Enhanced photovoltaic performance with co-sensitization of quantum dots and an organic dye in dye-sensitized solar cells

CdSe quantum dots of two different sizes exhibiting a maximum emission at 495 nm (CdSe495) and 545 nm (CdSe545) were combined with di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,2-bipyridyl-4,4-dicarboxylato) ruthenium(II) (N719) or 2-cyano-3-{5-[7-(4-diphenylamino-phenyl) benzo[1,2,5]thiadiazol-4-yl]-thiophen-2-yl}-acrylic acid (TBTCA) resulting in four novel hybrid organic-inorganic sensitizers, which were used in the fabrication of dye-sensitized solar cells. The results showed that with N719, both the CdSe dots decreased the power conversion efficiencies when compared to a standard device consisting only of N719 as the sensitizer. With the organic dye TBTCA, CdSe545 showed no significant effect, while CdSe495 interacted favorably, leading to a 25% increase in power conversion efficiency compared to a device sensitized solely by TBTCA. Studies on excited-state lifetimes of N719 in the presence of CdSe did not distinguish between energy and/or charge transfer mechanisms. On the other hand, time correlated single photon counting experiments on the photoelectrodes suggest that the advantages due to the CdSe495-TBTCA combination could be ascribed to FRET from quantum dots to the organic dye and to a further contribution, as suggested by IPCE spectra, consisting of electron transfer via cascade from the LUMO level of TBTCA to CdSe495 to TiO2, which produces a higher flux of electrons in the external circuit.