Improved Hybrid Solar Cells via in situ UV Polymerization

Abstract

One approach for making inexpensive inorganic–organic hybrid photovoltaic (PV) cells is to fill highly ordered TiO₂ nanotube (NT) arrays with solid organic hole conductors such as conjugated polymers. Here, a new in situ UV polymerization method for growing polythiophene (UV-PT) inside TiO₂ NTs is presented and compared to the conventional approach of infiltrating NTs with pre-synthesized polymer. A nanotubular TiO₂ substrate is immersed in a 2,5-diiodothiophene (DIT) monomer precursor solution and then irradiated with UV light. The selective UV photodissociation of the CI bond produces monomer radicals with intact π-ring structure that further produce longer oligothiophene/PT molecules. Complete photoluminescence quenching upon UV irradiation suggests coupling between radicals created from DIT and at the TiO₂ surface via a charge transfer complex. Coupling with the TiO₂ surface improves UV-PT crystallinity and π–π stacking; flat photocurrent values show that charge recombination during hole transport through the polymer is negligible. A non-ideal, backside-illuminated setup under illumination of 620-nm light yields a photocurrent density of ≈5 µA cm²—surprisingly much stronger than with comparable devices fabricated with polymer synthesized ex situ. Since in this backside architecture setup we illuminate the cell through the Ag top electrode, there is a possibility for Ag plasmon-enhanced solar energy conversion. By using this simple in situ UV polymerization method that couples the conjugated polymer to the TiO₂ surface, the absorption of sunlight can be improved and the charge carrier mobility of the photoactive layer can be enhanced.