Optical properties of oligothiophene substituted diketopyrrolopyrrole derivatives in solid phase : joint J and H-type aggregation

Photophysical properties of diketopyrrolopyrrole derivatives substituted with oligothiophenes are investigated. All compounds are found to be fluorescent both in solution and in the solid phase. At low temperature in the solid, fluorescence originates from excimer-like excited states. Comparison of absorption and fluorescence excitation spectra taken under matrix isolated conditions and on solid films show the presence of both J- and H-type absorption bands in the solid phase. Quantum-chemical calculations, including exciton-phonon coupling to account for deviations from the Born-Oppenheimer approximation, are performed to simulate the band shape of the lowest absorption band in the molecular solid. The joint presence of J- and H-bands is explained by the presence of two molecules in the unit cell. The Davydov splitting is substantial for molecules with linear alkyl substituents on the nitrogen atom (on the order of 0.2 eV) but can be reduced to almost zero by introducing branching at the ß-carbon of the alkyl side chain

Optical properties of oligothiophene substituted diketopyrrolopyrrole derivatives in solid phase : joint J and H-type aggregation

Photophysical properties of diketopyrrolopyrrole derivatives substituted with oligothiophenes are investigated. All compounds are found to be fluorescent both in solution and in the solid phase. At low temperature in the solid, fluorescence originates from excimer-like excited states. Comparison of absorption and fluorescence excitation spectra taken under matrix isolated conditions and on solid films show the presence of both J- and H-type absorption bands in the solid phase. Quantum-chemical calculations, including exciton-phonon coupling to account for deviations from the Born-Oppenheimer approximation, are performed to simulate the band shape of the lowest absorption band in the molecular solid. The joint presence of J- and H-bands is explained by the presence of two molecules in the unit cell. The Davydov splitting is substantial for molecules with linear alkyl substituents on the nitrogen atom (on the order of 0.2 eV) but can be reduced to almost zero by introducing branching at the ß-carbon of the alkyl side chain

Energetics of Electron\u2013Hole Separation at P3HT/PCBM Heterojunctions

The energetics of electronhole separation at the prototypical donor-acceptor interface P3HT/PCBM is investigated by means of a combination of molecular dynamics simulations, quantum-chemical methods, and classical microelectrostatic calculations. After validation against semiempirical Valence Bond/Hartree-Fock results, microelectrostatic calculations on a large number of electron-hole (e-h) pairs allowed a statistical study of charge separation energetics in realistic morphologies. Results show that charge separation is an energetically favorable process for about 50% of interfacial e-h pairs, which provides a rationale for the high internal quantum efficiencies reported for P3HT/PCBM heterojunctions. Three effects contribute to overcome the Coulomb attraction between electron and hole: (i) favorable electrostatic landscape across the interface, (ii) electronic polarization, and (iii) interface-induced torsional disorder in P3HT chains. Moreover, the energetic disorder due to the PCBM polar group is shown to play a key role in increasing the dissociation probability