Template assisted nanoporous TiO_2 nanoparticles: The effect of oxygen vacancy defects on photovoltaic performance of DSSC and QDSSC

摘要

We present a detailed study on the photovoltaic performance of dye and quantum dot (QD) sensitized solar cells fabricated using nanoporous TiO2 photoanodes prepared by a template approach. Nanoporous TiO2 particles (TNPO) are prepared by sol-gel method using dextran (TNPO-D) and glucose (TNPO-G) as templates. The decomposition of templating agents during calcination introduces nanopores (similar to 10 nm) in TiO2 nanoparticles as evidenced from the HRTEM studies. Oxygen vacancies are also introduced in TiO2 nanoparticles due to prevailing reducing ambient during decomposition. Oxygen vacancy (V-O) related defects are evidenced from EPR spectra from a strong absorption signal observed at g-value of 1.994 related to Ti3+-V-O-Ti3+ defect sites. The defect concentration estimated from EPR spectra is found to be similar to 5 to 15 times larger for nanoporous TiO2 particles compared to Degussa P25-TiO2. TNPO samples also show enhanced defect level photoluminescence emission. The photovoltaic efficiency (eta) and the IPCE of DSSC and QDSSC are strongly influenced by the defect concentration and seen to decrease with increasing V-O in TiO2 nanoparticles. Twofold lower eta is found in both DSSC (similar to 6.38% to 3.66%) and QDSSC (similar to 1.69% to 0.46%) devices made using nanoporous TiO2 compared to P25-TiO2 photoanode solar cells. P25-TiO2 also shows significant decrease in eta when oxygen vacancies are introduced by hydrogen annealing process, which reveal detrimental effect of V-O on the photovoltaic properties. EIS studies suggest increased interfacial resistance in solar cells made from nanoporous TiO2 photoanodes.