Low-cost transparent graphene electrodes made by ultrasonic substrate vibration-assisted spray coating (SVASC) for thin film devices

Microstructural and Nanostructural Evolution of Light Harvester Perovskite Thin Film under the Influence of Ultrasonic Vibrations

A key step of inexpensive and scalable perovskite thin-film formation is defect-free fabrication through low-cost and facile post-treatment processes. Methods using high annealing temperatures are not favorable for the scale-up of solution-processed thin-film solar cells, particularly on plastic/flexible substrates. This contribution analyzes the effect of ultrasonic vibrations, a recently developed low-cost post-treatment process, on thin-film quality. Ultrasonic vibrations were applied to as-spun CH₃NH₃PbI₃ perovskite thin films prepared with various solvents and antisolvents deposited on substrates with compact and mesoporous textures. Then, mechanisms of solvent evaporation, nucleation, and crystallization of perovskite grains were characterized during ultrasonic vibration. These studies demonstrate that ultrasonic vibration at low temperature facilitates heterogeneous crystallization of perovskite grains with a higher conversion of nuclei into crystal, compared with the conventional annealing process. Topographic scanning electron microscopy images confirm the dense and fully covered thin films after the evaporation of solvent. Furthermore, it is shown that crystal orientation does not change with the choice of solvent, eliminating the effect of solvent on the deposition of thin-film perovskites with this method. Therefore, this ultrasonic vibration post-treatment method is applicable to any solution-processed material and deposition technique, and it can be used to fabricate a range of thin-film devices and printed electronics.

Layer-by-layer hybrid chemical doping for high transmittance uniformity in graphene-polymer flexible transparent conductive nanocomposite

A traditional transparent conducting film (TCF) such as indium tin oxide (ITO) exhibits poor mechanical flexibility and inconsistent transmittance throughout the UV-VIS-NIR spectrum. Recent TCFs like graphene films exhibit high sheet resistance (R_s) due to defect induced carrier scattering. Here we show a unique hybrid chemical doping method that results in high transmittance uniformity in a layered graphene-polymer nanocomposite with suppressed defect-induced carrier scattering. This layer-by-layer hybrid chemical doping results in low R_s (15 Ω/sq at >90% transmittance) and 3.6% transmittance uniformity (300–1000 nm) compared with graphene (17%), polymer (8%) and ITO (46%) films. The weak localization effect in our nanocomposite was reduced to 0.5%, compared with pristine (4.25%) and doped graphene films (1.2%). Furthermore, negligible R_s change (1.2 times compared to 12.6 × 10³ times in ITO) and nearly unaltered transmittance spectra were observed up to 24 GPa of applied stress highlighting mechanical flexibility of the nanocomposite film.