| Summary: | 博士 === 國立清華大學 === 材料科學工程學系 === 99 === In the study, we incorporate carrier-controlling layers, including electron-blocking layer (EBL), hole-transporting layer (HTL) and hole-modulating layer (HML), to develop and fabricate highly efficient and very-high color rendering index (CRI) organic light-emitting diodes (OLEDs). The investigated devices are shown as follow.
In Part I, we demonstrate an efficient orange-red phosphorescent OLED using a novel host, 2,7-bis(carbazo-9-yl)-9,9-ditolyfluorene, doped with tris(2-phenylquinoline) iridium(III), as well as a thin EBL of 1,1-bis-(4-bis(4-methylphenyl)-aminophenyl)-cyclohexane (TAPC) is deposited prior to the emissive layer. The resulting device exhibits a current efficiency of 44.8 cd/A at 1,000 cd/m2. This high efficiency may be attributed to the adoption of the host, which favors the injection of holes, as well as the emissive-layer architecture enabling excitons to form on the host and hence favoring efficient energy-transfer from host to guest. The TAPC layer helps to modulate excessive holes to be injected into the emissive layer and to confine the electrons, which would in turn balance the injection of both carriers and improve the device efficiency, either at low or high voltages.
In Part II, we demonstrated the use of double HTLs, poly(3,4-ethylene-dioxythiophene)-poly-(styrenesulfonate) (PEDOT:PSS) and N,N’-bis-(1-naphthyl)-N,N’-diphenyl-1,10-biphenyl-4-4’-diamine (NPB), to modify the hole injection characteristics, to balance the injection of carriers, and consequently to improve the device efficiency. With the addition of a 7.5 nm second HTL (NPB), the resultant power-efficiency at 100 cd/m2 was increased from 11.9 to 18.9 lm/W, an improvement of 59%. The improvement was even more marked at 1,000 cd/m2, i.e. that the power-efficiency was increased from 9.1 to 16.5 lm/W, an improvement of 81%. The marked efficiency improvement may be attributed to a better balance of carrier-injection in the desired emissive zone since the addition of the NPB layer in between the first HTL and the EML may have effectively reduced the injection of excessive holes into the EML due to the relatively high energy-barrier to holes, which was 0.5 eV, at the interface of the two HTLs.
In Part III, we demonstrate an efficient very-high CRI OLED with a CRI of 98 and an efficacy of 8.3 lm/W at 100 cd/m2, or a CRI of 96 and an 5.2 lm/W at 1,000 cd/m2. The very high CRI may be attributed to the successful deposition and emission of the two full-spectrum complementary white emissive layers, especially as a thin HML is inserted in between to regulate the injection of carriers. Without the interlayer, the resultant CRI drops to 73 and efficacy to 3.6 lm/W at 1,000 cd/m2. The employment of the carrier regulating layer also helps disperse the injected carriers, leading recombination to occur in a wider area and hence a higher efficiency. Moreover, it is worth mentioning that the CRI remained around 96 to 98 throughout the brightness range over which we investigated. The CIE coordinates of the very high CRI OLED also fall within the pure-white region and are very close to daylight (Planckian) locus, indicating the device to possess the characteristics required for high-quality illumination.
In summary, the main contribution of this study is to markedly increase the power and current efficiencies of white and orange-red OLEDs and to fabricate a efficient very-high CRI OLED by using carrier-controlling layers. In the study of very-high CRI OLED, we use 1,3,5-trisN-phenylbenzimidazol-2-ylbenzene, a general electron-transporting layer, as the hole-modulating layer. This is a novel concept on the design of device structure. This also implies that any materials will have unanticipated functions if we use it at a different position in device. This is a very useful concept for the researchers in the field of organic electronics.
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