Kenichi Goushi

Highly efficient organic light-emitting diodes from delayed fluorescence

The inherent flexibility afforded by molecular design has accelerated the development of a wide variety of organic semiconductors over the past two decades. In particular, great advances have been made in the development of materials for organic light-emitting diodes (OLEDs), from early devices based on fluorescent molecules to those using phosphorescent molecules. In OLEDs, electrically injected charge carriers recombine to form singlet and triplet excitons in a 1:3 ratio; the use of phosphorescent metal–organic complexes exploits the normally non-radiative triplet excitons and so enhances the overall electroluminescence efficiency. Here we report a class of metal-free organic electroluminescent molecules in which the energy gap between the singlet and triplet excited states is minimized by design, thereby promoting highly efficient spin up-conversion from non-radiative triplet states to radiative singlet states while maintaining high radiative decay rates, of more than 106 decays per second. In other words, these molecules harness both singlet and triplet excitons for light emission through fluorescence decay channels, leading to an intrinsic fluorescence efficiency in excess of 90 per cent and a very high external electroluminescence efficiency, of more than 19 per cent, which is comparable to that achieved in high-efficiency phosphorescence-based OLEDs.

100% phosphorescence quantum efficiency of Ir(III) complexes in organic semiconductor films

We demonstrate that three Ir(III) complexes used as principal dopants in organic electrophosphorescent diodes have very high photoluminescence quantum efficiency (ηPL) in a solid-state film. The green emitting complex, fac-tris(2-phenylpyridinato)iridium(III) [Ir(ppy)3], the red-emitting bis[2-(2′-benzothienyl)pyridinato-N,C3′] (acetylacetonato)iridium(III) [Btp2Ir(acac)], and the blue complex bis[(4,6-difluorophenyl)pyridinato-N,C2](picolinato)iridium(III) (FIrpic) were prepared as codeposited films of varying concentration with 4,4′-bis(N-carbazolyl)-2,2′-biphenyl, a commonly used host material. The maximum ηPL values for Ir(ppy)3, Btp2Ir(acac), and FIrpic were, respectively, 97%±2% (at 1.5mol%), 51%±1% (at 1.4mol%), and 78%±1% (at 15mol%). Furthermore, we also observed that the maximum ηPL of FIrpic reached 99%±1% when doped into the high triplet energy host, m-bis(N-carbazolyl)benzene, at an optimal concentration of 1.2mol%.

Triplet exciton confinement and unconfinement by adjacent hole-transport layers

To understand confinement of the triplet exciton of Ir(ppy)3 by hole-transport layers, we compared energy-dissipative processes of the triplet exciton of Ir(ppy)3 which is doped into 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (α-NPD), 4,4′-bis [N-(p-tolyl)-N- phenyl-amino]biphenyl (TPD), 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC), and 4,4′-N,N′-dicarbazole-biphenyl hosts. Significant energy transfer from Ir(ppy)3 into the triplet levels of α-NPD was observed. In the case of the TPD host, however, partial confinement of the Ir(ppy)3 triplet exciton was observed. This result suggests both forward and backward energy transfer from Ir(ppy)3 to the TPD triplet levels. Furthermore, employing TAPC as a hole-transport layer achieved strong confinement of the Ir(ppy)3 triplet exciton. One conclusion from these results is that electrophosphorescence efficiency is well correlated with the triplet energy level of the hole-transport layer host materials.

Efficient organic light-emitting diodes through up-conversion from triplet to singlet excited states of exciplexes

Enhanced electroluminescence efficiency is achieved in organic light-emitting diodes through delayed fluorescence of the exciplex state formed between 4,4′,4′′-tris[3-methylphenyl(phenyl)amino]-triphenylamine (m-MTDATA) as an electron-donating material and 2,8-bis(diphenylphosphoryl)dibenzo-[b,d]thiophene (PPT) as an electron-accepting material. The devices exhibited maximum external electroluminescence quantum and power efficiencies of 10.0% and 47.0 lm/W, respectively.

100% fluorescence efficiency of 4,4′-bis[(N-carbazole)styryl]biphenyl in a solid film and the very low amplified spontaneous emission threshold

We examined the amplified spontaneous emission (ASE) characteristics of fluorescent styrylbenzene derivatives doped into a 4,4′-di(N-carbazolyl)biphenyl (CBP) host. In particular, 4,4′-bis[(N-carbazole)styryl]biphenyl (BSB-Cz) demonstrated an extremely low ASE threshold of Eth=0.32±0.05μJ∕cm2. We observed that the 6 wt %-BSB-Cz:CBP film has an ultimate photoluminescence (PL) quantum efficiency of ϕPL=100% and a short transient lifetime of τf=1.0±0.1ns, leading to a large radiative decay rate of kr=1×109s−1. We demonstrated that the extremely low ASE threshold originated from the large radiative decay rate of BSB-Cz.

Improvement of electroluminescence performance of organic light-emitting diodes with a liquid-emitting layer by introduction of electrolyte and a hole-blocking layer.

Research on organic light-emitting diodes (OLEDs) containing solid state organic semiconductors has advanced signifi cantly in the past twenty years, resulting in indispensible technology for fl at panel displays [ 1 ] and lighting applications. [ 2 ] This is because OLEDs exhibit not only high-performance light-emitting characteristics [ 3 , 4 ] but also provide unique benefi ts such as light weight and fl exibility, [ 5 , 6 ] which are diffi cult to realize using inorganic LEDs. Taking into account the advantages of organic materials, we developed a unique OLED containing a neat liquid semiconductor, i.e., a liquid OLED that can be used for lighting and display applications. [ 7 ] It is expected that liquid OLEDs will allow the realization of truly fl exible displays because detachment between the liquid emitting layer and electrodes does not occur even when the devices are bent signifi cantly. Furthermore, liquid emitters degraded by long-term use in an OLED can be replaced by a fl ow of fresh organic liquid emitters, removing the problem of OLED degradation resulting from the decomposition of organic materials (see Figure S1 of Supporting Information). Figure 1 demonstrates electroluminescence (EL) from a liquid OLED. Here, a liquid emitter was passed from the top to the bottom of the device by capillary action between two patterned indium tin oxide (ITO) glass substrates separated by a gap of 5.0 ± 0.5 μ m. After injection of the liquid emitter at the top, it penetrates into the gap between the substrates by capillary action and fl ows downward (Figure 1(i)) . The location of the liquid emitter can be detected by photoluminescence as shown in Figure 1(i)–(iv) . EL was not observed before the liquid emitter reaches the area where the two ITO electrodes overlap, as shown in Figure 1(v) . In contrast, intense EL was observed when the liquid emitter reached the area between the electrodes (Figure 1(vi)) . The area exhibiting EL then expanded gradually

Ultraviolet amplified spontaneous emission from thin films of 4,4′-bis(9-carbazolyl)-2,2′-biphenyl and the derivatives

We demonstrate 394 nm ultraviolet amplified spontaneous emission (ASE) with a low pumping power threshold of Eth=1.3±0.2 μJ/cm2, from a thin film of an organic semiconductor 4,4′-bis(9-carbazolyl)-2,2′-biphenyl (CBP) under the pulse excitation of a N2 gas laser (337 nm). 3-methyl and 3,6-dimethyl substituted CBP derivatives also exhibited pronounced ASE in the deep-blue region of 401 and 406 nm and ASE thresholds of less than Eth=2 μJ/cm2. We also examined the ASE characteristics of N,N′-di(m-tolyl)-N,N′-diphenylbenzidine (TPD), N,N,N′,N′-tetraphenylbenzidine (DPABP) and N,N′-di(α-naphtyl)-N,N′-diphenylbenzidine (α-NPD). While TPD and DPABP showed low ASE thresholds, α-NPD did not show any ASE. We show that the large radiative decay rate (kf) of DPABP and TPD, which is derived from their short fluorescence lifetime (τf) and large quantum efficiency (ηf), leads to a low ASE threshold. On the other hand, the lack of ASE from α-NPD is ascribable to the small kf of 0.8±0.1×108 s−1, which is due to the rather ...

Temperature dependence of photoluminescence properties in a thermally activated delayed fluorescence emitter

Using steady-state and time-resolved photoluminescence (PL) spectroscopy, we have investigated the temperature dependence of PL properties of 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyano-benzene (4CzIPN), which have a small energy gap between its singlet and triplet excited states and thus exhibits efficient thermally activated delayed fluorescence [H. Uoyama et al., Nature 492, 235 (2012)]. Below around 100 K, PL quantum efficiency of 4CzIPN thin films is largely suppressed and strong photoexcitation intensity dependence appears. These features can be explained by using rate equations for the densities of singlet and triplet excited states considering a triplet-triplet annihilation process.

Toward continuous-wave operation of organic semiconductor lasers

Organic semiconductor laser operating in the quasi-CW regime at 80 MHz and under 30 ms long pulse photoexcitation is demonstrated. The demonstration of continuous-wave lasing from organic semiconductor films is highly desirable for practical applications in the areas of spectroscopy, data communication, and sensing, but it still remains a challenging objective. We report low-threshold surface-emitting organic distributed feedback lasers operating in the quasi–continuous-wave regime at 80 MHz as well as under long-pulse photoexcitation of 30 ms. This outstanding performance was achieved using an organic semiconductor thin film with high optical gain, high photoluminescence quantum yield, and no triplet absorption losses at the lasing wavelength combined with a mixed-order distributed feedback grating to achieve a low lasing threshold. A simple encapsulation technique greatly reduced the laser-induced thermal degradation and suppressed the ablation of the gain medium otherwise taking place under intense continuous-wave photoexcitation. Overall, this study provides evidence that the development of a continuous-wave organic semiconductor laser technology is possible via the engineering of the gain medium and the device architecture.

Organic field effect transistor using pentacene single crystals grown by a liquid-phase crystallization process.

Nearly perfect pentacene single crystals with wide terraces several micrometers in width were grown by crystallization from a pentacene-containing trichlorobenzene solution. Organic field-effect transistors (OFETs) were fabricated with the pentacene single crystals and characterized for their electrical properties. The field effect mobility was found to be in the range of 0.4-0.6 cm(2)/V x s, which is comparable to that of OFETs fabricated with pentacene single crystals prepared by a physical vapor-phase growth method. The results described in this paper clearly demonstrate that the crystallization of organic semiconductors from solution is a promising chemical method for device processing of OFETs.