Dresden 2006 – scientific programme

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HL: Halbleiterphysik

HL 32: Photovoltaic

HL 32.7: Talk

Wednesday, March 29, 2006, 16:00–16:15, BEY 118

Efficiency limitations in dye-sensitized solar cells with ionic liquids — •F. Einsele¹, M. Hlusiak¹, U. Rau¹, R. Sastrawan², R. Kern², and A. Hinsch² — ¹Institut für Physikalische Elektronik, Universität Stuttgart, Pfaffenwaldring 47, 70569 Stuttgart — ²Fraunhofer Institut für Solare Energiesystem (ISE), Heidenhofstraße 2, 79110 Freiburg

Dye-sensitized solar cells (DSCs) consist of a layer of nanoporous TiO₂. A monolayer of dye molecules covers the TiO₂ nanoparticles. Photoexcitation of the dye and electron transfer into the conduction band of the TiO₂ makes up the primary photovoltaic action. The I⁻/I₃⁻ redox couple of an electrolyte (EL) oxidizes the dye molecule. Standard high-efficiency DSCs use an EL with highly volatile acetonitrile as a solvent. One major research goal is the replacement of this EL by a non-volatile EL. Here the use of ionic liquids is a major option. Unfortunately, the efficiencies, especially the short circuit current densities j_SC, obtained with these EL so far are much lower than those of the standard devices. The present contribution investigates possible limitations to j_SC by means of electrical measurements. We find that the diffusion constant D_T of I₃⁻ in ionic liquids is 1.8x10⁻⁷ cm²/s, i.e. about two orders of magnitude smaller than 1.2x10⁻⁵ cm²/s in acetonitrile. A further limitation is provided by the TiO₂ network and in an additional light-scattering ZrO₂ layer. We observe that the effective diffusion constant D_eff in the TiO₂ network is about 40% of the bulk value and reduces to 16% in the ZnO₂ layer. A simple model unveils that the maximum achievable short circuit current density j_SC,max is about 8 to 10 mA/cm² in this case.