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We present a study on the performance and analysis of hybrid solar cells comprising a planar heterojunction between between a conjugated donor polymer, P3HT or PCPDTBT, and hydrogenated amorphous silicon (a-Si:H). A comparison of the modeled absorption spectra of the layer stack with the measured external quantum efciency is used to investigate the contribution of the inorganic and organic material to the photocurrent generation in the device. Although both materials contribute to the photocurrent, the devices exhibit poor quantum e ciencies and low short circuit currents. Bandstructure simulations of the hybrid layer structure reveal that an unfavorable electric eld distributionwithin the planarmultilayer structure limits the performance. Using electroabsorption measurements we can show that the electric eld is extremelyweak in the amorphous siliconbut strong in the organicmaterial. The situation changes drasticallywhen the conjugated polymer is p-doped. Doping not only increases the conductivity of the organic material, but also restores the electric eld in the amorphous silicon layer. Optimized hybrid solar cells comprising thin doped P3HT layers exhibit energy conversion e ciencies (ECE) up to 2.8 %. || S. Schaefer, S. Albrecht, D. Neher: Universitat Potsdam, Institute of Physics and Astronomy, Soft Matter Physics, D-14476 Potsdam, Germany T. F. Schulze, E. Conrad, L. Korte, B. Rech: Department of Silicon Photovoltaics, Helmholtz Center Berlin for Materials and Energy, Kekulestr. 5, D-12489 Berlin, Germany J. Wordenweber, A. Gordijn: IEK5-Photovoltaik, Forschungszentrum Julich, D-52425 Julich, Germany U. Scherf, I. Dumsch: Bergische Universitat Wuppertal, Macromolecular Chemistry and Institute for Polymer Technology, GaussStrasse 20, D-42097 Wuppertal, Germany