Facile Solution Dropping Method: A Green Process for Dyeing TiO2 Electrodes of Dye-Sensitized Solar Cells with Enhanced Power Conversion Efficiency

Abstract

A simple solution dropping method was established for sensitizing TiO2 in the fabrication of dye-sensitized solar cells (DSCs). As compared with the conventional immersion dyeing process, this solution dropping method is very fast, taking less than similar to 5 min vs >5-10 h typically required in the traditional immersion dyeing process. There is much less organic solvent and dye substance (95% less) used in the dyeing TiO2 process and hence significantly less disposal of chemical wastes from device fabrications. Therefore, this facile solution dropping method is a greener process than the immersion dyeing process. Moreover, the solution dropping method is superior to the immersion dyeing process in terms of power conversion efficiency (PCE) of the device. We have acquired compelling evidence through dye uptake assessment of TiO2 electrodes, depth profile assay by SEM-EDX, and charge dynamic characteristics from transient photovoltage/photocarrent analysis indicating that the elevated dye loading level of a TiO2 electrode is the main cause responsible for increasing short-circuit current and hence the PCE of DSCs. Three types of dye were used in this study to demonstrate the superiority of the solution dropping method, including classical N719 (a ruthenium transition metal complex), 1P-PSS (a metal free organic dye), and newly synthesized ATT (a beta-pyrrole carbon-conjugated zinc tetraphenylporphyrin). Using the solution dropping method, dye uptake was improved from 3.0 X 10(-7) to 9.9 X 10(-7) mole cm(-2), 5.3 X 10(-7) to 6.6 X 10(-7) mole cm(-2), and 4.7 X 10(-8) to 3.92 X 10(-7) mole cm(-2) for N719, 1P-PSS, and ATT, respectively. In addition, the PCEs, averaged from 30 or 40 tested devices each with a 0.4 cm X 0.4 cm active area, were all improved from 8.1% to 8.5%, 5.9% to 6.6%, and 4.196 to 6.7% for N719, 1P-PSS, and ATT, respectively.