Single Component Dye-Sensitized Solar Cells Enabled by Copper Chemistry: Introduction of the Retro Cell

The possibility of utilizing a single molecule to act as both a chromophore and a redox shuttle in a new configuration of a dye-sensitized solar cell (DSSC) is investigated. This design, termed a retro cell, exploits the copper chromophore, [Cu(dsbtmp)₂]⁺ (dsbtmp = bis(2,9-di(sec-butyl)-3,4,7,8-tetramethyl-1,10-phenanthroline), which has been shown to have excited state lifetimes in excess of a microsecond, enabling sensitization of TiO₂ while dissolved in solution. The oxidized chromophore can then diffuse to the counter electrode to be regenerated. This concept simplifies the device components and fabrication and eliminates a charge transfer step compared to that of traditional DSSCs. Initial investigations show the concept is viable; however, the performance is suboptimal. We have found the addition of 4-tert-butylpyridine (TBP) to the electrolyte plays a crucial role in enabling solar energy conversion. Evidence of TBP displacing one of the dsbtmp ligands in the oxidized [Cu(dsbtmp)₂]²⁺ complex has been presented, which likely plays an important role in reducing recombination and enabling charge collection. The performance-limiting steps and routes to improved performance and viability of a retro cell are further discussed.