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Experimental and theoretical electronic structure of quinacridone

The energy positions of frontier orbitals in organic electronic materials are often studied experimentally by (inverse) photoemission spectroscopy and theoretically within density functional theory. However, standard exchange-correlation functionals often result in too small fundamental gaps, may lead to wrong orbital energy ordering, and do not capture polarization-induced gap renormalization. Here, we examine these issues and a strategy for overcoming them by studying the gas phase and bulk electronic structure of the organic molecule quinacridone (5Q), a promising material with many interesting properties for organic devices. Experimentally, we perform angle-resolved photoemission spectroscopy (ARUPS) on thin films of the crystalline $β$-phase of 5Q. Theoretically, we employ an optimally-tuned range-separated hybrid functional (OT-RSH) within density functional theory. For the gas phase molecule, our OT-RSH result for the ionization potential (IP) represents a substantial improvement over the semi-local PBE and the PBE0 hybrid functional results, producing an IP in quantitative agreement with experiment. For the bulk crystal, we take into account the correct screening in the bulk, using the recently developed OT-SRSH approach, while retaining the optimally-tuned parameters for the range-separation and the short-range Fock exchange. This leads to a band gap narrowing due to polarization effects and results in a valence band spectrum in excellent agreement with experimental ARUPS data, with respect to both peak positions and heights. Finally, full-frequency $G_0W_0$ results based on a hybrid functional starting point are shown to agree with the OT-SRSH approach, improving substantially on the PBE-starting point.