The influence of nitric acid on electron transport and recombination for non-sintering Tio2 photoanodes

Nitric acid was added to binder-free TiO2 paste for the preparation of plastic TiO2 dye-sensitized photoanode at low temperature on conductive indium–tin oxide (ITO)-coated polyethylene naphthalate (PEN) substrate. The influence of nitric acid on the electron transport within the cells was scrutinized. It was found that the electron transport was accelerated by means of increasing nitric acid contents. Rheological behavior testing revealed that the increasing concentration of nitric acid leaded to a decrease of viscosity of the paste and then increased the coordination number within the photoanode, which represent the possible electron transfer pathways in the photoanode. This was confirmed by scanning electronic microscopy (SEM) results. Electrochemical impedance spectroscopy (EIS) results showed that the charge transport resistance in the TiO2 film (Rt) decreased gradually when the nitric acid content increased from 0 to 0.1 M, which was attributed to the increasing coordination number of TiO2 particles in the nanoporous film. Meanwhile, the increasing NO3− will prohibit the electron recombination between TiO2 and electrolyte proved by EIS measurements. However, excessive nitric acid also leaded to a corrosion of the ITO substrate and impaired the photovoltaic performance of the flexible devices. Hence, the devices prepared with nitric acid content of 0.025 M achieved the highest overall energy conversion efficiency of 5.30%.