Lixin Xiao

Recent Progresses on Materials for Electrophosphorescent Organic Light‐Emitting Devices

Although organic light-emitting devices have been commercialized as flat panel displays since 1997, only singlet excitons were emitted. Full use of singlet and triplet excitons, electrophosphorescence, has attracted increasing attentions after the premier work made by Forrest, Thompson, and co-workers. In fact, red electrophosphorescent dye has already been used in sub-display of commercial mobile phones since 2003. Highly efficient green phosphorescent dye is now undergoing of commercialization. Very recently, blue phosphorescence approaching the theoretical efficiency has also been achieved, which may overcome the final obstacle against the commercialization of full color display and white light sources from phosphorescent materials. Combining light out-coupling structures with highly efficient phosphors (shown in the table-of-contents image), white emission with an efficiency matching that of fluorescent tubes (90 lm/W) has now been realized. It is possible to tune the color to the true white region by changing to a deep blue emitter and corresponding wide gap host and transporting material for the blue phosphor. In this article, recent progresses in red, green, blue, and white electrophosphorescent materials for OLEDs are reviewed, with special emphasis on blue electrophosphorescent materials.

Direct Observation of Long Electron-Hole Diffusion Distance in CH3NH3PbI3 Perovskite Thin Film

In high performance perovskite based solar cells, CH3NH3PbI3 is the key material. We carried out a study on charge diffusion in spin-coated CH3NH3PbI3 perovskite thin film by transient fluorescent spectroscopy. A thickness-dependent fluorescent lifetime was found. By coating the film with an electron or hole transfer layer, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) or 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) respectively, we observed the charge transfer directly through the fluorescence quenching. One-dimensional diffusion model was applied to obtain long charge diffusion distances in thick films, which is ~1.7 μm for electrons and up to ~6.3 μm for holes. Short diffusion distance of few hundreds of nanosecond was also observed in thin films. This thickness dependent charge diffusion explained the formerly reported short charge diffusion distance (~100 nm) in films and resolved its confliction to thick working layer (300–500 nm) in real devices. This study presents direct support to the high performance perovskite solar cells and will benefit the devices’ design.

Understanding the solvent-assisted crystallization mechanism inherent in efficient organic–inorganic halide perovskite solar cells

N,N-Dimethylformamide (DMF) has been shown to be an efficient precursor solvent for the one-step deposition of perovskite thin films in photovoltaic applications. Here, the specific advantage DMF introduces during the perovskite crystallization process is elucidated through comparison with dimethylacetamide (DMAc), one of its homologues. The unique presence of a DMF-induced intermediate phase was verified for the first time and its positive functions to inhibit uncontrolled perovskite precipitation and facilitate homogeneous nucleation were demonstrated. When combined with a double blocking layer structure to prevent shunting, our planar heterojunction (PHJ) perovskite solar cells achieved a high power conversion efficiency of up to 13.8%. Our results uncover the origin of the widespread adoption of DMF in perovskite thin film deposition, and represent a helpful step towards judicious perovskite morphological control.

On chip, high-sensitivity thermal sensor based on high-Q polydimethylsiloxane-coated microresonator

A high-sensitivity thermal sensing is demonstrated by coating a layer of polydimethylsiloxane (PDMS) on the surface of a silica toroidal microresonator on a silicon wafer. Possessing high-Q whispering gallery modes (WGMs), the PDMS-coated microresonator is highly sensitive to the temperature change in the surroundings. We find that, when the PDMS layer becomes thicker, the WGM experiences a transition from redshift to blueshift with temperature increasing due to the negative thermal-optic coefficient of PDMS. The measured sensitivity (0.151 nm/K) is one order of magnitude higher than pure silica microcavity sensors. The ultrahigh resolution of the thermal sensor is also analyzed to reach 10−4 K.

Plasmonic-enhanced perovskite solar cells using alloy popcorn nanoparticles

This article demonstrates a significant broadband enhancement of light absorption and improvement of photon-generated-charge transfer in CH3NH3PbI3 perovskite solar cells by incorporating plasmonic Au–Ag alloy popcorn-shaped nanoparticles (NPs). Compared to conventional nanoparticles and nanorods, these popcorn-shaped NPs have many fine structures. The devices maximum power conversion efficiency (PCE) increases from 8.9% to 10.3%, namely 15.7% enhancement, with the aid of plasmonic popcorn-shaped NPs.

Formation of ultrasmooth perovskite films toward highly efficient inverted planar heterojunction solar cells by micro-flowing anti-solvent deposition in air

Ultrasmooth perovskite thin films are prepared by a solution-based one-step micro-flowing anti-solvent deposition (MAD) method carried out in air with simplicity and practicability. Engaging inert gas blow and anti-solvent drips as accelerators, ultrafast crystallizing, thickness controllable, and high quality methylammonium lead iodide films are prepared with a least root mean square roughness of 1.43 nm (1.95 nm on average), achieving the smoothest surface morphology to the best of our knowledge, as well as a rather compact perovskite layer with a high coverage ratio. Perovskite films formed from MAD require no annealing procedure to ultimately crystallize, realizing a very fast crystallizing procedure within few seconds. By controlling the thickness of perovskite films, superior photovoltaic performance of solar cells with a large fill factor of 0.8 and a PCE of 15.98% is achieved without a glovebox. MAD technology will benefit not only highly efficient photovoltaic devices, but also perovskite-based hybrid optoelectronic devices with field effect transistors and light emitting diodes as well.

A pure blue emitter (CIEy ≈ 0.08) of chrysene derivative with high thermal stability for OLED

A chrysene derivative, BPCC (6,12-bis(9-phenyl-9H-carbazol-3-yl)chrysene), possessing high thermal stability with a high glass transition temperature (Tg = 181 °C) was synthesized. Carbazole groups were introduced to improve its hole transporting properties and suppress crystallization. The device using BPCC as the emitter shows pure saturated blue emission color with coordinates of (0.16, 0.08). Furthermore, utilizing transporting materials with high triplet energy, e.g., TAPC (1,1-bis[4-[N,N-di(p-tolyl)aminophenyl]cyclohexane) and TemPPB (1,2,4,5-tetra(3-pyrid-3-yl-phenyl)benzene) as the hole transport layer (HTL) and electron transport layer (ETL), respectively, the maximum external quantum efficiency (EQE) is 4.9%. It shows a great potential as a highly efficient pure blue emitter for organic light-emitting devices (OLEDs).

Highly efficient organic light emitting devices with insulator MnO as an electron injecting and transporting material

A facile way to fabricate highly efficient organic light emitting devices with insulator MnO as an electron injecting and transporting material was devised, which eliminates the problem of the oxidation of reactive dopants. The power efficiency of 1.1lm∕W by inserting 3-nm-thick MnO as the electron injecting layer was obtained, higher than the 0.8lm∕W efficiency for the reference device with 0.5-nm-thick LiF. A thermal coevaporation layer containing 10% weight of MnO and tris(8-hydroxyquinolato)aluminum (Alq3) as the electron transporting layer showed more efficient electron transport ability, with turn-on voltage of 3.8V, lower than 7.4V for the intrinsic Alq3.

Spirobifluorene derivative: a pure blue emitter (CIEy ≈ 0.08) with high efficiency and thermal stability

A spirobifluorene derivative containing phenanthrene moiety, 2,7-di(phenanthren-9-yl)-9,9′-spirobifluorene (DPSF), has been synthesized. It shows absorption peaks at 254 nm, 310 nm, and 327 nm and a fluorescence peak at 383 nm in CHCl3 that shifts to 398 nm in the film state. The quantum yield is 0.79 calibrated with a standard of coumarin 102 (0.93). A pure blue emission at Commission Internationale de l′Eclairage (CIE) (0.15, 0.08), has been achieved using DPSF as the emitter, poly(3,4-ethylene dioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS) as the hole injecting layer, 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino] biphenyl (NPB) as the hole transporting layer, and 1,3,5-tris(N-phenylbenzimidazol-2-yl)-benzene (TPBI) mixing with 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole (PBD) (2:1) as the electron transporting material. The maximum current efficiency (CE) and power efficiency (PE) of the DPSF device are 3.24 cd A−1 and 2.54 lm W−1, corresponding to 5.41% of maximum external quantum efficiency (EQE). The spirobifluorene derivative show high thermal stabilities, 178 °C for the glass transition temperature (Tg) and 503 °C for the decomposition temperature (Td). The synthesized spirobifluorene derivative shows potential application as a highly efficient pure blue emitter for organic light emitting devices (OLED).

A weak electron transporting material with high triplet energy and thermal stability via a super twisted structure for high efficient blue electrophosphorescent devices

A high triplet energy (ET = 3.2 eV) electron transporting/hole blocking (ET/HB) material, 1,2,4,5-tetra(3-pyrid-3-yl-phenyl)benzene (TemPPB) with a super twisted structure and high thermal stability has been synthesized. An external quantum efficiency (EQE) of 19.6% was achieved by using TemPPB as the ET/HB material in a blue electrophosphorescent device, much higher than the EQE of 12.5% for the device using the conventional ET material, 3-(4-biphenyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ). In addition, the weak ET property of TemPPB resulting from its super twisted structure can be enhanced via n-type doping with LiF. An EQE of 24.5% was achieved by combining n-type doping and a double-emission layer. This shows an alternative way to design ET/HB materials with high ET and improved thermal stability for blue electrophosphorescent devices.