Influence of spray conditions on droplet charge per unit volume for electrospray deposition

• The influence on the charge density of several electrospray parameters was investigated.
• The escaped charge density is estimated from the relationship between spraying distance and charge density.
• An additive solvent is found to affect efficient charging in electrospray deposition.

In electrospray deposition (ESD), the droplet charge per unit volume is an important parameter for optimizing a coating condition. Previous research has investigated the electrostatic charge on droplets both theoretically and experimentally. Recently, an additive solvent technique was proposed that yields smooth organic thin films, and several types of high-performance organic devices were demonstrated. However, investigating just the droplet charge per unit volume is insufficient to understand the advantages of the additive solvent. In the present work, we investigate the influence on the droplet charge per unit volume of several parameters such as applied voltage, liquid flow rate, and spraying distance to optimize the charging conditions even at a high liquid flow rate, which leads to a high deposition rate. The escaped droplet charge per unit volume is estimated from the relationship between spraying distance and droplet charge per unit volume. In addition, we also estimate how an additive solvent and a polymer affect efficient charging in ESD. The droplet charge per unit volume becomes high upon using the additive solvent, even at a low concentration (10 vol%). The additive solvent technique is found to be a useful tool for realizing high the droplet charge per unit volume, which results in a large spray diameter. However, because of its low concentration in the solvent, the polymer has little effect on the droplet charge per unit volume.