Bandgap-Determination from Autoionization Data in Molecular Crystals

Abstract

An interpretation of action spectra for intrinsic photo-conduction in crystalline anthracene and tetracene is presented. It is shown (i) that the primary dissociation probability of a molecular excited state does not depend on photon energy as long as its energy exceeds the energy of the chargetransfer state and (ii) that excess energy of the excited state is dissipated during thermalization of the generated electron-hole pair in course of a randomwalk process. The energy balance for the complete autoionization process allows precise determination of the band-gap. The values E , = 4.00 &0.02 for anthracene and E , = 3.11 1 0 . 0 3 eV for tetracene are obtained.

Modeling the electronic structure of organic materials: a solid-state physicist’s perspective

“…It is interesting to notice that both quantities vary by an almost identical amount in comparison to their values in gas phase. The fundamental gap becomes 3.95 eV, in excellent agreement with available measurements for transport gaps in ANT [128][129][130]. It is worth noting that this good match is enabled by the low dispersion of the bands in the ANT crystal (see figure 1(c)), which is in turn an consequence of the localized character of electronic states of the constituents.…”

“…The computed band-gap, 4.30 eV, indicates a substantial renormalization with respect to result obtained for the isolated molecule (6.94 eV, see figure 1(a)) as a consequence of the screening exerted by the molecules forming the crystal [127]. Notice that the aforementioned value, 4.30 eV, overestimates the experimental band gap of the ANT crystal, ranging between 3.90 and 4.00 eV [128][129][130]. We can ascribe this discrepancy to the fact that the band structure reported in figure 1(c) is obtained adopting the GGA in DFT, with the QP correction to the band gap included by means of an empirical scissors operator, see details in the appendix.…”