Koen Vandewal: Inter-molecular charge-transfer states for organic opto-electronics

Intermolecular charge transfer (CT) states at the interface between electron-donating and electron-accepting (A) materials in organic thin films are characterized by absorption and emission bands within the optical gap of the interfacing materials. Depending on the used donor and acceptor materials, CT states can be very emissive, or generate free carriers at high yield. The former can result in rather efficient organic light emitting diodes (OLED), via thermally activated delayed fluorescence, while the latter property is exploited in organic photovoltaic (OPV) devices and photodetectors. In this contribution, we will discuss the fundamental properties of CT states and link them to OLED, OPV and photodetector device performance. We will discuss the influence of intra- and inter-molecular properties, such as the energy of the CT state, the electronic coupling between electron donor and acceptor, the molecular reorganization energy as well as non-radiative triplet states on radiative and non-radiative free carrier recombination. Furthermore, we introduce a new device concept, using an optical cavity resonance effect to boost CT absorption at photon energies below the optical gap of both donor and acceptor, enabling narrow-band, near infrared (NIR) photo-detection. Our findings imply that the power conversion efficiency of organic photovoltaics and maximum achievable detectivities for organic NIR detectors are limited by the presence of high energy vibrational modes and electron-phonon coupling.