Effects of Solvents on ITO Cracks in Ultrasonic Cleaning of ITO-Coated Flexible Substrates for Polymer Solar Cells

We report an experimental optimization of the cleaning process in the fabrication of flexible polymer solar cells using an indium tin oxide (ITO) coated poly(ethylene naphthalate) (PEN) substrate. The effects of the solubility parameters of the cleaning solvents on the occurrence of ITO cracks have been systematically investigated. The experimental results showed that the ITO cracks caused during the ultrasonic cleaning process were mainly affected by differences between the solubility parameters of the solvent and that of the PEN substrate, not by direct interaction between the solvent and the ITO layer. Our results provide a simple method for reducing ITO cracks during wet cleaning processes by selecting solvents with solubility parameters which differ significantly from those of the flexible substrates.