The electronic structure of rubrene single crystals was studied by angle-resolved ultraviolet photoelectron spectroscopy. A clear energy dispersion of the highest occupied molecular orbital-derived band was observed, and the dispersion width was found to be 0.4 eV along the well-stacked direction. The effective mass of the holes was estimated to be 0.65(+/-0.1)m0. The present results suggest that the carrier conduction mechanism in rubrene single crystals can be described within the framework of band transport.

Influence of the direction of spontaneous orientation polarization on the charge injection properties of organic light-emitting diodes

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 C 60 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/C 60 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 C 60 case while an orientation dependent intermolecular coupling seems to change the origin of the photovoltaic gap in the DIP case.

A tris(7-propyl-8-hydroxyquinolinato) aluminum [Al(7-Prq) 3 ] film shows negative giant surface potential (GSP) because of spontaneous orientation polarization. The polarity of this film is opposite to those of tris-(8-hydroxyquinolate) aluminum films. In Al(7-Prq) 3-based organic light-emitting diodes, negative GSP leads to the positive interface charge and governs the electron injection and accumulation properties. In addition, a high resistance to the electron injection at the Al(7-Prq) 3 /Ca interface is suggested possibly because of the negative polarization charge at the interface. These results show the importance of orientation polarization in controlling the charge injection and accumulation properties and potential profile of the resultant devices. V

We investigate different parameters influencing the occurrence of s-shaped current voltage (j-V) characteristics in planar heterojunction organic solar cells. It is shown how substrate modification, purity of the active organic material as well as variation of the top contact can affect the shape of the j-V curves. The studies are performed on vacuum-evaporated planar heterojunction solar cells with diindenoperylene (DIP) as electron donor and fullerene C 60 as acceptor. The focus is on the fill factor and forward current being the most direct indicators for s-shapes in j-V curves. We find that the main effect of substrate heating during film growth can be assigned to changes in energy barriers rather than to the modification of morphology and crystallinity, which is also influenced by elevated substrate temperatures. The decisive role of the barrier height between the anode work function and the HOMO (i.e., highest occupied molecular orbital) level of the donor is approved by comparing hole-injection layers with different work functions. By using donor materials of different purity we find a correlation between charge carrier mobilities and fill factors. Finally, it is demonstrated that an exciton blocking interlayer is essential to get high fill factors when aluminum is used as top contact, but is dispensable for samarium as cathode material. This finding can be ascribed to the protective effect of the interlayer from aluminum diffusion into the active semiconductor rather than to its role as exciton diffusion barrier. V