Charge carrier generation and drift dynamics have been investigated in two types of dye:fullerene heterojunctions: vacuum-deposited merocyanine:C60 and solution-processed merocyanine:PC61BM blends by combining electric-field-induced fluorescence quenching and ultrafast time-resolved carrier drift measurements. We demonstrate that interfacial charge transfer (CT) states are strongly heterogeneous with energies dependent on the acceptor material and its domain sizes. Interfacial CT states on large C60 domains have low energies, while CT states on PC61BM domains have larger energies, which are weakly dependent on the domain sizes. We distinguish two interfacial CT state dissociation pathways: (i) ultrafast, weakly dependent on the electric field and (ii) slow field-assisted dissociation during entire CT state lifetime. We attribute process i to low-energy, weakly bound CT states on large fullerene domains and process ii to strongly bound CT states on small domains or single fullerene molecules. The electron mobility in films with 50% of C60 is several times higher than in the films with PC61BM and orders of magnitude higher than the hole mobility. We conclude that efficient carrier generation at low electric fields typical for operating solar cells relies on unperturbed motion of highly mobile electrons; thus, fast motion and extraction of electrons are crucial for efficient solar cells.

Excited-state relaxation processes in neat merocyanine MD376 films and their blends with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) have been investigated by means of steady-state and time-resolved absorption and fluorescence spectroscopy. Formation of collective states with intermolecular charge transfer character (CTM–M) during the initial several picoseconds after excitation determines the excited-state dynamics of neat MD376 films. Blending of MD376 with PCBM leads to quenching of its intrinsic fluorescence and to the appearance of a new fluorescence band caused by interfacial charge transfer states between MD376 and PCBM molecules (CTM–F). Generation of free charge carriers in films with high PCBM concentration causes quenching of all fluorescent states.

We report a joint theoretical and experimental study of the electronic properties of triphenylene based polycyclic aromatic hydrocarbons. Their aggregation tendency is suppressed by phenyl- or diphenylamino-substitution. The influence of the substituents on the absorption properties is investigated by time-dependent density functional theory (TD-DFT). Upon chemical modification of the triphenylene core, the singlet–triplet energy gap can be reduced by up to 0.4 eV. This prediction is spectroscopically verified. As a demonstration of the potential of these materials, Ir(III) doped phosphorescent organic light-emitting diodes (OLEDs) are tested, and limits of the performance are investigated. We achieve efficiencies above 30 Cd/A for simple, green-emitting two layer devices.Graphical abstractHighlights► We model influence of substitution on electronic properties of triphenylene based polycyclic aromatic hydrocarbons. ► Predictions are verified by spectroscopy. ► The singlet–triplet energy gap is tuned by up to 0.4 eV. ► Efficient Ir(III) doped phosphorescent organic light-emitting diodes (OLEDs) are demonstrated.