Comparison between theoretical and experimental electronic properties of some popular donor polymers for bulk-heterojunction solar cells

The theoretical estimation of energy levels and energy gaps of conjugated polymers for polymer solar cells represents in principle an useful tool for an a priori screening of new donor systems. Fourteen polymers, whose energy gaps vary in the range 1.2–3.1 eV have been selected and their HOMO, LUMO and gap energies have been calculated by applying Density Functional Theory methods. In spite of the variety of the examined molecular structures, nice correlations between theoretical and experimental electronic parameters were found. In particular, optical gaps and, to a lesser extent, electrochemical gap very well correlate with theoretical gaps, while for the other parameters (oxidation and reduction potentials) the general trend is reproduced. It is shown that, in general, the theoretical energies of the base repeating units have values close to the experimental energies, but the linear fittings are better when the theoretical data of much longer chains are considered: infinitely long chains must be used to predict the optical gaps, while long oligomers are more appropriate to estimate the electrochemical properties. Criteria and relationships for the prediction of energy data from theoretical ones are provided.