High-performance organic upconversion device with 12% photon to photon conversion efficiency at 980†nm and bio-imaging application in near-infrared region.

We demonstrated an organic upconversion device (UCD) successfully converted input NIR light (850-1310 nm) into 524 nm green emission. The UCD under 980 nm light irradiation exhibits a high photon to photon conversion efficiency of 12%. In addition, the linear dynamic range reaches 72.1 dB and the maximum on/off ratio of luminance reaches 4.4×104, which guarantee the clarity of imaging from 850 to 1310 nm. The UCD in this work has the characteristics of high efficiency and long wavelengths detection, and it makes some senses for long wavelengths NIR bio-imaging in further researches.

Characterizing the Conformational Distribution in an Amorphous Film of an Organic Emitter and Its Application in a "Self-Doping" Organic Light-Emitting Diode.

The conformational distribution and mutual interconversion of thermally activated delayed fluorescence (TADF) emitters significantly affect the exciton utilization. However, their influence on the photophysics in amorphous film states is still not known due to the lack of a suitable quantitative analysis method. Herein, we used temperature-dependent time-resolved photoluminescence spectroscopy to quantitatively measure the relative populations of the conformations of a TADF emitter for the first time. We further propose a new concept of "self-doping" for realizing high-efficiency nondoped OLEDs. Interestingly, this "compositionally" pure film actually behaves as a film with a dopant (quasi-equatorial form) in a matrix (quasi-axial form). The concentration-induced quenching that may occur at high concentrations is thus expected to be effectively relieved. The "self-doping" OLED prepared with the newly developed TADF emitter TP2P-PXZ as a neat emitting layer realizes a high maximum external quantum efficiency of 25.4 % and neglectable efficiency roll-off.

Centimeter-Long Single-Crystalline Si Nanowires.

The elongation of free-standing one-dimensional (1D) functional nanostructures into lengths above the millimeter range has brought new practical applications as they combine the remarkable properties of nanostructured materials with macroscopic lengths. However, it remains a big challenge to prepare 1D silicon nanostructures, one of the most important 1D nanostructures, with lengths above the millimeter range. Here we report the unprecedented preparation of ultralong single-crystalline Si nanowires with length up to 2 cm, which can function as the smallest active material to facilitate the miniaturization of macroscopic devices. These ultralong Si nanowires with augmented flexibility are of emerging interest for flexible electronics. We also demonstrate the first single-nanowire-based wearable joint motion sensor with superior performance to reported systems, which just represents one example of novel devices that can be built from these nanowires. The preparation of ultralong Si nanowires will stimulate the fabrication and miniaturization of electric, optical, medical, and mechanical devices to impact the semiconductor industry and our daily life in the near future.

Intermolecular Charge-Transfer Transition Emitter Showing Thermally Activated Delayed Fluorescence for Efficient Non-Doped OLEDs.

A novel molecular model of connecting electron-donating (D) and electron-withdrawing (A) moieties via a space-enough and conjugation-forbidden linkage (D-Spacer-A) is proposed to develop efficient non-doped thermally activated delayed fluorescence (TADF) emitters. 10-(4-(4-(4,6-diphenyl-1,3,5-triazin-2-yl) phenoxy) phenyl)-9,9-dimethyl-9,10-dihydroacridine (DMAC-o-TRZ) was designed and synthesized accordingly. As expected, it exhibits local excited properties in single-molecule state as D-Spacer-A molecular backbone strongly suppress the intramolecular charge-transfer (CT) transition. And intermolecular CT transition acted as the vital radiation channel for neat DMAC-o-TRZ film. As in return, the non-doped device exhibits a remarkable maximum external quantum efficiency (EQE) of 14.7 %. These results prove the feasibility of D-Spacer-A molecules to develop intermolecular CT transition TADF emitters for efficient non-doped OLEDs.

Synthesis, Structure, and Photophysical Properties of Two Four-Coordinate Cu(I)-NHC Complexes with Efficient Delayed Fluorescence.

Two luminescent cationic heteroleptic four-coordinate Cu(I) complexes supported by N-heterocyclic carbene ligand and diphosphine ligand were successfully prepared and characterized. These complexes adopt typical distorted tetrahedral configuration and have high stability in solid state. Quantum chemical calculations show carbene units have contributions to both highest occupied molecular orbitals and lowest unoccupied molecular orbitals of these Cu(I)-NHC complexes, the lowest-lying singlet and triplet excitations (S0 → S1 and S0 → T1) of [Cu(Pyim)(POP)](PF6) are dominated by metal-to-ligand charge transfer (MLCT) transition, while the S0 → S1 and S0 → T1 excitations of [Cu(Qbim)(POP)](PF6) are mainly MLCT and ligand-centered transitions, respectively. These Cu(I)-NHC complexes show efficient long-lifetime emissions (λem = 520 nm, τ = 79.8 µs, Φ = 0.56 for [Cu(Pyim)(POP)](PF6), λem = 570 nm, τ = 31.97 µs (78.99%) and 252.2 µs (21.01%), Φ = 0.35 for [Cu(Qbim)(POP)](PF6)) in solid state at room temperature, which are confirmed as delayed fluorescence by investigating the emissions at 77 K.

Theoretical investigation of the singlet-triplet splittings for carbazole-based thermally activated delayed fluorescence emitters.

Accurate prediction of singlet-triplet splittings (ΔESTs) is a key issue for the design of thermally activated delayed fluorescence (TADF) emitters. Due to the evident changes between ground- and excited-state electronic structures, the ΔESTs of carbazole (Cz)-based TADF emitters can't be accurately predicted based on the current optimal Hartree-Fock percentage (HF%) (OHF) method. To address this issue, here, we used the adiabatic excitation energy method to accurately predict the ΔESTs of the TADF emitters with different geometries of ground- and excited-states by calculating the minimum potential energy differences between the ground- and excited-states considering the relaxation effect. With the optimized excited states using the B3LYP functional, the theoretically calculated values of ΔESTs based on our method are well consistent with the experimental results for the Cz-based TADF emitters, obviously improved compared with the calculated results based on the OHF method. These results indicate that the adiabatic excitation energy method with the B3LYP functional is a general and accurate way to predict the ΔESTs of Cz-based TADF emitters.

Unraveling non-radiative decay channels of exciplexes to construct efficient red emitters for organic light-emitting diodes.

Exciplex emitters naturally have thermally activated delayed fluorescence characteristics due to their spatially separated molecular orbitals. However, the intermolecular charge transfer potentially induces diverse non-radiative decay channels, severely hindering the construction of efficient red exciplexes. Thus, a thorough comprehension of this energy loss is of paramount importance. Herein, different factors, including molecular rigidity, donor-acceptor interactions and donor-donor/acceptor-acceptor interactions, that impact the non-radiative decay were systematically investigated using contrasting exciplex emitters. The exciplex with rigid components and intermolecular hydrogen bonds showed a photoluminescence quantum yield of 84.1% and a singlet non-radiative decay rate of 1.98 × 106 s-1 at an optimized mixing ratio, respectively, achieving a 3.3-fold increase and a 70% decrease compared to the comparison group. In the electroluminescent device, a maximum external quantum efficiency of 23.8% was achieved with an emission peak of 608 nm, which represents the state-of-the-art organic light-emitting diodes using exciplex emitters. Accordingly, a new strategy is finally proposed, exploiting system rigidification to construct efficient red exciplex emitters that suppress non-radiative decay.

High-Quality Artery Monitoring and Pathology Imaging Achieved by High-Performance Synchronous Electrical and Optical Output of Near-Infrared Organic Photodetector.

Near-infrared organic photodetectors (NIR-OPDs) are significant technologies in emerging biomedicine applications for uniquely wearable, noninvasive, low-cost advantages. However, biosignals are weak and changing rapidly so practical biodetection and bioimaging are still challenging for NIR-OPDs. Herein, high-performance NIR-OPDs with synchronous optical output are realized by recombining anode-injected electrons with photogenerated holes on emitters. Owing to high detection performance of 4.5 × 1012 Jones detectivity and 120 kHz -3 dB bandwidth, five arteries are monitored by transmission-type method and cardiac cycle is analyzed. Importantly, the synchronous optical output is direct emission demonstrating outstanding photon conversion efficiency approaching 20% and luminance signal-to-noise ratio over 8000. Consequently, pathology imaging is directly developed without complex readout circuits and arrays from which squamous metaplasia of cervix and carcinoma of large intestine are observed clearly. The NIR-OPD demonstrates strategies for high-performance synchronous electrical/optical output and directly imaging. Biomedicine applications implemented here are high level, representing important steps for NIR-OPDs toward providing clues for clinical diagnosis.

Highly Efficient OLEDs by Using a Brominated Thermally Activated Delayed Fluorescent Host due to Balanced Carrier Transport and Enhanced Exciton Upconversion.

Thermally activated delayed fluorescent (TADF) materials are naturally bipolar and can potentially serve as hosts. However, triplet excitons in TADF materials are long-lived and prone to unfavorable bimolecular processes. Implementing an efficient reverse system intersection (RISC) process is an effective solution. Moreover, although the general TADF host is bipolar, polarity differences still cause a mobility imbalance. In this work, we designed and synthesized a novel TADF host material, 11-(3-(4-(3-bromophenyl)-6-phenyl-1,3,5-triazin-2-yl)phenyl)-12,12-dimethyl-11,12-dihydroindeno[2,1-a]carbazole (Br-DMIC-TRZ). The upconversion of the TADF host and its doped films is facilitated due to enhanced spin-orbit coupling (SOC) induced by bromine, which exhibits a higher rate of RISC. This progress facilitates the involvement of more triplet excitons in luminescence. Meanwhile, the attachment of bromine to the acceptor fragment of TADF enhances the electron mobility, where hole mobility and electron mobility are more comparable. Enhanced exciton upconversion and balanced carrier transport allow devices formed based on brominated TADF hosts to outperform other hosts. The Br-TADF-based devices with three dopants sensitized achieved improvements of 29.8, 21.4, and 24.4% compared to the DMIC-TRZ-based device. This work provides a feasible molecular design strategy for further developing efficient hosts.

Efficient Near-Infrared Organic Photodetectors with Spectral Response up to 1600 nm for Accurate Alcohol Concentration Detection.

The development of near-infrared organic photodetectors (NIR-OPDs) in 1000-1700 nm is essential for medical monitoring, food quality inspection, machine vision, and biomedical imaging. However, when solving the high dark current density (JD) in bulk-heterojunction (BHJ) NIR-OPDs based on narrow-bandgap systems, it is often accompanied by photocurrent loss, which is a great challenge in achieving high-performance NIR-OPDs. Here, an ideal hybrid pseudo-PHJ (planar-heterojunction)/BHJ structure is proposed to overcome this challenge, which is introducing the N2200 layer between the cathode and BHJ. The introduction of the N2200 raises the external charge injection barrier and reduces the trap density, thus achieving significant suppression of JD (6.22 × 10-7 A cm-2 at -0.2 V bias, about 2 orders of magnitude lower compared to the BHJ NIR-OPDs). Meanwhile, the hybrid structure combines the advantages of PHJ and BHJ, thus maintaining a high photocurrent, resulting in responsivity and detectivity of 18.71 mA W-1 and 4.19 × 1010 Jones, respectively, at 1400 nm at -0.2 V bias, which is superior to the performance of BHJ NIR-OPDs. And the hybrid structured NIR-OPDs are proven to rapidly quantify the alcohol content of mixtures to within 2% accuracy, which exhibits great potential for application in the food and pharmaceutical industries.

A high-performance dual-functional organic upconversion device with detectivity approaching 1013 Jones and photon-to-photon efficiency over 20.

Organic upconversion devices (UCDs) are a cutting-edge technology and hot topic because of their advantages of low cost and convenience in the important applications of near-infrared (NIR) detection and imaging. However, to realize utilization of triplet excitons (T1), previous UCDs have the drawback of heavily relying on toxic and costly heavy-metal-doped emitters. More importantly, due to poor performance of the detecting unit and/or emitting unit, improving their detectivity (D*) and photon-to-photon conversion efficiency (ηp-p) is still a challenge for real applications. Here, we report a high-performance dual-functional purely organic UCD that has an outstanding D* approaching 1013 Jones and a high ηp-p of 20.1% in the NIR region, which are some of the highest values among those reported for UCDs. The high performance is credited to the excellent D* of the detecting unit, exceeding 1014 Jones, and is also attributed to efficient T1 utilization via a dual reverse intersystem crossing channel and high optical out coupling achieved via a high horizontal dipole ratio in the emitting unit. The high D* and ηp-p enable the UCD to detect 850 nm light at as little as 0.29 µW cm-2 and with a high display contrast of over 70 000 : 1, significantly improving the potential of practical applications of UCDs in NIR detection and imaging. Furthermore, a fast rise time and fall time of 8.9 and 14.8 µs are also achieved. Benefiting from the high performance, consequent applications of low-power pulse-state monitoring and fine-structure bio-imaging are successfully realized with high quality results by using our organic UCDs. These results demonstrate that our design not only eliminates dependence of UCDs on heavy-metal emitters, but also takes their performance and applications to a high level.

Novel bipolar host materials based on 1,3,5-triazine derivatives for highly efficient phosphorescent OLEDs with extremely low efficiency roll-off.

Recently, bipolar host materials have attracted considerable attention because they can achieve balanced charge injection/transport in phosphorescent organic light emitting diodes (PhOLEDs) and consequently obtain excellent device performance. In this work, two bipolar host materials, namely, 3-(4,6-diphenyl-1,3,5-triazin-2-yl)-9-phenyl-9H-carbazole (DPTPCz) and 3-(4,6-diphenoxy-1,3,5-triazin-2-yl)-9-phenyl-9H-carbazole (DPOTPCz), have been designed, synthesized and characterized. With high triplet energy levels of 2.78 and 2.86 eV for DPTPCz and DPOTPCz, respectively, two compounds are considered promising bipolar host materials for PhOLEDs. Blue and green PhOLEDs based on these two new compounds show excellent performances. The phosphorescent devices based on DPTPCz exhibit maximum external quantum efficiencies of 14.4% (for blue device) and 21.2% (for green device), and maintain high efficiencies of 11.9% and 20.0% even at a high luminance of 10,000 cd m(-2).

Novel Dark Current Reduction Strategy via Deep Bulk Traps for High-Performance Solution-Processed Organic Photodetectors.

The performance improvement of the organic photodetectors (OPDs) focuses on suppressing the dark current density (Jd) to improve the specific detectivity. In this work, a dark current reduction strategy relying on constructing limited deep traps in the active layer to suppress charge injection rate was newly proposed. And an optimization method has been successfully demonstrated on the solution-processed OPDs accordingly. Compared with the Jd expressed by the OPD with the shallow trap system, the device with deep bulk traps exhibits a dramatically reduced dark current while ensuring high responsivity. At a bias of -2 V, the optimized photodiode with a Jd down to 1.4 × 10-5 mA cm-2 and a maximum responsivity of 0.42 A W-1 @620 nm was realized, leading to a maximum detectivity calculated from shot noise of 6.23 × 1012 Jones. This value is 49-fold higher than that of the original OPD with the same structure. The effects of deep traps inside the semiconductor film on injected carriers and photogenerated carriers are well explained by the relative positions of the initial hopping levels. A better understanding of charge transport regimes in OPD helps to open new approaches for constructing high-performance OPD toward practical applications.

Additive-Induced Vertical Component Distribution Enables High-Performance Sequentially Cast Organic Solar Cells.

Modulation of the active layer morphology to form a vertical component distribution structure is an effective way of improving the efficiency of organic solar cells (OSCs). In this paper, a layer-by-layer (LbL) spin-coating method was adopted combined with an additive strategy to achieve the purpose of precisely adjusting the morphology, and finally, high-performance OSCs based on a D18-Cl/Y6 system were achieved. After adding n-octane in D18-Cl, D18-Cl+/Y6 devices realized a PCE of 17.70%, while with the incorporation of 1-fluoronaphthalene (FN) in Y6, D18-Cl/Y6+ devices obtained a power conversion efficiency (PCE) of 17.39%, both higher than the control devices (16.66%). The former resulted in a more orderly arrangement of D18-Cl, forming a suitable phase separation morphology, and the latter improved the crystallization of Y6, which facilitated carrier transport. Furthermore, the dual-additive-treated D18-Cl+/Y6+ bilayer devices with n-octane doping in the donor and FN in the acceptor had a more desirable vertical morphology, exhibiting an excellent PCE of 18.16% with an improved JSC of 27.17 mA cm-2 and FF of 76.88%, one of the highest efficiencies for LbL OSCs. The results demonstrated that combining the LbL spin-coating method with the additive strategy is a valid way to achieve hierarchical morphology control and enhance device performance, which is of great significance for the fabrication and development of OSCs.

A novel orange-red thermally activated delayed fluorescence emitter with high molecular rigidity and planarity realizing 32.5% external quantum efficiency in organic light-emitting diodes.

Simultaneous optimization of photoluminescence quantum yield (ΦPL) and horizontally oriented dipoles (Θ‖) is considerably challenging for orange and red thermally activated delayed fluorescence (TADF) emitters, due to the conflicts between enhancing molecular rigidity and improving molecular planarity. Herein, a novel orange-red TADF emitter 10-(dipyrido[3,2-a:2',3'-c]phenazin-11-yl)-10H-spiro[acridine-9,9'-fluorene] (SAF-2NP) was constructed with a donor-acceptor structure. The highly rigid donor and acceptor segments ensure the overall rigidity of the emitter. More importantly, the quasi-coplanar structure between the acceptor and the fluorene moiety in the donor unit enlarges the molecular plane without weakening rigidity. Consequently, SAF-2NP exhibited extremely high ΦPL and Θ‖ of 99% and 85%, respectively. The optimal organic light-emitting diode using SAF-2NP as the emitter and 4,4'-di(9H-carbazol-9-yl)-1,1'-biphenyl (CBP) as the host demonstrated an unparalleled external quantum efficiency of 32.5% and a power efficiency of 85.2 lm W-1 without any extra light extraction structure. This work provides a feasible strategy to establish efficient orange and red TADF emitters with both high rigidity and planarity.

Thermally activated delayed fluorescence exciplex emitters for high-performance organic light-emitting diodes.

Owing to their natural thermally activated delayed fluorescence (TADF) characteristics, the development of exciplex emitters for organic light-emitting diodes (OLEDs) has witnessed booming progress in recent years. Formed between electron-donating and electron-accepting molecules, exciplexes with intermolecular charge transfer processes have unique advantages compared with unimolecular TADF materials, offering a new way to develop high-performance TADF emitters. In this review, a comprehensive overview of TADF exciplex emitters is presented with a focus on the relationship between the constituents of exciplexes and their electroluminescence performance. We summarize and discuss the latest and most significant developments of TADF exciplex emitters. Notably, the design principles of efficient TADF exciplex emitters are systematically categorized into three systems within this review. These progressive achievements of TADF exciplex emitters point out future challenges to trigger more research endeavors in this growing field.

Nonconjugated Triptycene-Spaced Donor-Acceptor-Type Emitters Showing Thermally Activated Delayed Fluorescence via Both Intra- and Intermolecular Charge-Transfer Transitions.

Thermally activated delayed fluorescence (TADF) emitters have aroused considerable attention, particularly for their great potential in organic light-emitting diodes (OLEDs). In typical TADF molecules, intramolecular charge transfer (CT) between electron-donor (D) and electron-acceptor (A) moieties is the dominant transition. Actually, CT transitions can possibly occur between different molecules as well. Herein, we used a nonconjugated triptycene (TPE) moiety to space D and A moieties and developed two novel emitters tBuDMAC-TPE-TRZ and tBuDMAC-TPE-TTR to explore the roles of intra- and intermolecular CT transitions. Along with weak intramolecular CT transitions, intermolecular CT transitions are dominant for tBuDMAC-TPE-TRZ and tBuDMAC-TPE-TTR neat films. Particularly, tBuDMAC-TPE-TRZ showed a high maximum external quantum efficiency of 10.0% in a nondoped solution-processed OLED, which was evidently higher than that of a corresponding 10 wt % tBuDMAC-TPE-TRZ-doped OLED with 4,4',4″-tris(carbazol-9-yl)triphenylamine (TCTA) as the host matrix. The results prove that intermolecular CT transitions indeed participate in the CT transition process in these systems and they are helpful to enhance the electroluminescence performance of emitting systems with weak intramolecular CT transitions.

Single-Crystalline Silicon Frameworks: A New Platform for Transparent Flexible Optoelectronics.

Single-crystalline silicon (sc-Si) is the dominant semiconductor material for the modern electronics industry. Despite their excellent photoelectric and electronic properties, the rigidity, brittleness, and nontransparency of commonly used silicon wafers limit their application in transparent flexible optoelectronics. In this study, a new type of Si microstructure, named single-crystalline Si frameworks (sc-SiFs), is developed, through a combination of wet-etching and microfabrication technologies. The sc-SiFs are self-supported, flexible, lightweight, tailorable, and highly transparent. They can withstand a small bending radius of less than 0.5 mm and have a transparency of up to 96% in all wavelength ranges, owing to the hollowed-out framework structures. Thus, the sc-SiFs provide a new platform for high-performance transparent flexible optoelectronics. Taking transparent flexible photodetectors (TFPDs) as an example, substrate-free and self-driven TFPDs are achieved based on the sc-SiFs. The devices exhibit superior performance compared to other reported TFPDs and reveal the great potential for integrated optoelectronic applications. The development of sc-SiFs paves the way toward the fabrication of high-performance transparent flexible devices for a host of applications, including e-skins, the Internet of Things, transparent flexible displays, and artificial visual cortexes.

Introducing Trifluoromethyl to Strengthen Hydrogen Bond for High Efficiency Organic Solar Cells.

Nowadays, the ternary strategy has become a common way to improve the power conversion efficiency (PCE) of organic solar cells (OSCs). The intermolecular interaction between the third component and donor or acceptor plays a key role in achieving a high performance. However, hydrogen bond as a strong intermolecular interaction is rarely considered in ternary OSCs. In this work, we introduce trifluoromethyl on a newly synthesized small molecular DTBO to strength hydrogen bonds between DTBO and IEICO-4F. Due to the existence of hydrogen bonds has a strong impact on electrostatic potential (ESP) and benefits π-π stacking in the active layer, the ternary OSCs show superior charge extraction and low charge recombination. In DTBO, PTB7-Th and IEICO-4F based ternary devices, the PCE increases from 11.02 to 12.48%, and short-circuit current density (J SC ) increases from 24.94 to 26.43 mA/cm2 compared with typical binary devices. Moreover, the addition of DTBO can realize an energy transfer from DTBO to PTB7-Th and broaden the absorption spectrum of blend films. Grazing-incidence wide-angle X-ray scattering (GIWAXS) patterns show that the π-π stacking distance of IEICO-4F decreased after adding 10 wt% DTBO. The effect of the hydrogen bond is also achieved in the PM6: Y6 system, showing 16.64% efficiency by comparison to the 15.49% efficiency of binary system. This work demonstrates that introduce trifluoromethyl to enhance hydrogen bond which improve π-π stacking can achieve higher performance in OSCs.

6,12-Dihydro-6,12-diboradibenzo[def,mno]chrysene: A Doubly Boron-Doped Polycyclic Aromatic Hydrocarbon for Organic Light Emitting Diodes by a One-Pot Synthesis.

One-pot synthesis of a new doubly boron-doped polycyclic aromatic hydrocarbon of 6,12-dimesityl-6,12-dihydro-6,12-diboradibenzo[def,mno]chrysene (MDBDBC) was reported. MDBDBC features a rigid planar electron-deficient core structure and demonstrates good chemical and thermal stabilities. A low-lying LUMO of -3.53 eV, a low locally excited triplet energy of 1.92 eV, as well as green electroluminescence with maximum EQE of 4.9% were found for MDBDBC, suggesting its potential as an n-type unit for future organic light emitting diode applications.