We compare the performance of heterojunction solar cells formed from p-type
organic semiconductor poly (3-hexylthiophene), P3HT, and sintered nano-crystalline titanium dioxide (nc-TiO₂) films. The solar cells were from two batches (A and B) of sol-gel materials and some were sensitized with a ruthenium dye. Atomic force microscopy shows that the RMS roughness of the nc-TiO₂ layers films was 12.7 to 20.5 nm with 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 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 (EQE) 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 ~0.2V in open-circuit voltage compared with the smoother films of nc-TiO₂ .
We have also investigated the properties of the P3HT/nc-TiO₂  interface and the effect of ambient conditions on the parameters of our solar cells. Good
rectification was achieved in the air in the dark. The capacitance of the devices
increased with increase of the applied voltage for the sensitized (batch A) TLSCs
and non-sensitized (batch A, B) DLSCs devices. This change in the capacitance
indicates the existence of band bending and the presence of a depletion region at the interface between P3HT and nc-TiO₂ . Therefore, the junction capacitance is dominated by the depletion capacitance. However, different results were obtained in the vacuum: 1) there was a significant decrease of the dark-current and the loss of rectification, 2) under illumination, the open circuit voltage was zero and the photocurrent larger than in air, and 3) the capacitance was not affected by varying the applied voltage because there is a decrease in the band bending at interface between P3HT and nc-TiO₂  and a disappearance of the depletion region at the interface. The sensitized devices from batch B TLSCs exhibit solar cells characteristics both in vacuum and in air with Voc of ~0.8V. The capacitance remained constant when the voltage increased.