Interface Modification and Exceptionally Fast Regeneration in Copper Mediated Solar Cells Sensitized with Indoline Dyes

The photovoltaic performance of solar cells sensitized with indoline D205 dye and its new derivative comprising an alkoxysilyl anchoring unit (D205Si) in the [Cu(tmby)(2)](TFSI)(2/1) (tmby = 4,4',6,6'-tetramethyl-2,2'-bipyridine, TFSI = bis(trifluoromethane)sulfonimide) redox couple mediated systems was studied in the presence of various titania/dye/electrolyte interface modifications. Cucurbit[7]uril (CB7) was employed to encapsulate dye molecules, creating an electronically insulating layer, suppressing electron interception by redox mediator, and leading to the increase in the electron lifetime in the titania conduction band. For example, the electron lifetime increased from 2.2 to 6.5 ms upon CB7 encapsulation of D205 cells at 0.9 V voltage. Further, molecular multicapping was optimized to minimize dye desorption and prevent electron recombination. As a result, photovoltaic performance was found to be enhanced by the interface modifications in most cases, especially when applied to the alkoxysilyl anchoring derivative. The charge transfer processes (dye regeneration, titania-dye and titania-redox mediator recombination) in the above-mentioned system and in the reference [Co(bpy)(3)](TFSI)(3/2) (bpy = 2,2'-bipyridine) redox couple mediated systems were investigated by means of small light perturbation electron lifetime measurements, electrochemical impedance spectroscopy, and nanosecond and femtosecond transient absorption spectroscopies. Indoline dyes were also found to be outstandingly fast regenerated by the Cu-based mediator (time constant shorter than 100 ns), which may open new opportunities for sensitizer improvements.