n‐Type—electron transporting—conduction is demonstrated for Schottky diodes and field effect transistors based on N,N′‐diphenyl‐3,4,9,10‐perylenetetracarboxylic‐dimide. The role of the electron‐injecting electrode is discussed, as are the potential improvements in performance offered by devices combining a p‐type semiconducting layer with an n‐type, electron transporting organic compound.

The full UV-visible dielectric tensor and the corresponding directions of the principal axes of triclinic tetracene crystals are reported as deduced either by polarized absorption and ellipsometry measurements or by calculations based on the molecular and crystallographic data. The results allow the attribution of the polarized bands observed in both absorption and photoluminescence emission spectra. In particular, the spectral line shape and polarization of the emission are found to depend on the sample thickness, and the effect is attributed to the modification of the state of polarization of the emitted light during its propagation inside the crystal. Indeed, the directions of polarization of the lowest optical transitions and the directions of the principal axes of the dielectric tensor are demonstrated not to coincide, in contrast to the assumptions typically made in the literature, thus causing the mixed transverse/longitudinal character of light propagation.

Exciton-phonon (EP) coupling in molecular aggregates is reexamined in cases where extended intermolecular interactions result in low-energy excitons with high effective masses. The analysis is based on a single intramolecular vibrational mode with frequency omega0 and Huang-Rhys factor lambda2. When the curvature Jc at the exciton band bottom is much smaller than the free-exciton Davydov splitting W, the strength of the EP coupling is determined by comparing the nuclear relaxation energy lambda2omega0 with the curvature. In this way, weak (lambda2omega0<<4piJc), intermediate I (lambda2omega0 approximately 4piJc), and strong I (lambda2omega0>>4piJc) coupling regimes are introduced. The conventional intermediate (lambda2omega0 approximately W) and strong (lambda2omega0>>W) EP coupling regimes originally defined by Simpson and Peterson [J. Chem. Phys. 26, 588 (1957)] are based solely on the Davydov splitting and are referred to here as intermediate II and strong II regimes, respectively. Within the intermediate I and strong I regimes the near degeneracy of the low-energy excitons allows efficient nonadiabatic coupling, resulting in a spectral splitting between the b- and ac-polarized first replicas in the vibronic progression characterizing optical absorption. Such spectral signatures are clearly observed in OT4 thin films and crystals, where splittings for the lowest energy mode with omega0=161 cm(-1) are as large as 30 cm(-1) with a small variation due to sample disorder. Numerical calculations using a multiphonon BO basis set and a Hamiltonian including linear EP coupling yield excellent agreement with experiment.

International audienceThe optical response of rubrene crystals is described by providing the full UV-VIS dielectric tensor. Consistently with the molecular and crystal symmetries, the lowest crystal transition originates from an Au molecular transition, it is polarized along the c axis (normal to the larger crystal face), and it is the origin of the emission, which is self-guided towards the edge where the corresponding polarization and intensity angular distribution are detected. By contrast, the Bu molecular transitions give rise to Davydov states described by the other two components of the diagonal dielectric tens