The emergence of copper(I)-based dye sensitized solar cells

Since the discovery of Grätzel-type dye sensitized solar cells (DSCs) in the early 1990s, there has been an exponential growth in the number of publications dealing with their optimization and new design concepts. Conventional Grätzel DSCs use ruthenium(II) complexes as sensitizers, and the highest photon-to-electrical current conversion efficiency for a ruthenium dye is ≈12%. However, ruthenium is both rare and expensive, and replacement by cheaper and more sustainable metals is desirable. In this Tutorial Review, we describe strategies for assembling copper(I) complexes for use as dyes in DSCs, a research area that has been active since ≈2008. We demonstrate design principles for (I) ligands to anchor the complex to a semiconductor surface and promote electron transfer from dye to semiconductor, and (II) ancillary ligands to tune the light absorption properties of the dye and facilitate electron transfer from electrolyte to dye in the DSC. We assess the progress made in terms of light-harvesting and overall photoconversion efficiencies of copper(I)-containing DSCs and highlight areas that remain ripe for development and improvement.