In organic light‐emitting diodes (OLEDs), horizontal orientation of the emissive transition dipole moment (TDM) can improve light outcoupling efficiency by up to 50% relative to random orientation. Therefore, there have been extensive efforts to identify drivers of horizontal orientation. The aspect ratio of the emitter molecule and the glass‐transition temperature (Tg) of the films are currently regarded as particularly important. However, there remains a paucity of systematic studies that establish the extent to which these and other parameters control orientation in the wide range of emitter systems relevant for state‐of‐the‐art OLEDs. Here, recent work on molecular orientation of fluorescent and thermally activated delayed fluorescent emitters in vacuum‐processed OLEDs is reviewed. Additionally, to identify parameters linked to TDM orientation, a meta‐analysis of 203 published emitter systems is conducted and combined with density‐functional theory calculations. Molecular weight (MW) and linearity are identified as key parameters in neat systems. In host–guest systems with low‐MW emitters, orientation is mostly influenced by the host Tg, whereas the length and MW of the emitter become more relevant for systems involving higher‐MW emitters. To close, a perspective of where the field must advance to establish a comprehensive model of molecular orientation is given.

color-tunable emission, they are particularly attractive for applications in self-emissive flat-panel displays. [2,3] Due to the compatibility of OLED technology with low-cost production approaches, such as solutionbased printing methods, OLEDs are also considered for large-area, thin, lightweight, and glare-free ambient illumination. [4-7] In the future, commercial devices may also exploit the fact that OLEDs are compatible with a range of different substrates, including mechanically flexible ones. [8-10] There has also been interest in adapting OLED technology for applications beyond the display and lighting sector, e.g., for biomedical use or optical communications. [11-16] Meanwhile, a number of other material platforms, e.g., perovskites and quantum dots, quickly gain prominence for thin-film LED light-sources, and each of these have specific benefits and challenges when compared to OLED technology. [17-20] In order to compare different materials and device architectures, reliable and accurate measurements of device efficiency are crucial. Yet, there are a number of difficulties associated with this, which mainly arise from the extended emissive area of OLEDs and other thin-film LEDs, from the use of transparent and often light-guiding substrates, and from the fact that the angular emission characteristics can vary drastically between devices. Despite decades of research into OLEDs, the community still uses a number of different measurement techniques and there is no universal standard. [21] Up to this date, a significant number of publications report efficiency estimates from oversimplified measurements and a substantial fraction of these appear to overestimate the real device efficiency. Here, we describe the implementation of a goniometer-based measurement setup that can record the electroluminescence spectrum of an OLED or other LED with extended emissive area under different angles to accurately determine their emission characteristics and efficiency. We explain in detail the design, assembly, and alignment of the goniometer and the procedure used to extract the device efficiency from the measured data. We provide examples that illustrate how the angle-resolved measurement leads to a significantly more accurate efficiency value, compared to, e.g., just recording the intensity emitted in the forward direction. Our motivation for using an angleresolved method over other approaches, in particular over a measurement based on an integrating sphere, is that it is more flexible and less prone to calibration artefacts. In addition, the The accurate characterization of thin-film light emitting diodes (LEDs)-including organic light emitting diodes (OLEDs), perovskite LEDs, and quantum dot LEDs-is crucial to the understanding of the factors that influence their efficiency and thus to the fabrication of LEDs with improved performance and stability. In addition, detailed information about the angular characteristics of LED emission is useful to assess the suitability of individual architectures, e.g., for display ap...

Single-molecule experiments in organic solvents provide new insight into the conformational dynamics of conjugated polymers.