Characteristics of water-soluble polythiophene: TiO ₂ composite and its application in photovoltaics

We have studied the characteristics of composites of an environmentally friendly water-soluble polythiophene sodium poly[2-(3-thienyl)-ethoxy-4- butylsulfonate] (PTEBS) and Ti O2. We observed that the ultraviolet-visible absorption spectrum of low molecular weight PTEBS is redshifted possibly due to the formation of aggregates. Cyclic voltammetry reveals the values of highest occupied molecular orbitals and lowest unoccupied molecular orbitals for PTEBS. A factor of 7 in photoluminescence quenching indicates that the exciton dissociation and charge separation occur successfully at the PTEBS:Ti O2 (1:1 by weight) interface. This enhances the possibility that the separated charges will reach the electrodes before recombining. Scanning electron micrograph images show how the PTEBS and Ti O2 are interconnected and form paths to the electrodes to improve charge transport. Photovoltaic devices with Ti O2: PTEBS composite achieved an energy conversion efficiency of η=0.015%, a short circuit current of JSC =0.22 mA cm2, an open circuit voltage of VOC =0.72 V, and a fill factor of FF=0.29 under ∼300 mW cm2 white light illumination.