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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Intersystem Crossing and Triplet Fusion in Singlet-FissionDominated Rubrene-Based OLEDs Under High Bias Current Xiantong Tang, Yeqian Hu, Weiyao Jia, Ruiheng Pan, Junquan Deng, Jinqiu Deng, Zhenghong He, and Zuhong Xiong* School of Physical Science and Technology, MOE Key Laboratory on Luminescence and Real-Time Analysis, Southwest University, Chongqing 400715, People’s Republic of China S Supporting Information *
ABSTRACT: Singlet fission is usually the only reaction channel for excited states in rubrene-based organic lightemitting diodes (OLEDs) at ambient temperature. Intriguingly, we discover that triplet fusion (TF) and intersystem crossing (ISC) within rubrene-based devices begin at moderate and high current densities (j), respectively. Both processes enhance with decreasing temperature. This behavior is discovered by analyzing the magneto-electroluminescence curves of the devices. The j-dependent magneto-conductance, measured at ambient temperature indicates that spin mixing within polaron pairs that are generated by triplet-charge annihilation (TQA) causes the occurrence of ISC, while the high concentrations of triplets are responsible for generating TF. Additionally, the reduction in exciton formation and the elevated TQA with decreasing temperature may contribute to the enhanced ISC at low temperatures. This work provides considerable insight into the different mechanisms that occur when a high density of excited states exist in rubrene and reasonable reasons for the absence of EL efficiency roll-off in rubrene-based OLEDs. KEYWORDS: magneto-electroluminescence, singlet fission, triplet fusion, intersystem crossing, rubrene
1. INTRODUCTION Rubrene (5,6,11,12-tetraphenylnaphthacene) is an organic semiconductor material that can be used as a highly efficient fluorescent guest,1 undergoes singlet exciton fission,2 and has a high carrier mobility (up to 40 cm2/(V s)) in the single-crystal phase.3 Thus, it is commonly used in organic light emitting diodes (OLEDs),2,4,5 organic photovoltaic cells (OPVs),6 and organic field-effect transistors (OFETs).3 However, the widespread commercialization of OLEDs is hampered by efficiency roll-off (i.e., OLEDs exhibit low efficiency at high current densities (j) and high brightness).7−10 As charge carrier injection and transport of high j in certain OLEDs (e.g., organic laser diodes) is a principle requirement,11,12 a deeper understanding of the excited-state processes that occur within rubrene-based OLEDs driven at high j is necessary for extending their use to a variety of applications. Recently, magnetic field effects (MFEs) have been used as a useful method to research the dynamics of excited states in rubrene. For example, Piland et al. confirmed the presence of singlet fission through examining the MFEs on the fluorescence decay.13 Tarasov et al. studied the role of spin−lattice relaxation in MEFs on the photoluminescence (i.e., MPL) of rubrene films.14 Chen et al. used magneto-electroluminescence (MEL) to investigate the microscopic mechanisms for the half-bandgap electroluminescence in bifunctional rubrene/C60 OLEDs.4 In fact, as one kind of MFEs, MEL has been widely used as a “fingerprint” tool to analyze the complex spin-related processes © XXXX American Chemical Society
that occur in different excited-state reaction channels in OLEDs, including intersystem crossing (ISC),15,16 reverse intersystem crossing (RISC),17−21 singlet fission (SF),2,14 and triplet fusion (TF).18,22−24 All of these reaction channels possess their own characteristic MEL curves and are influenced by charge injection or temperature. Therefore, the use of MEL to investigate the evolution of different reaction channels under high current bias is important in OLEDs. In conventional OLEDs, the spin mixing of polaron pairs (i.e., ISC), can be observed using MEL because of the small energy gap between the singlet (PPS) and triplet (PPT) polaron-pair states.16,25 However, ISC cannot be observed easily in rubrene-based OLEDs because of fast exciton formation (1 × 10−9 to 1 × 10−11 s)26 and SF (1 × 10−14 to 1 × 10−11 s).27,28 As Bai et al. pointed out that SF was the only reaction channel at ambient temperature but it could coexist with TF at low temperatures because of prolonged triplet exciton lifetime and the energy resonance (ES ≈ 2ET) within rubrene molecule (ES and ET denote the energy levels of singlet and triplet excitons, respectively).2 Here, we systematically investigated the dynamics of various microscopic processes in rubrene-based OLEDs operated within broad injection current range by performing j-dependent Received: November 20, 2017 Accepted: December 19, 2017
A
DOI: 10.1021/acsami.7b17695 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces MEL/MC at ambient and low temperatures. Surprisingly, at ambient temperature, we observed that TF and ISC start to occur at moderate j (1.7−16.7 mA/cm2) and high j (≥16.7 mA/cm2), respectively. The MEL curves of devices that were subject to varying temperatures exhibited more pronounced ISC at lower temperatures. Furthermore, we also found that the interfaces around the active layer of rubrene have little influences on the MELs. These MEL curves were close fits to a combination of Lorentzian and non-Lorentzian functions. The observed j and temperature dependence of ISC, in addition to TF presence at ambient temperature, have not been observed in rubrene-based OLEDs. Our experimental results provide considerable insights into the exciton dynamics in rubrene-based devices and potentially give rise to novel application in spin-optoelectronic devices.
2. EXPERIMENTAL SECTION
Figure 1. (a) Molecular structures of m-MTDATA, rubrene, and Bphen. (b) Energy levels of the materials used in m-MTDATA/ rubrene/Bphen device. (c) Normalized EL spectra of m-MTDATA/ rubrene/Bphen device at different temperatures. (d) Brightness− current characteristics of m-MTDATA/rubrene/Bphen device at different temperatures. The inset shows a double logarithmic j−V curve for m-MTDATA/rubrene/Bphen device at 300 K.
The devices used in this study were fabricated using organic molecular beam deposition under high vacuum (∼10−6 Pa). Device had the following structure: indium tin oxide (ITO)/poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (40 nm)/4,4′,4′-tris(N-3-methylphenyl-N-phenylamino)triphenylamine (m-MTDATA) (60 nm)/rubrene (30 nm)/4,7-diphenyl-1,10-phenanthroline (Bphen) (50 nm)/lithium fluoride LiF(1 nm)/Al(120 nm), with an active area of 6 mm2. A control device, which respectively used N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB) and bathocuproine (BCP) in replace of the m-MTDATA and Bphen, was fabricated to enable comparison of excited state phenomena. These materials possess higher hole (NPB) and electron (BCP) mobility. The structure of this device was as follows: ITO/PEDOT:PSS (40 nm)/NPB (60 nm)/rubrene (30 nm)/BCP (50 nm)/LiF (1 nm)/Al (120 nm). The thickness of the functional layers was monitored using quartz oscillators at a controlled rate between 0.01 and 0.5 nm/s for the organic materials and LiF and 1.0 nm/s for the Al layer. The devices were mounted on the coldfinger of the close-cycle cryostat (Janis: CCS-350S, temperature range: 20−300 K), which was located between the two poles of an electromagnet (Lakeshore: EM 647). The applied voltage was powered by a Keithley 2400. The EL spectra were measured using a SpectraPro-2300i spectrum unit. MEL and MC are defined as MEL = ΔEL/EL = [EL(B) − EL(0)]/EL(0) and MC = ΔI/ I= [I(B) − I(0))/I(0), respectively.29
function of current at temperatures ranging from 20 to 300 K is shown in Figure 1d. It can be seen that the EL intensity significantly increases by over a factor of 8 with decreasing temperatures from 300 down to 20 K at the current of 100 μA. To understand the origin of increased EL intensity at low temperature, we analyzed the double logarithmic j−V curve shown in the inset of Figure 1d. As can be seen, it includes three distinct regimes: (1) Ohmic, (2) trap-filled space charge limited current (TF-SCLC), and (3) trap-filled limit current (TFL-SCLC) regimes.31,32 This indicated that traps which inevitably formed during device fabrication process existed at low injection current but some new traps were formed at higher injection currents. According to the reports in the literatures, the amounts of traps can increase with decreasing temperature,29 and physical defects of the devices reduce the free triplet lifetime and can also reduce the intensity of delayed fluorescence by immobilizing the triplet excitons.33 Therefore, if defect trapping is dominated in the devices, the EL intensity of rubrene-based OLEDs would be reduced with decreasing temperature, but this is contrary to the temperature dependence of brightness-current curves presented in Figure 1d. Thus, we can conclude that defect trappings have no significant effects on the EL emission intensity from our devices. Factually, the phonon coupling becomes weak as the temperature decreases, which prolongs the lifetime of triplet exciton. In this case, the TF process is promoted, giving rise to extra contribution to the luminescent efficiency through emitting the delayed luminescence. On the other hand, SF is dominated at room temperature and becomes weakened at low temperatures because it is an endothermic process in rubrene. Therefore, the increased EL intensity with decreasing temperature from 300 to 20 K could be attributed to the overall effects of both decreased SF and increased TF. 3.2. MEL Responses of m-MTDATA/Rubrene/Bphen and NPB/Rubrene/BCP Devices. The current dependence of the MEL responses for m-MTDATA/rubrene/Bphen device is shown in Figure 2. The typical MELs in Figure 2a first decrease to the minimum of −1.3% at Bmin ≈ 20 mT, and then rise with increasing field B. After returning to the zero-field value of 0%
3. RESULTS AND DISCUSSION 3.1. Optical and Electrical Properties of m-MTDATA/ Rubrene/Bphen Device. The molecular structures of mMTDATA, rubrene, and Bphen are illustrated in the Figure 1a and their energy level alignments within m-MTDATA/ rubrene/Bphen device are shown in Figure 1b. PEDOT:PSS, m-MTDATA, rubrene, and Bphen were used as the hole injection layer, hole transport layer, luminous layer, and electron transport layer, respectively. The low ionization potential of m-MTDATA (5.1 eV) is less than that of rubrene (5.4 eV) and can result in holes being the majority carriers in the rubrene layer. This situation can lead to the reaction between long lifetime triplet excitons and holes. Conversely, electrons are thought to be minority carriers because of the high cathode injection barrier (0.8 eV). The charge blocking behavior of Bphen (hole blocking) and m-MTDATA (electron blocking) ensures that an abundance of charge carriers and excited states are present in the rubrene layer. The effects of temperature on the normalized EL spectra of m-MTDATA/rubrene/Bphen device are depicted in Figure 1c. The EL extended from 450 to 800 nm with a maximum at ∼565 nm and a shoulder at ∼605 nm, which was consistent with rubrene emission.2,13,30 The EL emission intensity as a B
DOI: 10.1021/acsami.7b17695 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
this new structure may result from ISC (see the analysis in section 3.3). The materials used in NPB/rubrene/BCP device have higher hole and electron mobility when compared with those in mMTDATA/rubrene/Bphen device. These materials were chosen to further demonstrate the findings observed in mMTDATA/rubrene/Bphen device. The hole mobility of NPB (∼5.1 × 10−4 cm2/(V s))36 is an order of magnitude higher than that of m-MTDATA (3.0 × 10−5 cm2/(V s)),37,38 as is the electron mobility of BCP (1.1 × 10−3 cm2/(V s))39 when compared with Bphen (4.2 × 10−4 cm2/(V s)).39 BCP served as the electron-transport layer in NPB/rubrene/BCP device. Therefore, it was expected that increased densities of electrons and holes in NPB/rubrene/BCP device may lead to enhanced excited state phenomenon (ISC) at high injection currents. The MEL curves for NPB/rubrene/BCP device at ambient temperature and different currents (Figure 3a−d) exhibited Figure 2. MEL curves of m-MTDATA/rubrene/Bphen device at different injection currents (300 K). (a, c) MEL curves at low and high injection currents as functions of high magnetic field (B), respectively. (b) Low-field variations in the MEL curves shown in a. (d) Low-field variations in the MEL curves shown in c. The MEL curves in b and d have been moved properly for clear observation. The corresponding raw data without shifts in Figure 2d and the descriptions for how to shift these data are added in Figure S1 and Supporting Text 1, respectively.
at Bzero‑cross ≈ 41 mT, the MELs monotonically increase to the maximum of +18% and show nonsaturation trends up to 300 mT. These line-shapes are very similar to those of MPL obtained from neat rubrene film13,34 or codeposited rubrene films.35 Very recently, Hodges et al. have also attained similar MPL curves from amorphous rubrene film employed for wide field magnetic luminescence imaging.34 As concluded before,13,34,35 these traces are the characteristic curves originated from B-mediated SF process in rubrene. In fact, there is no difference in the SF process occurred whether from rubrenebased OLEDs or from a single layer of rubrene film, only if the rubrene films used in these two cases are the same material states (e.g., amorphous, poly crystal, or single crystal). The detailed explanation about SF-induced MEL is displayed in Supporting Text 2. Additionally, the line-shape and amplitude of the MEL curves shown in Figure 2a, b did not change significantly when the current was increased from 5 μA (0.08 mA/cm2) to 100 μA (1.67 mA/cm2). Therefore, the device injection currents did not have a significant impact on SF process. To investigate the influence of high j on the MEL behavior, we measured the high- and low-field MEL responses from 250 μA (4.17 mA/cm2) to 4500 μA (75.00 mA/cm2), as shown in Figure 2c, d, respectively. The MEL curves at high injection currents (Figure 2c) exhibited a significant dependence on j when compared with those at low injection currents (Figure 2a). Notably, the curve measured at an injection current of 4500 μA showed growth in the EL intensity with increasing |B| by 6.5% at 300 mT. The corresponding MEL value of 17% at an injection current of 250 μA was ∼10.5% higher than that at 4500 μA. This may have been caused by enhanced TF, because TF and SF have reverse MEL variation tendency.13 An expansion of the MEL curves (|B| < 50 mT) in Figure 2c revealed a new structure when |B| < 8 mT (Figure 2d). As the characteristic field of ISC is about 8 mT,22,24 it is plausible that
Figure 3. (a) High-field variation of the MEL measured at low j. (b) Expansion of the low-field variation of the MEL in a. (c) High-field variation of the MEL measured at high j. The inset shows the MEL at 2500 μA and ultrasmall fields (|B| < 3 mT). (d) Expansion of the lowfield variation of the MEL in c. (e) The MEL responses obtained by subtracting the MEL curve acquired at 100 μA in Figure 3a from each curve in Figure 3c. (f) Current-dependent magnitude of the MEL curves caused by SF, TF, and ISC. All measurements on NPB/ rubrene/BCP device were performed at ambient temperature. The MEL curves in b and d have been moved properly for clear observation.
similar trends to those observed in m-MTDATA/rubrene/ Bphen device (Figure 2a−d). Therefore, the SF channel in NPB/rubrene/BCP device was also independent of current, whereas TF and ISC occurred at moderate and high j, respectively. Notably, the new structure observed in the MEL at low fields from NPB/rubrene/BCP device became more prominent than that observed in the MEL from mMTDATA/rubrene/Bphen device. That is, this structure was visible at 2000 μA (33.33 mA/cm2) in m-MTDATA/rubrene/ Bphen device, but could be observed around 1000 μA in the NPB/rubrene/BCP device. In addition to the increased C
DOI: 10.1021/acsami.7b17695 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces intensity of this structure, at 2500 μA (41.67 mA/cm2) and at ultra-small fields (|B| < 3 mT), the NPB/rubrene/BCP device exhibited another fine structure (inset, Figure 3c). Nguyen et al. reported that this ultra-small field MEL response was the result of a hyperfine-induced ISC process in poly(dioctyloxy) phenyl vinylene (DOO-PPV) and deuterated aluminum tris(8hydroxyquinoline) (D-Alq3)-based OLEDs.40,41 Consequently, it is reasonable to conclude that hyperfine interactions (HFI) in polaron-pair species play a significant role in the low-field MEL response in rubrene-based devices. The new structures observed in the MEL curves shown in Figures 3d and 2d can be attributed to ISC. To quantify the variations in the current dependence of the TF and ISC channels, assuming that SF does not change with changing currents (Figures 2a and 3a), we subtracted the MEL curve obtained at 100 μA from the MEL curves at high injection currents (500 to 2500 μA), as shown in Figure 3e. A sharp increase in the EL was observed at |B| < 20 mT, followed by a significant decrease at |B| > 20 mT. These MEL curves demonstrated characteristic magnetic-field responses, which comprise the low-field effect (LFE) caused by ISC and the high-field effect (HFE) caused by TF.5,22−24 Both of them can be defined as LFEISC = MEL(8 mT) − MEL(0 mT) and HFETF = MEL(20 mT) − MEL(300 mT), respectively. The values for the HFETF and LFEISC that corresponded to different injection currents, as determined by the relationships given above, are displayed in Figure 3f. When the driving current was increased from 500 to 2500 μA, the MEL values for the TF channel increased from 2% to 7.5%. Analogously, the ISC was observed to increase by approximately 1.2% (from 0.25% at 500 μA to 1.45% at 2500 μA). Thus, ISC occurred and enhanced at high j. As ISC is weaker than both SF and TF in rubrene-based OLEDs, it is difficult to observe under usual operating conditions (i.e., low j at ambient temperature). 3.3. Magneto-conductance (MC) Response and Analysis of Microscopic Processes. The MC of m-MTDATA/ rubrene/Bphen device, when drove at different currents, was measured to understand the origins of the ISC channel. The MC curves, which could be attributed to the effects of TQA,42 exhibited characteristic negative non-Lorentzian line-shapes with full width at quarter maximum values of ∼110 mT for all currents examined (Figure 4a). Additionally, the absolute value of the MC at B = 300 mT initially decreased and then increased at higher currents. This nonmonotonic variation is depicted in Figure 4b. These results, combined with the analysis of the j−V curve (section 2.1) suggested that the downward trend in Figure 4b (5−100 μA) may have been caused by traps being filled by holes or electrons. At currents larger than 100 μA, further increases in the injection current led to increased densities of reacted triplets and holes, which led to enhanced TQA. This explains the increasing trend observed in the absolute value of the MC (B = 300 mT) from 250 μA to 4500 μA (Figure 4b). Furthermore, an expansion of the MC curves (|B|
