Design of an organic redox mediator and optimization of an organic counter electrode for efficient transparent bifacial dye-sensitized solar cells

A new thiolate/disulfide mediator was designed and synthesized by employing DFT calculations as a guide. It possesses high transparency to visible light, a very attractive feature for bifacially active transparent DSCs that require a highly transparent counter electrode (CE). Compared to the reported and most promising thiolate/disulfide mediator T-/T-2, this new analogous mediator produced a major enhancement in open circuit potential (V-OC) by about 40 mV and correspondingly a higher power conversion efficiency (eta) for DSCs. Furthermore, a highly uniform and transparent (transmittance > 91%) poly(3,4-ethylenedioxythiophene) (PEDOTBE) CE was prepared and could efficiently catalyze the reduction of the disulfide. Based on the novel transparent redox couple and PEDOTBE CE, a new type of iodine-free and Pt-free transparent bifacial DSC was successfully fabricated. This new bifacial device could not only yield a promising front-illuminated eta of 6.07%, but also produce an attractive eta as high as 4.35% for rear-side irradiation, which exceeds the rear-illuminated eta of 3.93% achieved for the same type of device, employing the dark-colored I-/I-3(-) electrolyte.