Large Pore Size and High Porosity of TiO2 Photoanode for Excellent Photovoltaic Performance of CdS Quantum Dot Sensitized Solar Cell

While holding great potential as sunlight absorbers, quantum dots (QDs), which are generally much larger than dye molecule in size, which makes it more difficult to deposit them on the surface of TiO2. As a result, relatively low QD loading is now one of the most challenging issues for improving the photovoltaic performance of QD-sensitized solar cells (QDSSC). In this study, TiO2 photoanodes with different pore sizes and porosities were constructed by systematically varying the solid content of the TiO2 paste. It was confirmed that reducing the solid content resulted in both larger pore sizes and higher porosities. CdS quantum dots were then deposited on these different electrodes by the successive ionic layer adsorption and reaction (SILAR) method, with either 4 or 7 repetitive cycles. By correlating the photovoltaic performances of QDSSCs with different solid contents of TiO2 paste and number of SILAR cycles of CdS QD deposition, it was found that the combination of 7 SILAR cycles with 10% electrode solid content yielded the highest overall energy conversion efficiency. In particular this cell exhibited an outstanding open-circuit photovoltage up to 640 mV using a polysulfide electrolyte, which currently ranks the highest among reported literature. This outcome is due to the fact that a 10%-solid-content provided the largest pore sizes and the highest porosity for the QDs deposition, while the 7 SILAR cycles guaranteed the sufficient CdS QD loading which is favorable for light harvesting.