Energy bands and acceptor binding energies of GaN

The energy bands of zinc-blende and wurtzite GaN are calculated with the empirical pseudopotential method, and the pseudopotential parameters for Ga and N atoms are-given. The calculated energy bands are in agreement with those obtained by the ab initio method. The effective-mass theory for the semiconductors of wurtzite structure is established, and the effective-mass parameters of GaN for both structures are given The binding energies of acceptor states are calculated by solving strictly the effective-mass equations. The binding energies of donor and acceptor are 24 and 142 meV for the zinc-blende structure, 20 and 131, and 97 meV for the wurtzite structure, respectively, which are consistent with recent experimental results. It is proposed that there are two kinds of acceptor in wurtzite GaN. One kind is the general acceptor such as C, which substitutes N, which satisfies the effective-mass theory. The other kind of acceptor includes Mg, Zn, Cd, etc., the binding energy of these accepters is deviated from that given by the effective mass theory. In this report, wurtzite GaN is grown by the molecular-beam epitaxy method, and the photoluminescence spectra were measured. Three main peaks are assigned to the donor-acceptor transitions from two kinds of accepters. Some of the transitions were identified as coming from the cubic phase of GaN, which appears randomly within the predominantly hexagonal material. [S0163-1829(99)15915-0].

Donor acceptor pair in molecular beam epitaxy grown GaN

We have recently found evidence of new donor acceptor pair (DAP) luminescence in molecular beam epitaxy (MBE) grown films. A variety of nominally undoped samples have been studied by photoluminescence (PL) over a temperature range of 5-300 K. The samples show intensive luminescence al energies of 3.404-3.413 eV varying with different sample at 5 K, as well as a fairly strong (DX)-X-0 line at low temperature. We attribute the Line at 3.404-3.413 eV to DAP recombination which is over 0.1 eV different from the well known DAP caused by ME-doping in GaN. The DAP line shows fine structure. it even predominates in one particular sample. The peak position shifts to higher energy with temperature increasing from 5 up to 70 K, and as the excitation laser intensity increases. The data are consistent with DAP luminescence involving an acceptor level of about 90 meV (presumably carbon) above the valence band edge in GaN. It is much shallower than the acceptor level of 250 meV produced by the p-type dopant Mg which is commonly used at present. (C) 1997 Elsevier Science S.A.

Energy band and acceptor binding energy of GaN and AlxGa1-xN

The hole effective-mass Hamiltonian for the semiconductors of wurtzite structure is established, and the effective-mass parameters of GaN and AlxGa1-xN are given. Besides the asymmetry in the z and x, y directions, the linear term of the momentum operator in the Hamiltonian is essential in determining the valence band structure, which is different from that of the zinc-blende structure. The binding energies of acceptor states are calculated by solving strictly the effective-mass equations. The binding energies of donor and acceptor for wurtzite GaN are 20 and 131, 97 meV, respectively, which are inconsistent with the recent experimental results. It is proposed that there are two kinds of acceptors in wurtzite GaN. One kind is the general acceptor such as C, substituting N, which satisfies the effective-mass theory, and the other includes Mg, Zn, Cd etc., the binding energy of which deviates from that given by the effective-mass theory. Experimentally, wurtzite GaN was grown by the MBE method, and the PL spectra were measured. Three main peaks are assigned to the DA transitions from the two kinds of acceptor. Some of the transitions were identified as coming from the cubic phase of GaN, which appears randomly within the predominantly hexagonal material. The binding energy of acceptor in ALN is about 239, 158 meV, that in AlxGa1-xN alloys (x approximate to 0.2) is 147, 111 meV, close to that in GaN. (C) 2000 Published by Elsevier Science S.A. All rights reserved.

Manipulating charge-transfer character with electron-withdrawing main-group moieties for the color tuning of iridium electrophosphors

A new and synthetically versatile strategy has been developed for the phosphorescence color tuning of cyclometalated iridium phosphors by simple tailoring of the phenyl ring of ppy (Hppy=2-phenylpyridine) with various main-group moieties in [Ir(ppy-X)(2)(acac)] (X=B(Mes)(2), SiPh3, GePh3, NPh2, POPh2, OPh, SPh, SO2Ph). This can be achieved by shifting the charge-transfer character from the pyridyl groups in some traditional iridium ppy-type complexes to the electron-withdrawing main-group moieties and these assignments were supported by theoretical calculations.

Red-light-emitting iridium complexes with hole-transporting 9-arylcarbazole moieties for electrophosphorescence efficiency/color purity trade-off optimizationE

The synthesis, structures, photophysics, electrochemistry and electrophosphorescent properties of new red phosphorescent cyclometalated iridium(III) isoquinoline complexes, bearing 9-arylcarbazolyl chromophores, are reported. The functional properties of these red phosphors correlate well with the results of density functional theory calculations

Red-Light-Emitting Iridium Complexes with Hole-Transporting 9-Arylcarbazole Moieties for Electrophosphorescence Efficiency/ Color Purity Trade-off Optimization

The synthesis, structures, photophysics, electrochemistry and electrophosphorescent properties of new red phosphorescent cyclometalated iridium(III) isoquinoline complexes, bearing 9-arylcarbazolyl chromophores, are reported. The functional properties of these red phosphors correlate well with the results of density functional theory calculations. The highest occupied molecular orbital levels of these complexes are raised by the integration of a carbazole unit to the iridium isoquinoline core so that the hole-transporting ability is improved in the resulting complexes relative to those with I-phenylisoquinoline ligands. All of the complexes are highly thermally stable and emit an intense red light at room temperature with relatively short lifetimes that are beneficial for highly efficient organic light-emitting diodes (OLEDs).

Band Gap Tunable, Donor-Acceptor-Donor Charge-Transfer Heteroquinoid-Based Chromophores: Near Infrared Photoluminescence and Electroluminescence

A series of D-pi-A-pi-D type of near-infrared (NIR) fluorescent compounds based on benzobis(thia diazole) and its selenium analogues were synthesized and fully characterized by H-1 and C-13 NMR, high-resolution mass spectrometry, and elemental analysis. The absorption fluorescence, and electrochemical properties were also studied. Photoluminescence of these chromophores ranges from 900 to 1600 nm and their band gaps are between 1.19 and 0.56 eV.

Monodisperse Co-oligomer Approach toward Nanostructured Films with Alternating Donor-Acceptor Lamellae

A series of donor-acceptor (D-A) co-oligomers with oligo(fluorene-alt-bithiophene) and perylene diimide as donor and acceptor segments, respectively, have been designed and synthesized. They can self-assembly into alternating D-A lamellar nanostructured films with the periods depending on the molecular length. These films have been successfully used in fabrication of high-performance single-molecular solar cells with power conversion efficiency up to 1.50%.

Synthesis, characterization, photophysics and electrophosphorescent applications of phosphorescent platinum cyclometalated complexes with 9-arylcarbazole moieties

A series of cyclometalating platinum(II) complexes with substituted 9-arylcarbazolyl chromophores have been synthesized and characterized. These complexes are thermally stable and most of them have been characterized by X-ray crystallography. The phosphorescence emissions of the complexes are dominated by (MLCT)-M-3 excited states. The excited state properties of these complexes can be modulated by varying the electronic characteristics of the cyclometalating ligands via substituent effects, thus allowing the emission to be tuned from bright green to yellow, orange and red light. The correlation between the functional properties of these metallophosphors and the results of density functional theory calculations was made. Because of the propensity of the electron-rich carbazolyl group to facilitate hole injection/transport, the presence of such moiety can increase the highest occupied molecular orbital levels and improve the charge balance in the resulting complexes relative to the parent platinum(II) phosphor with 2-phenylpyridine ligand.

Phosphorescence Color Tuning by Ligand, and Substituent Effects of Multifunctional Iridium(III) Cyclometalates with 9-Arylcarbazole Moieties

The synthesis, isomeric studies, and photophysical characterization of a series of multifunctional cyclometalated iridium(III) complexes containing a fluoro- or methyl-substituted 2[3-(N-plienylcarbazolyl)]pyridine molecular framework are presented. All of the complexes are thermally stable solids and highly efficient electrophosphors. The optical, electrochemical, photo-, and electrophosphorescence traits of these iridium phosphors have been studied in terms of the electronic nature and coordinating site of the aryl or pyridyl ring substituents. The correlation between the functional properties of these phosphors and the results of density functional theory calculations was made. Arising from the propensity of the electron-rich carbazolyl group to facilitate hole injection/transport, the presence of such a moiety can increase the highest-occupied molecular orbital levels and improve the charge balance in the resulting complexes relative to the parent phosphor with 2-phenylpyridine ligands. Remarkably, the excited-state properties can be manipulated through ligand and substituent effects that allow the tuning of phosphorescence energies from bluish green to deep red.

Organic photovoltaic cells using hexadecafluorophthalocyaninatocopper (F16CuPc) as electron acceptor material

We fabricated organic photovoltaic cells by using hexadecafluorophthalocyaninatocopper (F16CuPc) as electron acceptor material and para-sexiphenyl (p-6P) as electron donor material. F16CuPc has wide absorption spectrum from 550 nm to 850 nm, which covers the maximum of solar photo flux. The measurement of their external quantum efficiency (EQE) demonstrated that the photocurrent comes from the excitons created in F16CuPc, which were separated into free electrons and holes at heterojunction interface of p-6P and F16CuPc. Moreover, F(16)FuPc with excellent air-stability improved the environmental stability of photovoltaic cells, and the unencapsulated cells exhibited the shelf lifetime of exceeding a week.

Green light-emitting polyfluorenes with improved color purity incorporated with 4,7-diphenyl-2,1,3-benzothiadiazole moieties

By incorporating 4,7-diphenyl- 2,1,3 benzothiadiazole instead of 2,1,3-benzothiadiazole into the backbone of polyfluorene, we developed a novel series of green light- emitting polymers with much improved color purity. Compared with the state-of-the-art green light-emitting polymer, poly(fluorene-co-benzothiadiazole) (lambda max = 537 nm), the resulting polymers (lambda(max) = 521 nm) showed 10-20 nm blueshifted electroluminescence (EL) spectra and greatly improved color purity because the insertion of two phenylene units between the 2,1,3-benzothiadiazole unit and the fluorene unit reduced the effective conjugation length in the vicinity of the 2,1,3-benzothiadiazole unit. As a result, the resulting polymers emitted pure green light with CIE coordinates of (0.29, 0.63), which are very close to (0.26, 0.65) of standard green emission demanded by the National Television System Committee (NTSC). Moreover, the insertion of the phenylene unit did not affect the photoluminescence (PL) and EL efficiencies of the resulting polymers. PL quantum efficiency in solid films up to 0.82 was demonstrated. Single-layer devices (ITO/PEDOT/ polymer/Ca/Al) of these polymers exhibited a turn-on voltage of 4.2 V, luminous efficiency of 5.96 cd A(-1) and power efficiency of 2.21 lm W-1. High EL efficiencies and good color purities made these polymers very promising for display applications.

Synthesis and characterization of white-light-emitting polyfluorenes containing orange phosphorescent moieties in the side chain

Two orange phosphorescent iridium complex monomers, 9-hexyl-9-(iridium (III)bis(2-(4'-fluorophenyl)-4-phenylquinoline-N, C-2')(tetradecanedionate-11,13))-2,7-dibromofluorene (Br-PIr) and 9-hexyl-9-(iridium(III)bis(2-(4'-fluorophenyl)-4-methylquinoline-N, C-2')(tetradecanedionate-11,13))-2,7-dibromofluorene (Br-MIr), were successfully synthesized. The Suzuki polycondensation of 2,7-bis(trimethylene boronate)-9,9-dioctylfluorene with 2,7-dibromo-9,9-dioetylfluorene and Br-Plr or Br-MIr afforded two series of copolymers, PIrPFs and MIrPFs, in good yields, in which the concentrations of the phosphorescent moieties were kept small (0.5-3 mol % feed ratio) to realize incomplete energy transfer. The photoluminescence (PL) of the copolymers showed blue- and orange-emission peaks. A white-light-emitting diode with a configuration of indium tin oxide/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/PIr05PF (0.5 mol % feed ratio of Br-PIr)/Ca/Al exhibited a luminous efficiency of 4.49 cd/A and a power efficiency of 2.35 lm/W at 6.0 V with Commission Internationale de L'Eclairage (CIE) coordinates of (0.46, 0.33). The CIE coordinates were improved to (0.34, 0.33) when copolymer MIr10PF (1.0 mol % feed ratio of Br-MIr) was employed as the white-emissive layer. The strong orange emission in the electroluminescence spectra in comparison with PL for these kinds of polymers was attributed to the additional contribution of charge trapping in the phosphorescent dopants.