Improving uniformity and nanostructure of solution-processed thin films using ultrasonic substrate vibration post treatment (SVPT).

The main goal of this paper is to introduce a novel mechanical method herein terms as substrate vibration post treatment (SVPT) technique, powered by ultrasonic vibration imposed on the substrate to enhance the characteristics and functionality of spun-on thin films or thin films made by similar casting techniques, such as drop and dip coating. In this technique, the as-casted wet films are placed on a substrate vibrated by an ultrasonic transducer with controlled power and duration to improve the film characteristics, such as uniformity and nanostructure. The performance of this technique is examined on spun-on PEDOT: PSS thin films used in polymer and perovskite solar cells and unprecedented results are presented. We first explore the influence of the vibration duration time on the characteristics of the films made by pristine PEDOT: PSS solution, where it is found that the optimized vibration duration for the pristine PEDOT: PSS film is about 10s, resulting in significant increase in the film electrical conductivity and lowered thickness and roughness. In order to further test the generality and merit of the method, thin films made using PEDOT: PSS solution modified with various types of surfactants and cured by the SVPT are studied. The results show that the application of the SVPT method combined with surfactant modification leads to an impressive twelve-fold increase in the conductivity of the PEDOT: PSS thin films compared with that of the pristine non-vibrated PEDOT: PSS thin films. The sole effect of the SVPT is a four-fold increase in the conductivity of pristine PEDOT: PSS film compared with that of the non-vibrated film. This remarkable enhancement in conductivity is further explained by the AFM phase images of PEDOT: PSS films, showing that the ultrasonic energy could loosen the Coulomb forces between PEDOT and PSS chains, resulting in phase separation and localized reordering of the conducting PEDOT chains leading to an increase in the electrical conductivity of the film. Highly conductive PEDOT: PSS thin film is a viable candidate as electrodes in emerging solution-processed solar cells.