Zhidong Lou

Electric field-modulated amplified spontaneous emission in waveguides based on poly [2-methoxy-5-(2′-ethylhexyloxy)-1, 4-phenylene vinylene]

We report amplified spontaneous emission (ASE) in an optically pumped symmetric thin film waveguide based on poly [2-methoxy-5-(2′-ethylhexyloxy)-1, 4-phenylene vinylene] (MEH-PPV) and electric-field modulation of the ASE in the MEH-PPV thin film. The electric-field quenching of the ASE at 620 nm and the spontaneous emission (SE) at 580 nm was observed, which increased with the electric field. The ASE intensity was more effectively quenched than the SE intensity as the field increased. The field dependence of the ASE and the SE intensity can be attributed to field-induced dissociation of photogenerated excitons in the MEH-PPV thin film.

Improvement of amplified spontaneous emission performance of conjugated polymer waveguides with a low loss cladding

We report the improvement of the amplified spontaneous emission (ASE) performance in the optically pumped symmetric thin film waveguide glass/indium-tin oxide (ITO)/SiO2/poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene]/SiO2/Al or Ag by optimizing the thickness of SiO2 and replacing the Al electrode with Ag, where SiO2 acted as a spacer layer to prevent the ASE at 620 nm from being destructed. The results show that the SiO2/Ag cladding reduced the ASE threshold to 4 μJ/pulse compared with the SiO2/Al cladding, owing to the higher work function and reflection rate as well as lower absorption loss of the Ag electrode. No photoluminescence at 580 nm being observed makes it possible for an electric field to only modulate the ASE at 620 nm in the device with the SiO2/Ag cladding. The electric-field quenching of the ASE has been observed, which increases with the electric field. The field dependence of ASE can be attributed to field-induced dissociation of photogenerated excitons in the polymer waveguides.

Luminescent Properties of A Novel Terbium Complex Tb(o-BBA)3(phen)

A novel rare earth complex of terbium ion with 2-benzoylbenzoic acid and 1, 10-phenathroline (Tb(o-BBA)3 (phen), o-BBA = 2-benzoylbenzoic acid, phen = 1,10-phenathroline) was used as an electroluminescent material for the first time. The Tb complex was blended with poly(N-vinylcarbazole) (PVK) in different weight ratios and spinn to coated into films (noted as PVK:Tb films). The photoluminescence (PL) properties of films were investigated and the optimum weight ratio between PVK and Tb(o-BBA)3(phen) was found to be 3:1. Monolayer devices with the structure ITO/PVK: Tb/Al were fabricated and emitted green light, which was characteristic of Tb3+ emission. The results show that mechanisms for PL and EL are different. The PL is considered to be caused because of energy transfer and direct excitation to the Tb(o-BBA)3(phen) molecule, while EL is mainly on charging trapping.

High sensitivity and fast response solution processed polymer photodetectors with polyethylenimine ethoxylated (PEIE) modified ITO electrode

Most organic photodetectors utilize a bulk heterojunction (BHJ) photo-active film due to its high exciton dissociation efficiency. However, the low dark current density, a key role in determining the overall performance of photodetectors, is hardly achieved in the BHJ structure since both the donor and acceptor domains are in contact with the same electrode. The most popular strategy to overcome this problem is by fabricating bilayer or multilayer devices. However, the complicated fabrication process is a challenge for printing electronics. In this work, we demonstrate a solution processed polymer photodetector based on a poly (3-hexylthiophene) (P3HT): (phenyl-C61-butyric-acid-methyl-ester) (PC61BM) blend film with polyethylenimine ethoxylated (PEIE) modified ITO electrode. The transparent PEIE efficiently blocks the unnecessary electronic charge injection between the active film and the electrode, which dramatically decrease the dark current. Under illumination, the photoexcited charges accumulated in the PEIE modified ITO region finally can tunnel through the barrier with the help of the applied reverse bias, leading to a large photocurrent. Therefore, the resulting polymer photodetector shows a 2.48 × 104 signal-to-noise ratio (SNR) under -0.3 V bias and an 11.4 MHz bandwidth across the visible spectra under a small reverse bias of 0.5 V. The maximum EQE of 3250% in the visible wavelength is obtained for the polymer photodetector at -1 V under 370 nm (3.07 μW/cm2) illumination. This solution processed polymer photodetector manufacturing is highly compatible with the flexible, low-cost, and large area organic electronic technologies.

In situ growth of blue-emitting thin films of cerium-doped barium chloride hydrate at low temperatures

Blue emission was observed from thin films of barium chloride hydrate doped with cerium. The films were deposited by spray pyrolysis of aqueous solutions with substrate temperatures between 250 and 450 °C. The cathodoluminescence (CL) spectrum consists of two peaks at 443 and 485 nm due to 4f-5d transitions of cerium ion. The dependence of the emission band on deposition temperature and Ce/Ba ratio is discussed. The CL luminance and luminous efficiency at 5 kV were 120 cd/m2 and 0.48 lm/W, respectively, for the films deposited at temperatures as low as 250 °C. The results indicate barium chloride hydrate doped with cerium is a blue phosphor that exhibits relatively efficient luminescence after processing at low temperatures.

Preparation of CaGa2S4:Ce TFEL devices with inter-layer reaction

Abstract Ce 3+ -doped alkaline earth thiogallates with blue emission have been intensively investigated to fabricate full-color thin film electroluminescence (TFEL) devices, with sputtering, atom layer epitaxy (ALE) and molecular beam epitaxy (MBE) being the methods used. In this paper, a new way of preparing CaGa 2 S 4 :Ce TFEL devices, based on inter-layer reaction is first reported. CaS:Ce and Ga 2 S 3 were deposited by electron beam evaporation, layer by layer, and CaGa 2 S 4 :Ce thin film was formed by post processing. The results of X-ray diffraction, scanning transmission electron microscopy (STEM) microphotographs and energy dispersive X-ray spectroscopy (EDX) show that CaGa 2 S 4 :Ce thin film with good crystallinity and stoichiometry were obtained. Photoluminescence (PL) and electroluminescence (EL) of these devices were investigated.

Ligand-free rutile and anatase TiO2 nanocrystals as electron extraction layers for high performance inverted polymer solar cells

Ligand-free rutile and anatase TiO2 nanocrystals have been synthesized through a hydrolytic sol–gel reaction. The morphology, crystal structure, elemental composition and band structure of the obtained nanocrystals are characterized by transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and UV-visible absorption spectroscopy. These two kinds of nanocrystals could serve as electron extraction layers for improving the performance in inverted polymer solar cells. Compared with the device fabricated by using amorphous TiO2 (6.11%) and rutile TiO2 (6.93%), the device based on anatase TiO2 shows a significant enhancement in power conversion efficiency (7.85%). Meanwhile, the ideal current–voltage model for a single heterojunction solar cell is applied to clarify the junction property of the cell. The model demonstrates that the device based on anatase TiO2 has effective electron extraction and hole-blocking properties.

Investigation on Thermal Degradation Process of Polymer Solar Cells Based on Blend of PBDTTT-C and BM

The effects of thermal treatment on the photovoltaic performance of conventional and inverted polymer solar cells (PSCs) based on the combination of poly[(4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b;4,5-b′]dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thie-no[3,4-b]thiophene))-2,6-diyl] (PBDTTT-C) and [6,6]-phenyl C70-butyric acid methyl ester (PC70BM) are investigated. The transient photoconductivity, the absorption spectra, and the transmission electron microscopy (TEM) images have been employed to study the thermal degradation of the inverted PSCs. The degradation is attributed to the inefficient charge generation and imbalance in charge-carrier transport, which is closely associated with the morphological evolution of the active layer with prolonged heating time.

High sensitivity, fast response and low operating voltage organic photodetectors by incorporating a water/alcohol soluble conjugated polymer anode buffer layer

Low dark current density plays a key role in determining the overall performance of organic photodetectors (OPDs). However, both the donor domains and acceptor domains in the bulk heterojunction, which has high exciton dissociation efficiency, are in contact with the two electrodes. Therefore, the undesirable charge injection from the electrodes to the active layer is hard to avoid, leading to a high dark current density in most OPDs. In this work, we fabricate the OPDs based on a conventional poly(3-hexylthiophene) (P3HT)/(phenyl-C61-butyric-acid-methyl-ester) (PC61BM) bulk heterojunction. By incorporating a water/alcohol soluble conjugated polymer (WSCP), poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN), interlayer between the anode and the active layer, the dark current density is effectively reduced from 0.07 mA cm−2 to 1.92 × 10−5 mA cm−2 under a −0.5 V bias. The resulting OPDs show a 1.93 × 105 signal-to-noise ratio (SNR), a 10 MHz bandwidth, and a 9.10 × 1012 Jones detectivity at a low reverse bias of −0.5 V (at 550 nm). Our research provides a promising way for high performance OPDs.

Enhanced amplified spontaneous emission from morphology-controlled organic–inorganic halide perovskite films

Organic–inorganic tri-halide perovskites (MAPbX3, where MA = CH3NH3, and X = Cl, Br, I) have shown promise as laser gain media. The laser characteristics of perovskite films are susceptible to their crystallinity and morphology. Herein, we demonstrate the morphology and crystallinity of perovskite films could be well-controlled in a modified sequential deposition process by using binary solvent mixtures involving N,N-dimethylmethanamide (DMF) and dimethylsulfoxide (DMSO). The highly crystalline and notably smooth MAPbI3 films on glass substrates yield outstanding amplified spontaneous emission (ASE) performance. The binary solvent engineering approach opens up new opportunities for the development of low-cost and high-efficient ASE devices based on perovskite materials.