Evaluation of limiting factors affecting photovoltaic performance of low-temperature-processed TiO₂ films in dye-sensitized solar cells

Limiting factors affecting photovoltaic performance of dye-sensitized solar cell employing low-temperature-processed TiO₂ films were investigated. TiO₂ films were prepared at a low temperature of 200 °C using the normal alcohol-containing binder-free TiO₂ paste (LT200). Their photovoltaic performance was compared to a high-temperature (550 °C) annealed TiO₂ film prepared using a polymer binder containing TiO₂ paste (HT550). Compared to the proportional increase in conversion efficiency with TiO₂ film thickness upto 14 μm for HT550, the increase in efficiency was terminated at relatively smaller thickness of about 8 μm for LT200 mainly due to unaugmented photocurrent. From the transient photocurrent-voltage studies, the electron transport rate was found to be almost identical, while charge recombination was one order of magnitude faster for LT200. Consequently, the electron diffusion length was more than 2-3 times shorter for LT200 than for HT550. Electron diffusion length and electron life time obtained from electrochemical impedance analysis were well consistent with those observed from transient measurement. Density of states (DOS) was evaluated to be shallow and narrow in LT200, which was responsible for limiting photovoltaic performance in the low-temperature processed TiO₂ film. Thats why: The limited photovoltaic performance for the low-temperature- processed TiO₂ films (LT200) is due to shorter diffusion length, associated with shorter electron life time, together with a shallow and narrow density of states compared to high-temperature-processed TiO₂ films (HT550).