Decoupling Charge Transport and Electroluminescence in a High Mobility Polymer Semiconductor

Effects of Molecular Encapsulation on the Photophysical and Charge Transport Properties of a Naphthalene Diimide Bithiophene Copolymer

Engineering the molecular structure of conjugated polymers is key to advancing the field of organic electronics. In this work, we synthesized a molecularly encapsulated version of the naphthalene diimide bithiophene copolymer PNDIT2, which is among the most popular high charge mobility organic semiconductors in n-type field-effect transistors and non-fullerene acceptors in organic photovoltaic blends. The encapsulating macrocycles shield the bithiophene units while leaving the naphthalene diimide units available for intermolecular interactions. With respect to PNDIT2, the encapsulated counterpart displays an increased backbone planarity. Molecular encapsulation prevents preaggregation of the polymer chains in common organic solvents, while it permits π-stacking in the solid state and promotes thin film crystallinity through an intermolecular-lock mechanism. Consequently, n-type semiconducting behavior is retained in field-effect transistors, although charge mobility is lower than in PNDIT2 due to the absence of the fibrillar microstructure that originates from preaggregation in solution. Hence, molecularly encapsulating conjugated polymers represent a promising chemical strategy to tune the molecular interaction in solution and the backbone conformation and to consequently control the nanomorphology of casted films without altering the electronic structure of the core polymer.

Molecular Regulation of Polymeric Raman Probes for Ultrasensitive Microtumor Diagnosis and Noninvasive Microvessle Imaging

Raman imaging is a powerful tool for the diagnosis of cancers and visualization of various biological processes. Polymers possessing excellent biocompatibility are promising probes for Raman imaging. However, few polymers are reported to serve as Raman probes for in vivo imaging, mainly due to the intrinsic weak Raman signal intensity and fluorescence interference of these polymers. Herein, a poly(indacenodithiophene-benzothiadiazole) (IDT-BT) polymer is presented, which emits unprecedentedly strong Raman signals under the near-infrared wavelength (785 nm) excitation, thus functioning as a Raman probe for ultrasensitive in vivo Raman imaging. Further mechanistic studies unveil that the unique Raman feature of the IDT-BT polymer relies on molecularly regulating its absorbance edge adjacent to the desired excitation wavelength, thus avoiding fluorescence interference and simultaneously emitting strong Raman scattering under preresonant excitation. Taking advantage of this discipline, the IDT-BT polymeric probe successfully realizes intraoperative Raman imaging of micrometastasis as small as 0.3 mm × 0.3 mm, comparable to the most sensitive Raman probes currently reported. Impressively, the IDT-BT enables noninvasive microvascular imaging, which is not achieved using other Raman probes. This work opens a new avenue toward the development of polymeric Raman probes for in vivo Raman imaging.

Fluorescence band exchange narrowing in a series of squaraine oligomers: energetic vs. structural disorder

The influence of oligosquaraine chain length on the energies and shape of absorption and emission bands and the exciton coherence length is studied in CHCl₃ where the oligomers adopt a random coil structure. From the observed fluorescence band narrowing an effective coherence length of N_coh = 2.5 was estimated for the nonamer. Applying a theoretical Frenkel exciton model the absorption and emission spectra were simulated which confirmed the experimental results. From the relative amplitude of the 00 peak to the vibronic shoulder the coherence length was estimated which yields a somewhat higher saturation value of N_coh≈ 3 for the nonamer, which is in very good agreement with the theoretical amplitude ratio. The coherence length is much smaller than the geometrical length because the electronic delocalisation is reduced by structural disorder. Taking into account the energetic (diagonal) and structural (off-diagonal) disorder we observed a different influence on the absorption and fluorescence spectra. For the emission spectra, exciton delocalisation leads to a narrowing of the band caused by averaging over energetic disorder, but for the absorption band the spectra are broadened by excitonic splitting and structural disorder.

Non-toxic near-infrared light-emitting diodes

Harnessing cost-efficient printable semiconductor materials as near-infrared (NIR) emitters in light-emitting diodes (LEDs) is extremely attractive for sensing and diagnostics, telecommunications, and biomedical sciences. However, the most efficient NIR LEDs suitable for printable electronics rely on emissive materials containing precious transition metal ions (such as platinum), which have triggered concerns about their poor biocompatibility and sustainability. Here, we review and highlight the latest progress in NIR LEDs based on non-toxic and low-cost functional materials suitable for solution-processing deposition. Different approaches to achieve NIR emission from organic and hybrid materials are discussed, with particular focus on fluorescent and exciplex-forming host-guest systems, thermally activated delayed fluorescent molecules, aggregation-induced emission fluorophores, as well as lead-free perovskites. Alternative strategies leveraging photonic microcavity effects and surface plasmon resonances to enhance the emission of such materials in the NIR are also presented. Finally, an outlook for critical challenges and opportunities of non-toxic NIR LEDs is provided.

Ultrafast Energy Transfer Dynamics in a Squaraine Heterotriad

A squaraine heterotriad consisting of three different covalently linked squaraine chromophores was synthesized, and its absorption spectra were interpreted in terms of Kasha's exciton coupling theory. Using the exciton couplings derived from model dyads (ca. 700 cm^-1) as the input, we were able to predict the exciton state energies of the heterotriad. Transient absorption spectroscopy with femtosecond time resolution showed that excitation of the highest exciton state populates a state mainly localized at one terminal squaraine chromophore, and energy transfer to the lowest exciton state localized at the other terminal squaraine occurs within 30 fs. Field-induced surface hopping dynamics simulations support the assumption of ultrafast energy transfer. Moreover, they show the close relationship between internal conversion and energy transfer in the intermediate to weak coupling regime. The latter is a consequence of excitation localization caused by molecular vibrations.

Performance Limits of an Alternating Current Electroluminescent Device

The use of an alternating current (AC) voltage is a simple, versatile method of producing electroluminescence from generic emissive materials without the need for contact engineering. Recently, it was shown that AC-driven, capacitive electroluminescent devices with carbon nanotube network contacts can be used to generate and study electroluminescence from a variety of molecular materials emitting in the infrared-to-ultraviolet range. Here, performance trade-offs in these devices are studied through comprehensive device simulations and illustrative experiments, enhancing understanding of the mechanism and capability of electroluminescent devices based on alternating as opposed to direct current (DC) schemes. AC-driven electroluminescent devices can overcome several limitations of conventional DC-driven electroluminescent devices, including the requirement for proper alignment of material energy levels and the need to process emitting materials into uniform thin films. By simultaneously optimizing device geometry, driving parameters, and material characteristics, the performance of these devices can be tuned. Importantly, the turn-on voltage of AC-driven electroluminescent devices approaches the bandgap of the emitting material as the gate oxide thickness is scaled, and internally efficient electroluminescence can be achieved using low-mobility single-layer emitter films with varying thicknesses and energy barrier heights relative to the contact.

Chain Coupling and Luminescence in High-Mobility, Low-Disorder Conjugated Polymers

Optoelectronic devices based on conjugated polymers often rely on multilayer device architectures, as it is difficult to design all the different functional requirements, in particular the need for efficient luminescence and fast carrier transport, into a single polymer. Here we study the photophysics of a recently discovered class of conjugated polymers with high charge carrier mobility and low degree of energetic disorder and investigate whether it is possible in this system to achieve by molecular design a high photoluminescence quantum yield without sacrificing carrier mobility. Tracing exciton dynamics over femtosecond to microsecond time scales, we show that nearly all nonradiative exciton recombination arises from interactions between chromophores on different chains. We evaluate the temperature dependence and role of electron-phonon coupling leading to fast internal conversion in systems with strong interchain coupling and the extent to which this can be turned off by varying side chain substitution. By sterically decreasing interchain interaction, we present an effective approach to increase the fluorescence quantum yield of low-energy gap polymers. We present a red-NIR-emitting amorphous polymer with the highest reported film luminescence quantum efficiency of 18% whose mobility concurrently exceeds that of amorphous-Si. This is a key result toward the development of single-layer optoelectronic devices that require both properties.

Energy Transfer-Assisted Whispering Gallery Mode Lasing in Conjugated Polymer/Europium Hybrid Microsphere Resonators

Lanthanide metal complexes display luminescence with narrow bandwidth. Here, we present coupling of the luminescence from europium ion (Eu³⁺ ) with whispering gallery modes (WGMs) in conjugated polymer microsphere resonators. Self-assembly of fluorene-terpyridine alternating copolymer, coordinated by Eu³⁺ (F8tpy-Eu³⁺ ), forms well-defined microspheres with an average diameter of 3.2 μm. Upon focused laser excitation, a microsphere of F8tpy copolymer displays WGM photoluminescence (PL) at a wide spectral range from 420 to 680 nm. In contrast, F8tpy-Eu³⁺ hybrid microspheres exhibit sharp WGM PL at a narrow spectral range of 615-630 nm, which is characteristic of luminescence from Eu³⁺ . The PL behavior indicates that photoinduced energy transfer from F8tpy to Eu³⁺ occurs efficiently. Furthermore, the intensity of the PL peak increases nonlinearly upon strong pumping, indicating that a lasing action appears with the threshold of 1.85 mJ cm^-2 . These results will pave the way for developing microlasers and photonic devices from soft organic materials.

fs–ps Exciton dynamics in a stretched tetraphenylsquaraine polymer

A squaraine polymer shows surprisingly fast light induced energy transfer between two different structural sections on the ps/fs time scale.

Organic Semiconductor Single Crystals for Electronics and Photonics

Organic semiconducting single crystals (OSSCs) are ideal candidates for the construction of high-performance optoelectronic devices/circuits and a great platform for fundamental research due to their long-range order, absence of grain boundaries, and extremely low defect density. Impressive improvements have recently been made in organic optoelectronics: the charge-carrier mobility is now over 10 cm² V^-1 s^-1 and the fluorescence efficiency reaches 90% for many OSSCs. Moreover, high mobility and strong emission can be integrated into a single OSSC, for example, showing a mobility of up to 34 cm² V^-1 s^-1 and a photoluminescence yield of 41.2%. These achievements are attributed to the rational design and synthesis of organic semiconductors as well as improvements in preparation technology for crystals, which accelerate the application of OSSCs in devices and circuits, such as organic field-effect transistors, organic photodetectors, organic photovoltaics, organic light-emitting diodes, organic light-emitting transistors, and even electrically pumped organic lasers. In this context, an overview of these fantastic advancements in terms of the fundamental insights into developing high-performance organic semiconductors, efficient strategies for yielding desirable high-quality OSSCs, and their applications in optoelectronic devices and circuits is presented. Finally, an overview of the development of OSSCs along with current challenges and future research directions is provided.