Effect of defects in TiO2 nanotube thin film on the photovoltaic properties of quantum dot-sensitized solar cells

In the liquid-phase-deposition (LPD) method, the deposition temperature is considered to be one of the most important factors in TiO2 nanotube crystal growth. We investigated the effects of the deposition temperature on the surface morphology and defects in TiO2 nanotube (NT-TiO2) thin film electrodes utilizing scanning-electron-microscopy (SEM), X-ray diffraction (XRD), and photoluminescence (PL), together with the effects of these on the photovoltaic characteristics of CdSe quantum dot (QD)-sensitized NT-TiO2 solar cells. In addition, we studied the effect of these defects on the physical properties, such as the carrier recombination and electron transport at the TiO2 and TiO2/QD interface. NT-TiO2 electrodes prepared at low temperatures have a more uniform surface and lower defects than those prepared at high temperatures. From the PL measurements and the photovoltaic characterization such as shunt resistance (Rsh) and open circuit voltage decay (OCVD), these defects can act as carrier recombination centers. The defect density increases with increasing deposition temperature, leading to an increase in carrier recombination. Series resistances (Rs) of the solar cells with NT-TiO2 electrodes prepared at high temperatures were larger than those of the solar cells with NT-TiO2 electrodes prepared at low temperatures, suggesting that the defects can also affect the carrier transport characteristics. Eventually, CdSe QD-sensitized NT-TiO2 solar cells employing NT-TiO2 prepared at low temperatures showed higher conversion efficiencies than those prepared at high temperatures.