Role of Carbon Nanotubes in Dye-Sensitized TiO2-Based Solar Cells

Incorporation of low-dimensional carbon nanostructures such as carbon nanotubes (CNTs) and graphene sheets into the semiconductor electrodes is a common approach to improve the charge collection and photovoltaic performance of dye-sensitized solar cells. In this work, we clarify the role of CNTs in the semiconductor electrodes by investigating and comparing the electronic process in the dye-sensitized TiO2-based photovoltaic devices. The results show that the formed CNT-TiO2 Schottky junction plays a crucial role in the photovoltaic characteristics. According to the thermionic emission theory, the variation of the photocurrent over the voltage of the cells strongly depends on the height of the Schottky barrier. When the output voltage is low, the intrinsic one-dimensional carbon nanostructures can facilitate electron Voltage transport. With the voltage of the cell increasing, the energy dissipation on the Schottky junction increases dramatically and CNTs gradually lose the role of electron transport channels. At the high voltage range, however, leakage of electrons via the CNTs becomes predominant. By virtue of the charge transport channels of CNTs, increments of 44% in photocurrent at short-circuit condition and 18.7% in the overall energy conversion are achieved. Our results provide a basic understanding of the role of CNTs in solar energy conversion.