Full coverage all-inorganic cesium lead halide perovskite film for high-efficiency light-emitting diodes assisted by 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene

A full coverage all-inorganic cesium lead halide perovskite CsPbBr₃ film is achieved by introducing a small organic molecule material, 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB), as a solution additive. The light-emitting diode (LED) using this CsPbBr₃:TmPyPB perovskite film as light emitting layer exhibit improved electroluminescent (EL) performance with the maximum brightness of 22309 cd/m², highest current efficiency of 8.77 cd/A, and external quantum efficiency (EQE) of 2.27%, which are 8.6, 10.2 and 10.3 times to that of neat CsPbBr₃ film based LED, respectively. The enhanced EL performances are ascribed to less current leakage due to full coverage, and improved electron transporting in the CsPbBr₃:TmPyPB perovskite film.     

Thermally stable carboline derivative as a host material for blue phosphorescent organic light-emitting diodes

A α-carboline based high triplet energy material, 9,9′-(5′-(carbazol-9-yl)-[1,1′:3′,1″-terphenyl]-3,3″-diyl)di-α-carboline (2CbCzT), was designed and synthesized as the thermally stable host material for blue phosphorescent organic light-emitting diodes (PHOLEDs). The 2CbCzT host showed high glass transition temperature of 149 °C and high decomposition temperature of 518 °C at 5% weight loss. In addition, the 2CbCzT exhibited bipolar charge transport properties due to hole transport type carbazole and electron transport type α-carboline units. Blue PHOLEDs were developed using the high triplet energy 2CbCzT host material and a high quantum efficiency of 22.1% was obtained.     

Thermally stable indoloacridine type host material for high efficiency blue phosphorescent organic light-emitting diodes

High triplet energy materials derived from carbazole or α-carboline modified indoloacridine were synthesized and device characteristics of blue triplet emitter doped devices were investigated. The indoloacridine derived host materials showed a high triplet energy above 2.80 eV and a high glass transition temperature over 170 °C due to rigid nature of the molecular structure. The indoloacridine based host materials could approach high external quantum efficiency above 20% in blue phosphorescent organic light-emitting diodes.     

Influence of GaN material characteristics on device performance for blue and ultraviolet light-emitting diodes

An analysis of blue and near-ultraviolet (UV) light-emitting diodes (LEDs) and material structures explores the dependence of device performance on material properties as measured by various analytical techniques. The method used for reducing dislocations in the epitaxial III-N films that is explored here is homoepitaxial growth on commercial hybride vapor-phase epitaxy (HVPE) GaN substrates. Blue and UV LED devices are demonstrated to offer superior performance when grown on GaN substrates as compared to the more conventional sapphire substrate. In particular, the optical analysis of the near-UV LEDs on GaN versus ones on sapphire show substantially higher light output over the entire current-injection regime and twice the internal quantum efficiency at low forward current. As the wavelength is further decreased to the deep-UV, the performance improvement of the homoepitaxially grown structure as compared to that grown on sapphire is enhanced.     

Black phosphorus doped Poly(triarylamine) as hole transport layer for highly efficient perovskite solar cells

Charge transport layer plays a critical role in high-performance perovskite solar cells (PSCs). Herein, few-layered 2D black phosphorus (BP) nanosheet doped poly(triarylamine) (PTAA) is employed as hole transport layer for PSCs. The BP:PTAA significantly improves charge extraction at perovskite/BP:PTAA interface together with the smaller energy barrier, the increased conductivity of the PTAA film, and the formation of the high-quality perovskite film with enlarged crystal gain size, which suppress the interfacial charge recombination and trap-assisted recombination. As a result, the champion device using BP:PTAA produces the higher power conversion efficiency of 20.49% than the control device of 18.26%. Moreover, the remarkable improvement in device stability has been demonstrated attributed to the more hydrophobicity of BP:PTAA and the perovskite layer with less defect states. This work provides an effective hole transport layer for PSCs, which is comparable with the commonly used 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ):PTAA.     

双空穴注入层对有机电致蓝光器件性能的改善

CuI/CuPc被采用作为有机电致蓝光CBP:BCzVBi器件 的双空穴注入层。采用双空穴注入层后使得CBP:BCzVBi蓝光器件的启亮电压降低至 3.4 V,较采用CuPc单空穴注入层的CBP:BCzVBi蓝光器件低0.4 V。在驱动电流20 mA/cm²的情况下,与单空穴注入层器件 相比,采用该双空穴注入层结构使得器件电流效率提升约19%,亮度 增加约17%,驱动电压降低0.9 V。采用Fowler -Nordheim (F-N)隧穿注入理论对器件空穴注入电流的影响因素进行了分析,发现双空穴 注入层形成的能级台阶可以有效地改善发光器件的空穴注入效率,进而起到改善器件发光电 流效率和降低驱动电压的目的。     

9-(Pyridin-3-yl)-9H-carbazole derivatives as host materials for green phosphorescent organic light-emitting diodes

9-(Pyridin-3-yl)-9H-carbazole (PyCz) derivatives were synthesized as bipolar host materials for green phosphorescent organic light-emitting diodes (PHOLEDs) and the device performance was investigated. The PyCz core was modified with carbazole and diphenylamine to prepare the bipolar host materials. Bipolar charge transport properties were observed in the PyCz derivative with the carbazole unit and a high quantum efficiency of 21.3% was obtained in green PHOLEDs.     

Enhanced flexibility and stability of PEDOT:PSS electrodes through interfacial crosslinking for flexible organic light-emitting diodes

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films have drawn extensive attention as one of the most promising flexible transparent conductive electrodes to replace traditional indium tin oxide. However, some critical issues, such as weak adhesion, vulnerability to moisture and detrimental acidic property, need to be addressed before the practical application and industrialization. Here, we propose a facile and effective strategy of interfacial crosslinking to further improve the flexibility and stability of PEDOT:PSS electrodes with high transparency and conductivity by introducing polyethyleneimine ethoxylated (PEIE) on the surface. The flexibility and stability of PEDOT:PSS electrodes with PEIE overcoating layer are significantly improved, which can be attributed to the interfacial crosslinking reaction between PEIE and PSS. Finally, flexible organic light-emitting didoes (OLEDs) are constructed based on the PEDOT:PSS electrodes modified by PEIE, and current efficiency is enhanced from 20.5 to 76.4 cd/A with a 2.7-fold enhancement, owning to the improved carrier balance. This study confirms that PEIE is effective in protecting the PEDOT:PSS films from mechanical damage and moisture attack, while maintaining the high conductive and transmittance, and illustrates a promising future in low-cost flexible optoelectronic devices employing PEDOT:PSS electrodes.     

Chemical encapsulation of perovskite film by tetra-thiol copper(II) porphyrin for stable and clean photovoltaics

The undersaturated lead or iodide ions can produce defects during the preparation of polycrystalline perovskite films, the defects critically influenced photovoltaic device efficiency and stabilities. The leakage of toxic lead also has certain environmental safety risks. In response to the above problems, we designed a molecular modification strategy for problems. A tetra-thiol copper porphyrin (CuP) molecule was synthesized and used to modify the perovskite film. The Raman spectroscopy tests determined that CuP can be anchored on the surface of perovskite film through the Pb–S bonds between the thiol of CuP and lead on perovskite film surface. The density functional theory (DFT) calculation indicate that CuP can adsorb iodide ion and iodine, then effectively reduced iodide ions and iodine vacancies. In addition, the photoluminescence and space-charge limited current measurements shown that CuP can effectively passivate defects and improve film quality. Finally, the CuP modified PSCs enhancement of the power conversion efficiencies, the best efficiency up to 20.7%. The stabilities of humidity, thermal and photo also improved. Moreover, the surface encapsulation of CuP can also effectively prevent the leakage of lead. This work provides a strategy to fabricate stable and clean PSCs by the in situ chemical coordination on perovskite films.     

The modification of silver anode by an organic solvent (tetrahydrofuran) for top-emissive polymer light-emitting diodes

An organic solvent, tetrahydrofuran (THF), was employed to modify the Ag anode of a top-emissive polymer light-emitting diode (T-PLED) for improving the hole injection capability and the performance of a T-PLED device. The X-ray photoelectron spectroscope analysis shows that the THF molecules were chemically adsorbed on the Ag surface, forming oxygen-rich species by substrate-catalyzed decomposition. The THF-modification were found to enhance the hole injection on the Ag anode, decrease the threshold voltage, and increase the light intensity and luminous efficiency of a T-PLED device, attributing mainly to the increase of work function of the Ag anode.     

Optical and electrical characterization of (Ga,Mn)N/InGaN multiquantum well light-emitting diodes

(Ga,Mn)/N/InGaN multiquantum well (MQW) diodes were grown by molecular beam epitaxy (MBE). The current-voltage characteristics of the diodes show the presence of a parasitic junction between the (Ga,Mn)N and the n-GaN in the top contact layer due to the low conductivity of the former layer. Both the (Ga,Mn)N/InGaN diodes and control samples without Mn doping show no or very low (up to 10% at the lowest temperatures) optical (spin) polarization at zero field or 5 T, respectively. The observed polarization is shown to correspond to the intrinsic optical polarization of the InGaN MQW, due to population distribution between spin sublevels at low temperature, as separately studied by resonant optical excitation with a photon energy lower than the bandgap of both the GaN and (Ga,Mn)N. This indicates efficient losses in the studied structures of any spin polarization generated by optical spin orientation or electrical spin injection. The observed vanishing spin injection efficiency of the spin light-emitting diode (LED) is tentatively attributed to spin losses during the energy relaxation process to the ground state of the excitons giving rise to the light emission.     

Blade coating of Tris(8-hydroxyquinolinato)aluminum as the electron-transport layer for all-solution blue fluorescent organic light-emitting diodes

Organic light-emitting diodes (OLEDs) with a low driving voltage and efficient blue fluorescence were fabricated through blade coating. Tris(8-hydroxyquinolinato)aluminum (Alq₃) is a relatively stable electron-transporting material commonly used in evaporation. However, depositing Alq₃ through a solution process is difficult because of its extremely low solubility organic solvents, a result of its symmetrical molecular structure. In this study, Alq₃ was successfully deposited through blade coating at a very low concentration below 0.1wt%. The OLEDs contained co-dopants BUBD-1 and p-bis(p-N,N-diphenyl-aminostyryl)benzene (DSA-Ph), and a high-band-gap host 2-methyl-9,10-bis(naphthalen-2-yl)anthracene (MADN) as the emission layer with the following structure: ITO/PEDOT:PSS (40 nm)/VB-FNPD (30 nm)/MADN:2% BUBD-1:1% DSA-Ph (50 nm)/TPBI (30 nm)/LiF (0.8 nm)/Al (100 nm)or ITO/PEDOT:PSS (40 nm)/VB-FNPD (30 nm)/MADN:3% BUBD-1 (50 nm)tris(8-hydroxyquinolinato)aluminum (Alq₃; 10 nm)/LiF (0.8 nm)/Al (100 nm). 2,7-disubstituted fluorene-based triaryldiamine(VB-FNPD)is the cross-linking transporting material. The device exhibited a peak current efficiency of 5.67 cd/A for Alq₃ and 5.76 cd/A for TPBI. The device with Alq₃ has operated lifetime seven times higher than the device with TPBI.     

Different orientation of the transition dipole moments of two similar Pt(II) complexes and their potential for high efficiency organic light-emitting diodes

The efficiency of organic light-emitting diodes (OLEDs) is especially limited by their low light outcoupling efficiency. An approach for its enhancement is the use of horizontally oriented emitter molecules with respect to the substrate. In this study we quantitatively determine the orientation of the optical transition dipole moments in doped films of two similar phosphorescent Pt(II) complexes having a linear molecular structure. These emitters are employed in OLED devices and their efficiency is analyzed by optical simulations. For an OLED with slightly more horizontally oriented emitter molecules an external quantum efficiency (η_EQE) of 15.8% at low current-density is realized, indicating a relative improvement of outcoupling efficiency of 5.3% compared to the isotropic case. However, a very similar complex adopting isotropic molecular orientation yields η_EQE of only 11.5% implying an imperfect charge carrier balance in the OLED device and a shift of the recombination zone. Furthermore, we highlight the enormous potential of horizontal molecular orientation of emitting molecules in OLEDs.     

Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate films with low conductivity and low acidity through a treatment of their solutions with probe ultrasonication and their application as hole transport layer in polymer solar cells and perovskite solar cells

Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) has been widely used as the hole transport material in optoelectronic devices. To avoid the cross talk among different crossbars, PEDOT:PSS with low conductivity is required. It thus has a high loading of the non-conductive PSSH. The PSSH-to-PEDOT weight ratio is 6 for Clevios P VP Al 4083 that is the most popular polymer as the hole transport layer. However, the acidic PSSH brings severe problems to the device stability and performance. Here, PEDOT:PSS solutions with low acidity can be prepared through a facile treatment of PEDOT:PSS solution by probe ultrasonication. Two grades of PEDOT:PSS, Clevios PH1000 and Clevios P, with a PSSH-to-PEDOT weight ratio of 2.5 were treated by probe ultrasonication. The ultrasonication can lower the viscosity and the colloidal sizes of PEDOT:PSS solutions and conductivity of PEDOT:PSS films. The pH value of probe-ultrasonicated Clevios P was 2.12, higher than that (1.77) of pristine Clevios P VP Al 4083. The ultrasonication-treated PEDOT:PSS solutions were used as hole transport layer in polymer solar cells and perovskite solar cells. The photovoltaic performances of these solar cells are comparable to that of control devices employing Clevios P VP Al 4083 PEDOT:PSS as the hole transport layer.     

Optical performance of efficient blue/near UV nitropyridine-conjugated anthracene (NAMA) based light emitting diode

9-[(5-nitropyridin-2-aminoethyl) iminiomethyl]-anthracene (NAMA) was synthesized for the first time from the reaction between 9-antracene carboxaldehyde and 2-(2-aminoethylamino)-5-nitropyridine under mild reaction conditions. The structure of the nitropyridine-conjugated anthracene (NAMA) was characterized using ¹H-NMR, ¹³C-NMR and elemental analysis techniques. Furthermore, electroluminescent behavior of the NAMA was investigated by using a material based on polycyclic aromatic hydrocarbons (PAHs) in organic light-emitting diodes (OLEDs). It was observed that the NAMA exhibits blue/near UV emission at 410 nm with a high density signal. The optimized device structure, ITO/CuPc/α−NPD/NAMA/Alq₃/LiF/Al is characterized by blue/near UV electroluminescence (EL) and a high current density with 7735 cd m⁻² maximum brightness at approximately 10.4 V. The emitting color of the device showed the blue/near UV emission (x, y) = (0.14, 0.10) at 277.1 mA cm⁻² in CIE (Commission Internationale de l’Eclairage) chromaticity coordinates with a long operational lifetime (180 h).     

Edge-emitting electroluminescence polarization investigation of InGaN/GaN light-emitting diodes grown by metal-organic chemical vapor deposition on sapphire (0001)

The edge-emitting electroluminescence (FL) state of polarization of blue and green InGaN/GaN light-emitting diodes (LEDs) grown in EMCORE’s commercial reactors was studied and compared to theoretical evaluations. Blue (∼475 nm) LEDs exhibit strong EL polarization, up to a 3:1 distinction ratio. Green (∼530 nm) LEDs exhibit smaller ratios of about 1.5:1. Theoretical evaluations for similar InGaN/GaN superlattices predicted a 3:1 ratio between light polarized perpendicular (E⊥c) and light polarized parallel (E‖c) to the c axis. For the blue LEDs, a quantum well-like behavior is suggested because the E⊥c mode dominates the E‖c mode 3:1. In contrast, for the green LEDs, a mixed quantum well (QW)-quantum dot (QD) behavior is proposed, as the ratio of E⊥c to E‖c modes drops to 1.5:1. The EL polarization fringes were also observed, and their occurrence may be attributed to a symmetric waveguide-like behavior of the InGaN/GaN LED structure. A large 40%/50% drop in the surface root mean square (RMS) from atomic force microscopy (AFM) scans on blue/green LEDs with and without EL fringes points out that better surfaces were achieved for the samples exhibiting fringing. At the same time, a 25%/10% increase in the blue/green LED photoluminescence (PL) intensity signal was found for samples displaying EL interference fringes, indicating superior material quality and improved LED structures.     

Enhanced Photorefractivity of Poly(N‐vinylcarbazole)‐Based Composites through Electric‐Field Treatments and Ionic Liquid Doping

It is shown that the photorefractive (PR) performance of polymer composites based on poly(N‐vinylcarbazole) can be improved when samples are subjected to an electric field for a certain time, i.e. conditioned, previous to the PR characterization. It is also found that for conditioned samples the addition of an organic ionic liquid to the PR composition allows to obtain PR effect without the need of using a sensitizer. The typical electric field treatment time at room temperature and at a field of 20 V µm^?1 is 20 min. This procedure leads to a decrease of dark conductivity and an increase of photoconductivity, and consequently an increase of conductivity contrast. This results in higher PR two‐beam‐coupling gain coefficients and shorter response times, particularly at low fields. Dependencies of the process dynamics on impurities, applied field strength, temperature and the presence of an organic ionic liquid are examined in detail. It is remarkable the significant increase of the PR gain coefficients, and more drastically of the net gain coefficients, observed at low fields (<55 V µm^?1), when an ionic organic liquid such as benzalkonium chloride is added to unsensitized conditioned PR composites. These findings open a new route to improve the PR performance, not only of PVK‐based composites, but also of other types of organic materials, the main advantage being that no sensitizer is needed.     

Controlling Polymorphism in Poly(3‐Hexylthiophene) through Addition of Ferrocene for Enhanced Charge Mobilities in Thin‐Film Transistors

Crystalline organic molecules often exhibit the ability to assemble into multiple crystal structures depending on the processing conditions. Exploiting this polymorphism to optimize molecular orbital overlap between adjacent molecules in the unit lattice is an effective method for improving charge transport within the material. In this study, grazing incident X‐ray diffraction was employed to demonstrate the formation of tighter π‐π stacking poly(3‐hexylthiophene‐2,5‐diyl) polymorphs in films spin coated from ferrocene‐containing solutions. As a result, the addition of ferrocene to casting solutions yields thin‐film transistors which exhibit approximately three times higher source‐drain currents and charge mobilities than neat polymer devices. Nevertheless, XPS depth profiling and NMR analyses of the active layer reveal that all ferrocene is removed during the spin coating process, which may be an essential factor to achieve high mobilities. Such insights gleaned from ferrocene/poly(3‐hexylthiophene‐2,5‐diyl) mixtures can serve as a template for selection and optimization of other small molecule/polymer systems with greater baseline charge mobilities.