Efficient Trilayer Phosphorescent Organic Light-Emitting Devices Without Electrode Modification Layer and Its Working Mechanism
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At present, numerous functional layers are introduced to improve the carrier injection and balance the carrier transport in organic light-emitting devices (OLEDs). Although it may be a good way to enhance the efficiency of devices, the introduction of functional layers would also result in extra process and long manufacture period. Actually, with the enrichment of material system, many appropriate materials could be chosen to share two or even more functions in OLEDs. Here, via impedance spectroscopy and transient electroluminescence analysis, di-[4-(N,N-ditolyl-amino)-phenyl] cyclohexane (TAPC) and 4,7-diphenyl-1,10-phenanthroline (Bphen) are demonstrated to serve as carrier injection and transport layers simultaneously. As a result, efficient trilayer OLEDs are achieved with comparable performances to conventional multilayer devices. Further studies have also been carried out to analyze the recombination and quenching mechanisms in devices. TAPC can block electrons effectively, while Bphen avoids the accumulation of holes. It makes carriers in emitting layer become more balanced, resulting in the reduction of efficiency roll-off.
KeywordsOrganic light-emitting device Trilayer Impedance spectroscopy Transient analysis
2,6-Bis(3-(carbazol 9,9′-[4′-(2-ethyl-1H-benzimidazol-1-yl)-9-yl) phenyl) pyridine
Anode modification layer
Current efficiency-luminance-external quantum efficiency
Cathode modification layer
External quantum efficiency
Bis [(4,6-difluorophenyl)-pyridinato-N,C2′] (picolinato) Ir(III)
Highest occupied molecular orbital
Internal quantum efficiency
Iridium (III) bis-(2-methyldibenzo-[f, h] quinoxaline) (acetylacetonate)
Indium tin oxide
Lowest unoccupied molecular orbital
Organic light-emitting devices
Phosphorescent white OLEDs
It is known to all that organic light-emitting devices (OLEDs) have attracted considerable attention for solid-state lighting, full color displays, and so on. A good deal of functional layers, such as the anode modification layer (AML), cathode modification layer (CML), hole-blocking layer (HBL), and electron-blocking layer (EBL), have been introduced in the OLEDs to achieve high-efficiency and low turn-on voltage. The AML and CML are used to enhance the hole or electron injection, respectively [1, 2]. While the HBL and EBL can efficiently block the diffusion of the exciton from the luminescent layer into the transport layer . Obviously, the multilayer structure becomes a frequently used way to improve device performance. However, since one more layer means an extra preparation process, excess function layers would also cause the long period and high cost that limit the development of their industrialization. With the improvement of the organic material system, some materials could play multiple roles in OLEDs due to their prominent properties. For example, deoxyribonucleic acid-cetyltrimetylammonium complex can act as hole-transporting layers (HTL) because of high hole mobility, meanwhile the low lowest unoccupied molecular orbital (LUMO) energy level makes it suit for the EBL . 4,4′,4″-Tris (carbazol-9-yl)-triphenylamine (TCTA) is usually used to be HTL; besides, it can also serve as the host in emitting layer (EML) because of its high triplet energy [5, 6]. Hence, it is possible to simplify the structure without sacrificing the device performance by choosing appropriate material. However, few studies have been carried out on phosphorescent white OLEDs (PHWOLEDs) with simple structure [7, 8].
More recently, capacitance characteristics based on impedance spectroscopy (IS) measurement has been a widely used tool to investigate the physical mechanisms of OLEDs. The inflection point of the first peak in capacitance–voltage (C-V) curves has been reported to be corresponded to the turn-on voltage of OLEDs. It is also a very sensitive probe of carrier accumulation caused by the barrier in the interface of organic layers or the imbalance of charge injection and transport in devices [9, 10, 11, 12, 13, 14, 15, 16, 17]. Meanwhile, transient electroluminescence (EL) has also been the subject of intense technological as well as fundamental research, because transient EL studies have generated insight into the internal working mechanism in OLEDs. Transient EL is investigated by driving the devices with short, rectangular voltage pulses. The response times obtained from transient EL characteristics of devices provides an essential criterion for their application [18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28].
In this paper, via impedance spectroscopy and transient analysis, we confirm that di-[4-(N,N-ditolyl-amino)-phenyl] cyclohexane (TAPC) and 4,7-diphenyl-1,10-phenanthroline (Bphen) can be used to play multiple roles in OLEDs. Combined with bipolar transport material 4,4′-N,N′-dicarbazole-biphenyl (CBP), we fabricate efficient trilayer PHOLEDs. Obviously, the performance of trilayer OLED is comparable with the common multilayer OLEDs and even possesses better efficiency roll-off. It is interpreted by the mathematical model of exciton-quenching mechanisms. Subsequently, we focus on the carrier recombination and exciton-quenching mechanisms which occurred in monochromatic phosphorescent devices in order to proceed the further optimization of the structure. With the existence of Langevin and trap-assisted recombination in CBP-doped tris(2-phenylpyridine) iridium [Ir(ppy)3] and iridium (III) bis-(2-methyldibenzo-[f, h] quinoxaline) (acetylacetonate) [Ir(MDQ)2(acac)], two exciton-quenching mechanisms, i.e., triplet–triplet annihilation (TTA) and triplet–polaron annihilation (TPA), can be observed via the mathematical model.
The small molecular organic materials used in our experiments are purchased from Luminescence Technology Corporation, i.e., TAPC, Bphen, 1,3,5-tri (m-pyrid-3-yl-phenyl) benzene (TmPyPB), and CBP. The phosphorescent dopant Ir(ppy)3, Ir(MDQ)2(acac) and bis [(4,6-difluorophenyl)-pyridinato-N,C2′] (picolinato) Ir(III) (FIrpic), and poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS, PH8000) are obtained from Xi’an p-OLED. Thus, all materials and solvents are commercially available and used as received without further purification.
All devices are prepared on glass substrates covered with patterned indium tin oxide (ITO) stripes. Before film deposition, the ITO glass substrates are subjected to a routine cleaning process with rinsing in Decon 90, deionized water, drying in an oven, and finally treated in a plasma cleaner chamber for about 5 min. The PEDOT:PSS films are fabricated by spin coating from aqueous solution before depositing with the thickness to be approximately 40 nm, and then the PEDOT:PSS films are all annealed at 120 °C for 10 min.
All organic layers and cathode are evaporated by thermal vapor deposition using resistively heated tungsten filament and metal boats under high vacuum (~ 5 × 10−4 Pa) at a rate of 1–2 Å s−1 monitored in situ with a quartz oscillator. The cathode we used in our experiments is Mg:Ag (15:1) alloy, which is controlled independently by separate thin-film deposition monitors, so does the doping process in EML. Finally, four active areas of the devices on each substrate were 10 mm2, which is decided by the overlap between the anode and cathode via using a shadow mask [24, 25].
Luminance–current density–voltage characteristics and spectra of unpackaged devices are measured simultaneously using Goniophotometric Measurement System based on spectrometer (GP-500, Otsuka Electronics Co. Osaka, Japan) in air at room temperature.
For the transient voltage decay measurement, high-speed switching diode (Philips, 1N4531) and arbitrary waveform generator (Rigol, DG5102) are connected with our devices in series orderly, and the transient voltage of the devices is recorded by a digital oscilloscope (Rigol, DS4054) after a consecutive signal averaging. In the transient EL measurement, the tested devices are driven by pulsed voltage with a pulse width of 1 ms using arbitrary waveform generator (Rigol, DG5102) as an electrical switch for driving tested devices and a trigger signal for starting the collection of EL signals. The transient EL response was detected and collected by using an avalanche photodiode (C30902) and time-correlated single-photon counting system.
The capacitance–voltage (C-V) characteristics are measured with an Impedance Analyzer (TH2829C, Changzhou Tonghui Electronic Co., Ltd., China) with oscillating amplitude of 100 mV and the repetition rate of 1 kHz. The range of dc bias applied by this setup allows sweeping from 0 to + 10 V .
Results and Discussion
Efficient OLEDs Simplified Without AML
Efficient OLEDs Simplified Without CML
Performance Comparison Between Simple Trilayer and Multilayer OLEDs
For Eq. (4), we deem that charge carriers recombine via Langevin recombination with the rate KL, where q is the elementary charge, μe/h is the mobility, εr is the relative permittivity, and ε0 is the permittivity of free space. The triplet and polaron densities, nT and nP, were calculated by Eqs. (5) and (6), where KTT and KTP are the rate constants describing the kinetics of the TTA and TPA process. Actually, the internal quantum efficiency (IQE) is the ratio of radiative decaying triplets over the number of injected electrons from Eq. (7). For simplification, we do not consider light outcoupling. Moreover, the electric efficiency and the PL quantum efficiency at low current density are set to 1. Hence, the calculated IQE is used to compare with experimental EQE .
Analysis to the Mechanism of Exciton Recombination in Monochrome PHOLEDs
As we all know that the low concentration of phosphorescent dopant molecules leads to the long intermolecular distance, it is generally believed that phosphorescent materials act as trapping for the charge carrier. Therefore, there are two recombination mechanisms in EML of PHOLEDs, Langevin recombination I and trap-assisted recombination II. For the former, when the device is driven by applied voltage, a mass of carriers inject continuously into EML. The holes transfer through the host material, followed by an accumulation in the interface of EML/ETL. On account of a good matching to energy levels between ETL and cathode, most electrons flow through ETL up to EML and then recombine with the stored charge. In this case, excitons generated in the host material transfer to the dopant by the Förster and/or Dexter mechanisms; therefore, it belongs to the bimolecular recombination. The latter recombination zone is located in dopant due to the shallow-energy-level trapping formed by phosphorescent guest .
The recombination behaviors are investigated via the transient EL measurements. Normalized intensity of transient EL shown in Fig. 5b, c is tested by changing the reverse bias (0 V, − 1 V, − 3 V, and 5 V) after the applied voltage turning off, while the voltage pulse height corresponds to a current density of 90 mA cm−2. The voltage pulse width is 1 ms, and the pulse frequency is 100 Hz. As shown in Fig. 5b, c, the rise time of green and red devices slow down with the increase of reverse bias. However, this phenomenon does not occur in the other two devices. The reverse bias would take the captured carriers out of the trapping sites, and then the trapped carriers will make less contribution to EL intensity. So, we infer that trap-assisted recombination probably consists in devices fabricated by CBP-doped Ir(MDQ)2(acac) or Ir(ppy)3 due to the existence of the trapped charges .
More holes inject with the augment of applied voltage; besides the trapped ones, most of them get to store at the interface of EML/Bphen. Therefore, both of the C-V curves of green and red devices rise again. At this point, the Langevin recombination has happened in the EML causing the reduction of internal stored carriers. When the dissipative rate of charges exceeds their injection rates, the accumulated charges reduce rapidly and the C-V curve exhibits a sharp drop. The recombination process is shown in Fig. 6b, c. For comparison, only one strong peak appears in the capacitance characterize of the blue device, indicating that only the Langevin recombination occurs in the EML. Schematic energy-level diagrams with the recombination mechanism are shown in Fig. 6d.
We can also verify our results via the mathematical model mentioned above. It is well known that TTA is caused by high triplet density, while the high Langevin recombination rate would reduce the triplet density. So, the TTA can be associated with the Langevin recombination. TPA depends on the charge trapping characteristics of the host–guest system: when the emitter molecules constitute a trapping site for polarons within the host, accelerated TPA can be expected .
The corresponding contribution of TTA and TPA to the overall annihilation for the two devices with the EML of CBP:Ir(ppy)3 and CBP:Ir(MDQ)2(acac) is shown in Fig. 6e, f. The calculated IQE is coincident to the measured EQE; moreover, the distinction between IQE and EQE curves at low bias voltage is caused by leak current. For the two devices, the polaron density is larger than the triplet density when the current density is below 5 mA cm−2. Therefore, we believe that there are two quenching processes on operation condition, meaning that two recombination types occur in the EML. A higher percentage of TPA occurs in the red device, reflecting the stronger trap-assisted recombination [33, 34].
In terms of the quenching process discussed above, it is obvious that TTA and TPA may dramatically decrease the efficiency of phosphorescent OLEDs. Therefore, in order to research the effect on device performance by changing host material, we prepare red devices with different hosts, i.e., CBP, TCTA, 2,6-bis(3-(carbazol 9,9′-[4′-(2-ethyl-1H-benzimidazol-1-yl)-9-yl) phenyl)pyridine [26DCzPPy] and 2,2′[2″-1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) [TPBi]. When CBP is used as the host, the TTA and TPA are efficiently limited. Therefore, the CBP is chose to act as the host in this work.
Single-Layer White OLEDs
In summary, efficient phosphorescent OLEDs have been prepared based on a simple trilayer structure (TAPC/EML/Bphen). We simplify the devices gradually via impedance spectroscopy and transient measurement. The EL performances of trilayer devices could be still comparable to the conventional devices with modification layers. Langevin recombination and trap-assisted recombination are certified to be existed in red and green phosphorescent devices by capacitance–voltage measurement. In addition, mathematical model is used to describe the TTA and TPA quenching processes, which are relevant to the two recombination types mentioned above. Based on the above analysis, we obtain the efficient WOLEDs with low roll-off. These results demonstrate an effective approach towards simplified OLED with high efficient and low cost.
This work was supported by the National Natural Science Foundation of China (Grant Nos. 61475060, 61774074, and 61474054).
Availability of Data and Materials
The datasets supporting the conclusions of this article are included within the article.
XMP designed and conducted most of the experiments, analyzed the data, and prepared the manuscript. HWF contributed to transient EL and transient voltage decay measurement. JXZ helped with the white OLED fabrication. SHL helped with the mathematical simulation and prepared the manuscript. LTZ contributed to capacitance–voltage characteristics and data analysis. WFX initiated the study, designed all the experiments, analyzed the data, and prepared the manuscript. All authors discussed the results and commented on the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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