Like in their inorganic counterparts, the working principle of organic light-emitting diodes relies on the radiative recombination of injected electron–hole pairs in a solid, a process termed electroluminescence. In detail, however, the nature of electronic excitations and the structural peculiarities of organic molecular materials lead to marked differences concerning their opto-electronic properties. This holds true particularly for emitter materials in OLEDs, where spin-selection rules and molecular orientation are important parameters in determining their efficiency of converting electrical current into visible light. Our group is developing methods to analyze and improve OLED efficiency by combining experimental studies with numerical simulation. Recent research highlights have been the discovery of non-isotropic emitter orientation in molecular guest-host systems and its implementation in phosphorescent OLEDs as well as devices based on thermally activated delayed fluorescence (TADF).

Owing to the excitonic nature of photoexcitations in organic semiconductors, the working mechanism of organic solar cells relies on the donor-acceptor (D/A) concept enabling photoinduced charge transfer at the interface between two organic materials with suitable energylevel alignment. However, the introduction of such a heterojunction is accompanied by additional energy losses compared to an inorganic homojunction cell due to the presence of a charge-transfer (CT) state at the D/A interface. We are investigating the interrelation of film structure and morphology, electronic structure at interfaces as well as charge and exciton transport properties of a number of prototypical molecular D/A pairs and their performance in organic photovoltaic cells (OPVs). In particular, we develop quantitative methods to extract information on the mechanisms of charge generation and recombination in OPVs as well as their energy losses determining the overall power conversion efficiency.