以昇陳

The salt of the cation (Mes₂B⁺; Mes: mesitylene) and the anion [B(C₆F₅)₄]⁻ is introduced as superior p‐type dopant for organic semiconductors. The doping mechanism involves electron transfer from the semiconductor to Mes₂B⁺, and the positive charge is stabilized by [B(C₆F₅)₄]⁻. For poly(3‐hexylthiophene), the anion even stabilizes bipolarons. The effective electron affinity of Mes₂B⁺[B(C₆F₅)₄]⁻ is estimated to be 5.9 eV.

Abstract

Molecular doping allows enhancement and precise control of electrical properties of organic semiconductors, and is thus of central technological relevance for organic (opto‐) electronics. Beyond single‐component molecular electron acceptors and donors, organic salts have recently emerged as a promising class of dopants. However, the pertinent fundamental understanding of doping mechanisms and doping capabilities is limited. Here, the unique capabilities of the salt consisting of a borinium cation (Mes₂B⁺; Mes: mesitylene) and the tetrakis(penta‐fluorophenyl)borate anion [B(C₆F₅)₄]⁻ is demonstrated as p‐type dopant for polymer semiconductors. With a range of experimental methods, the doping mechanism is identified to comprise electron transfer from the polymer to Mes₂B⁺, and the positive charge on the polymer is stabilized by [B(C₆F₅)₄]⁻. Notably, the former salt cation leaves during processing and is not present in films. The anion [B(C₆F₅)₄]⁻ even enables the stabilization of polarons and bipolarons in poly(3‐hexylthiophene), not yet achieved with other molecular dopants. From doping studies with high ionization energy polymer semiconductors, the effective electron affinity of Mes₂B⁺[B(C₆F₅)₄]⁻ is estimated to be an impressive 5.9 eV. This significantly extends the parameter space for doping of polymer semiconductors.

The emergence of nonfullerene electron acceptors has rejuvenated the field of organic photovoltaics, with device efficiencies over 18% and 20% in sight. In this essay, the basic properties of these new nonfullerene acceptors are discussed. Perspectives and suggestions for further research endeavors toward successful commercialization are also provided.

Abstract

Efficient, low‐cost, and low‐embodied energy photovoltaics are key enablers of the global decarbonization agenda. In addition to the market‐leading crystalline silicon technology, several other promising candidates are under active investigation with the perovskites leading the way with single‐junction efficiencies exceeding 25% at the lab‐scale. So‐called organic photovoltaics (solar cells based upon organic semiconductors), particularly those that can be solution processed, have long promised the Nirvana of ultralow cost and very short energy payback times. However, relatively low efficiencies, poor long‐term stability, and issues with manufacturing at scale have so far prevented truly meaningful commercialization of the technology. The recent emergence of the so‐called nonfullerene electron acceptors is potentially about to shift this dynamic—they have delivered a step change in performance in a relatively short period of time. In this Essay, the basic properties of these new materials, their pros and cons, what we know and what we do not know are explored.

Solid‐state ¹⁹F magic angle spinning nuclear magnetic microscopy and elemental mapping are introduced to probe the structures of ternary and quaternary blends. The presence of the individual guest paths minimizes the influence on charge generation and transport of the host system, allowing cooperation of the parallel‐like subcells, producing impressive 17.2% efficiency via a quaternary strategy.

Abstract

Ternary strategies show over 16% efficiencies with increased current/voltage owing to complementary absorption/aligned energy level contributions. However, poor understanding of how the guest components tune the active layer structures still makes rational selection of material systems challenging. In this study, two phthalimide based ultrawide bandgap polymer donor guests are synthesized. Parallel energies between the highest occupied molecular orbitals of host and guest polymers are achieved via incorporating selnophene on the guest polymer. Solid‐state ¹⁹F magic angle spinning nuclear magnetic spectroscopy, graze‐incidence wide‐angle X‐ray diffraction, elemental transmission electron microscopy mapping, and transient absorption spectroscopy are combined to characterize the active layer structures. Formation of the individual guest phases selectively improves the structural order of donor and acceptor phase. The increased electron mobility in combination with the presence of the additional paths made by the guest not only minimizes the influence on charge generation and transport of the host system but also contributes to increasing the overall current generation. Therefore, phthalimide based polymers can be potential candidates that enable the simultaneous increase of open‐circuit voltage and short‐circuit current‐density via fine‐tuning energy levels and the formation of additional paths for enhancing current generation in parallel‐like multicomponent organic solar cells.

The electric field is demonstrated to enhance the photo‐thermoelectric effect by promoting the exciton separation efficiency with a coupled modulation process. The increased photoinduced carrier concentration and abnormal trade‐off relationship of the TE parameters together lead to a more than 500% enhancement in the PF to 11.2 µW m⁻¹ K⁻².

Abstract

Modulating photophysical processes is a fundamental way for tuning performance of many organic devices. However, it has not been explored as an effective strategy to manipulate the thermoelectric (TE) conversion of organic semiconductors (OSCs) owing to their critical requirement to carrier concentration (>10¹⁸ cm⁻³) and the fact of low exciton separation efficiency in single element OSCs. Here, an electric field modulated photo‐thermoelectric (P‐TE) effect in an n‐type OSC is demonstrated to realize a significant improvement of TE performance. The electrical and spectroscopy characterizations reveal that the electric field gating generates combined modulation of exciton separation, charge screening, and carrier recombination, which produces a more than ten times improvement of photoinduced carrier concentration. These coupled processes contribute to the unconventional Seebeck coefficient (S )‐electrical conductivity (σ) trade‐off relationship of the photoexcited films, therefore leading to a more than 500% enhancement in the power factor for n‐type OTE semiconductors. This work opens a unique way toward state‐of‐the‐art organic P‐TE materials for energy harvesting applications.

Linearly polarized light‐emitting diodes are demonstrated using few‐layer ReS₂, a 2D semiconductor with reduced in‐plane symmetry. Two excitonic electroluminescence peaks exhibiting high degrees of linear polarization of ≈80% are observed in near‐infrared frequencies. Hot hole injection through a hBN tunneling layer is shown to be key to the activation of hot exciton emission.

Abstract

An on‐chip polarized light source is desirable in signal processing, optical communication, and display applications. Layered semiconductors with reduced in‐plane symmetry have inherent anisotropic excitons that are attractive candidates as polarized dipole emitters. Herein, the demonstration of polarized light‐emitting diode based on anisotropic excitons in few‐layer ReS₂, a 2D semiconductor with excitonic transition energy of 1.5–1.6 eV, is reported. The light‐emitting device is based on minority carrier (hole) injection into n‐type ReS₂ through a hexagonal boron nitride (hBN) tunnel barrier in a metal–insulator–semiconductor (MIS) van der Waals heterostack. Two distinct emission peaks from excitons are observed at near‐infrared wavelength regime from few‐layer ReS₂. The emissions exhibit a degree of polarization of 80% reflecting the nearly 1D nature of excitons in ReS₂.

A highly efficient organic solar cell with a ternary architecture is successfully demonstrated by enhancing and balancing charge transport as well as matching integer charge transfer energy in a bulk heterojunction blend. As a result, a power conversion efficiency of 17.13% is obtained with the significantly improved fill factor of 0.813.

Abstract

Ternary architecture is one of the most effective strategies to boost the power conversion efficiency (PCE) of organic solar cells (OSCs). Here, an OSC with a ternary architecture featuring a highly crystalline molecular donor DRTB‐T‐C4 as a third component to the host binary system consisting of a polymer donor PM6 and a nonfullerene acceptor Y6 is reported. The third component is used to achieve enhanced and balanced charge transport, contributing to an improved fill factor (FF) of 0.813 and yielding an impressive PCE of 17.13%. The heterojunctions are designed using so‐called pinning energies to promote exciton separation and reduce recombination loss. In addition, the preferential location of DRTB‐T‐C4 at the interface between PM6 and Y6 plays an important role in optimizing the morphology of the active layer.

The emergence of nonfullerene electron acceptors has rejuvenated the field of organic photovoltaics, with device efficiencies over 18% and 20% in sight. In this essay, the basic properties of these new nonfullerene acceptors are discussed. Perspectives and suggestions for further research endeavors toward successful commercialization are also provided.

Abstract

Efficient, low‐cost, and low‐embodied energy photovoltaics are key enablers of the global decarbonization agenda. In addition to the market‐leading crystalline silicon technology, several other promising candidates are under active investigation with the perovskites leading the way with single‐junction efficiencies exceeding 25% at the lab‐scale. So‐called organic photovoltaics (solar cells based upon organic semiconductors), particularly those that can be solution processed, have long promised the Nirvana of ultralow cost and very short energy payback times. However, relatively low efficiencies, poor long‐term stability, and issues with manufacturing at scale have so far prevented truly meaningful commercialization of the technology. The recent emergence of the so‐called nonfullerene electron acceptors is potentially about to shift this dynamic—they have delivered a step change in performance in a relatively short period of time. In this Essay, the basic properties of these new materials, their pros and cons, what we know and what we do not know are explored.

Nature Photonics, Published online: 20 July 2020; doi:10.1038/s41566-020-0664-3

Nature Energy, Published online: 20 July 2020; doi:10.1038/s41560-020-0652-3

A systematic study of a series of polymer:non‐fullerene acceptor blends is conducted to unify the cumulative effects of voltages losses, charge generation efficiencies, non‐geminate recombination and extraction dynamics, and nuanced morphological differences to the device performance. Deconvolution of the major loss processes in these blends and their connections to the nuanced bulk‐heterojunction morphology and energetics are established.

Abstract

Even though significant breakthroughs with over 18% power conversion efficiencies (PCEs) in polymer:non‐fullerene acceptor (NFA) bulk heterojunction organic solar cells (OSCs) have been achieved, not many studies have focused on acquiring a comprehensive understanding of the underlying mechanisms governing these systems. This is because it can be challenging to delineate device photophysics in polymer:NFA blends comprehensively, and even more complicated to trace the origins of the differences in device photophysics to the subtle differences in energetics and morphology. Here, a systematic study of a series of polymer:NFA blends is conducted to unify and correlate the cumulative effects of i) voltage losses, ii) charge generation efficiencies, iii) non‐geminate recombination and extraction dynamics, and iv) nuanced morphological differences with device performances. Most importantly, a deconvolution of the major loss processes in polymer:NFA blends and their connections to the complex BHJ morphology and energetics are established. An extension to advanced morphological techniques, such as solid‐state NMR (for atomic level insights on the local ordering and donor:acceptor ππ interactions) and resonant soft X‐ray scattering (for donor and acceptor interfacial area and domain spacings), provide detailed insights on how efficient charge generation, transport, and extraction processes can outweigh increased voltage losses to yield high PCEs.

Solid‐state ¹⁹F magic angle spinning nuclear magnetic microscopy and elemental mapping are introduced to probe the structures of ternary and quaternary blends. The presence of the individual guest paths minimizes the influence on charge generation and transport of the host system, allowing cooperation of the parallel‐like subcells, producing impressive 17.2% efficiency via a quaternary strategy.

Abstract

Ternary strategies show over 16% efficiencies with increased current/voltage owing to complementary absorption/aligned energy level contributions. However, poor understanding of how the guest components tune the active layer structures still makes rational selection of material systems challenging. In this study, two phthalimide based ultrawide bandgap polymer donor guests are synthesized. Parallel energies between the highest occupied molecular orbitals of host and guest polymers are achieved via incorporating selnophene on the guest polymer. Solid‐state ¹⁹F magic angle spinning nuclear magnetic spectroscopy, graze‐incidence wide‐angle X‐ray diffraction, elemental transmission electron microscopy mapping, and transient absorption spectroscopy are combined to characterize the active layer structures. Formation of the individual guest phases selectively improves the structural order of donor and acceptor phase. The increased electron mobility in combination with the presence of the additional paths made by the guest not only minimizes the influence on charge generation and transport of the host system but also contributes to increasing the overall current generation. Therefore, phthalimide based polymers can be potential candidates that enable the simultaneous increase of open‐circuit voltage and short‐circuit current‐density via fine‐tuning energy levels and the formation of additional paths for enhancing current generation in parallel‐like multicomponent organic solar cells.

The salt of the cation (Mes₂B⁺; Mes: mesitylene) and the anion [B(C₆F₅)₄]⁻ is introduced as superior p‐type dopant for organic semiconductors. The doping mechanism involves electron transfer from the semiconductor to Mes₂B⁺, and the positive charge is stabilized by [B(C₆F₅)₄]⁻. For poly(3‐hexylthiophene), the anion even stabilizes bipolarons. The effective electron affinity of Mes₂B⁺[B(C₆F₅)₄]⁻ is estimated to be 5.9 eV.

Abstract

Molecular doping allows enhancement and precise control of electrical properties of organic semiconductors, and is thus of central technological relevance for organic (opto‐) electronics. Beyond single‐component molecular electron acceptors and donors, organic salts have recently emerged as a promising class of dopants. However, the pertinent fundamental understanding of doping mechanisms and doping capabilities is limited. Here, the unique capabilities of the salt consisting of a borinium cation (Mes₂B⁺; Mes: mesitylene) and the tetrakis(penta‐fluorophenyl)borate anion [B(C₆F₅)₄]⁻ is demonstrated as p‐type dopant for polymer semiconductors. With a range of experimental methods, the doping mechanism is identified to comprise electron transfer from the polymer to Mes₂B⁺, and the positive charge on the polymer is stabilized by [B(C₆F₅)₄]⁻. Notably, the former salt cation leaves during processing and is not present in films. The anion [B(C₆F₅)₄]⁻ even enables the stabilization of polarons and bipolarons in poly(3‐hexylthiophene), not yet achieved with other molecular dopants. From doping studies with high ionization energy polymer semiconductors, the effective electron affinity of Mes₂B⁺[B(C₆F₅)₄]⁻ is estimated to be an impressive 5.9 eV. This significantly extends the parameter space for doping of polymer semiconductors.

Nature Photonics, Published online: 29 June 2020; doi:10.1038/s41566-020-0653-6

The first example of a UV‐emissive thermally activated delayed fluorescence emitter, namely CZ‐MPS, is demonstrated successfully. An organic light‐emitting diode using CZ‐MPS as the guest material can emit efficient UV light with an emission maximum (λ_EL) of 389 nm as well as a record‐breaking total external quantum efficiency (EQE_max) of 9.3%.

Abstract

Owing to the difficulty in acquiring compounds with combined high energy bandgaps and lower‐lying intramolecular charge‐transfer excited states, the development of ultraviolet (UV) thermally activated delayed fluorescence (TADF) materials is quite challenging. Herein, through interlocking of the diphenylsulfone (PS) acceptor unit of a reported deep‐blue TADF emitter (CZ‐PS) by a dimethylmethylene bridge, CZ‐MPS, a UV‐emissive TADF compound bearing a shallower LUMO energy level and a more rigid structure than those of CZ‐PS is achieved. This represents the first example of a UV‐emissive TADF compound. Organic light‐emitting diode (OLED) using CZ‐MPS as the guest material can emit efficient UV light with emission maximum of 389 nm and maximum total external quantum efficiency (EQE_max) of 9.3%. Note that this EQE_max value is twice as high as the current record EQE_max (4.6%) for UV‐OLEDs. This finding may shed light on the molecular design strategy for high‐performance UV‐OLED materials.

High‐performance semitransparent organic solar cells are achieved through combined design efforts on the formulation of near‐infrared ternary blends and optical control over photonic reflectors, which exhibit excellent features of power generation, they being see‐through, and infrared reflection.

Abstract

Clean energy production and saving play vital impacts on the sustainability of the global community. Herein, high‐performance semitransparent organic solar cells (ST‐OSCs) with excellent features of power generation, being see‐through, and infrared reflection of heat dissipation, with promising perspectives for building‐integrated photovoltaics (BIPVs) are reported. To simultaneously improve average visible transmittance (AVT) and power conversion efficiency (PCE), formally in a trade‐off relationship, of ST‐OSCs, new ternary blends with alloy‐like near‐infrared (NIR) acceptors are employed, which are effective to improve device efficiency while maintaining visible absorption unchanged, resulting in PCEs of 16.8% for opaque devices and 13.1% for semitransparent OSCs (AVT of 22.4% and infrared photon radiation rejection (IRR) of 77%). Further, multifunctional ST‐OSCs are realized via introducing simple, yet effective photonic reflectors, together with optical simulation, leading to not only perfect fitting of the visible transmittance peak (555 nm) to the photopic response of the human eye but also an excellent IRR of 90% (780–2500 nm), along with 23% AVT and over 12% PCE. This is thought to be the best‐performing multifunctional ST‐OSC with promising prospects as BIPVs in terms of power generation, heat dissipation, and being see‐through.

An optimal power conversion efficiency (PCE) of 17.4% is achieved in the optimized ternary organic photovoltaics (OPVs) with two well‐compatible acceptors (BTP‐4F‐12 and MeIC) and one wide bandgap donor (PM6), resulting from simultaneously improved J _SC, fill factor (FF), and V _OC. The energy loss of ternary OPVs is minimized compared with the two binary OPVs, which is an important development for PCE improvement of ternary OPVs.

Abstract

A power conversion efficiency (PCE) of 16.2% is achieved in PM6:BTP‐4F‐12 based organic photovoltaics (OPVs). On the basis of efficient binary OPVs, a series of ternary OPVs are constructed by incorporating MeIC as the third component. The open circuit voltages (V _OCs) of ternary OPVs can be gradually increased along with the incorporation of MeIC, suggesting the formation of an alloy state between BTP‐4F‐12 and MeIC with good compatibility. The energy loss (E _loss) of ternary OPVs can be decreased compared with that of two binary OPVs, contributing to the V _OC improvement of ternary OPVs. The short circuit current density (J _SC) and fill factor (FF) of ternary OPVs can also be simultaneously enhanced with MeIC content up to 10 wt% in acceptors, leading to 17.4% PCE of the optimized ternary OPVs. The J _SC and FF improvement of ternary OPVs is thought to result from the optimized ternary active layers with more efficient photon harvesting, exciton dissociation and charge transport. The 17.4% PCE and 79.2% FF is among the top values of ternary OPVs. This work indicates that a ternary strategy is an emerging method to simultaneously minimize E _loss and optimize photon harvesting as well as improve the morphology of active layers for realizing performance improvement for OPVs.

An extension of conjugated antenna units relating transition from high‐order singlet excited states in heavy‐atom‐free chromophores greatly enhances the phosphorescence rate (kp) without a large increase of the vibration‐based radiationless transition rate (knr(RT)) with large triplet yield, which results in a room‐temperature phosphorescence (RTP) yield (Φ_p(RT)) of 50%, as well as an RTP lifetime (τ_p(RT)) of 1.0 s.

Abstract

Persistent (lifetime > 100 ms) room‐temperature phosphorescence (pRTP) is important for state‐of‐the‐art security and bioimaging applications. An unclear relationship between chromophores and physical parameters relating to pRTP has prevented obtaining an RTP yield of over 50% and a lifetime over 1 s. Here highly efficient pRTP is reported under ambient conditions from heavy atom‐free chromophores. A heavy atom‐free aromatic core substituted with a long‐conjugated amino group considerably accelerates the phosphorescence rate independent of the intramolecular vibration‐based nonradiative rate from the lowest excited triplet state. One of the designed heavy atom‐free dopant chromophores presents an RTP yield of 50% with a lifetime of 1 s under ambient conditions. The afterglow brightness under strong excitation is at least 10⁴ times stronger than that of conventional long‐persistent luminescence emitters. Here it is shown that highly efficient pRTP materials allow for high‐resolution gated emission with a size of the diffraction limit using small‐scale and low‐cost photodetectors.

Solid‐state triplet–triplet‐annihilation‐based photon‐upconversion systems are subject to losses from back transfer, molecular aggregation, and triplet–charge annihilation. Following strategies provided to mitigate these losses, a dry‐processed solid‐state device having comparable upconversion efficiency and threshold intensity to solution‐processed solid‐state systems is developed, offering a route for high‐performance upconversion devices compatible with applications sensitive to solvent damage.

Abstract

Photon upconversion via triplet–triplet annihilation (TTA) has achieved high efficiencies in solution and within polymer matrices that support molecular migration systems. It has diverse potential applications including bioimaging, optical sensors, and photovoltaics. To date, however, the reported performance of TTA in rigid solid‐state systems is substantially inferior, which may complicate the integration of TTA in other solid‐state devices. Here, solid‐state loss mechanisms in a green‐to‐blue upconversion system are investigated, and three specific losses are identified: energy back transfer, sensitizer aggregation, and triplet–charge annihilation. Strategies are demonstrated to mitigate energy back transfer and sensitizer aggregation, and a completely dry‐processed solid‐state TTA upconversion system having an upconversion efficiency of ≈2.5% (by the convention of maximum efficiency being 100%) at a relatively low excitation intensity of 238 mW cm⁻² is reported. This device is the first demonstration of dry‐processed solid‐state TTA comparable to solution‐processed solid‐state systems. The strategies reported here can be generalized to other upconversion systems and offer a route to achieving higher‐performance solid‐state TTA upconversion devices that are compatible with applications sensitive to solvent damage.

Newly developed organic, air‐stable radicals and radical polymers, as well as their properties and applications in flexible electronic devices, are reviewed. Recently, these materials have shown high performance in batteries, photovoltaics, transistors, light‐emitting diodes, and photothermal devices. A short perspective is also given.

Abstract

In the last few years, air‐stable organic radicals and radical polymers have attracted tremendous attention due to their outstanding performance in flexible electronic devices, including transistors, batteries, light‐emitting diodes, thermoelectric and photothermal conversion devices, and among many others. The main issue of radicals from laboratory studies to real‐world applications is that the number of known air‐stable radicals is very limited, and the radicals that have been used as materials are even less. Here, the known and newly developed air‐stable organic radicals are summarized, generalizing the way of observing air‐stable radicals. The special electric and photophysical properties of organic radicals and radical polymers are interpreted, which give radicals a wide scope for various of potential applications. Finally, the exciting applications of radicals that have been achieved in flexible electronic devices are summarized. The aim herein is to highlight the recent achievements in radicals in chemistry, materials science, and flexible electronics, and further bridge the gap between these three disciplines.

A small molecule of 4,4′,4″,4′″‐(pyrazine‐2,3,5,6‐tetrayl) tetrakis (N,N‐bis(4‐methoxyphenyl) aniline) (PT‐TPA) is applied to effectively p‐dope the FA _x MA_{1− x} PbI₃ (FA:HC(NH₂)₂; MA:CH₃NH₃) perovskite surface, with obvious conductivity and carrier concentration increase. After applying PT‐TPA into perovskite solar cells, the doping‐induced band bending at the perovskite surface facilitates hole extraction to the hole‐transport layer and expels electrons toward the cathode, which reduces surface charge recombination. The optimized devices demonstrate a stabilized efficiency of 22.9%.

Abstract

Tailoring the doping of semiconductors in heterojunction solar cells shows tremendous success in enhancing the performance of many types of inorganic solar cells, while it is found challenging in perovskite solar cells because of the difficulty in doping perovskites in a controllable way. Here, a small molecule of 4,4′,4″,4″′‐(pyrazine‐2,3,5,6‐tetrayl) tetrakis (N,N‐bis(4‐methoxyphenyl) aniline) (PT‐TPA) which can effectively p‐dope the surface of FA _x MA_{1− x} PbI₃ (FA: HC(NH₂)₂; MA: CH₃NH₃) perovskite films is reported. The intermolecular charge transfer property of PT‐TPA forms a stabilized resonance structure to accept electrons from perovskites. The doping effect increases perovskite dark conductivity and carrier concentration by up to 4737 times. Computation shows that electrons in the first two layers of octahedral cages in perovskites are transferred to PT‐TPA. After applying PT‐TPA into perovskite solar cells, the doping‐induced band bending in perovskite effectively facilitates hole extraction to hole transport layer and expels electrons toward cathode side, which reduces the charge recombination there. The optimized devices demonstrate an increased photovoltage from 1.12 to 1.17 V and an efficiency of 23.4% from photocurrent scanning with a stabilized efficiency of 22.9%. The findings demonstrate that molecular doping is an effective route to control the interfacial charge recombination in perovskite solar cells which is in complimentary to broadly applied defect passivation techniques.

An extension of conjugated antenna units relating transition from high‐order singlet excited states in heavy‐atom‐free chromophores greatly enhances the phosphorescence rate (kp) without a large increase of the vibration‐based radiationless transition rate (knr(RT)) with large triplet yield, which results in a room‐temperature phosphorescence (RTP) yield (Φ_p(RT)) of 50%, as well as an RTP lifetime (τ_p(RT)) of 1.0 s.

Abstract

Persistent (lifetime > 100 ms) room‐temperature phosphorescence (pRTP) is important for state‐of‐the‐art security and bioimaging applications. An unclear relationship between chromophores and physical parameters relating to pRTP has prevented obtaining an RTP yield of over 50% and a lifetime over 1 s. Here highly efficient pRTP is reported under ambient conditions from heavy atom‐free chromophores. A heavy atom‐free aromatic core substituted with a long‐conjugated amino group considerably accelerates the phosphorescence rate independent of the intramolecular vibration‐based nonradiative rate from the lowest excited triplet state. One of the designed heavy atom‐free dopant chromophores presents an RTP yield of 50% with a lifetime of 1 s under ambient conditions. The afterglow brightness under strong excitation is at least 10⁴ times stronger than that of conventional long‐persistent luminescence emitters. Here it is shown that highly efficient pRTP materials allow for high‐resolution gated emission with a size of the diffraction limit using small‐scale and low‐cost photodetectors.

Linearly polarized light‐emitting diodes are demonstrated using few‐layer ReS₂, a 2D semiconductor with reduced in‐plane symmetry. Two excitonic electroluminescence peaks exhibiting high degrees of linear polarization of ≈80% are observed in near‐infrared frequencies. Hot hole injection through a hBN tunneling layer is shown to be key to the activation of hot exciton emission.

Abstract

An on‐chip polarized light source is desirable in signal processing, optical communication, and display applications. Layered semiconductors with reduced in‐plane symmetry have inherent anisotropic excitons that are attractive candidates as polarized dipole emitters. Herein, the demonstration of polarized light‐emitting diode based on anisotropic excitons in few‐layer ReS₂, a 2D semiconductor with excitonic transition energy of 1.5–1.6 eV, is reported. The light‐emitting device is based on minority carrier (hole) injection into n‐type ReS₂ through a hexagonal boron nitride (hBN) tunnel barrier in a metal–insulator–semiconductor (MIS) van der Waals heterostack. Two distinct emission peaks from excitons are observed at near‐infrared wavelength regime from few‐layer ReS₂. The emissions exhibit a degree of polarization of 80% reflecting the nearly 1D nature of excitons in ReS₂.