Enhanced open circuit voltage by hydrophilic ionic liquids as buffer layer in conjugated polymer–nanoporous titania hybrid solar cells

Abstract

We demonstrate the fabrication of a nanoporous titania (NP-TiO₂) network structure by using a polystylene-block-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymer template and modifying the surface of NP-TiO₂ with ionic liquids (ILs), bmim-BF₄ and benmim-Cl. The effect of the molecular weight of PS-b-P4VP on the morphology of the NP-TiO₂ and IL-modified NP-TiO₂ are characterized by scanning electron microscopy and contact angle measurements. Subsequently, hybrid solar cells are fabricated using MEH-PPV and NP-TiO₂, and the effect of IL layers and IL concentrations on device performances are evaluated under AM 1.5 G illumination. The devices containing bmim-BF₄ and benmim-Cl show drastically enhanced open circuit voltages (V_oc) of 1.05 V and 0.91 V, respectively, while the reference device without an IL layer exhibits a V_oc of 0.60 V. Significantly improved V_oc can be attributed to the change in interfacial energy levels by formation of ionic double layers at the TiO₂/IL and at the IL/MEH-PPV interfaces. We also observed the trend that short circuit current decreased and V_oc increased with increasing IL concentration, which is ascribed to interruption of charge transfer from MEH-PPV to TiO₂ and the change in interfacial energy level by shifting the vacuum level, respectively.