Iron-doped titania thin films with enhanced photovoltaic efficiency: Effects of iron concentration and rectifying layer

Iron-doped titania thin films were fabricated by spin-coating from sol-gel pastes of iron nitrate and titanium isopropoxide and evaluated for photovoltaic conversion in a solar cell. The Fe addition induced a red shift in optical absorption, an electronic interaction between Fe and Ti atoms and an increase in donor density, indicating the excitation of electrons from Fe donors. All Fe-added titania films were shown more hydrophilic than the titania film and produced higher photocurrents when irradiated by visible light. Under ultraviolet illumination, low amounts of Fe dopants promoted photocurrent yield because they served as electron traps for enhancing charge carrier separation, and the yield was lowered at higher Fe concentrations due to charge carrier recombination at Fe sites. Increasing Fe content led to a decrease in open-circuit photovoltage. Addition of a Fe-TiO2 film between the TiO2 layer and the conductive substrate caused a marked increase in photocurrent. The improved cell performance is related to the behavior of rectifying electron flow through two oxide layers, composed of electron transfer and diffusion. Results demonstrate that the Fe doping provides photoactive traps to mediate electron transport for enhancing quantum conversion efficiency. Mechanistic aspects about the role of the Fe traps are discussed.