Water-processed self-assembles of monolayers as interface modifier for ZnO/P3HT hybrid solar cells

Benzoic acid based molecules were used for surface modification of ZnO nanorods as self-assembled monolayers (SAMs) in order to improve the interface interaction between ZnO and poly(3-hexylthiophene) in hybrid solar cells. The dipole moment of the molecules and the solvent used for the surface modification were investigated in relation to the performance of the hybrid devices. A linear relationship between the dipole moment of the interface modifiers and the open-circuit voltage (Voc) of the devices were found. The Voc are enhanced by the use of the molecules with their dipole moment pointing away from the ZnO surface and vice versa. The enhancement in Voc is attributed to the shift of the vacuum level of ZnO when the surface is modified with the SAMs. When water was used as the solvent for surface modification process, aggregate was found in the solution state. This implies an ordered orientation of SAMs attached onto the ZnO surface, resulting in the improvement of the Voc. An enhancement in the incident photon to current efficiency spectra was also obtained in the devices prepared from aqueous solution. The SAMs prepared from aqueous solution have better charge collecting properties in comparison with that prepared from ethanol solution.