Improved inverted-organic-solar-cell performance via sulfur doping of ZnO films as electron buffer layer

Abstract

Sol-gel methods have been widely reported for the design and synthesis of functional materials, and they exhibit excellent potential for nanoscale film deposition. In this study, we investigate sol-gel-processed ZnO films as electron buffer layers (EBLs) for inverted organic solar cells. We optimize the ZnO film thickness at 25 nm by adjusting the precursor/solvent ratio, and we evaluate its effect on the photovoltaic (PV) performance parameters. Furthermore, we investigate the doping effect of sulfur in ZnO film in terms of the power-conversion efficiency (PCE) of the cells. We select thiophene as a potential sulfur dopant. An inverted organic solar cell fabricated with a ~4 at% sulfur-doped ZnO EBL exhibited a PCE of 4.16%, which is a 10% increase compared with a reference device containing a pristine ZnO EBL. The doping of sulfur improved the electrical properties of the ZnO films, as confirmed by measurements of the Hall electron mobility and carrier density. The device with 4 at% sulfur-doped ZnO EBL was found to be more stable than the reference ZnO device by maintaining its PCE at 74% of the original value for 264 h.