Improvement of Light Harvesting by Addition of a Long-Wavelength Absorber in Dye-Sensitized Solar Cells Based on ZnO and Indoline Dyes

A pathway to improve the light-harvesting efficiency of dye-sensitized solar cells based on porous electrodeposited ZnO films is presented. Cosensitization with the indoline dyes D149 and D131 and the squaraine dye SQ2 leads to panchromatic light harvesting and photovoltaic activity. When coadsorbates are employed to prevent dye aggregation on the ZnO surface, an increase in the short-circuit photocurrent and overall efficiency compared to reference cells with D149, or D149 and D131 is attained. The overall performance of cosensitized cells containing SQ2, however, is limited by low open-circuit voltages and fill factors. By use of electrochemical impedance spectroscopy, current-voltage characterization in the dark and under illumination, and charge extraction measurements, recombination reactions and the distribution of trap states in the ZnO films are investigated, and the origins of the limited open-circuit voltages and fill factors in cells with SQ2 are revealed.