Accurate determination of HOMO energies in conjugated diketopyrrolopyrrole-based polymers for predicting the open-circuit voltage of organic photovoltaic devices

Diketopyrrolopyrrole (DPP) polymers have extensively been used as electron donor materials in organic solar cells. For a diverse set of 19 DPP based polymers we have determined (i) the highest-occupied molecular orbital (HOMO) energies, derived from the oxidation potential as obtained for thin films from square-wave voltammetry (SWV), and (ii) the ionization potential as obtained from ultraviolet photoelectron spectroscopy (UPS), and from density functional theory (DFT) calculations. The empirical relationship obtained from our combined SWV-UPS study between the oxidation and ionization potential shows a slope for DPP polymers which is significantly lower than reported before for other organic semiconductors, while the intercept is the same as reported before. The HOMO energies, that span a range of −5.4 to −4.5 eV, as derived using SWV are found to show an excellent correlation with the open-circuit voltage of optimized solar cells in which the polymer studied forms a blend with PCBM, with deviations of at most 0.1 eV. A weaker correlation was found with the HOMO energies as obtained from UPS and DFT. Hence, the oxidation potential from SQV is a better empirical predictor for the open-circuit of the solar cell.