Silane derivatives with wide energy gap (≈3.5 eV) containing different electron-withdrawing groups of quinoline and naphthyridine are synthesized and used as the electron transporting materials. The different electron transporting and hole/exciton blocking properties of the silane derivatives are investigated via multilayered structure of organic electrophosphorescent devices by using fac-tris(2-phenylpyridine)iridium (Ir(ppy)₃) as the phosphorescent emitter. 15.4% of maximum external quantum efficiency (EQE) corresponding to 56.2 cd A⁻¹ of maximum current efficiency is obtained with a maximum power efficiency of 58.9 lm W⁻¹ by employing di-(4-(1,8-naphthyridin-2-yl)phenyl)diphenylsilane (DNPS) as the electron transporting material, combining with 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline as the hole blocking layer, which is higher than the performance of conventional Alq₃ device. When changing naphthyridine of DNPS to the electron-withdrawing group of quinoline (di-(4-(isoquinolin-4-yl)phenyl)diphenylsilane), only 11.4% of maximum EQE with 41.4 cd A⁻¹ of maximum current efficiency and 32.5 lm W⁻¹ of a maximum power efficiency is obtained. These indicate that the electron transporting ability increases while the electron-withdrawing group changes from quinoline to naphthyridine, which is also consistent with the calculated reorganization energy.

Deep-blue fluorescent compounds are particularly important in organic light-emitting devices (OLEDs). A donor–accepotor (DA)-type blue-emitting compound, 1-(10-(4-methoxyphenyl)anthracen-9-yl)-4-(10-(4-cyanophenyl)anthracen-9-yl)benzene (BD3), is synthesized, and for comparison, a nonDA-type compound, 1,4-bis(10-phenylanthracene-9-yl)benzene (BD1) and a weak DA-type compound, 1-(10-phenylanthracen-9-yl)-4-(10-(4-cyanophenyl)anthracen-9-yl)-benzene (BD2), are also synthesized. The twisted conformations of the two anthracene units in the compounds, confirmed by single crystal X-ray analysis, effectively prevent π-conjugation, and the compound shows deep-blue photoluminescence (PL) with a high PL quantum efficiency, almost independent of the solvent polarity, resulting from the absence of an intramolecular charge transfer state. The DA-type molecule BD3 in a non-doped device exhibits a maximum external quantum efficiency (EQE) of 4.2% with a slight roll-off, indicating good charge balance due to the DA-type molecular design. In the doped device with 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) host, the BD3 exhibits higher EQE than 10% with Commission International de L'Eclairge (CIE) coordinates of (0.15, 0.06) and a narrow full-width at half-maximum of 45 nm, which is close to the CIE of the high definition television standard blue.

A series of (vinylphenyl)pyridine-based polymer binders, PVPh2Py, PVPh3Py, and PVPh4Py, are designed and synthesized and it is found that mixtures of Liq and the polymers exhibit superior electron injection characteristics as ultrathin (1.6 nm) electron injection layer (EIL) films. They are comparable to those of EILs composed only of Liq. The addition of the polymers does not deteriorate the performance of Liq EILs. Additionally, when the EIL thickness is increased from 1.6 nm to 16 nm, the driving voltages increase and the external quantum efficiencies decrease. The increase in the voltage and decrease in the EQE are suppressed in the device with mixed EILs compared to those observed for the device composed of 100 wt% Liq. Furthermore, the position of the nitrogen in the pyridine ring is considered to influence the electron transport properties of the EILs. The mixing PVPh4Py with Liq improves the driving voltage of the fabricated devices, even with a thick mixed EIL. This reduced dependence of the performance of EILs on their thickness will be advantageous for the coating of large areas using solution processes.

A series of 1,8-naphthyridine derivatives is synthesized and their electron-transporting/injecting (ET/EI) properties are investigated via a multilayered electrophosphorescent organic light-emitting device (OLED) using fac-tris(2-phenylpyridine)iridium [Ir(ppy)₃] as a green phosphorescent emitter doped into a 4,4′-N,N′-dicarbazolebiphenyl (CBP) host with 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (a-NPD) as the hole-transporting layer, and poly(arylene ether sulfone) containing tetraphenylbenzidine (TPDPES) doped with tris(4-bromophenyl)ammonium hexachloroantimonate (TBPAH) as the hole-injecting layer. The turn-on voltage of the device is 2.5 V using 2,7-bis[3-(2-phenyl)-1,8-naphthyridinyl]-9,9-dimethylfluorene (DNPF), lower than that of 3.0 V for the device using a conventional ET material. The maximum current efficiency (CE) and power efficiency (PE) of the DNPF device are much higher than those of a conventional device. With the aid of a hole-blocking (HB) and exciton-blocking layer of bathocuproine (BCP), 13.2–13.7% of the maximum external quantum efficiency (EQE) and a maximum PE of 50.2–54.5 lm W⁻¹ are obtained using the naphthyridine derivatives; these values are comparable with or even higher than the 13.6% for conventional ET material. The naphthyridine derivatives show high thermal stabilities, glass-transition temperatures much higher than that of aluminum(III) bis(2-methyl-8-quinolinato)-4-phenylphenolate (BAlq), and decomposition temperatures of 510–518 °C, comparable to or even higher than those of Alq₃.

Organic light-emitting devices (OLEDs) are expected to be adopted as the next generation of general lighting because they are more efficient than fluorescent tubes and are mercury-free. The theoretical limit of operating voltage is generally believed to be equal to the energy gap, which corresponds to the energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for the emitter molecule divided by the electron charge (e). Here, green OLEDs operating below a theoretical limit of the energy gap (E_g) voltage with high external quantum efficiency over 20% are demonstrated using fac-tris(2-phenylpyridine)iridium(III) with a peak emission wavelength of 523 nm, which is equivalent to a photon energy of 2.38 eV. An optimized OLED operates clearly below the theoretical limit of the E_g voltage at 2.38 V showing 100 cd m⁻² at 2.25 V and 5000 cd m⁻² at 2.95 V without any light outcoupling enhancement techniques.

Organic light-emitting devices (OLEDs) are solid-state light-emitting devices based on organic semiconductors. Recent rapid advances in materials chemistry have enabled white OLEDs to be used for general lighting and large-area flat panel display. White OLED panel efficacy has reached 90 lm W^–1, and a tandem white OLED panel has achieved a lifetime of over 100000 h at 1000 cd m^–2. LG is set to launch a 55″ OLED TV in 2013, and OLEDs will be expected to make bigger breakthroughs. Although white OLED panels show superior performance, there is still much room (nearly 160 lm W^–1) for improvement, in view of the theoretical limit of 248 lm W^–1. To reach this objective, OLEDs need to achieve three goals: (1) high internal quantum efficiency, (2) low operation voltage, and (3) high light-outcoupling efficiency at the same time. For organic chemists creating new organic semiconductors, issues (1) and (2) are particularly important because these relate to materials chemistry. Here we review recent developments in phosphorescent OLED technology, especially from materials chemistry.

A series of 2-methylpyrimidine skeleton-based electron-transporting derivatives (BPyMPM) are designed and synthesized to investigate the influence of substituted pyridine rings on the physical properties and electron mobilities (μ_e). The only structural difference is the position of substituted pyridine rings. The melting point (Tm) of B4PyMPM is estimated to be ca. 50 °C higher than that of B3PyMPM, and ca. 120 °C higher than that of B2PyMPM. The ionization potential is observed to increase in the order B2PyMPM (6.62 eV) < B3PyMPM (6.97 eV) < B4PyMPM (7.30 eV), measured using ultraviolet photoelectron spectroscopy. Furthermore, time-of-fight measurements of vacuum-deposited films demonstrate that the μ_e at 298 K of B4PyMPM is 10 times higher than that of B3PyMPM and 100 times higher than that of B2PyMPM. To extract the charge transport parameters, the temperature and field dependencies of μ_e are investigated. Using Bässler's disorder formalism, the degree of energetic disorder is estimated to decrease in the order B2PyMPM (91 meV) > B3PyMPM (88 meV) > B4PyMPM (76 meV), and the positional disorder is 2.7 for B2PyMPM, and < 1.5 for B3PyMPM and B4PyMPM.

Three triphenyl benzene derivatives of 1,3,5-tri(m-pyrid-2-yl-phenyl)benzene (Tm2PyPB), 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (Tm3PyPB) and 1,3,5-tri(m-pyrid-4-yl-phenyl)benzene (Tm4PyPB), containing pyridine rings at the periphery, are developed as electron-transport and hole/exciton-blocking materials for iridium(III) bis(4,6-(di-fluorophenyl)pyridinato-N,C^2′)picolinate (FIrpic)-based blue phosphorescent organic light-emitting devices. Their highest occupied molecular orbital and lowest unoccupied molecular orbital (LUMO) energy levels decrease as the nitrogen atom of the pyridine ring moves from position 2 to 3 and 4; this is supported by both experimental results and density functional theory calculations, and gives improved electron-injection and hole-blocking properties. They exhibit a high electron mobility of 10⁻⁴–10⁻³ cm² V⁻¹ s⁻¹ and a high triplet energy level of 2.75 eV. Confinement of FIrpic triplet excitons is strongly dependent on the nitrogen atom position of the pyridine ring. The second exponential decay component in the transient photoluminescence decays of Firpic-doped films also decreases when the position of the nitrogen atom in the pyridine ring changes. Reduced driving voltages are obtained when the nitrogen atom position changes because of improved electron injection as a result of the reduced LUMO level, but a better carrier balance is achieved for the Tm3PyPB-based device. An external quantum efficiency (EQE) over 93% of maximum EQE was achieved for the Tm4PyPB-based device at an illumination-relevant luminance of 1000 cd m⁻², indicating reduced efficiency roll-off due to better confinement of FIrpic triplet excitons by Tm4PyPB in contrast to Tm2PyPB and Tm3PyPB.