Over the years, organic optoelectronics have evolved into a mature technology with a wide range of applications. Their building blocks, namely organic semiconductors, are distinctly different from ...their inorganic counterparts due to orientational degrees of freedom that offer unique possibilities for tailoring their properties in thin‐film structures. In this article, the many facets that molecular orientation has for fundamental aspects, like film growth as well as optical and electrical behavior, are reviewed and the implications of molecular orientation for device application are discussed.
In this article, the many facets of molecular orientation in organic optoelectronics are reviewed. Their building blocks are distinctly different from their inorganic counterparts due to orientational degrees of freedom that offer unique possibilities for tailoring their properties in thin‐film structures. The authors elucidate different orientational properties, tailored measurements, and discuss the implications for device application.
A simplified state model and associated rate equations are used to extract the reverse intersystem crossing and other key rate constants from transient photoluminescence measurements of two high ...performance thermally activated delayed fluorescence materials. The values of the reverse intersystem crossing rate constant are in close agreement with established methods, but do not require a priori assumption of exponential decay kinetics, nor any additional steady state measurements. The model is also applied to measurements at different temperatures and found to reproduce previously reported thermal activation energies for the thermally activated delayed fluorescence process. Transient absorption measurements provide independent confirmation that triplet decay channels (neglected here) have no adverse effect on the fitting.
We report controllable anisotropic light emission of photons originating from vertically aligned transition dipole moments in spun-cast films of CsPbBr3 nanocubes. By depositing films of nanocrystals ...on precoated substrates we can control the packing density and resultant radiation pattern of the emitted photons. We develop a technical framework to calculate the average orientation of light emitters, i.e., the angle between the transition dipole moment vector (TDM) and the substrate. This model is applicable to any emissive material with a known refractive index. Theoretical modeling indicates that oriented emission originates from an anisotropic alignment of the valence band and conduction band edge states on the ionic crystal lattice and demonstrates a general path to model the experimentally less accessible internal electric field of a nanosystem from the photoluminescent anisotropy. The uniquely accessible surface of the perovskite nanoparticles allows for perturbation of the normally isotropic emissive transition. The reported sensitive and tunable TDM orientation and control of emitted light will allow for applications of perovskite nanocrystals in a wide range of photonic technologies inaccessible to traditional light emitters.
Alignment of the emissive transition dipoles in organic guest–host systems is a powerful tool to enhance light outcoupling from organic light-emitting diodes. Here, we study the orientation mechanism ...of four nonpolar dyes in neat films and dye-doped guest–host systems with varying emitter concentrations. We find that the key factors controlling their alignment in thermally evaporated thin films are their shape anisotropy and the ratio between the temperature of the substrate, on which the film is grown, and the glass transition temperature of the guest–host system. However, care has to be taken when a mixed cohost system with largely different glass transition temperatures (T g) of its constituents is used. In this case, the emitter orientation may not follow the effective T g,mix of the mixed host. In addition, we suggest using the principal moments of inertia of a molecule for judging its resilience toward reorientation, and the derived aspect ratio to characterize its shape anisotropy.
It is demonstrated that dipolar doping of hole transport layers (HTLs) controls the density and polarity of the accumulated charge at the critical interface between the HTL and the emission layer ...(EML) in organic light‐emitting diodes (OLEDs). Dipolar doping enables spontaneous orientation polarization (SOP) even in nonpolar HTL, and consequently compensates for the negative interface charge originating from the SOP of the adjacent layer. This concept is applied to a phosphorescent OLED, where bis‐4(N‐carbazolyl)phenylphosphine oxide (BCPO) is employed as a polar dopant for the HTL. The net interface charge is completely compensated at ≈29.5% of doping and further doping even facilitates the positive interface charge. The luminescence loss due to triplet‐polaron quenching is observed for both hole and electron accumulations, and it is suppressed by reducing the net interface charge density. On the other hand, the carrier balance factor linearly decreases with increasing doping ratio of BCPO. The results suggest that besides the energy level offset, SOP and permanent dipole moment of the materials should also be taken into account for realizing efficient carrier blocking interfaces. Dipolar doping is a versatile tool to tune charge accumulation, and to study its influence on device performance as well as the role of SOP in OLEDs.
A method to control the density and polarity of the accumulated charge at the critical interface in organic light‐emitting diodes is demonstrated. The luminescence loss due to triplet‐polaron quenching is suppressed by reducing the net interface charge density. Impacts of spontaneous orientation polarization on the charge blocking ability are also demonstrated.
Colloidal cesium lead halide perovskite nanocrystals exhibit unique photophysical properties including high quantum yields, tunable emission colors, and narrow photoluminescence spectra that have ...marked them as promising light emitters for applications in diverse photonic devices. Randomly oriented transition dipole moments have limited the light outcoupling efficiency of all isotropic light sources, including perovskites. In this report we design and synthesize deep blue emitting, quantum confined, perovskite nanoplates and analyze their optical properties by combining angular emission measurements with back focal plane imaging and correlating the results with physical characterization. By reducing the dimensions of the nanocrystals and depositing them face down onto a substrate by spin coating, we orient the average transition dipole moment of films into the plane of the substrate and improve the emission properties for light emitting applications. We then exploit the sensitivity of the perovskite electronic transitions to the dielectric environment at the interface between the crystal and their surroundings to reduce the angle between the average transition dipole moment and the surface to only 14° and maximize potential light emission efficiency. This tunability of the electronic transition that governs light emission in perovskites is unique and, coupled with their excellent photophysical properties, introduces a valuable method to extend the efficiencies and applications of perovskite based photonic devices beyond those based on current materials.
Spontaneous orientation polarization (SOP) of amorphous organic semiconducting films has attracted much attention because of its frequent observation in common organic light-emitting diodes (OLEDs) ...and potential influences on the device properties of OLEDs. On the other hand, the formation mechanism of SOP has been controversial for a long time, ever since its discovery in 2002. Recently, the formation mechanism of SOP was explained in terms of the surface equilibration mechanism of vapor-deposited glasses, and the understanding of SOP has progressed significantly. Based on the improved understanding, some active control methods of SOP have been demonstrated and further influences on the device performance of OLEDs were revealed, suggesting that higher efficiency can be achieved by managing SOP properly. Furthermore, some applications of SOP have also been proposed, such as a self-assembled electret and a tool for evaluating materials properties. In this paper, recent progress in the understanding of SOP and its applications to devices are reviewed.
•Recent progress in understanding of spontaneous orientation polarization (SOP) in organic semiconducting films is reviewed.•Formation mechanism and active control methods of SOP are discussed.•Influences of SOP on the device performance of organic light-emitting diodes are discussed.•Applications of SOP are introduced.
Due to their thin amourphous structure, unique electrical properties, and the associated variety of possible applications, OLEDs can now be found in smartphones, TVs, laptops, and wearables. While ...already big steps have been made in optimizing and understanding the properties influencing the external quantum efficiency (EQE), there is still room for improvement, especially when it comes to finding design principles for new emitter complexes. One contributer to the EQE here is the molecular orientation of the emitter in a given host matrix. In this work we study the viability of using molecular modeling approaches in sampling these emitter orientations for a set of already published homoleptic Ir carbene emitters and a set of emitter materials synthesized at Merck KGaA, Darmstadt, Germany, comprising both homoleptic and heteroleptic Ir(ppy)3 derivatives. We combine these simulations with different measurements for the orientation parameter and EQE, all performed with the same material stack under the same conditions. We observe a good agreement between simulation and experiment and find that the horizontal orientation of emitter molecules seems to be the main factor contributing to a higher EQE.
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