Acknowledgements
The authors thank Jacek Gasiorowski, Philipp Stadler and
Christoph Ulbricht for fruitful discussions and suggestions. The
work was financially supported by the Austrian Science Foundation
(FWF Project S 9711-N20). C. Yumusak also gratefully
acknowledges the financial support of European Science Foundation
(ESF Organisolar Project) for her scientific visit to Linz
Institute for Organic Solar Cells (LIOS)