Microscopic models for self-trapped excitons in α-PTCDA

The three slowest radiative recombination channels observed in photoluminescence (PL) spectra of α-PTCDA single crystals can be assigned to radiative recombination between two oppositely charged molecules in different relative positions, and to an excimer in a stack geometry. For both kinds of self-trapped excitons, we construct dimer geometries involving changes of the inter-molecular distance and internal deformations of each molecule. The Stokes shift resulting from self-trapping along the inter-molecular separation is analysed with a combination of second-order Møller-Plesset theory (MP2) and configuration interaction of singles (CIS), and the red shift arising from the internal deformations with time-dependent density functional theory (TD-DFT). The combination of the observed PL energies with these microscopic calculations of the Stokes shift allows an assignment of the lowest charge transfer transition in the undeformed crystal. Along the stacking direction, it occurs at 2.06 eV, far below previous estimates based on micro-electrostatic calculations.