First-Order Analysis of Dye Sensitized Solar Cell Performance Tuning via Oxygen Partial Pressure Variation during Annealing

Dye sensitized solar cells (DSSC) can be a low cost alternative to conventional solar cells. The nanoscale semiconductor TiO2 is annealed during DSSC processing to create electron pathways to a thin conductive oxide layer (TCO). An unfortunate consequence of air annealing is a simultaneous decrease in the electrical conductivity of the TCO layer. Since electron transport in DSSC's is strongly dependent on oxygen vacancy density, DSSC performance can be tuned by adjusting the oxygen content during annealing because of the resulting decrease (or increase) in device resistances. Indium tin oxide (ITO) conductive glass substrates were annealed in different oxygen level environments. DSSC's were fabricated from TiO2 films that were also annealed in different oxygen level environments. The resistances of the ITO and performance of the DSSC were measured. Results showed that low oxygen annealing during DSSC processing prevents significant increases in the resistivity of ITO. It also causes an increase in open circuit voltage (VOC) and a decrease in short circuit current (ISC). Previous work suggests that the conductivity of TiO2 increases in low oxygen annealing. This increased electrical conductivity was modeled as a reduced TiOx layer formed during low oxygen annealing. It was posited that this conductive TiOx layer has the desirable effect of increased shunt resistance. However, the conductive layer can also restrict electron flow from the electrolyte to the dye either by reducing electron drift and diffusion. These resulted in the changes in VOC and ISC observed. This hypothesis was verified through the use of a novel "Conductive Layer Resistance Model".