The preparation of 27 different derivatives of C60 and C70 fullerenes possessing various aryl (heteroaryl) and/or alkyl groups that are appended to the fullerene cage via a cyclopropane moiety and their use in bulk heterojunction polymer solar cells is reported. It is shown that even slight variations in the molecular structure of a compound can cause a significant change in its physical properties, in particular its solubility in organic solvents. Furthermore, the solubility of a fullerene derivative strongly affects the morphology of its composite with poly(3‐hexylthiophene), which is commonly used as active material in bulk heterojunction organic solar cells. As a consequence, the solar cell parameters strongly depend on the structure and the properties of the fullerene‐based material. The power conversion efficiencies for solar cells comprising these fullerene derivatives range from negligibly low (0.02%) to considerably high (4.1%) values. The analysis of extensive sets of experimental data reveals a general dependence of all solar cell parameters on the solubility of the fullerene derivative used as acceptor component in the photoactive layer of an organic solar cell. It is concluded that the best material combinations are those where donor and acceptor components are of similar and sufficiently high solubility in the solvent used for the deposition of the active layer.

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In order to study the influence of the organic solar cell device layout on the photovoltaic parameters, we systematically varied its geometry. By knowledge of all sheet resistances in the device, we were able to correlate the series resistance with the geometry of the device using a simple model for its calculation. Deviations between experiment and calculation could be related with the solar cell geometry and understood by postulating curved transport ways of the current within the largely resistive ITO‐layer. Thus, a further refinement of the calculation is required in order to minimize the deviation between calculation and experiment. Short solar cell lengths and ITO‐bridges yield minimal series resistance and best conversion efficiency. A schematic cross‐section of a polymer solar cell displays the current flow through the device. The corresponding contributions to the series resistance are depicted as well.

During the last decade polymer solar cells have undergone a steady increase in overall device efficiency. To date, essential efficiency improvements of polymer–fullerene solar cells require the development of new materials. Whilst most research efforts aim at an improved or spectrally extended absorption of the donor polymer, not so much attention has been paid to the fullerene properties themselves. We have investigated a number of structurally related fullerenes, in order to study the relationship between chemical structure and resulting polymer–fullerene bulk heterojunction photovoltaic properties. Our study reveals a clear connection between the fullerene solubility as material property on one hand and the solar cells short circuit photocurrent on the other hand. The tendency of the less soluble fullerene derivates to aggregate was accounted for smaller current densities in the respective solar cells. Once a minimum solubility of approx. 25 mg/ml in chlorobenzene was overcome by the fullerene derivative, the short circuit current density reached a plateau, of about 8–10 mA/cm2. Thus the solubility of the fullerene derivative directly influences the blend morphology and displays an important parameter for efficient polymer–fullerene bulk heterojunction solar cell operation. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)