Heavily Doped, Charge-Balanced Fluorescent Organic Light-Emitting

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Heavily doped, charge balanced fluorescent organic light-emitting diodes from direct charge trapping of dopants in emission layer Sang Ho Rhee, Sung Hyun Kim, Hwang Sik Kim, Jun Young Shin, Jeeban Bastola, and Seung Yoon Ryu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b04519 • Publication Date (Web): 07 Jul 2015 Downloaded from http://pubs.acs.org on July 12, 2015

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ACS Applied Materials & Interfaces

Heavily doped, charge balanced fluorescent org anic light-emitting diodes from direct charge trapping of dopants in emission layer Sang Ho Rhee,†,§ Sung Hyun Kim, ‡,§ Hwang Sik Kim,† Jun Young Shin,† Jeeban Bastola,† and Seung Yoon Ryu†,*

†

Department of Information Communication & Display Engineering, Division of Mechanics and ICT Convergence Engineering, Sunmoon University, 221, Sunmoon-ro, Tangjeong-myeon, Asan, Chungnam, 336708, Republic of Korea ‡ Department of Chemistry, College of Natural Sciences, Seoul National University, Seoul 151-747, Republic of Korea

ABSTRACT We studied the effect of direct charge trapping at different doping concentrations on the device performance in tris(8-hydroxyquinoline) aluminum (Alq3):10-(2-benzothiazolyl)2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,

5H,

11H-(1)-benzopyropyrano(6,7-8-

i,j)quinolizin-11-one (C545T) as a host-dopant system of a fluorescent organic light-emitting diode. With increasing C545T doping concentration, trap sites could lead to the promotion of hole injection and the suppression of electron injection due to the electron-transport-character of Alq3 host for each carriers, as confirmed by hole- and electron-only devices. Direct charge injection of hole carriers from the hole transport layer into C545T dopants and the charge trapping of electron carriers are the dominant process to improve the charge balance and the corresponding efficiency. The shift of the electroluminescence (EL) spectra from 519 nm to 530 nm was confirmed the exciton formation route from Förster energy transfer of host-1-

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dopant system to direct charge trapping of dopant only emitting system. Variation in the doping concentration plays how the dopant roles in the fluorescent hostdopant system. Even though concentration quenching in fluorescent dopants is unavoidable, relatively heavy doping is necessary to improve the charge balance and efficiency and to investigate the relationship between direct charge trapping and device performance. Heavy doping at 6% doping ratio also generates heavy exciton quenching and excimer exciton because of close enough among excitons and dipole-dipole interaction. The optimum device performance was achieved with a 4% doped device, retaining the high efficiency of 12.5 cd/A from 100 cd/m2 up to 15,000 cd/m2.

*

Corresponding authors

Prof. Dr. Seung Yoon Ryu†,* †

Department of Information Communication & Display Engineering, Division of Mechanics and ICT Convergence Engineering, Sunmoon University, Asan, Chungnam, 336-708, Republic of Korea, Tel) +82-41-530-2295, [email protected]

Author Contributions §

These authors are equally contributed to this paper.

Keywords: hole/electron injection, fluorescent OLEDs, hole-/electron-only devices, direct charge trapping, trap sites, charge balance

-2-


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I. INTRODUCTION In organic light-emitting diodes (OLEDs), there are two main ways to form excitons. The one way is from Fӧrster energy transfer1 from host to dopant molecules and the other one is from direct recombination between holes and electrons at the dopant molecules.2 The management of doping concentration and energy level alignment through the device structure has been widely developed as an approach to improve device performance, making it possible to lead this technology towards practical applications.3-6 However, there is still a demand to understand the quantitative contribution in these two light-emission mechanism in terms of singlet excitons formation and quenching, charge injection, charge balance, and so on. This demand is attributed to the benefits of the simple device structure, the exclusion of the costly phosphorescent dopants, and a relatively low fabrication cost.7-12 Low doping concentration is generally used to obtain the high efficiency of these devices because its additional loading of dopants gives rise to heavier self-quenching.10,11 Tris(8-hydroxyquinoline) aluminum (Alq3):10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,

5H,

11H-(1)-

benzopyropyrano(6,7-8-i,j)quinolizin-11-one (C545T) is the most commonly used host: dopant in green-emitting fluorescent organic light-emitting diodes (FLOLEDs). So far, device performance has been shown to increase the efficiency mainly through control of the thickness and the charge injection/transport capability using by the different energy level layers instead of the commonly used layers in the alternative device structure.7,13-16 A. B. Chwang et al. proposed a graded and mixed stack of an Alq3:C545T (1 wt %) emitting layer (EML) with hole and electron transport characters, resulting in an efficiency of 10 cd/A at 1000 cd/m2.7 J. X. Sun et al. obtained a maximum current efficiency of 19.6 cd/A with an Alq3:C545T (2 wt %) due to micro-cavity effects induced by the cathode modification in -3-



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tandem FLOLEDs.14 From those approaches, as well as other research reports, the highest current efficiencies were obtained by enhancing Fӧrster energy transfer with low doping concentration of C545T in Alq3 host13-16 and/or by increasing the hole current density via electrode modification.8-11 Generally, 4~5% doping in EML has been widely investigated in the case of phosphorescent OLEDs (PHOLEDs) through long range dexter energy transfer compared to low doping (less than 1%) of FLOEDs with short-range Fӧrster energy transfer by dipole-dipole interaction. In the ideal case, host-guest energy transfer with 1% dopant concentration in PHOLEDs was unusually observed due to the combination of efficient Fӧrster energy transfer1 and dexter energy transfer between host singlet and the metal-to ligand charge-transfer (MLCT) state.17 That means, conventional concept that heavy doping in PHOLEDs and low doping in FLOLEDs was adopted to increase the efficiency could be reconsidered. On the other hand, direct charge trapping was developed to increase charge recombination efficiency and to reduce excessive charges for dissipation through the device, which is the primary emission mechanism in highly efficient PHOLEDs with the appropriate doping concentration.17-20 Heavily doped phosphorescent EML up to 20 % without hole injection layer (HIL) and hole transport layer (HTL) were investigated for direct hole injection into triplet dopants.20 In addition, Z. Liu et al. they investigated how the device performance varied with doping concentration up to 23%, the highest efficiency of 14.3 cd/A was obtained for 1% doping of C545T through the modification of the anode with transition metals in FLOLEDs.13 The current density of the device decreased with increasing doping concentration due to the gradual reduction of current injection capability. They asserted that the high efficiency of 7.5 cd/A in the 4% doped device originated from the reduction in the -4-


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current density as compared to the 1% doped device, when a nickel oxide (Ni2O3) was used to modify the ITO. However, they could not fully investigate the mechanism of efficient device performance which was originated from direct carrier injection into dopants in EML. Simply using a metal oxide-modified ITO anode without HIL and HTL was proposed and the current density decreased as the fluorescent doping concentration increased was reported.13 However, the current density in PHOLEDs was enhanced as the phosphorescent doping concentration was increased in EML at their previous work.20 In that case, they clearly asserted the device mechanism for improved devices was direct charge injection into dopants. Regardless, the direct charge trapping mechanism renders the device structure simple and the operating voltage low with maintaining the high efficiency. There has been considerable effort in understanding the influence of charge injection/transport and the increased efficiency in Alq3:C545T host-dopant systems. Nevertheless detailed relationship between the contribution of the hole/electron injection and the device performance was not fully investigated in terms of direct charge trapping with relatively heavy doping in FLOLEDs. Herein, we simply showed the correlation between direct charge trapping and device performance according to the operating voltage without complicated investigation. The detailed investigation of the contribution between hole and electron injection as a function of the doping concentration was studied by using hole-/electron-only devices (HODs/EODs) in an Alq3:C545T host-dopant system of FLOLEDs. A relatively high doping concentration is considered as charge traps at the dopants since the dopant performs the role of charge injection/transport. The optimal doping concentration makes it possible to balance between holes and electrons through direct charge injection, resulting in higher current efficiency despite heavier self-quenching. -5-



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II. EXPERIMENTAL SECTION The devices were fabricated with a high vacuum (~2ⅹ10-7 torr) evaporation process where constituent organic materials were thermally-deposited onto cleaned ITO. For the sample cleaning, the ITO glass was ultrasonically cleaned with acetone, isopropanol (IPA) under sonication at 40 kHz, dipping in boiled IPA and was finally followed to ultraviolet (UV) ozone treatment. The devices were fabricated with a configuration of ITO/1,4,5,8,9,11hexaazatriphenylene-hexacarbonitrile (HAT-CN, 60 nm)/N,N-bis-(1-naphthyl)-N,N′-diphenyl1,1′-biphenyl-4,4′diamine (NPB, 30 nm)/Alq3:C545T(30 nm, X %)/2,9-dimethyl-4,7diphenyl-1,10-phenanthroline (BPhen, 25 nm)/LiF(1 nm)/Al (130 nm), in which the doping concentration, X, of C545T varied between 0, 1, 2, 4, and 6% in a high-vacuum chamber. The metal layer was deposited by using a shadow mask with an area of 0.04 cm2. The thickness of layers was confirmed by spectroscopic ellipsometry (SE). The device structure of the HODs and EODs were constructed as ITO/HATCN(30 nm)/NPB(60 nm)/ Alq3:C545T(60 nm, X%)/MoO3(10 respectively.

nm)/Al The

and

current

ITO/Alq3:C545T(120

nm,

X%)/Bphen(25

density-luminance-voltage

characteristics

nm)/LiF/Al, and

the

electroluminescence (EL) spectra of the devices were respectively obtained using a Keithley 2400 voltmeter and a Minolta CS-1000 spectrometer, respectively.

III. RESULTS AND DISCUSSION Figure 1(a) and (b) show the schematic device structure and energy band diagram of an Alq3:C545T host-dopant system in a FLOLED, respectively. The EML consists of Alq3 and -6-


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C545T, which are an electron-transport-character host material, and a well-known green fluorescent-emitting dopant material, respectively. In general, excitons are formed on the host and are transferred to the dopant through Förster energy transfer process, leading to efficient fluorescence. Based on the device configuration and the energy level diagram, it is assumed that the dopants have a role as trap sites for both electrons and holes, limiting the current density of the devices as the doping concentration increases. In addition to electron-transportcharacter of EML materials, the mobility of electrons in the electron transport layer is faster than that of holes in the hole transport layer(µNPB

Heavily Doped, Charge-Balanced Fluorescent Organic Light-Emitting

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