Lately, a lot of attention has been paid to metal oxide-organic hybrid solar cells. In these devices, conjugated polymers replace the typically transparent hole transporter as usually used in solid-state dye-sensitized solar cells in order to maximize the photon absorption efficiency. However, to unleash the full potential of hybrid solar cells it is imperative to push the photocurrent contribution of the absorbing polymer. The tremendous success of dye-sensitized solar cells and organic

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... ltaic as low-cost alternative to conventional silicon-based photovoltaic has led to the introduction of hybrid metal oxide-polymer solar cells. 1 Hybrid solar cells combine high surface area n-type metal oxide electrodes with hole transporting conjugated polymers. Most commonly, a thin film of sintered TiO 2 nanoparticles with diameters around 20 nm is used as n-type electrode, similar to the TiO 2 nanostructures employed in dye-sensitized solar cells. Frequently, ZnO is used instead of TiO 2 , which is chemically less stable but exhibits similar energy levels and typically shows higher charge carrier mobilities. Besides particle films, also ordered nanostructures such as nanowire and nanotube arrays have been employed. The nanostructured metal oxide film can be decorated with a self-assembled monolayer of dye, which enables rapid electron injection into the metal oxide upon photon absorption. Alternatively, the metal oxide can be coated with a thin layer of an inorganic absorber like crystalline Sb 2 S 3 or CdSe. 2 As polymeric compound of a hybrid solar cell, materials known from organic photovoltaic are employed, most commonly poly(3-hexylthiophene) (P3HT). Metal oxide-polymer hybrid solar cells have reached power conversion efficiencies close to 4% with dye-sensitized TiO 2 and 6.2% with Sb 2 S 3 as absorber coating on TiO 2 nanoparticle films. 3 These efficiencies are impressive considering that hybrid solar cells are a relatively new technology. However, compared to dye-sensitized solar cells, which have reached efficiencies well above 12%, and organic polymer-fullerene solar cells with close to 10% there appears to be room for drastic improvements. 4 In dye-sensitized solar cells the TiO 2 forms a highly connected network, which allows the charges to be transported to the electrodes after electron injection from the excited dye. However, the overall absorption is limited unless very thick TiO 2 electrodes are employed. Infiltration of the hole transporter into the TiO 2 electrode is challenging, which limits its thickness to a few μm. Higher extinction coefficients are provided by the active films of organic solar cells, which consist of a blended structure of conjugated polymers and fullerenes. However, the thickness of the active layers usually has to be about 100 nm in order to allow loss-free charge transport through the active layer due to its uncontrolled internal morphology. The hybrid solar cell concept combines the advantages of dye-sensitized and organic solar cells, i.e., consistent charge transport pathways through a nanostructured metal oxide electrode and high extinction coefficients due to absorbing polymers as hole transporters. However, their full potential has not been unleashed. The main reason is that the advantage of the polymeric hole transporter is not exhausted yet. In many cases the polymer serves only as hole transporter and does a Author to whom correspondence should be addressed. Electronic