Effect of triplet multiple quantum well structures on the performance of blue phosphorescent organic light-emitting diod
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NANO EXPRESS
Open Access
Effect of triplet multiple quantum well structures on the performance of blue phosphorescent organic light-emitting diodes Seokjae Lee1†, Jaryong Koo1†, Gunwoo Hyung1†, Donghwan Lim1, Donghyung Lee1, Kumhee Lee2†, Seungsoo Yoon2*, Wooyoung Kim3 and Youngkwan Kim1*
Abstract We investigate multiple quantum well [MQW] structures with charge control layers [CCLs] to produce highly efficient blue phosphorescent organic light-emitting diodes [PHOLEDs]. Four types of devices from one to four quantum wells are fabricated following the number of CCLs which are mixed p- and n-type materials, maintaining the thickness of the emitting layer [EML]. Remarkably, such PHOLED with an optimized triplet MQW structure achieves maximum luminous and external quantum efficiency values of 19.95 cd/A and 10.05%, respectively. We attribute this improvement to the efficient triplet exciton confinement effect and the suppression of triplet-triplet annihilation which occurs within each EML. It also shows a reduction in the turn-on voltage from 3.5 V (reference device) to 2.5 V by the bipolar property of the CCLs. Background Due to their high efficiency, phosphorescent organic light-emitting diodes [PHOLEDs] are promising lightemitting materials in organic light-emitting diodes [OLEDs]. An internal quantum efficiency of 100% could be realized in red and green PHOLEDs [1,2]. However, the performance of blue PHOLEDs still needs to be improved for lighting applications. Light emission in PHOLEDs depends on the properties of the organic material in the devices [3,4]. In particular, the energy level of the charge transport, host, and emitter materials influences the light-emitting efficiency. Besides, many different device architectures have attempted to improve the light-emitting efficiency of PHOLEDs. Hole and electron blocking layers or triplet exciton blocking layers [TEBLs] in PHOLEDs were introduced to confine both carriers and excitons within emitting layers [EMLs] [5]. A double emitting layer structure was also employed in OLEDs by utilizing phosphorescent materials doped in * Correspondence: [email protected]; [email protected] † Contributed equally 1 Department of Information Display, Hongik University, Seoul, 121-791, South Korea 2 Department of Chemistry, Sungkyunkwan University, Suwon, 440-746, South Korea Full list of author information is available at the end of the article
two different hosts. As a result, these ways were effective in providing higher efficiency in PHOLEDs [6]. Another way to achieve high efficiency in OLEDs is to confine excitons inside the EML using the multiple quantum well [MQW] structure [7]. Only a few reports concerning the MQW structure with good carrier and exciton confinement ability have been presented on OLEDs until quite recently. For example, Qiu et al. [7] improved the charge balance by utilizing an organic MQW structure to decelerate hole transportation. Huang et al. [8] used MQW structures to increase the carrier recombination efficiency, where both charges and excitons we
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