V oc from a Morphology Point of View: the Influence of Molecular Orientation on the Open Circuit Voltage of Organic Planar Heterojunction Solar Cells

The film morphology and device performance of planar heterojunction solar cells based on the molecular donor material α-sexithiophene (6T) are investigated. Planar heterojunctions of 6T with two different acceptor molecules, the C60 fullerene and diindenoperylene (DIP), have been prepared. The growth temperature of the 6T bottom layer has been varied between room temperature and 100 °C for each acceptor. By means of X-ray diffraction and X-ray absorption, we show that the crystallinity and the molecular orientation of 6T is influenced by the preparation conditions and that the 6T film templates the growth of the subsequent acceptor layer. These structural changes are accompanied by changes in the characteristic parameters of the corresponding photovoltaic cells. This is most prominently observed as a shift of the open circuit voltage (V oc): In the case of 6T/C60 heterojunctions, V oc decreases from 0.4 to 0.3 V, approximately, if the growth temperature of 6T is increased from room temperature to 100 °C. By contrast, V oc increases from about 1.2 V to almost 1.4 V in the case of 6T/DIP solar cells under the same conditions. We attribute these changes upon substrate heating to increased recombination in the C60 case while an orientation dependent intermolecular coupling seems to change the origin of the photovoltaic gap in the DIP case.