Inverted organic photovoltaic cells using three-dimensionally interconnected TiO₂ nanotube arrays

Here we describe a simple sol-gel method to fabricate inverted organic photovoltaics (OPV) using interconnected TiO₂ nanotubes (inter-TiO₂ NT) as an efficient electron transport layer. Threedimensionally inter-TiO₂ NT arrays were prepared by spin-coating a TiO₂ precursor solution on the ZnO nanorod (NR) template grown via the liquid phase deposition method. Upon etching of ZnO NRs, inter-TiO ₂ NT arrays were generated, as confirmed by X-ray diffraction (XRD), energy-filtering transmission electron microscopy (EF-TEM) and field-emission scanning electron microscopy (FE-SEM). A blend of poly(3-hexylthiophene) (P3HT) and phenyl-C₆₁-butyric acid methyl ester (PCBM) deeply infiltrated into the pores of inter-TiO₂ NT, as revealed by FE-SEM and atomic force microscopy (AFM) images. The power conversion efficiency (PCE) of inter-TiO₂ NT-based inverted OPV reached 3.0% at an air mass of 1.5 (100 mW/cm²), which is a 25% performance improvement compared to flat TiO₂ films derived from the sol-gel process or commercial paste. The efficiency improvement arises from facilitated charge separation and collection due to the increased TiO₂ interface area and efficient transport pathway.