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Limited stability of blue quantum dot light-emitting diodes (QLEDs) under current stress impedes commercialization. Multi-layer structures of the state-of-the-art blue QLEDs pose significant difficulty in the fundamental understanding of degradation mechanisms. Here, by applying transient electroluminescence measurements, we disentangle charge transport in both pristine and degraded blue QLEDs. By varying thicknesses of the charge transport layers and the emissive layer, respectively, we show that the charge transport in pristine QLEDs is primarily dominated by holes. Furthermore, the degradation of QLEDs under electrical stress is governed by the decrease of hole transport in the emissive quantum dot layer due to the formation of hole traps. [ABSTRACT FROM AUTHOR]

Precision graphene nanoribbons (GNRs) offer distinctive physicochemical properties that are highly dependent on their geometric topologies, thereby holding great potential for applications in carbon‐based optoelectronics and spintronics. While the edge structure and width control has been a popular strategy for engineering the optoelectronic properties of GNRs, non‐hexagonal‐ring‐containing GNRs remain underexplored due to synthetic challenges, despite offering an equally high potential for tailored properties. Herein, we report the synthesis of a wavy GNR (wGNR) by embedding periodic eight‐membered rings into its carbon skeleton, which is achieved by the A2B2‐type Diels–Alder polymerization between dibenzocyclooctadiyne (6) and dicyclopenta[e,l]pyrene‐5,11‐dione derivative (8), followed by a selective Scholl reaction of the obtained ladder‐type polymer (LTP) precursor. The obtained wGNR, with a length of up to 30 nm, has been thoroughly characterized by solid‐state NMR, FT‐IR, Raman, and UV/Vis spectroscopy with the support of DFT calculations. The non‐planar geometry of wGNR efficiently prevents the inter‐ribbon π–π aggregation, leading to photoluminescence in solution. Consequently, the wGNR can function as an emissive layer for organic light‐emitting electrochemical cells (OLECs), offering a proof‐of‐concept exploration in implementing luminescent GNRs into optoelectronic devices. The fast‐responding OLECs employing wGNR will pave the way for advancements in OLEC technology and other optoelectronic devices. [ABSTRACT FROM AUTHOR]

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Organic emitters that exhibit room‐temperature phosphorescence (RTP) in neat films have application potential for optoelectronic devices, bio‐imaging, and sensing. Due to molecular vibrations or rotations, the majority of triplet excitons recombine rapidly via non‐radiative processes in purely organic emitters, making it challenging to observe RTP in amorphous films. Here, a chemical strategy to enhance RTP in amorphous neat films is reported, by utilizing through‐space charge‐transfer (TSCT) effect induced by intramolecular steric hindrance. The donor and acceptor groups interact via spatial orbital overlaps, while molecular motions are suppressed simultaneously. As a result, triplets generated under photo‐excitation are stabilized in amorphous films, contributing to phosphorescence even at room temperature. The solvatochromic effect on the steady‐state and transient photoluminescence reveals the charge‐transfer feature of involved excited states, while the TSCT effect is further experimentally resolved by femtosecond transient absorption spectroscopy. The designed luminescent materials with pronounced TSCT effect show RTP in amorphous films, with lifetimes up to ≈40 ms, comparable to that in a rigid polymer host. Photoluminescence afterglow longer than 3 s is observed in neat films at room temperature. Therefore, it is demonstrated that utilizing intramolecular steric hindrance to stabilize long‐lived triplets leads to phosphorescence in amorphous films at room temperature. [ABSTRACT FROM AUTHOR]

Already in 2012, Blom et al. reported (Nature Materials 2012, 11, 882) in semiconducting polymers on a general electron-trap density of ≈3 × 1017 cm−3, centered at an energy of ≈3.6 eV below vacuum. It was suggested that traps have an extrinsic origin, with the water-oxygen complex [2(H2O)-O2] as a possible candidate, based on its electron affinity. However, further evidence is lacking and the origin of universal electron traps remained elusive. Here, in polymer diodes, the temperature-dependence of reversible electron traps is investigated that develop under bias stress slowly over minutes to a density of 2 × 1017 cm−3, centered at an energy of 3.6 eV below vacuum. The trap build-up dynamics follows a 3rd-order kinetics, in line with that traps form via an encounter between three diffusing precursor particles. The accordance between universal and slowly evolving traps suggests that general electron traps in semiconducting polymers form via a triple-encounter process between oxygen and water molecules that form the suggested [2(H2O)-O2] complex as the trap origin. IMPACT STATEMENT: Formation of universal electron traps in polymer light-emitting diodes is a dynamic process that occurs via a slow triple-encounter between trap precursor species, with the water-oxygen [2(H2O)-O2] complex as a likely candidate. [ABSTRACT FROM AUTHOR]

Although quantum dot light‐emitting diodes (QLEDs) are extensively studied nowadays, their charge transport mechanism remains a subject of ongoing debate. Here, the hole transport in blue quantum dots (QDs) (CdZnSe/ZnSe/ZnS/CdZnS/ZnS based) is investigated by combining current‐voltage and transient electroluminescence measurements. The study demonstrates that the hole transport in QD thin films is characterized by a trap‐free space‐charge‐limited current with a zero‐field room temperature mobility of 4.4 × 10−11 m2 V−1 s−1. The zero‐field hole mobility is thermally activated with an activation energy of 0.30 eV. Applying the Extended Gaussian Disorder model provides a consistent description of the QD hole current as a function of voltage and temperature. The QD hole mobility is characterized by a hopping distance of 2.8 nm in a Gaussian broadened density of states with a width of 0.12 eV. [ABSTRACT FROM AUTHOR]

Thermally activated delayed fluorescence (TADF) emitters potentially can provide organic light‐emitting diodes with 100% internal quantum efficiency by harvesting triplet excitons. Generally, TADF emitters are small molecules that are not applicable for solution processability. The addition of tert‐butyl groups to the periphery of TADF emitters has proven to improve their solubility in various organic solvents, reduce aggregation‐induced quenching, and enhance the photoluminescence quantum yield (PLQY). This article studies the photophysical influence of the tert‐butyl group attached to an emitter with a carbazole acceptor and a triazine donor. The resulting t3CzTrz‐F is a blue–green TADF emitter, in which the addition of a tert‐butyl group increases the rate of reverse intersystem crossing (rISC), while simultaneously decreasing the nonradiative decay rate substantially. In addition, dilution of t3CzTrz‐F in a host matrix in film results in an enhanced PLQY, which is associated with a decrease in the nonradiative decay constant, while there is no change in the rISC rate. Through a solid‐state NMR study, the change in rISC and nonradiative rate upon tert‐butylation by enlarged intermolecular spacing and reduced vibrational and rotational freedom is rationalized, resulting in improved photophysical performance. [ABSTRACT FROM AUTHOR]

For understanding the operation of perovskite solar cells and light-emitting diodes, knowledge of the dielectric properties is indispensable. The dielectric properties of perovskites are frequency dependent due to the presence of moving ions, which complicates the interpretation of impedance spectra. Using Au/CsPbI2Br/Au capacitors with varied layer thickness as a model system, we demonstrate that in the dark, an extended Maxwell circuit consistently describes the impedance data. From the thickness dependence of the resistivities, both the electronic and ionic conductivities are obtained, whereas the combination of electronic and ionic capacitances with the characteristic frequencies for space-charge formation determines the ion diffusion coefficient and ion density. At low frequencies, a slow transient process with a fixed time constant of ∼0.1 s occurs, governed by the electronic conductivity, being independent of illumination strength and sample thickness. As a possible mechanism, we propose the spatial reorganization of ions within the ion accumulation layer at the electrode/perovskite interface. [ABSTRACT FROM AUTHOR]

The conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is of great interest for a variety of applications in flexible and stretchable electronics, due to its high conductivity, optical transparency, mechanical properties, commercial availability and biocompatibility. However, to advance novel applications on conformable substrates, the development of innovative and reliable methods for processing conductive polymers is vital. Here, a fast, dependable and easy PEDOT:PSS transfer-printing method onto flexible, transparent and biodegradable poly(vinylalcohol) (PVA):glycerol targets is presented. In contrast to previous work, this method neither depends on acid, nor doping, nor surface soaking treatments. Furthermore, this approach is compatible with common strategies to induce stretchability and increase the conductivity of PEDOT:PSS. To ensure the applicability to next-generation flexible and stretchable electronics, the electrical, morphological and adhesion properties before and after printing are characterized. Additionally, a straightforward methodology to pattern transfer-printable PEDOT:PSS structures is established, using an O2-plasma treatment. Furthermore, the water-soluble targets allow for environmental-friendly disposal. [ABSTRACT FROM AUTHOR]

Intermolecular interactions in π-stacked chromophores strongly influence their photophysical properties, and thereby also their function in photonic applications. Mixed electronic and vibrational coupling interactions lead to complex potential energy landscapes with competitive photophysical pathways. Here, we characterize the photoexcited dynamics of the small molecule semiconductor copper pthalocyanine (CuPc) in solution and in thin film, the latter comprising two different π-stacked architectures, α-CuPc and β-CuPc. In solution, CuPc undergoes ultrafast intersytem crossing (ISC) to the triplet excited state. In the solid state, both α-CuPc and β-CuPc morphologies exhibit a mixing between Frenkel and charge-transfer excitons (Frenkel–CT mixing). We find that this mixing influences the photophysical properties differently, based on morphology. In addition to ISC, α-CuPc demonstrates symmetry-breaking charge transfer, which furthermore depends on excitation wavelength. This mechanism is not observed in β-CuPc. These results elucidate how molecular organization mediates the balance of competitive photexcited decay mechanisms in organic semiconductors. [ABSTRACT FROM AUTHOR]