Quantum Efficiency Of Devices Research Articles

Synthesis and characterization of new fluorene-acceptor alternating and random copolymers for light-emitting applications

A series of novel light-emitting copolymers consisted of 9,9-dihexylfluorene ( F) and different acceptor segments, including quinoxaline ( Q), 2,1,3-benzothiadiazole ( BT) and thieno[3,4- b]-pyrazine ( TP), were synthesized by the palladium-catalyzed Suzuki coupling reaction. Three fluorene-acceptor alternating copolymers ( PFQ, PFBT, PFTP) and six F- TP ( PFTP0.5– PFTP35) random copolymers were investigated and compared with the parent polyfluorene ( PF). The experimental results suggest that the acceptor strength or content significantly affect the electronic and optoelectronic properties. The optical absorption maxima of the PF, PFQ, PFBT, and PFTP are 368, 416, 470, and 578 nm, respectively, which indicates the significance of intramolecular charge transfer. The electrochemical band gap also shows a similar trend. The incorporation of the acceptor into the PF lowers the LUMO level and thus could improve the electron-accepting ability of the PF. The emission maxima on the photoluminescence (PL) spectra of the PF, PFQ, PFBT, and PFTP films are 424, 493, 540, and 674 nm, which correspond to the color of blue, green, yellow, and red, respectively. It suggests that the full color of emission can be achieved by different acceptors. The significant positive solvatochromism on the PL spectra in different polar solvents suggests the efficient intramolecular charge transfer in PFTP. However, such charge transfer or heavy-atom effect results in fluorescence quenching and thus reduces the PL efficiencies. By random copolymerizing the TP into the PF, the PL efficiency could be improved. A significantly reduction on the PF emission peak with increasing the TP content suggests the energy transfer between the PF and TP segments. Besides, the characteristics on the electroluminescence (EL) devices of ITO/PEDOT:PSS/emissive layer/Ca/Ag suggest that such energy transfer results in the complete quenching of the PF emission at only 1% TP content in the PFTP01. The maximum external quantum efficiency (EQE) of the EL device based on the PFTP01 is superior to that of the PF due to the reduced LUMO level in matching with the Ca. The CIE 1931 coordinates of the PFTP01 based EL device under the condition of maximum EQE is (0.66, 0.31), which is close to the standard red of (0.66, 0.34) demanded by the National Television System Committee (NTSC). The luminescence characteristics based on the prepared polymers depend on the Förster energy transfer or the intramolecular charge transfer, or heavy-atom fluorescence quenching. The present study suggests that the tuning of the electronic and optoelectronic properties could be achieved by incorporating various acceptors or content into the polyfluorenes.

Effects of scintillator on the detective quantum efficiency (DQE) of a digital imaging system

To compare the effects of scintillator on the detective quantum efficiency (DQE) of a charge-coupled device (CCD) digital intraoral radiographic system. Three screens composed of 3 different scintillator materials, namely europium-doped lutetium oxide (Lu2(O3):Eu3+), transparent optical ceramic (TOC), thallium-doped cesium iodide (CsI:Tl; CsI), and terbium-doped gadolinium oxysulfide (Gd2(O2)S:Tb; GOS) were compared, in turn, in combination with a CCD detector having square pixels with height and width dimensions of 19.5 microm. DQE was investigated using the slanted-slit-derived MTF and surrogate signal-to-noise ratio (SNR) measurements derived from calculations of the mean and standard deviations from the mean pixel values of multiple random patches from various uniform exposures. An Irix x-ray generator operated at 70 kVp and 8 mA, with a nominal focal spot size of 0.7 mm and 2.5 mm Al equivalent filtration, was used in making all exposures. Using TOC, the peak DQE was 62% at 5 cycles/mm. For CsI, the peak DQE was 22% at 2 cycles/mm. With GOS, the peak DQE was 10% at 1 cycle/mm. Under identical experimental settings, TOC consistently resulted in higher DQE than CsI and commercially available GOS scintillators combined with the same high-resolution solid-state detector.

Internal quantum efficiency of high-brightness AlGaInP light-emitting devices

The internal quantum efficiency of (AlxGa1−x)0.5In0.5P light-emitting devices (LEDs), with an emission wavelength ranging from 650 to 560 nm, is determined by means of a model that takes into account the radiative and nonradiative recombination in the active layer, the diffusive leakage of carriers into the confining layers, and the influence of photon recycling on the light extraction efficiency. The evaluation is based on measurements of the external quantum efficiency of the LEDs as a function of the operating current and temperature. The analysis provides the wavelength dependence of both the nonradiative recombination as well as the carrier leakage.

Generalized models of the spectral response of the voltage for the extraction of recombination parameters in silicon devices

Analytical models of the spectral response of the voltage of silicon devices have been generalized using the concept of the internal quantum efficiency of the semiconductor. This allows the extension of models used in the analysis of the internal quantum efficiency to the spectral response of the voltage. Existing models for the spectral response of the voltage that are largely employed in the surface photovoltage technique are shown to be special cases that approximate the internal quantum efficiency. A more sophisticated model of the internal quantum efficiency, the model of Isenberg, and a model that allows the analysis of the internal quantum efficiency of rear-illuminated devices have been adapted to the spectral response of the voltage. This paves the way to analyze solar cells, Schottky devices, or chemically treated silicon wafers independently of the light intensity and using front or rear illumination. The models have been validated by comparing the analysis of the spectral response of the short-circuit current and the open-circuit voltage with computer simulations for a wide range of solar cells. The model of Isenberg has been found to give in general the best prediction of the recombination parameters for both the spectral photocurrent and photovoltage. The analysis of the spectral response of the voltage can often unveil the surface recombination rate where the spectral photovoltage fails to produce any output. The measurement of rear-junction cells showed that the surface recombination rate can be predicted equally well with either method.

Charge carrier photogeneration in conjugated polymer PhPPV/R6G composite system

The spectral and polarity dependence of the quantum yield of charge carrier photo-generation was studied by steady-state photocurrent measurement in a single layer PhPPV film, double layer film of PhPPV and R6G and doped film of PhPPV with R6G The intrinsic and extrinsic charge carrier photogeneration was observed. The result indicates that the quantum efficiency of the double layer device is higher than that of single layer device under reverse bias, but it is opposite under forward bias. The yield of charge carrier photogeneration of the doped film is higher than that of the other two films at both forward and reverse bias because of the increased interface area between the electron donor and acceptor.

Discrete hopping model of exciton transport in disordered media

A model of dispersive exciton transport has been developed for a medium with exciton energy levels randomly distributed in both space and energy scale. For a boxcar density of states of excitons an analytical solution is given describing the exciton density as a function of time and the proximity to the exciton quenching interfaces. The model parameters, such as exciton lifetime, effective number of exciton energy levels within inhomogeneously broadened density of states, and hopping distance and hopping rate constants, could be determined using time-resolved photoluminescence data. The developed model is verified via comparison with experimental data on the time-dependent photoluminescence decay of the conjugated polymer MEH-PPV and on the spectrum of internal quantum efficiency of the heterojunction photovoltaic device based on the MEH-PPV/TiO2 nanostructure.

Application of large area avalanche photodiodes for alpha liquid scintillation counting

In order to improve the resolution in alpha liquid scintillation and to promote this method for the measurement of actinides in environment, a silicon Large Area Avalanche Photodiode (LAAPD) has been tested as liquid scintillation detector. The set-up consists of a LAAPD (16 mm diameter) coupled to a thin vial containing alpha-emitters and a liquid scintillation cocktail. After optimization of several parameters like bias voltage, temperatures, counting geometry and composition of scintillating cocktail, energy resolutions are found to be better than those obtained with photomultiplier tubes: 5 % (200 keV FWHM) for 232Th and 3.9 % (260 keV FWHM) for 216Po. The improvement in energy resolution is not due to higher quantum efficiency of the silicon device but to better uniformity of the spatial photoresponse. A few energy spectra of thorium, plutonium and americium are given as example.

Continuous-wave operation of all-epitaxial InP-based 1.3 µm VCSELs with 57% differential quantum efficiency

All-epitaxial InP-based 1.3 µm VCSELs with a record-high continuous-wave differential quantum efficiency (57%) for single active region long-wavelength devices are demonstrated. Low-loss optical mode confinement is achieved through a selectively etched undercut tunnel-junction aperture. Singlemode continuous-wave lasing was observed up to 87°C and the room-temperature output power was 1.1 mW at a current of 4.1 mA and a wavelength of 1.305 µm.

White Organic Light-Emitting Devices Based on2-(2-Hydroxyphenyl) Benzothiazole and Its Chelate Metal Complex

We present three kinds of organic light-emitting devices (OLED)fabricated to achieve the emission of bright and pure white light.Device A, with a double-layered structure using 2-(2-hydroxyphenyl)benzothiazole (HBT) and poly (N-vinylcarbazole) (PVK) as the emittinglayer (EML) and the hole transport layer (HTL) respectively, couldrealize the blue-green light emission. Bis-(2-(2-hydroxyphenyl)benzothiazole)zinc (Zn(BTZ)2), synthesized with zinc acetatedihydrate and HBT to form a complex, is used as main EMLs in a similarstructure to fabricate devices B and C. Bright and pure white lightemissions can be obtained from device C which was fabricated with agreen-white emitting host Zn(BTZ)2 and red dopant5,6,11,12-tetraphenylnaphthacene (rubrene). The maximum quantumefficiency of device C could reach 0.63%, and the correspondingbrightness and CIE coordinates were 4000 cd/m2 and (x = 0.341, y = 0.334) at the driving voltage of 20 V.

Uncooled operation of type-II InAs∕GaSb superlattice photodiodes in the midwavelength infrared range

We report high performance uncooled midwavelength infrared photodiodes based on interface-engineered InAs∕GaSb superlattice. Two distinct superlattices were designed with a cutoff wavelength around 5μm for room temperature and 77 K. The device quantum efficiency reached more than 25% with responsivity around 1A∕W. Detectivity was measured around 109cmHz1∕2∕W at room temperature and 1.5×1013cmHz1∕2∕W at 77 K under zero bias. The devices were without antireflective coating. The device quantum efficiency stays at nearly the same level within this temperature range. Additionally, Wannier–Stark oscillations in the Zener tunneling current were observed up to room temperature.

Multiple roles of bathocuproine employed as a buffer-layer in organic light-emitting diodes

Efficiency and brightness and carriers injection have been obviously improved by using bathocuproine (BCP) as a buffer-layer in organic light-emitting diodes. Compared with the bufferless device, the quantum efficiency of device ITO/NPB (10 nm)/Alq 3 (10 nm)/BCP (2.4 nm)/Al has increased four times at the same current density (32 mA/cm 2). Moreover, the buffer layer has changed the current-voltage properties and the turn-on voltage has obviously decreased. Considering BCP and Al 3+ can react conveniently under room temperature, we suggest that a complex cathode structure of BCP/[(Al) x (BCP) y ] 3 x+ /Al has formed under electric field and the new cation [(Al) x (BCP) y ] 3 x+ at the BCP/Al interface has improved the internal electric field and then enhanced the electrons injection. we conclude that: for a very thin (<1 nm) BCP buffer layer, improving electron injection will principally responsible to the improvement of the performance of the OLEDs; for a thicker BCP layer, there will be a synthetic function of BCP: improving electron injection, hole-blocking and electron-transporting.

High-efficiency 1.3μmInGaAs∕GaAs quantum-dot microcavity light-emitting diodes grown by metalorganic chemical vapor deposition

We investigate the optical properties of quantum-dot (QD) microcavity light-emitting diodes (MCLED) operating at 1.3μm at room temperature. The active medium consists of a single layer of InGaAs quantum dots, directly grown in a GaAs matrix by metalorganic chemical vapor deposition. Electrical injected QD MCLEDs were fabricated by exploiting a hybrid technology, which employs epitaxial and oxide-based mirrors. Such technology allows us to preserve the QD properties and leads to a wider optical bandwidth of the hybrid top distributed Bragg reflector with respect to the fully epitaxial mirror, resulting in photon recycling of the overall QDs spectrum, thus improving the efficiency of the device. The devices exhibit bright electroluminescence peaked at 1.29μm, with a full width at half maximum of 10 meV. The room-temperature external quantum efficiency of such devices is 0.52%, higher than that reported in the literature for QD MCLEDs operating at 1.3μm at room temperature.

Highly efficient green polymer light-emitting diodes through interface engineering

— By interface engineering, we have improved the quantum efficiency for green polyfluorene polymer light-emitting diodes by three fold: the efficiency improved from 9 to 28 cd/A. This interface engineering was achieved by inserting a thin layer of calcium (2) acetylacetonate, denoted as Ca(acac)2, at the polymer/metal cathode interface. The Ca(acac)2 layer behaves in a multifunctional way. It assists the electron injection by lowering the electron-injection barrier. In the meanwhile, hole injection is enhanced by the accumulated electrons in the polymer layer. This effect is believed to lower the barrier height for hole-injection through the Schottky effect. Finally, the Ca(acac)2 layer works as a hole block layer to block the holes. As a result of the charge balance and charge confinement, the device quantum efficiency increases dramatically.

Description of exciton transport in a TiO2/MEH–PPV heterojunction photovoltaic material

A model for exciton transport in conjugated polymers has been developed taking into account inhomogeneous broadening of exciton energy level. Time-resolved photoluminescence data were used to determine the parameters of the model, such as exciton lifetime, effective number of exciton energy levels within inhomogeneously broadened density of states, hopping distance and hopping rate constants for excitons. Using the developed model a non-monoexponential decay of photoluminescence of the conjugated polymer deposited on TiO2 substrate was described, and the spectrum of internal quantum efficiency of the heterojunction photovoltaic device based on the conjugated polymer with a nanocrystalline TiO2 layer was predicted.

Improved device efficiency and color purity: Spectral redshift and line narrowing for poly [2-methoxy,5-(2-ethylhexyloxy)-1,4-phenylenevinylene] via blending with phenyl-substituted poly [p-phenylene vinylene] derivatives

Highly efficient light-emitting diodes with redshifted narrow emission were realized based on blends of poly [2-methoxy,5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and phenyl-substituted poly [p-phenylene vinylene] derivative (P-PPV). EL emissions of blend device were redshifted about 18nm from the peak emission of 582nm for neat MEH-PPV device to around 600nm for devices from the blend film. The full width at half maximum of MEH-PPV emission from blend film was reduced from 91 to 36nm. The external quantum and luminous efficiencies of devices from blend films (MEH-PPV:P-PPV=20:80) reached 5.27% and 4.71cd∕A, respectively. The effective energy transfer and solid-state dilution effect of MEH-PPV in P-PPV host were thought of as the origin of the performance enhancement.

Precise Measurement of External Quantum Efficiency of Organic Light-Emitting Devices

We report on the direct measurement of the external quantum efficiency of electroluminescence of organic light-emitting devices with different emission colors and patterns using an integrating sphere and a highly sensitive charge-coupled device spectral analyzer. Using this direct-measurement method, precise external quantum efficiency can be obtained more easily and more quickly than using the conventional luminance-conversion method. This method is shown to be very useful for determining the external quantum efficiency of organic light-emitting devices.

High-Efficiency Saturated Red Emitting Polymers Derived from Fluorene and Naphthoselenadiazole

A variety of novel light-emitting copolymers derived from 9,9-dioctylfluorene (DOF) and 2,1,3-naphthozoselenadiazole (NSeD) were prepared by the palladium-catalyzed Suzuki coupling reaction. The feed ratios of DOF to NSeD were 99.9:0.1, 99.5:0.5, 99:1, 98:2, 95:5, and 85:15. All of the polymers are soluble in common organic solvents and highly fluorescent in solid state. Devices based on the copolymers emit saturated red light, and the emission slightly red-shifted gradually with increasing NSeD's contents. The maximal external quantum efficiency of the polymer light-emitting devices (PLED) reaches 3.1%, and luminous efficiency is greater than 1.0 cd/A with emission maximum at 657 nm and Commission Internationale de L'Eclairage (CIE) coordinates of (0.64, 0.33). This is the highest efficiency with saturated red emission for a single-layer device with nonblend type emitter reported so far in the scientific literature. This indicates that the new EL polymers based on fluorene and naphthoselenadiazole are promising as a red emitter in polymer light-emitting displays.

Characteristics in Bilayer Organic Light-Emitting Diodes with Variation of the Film Thickness

We fabricated bilayer polymer light-emitting diodes(PLED)with Poly(9,9′-dihexylfluorene-2,7-divinylene-m-phenylenevinylene-stat-p-phenylenevinylene)(PDHFMPPV)and poly(N-vinylcarbazole)(PVK)as an emitting layer and a hole transport layer, respectively. The thickness of each layer was varied, while the total thickness was controlled to be constant as 110 nm. We observed electroluminescence, photoluminescence and current–voltage-luminescence characteristics of these bilayer devices and a single layer device with PDHFMPPV polymer as a light emitting layer for comparison. The quantum efficiencies of the bilayer devices were enhanced up to two about orders magnitude compared with the single-layer device.

Highly Efficient Blue-Light-Emitting Diodes from Polyfluorene Containing Bipolar Pendant Groups

A highly efficient blue-light-emitting copolymer with bulky hole-transporting triphenylamine (TPA) and electron-transporting oxadiazole (OXD) pendant groups at the C-9 position of fluorene was synthesized. The results from photoluminescence and electrochemical measurements reveal that both the side chains and the polyfluorene main chain retain their own electronic characteristics in the copolymer. It shows a pure blue emission with no aggregates or excimers formed even after being annealed at 150 °C under nitrogen for 20 h. In addition, it also demonstrates improved charge injection and balanced charge transport in electroluminescence. The maximum external quantum efficiency of a single-layer device using this copolymer as the emitting layer is 1.21% (at a brightness of 354 cd/m2 with driving voltage of 7.6 V). The maximum luminance of the device reaches 4080 cd/m2 at a bias of 12.0 V and a current density of 640 mA/cm2.

Nanophase-Separated Blends of Acceptor and Donor Conjugated Polymers. Efficient Electroluminescence from Binary Polyquinoline/Poly(2-methoxy-5-(2‘-ethylhexyloxy)-1,4-phenylenevinylene) and Polyquinoline/Poly(3-octylthiophene) Blends

Binary blends of the acceptor conjugated polymer poly(2,2‘-(3,3‘-dioctyl-2,2‘-bithienylene)-6,6‘-bis(4-phenylenequinoline)) (POBTPQ) with donor conjugated polymer poly(2-methoxy-5-(2‘-ethylhexyloxy)-1,4-phenylenevinylene) (MEH−PPV) or poly(3-octylthiophene) (POT) were nanophase-separated and observed to exhibit efficient electroluminescence in light-emitting-diodes. The 90−120 nm phase-separated morphology of MEH−PPV-containing blends was characteristic of dimixing by spinodal decomposition whereas that of POT-containing blends consisted of nucleation and growth type spherical domains (50−400 nm) dispersed in a matrix. Efficient Förster energy transfer was observed in both blend systems. Voltage-tunable orange-red ↔ yellow ↔ green electroluminescence was observed in the POBTPQ:MEH−PPV blend diodes at a composition of 10−80 wt % MEH−PPV. Only red emission characteristic of POT was observed from POBTPQ:POT blend devices due to efficient energy transfer from POBTPQ. Large enhancements in performance of the bipolar blend light-emitting diodes were observed compared to that of the homopolymer diodes. The compositional dependence of luminance and external quantum efficiency of the blend devices was very different for the MEH−PPV and POT blends, reflecting the difference in morphology. Electric-field-induced photoluminescence quenching confirmed the bipolar charge transport in the blends and associated improved electron−hole recombination and device efficiencies. These results demonstrate that efficient electroluminescence can be achieved in bipolar blends of conjugated polymers and that nanophase-separated morphology is essential to voltage-tunable multicolor light emission in blend devices.