Enhanced charge extraction in inverted hybrid photovoltaic cells assisted by graphene nanoflakes

We use graphene nanoflakes (GNFs) to greatly enhance the charge extraction out of a photoactive blend in inverted hybrid poly(3-hexylthiophene):(6,6)-phenyl C-61 butyric acid methyl ester (P3HT:PCBM)/ZnO-nanorod photovoltaic cells. Instead of a continuous film, solution processed GNFs with dimensions less than 200 nm x 200 nm are homogeneously scattered on top of the well-aligned ZnO-nanorods. Those GNFs play key roles, they serve as an electron drain to collect electron flow out to ZnO-nanorods, enhance the carrier mobility of the device and promote holes to drift toward the surface in contact with the cathode. As a result, there is a large enhancement in photocurrent and photovoltage of 35% and 27%, respectively, leading to an improved cell efficiency by up to about 100%.