The Effect of an Atomically Deposited Layer of Alumina on NiO in P-type Dye-Sensitized Solar Cells

We present a systematic investigation of the fundamental effects of an atomically deposited alumina (AlOxHy) onto the NiO films in p-type dye-sensitized solar cells (p-DSCs). With P1 as the sensitizing dye and 0.1 M I-2 and 1.0 M Lil in 3-methoxypropionitrile as the electrolyte, one atomic layer deposition (ALD) cycle of alumina was used to achieve a 74% increase in the overall conversion efficiency of a NiO-based DSC. The open circuit voltage of the cells increased from 0.11 to 0.15 V, and the short circuit current density increased from 0.83 to 0.95 mA/cm(2). Adsorption isotherm studies were performed to show that the amount of dye adsorbed on the NiO-alumina film is slightly lower than the amount adsorbed on the nontreated NiO film. The increased J(sc), was therefore assigned to the increased efficiency of carrier collection at the semiconductor-FTO interface. Our study of the photocurrent onset potentials of NiO and NiO-alumina films with the chopped light measurement technique showed no definitive difference in the onset potential values. However, the DSCs based on NiO-alumina showed a higher recombination resistance value from the electrochemical impedance studies and a higher diode ideality factor from the V-oc versus ln(light intensity) plots as compared to the DSCs based on untreated NiO. It has thus been established that the increase in V-oc upon alumina treatment arises due to a higher resistance for electron-hole recombination across NiO surface locally.