Improving P3HT:PCBM absorber layers by blending TiO2/CdS nanocomposites for application in photovoltaic solar cells

Abstract

In this work we propose the use of the semiconducting TiO2/CdS nanocomposite (NC) to improve the optical and electrical properties of the P3HT:PCBM polymeric system employed as the absorber layer in organic photovoltaic solar cells. Therefore, we report a methodology for obtaining the TiO2/CdS-NC, its incorporation into precursor solutions containing P3HT and PCBM polymers, and the fabrication of hybrid P3HT:PCBM:TiO2/CdS-NC absorber layers. The effect of the mass ratio between the TiO2/CdS-NC and the polymeric system was studied. XRD measurements conducted on the TiO2/CdS-NC showed that the TiO2 component has a tetragonal crystalline structure (anatase phase) with a nanocrystalline size of 15.8 nm, while the CdS component has the hexagonal close-packed crystalline structure with a nanocrystalline size of 14.9 nm. XPS analysis confirmed the formation of stoichiometric TiO2 and CdS compounds. The formation of any other compound comprising the elements Ti, O, Cd and S was not found, however, elements Cl, Br, N and C were observed at trace levels, which come from the capping agent that was used to obtain the nanocomposite. Measurements on the hybrid P3HT:PCBM:TiO2/CdS-NC absorber layers through UV-vis spectroscopy showed a shift on the absorption edge and an improved light absorption for wavelengths below 550 nm. The four-point probe measurements showed a decrease of the electrical resistivity from 8.5 × 108 Ω-cm in pristine P3HT:PCBM to a minimum of 4 × 105 Ω-cm in the hybrid absorber layers. Both the light absorption and the electrical resistivity are modulated by the mass ratio between the TiO2/CdS-NC and the P3HT:PCBM polymeric system.