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Recombination Zone Control without Sensing Layer and the Exciton Confinement in Green Phosphorescent OLEDs by Excluding Interface Energy Transfer Justin Jesuraj Periyanayagam, Hassan Hafeez, Dong Hyun Kim, Jong Chan Lee, Won Ho Lee, Dae Keun Choi, Chul Hoon Kim, Myungkwan Song, Chang Su Kim, and Seung Yoon Ryu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11039 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 18, 2018
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The Journal of Physical Chemistry
Recombination Zone Control without Sensing Layer and the Exciton Confinement in Green Phosphorescent OLEDs by Excluding Interface Energy Transfer
P. Justin Jesuraj1,a, Hassan Hafeez1,a, Dong Hyun Kim1, Jong Chan Lee1, Won Ho Lee1, Dae Keun Choi1, Chul Hoon Kim2, Myungkwan Song3, Chang Su Kim3 and Seung Yoon Ryu1,*
1Divison of Display and Semiconductor Physics, Display Convergence, College of Science and Technology, Korea University Sejong Campus 2511 Sejong-ro, Sejong City, 30019 (Republic of Korea) 2Department of Advanced Materials Chemistry, College of Science and Technology, Korea University Sejong Campus 2511 Sejong-ro, Sejong City, 339-770, Republic of Korea 3Advanced Functional Thin Films Department, Korea Institute of Materials Science (KIMS) Changwon, 51508 (Republic of Korea)
Abstract Herein, we report the confinement of recombination zone (RZ) in green phosphorescent organic light emitting-diodes (Ph-OLEDs) for enhanced efficiency by varying the emission layer (EML) thickness and through quantum well (QW) insertion. At low thickness of EML, the efficiency is reduced owing to the diffusion of the RZ towards the EML/hole transport layer interface (HTL), which was revealed through the presence of exciton blocking layer (EBL) [TCTA-Tris(4-carbazoyl-9ylphenyl)amine] excitation accompanied by a blue shift in electroluminescence (EL). Further increase in the thickness of the EML caused the RZ to move towards the cathode, which was determined based on the disappearance of TCTA emission and the corresponding red shift observed in EL spectra. The solid state and time resolved area normalized photoluminescence emission spectra (TRANES) investigations further corroborate the RZ movement tactics along with TCTA excimer generation and exciplex generation between TCTA and Ir(ppy)3. The superior quantum and
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current efficiency of 14.4% and 50 cd/A, respectively, was extracted determined for the device with an EML thickness of 15 nm due to the confinement of the RZ in the EML. The addition of (EML/interlayer/EML) QW facilitates improved charge balance in the Ph-OLED and further assists in the confinement of the RZ in the EML. Because of QW, a slight increment in quantum (14.6%) and current efficiency (52 Cd/A) was observed. Without using any sensing layers, movement of the RZ was successfully monitored and confined in the EML to realize enhanced efficiency in green Ph-OLEDs. a
These authors equally contributed to this paper.
*
Corresponding author List
Prof. Dr. Seung Yoon Ryu1,* 1
Division of Display and Semiconductor Physics, College of Science and Technology,
Korea University Sejong Campus 2511 Sejong-ro, Sejong City, 339-770, Republic of Korea, Tel) +82-44-860-1376,
[email protected] 2 ACS Paragon Plus Environment
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The Journal of Physical Chemistry
I. INTRODUCTION
Organic light emitting devices (OLEDs) with high external quantum efficiency (ηext) are indispensable for futuristic solid-state light technologies. The value of ηext depends on internal quantum efficiency of the molecule, charge balance, and out coupling efficiency of the architecture.1-6 Especially, the ηext of OLEDs drops at higher luminance called as ‘efficiency roll-off’ occurs due to lack of exciton confinement in the emissive layer (EML)7-8 and triplet induced exciton quenching.9-11 Triplet based losses occur through a triplet-triplet annihilation (TTA) and tripletpolaron annihilation (TPA) mechanism which causes decrement in the internal quantum efficiency of EML. However, the inability to confine the exciton density in the EML can induce a significant drop in device efficiency8, 12-16 despite having an efficient EML. Hence, it is essential to confine the exciton density or recombination zone (RZ) in the EML for enhanced efficiency in the OLED architecture. U.S. Bhansali et al.17 controlled the RZ and color coordinates in white OLED with fluorophore/Phosphor emitters by varying the electron transport layer (ETL) thickness. Recently, W. Song et al.18 proposed a step wise energy level doping of emitters in same host to confine the carrier RZ and also reduced the TTA and TPA in Phosphorescent OLED for achieving higher quantum efficiencies. In general, the movement or change in RZ is carefully examined using a sensing layer8,
19-22
in the EML, which usually consists of different guest or emission
candidates. The movement of the RZ has been observed by comparing the device electroluminescence (EL) pertaining to the EML and different sensing positions of sensing layers. Although incorporation of an emissive sensing layer is beneficial for
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OLEDs to realize a RZ shift, the device emission generally consists of contributions from both the EML and sensing layers, which have improper color coordinates,21-22 which disturbs the monochromatic nature of devices. The use of a sensing layer in the EML is adequate for confirming the exact RZ location, but it requires extra device fabrication steps, making the process more costly and time-consuming. Hence, monitoring the RZ movement in monochromatic OLEDs without employing any sensing layers is desirable. Some theoretical and empirical models have been proposed in the literature to quantify the possible variation of the RZ.19, 23 Mostly, the RZ shift is suppressed by placing additional blocking layers near the EML/transport layer interface. However, it is necessary to ensure that the insertion of blocking layers does not alter charge balance in the EML. Careful examination of the interfacial energy levels and recombination may be useful for the optimization of the RZ in OLEDs. Recently, exciplex formation within the EML24-25 and EML/transport layer2627
was found to play a major role in determining OLED efficiency. Hence, it is
necessary to examine the photophysics of the transport layer/EML interface in order to realize the recombination effects while RZ shift towards them. Here, we present a simple and effective method to detect RZ movement in green phosphorescent OLEDs (Ph-OLEDs) using various EML thicknesses. Furthermore, the RZ shift tactics were tested by employing a quantum well (QW) structure consisting of an EML/exciton blocking layer (EBL)/EML. Further, solid state and time resolved area normalized photoluminescence emission spectra (TRANES) analyses were adapted to examine the RZ movement and possible exciton generation in terms of excimers or exciplexes at the EML/EBL interface. To our knowledge, there have been no reports on the energy transfer from TCTA to the EML layers to
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The Journal of Physical Chemistry
date. The variation observed in the EL spectra with various EML thicknesses and QW infusions together with their associated color coordinates were compared with a goal of enhancing the performance of green Ph-OLED via efficient exciton confinement in the EML.
II. EXPERIMENTAL SECTION Green Ph-OLED fabrication was initiated with commercial indium tin oxide (ITO) coated glass substrates. The ITO was cleaned and treated with ultraviolet ozone for 10 minutes to remove any impurities and to enhance its surface qualities. Lithium fluoride (LiF)/aluminum was deposited as a bilayer cathode for the OLEDs. Poly(3,4ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) (40 nm) and N,N-bis(1-naphthyl)-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine
(NPB)
(20
nm)
were
deposited as the hole injection layer and HTL, respectively. 2,2′,2"-(1,3,5benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi, 10 nm) was utilized as the ETL owing to its high electron mobility nature. Tris(4-carbazoyl-9-ylphenyl)amine (TCTA) was incorporated as an EBL with 10 nm thickness at the EML/HTL interface. The host guest system consisting of 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl (CBP) and Tris[2-phenylpyridinato-C2,N]iridium(III)[Ir(ppy)3] was used as the EML in the green Ph-OLED with various thickness (10, 15, 20 and 30 nm). Green Ph-OLED with a QW consisting of an EML (7.5 nm)/TCTA (5 nm)/EML (7.5 nm) architecture was fabricated to compare the RZ movement. The device active area was maintained at 4 mm2 in each OLEDs. Solid state and time-resolved photoluminescence spectrum measurement unit consisting of 350 nm (
