Optimizing the Energy O ff set between Dye and Hole-Transporting Material in Solid-State Dye-Sensitized Solar Cells

The power-conversion efficiency of solid-state dye-sensitized solar cells can be optimized by reducing the energy offset between the highest occupied molecular orbital (HOMO) levels of dye and hole-transporting material (HTM) to minimize the loss-in-potential. Here, we report a study of three novel HTMs with HOMO levels slightly above and below the one of the commonly used HTM 2,2′,7,7′tetrakis(N,N-di-p-methoxyphenylamino)-9,9′-spirobifluorene (spiro-OMeTAD) to systematically explore this possibility. Using transient absorption spectroscopy and employing the ruthenium based dye Z907 as sensitizer, it is shown that, despite one new HTM showing a 100% hole-transfer yield, all devices based on the new HTMs performed worse than those incorporating spiro-OMeTAD. We further demonstrate that the design of the HTM has an additional impact on the electronic density of states present at the TiO2 electrode surface and hence influences not only holebut also electron-transfer from the sensitizer. These results provide insight into the complex influence of the HTM on charge transfer and provide guidance for the molecular design of new materials.