Neodymium Chloride-Doped Perovskite Nanocrystals for Efficient Blue Light-Emitting Devices.

Metal halides doping of perovskite nanocrystals (NCs) has been shown to precisely control nonradiative pathways and to improve photoluminescence quantum yield (PLQY). Here, we report a trivalent lanthanide halide neodymium (III) chloride (NdCl3)-doped perovskite NCs prepared with a post-synthetic room temperature treatment for efficient blue light-emitting devices (LEDs). The Nd 3d and Cl 2p core peaks were observed in the NdCl3-doped NCs, which allowed for simultaneous doping of Nd3+ and Cl- into the pristine CsPbBr3 NCs. The NdCl3-doped NCs exhibited blue emission at a peak wavelength of 478 nm with a high PLQY of 97% in solution. We found that the Nd3+ cation incorporated into the NCs more effectively suppressed nonradiative recombination compared with common halide anion exchange from temperature dependence of optical properties. Blue LEDs based on NdCl3-doped NCs had an external quantum efficiency of 2.7%, which represents a considerable performance improvement compared with LEDs based on organic chloride salt-doped NCs.

Surface Crystal Growth of Perovskite Nanocrystals via Postsynthetic Lead(II) Bromide Treatment to Increase the Colloidal Stability and Efficiency of Light-Emitting Devices.

The surface modification of metal halide perovskite nanocrystals (NCs) significantly impacts their optical properties and colloidal stability. This subsequently affects the performance of light-emitting devices (LEDs). Therefore, numerous surface passivation techniques like ligand exchange and metal halide doping have been explored to passivate the surface defects of perovskite NCs and obtain highly efficient LEDs. In this study, we demonstrated the postsynthetic metal halide doping treatment using lead(II) bromide (PbBr2) to passivate the surface defects of the CsPbBr3 NCs at a moderate reaction temperature of 80 °C. The alkyl quaternary ammonium salt, didodecyldimethylammonium bromide (DC12AB), enabled the complete dissolution of PbBr2 in a nonpolar solvent, toluene. Because of surface crystal growth, the particle sizes of the PbBr2-doped CsPbBr3 NCs were higher than those of the as-synthesized CsPbBr3 NCs. The photoluminescence quantum yield of the CsPbBr3 NCs drastically increased from 26.8 to 83.9% after the PbBr2 doping treatment. Moreover, the PbBr2-doped CsPbBr3 NCs possessed long-term colloidal stability of more than 2 months that indicates the strong bonding between the NCs and ligands. We observed that the alkyl chain length of the quaternary alkyl ammonium salts affected the luminance and device stability during operations. In this study, a promising strategy was devised to achieve highly luminescent perovskite NCs with excellent colloidal stability that can enhance the performance of LEDs.

Molecular Orientations of Delayed Fluorescent Emitters in a Series of Carbazole-Based Host Materials.

Molecular orientation is one of the most crucial factors to boost the efficiency of organic light-emitting devices. However, active control of molecular orientation of the emitter molecule by the host molecule is rarely realized so far, and the underlying mechanism is under discussion. Here, we systematically investigated the molecular orientations of thermally activated delayed fluorescence (TADF) emitters in a series of carbazole-based host materials. Enhanced horizontal orientation of the TADF emitters was achieved. The degree of enhancement observed was dependent on the host material used. Consequently, our results indicate that π-π stacking, CH/n (n = O, N) weak hydrogen bonds, and multiple CH/π contacts greatly induce horizontal orientation of the TADF emitters in addition to the molecular shape anisotropy. Finally, we fabricated TADF-based organic light-emitting devices with an external quantum efficiency (ηext) of 26% using an emission layer with horizontal orientation ratio (Θ) of 79%, which is higher than that of an almost randomly oriented emission layer with Θ of 62% (ηext = 22%).

Doping of Tetraalkylammonium Salts in Polyethylenimine Ethoxylated for Efficient Electron Injection Layers in Solution-Processed Organic Light-Emitting Devices.

For efficient electron injection, a method to control the work functions (WFs) of ZnO electrodes in organic light-emitting devices (OLEDs) is reported in this study. First, ZnO was modified by doping of tetraalkylammonium salts (TRAX) into polyethylenimine ethoxylated (PEIE) for the WF control. Tetrabutylammonium salts (TBAX), where X = chloride, bromide, iodide, acetate, thiocyanate, and tetrafluoroborate anions, were doped into PEIE. A WF of nondoped PEIE-modified ZnO was 3.65 eV, whereas TBAX-doped PEIE-modified ZnO exhibited WFs ranging from 3.52 to 3.00 eV depending on the anion. TBAX salts exhibited different electron-donating capabilities depending on the anion, and the doping of TBAX with a large electron-donating capability exhibited a large WF reduction effect. In addition, tetraethyl- and tetrahexylammonium chlorides were doped into PEIE. PEIE doped with TRACl containing long alkyl chains exhibited a large WF reduction effect due to its low electron-accepting capabilities. In addition, the WF reduction mechanism was considered by the depth direction analysis of the PEIE:TBAX films. Finally, the ZnO/PEIE:TRAX bilayers were applied as electron injection layers in poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] emissive-layer-based OLEDs with an inverted structure. The ZnO/PEIE:TBAX devices with low WFs exhibited low driving voltages.

Control of Molecular Orientation in Organic Semiconductor Films using Weak Hydrogen Bonds.

Use of the intrinsic optoelectronic functions of organic semiconductor films has not yet reached its full potential, mainly because of the primitive methodology used to control the molecular aggregation state in amorphous films during vapor deposition. Here, a universal molecular engineering methodology is presented to control molecular orientation; this methodology strategically uses noncovalent, intermolecular weak hydrogen bonds in a series of oligopyridine derivatives. A key is to use two bipyridin-3-ylphenyl moieties, which form self-complementary intermolecular weak hydrogen bonds, and which do not induce unfavorable crystallization. Another key is to incorporate a planar anisotropic molecular shape by reducing the steric hindrance of the core structure for inducing π-π interactions. These synergetic effects enhance horizontal orientation in amorphous organic semiconductor films and significantly increasing electron mobility. Through this evaluation process, an oligopyridine derivative is selected as an electron-transporter, and successfully develops highly efficient and stable deep-red organic light-emitting devices as a proof-of-concept.

Purification of Perovskite Quantum Dots Using Low-Dielectric-Constant Washing Solvent "Diglyme" for Highly Efficient Light-Emitting Devices.

Cesium lead halide (CsPbX3, X = Cl, Br, or I) perovskite quantum dots (QDs) are known as ionic nanocrystals, and their optical properties are greatly affected by the washing solvent used during the purification process. Here, we demonstrate the purification process of CsPbBr3 perovskite QDs using low-dielectric-constant solvents to completely remove impurities, such as the reaction solvent and desorbed ligands. The use of the ether solvent diethylene glycol dimethyl ether (diglyme), having a low dielectric constant of ε = 7.23, as a poor solvent for reprecipitation allowed for multiple wash cycles, which led to high purity and high photoluminescence quantum yield for CsPbBr3 QDs. The light-emitting device constructed with the CsPbBr3 QDs and washed twice with diglyme (two-wash) showed a low turn-on voltage of 2.7 V and a peak external quantum efficiency of over 8%. Thus, the purification of perovskite QDs with multiple wash cycles using a low-dielectric-constant solvent is an effective approach for enhancing not only the optical properties but also the efficiency of perovskite quantum dot light-emitting devices.

Two-Dimensional Ca2Nb3O10 Perovskite Nanosheets for Electron Injection Layers in Organic Light-Emitting Devices.

We report in this article the application of calcium niobate (CNO) perovskite nanosheets for electron injection layers (EILs) in organic light-emitting devices (OLEDs). Four kinds of tetraalkylammonium hydroxides having different alkyl lengths were utilized as the exfoliation agents of a layered compound precursor HCa2Nb3O10 to synthesize CNO nanosheets, including tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide (TPAOH), and tetrabutylammonium hydroxide. CNO nanosheet EILs were applied in fluorescent poly[(9,9-di- n-octylfluorenyl-2,7-diyl)- alt-(benzo[2,1,3]thiadiazol-4,8-diyl)] (F8BT) organic light-emitting polymer-based devices. The effects of dispersion concentrations and alkyl chain length on the devices' performances were investigated. The results demonstrated that OLEDs' performances were related to the coverage ratio of the CNO nanosheets, their thicknesses, and their work function values. Among the four exfoliation agents, the device with CNO nanosheets exfoliated by TPAOH showed the lowest driving voltage. The OLEDs with the CNO nanosheet EILs showed lower driving voltages compared with the devices with conventional EIL material lithium 8-quinolate.

Colorful Squaraines Dyes for Efficient Solution-Processed All Small-Molecule Semitransparent Organic Solar Cells.

Semitransparent organic solar cells (ST-OSCs) exhibit highly promising applications to develop integrated photovoltaics and power-generating windows. However, the development of ST-OSCs is significantly lagging behind opaque OSCs, especially for all small-molecule ST-OSCs. Here, four unique squaraines dyes (IDPSQ, SQ-BP, D-BDT-SQ, and AzUSQ) were successfully used as donors in ST-OSCs, whose colors can be tuned from purple to blue, green, and dark green, respectively. While using ultrathin Ag as a transparent electrode, the ST-OSCs fabricated using IDPSQ:PC71BM, SQ-BP:PC71BM, D-BDT-SQ:PC71BM, and AzUSQ:PC71BM yield power conversion efficiencies (PCEs) of 2.96, 4.36, 4.91, and 1.71%, respectively, and their colors are purple, cyan, brown, and light brown, respectively. Compared to their opaque OSCs (PCEs of 3.95, 5.45, 5.84, and 1.91%, respectively), the reduction in the PCEs are as low as 25, 20, 16, and 10%, respectively. Furthermore, each of these ST-OSCs exhibit good average visible transmittance (AVT) of 25-30%, favorable CIE color coordinates, and a color rendering index (CRI) of 71-97%. Finally, by changing the thickness of the Ag electrode, an impressive PCE of 4.9% along with an AVT of 25% and a CRI of 97% can be obtained in D-BDT-SQ:PC71BM-based ST-OSCs, which is the highest PCE among all small-molecule ST-OSCs.

Conjugated Polyelectrolyte Blend with Polyethyleneimine Ethoxylated for Thickness-Insensitive Electron Injection Layers in Organic Light-Emitting Devices.

Electron injection layers (EILs) based on a simple polymer blend of polyethyleneimine ethoxylated (PEIE) and poly[(9,9-bis(3'-(( N, N-dimethyl)- N-ethylammonium)-propyl)-2,7-fluorene)- alt-2,7-(9,9-dioctylfluorene)] (PFN-Br) can suppress the dependence of organic light-emitting device (OLED) performance on thickness variation compared with single PEIE or PFN-Br EILs. PEIE and PFN-Br were compatible with each other and PFN-Br uniformly mixed in the PEIE matrix. PFN-Br in PEIE formed more fluorene-fluorene pairs than PFN-Br alone. In addition, PEIE:PFN-Br blends reduced the work function (WF) substantially compared with single PEIE or PFN-Br polymer. PEIE:PFN-Br blends were applied to EILs in fluorescent polymer-based OLEDs. Optimized PEIE:PFN-Br blend EIL-based devices presented lower driving voltages and smaller dependences of device performance on EIL thickness than single PEIE or PFN-Br-based devices. These improvements were attributed to electron-transporting fluorene moieties, increased fluorene-fluorene pairs working as channels of electron transport, and the large WF reduction effect of PEIE:PFN-Br blends.

Air-Stable and High-Performance Solution-Processed Organic Light-Emitting Devices Based on Hydrophobic Polymeric Ionic Liquid Carrier-Injection Layers.

A lot of research, mostly using electron-injection layers (EILs) composed of alkali-metal compounds has been reported with a view to increase the efficiency of solution-processed organic light-emitting devices (OLEDs). However, these materials have intractable properties, such as a strong affinity for moisture, which cause the degradation of OLEDs. Consequently, optimal EIL materials should exhibit high electron-injection efficiency as well as be stable in air. In this study, polymer light-emitting devices (PLEDs) based on the commonly used yellow-fluorescence-emitting polymer F8BT, which utilize poly(diallyldimethylammonium)-based polymeric ionic liquids, are experimentally and analytically investigated. As a result, the optimized PLED employing an EIL comprising poly(diallyldimethylammonium) bis(trifluoromethanesulfonyl)imide (poly(DDA)TFSI), which is expected to display good moisture resistance because of water repellency of fluorocarbon groups, exhibits excellent storage stability in air and electroluminescence performance with a low turn-on voltage of 2.01 V, maximum external quantum efficiency of 9.00%, current efficiency of 30.1 cd A-1 , and power efficiency of 32.4 lm W-1 . The devices with poly(DDA)TFSI show one of the highest efficiencies as compared to the reported standard PLEDs. Moreover, poly(DDA)TFSI is applied as a hole-injection layer (HIL). The optimized PLED using poly(DDA)TFSI as the HIL exhibits performances comparable to those of a device that uses a conventional poly(3,4-ethylenedioxy-thiophene):poly(4-styrenesulfonate) HIL.

Post-Treatment-Free Solution-Processed Reduced Phosphomolybdic Acid Containing Molybdenum Oxide Units for Efficient Hole-Injection Layers in Organic Light-Emitting Devices.

We report the development of solution-processed reduced phosphomolybdic acid (rPMA) containing molybdenum oxide units for post-treatment-free hole-injection layers (HILs) in organic light-emitting devices (OLEDs). The physical and chemical properties of rPMA, including its structure, solubility in several solvents, film surface roughness, work function, and valence states, were investigated. The formation of gap states just below the Fermi level of rPMA was observed. Without any post-treatment after the formation of rPMA films, OLEDs employing rPMA as an HIL exhibited a very low driving voltage and a high luminous efficiency. The low driving voltage was attributed to the energy level alignment between the gap states formed by reduction and the HOMO level of the hole-transport layer material N,N'-bis(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine.

A Series of Lithium Pyridyl Phenolate Complexes with a Pendant Pyridyl Group for Electron-Injection Layers in Organic Light-Emitting Devices.

We report a new series of lithium pyridyl phenolate complexes with a pendant pyridyl group, Li2BPP, Li3BPP, and Li4BPP, in which the pendant pyridines are substituted at the 2-, 3-, and 4-positions, respectively. The most important difference between these complexes is their molecular planarity; Li3BPP and Li4BPP adopt twisted bipyridine structures, whereas Li2BPP adopts a planar structure owing to the steric hindrance and chelating effect of bipyridine on the Li core. The planar structure leads to crystallization through π-π stacking interactions, and the small differences in the molecular structures of the pendant pyridine rings cause drastic differences in the physical properties of thin solid films of these complexes. We applied these complexes as electron-injection layers (EILs) in Ir(ppy)3-based organic light-emitting devices. When thin EILs were used, Li3BPP and Li4BPP afforded lower driving voltages than Li2BPP; the order of the driving voltages followed the order of their electron affinity values. Moreover, the dependence of driving voltage on the EIL thickness was investigated for each complex. Among the three LiBPP derivatives, Li2BPP-based devices showed almost negligible EIL thickness dependence, which may be attributable to the high crystallinity of Li2BPP. All LiBPP-based devices also showed higher stability than conventional 8-quinolinolato lithium-based devices.

High-Efficiency Perovskite Quantum-Dot Light-Emitting Devices by Effective Washing Process and Interfacial Energy Level Alignment.

All inorganic perovskites quantum dots (PeQDs) have attracted much attention for used in thin film display applications and solid-state lighting applications, owing to their narrow band emission with high photoluminescence quantum yields (PLQYs), color tunability, and solution processability. Here, we fabricated low-driving-voltage and high-efficiency CsPbBr3 PeQDs light-emitting devices (PeQD-LEDs) using a PeQDs washing process with an ester solvent containing butyl acetate (AcOBu) to remove excess ligands from the PeQDs. The CsPbBr3 PeQDs film washed with AcOBu exhibited a PLQY of 42%, and a narrow PL emission with a full width at half-maximum of 19 nm. We also demonstrated energy level alignment of the PeQD-LED in order to achieve effective hole injection into PeQDs from the adjacent hole injection layer. The PeQD-LED with AcOBu-washed PeQDs exhibited a maximum power efficiency of 31.7 lm W-1 and EQE of 8.73%. Control of the interfacial PeQDs through ligand removal and energy level alignment in the device structure are promising methods for obtaining high PLQYs in film state and high device efficiency.

Addition of Lithium 8-Quinolate into Polyethylenimine Electron-Injection Layer in OLEDs: Not Only Reducing Driving Voltage but Also Improving Device Lifetime.

Solution-processed electron injection layers (EILs) comprising lithium 8-quinolate (Liq) and polyethylenimine ethoxylated (PEIE) are highly effective for enhancing electron injection from ZnO to organic layers and improving device lifetime in organic light-emitting devices (OLEDs). Doping of Liq into PEIE further reduces the work function of zinc oxide (ZnO) by enhancing dipole formation. The intermolecular interaction between Liq and PEIE was elucidated by UV-vis absorption measurement and quantum chemical calculation. The OLEDs with ZnO covered with PEIE:Liq mixture exhibited lower driving voltage than that of the device without Liq. Furthermore, as doping concentration of Liq into PEIE increased, the device lifetime and voltage stability during constant current operation was successively improved.

A Solution-Processed Heteropoly Acid Containing MoO3 Units as a Hole-Injection Material for Highly Stable Organic Light-Emitting Devices.

We report hole-injection layers (HILs) comprising a heteropoly acid containing MoO3 units, phosphomolybdic acid (PMA), in organic light-emitting devices (OLEDs). PMA possesses outstanding properties, such as high solubility in organic solvents, very low surface roughness in the film state, high transparency in the visible region, and an appropriate work function (WF), that make it suitable for HILs. We also found that these properties were dependent on the postbaking atmosphere and temperature after film formation. When the PMA film was baked in N2, the Mo in the PMA was reduced to Mo(V), whereas baking in air had no influence on the Mo valence state. Consequently, different baking atmospheres yielded different WF values. OLEDs with PMA HILs were fabricated and evaluated. OLEDs with PMA baked under appropriate conditions exhibited comparably low driving voltages and higher driving stability compared with OLEDs employing conventional hole-injection materials (HIMs), poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate), and evaporated MoO3, which clearly shows the high suitability of PMA HILs for OLEDs. PMA is also a commercially available and very cheap material, leading to the widespread use of PMA as a standard HIM.

Efficient Electron Injection by Size- and Shape-Controlled Zinc Oxide Nanoparticles in Organic Light-Emitting Devices.

Three different sized zinc oxide (ZnO) nanoparticles were synthesized as spherical ZnO (S-ZnO), rodlike ZnO (R-ZnO), and intermediate shape and size ZnO (I-ZnO) by controlling the reaction time. The average sizes of the ZnO nanoparticles were 4.2 nm × 3.4 nm for S-ZnO, 9.8 nm × 4.5 nm for I-ZnO, and 20.6 nm × 6.2 nm for R-ZnO. Organic light-emitting devices (OLEDs) with these ZnO nanoparticles as the electron injection layer (EIL) were fabricated. The device with I-ZnO showed lower driving voltage and higher power efficiency than those with S-ZnO and R-ZnO. The superiority of I-ZnO makes it very effective as an EIL for various types of OLEDs regardless of the deposition order or method of fabricating the organic layer, the ZnO layer, and the electrode.

Molecular Interdiffusion between Stacked Layers by Solution and Thermal Annealing Processes in Organic Light Emitting Devices.

In organic light emitting devices (OLEDs), interfacial structures between multilayers have large impacts on the characteristics of OLEDs. Herein, we succeeded in revealing the interdiffusion in solution processed and thermal annealed OLEDs by neutron reflectometry. We investigated interfaces between a polymer under layer and small molecules upper layer. The small molecules diffused into the swollen polymer layer during the interfacial formation by the solution process, but the polymer did not diffuse into the small molecules layer. At temperatures close to the glass transition temperatures of the materials, asymmetric molecular diffusion was observed. We elucidated the effects of the interdiffusion on the characteristics of OLEDs. Partially mixing the interface improved the current efficiencies due to suppressed triplet-polaron quenching at the interface. Controlling and understanding the interfacial structures of the miultilayers will be more important to improve the OLED characteristics.