以昇陳

Persistent thermally activated delayed fluorescence and phosphorescence emission is achieved in single-component systems via utilizing the triplet exciton in the single compound. The dual persistent emission characteristics are tunable through the bromine atoms in different chemical environments and the organic luminophores are used for visual temperature detection and anti-counterfeiting.

Abstract

Organic persistent luminescent materials attract great attention, but most of them exhibit single luminescence color with one emission band, which limits their applications in optoelectronic, sensing, and bioimaging fields. Herein, an effective strategy to achieve dual persistent emission is proposed by utilizing the triplet excited state in the single-molecule 6,12-diphenyl-5,6,11,12-tetrahydroindolo[3,2-b]carbazole derivatives. Experimental data and theoretical calculations suggest that the triplet excited state operates as a triplet exciton reservoir to stockpile and provide long-lived excitons for room temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF) in aggregate state. Moreover, tunable emission color is achieved through the regulation of RTP and TADF emission by introducing aromatic and aliphatic bromine atoms. Finally, dual persistent emission with a long lifetime of 0.26 s and a persistent luminescence quantum yield of 10% is obtained in single-component materials, and these luminophores are used for visual temperature detection and anti-counterfeiting.

Nature Energy, Published online: 01 November 2021; doi:10.1038/s41560-021-00923-5

Organic solar cells processed from green solvents are easier to implement in manufacturing yet their efficiency is low. Chen et al. devise a guest molecule to improve the molecular packing, enabling devices with over 17% efficiency.

The similarities and differences in the charge carrier dynamics in organic solar cells and organic–inorganic hybrid metal halide perovskite solar cells, two leading technologies in thin-film photovoltaics, are discussed, linking these back to the intrinsic material properties of organic and perovskite semiconductors, and how these factors impact on photovoltaic device performance is elucidated.

Abstract

The charge carrier dynamics in organic solar cells and organic–inorganic hybrid metal halide perovskite solar cells, two leading technologies in thin-film photovoltaics, are compared. The similarities and differences in charge generation, charge separation, charge transport, charge collection, and charge recombination in these two technologies are discussed, linking these back to the intrinsic material properties of organic and perovskite semiconductors, and how these factors impact on photovoltaic device performance is elucidated. In particular, the impact of exciton binding energy, charge transfer states, bimolecular recombination, charge carrier transport, sub-bandgap tail states, and surface recombination is evaluated, and the lessons learned from transient optical and optoelectronic measurements are discussed. This perspective thus highlights the key factors limiting device performance and rationalizes similarities and differences in design requirements between organic and perovskite solar cells.

Three D–D type wide-bandgap donor polymers (PBDTT, PBDTT1Cl, and PBDTT2Cl) are designed and facilely synthesized. Organic solar cells (OSCs) based on PBDTT1Cl exhibit a high power conversion efficiency of 17% and a low nonradiative energy loss of 0.19 eV. In addition, PBDTT1Cl has a very low figure-of-merit and good universality, indicating its potential as a low-cost polymer donor for high-performance OSCs.

Abstract

Three regioregular benzodithiophene-based donor–donor (D–D)-type polymers (PBDTT, PBDTT1Cl, and PBDTT2Cl) are designed, synthesized, and used as donor materials in organic solar cells (OSCs). Because of the weak intramolecular charge-transfer effect, these polymers exhibit large optical bandgaps (>2.0 eV). Among these three polymers, PBDTT1Cl exhibits more ordered and closer molecular stacking, and its devices demonstrate higher and more balanced charge mobilities and a longer charge-separated state lifetime. As a result of these comprehensive benefits, PBDTT1Cl-based OSCs give a very impressive power conversion efficiency (PCE) of 17.10% with a low nonradiative energy loss (0.19 eV). Moreover, PBDTT1Cl also possesses a low figure-of-merit value and good universality to match with different acceptors. This work provides a simply and efficient strategy to design low-cost high-performance polymer donor materials.

Publication date: January 2022

Source: Materials Today Energy, Volume 23

Author(s): Wansi Li, Zhaoming Wang, Meng Zhang, Miao He, Si-Wei Zhang, Chen Yang, Yuan Wu, Jingzhou Li, Man-Chung Tang, Hongyan Fu, Guodan Wei

The combination of the dual roles of Mn²⁺ in generating red and near-infrared (NIR) emission and the efficient energy transfer from Ce³⁺ to Mn²⁺ in Lu₂BaAl₄SiO₁₂ garnet leads to an extra-broadband visible-NIR emitting phosphor for multifunctional light-emitting diode (LED) applications. This study demonstrates that Mn²⁺ with highly symmetric coordination can efficiently activate broadband NIR phosphors.

Abstract

The development of extra-broadband emitting phosphors is challenging but meaningful work. So far, however, phosphors that can be effectively excited by GaN-based blue light-emitting diode (LED) chips and emit from visible (VIS) to near-infrared (NIR) regions are still rare. Herein, this study designs an extra-broadband VIS-NIR emitting phosphor with emission band ranging from 460 nm to 880 nm (bandwidth >400 nm) upon 450 nm excitation, owing to an efficient energy transfer from Ce³⁺ to the red and NIR emitting Mn²⁺ ions in Lu₂BaAl₄SiO₁₂ (LBAS) host. By the analysis of extended X-ray absorption fine structure (EXAFS) spectra and fluorescence lifetimes, it is demonstrated that the NIR emission most probably originates from those Mn²⁺ occupying the dodecahedral sites with high symmetry rather than the exchange-coupled Mn²⁺-Mn²⁺ pairs. Furthermore, two single-phase phosphor-converted LEDs are fabricated by combining blue LEDs with LBAS:Ce³⁺,Mn²⁺ phosphors, and thanks to the extra-broadband emission, the resultant devices may realize multifunctional applications, such as in high-quality general lighting, NIR spectroscopy, and plant growth lighting.

An electron-withdrawing PZ-T unit is employed to incorporate into the PM6 polymer backbone as the third component, and a series of high-performance D-A₁-D-A₂ type terpolymers are synthesized by random copolymerization strategy. Among them, the PMZ-10:Y6-based polymer solar cells (PSCs) achieved an outstanding power conversion efficiency of 18.23%, which is the highest reported performance among the terpolymer-based PSCs so far.

Abstract

Recently, a random ternary copolymerization strategy has become a promising and efficient approach to develop high-performance polymer donors for polymer solar cells (PSCs). In this study, a low-cost electron-withdrawing unit, 2,5-bis(4-(2-ethylhexyl)thiophen-2-yl)pyrazine (PZ-T), is incorporated into the polymer backbone of PM6 as the third component, and three D-A₁-D-A₂ type terpolymers PMZ-10, PMZ-20, and PMZ-30 are synthesized by the random copolymerization strategy, with the PZ-T proportion of 10%, 20%, and 30%, respectively. The terpolymers exhibit downshifted highest occupied molecular orbital energy levels than PM6, which is beneficial for obtaining higher open-circuit voltage (V _oc) of the PSCs with the polymer as a donor. Importantly, the PSCs based on PMZ-10:Y6 demonstrate efficient exciton dissociation, higher and balanced electron/hole mobilities, desirable aggregation, and high power conversion efficiency of 18.23%, which is the highest efficiency among the terpolymer-based PSCs so far. The results indicate that the ternary copolymerization strategy with PZ-T as the second A-unit is an efficient approach to further improve the photovoltaic performance and reduce the synthetic cost of the D-A copolymer donors.

Thermally driven phase separation is found to improve the performance of solution-processed organic light-emitting diodes with an exciplex host:red phosphorescent light-emitting dendrimer emissive layer blend. The phase separated devices containing 2 wt% of the phosphorescent dendrimer are found to have maximum current, power, and external quantum efficiencies of 17.9 cd A⁻¹, 19.4 lm W⁻¹, and 14.8 ± 0.6%, respectively.

Abstract

Red-emitting organic light-emitting diodes (OLEDs) are important for displays and lighting, with the latter benefiting from solution processable materials, which would enable low embedded energy, scalable fabrication. Herein, the effect of annealing and phase separation on the performance of solution-processed OLEDs incorporating a light-emitting layer composed of the exciplex host, m-MTDATA:OXD-7, and a red phosphorescent light-emitting dendrimer, Ir(tD_Cpq)₃, is described. Solution-processed OLEDs containing an annealed emissive layer with a low dendrimer concentration (2 wt%) are found to have the best performance, which is higher than the device in which the light-emitting layer is not annealed. The improvement in the performance of the annealed device is ascribed to improved charge mobility within the emissive layer caused by phase separation of the OXD-7. The OLEDs containing annealed m-MTDATA:OXD-7:(2 wt%) Ir(tD_Cpq)₃ have maximum current, power, and external quantum efficiencies of 17.9 cd A⁻¹, 19.4 lm W⁻¹, and 14.8 ± 0.6%, respectively. The fact that the maximum external quantum efficiency (EQE) of 14.8% is larger than that expected based on the photoluminescence quantum yield (PLQY) and the normal out-coupling efficiency of 20% from a bottom-emitting device is determined to arise from the different pathways of exciton formation under photoexcitation and charge injection.

An effective light-deflecting pattern is introduced to nonfullerene organic solar cells to improve energy conversion efficiency in the blue region. The normal incidence light is guided into a cavity-like chamber and mostly captured by the active layer, resulting in broadband absorption enhancement. The optimized device based on all A-D-A type nonfullerene acceptors achieves the highest reported efficiency of approaching 18%.

Abstract

Using narrow bandgap nonfullerene acceptors (NFAs) can broaden the absorption spectrum of organic solar cells (OSCs) to the near-infrared region. However, the simultaneously decreased extinction coefficient of the active layer at the blue region results in inevitable light escaping and energy loss. Herein, a blazed grating-based device configuration consisting of a patterned rear electrode is employed to compensate for the low absorption of nonfullerene OSCs. Experimental results reveal that the normal incidence light, especially blue light, that bounces off the patterned rear electrode is concentrated in a large tilted angle and subsequently trapped in waveguide mode. Along with the excitation of surface plasmon polariton, the structured nonfullerene OSCs using a new-designed PM6:M36 active layer obtain the broadband absorption enhancement with 1.5 times increase at the blue region. The optimized device achieves an 8.95% increase in photocurrent and a champion power conversion efficiency of approaching 18%, which is the highest reported value among all the devices based on A-D-A type NFAs.

Optically switchable organic light-emitting diodes have been achieved by doping a diarylethene (DAE) derivative into the light-emitting polymer poly(9,9′-dioctylfluorene-alt-benzothiadiazole), F8BT. Switching is based on the reversible charge carrier trapping by the DAE.

Abstract

The design, fabrication, and characterization of optically switchable organic light-emitting diodes (OSOLEDs) based on the combination of the commercially available light-emitting polymer poly(9,9′-dioctylfluorene-alt-benzothiadiazole), F8BT, doped with a diarylethene derivative is reported. The photochromic activity of the dopant in the solid state has been investigated both via UV/vis absorption and photoluminescence spectroscopy, whereas the morphology of different blends is investigated via atomic force microscopy. OSOLEDs embedding dopant loadings of 1, 5, and 10 wt% exhibit optical responsivity with a maximum reversible optical threshold voltage shift of 4 V. The best performing devices containing 5 wt% dopant show a maximum current density and luminance ON/OFF ratio of ≈20 and ≈90, respectively. For the first time, the impact of the diarylethene isomerization on hole and electron transport has been decoupled and directly investigated, via the design, fabrication, and characterization of single-carrier switchable devices based on the same blends. Not only do these results confirm the photo-responsive trapping activity of the diarylethenes on both charge carriers, but they also demonstrate its asymmetry, with a predominant effect on electron transport that is over 3.4 times larger as compared to hole transport.

A 3D optical simulation study is reported that predicts average visible transmission and ideal J _SC values of semi-transparent organic photovoltaics (ST-OPVs) with porous silver nanowire top electrodes, thereby leading to the highest light utilization efficiency value in ST-OPVs reported to date.

Abstract

A key factor in improving semi-transparent organic photovoltaics (ST-OPVs) performance is achieving high light utilization efficiency (LUE). However, device performance can also be limited by the lack of understanding of light transmission and reflection within the device architecture, and the transmission of the top electrode in particular. Here, highly efficient ST-OPVs are reported via the rational design of silver nanowire (Ag NW) top electrodes via 3D optical simulation. Due to its careful consideration for the ST-OPV of the effect of the Ag NW networking structure, estimated average visible transmission (AVT) and ideal short-circuit current density values from 3D optical simulation closely match those from actual measurements. Optimized ST-OPVs with Ag NW porosity of 20% and active layer thickness of 150 nm exhibit LUE of 4.15% with a power conversion efficiency of 9.7% and AVT of 42.82%. This work achieves a record-high LUE in ST-OPVs reported to date and the first report introducing a 3D optical simulation study.

Alkyl-chain branching of non-fullerene acceptors flanking conjugated side-groups enables optimized optoelectronic and morphological properties, affording device performance of over 18%.

Abstract

Side-chain modifications of non-fullerene acceptors (NFAs) are essential for harvesting their full potential in organic solar cells (OSC). Here, an effective alkyl-chain-branching approach of the Y-series NFAs flanking meta-substituted phenyl side groups at the outer positions is demonstrated. Compared to BTP-4F-PC6 with linear m-hexylphenyl chains, two new acceptors named BTP-4F-P2EH and BTP-4F-P3EH are developed with bulkier alkyl chains branched at the β and γ positions, respectively. These branched chains result in altered molecular packing of the NFAs and afford higher open-circuit voltage of the devices. Despite the blue-shifted absorption of the branched-chain NFAs, their blends with PBDB-T-2F enable improved short-circuit current density for the corresponding devices owing to the more suitable phase separation and better exciton dissociation. Consequently, the OSCs based on BTP-4F-P2EH and BTP-4F-P3EH yield enhanced device performance of 18.22% and 17.57%, respectively, outperforming the BTP-4F-PC6-based ones (17.22%). These results highlight that the side-chain branching design of NFAs has great potential in optimizing molecular properties and promoting photovoltaic performance.

Homojunctions based on bipolar small π-conjugated molecules represent the ultimate stage of simplification of organic solar cells. Besides the fundamental questions posed by the possible direct photogeneration of charges–carriers and their transport, this concept can potentially contribute to the elimination of some of the major technical obstacles which limit the industrial scale development of organic photovoltaics.

Abstract

Single-material organic solar cells (SMOSCs) are on the forefront of research on organic photovoltaics (OPV). The generic term of SMOSCs encompasses a large variety of chemical structures implying very different basic concepts. Polydisperse «double cable» polymers and oligomers with acceptor groups linked to the conjugated backbone by a flexible spacer and donor–acceptor block copolymers are at present, the most investigated and efficient systems with spectacular progress in conversion efficiency achieved in the past 2–3 years. However, besides this mainstream SMOSCs research, a few recent publications describe OPV cells constituted of homojunctions based on small π-conjugated molecules. While the process of charge generation in such systems is still a matter of debate due in particular to the possible direct photogeneration of charge–carriers, devices with significant performance have been recently reported. After a brief overview of the most recent advances on the various types of SMOSCs, recent remarkable results on homojunction OPV cells based on small π-conjugated molecules are discussed in order to highlight the potential fundamental and technological interest of this emerging field of research.

Nature Communications, Published online: 03 November 2021; doi:10.1038/s41467-021-26700-2

‘Fluorescent chemisensors with fast response time and pronounced luminescence variation are important but have not been well investigated for self-assembled chiral systems. Here, the authors present a coassembled multiple component system that responds to SO2 derivatives, giving rise to dynamic aggregation behaviors and switchable luminescence as well as circularly polarized luminescence.

Nature Energy, Published online: 01 November 2021; doi:10.1038/s41560-021-00923-5

Organic solar cells processed from green solvents are easier to implement in manufacturing yet their efficiency is low. Chen et al. devise a guest molecule to improve the molecular packing, enabling devices with over 17% efficiency.

A biodegradable photothermal agent is developed by inserting cleavable disulfide moieties into a conjugative polymer, which affords biodegradability and excellent adsorption in both the near-infrared (NIR)-I and NIR-II windows, effectively inhibits tumor progression, and extends survival spans in a deep-seated ovarian cancer mouse model.

Abstract

Photothermal therapy holds great promise for cancer treatment due to its effective tumor ablation and minimal invasiveness. Herein a new class of biodegradable photothermal agents with effective adsorption in both near-infrared-I (NIR-I) and NIR-II windows is reported for deep tumor therapy. As demonstrated in a deep-seated ovarian cancer model, photothermal therapy using 1064 nm irradiation effectively inhibits tumor progression and prolongs survival spans. This work provides a new design of photothermal agents toward a more effective therapy of tumors.

Three kinds of small-dipole-molecule-containing electrolytes are developed for aqueous batteries. The developed electrolytes deliver an expanded electrochemical stability window (over 2.5 V), and their solvation-sheath structure and ion conductivity mechanism are explored. A constructed Li₄Ti₅O₁₂//LiMn₂O₄ battery in a glycerol-containing electrolyte exhibits high output voltage (2.45 V) and a specific capacity of 119.6 mAh g⁻¹ after 400 cycles.

Abstract

High-voltage aqueous rechargeable batteries are promising competitors for next-generation energy storage systems with safety and high specific energy, but they are limited by the absence of low-cost aqueous electrolytes with a wide electrochemical stability window (ESW). The decomposition of aqueous electrolytes is mainly facilitated by the hydrogen bond network between water molecules and the water molecules in the solvation sheath. Here, three types of small dipole molecules (small molecules containing a dipole; glycerol (Gly), erythritol (Et), and acrylamide (AM)) are reported to develop aqueous electrolytes with high safety and wide ESW (over 2.5 V) for aqueous lithium-, sodium-, and zinc-ion batteries, respectively. The solvation-sheath structures are explored by ab initio molecular dynamics (MD) simulations, demonstrating that three types of dipole molecules deplete the water molecules in the solvation sheath of the charge carrier and break the hydrogen bond network between the water molecules, thus effectively expanding the ESW. A battery constructed from lithium titanate and lithium manganate in Gly-containing electrolyte exhibits an output voltage of 2.45 V and retains a specific capacity of 119.6 mAh g⁻¹ after 400 cycles. This work provides another strategy for exploiting low-cost high-voltage electrolytes for aqueous energy-storage systems.

The near-infrared II (NIR-II)-imaging-guided photothermal therapy (PTT) of nanofluorophores is a promising approach for theranostic of orthotopic glioma. Brain-targeting nanofluorophores featuring novel molecular design of “backbone distortion + molecular rotor” are prepared. They can balance the photon energy flow to achieve advanced NIR-II imaging at 1550 nm and photo-to-heat ratio for PTT in orthotopic-glioma-bearing mice.

Abstract

A remaining challenge in the treatment of glioblastoma multiforme (GBM) is surmounting the blood–brain barrier (BBB). Such a challenge prevents the development of efficient theranostic approaches that combine reliable diagnosis with targeted therapy. In this study, brain-targeted near-infrared IIb (NIR-IIb) aggregation-induced-emission (AIE) nanoparticles are developed via rational design, which involves twisting the planar molecular backbone with steric hindrance. The resulting nanoparticles can balance competing responsiveness demands for radiation-mediated NIR fluorescence imaging at 1550 nm and non-radiation NIR photothermal therapy (NIR-PTT). The brain-targeting peptide apolipoprotein E peptide (ApoE) is grafted onto these nanoparticles (termed as ApoE-Ph NPs) to target glioma and promote efficient BBB traversal. A long imaging wavelength 1550 nm band-pass filter is utilized to monitor the in vivo biodistribution and accumulation of the nanoparticles in a model of orthotopic glioma, which overcomes previous limitations in wavelength range and equipment. The results demonstrate that the ApoE-Ph NPs have a higher PTT efficiency and significantly enhanced survival of mice bearing orthotopic GBM with moderate irradiation (0.5 W cm⁻²). Collectively, the work highlights the smart design of a brain-targeted NIR-II AIE theranostic approach that opens new diagnosis and treatment options in the photonic therapy of GBM.

The advancement of organic photovoltaics has been significantly aided by the emergence of near-infrared (NIR) materials, and they are also able to satisfy the varied criteria for various types of organic photovoltaic device architectures. Furthermore, the absorption window is the most fundamental criterion for developing novel NIR organic photoelectric materials.

Abstract

Near-infrared (NIR)-absorbing organic semiconductors have opened up many exciting opportunities for organic photovoltaic (OPV) research. For example, new chemistries and synthetical methodologies have been developed; especially, the breakthrough Y-series acceptors, originally invented by our group, specifically Y1, Y3, and Y6, have contributed immensely to boosting single-junction solar cell efficiency to around 19%; novel device architectures such as tandem and transparent organic photovoltaics have been realized. The concept of NIR donors/acceptors thus becomes a turning point in the OPV field. Here, the development of NIR-absorbing materials for OPVs is reviewed. According to the low-energy absorption window, here, NIR photovoltaic materials (p-type (polymers) and n-type (fullerene and nonfullerene)) are classified into four categories: 700–800 nm, 800–900 nm, 900–1000 nm, and greater than 1000 nm. Each subsection covers the design, synthesis, and utilization of various types of donor (D) and acceptor (A) units. The structure–property relationship between various kinds of D, A units and absorption window are constructed to satisfy requirements for different applications. Subsequently, a variety of applications realized by NIR materials, including transparent OPVs, tandem OPVs, photodetectors, are presented. Finally, challenges and future development of novel NIR materials for the next-generation organic photovoltaics and beyond are discussed.

Host-dopant interaction has a significant impact on the device performance of thermally activated delayed fluorescence (TADF) based organic light-emitting diodes (OLEDs). Current basic understanding of TADF and reports on the impact of host materials on photophysical properties/device performance of TADF emitters are reviewed, aiming to draw the attention of the research community from optoelectronics toward developing highly efficient TADF OLEDs.

Abstract

Organic light-emitting diodes (OLEDs) represent one of the most promising technologies for future displays and lighting sources, which have received extensive research attention. Purely organic thermally activated delayed fluorescence (TADF) emitters offer obvious advantages, including high efficiencies and low costs, and they are typically doped in host materials to achieve optimal device efficiencies. TADF emitters typically feature intramolecular charge transfer characteristics, and their excited states properties are sensitive to local environment, giving the implication that host materials can finely tune their emission properties. In recent years, the development of TADF emitters has been vigorous with abundant and fast-growing reports on new design concepts and molecular structures. Comparatively, research on the host materials for TADF OLEDs has lagged, and reports on host materials, especially those providing insights into host-dopant interactions are limited. This subject is at the interface of synthetic chemistry, physical chemistry, solid-state physics, and computational modeling, etc. In this review article, current basic understanding of TADF and the reports on the impact of host materials on the photophysical properties/device performance of TADF emitters are reviewed to provide insights into the host-dopant interactions, aiming to draw the attention of the research community from optoelectronics toward developing highly efficient TADF OLEDs.

Nature Communications, Published online: 26 October 2021; doi:10.1038/s41467-021-26439-w

Molecular designs improving the performance of thermally activated delayed fluorescence (TADF) emitters are greatly desired for optoelectronic applications. The authors propose a molecular geometry with donor and acceptor moieties facing each other separated by an anthracene bridge, giving rise to hot exciplex states producing color pure and fast TADF emission.

Supramolecular chemistry has provided a platform for the rational design of smart optical materials. Noncovalent bonding interactions, not only afford convenient and efficient approaches to fabricate emissive materials, but also endow them with dynamic reversibility and stimuli responsiveness. This review highlights the state-of-the-art noncovalent strategies available for the construction of color-tunable luminescent materials.

Abstract

Constructing multicolor photoluminescent materials with tunable properties is an attractive research objective on account of their abundant applications in materials science and biomedical engineering. By comparison with covalent synthesis, supramolecular chemistry has provided a more competitive and promising strategy for the production of organic materials and the regulation of their photophysical properties. By taking advantage of dynamic and reversible noncovalent bonding interactions, supramolecular strategies can, not only simplify the design and fabrication of organic materials, but can also endow them with dynamic reversibility and stimuli responsiveness, making it much easier to adjust the superstructures and properties of the materials. Occasionally, it is possible to introduce emergent properties into these materials, which are absent in their precursor compounds, broadening their potential applications. In an attempt to highlight the state-of-the-art noncovalent strategies available for the construction of smart luminescent materials, an overview of color-tunable materials is presented in this Review, with the emphasis being placed on the examples drawn from host–guest complexes, supramolecular assemblies and crystalline materials. The noncovalent synthesis of room-temperature phosphorescent materials and the modulation of their luminescent properties are also described. Finally, future directions and scientific challenges in the emergent field of color-tunable supramolecular emissive materials are discussed.

A strategy to boost the external quantum efficiency (EQE) of photomultiplication-type organic photodiodes (OPDs) is suggested by stabilizing trapped electron states within the active layer via electrostatic interactions. By introducing an ionic electrolyte interfacial layer and by the resulting built-in electrostatic interaction, an ultrahigh EQE (>2 000 000%), responsivity (>10 000 A W⁻¹), and visible specific detectivity (>10¹⁴ Jones) for OPDs is reported.

Abstract

A photomultiplication-type organic photodiode (PM-OPD), where an electric double layer (EDL) is strategically embedded, is demonstrated, with an exceptionally high external quantum efficiency (EQE) of 2 210 000%, responsivity of 11 200 A W⁻¹, specific detectivity of 2.11 × 10¹⁴ Jones, and gain–bandwidth product of 1.92 × 10⁷ Hz, as well as high reproducibility. A polymer electrolyte, poly(9,9-bis(3′-(N,N-dimethyl)-N-ethylammoinium-propyl-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene))dibromide is employed as a work-function-modifying layer of indium tin oxide (ITO) to construct an EDL-embedded Schottky junction with p-type polymer semiconductor, poly(3-hexylthiophene-diyl), resulting in not only advantageous tuning of the work function of ITO but also an enhancement of the electron-trapping efficiency due to electrostatic interaction between exposed cations and trapped electrons within isolated acceptor domains. The effects of the EDL on the energetics of the trapped electron states and thus on the gain generation mechanism are confirmed by numerical simulations based on the drift–diffusion approximation of charge carriers. The feasibility of the fabricated high-EQE PM-OPD especially for weak light detection is demonstrated via a pixelated prototype image sensor. It is believed that this new OPD platform opens up the possibility for the ultrahigh-sensitivity organic image sensors, while maintaining the advantageous properties of organics.

The triplet–triplet annihilation (TTA) process can recycle nonradiative triplet excitons to radiative singlet excitons and enhance the efficiency of fluorescent organic light-emitting diodes (OLEDs). Conventionally, the theoretical limit of delayed emission ratio by the TTA process is known to be 37.5% in anthracene-based molecules. In this work, 48% of delayed emission ratio is achieved by TTA with carefully designed blue OLEDs.

Abstract

Triplet harvesting is important for the realization of high-efficiency fluorescent organic light-emitting diodes (OLEDs). Triplet–triplet annihilation (TTA) is one triplet-harvesting strategy. However, for blue-emitting anthracene derivatives, the theoretical maximum radiative singlet-exciton ratio generated from the TTA process is known to be 15% in addition to the initially generated singlets of 25%, which is insufficient for high-efficiency fluorescent devices. In this study, nearly 25% of the radiative singlet-exciton ratio is realized by TTA using an anthracene derivative, breaking the theoretical limit. As a result, efficient deep-blue TTA fluorescent devices are developed, exhibiting external quantum efficiencies of 10.2% and 8.6% with Commission Internationale de l'Eclairage color coordinates of (0.134, 0.131) and (0.137, 0.076), respectively. The theoretical model provided herein explains the experimental results considering both the TTA and reverse intersystem crossing to a singlet state from higher triplet states formed by the TTA, clearly demonstrating that the radiative singlet ratio generated from TTA can reach 37.5% (total radiative singlet-exciton ratio: 62.5%), well above 15% (total 40%), despite the molecule having S₁, T₂ < 2T₁ < Q₁ energy levels, which will lead to the development of high-efficiency fluorescent OLEDs with external quantum efficiencies exceeding 28% if the outcoupling efficiency is 45%.