Effect of nanocrystalline-TiO2 morphology on the performance of polymer heterojunction solar cells

We compare the performance of ruthenium-dye-sensitized, heterojunction solar cells formed from the p-type organic semiconductor poly (3-hexylthiophene), P3HT, and sintered nanocrystalline-titanium dioxide ( nc-TiO2) films prepared from different batches of sol-gel material. Atomic force microscopy shows that the RMS roughness of the nc-TiO2 layers from these different batches ranged from 12.7 to 20.5 nm with the corresponding mean particle height ranging from 60 to 90 nm. Scanning electron microscopy reveals that these different topographies are linked to differences in the morphology of the sintered nc-TiO2 layers. Thermogravimetric analysis suggests that these structural differences may arise from differences in the solvent concentration in the nominally identical sol-gel solutions used for preparing the nc-TiO2 layer. The best solar cell performance, achieved with films showing the highest RMS roughness and a 'columnar' morphology, displayed the highest external quantum efficiencies (EQEs) reported for this combination of materials: 11%< EQE < 16% for the wavelength range 580-380 nm. However, the lower turn-on voltage for the dark, forward current in these devices leads to a reduction of similar to 0.2V in open-circuit voltage compared with the smoother films of nc-TiO2.