Calculation of solid state molecular ionisation energies and electron affinities for organic semiconductors

Energy level engineering has become one of the central ideas in organic optoelectronics; particularly in the field of organic photovoltaics. However, this paradigm has led to significant ambiguities and errors in the literature. To investigate and address these issues we calculate solid state ionisation energies and electron affinities of a range of organic molecular semiconductors from density functional theory and the polarizable continuum model. We show that the differences between these results and measurements from (inverse) photoemission are on the same scale as the differences between the measured values reported by different groups. We compare our results with in vacuo calculations and estimates of the ionisation energies and electron affinities from the eigenvalues of the Kohn–Sham equations. In vacuo calculations overestimate the ionisation energies measured in the solid state, but underestimate solid state electron affinities. However, the Kohn–Sham eigenvalues predict the measured ionisation energies nearly as well as the full calculation. However, we show that the apparent accuracy of the Kohn–Sham eigenvalues is fortuitous and arises from the cancellation of the errors due to the use of Kohn–Sham molecular orbital energies as predictions of ionisation energies and electron affinities and the neglect of the polarizable solid state environment. These results stress the importance of descriptions based on molecular states rather than molecular orbitals when designing and characterising materials for organic electronic and optoelectronic devices.

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► Ionisation energies (electron affinities) are not the same as HOMO (LUMO) energies.
► The solid-state environment reduces the IE and increases the EA.
► Calculating the IE and EA with the inclusion of the effect of the environment yields results that are within the experimental error of experimental results.
► The apparent success of standard HOMO/LUMO calculations results from a fortuitous cancellation of errors.