Impact of Polarization Effect on Exciton Binding Energies and Charge Transport for the Crystals of Chlorinated ITIC Derivatives

Exciton binding energy (Eb) and carrier mobility are the key parameters to determine organic photovoltaic performance. Here, the impact of polarization effect on the Eb and carrier mobility in the crystals of different end -group-chlorinated ITIC isomers have been investigated by the self-consistent quantum mechanics/embedded charge method. In contrast to large values and small fluctuation for the gas phase Eb, the Eb in the solid crystals is substantially decreased and ranges from 0.08 to 0.36 eV due to important charge polarization and distinct molecular packing. Because of the counteraction between the electrostatic effect of the hole and electron, a good linear relationship is found between the Eb and the charge induction term. Accordingly, the nonequivalent molecules in each crystal have similar Eb. However, their ionization potential and electron affinity can be much changed due to different electrostatic effects, leading to appreciable site energy differences. Thus, the carrier mobility is decreased to varying degrees, up to 3 orders of magnitude. Remarkably, benefiting from a three-dimensional dense packing, alpha-ITIC-2Cl possesses the smallest Eb and the highest electron mobility. These results underline that chlorination is an effective way to tune molecular packing to simultaneously reduce Eb and improve carrier mobility toward high-efficiency organic photovoltaics.