Surface engineering of ITO electrode with a functional polymer for PEDOT:PSS-free organic solar cells

In this report, surface engineering is used to improve the air-stability of direct polymer solar cells by replacing the PEDOT:PSS hole transporting layer (HTL) by a grafted polymeric layer. For that purpose a poly(3-hexylthiophene) bearing a triethoxysilane function at the end of the chain (P3HT-Si) was synthesized and anchored to the indium-tin oxide (ITO) electrode. The different characterization techniques used (UV-visible and X-ray photoemission spectroscopy) showed that the applied temperature during grafting is a determinant parameter to tune the P3HT-Si layer thickness/density. For low temperature grafting, polymeric chains were laying close to the surface while higher grafting temperature led to polymeric chains pointing upwards. This organization of the polymeric chains in the grafted layer has a direct impact on photovoltaic properties of the devices fabricated with P3HT-Si as HTL. The use of this new self-assembled HTL creates a hole selective membrane which drastically reduces the leakage current. As a result, the power conversion efficiency of such devices was improved compared to devices with bare ITO. Additionally, by replacing PEDOT:PSS by the hydrophobic P3HT-Si, the water penetration in the device is impeded which significantly improved the shelf lifetime of devices.