A DFT COMPUTATIONAL STUDY OF STRUCTURALLY RELATED POLY-THIOPHENES USED IN PHOTOVOLTAIC SYSTEMS

The electric properties of a large number of different complex composed systems (with particular focus on interfacial systems) are rationalized within a “Lego-like sum approach” of the electronic properties of the single components. Such a general modelistic approach span extremely different worlds: from Mott-Schottky barrier to Tafel plots in electrochemical systems [1,2]. In particular, in the field of organic semiconductors exploited to assemble photovoltaic devices, the open circuit potential is rationalized on the basis of the reciprocal HOMO LUMO energy differences between the donor and the acceptor partners [3]. Although such an approach seems by far too much crude in its strategy, the straight comparison of HOMO LUMO MOs energy levels, of the donor and acceptor building blocks, is yet the most widespread tools exploited in the modelization of photovoltaic organic based system [4]. In this work a series of seven thiophene based polymers (donors) are considered, whose structure was designed (both by introduction of ring structures of various chemical nature in the polymeric backbone structure – heteropolymers – and also by various lateral alkyl chains) so to suitably tune the electronic properties of the [6,6]-Phenyl C61 butyric acid methyl ester, often addressed as PCBM, (acceptor) and obtained a satisfactory solubility. Examples of the polymers structure are shown in Chart 1. The electronic properties of the mono-, di-, tri-meric oligomer species are reckoned and compared to experimental spectroscopic and electrochemical results.