Metal-Free Sensitizers for Dye-Sensitized Solar Cells.

This review focuses on our work on metal-free sensitizers for dye-sensitized solar cells (DSSCs). Sensitizers based on D-A'-π-A architecture (D is a donor, A is an acceptor, A' is an electron-deficient entity) exhibit better light harvesting than D-π-A-type sensitizers. However, appropriate molecular design is needed to avoid excessive aggregation of negative charge at the electron-deficient entity upon photoexcitation. Rigidified aromatics, including aromatic segments comprising fused electron-excessive and -deficient units in the spacer, allow effective electronic communication, and good photoinduced charge transfer leads to excellent cell performance. Sensitizers with two anchors/acceptors, D(-π-A)2 , can more efficiently harvest light, inject electrons, and suppress dark current compared with congeners with a single anchor. Appropriate incorporation of heteroaromatic units in the spacer is beneficial to DSSC performance. High-performance, aqueous-based DSSCs can be achieved with a dual redox couple comprising imidazolium iodide and 2,2,6,6-tetramethylpiperidin-N-oxyl, and/or using dyes of improved wettability through the incorporation of a triethylene oxide methyl ether chain.