Role of structural order at the P3HT/C60 heterojunction interface

•We explored the influence of crystal domain size on the band alignment of P3HT/C60.•We estimated the potential energy barrier at the heterojunction interface.•We identified the energy levels involve in the optical transition in the junction with TD-DFT.

The influence of structural order on the electronic and optical properties of C60/P3HT bulk heterojunctions (BHJs) was studied using first principles DFT and TDDFT methods. The electronic levels alignment between the two phases in the BHJs is mainly controlled by the interfacial dipole moment that shifts the P3HT electronic levels towards higher binding energies with respect to C60 levels. An increasing order translates into an increasing P3HT domains size, for which we considered different stacks of P3HT oligomers. A significant decrease of both the electronic (HOMOP3HT–LUMOC60) and the optical (HOMOP3HT–LUMOP3HT) band gap is observed with an increasing P3HT domain size. TDDFT approach was used to identify the orbitals involved in the electronic transitions, and to reveal that the reduction of the BHJ optical band gap cannot simply be predicted from the variation of the rrP3HT band gap. The lowest electronic transition in rrP3HT becomes optically forbidden due to the formation of H-aggregates.

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