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This work was funded by the EU Horizon 2020 MSC ITN “TADFlife” (Grant Agreement No. 812872) as well as by Deutsche Forschungsgemeinschaft (DFG, project no. Br 1728/20-1). ZZ acknowledges the financial support from Chinese Scholarship Council (CSC, 201606890009) for his PhD studies. Thermally-activated delayed fluorescence (TADF) emitters—just like phosphorescent ones—can in principle allow for 100% internal quantum efficiency of organic light-emitting diodes (OLEDs), because the initially formed electron-hole pairs in the non-emissive triplet state can be efficiently converted into emissive singlets by reverse intersystem crossing. However, as compared to phosphorescent emitter complexes with their bulky—often close to spherical—molecular structures, TADF emitters offer the advantage to align them such that their optical transition dipole moments (TDMs) lie preferentially in the film plane. In this report, we address the question which factors control the orientation of TADF emitters. Specifically, we discuss how guest-host interactions may be used to influence this parameter and propose an interplay of different factors being responsible. We infer that emitter orientation is mainly governed by the molecular shape of the TADF molecule itself and by the physical properties of the host—foremost, its glass transition temperature Tg and its tendency for alignment being expressed, e.g., as birefringence or the formation of a giant surface potential of the host. Electrostatic dipole-dipole interactions between host and emitter are not found to play an important role. Publisher PDF

The unique optical properties of lead halide perovskites have drawn significant attention towards their application in light emitting devices (LEDs) in recent years. While quantum yield, emission wavelength and stability are already in the focus of many research groups, the orientation of the emissive transition dipole moments (TDM) has rarely been investigated. As known from other thin film applications such as organic LEDs, this quantity can severely affect the light outcoupling of the device and thereby limit the external quantum efficiency. In this work, we investigate CsPbBr 3 nanoplatelets of variable thickness and determine the orientation of their TDMs from thin film radiation pattern analysis. We then apply optical simulations to elucidate the performance limits of perovskite based blue LEDs in prototypical device architectures. We find that with increasingly beneficial horizontal orientation, the maximum efficiency achievable increases to values close to 30%. However, since the photoluminescence quantum efficiency degrades considerably for decreasing thickness, the overall device efficiency does not significantly improve. Thus, for the currently available material sets we can conclude that while for nanocubes the non-ideal orientation limits device performance, devices with nanoplatelets are limited by non-optimal photoluminescence quantum yields.