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Paradigm Shifts in Dye-Sensitized Solar Cells

Efficiency limitations from the iodine-based electron shuttle have been overcome by replacing it with cobalt compounds and by using dyes that block recombination. Dye-sensitized solar cells (DSCs) have captured the imagination of a wide range of people, from middle-school students who make their first solar cell by sensitizing films of titania nanocrystals with berry juice to scientists and engineers who are striving to solve the world's energy problem. A schematic of a DSC is shown in the figure, panel A (1, 2). Until recently, practical DSCs—ones with efficiencies exceeding 11%—have had to rely on dyes that are expensive in that they contain the noble metal ruthenium (Ru). Efforts to increase cell efficiency by boosting the output voltage of the cell, and to decrease costs by eliminating the use of Ru, have run afoul of the enemy of all solar cells—the recombination of charge carriers before they are delivered to the electrodes. On page 629 of this issue, Yella et al. (3) report on a specially designed redox mediator containing cobalt (Co) complexes that enables a DSC efficiency of 12.3% under 1 sun of illumination.