So Kawata

Instant Low‐Temperature Cross‐Linking of Poly(N‐vinylcarbazole) for Solution‐Processed Multilayer Blue Phosphorescent Organic Light‐Emitting Devices

Poly(N-vinylcarbazole) undergoes cross-linking to highly solvent-resistant films through an oxidative coupling reaction, for which an annealing process takes only 3 min at 110 °C. This reaction allows the construction of a solution-processed multilayer OLED without a time-consuming annealing process. The maximum external quantum efficiency reaches 18%, and remains at 17%, even at a high brightness of 10 000 cd m(-2) for all-solution-processed blue OLEDs.

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.

Excimer-emitting single molecules with stacked π-conjugated groups covalently linked at the 1,8-positions of naphthalene for highly efficient blue and green OLEDs

Two efficient blue-light-emitting compounds, 1,8-bis(4-(N-carbazolyl)phenyl)naphthalene (BCzPN) and 1,8-bis(4-(10-phenylanthracen-9-yl)-phenyl)naphthalene (BPAPN), are designed and synthesized, in which two phenylcarbazole or diphenylanthracene units are closely stacked through bonding to the 1- and 8-positions of the naphthalene ring, resulting in strong intramolecular excimer emissions in solution or as a film. By utilizing BPAPN as an emitter, high efficiencies of 6 cd A−1 and 5.8% external quantum efficiency (EQE) at 100 cd m−2, and 8 cd A−1 and 5.8% EQE at 1000 cd m−2 are achieved in a non-doped blue device. By using BCzPN or BPAPN as a host, a DPAVBi-doped BCzPN based blue device gave high efficiencies of 15 cd A−1 and 6.5% EQE at 100 cd m−2, and 12 cd A−1 and 5.5% EQE at 1000 cd m−1, and a C545T-doped BPAPN based green device gave high efficiencies of 23 cd A−1 and 6.7% EQE at 100 cd m−2, and 22 cd A−1 and 6.7% EQE at 1000 cd m−2, respectively.

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 morphology control layer of a pyrene dimer enhances the efficiency in small molecule organic photovoltaic cells

Improved efficiency in organic photovoltaic (OPV) devices induced by a morphology control donor layer as an anode buffer layer (ABL) was achieved. We synthesized the pyrene dimer 1,1′-dipyrene (1DPy) and applied it in vacuum-evaporated small-molecule-type OPV devices based on pentacene. N,N′-Di(naphthalen-1-yl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine (NPD) was used for comparison in the ABLs. While NPD led to a decreased fill factor, the insertion of a 1DPy layer with only 1 nm thickness improved the power conversion efficiency (PCE) 1.5-fold compared with that of the device without 1DPy. Moreover, 1 nm thick 1DPy layers were also incorporated into devices with different donor materials: copper-phthalocyanine and tetraphenyldibenzoperiflanthene. All of the devices showed enhanced short-circuit current densities, which resulted in improvements in the power conversion efficiency. The 1DPy layer prevented hole accumulation owing to its island-like grain structure. The morphologies of the donor layers on top of the 1DPy layers were characterized using atomic force microscopy images and X-ray diffraction patterns. The roughness of the pentacene layer was decreased from 4.41 nm to 2.15 nm when a 1DPy layer was used, and the pentacene layer on 1DPy showed a tilted crystal phase. The tilted pentacene orientation may facilitate perpendicular carrier transport or exciton diffusion through an increase in molecular orbital overlap. 1DPy inserted as an ABL can be useful in controlling the morphology of the donor layer, thereby resulting in an improvement in the efficiency of the device.

Controlling the excited-state energy levels of 9,9′-bifluorenylidene derivatives by twisting their structure to attaining singlet fission character in organic photovoltaics

9,9′-Bifluorenylidene (BFN) derivatives, in which the two fluorene moieties are connected via a CC double bond at the 9-position, are expected to show singlet fission (SF) character in organic photovoltaics (OPVs). Due to steric repulsion between the hydrogen atoms at the 1,8- and 1′,8′-positions, the two fluorene planes of BFN are twisted by 31° with respect to each other. DFT calculations suggest that this deviation from coplanarity lowers the T1 energy level of BFN to approximately half that of S1. We synthesized a molecular alkylbithiophene-substituted BFN and a copolymer of diketopyrrolopyrrole and BFN. The optoelectronic properties of these compounds were investigated, and OPV devices using these BFNs as a p-type material were fabricated. The device composed of the polymeric BFN derivative achieved a power conversion efficiency of 4.9%. The observed negative dependence of the photocurrent on the magnetic field suggested that the triplet excitons of the molecular BFN derivatives contribute to the photocurrent in these OPV devices.

Organic Photovoltaics: Singlet Fission of Non-polycyclic Aromatic Molecules in Organic Photovoltaics (Adv. Mater. 8/2016)

On page 1585, Y.-J. Pu, J. Kido, and co-workers demonstrate the singlet fission of thienoquinoid compounds by measuring the magnetic-field dependence of the photocurrent of photovoltaic devices. Three kinds of thienoquinoid core and fluorene end-groups are chosen to satisfy the material conditions for singlet fission and organic photovoltaics at the same time. These materials aid in expansion of the molecular design of singlet fission materials.

Instant Low-Temperature Cross-Linking Poly( N -vinylcarbazole) for Solution-Processed Multilayer Organic Light-Emitting Devices

We report instant low-temperature cross-linking of PVK. The preparation and characterization of cross-linked PVK films will be presented as well as their application in solution-processed OLEDs with each organic layer processed from the solution.