Improved charge transport in inverted polymer solar cells using surface engineered ZnO-nanorod array as an electron transport layer

•Use of conjugated thioaromatic molecules to modify ZnO-nanorod surface.•The thioaromatic molecules improve the polymer-ZnO-nanorod compatibility.•Both the electron and hole mobility values of the devices are modulated.•Balanced charge transport of the device can be obtained by using longer and linearly arranged aromatic rings.

Interface control is an important issue in polymer based solar cells because the influence of interface property on bulk heterojunction transport can govern the device performance. We systematically select a series of thioaromatic molecules to tune the surface characters of the metal-oxide array in inverted ZnO-nanorod/poly(3-hexythiophene):(6,6)-phenyl C61 butyric acid methyl ester (P3HT:PCBM)/Ag devices. In addition to physically improving the compatibility between ZnO-nanorod and polymer blend contact junction, those conjugated thioaromatic molecules slightly modulate the nanostructured donor and acceptor percolated networks as revealed by the measured carrier mobility values whereas the modulations of photon absorption efficiency and the exciton dissociation rate are negligible. The balanced charge transport architecture can be obtained by using longer and linearly arranged aromatic rings. Additionally, both series resistance and shunt resistance are also improved. As high as ~80% enhancement in power conversion efficiency of the device has been achieved. The result decouples the contribution of the exciton dissociation rate from the bulk heterojunction charge transport process to device performance in inverted polymer solar cells and can be generalized to hybrid polymer-nanocrystal systems as well.