflection spectroscopy, [45][46][47][48] and PL spectroscopy. [45,49] In contrast to Raman spectroscopy, which is limited to small monitoring areas due to the need of a focused laser beams, PL can be used as a large area imaging method.…”