Control of Recombination Pathways in TiO₂ Nanowire Hybrid Solar Cells Using Sn⁴⁺ Dopants

Abstract

Hybrid nanostructures have shown increasing potential as a replacement for Si solar cells due to the availability of low-cost material combinations. However, up to now, hybrid solar cells, where photon absorption occurs in a semiconducting polymer and charge separation occurs at a metal oxide-polymer interface, show limited efficiencies. One limitation is caused by a relative low charge carrier mobility in the metal oxide. Here we addressed this issue and describe the use of a Sn:TiO2|TiO2 core–shell nanowire array to increase the charge-carrier mobility in the core of the nanowires while decreasing the charge-carrier recombination at the metal oxide–polymer interface due to fast electron extraction from this interface, driven by a cascaded conduction band energy from shell to core of the nanowires. These doped cores with an undoped shell structure resulted in impressive efficiency improvement in hybrid solar cells of 33% over the reference TiO2-based device. Additionally, this device structure resulted in a 17% increase in recombination lifetimes based on both photovoltage decay measurements and impedance spectroscopy. Recombination mechanisms are proposed for the core and core–shell systems to highlight the various effects of the Sn4+-doped TiO2 nanowire arrays. Doped core–shell structures have the potential for application in the hybrid-type devices without the limitations that are seen with the current dual metal oxide structures due to the seamless interface of the metal oxide host for direct transport of the electrons into the highly mobile core material.