Enhanced thin-film deposition uniformity during droplet evaporation: Effects of graphene particle size and concentration

Graphene has been widely recognized for its ability to enhance the efficiency and stability of solar cells, promoting extensive research into its application in thin films. This study employs the droplet deposition technique, utilizing the evaporation of a sessile droplet, to optimize the uniformity of particle deposition, with an emphasis on controlling film thickness and mitigating common challenges, such as the ‘coffee-ring’ effect. We evaluate the key performance parameters, including thickness distribution and surface characteristics, to develop strategies for improving deposition techniques. Side-view imaging offers insights into the changes in contact angle, diameter, and volume during evaporation. Also, we analyze particle distribution and thin-film thickness through top-view and cross-sectional images. Our findings reveal that larger graphene particles exhibit slower movement toward the contact line due to their increased mass, causing improved uniformity at higher concentrations and a reduction in the “coffee-ring” effect observed at lower concentrations. At high weight percentages, particle accumulation at the droplet's center results in increased thickness because of stronger cohesive forces. In contrast, reducing particle size at concentrations above 5 wt% promotes enhanced inter-particle interactions, yielding a homogeneous pattern and decreased thickness, while increasing surface tension and contact angle owing to the hydrophobic nature of graphene.