Parameters influencing charge separation in solid-state dye-sensitized solar cells using novel hole conductors

Solid-state dye-sensitized solar cells employing a solid organic hole-transport material (HTM) are currently under intensive investigation, since they offer a number of practical advantages over liquid-electrolyte junction devices. Of particular importance to the design of such devices is the control of interfacial charge transfer. In this paper, the factors that determine the yield of hole transfer at the dye/HTM interface and its correlation with solid-state-cell performance are identified. To this end, a series of novel triarylamine type oligomers, varying in molecular weight and mobility, are studied. Transient absorption spectroscopy is used to determine hole-transfer yields and pore-penetration characteristics. No correlation between hole mobility and cell performance is observed. However, it is found that the photocurrent is directly proportional to the hole-transfer yield. This hole-transfer yield depends on the extent of pore penetration in the dye-sensitized film as well as on the thermodynamic driving force Delta G(dye-HTM) for interfacial charge transfer. Future design of alternative solid-state HTMs should focus on the optimization of pore-filling properties and the control of interfacial energetics rather than on increasing material hole mobilities.