Electric-Field-Induced Excimer Formation at Interface of Deep-Blue

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Electric-Field-Induced Excimer Formation at Interface of DeepBlue Emission Poly(9,9-dioctyl-2,7-fluorene) with Polyelectrolyte or Its Precursor as Electron Injection Layer in Polymer Light Emitting Diode and Its Prevention for Stable Emission and Higher Performance Kuen-Wei Tsai, Yun-Chung Wu, Tzu-Hao Jen, and Show-An Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03378 • Publication Date (Web): 16 Jul 2018 Downloaded from http://pubs.acs.org on July 16, 2018

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ACS Applied Materials & Interfaces

Electric-Field-Induced Excimer Formation at Interface of Deep-Blue Emission Poly(9,9dioctyl-2,7-fluorene) with Polyelectrolyte or Its Precursor as Electron Injection Layer in Polymer Light Emitting Diode and Its Prevention for Stable Emission and Higher Performance

Kuen-Wei Tsai,‡ Yun-Chung Wu,‡ Tzu-Hao Jen and Show-An Chen*

Chemical Engineering Department, National Tsing-Hua University, Hsinchu 30013, Taiwan, Republic of China. *E-mail: [email protected]

KEYWORDS: β-phase polyfluorene (β-PFO), deep-blue emission, polyelectrolytes, electron injection layer (EIL), excimer.

ABSTRACT: Conjugated polyelectrolytes and their precursors as electron-injection layer (EIL) in polymer light emitting diode (PLED) have attracted extensive attention since they allow a use of environmentally stable high work-function metals as cathode with efficient electron injection. Here, for the first time, we find that an undesirable green emission component (470-650 nm) in EL spectra is observed during continuous operation of deep-blue emission β-phase poly(9,9dioctyl-2,7-fluorene) (β-PFO) device upon introducing polyelectrolyte poly[9,9-bis(6'-(18crown-6)methoxy)hexyl fluorene] chelating to potassium ion (PFCn6:K+) as EIL. This phenomenon

also

happens

to

non-chelating

ACS Paragon Plus Environment

PFCn6,

poly[(9,9-bis(3'-(N,N-

1

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dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) or even non-emissive poly[4-((18-crown-6)methoxy)methyl styrene] chelating to K+ (PSCn6:K+). It can be ascribed to electric-field-induction accompanied by thermal motion of highly polar side chain in the polyelectrolyte leading to local segmental alignment of PFO main chains at the emitting layer (EML)/EIL interface and thus formation of green emission excimer, which is supported by the following observations: appearance of green emission component using non-emissive PSCn6:K+ as EIL, absence of green emission component as the device is operated at low-temperature (78 K) at which molecular thermal motion are frozen, and absence of green emission upon introducing TPBi as buffer layer in between EML and EIL for prevention of direct contact of EML with polyelectrolyte or its precursor EIL.

1. INTRODUCTION The development of high performance blue polymeric light-emitting diodes (PLEDs), especially deep-blue PLED, is critical for realizing commercial applications of full-color display1 and solid-state lighting2-3. In order to achieve deep-blue emission, the emitting polymers should possess a wide enough bandgap (Eg). However, efficient carrier injection will become a very critical issue for the device performance of the intrinsic-large Eg molecules because of their relatively deep highest occupied molecular orbital (HOMO) and shallow lowest unoccupied molecular orbital (LUMO) levels. In the case of electron injection, a low work-function (EΦ) metal cathode can give a lower-barrier electron injection for better device performance. However, the low EΦ metal cathode can lead to poor device stability. For practical application, it is essential to utilize environmentally stable high EΦ (such as Al or Ag) as metal cathode for practically acceptable device lifetime. The utilization of conjugated


2

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polyelectrolyte or its precursor as interlayer between active layer and high EΦ metal cathode have been widely investigated in recent decades for PLEDs4-9 and polymer solar cells (PSCs)10-14 for promoting device performance. They are composed of conjugated polymer main chain and pendant high-polarity moieties such as sulfate-15-20, phosphonate-21-25, diethanolamino-26-30, dimethylamino-31-38, dimethylammonium-39-46 or crown ether47-51 as side chain. As for PLEDs, the introduction of conjugated polyelectrolyte or its precursor as an electron injection layer (EIL) has been confirmed to form an interfacial dipole19,25-27,31,32 to environmentally stable high EΦ metal cathode such as Al and Ag. Consequently, an electron-injection barrier is dramatically reduced because of interfacial dipole formation between EIL and high EΦ metal cathode, resulting in improved device performance comparable to or even better than that with low EΦ metal (such as Ca or Ba) cathode. For example, the luminous efficiencies (LEs) of the devices with

poly[(9'9-bis(6'-diethoxylphosphorylhexy)fluorene)25

and

poly[9,9-bis(2-(2-(2-

diethanolaminoethoxy)ethoxy)ethyl) fluorene29 as EILs and Al as cathode were obvious improved by factors of 1.4 and 3.8, respectively, in comparison with the cases with low EΦ metal Ba as cathode. Besides, in our previous work48, we have reported the use of water/methanolsoluble polyﬂuorene grafted with 18-crown-6 (Cn6) chelating to potassium ion (PFCn6:K+) as efficient EIL for deep-blue emission PLED based on β-phase PFO (β-PFO), giving the best record at that time for device performance with maximum brightness (Bmax), LE and external quantum efficiency (EQE) of 54800 cd/m2, 6.14 cd/A and 5.42 %, respectively. This high performance can be ascribed to, in addition to interfacial dipole effect, electron donation from the Cn6 can reduce K+ to form a stable “pseudometallic state”, enabling it to act an additional channel for electron injection. From the above reviews, it is clear that the use of conjugated polyelectrolytes opens a broad avenue for industrialization of PLEDs.


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From practical consideration, the stability of electroluminescence (EL) spectrum during continuous operation also plays a critical role for realizing commercial applications in full-color display and solid-state lighting. However, the previous reports for PFO devices only showed initial EL spectrum measured at certain applied voltages but did not investigate whether the emission color would still remain unchanged after prolonged operation. Here, for the first time, we investigate emission spectrum stability of deep-blue emission β-PFO-based PLED under continuous operation up to 120 minutes at a fixed bias, in which conjugated polyelectrolytes PFCn6:K+ and Al are used as EIL and cathode, respectively. We found that x + y of Commission Internationale d’Enclairage (CIEx+y) gradually changes from CIEx+y < 0.3 (deep-blue) to CIEx+y > 0.4 (sky-blue) under a fixed bias after operation for 120 minutes. Similar phenomenon is also observed at non-chelating PFCn6, PFCn6 chelating K+ from the various potassium salts (PFCn6:K+X-, where X- are F-, Cl-, Br-, Poly(acrylate anion) (PAA-)), and polyelectrolyte precursor

poly[(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-

dioctylfluorene)] (PFN) or even non-emissive poly[4-((18-crown-6)methoxy)methyl styrene] chelating to K+ (PSCn6:K+) as EIL. The occurrence of the variation in the EL spectra during continuous operation of β-PFO-based device is due to thermal motion of high-polarity side chain in the polyelectrolyte or its precursor under the electrical bias that results in the local segmental alignment of PFO main chains at emitting layer (EML)/EIL interface, and consequently induces the formation of PFO excimer and therefore the green emission component. This point of view is evidenced by the following experimental observations. (1) Green emission component appears even for the utilization of the non-emissive PSCn6:K+ as EIL, implying that the green emission component comes from the EML rather than the EIL. (2) Photoluminescence excitation (PLE) spectra remains unchanged during prolonged operation, indicating a formation of excimer, but


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not aggregate52. (3) Excimer formation is expected as judged from the absence of green emission component if the device is operated at low-temperature (78 K). (4) Green emission component does

not

appear

upon

introducing

2,2',2"-(1,3,5-phenylbenzenetriyl)tris[1-phenyl-1H-

benzimidazole] (TPBi) as buffer layer in between EML and EIL, implying the formation of excimer at EML/EIL interface.

2. RESULTS AND DISCUSSION 2.1. Growth of Green Emission Component in the Device with Polyelectrolyte or Its Precursor as EIL Using the conjugated polyelectrolyte PFCn6:K+ (with or without poly(ethylene oxide) (PEO)53) as EIL for β-PFO-based device has been found to provide a great improvement in device efficiency48. For studying the effects of PFCn6:K+ as EIL on emission stability of the device, we firstly investigate PFCn6:K+ without adding PEO for simplicity, though the addition gives promoted performance. The chemical structures of emitting polymer, polyelectrolytes and their precursors are shown in Figure 1. The experimental details including device fabrication and material synthesis are given in the Experimental Section. The normalized EL spectra from the device ITO/PEDOT:PSS (25 nm)/β-PFO (100 nm)/PFCn6:K+ (20 nm)/Al (70 nm) operated from 4 V to 6 V are shown in Figure 2a. A typical β-PFO deep-blue emission54 with very small variation under different applied voltages is observed, and these spectra are nearly identical to the device without PFCn6:K+ layer at 10 V (the dash line curve in Figure 2a). It illustrates that the high stability of deep-blue emission in EL spectra55,56 is contributed from the side chains locating beside β-PFO backbone that could hinder neighboring PFO main chains from segregation57. To


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explore the time-dependent emission of this device, we continuously monitor the EL spectra under a fixed low bias (4.5 V, at which the luminance is 1500 cd/m2 at initial) for 120 minutes. The deep-blue emission with CIEx,y (0.163, 0.106) is observed at the beginning as shown in Figure 2b, but the green emission component (470-650 nm) grows continuously to give the skyblue emission with CIEx,y (0.201, 0.299) at 120 minutes. Such shift in emission is completely different from the invariant EL spectra from the β-PFO-based device without PFCn6:K+ layer under the higher applied voltage (10 V) for the same operation period, 120 minutes (Figure 2c). Comparing Figure 2b and 2c, it is clear that the additional green emission component grows as operation time increases for the β-PFO-based device with PFCn6:K+ as EIL.

Figure 1. The chemical structures of emitting polymer (β-PFO), various polyelectrolytes and their precursors used in this study.


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Figure 2. Normalized EL spectra (normalized at 439 nm) of β-PFO-based device with: (a) PFCn6:K+ as EIL under different voltages (the dotted line indicates device without EIL at 10 V), (b) PFCn6:K+ as EIL at 4.5 V and (c) without EIL at 10 V for 120 minutes. The β-PFO-based device configuration with or without EIL is ITO/PEDOT:PSS (25 nm)/β-PFO (100 nm)/[with or without PFCn6:K+ (20 nm)]/Al (70 nm). In addition to potassium salt with counter-anion CO32- in PFCn6:K+, we also investigate the effect of various anions including F-, Cl-, Br-, and Poly(acrylate anion) (PAA-) on emission stability under a fixed bias for 1200 seconds. It shows that the same behavior of gradual growth of green component in the corresponding EL spectra for these anions is observed (Figure S1a-d, Supporting Information), in which the emission also shifts from deep-blue to CIEx+y > 0.3 region


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(Figure S1e, Supporting Information). It should be mentioned that the appearance of green emission component even occurs in the polymeric anion PAA-, in spite of its high molecular weight (viscosity-average molecular weight of poly(acrylic acid), Mv~450000 Dalton) as compared to those small-size counter-anions. These results indicate that regardless of the size of the counter-anions in PFCn6:K+ layer, a green emission component is observed in β-PFO-based device upon introducing polyelectrolytes as EIL. In fact, similar phenomenon is also observed for polyelectrolyte precursor PFCn6 and PFN as EILs. As shown in Figure 3a and 3b, the growth of green emission component also occurs by using these EILs, leading to the change in emission to non-deep-blue emission (CIEx+y > 0.3) within 15 minutes. The extent of additional growth of green emission component at 120 minutes for the device with PFCn6 as EIL is very similar to that in PFCn6:K+ case, indicating that the existence of metal ion and its counter-anion in EILs gives no significant influence on the growth of green component, though chelating K+ from potassium salt to the crown ether moieties in the PFCn6 can give a dramatic promotion in device performance48. For the device with PFN as EIL, the EL spectra show the more pronounced emission color shifting to bluish-green region than that with PFCn6 as can be seen from the growth of the 525 nm peak and weak growth at 500 nm during 120 minutes. One may argue that the growth of long-wavelength band with time could be due to the strong decrease of the short-wavelength band with time. However, it is unambiguously revealed that the growth of long-wavelength band rather than a strong decrease of short-wavelength band with time in the absolute EL intensity (Figure S2a and S2b, Supporting Information) even though the brightness of the β-PFO-based device with PFCn6:K+ as the EIL decays with time during continuous 120 minutes operation. Similarly, for the corresponding absolute PL intensity at


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initial and after device operation, the obvious growth of long-wavelength band and invariant intensity of short wavelength peak can also be clearly observed (Figure S2c, Supporting Information). Therefore, the growth of long-wavelength band with time resulting from the strong decrease of the short-wavelength band with time is unlikely to occur. In order to identify whether green emission component is originated from conjugated main chain in polyelectrolytes, we synthesize a water/alcohol-soluble non-conjugated polymer by grafting Cn6 on polystyrene (PSCn6) and its chelation to K+ (PSCn6:K+) (the synthesis details of PSCn6 is described in the Experimental Section). The β-PFO-based devices with PSCn6 or PSCn6:K+ as EIL are subjected to open-circuit voltage (VOC) measurements under simulated 100 mW/cm2 AM 1.5 G illumination32,49,51,58 as that with PFCn6 or PFCn6:K+ as EIL (Figure S3, Supporting Information). Their VOC values increase from 1.23 V (without EIL) to 2.03 V and 2.29 V in the devices with PSCn6 and PSCn6:K+ as EIL, respectively, which are close to those of PFCn6 and PFCn6:K+ (2.04 V and 2.35 V), respectively. This result indicates that PSCn6 and PSCn6:K+ are also able to form an interfacial dipole to Al cathode similar to PFCn6 and PFCn6:K+. Note that PSCn6:K+ is also conductive due to its Cn6:K+ moiety. The estimated work function of the K+ intercalated into the crown ether in PSCn6:K+ in Figure 6a. The caged K+ state was termed as the pseudometallic state with K 2p3/2 binding energy (293.29 eV) higher than that of the metallic state (292.60 eV) by 0.69 eV as reported in our previous work48, and its work function can be estimated as 2.99 eV here, which is 0.69 eV higher than that of K metal 2.3 eV. The reason for such assignment for PSCn6:K+ is due to that PS is non-conductive. Then we follow the preceding condition for device fabrication by using PSCn6K+ to replace PFCn6:K+, and also found that a growth of the green emission component after 120 minutes operation as shown in the normalized EL spectra of this device (Figure 3c). In general, the non-


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conjugated polymer does not emit any visible light, and the CIE value of x+y = 0.493 at 120 minutes for this device is very close to that using PFCn6 as EIL with the CIEx+y of 0.498 and PFCn6:K+ as EIL with the CIEx+y of 0.500, respectively, as shown in the CIEx+y verse time plots in Figure 3d. The slopes of the plots of the three devices with PFCn6, PFCn6:K+ and PSCn6:K+ as EIL are very close in the time range 60-120 minutes, indicating clearly that: (i) the additional green emission component originates from the β-PFO layer in those devices with either the conjugated PFCn6:K+ or non-conjugated PSCn6:K+ polyelectrolyte, (ii) the charge recombination zone does not extend into the PFCn6:K+ layer.

Figure 3. Normalized EL spectra (normalized at 439 nm) of β-PFO-based device with: (a) PFCn6, (b) PFN, and (c) PSCn6:K+ as EIL operated at 4.5 V for 120 minutes. (d) The plot of


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CIEx+y verse time from the devices with PFN, PFCn6, PFCn6:K+ and PSCn6:K+ as EIL, and without EIL. The β-PFO-based device configuration is ITO/PEDOT:PSS (25 nm)/β-PFO (100 nm)/EILs (20 nm)/Al (70 nm). From the above discussion, we can conclude that the existence of polyelectrolyte in the device will induce the green emission component originating from the EML under a fixed electric field. The emitting species that contributes to green emission will be discussed in more detail in the next section. Since the solvent for the preparation of the polyelectrolyte solution is the mixed polar solvent methanol/water (19:1, v/v), one may argue that this green emission component is due to the effect of mixed polar solvent spun on the β-PFO layer rather than the presence of polyelectrolyte. This factor can be eliminated because the device with only the mixed polar solvent (without the polyelectrolyte) spun on the EML shows the same deep-blue emission (Figure S4, Supporting Information) as that without the polar solvent treatment. 2.2. Characterization of Green Component Emission and Origin of Excimer Formation It is difficult for PFO to obtain pure and stable blue EL due to a presence of undesirable extra green emission. Two explanations have been given for the green emission: one was the excimer or aggregate emission, and the other was keto defect. Such phenomenon could be originated from: (i) electric-field-induction under assistance of side chain thermal relaxation motion giving an excimer formation with two growing peaks at 485 and 520 nm in the EL spectra52; (ii) end group-enhanced aggregation (PFO without end-capping) that leads to a strong green EL at 507 nm59; (iii) presence of oligomers as oligomers with molecular weight below 10000 Dalton as EML can give a green EL (main peak at 510 nm) because of aggregate formation60; and (iv) chemical defect on polyfluorene main chain (i.e. keto defect or fluorenone) which can emit green EL at around 535 nm61-66. The molecular weight (Mw) and low


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polydispersity index (PDI) of the PFO we used are 379000 Dalton and 1.55, respectively. The possibility of green emission resulting from a presence of oligomer67 is unlikely to occur. This viewpoint can be further supported by the absence of green emission in the EL spectra as shown in the Figure 2c for the device ITO/PEDOT:PSS/β-PFO/Al. As for a generation of green emission due to charge-transfer (CT) states activated by ions6, it is also unlikely to occur because an obvious green emission is observed in the β-PFO-based device even with polyelectrolyte precursor, such as PFCn6 and PFN, as EIL after prolonger operation. In order to understand the origin of the gradual growth of green component in the emission from β-PFO layer under continuous electric bias, we compare the normalized photoluminescence (PL) excited by the light at 382 nm and PLE spectra monitored at 525 nm (Figure 4a), respectively, from the device ITO/PEDOT:PSS/β-PFO/PFCn6:K+/Al at initial (right after device fabrication) and after use (after measurement for 120 minutes at 4.5 V) for identifying the origin of the green emission component. In the PL spectrum of the long wavelength region (470-650 nm), the intensity area of the PFO film in the device after use is obviously increased by 14.9 % in comparison with the initial PL of the same region. While no change in the intensity in the 420490 nm region of PLE is observed, indicating that aggregate68 does not form in PFO film. Therefore, it is reasonable to assign this green emission component to excimer formation in the EML after use. The inset in Figure 4a shows the additional green component emission in PL spectrum of the device after use (which is obtained by subtraction of the PL spectrum of the film at initial from that after use), which has the main peak and shoulder located at 495 and 525 nm, respectively, resembling to the growing part with time from 470-650 nm in the EL spectra for long operation time (Figure 2b). On the contrary, the device without EIL remains unchanged in the PL and PLE spectra after 120 minutes operation (Figure 4b). Therefore, we suggest that


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thermal relaxation motion of high-polarity side chains in the polyelectrolyte under applied electric field induction can lead to a reorientation of the polar moieties toward PFO main chains at the interface such that two neighboring PFO main chains at the interface are able to form PFO excimer, which is the origin of the green emission component. However, a very slight change in the PLE intensity at around 430 nm in Figure 4a is observed indicating that β-phase PFO fraction is increased from 4.7 % (as estimated by spectral deconvolution54 from the characteristic peak at 430 nm) to about 4.8 % after 120 minutes operation. The variation in the content of β-phase after the device operation is insignificant. It should be noted that the estimation of fraction of β-phase PFO in the device after use via UV-Vis spectroscopy is not possible due to presence of nontransparent and thick metal cathode. As a result, the content of β-phase in PFO from the prolonged operating device is estimated by the PLE spectroscopy, by which one can obtain the fraction of β-phase PFO in our study that contributes to the generation of photoluminescence. In fact, the comparison of β-phase content in PFO at different temperature via PLE spectroscopy was also reported in the previous study.69 Therefore, the utilization of a PLE spectroscopy to estimate the fraction of β-phase PFO in the device after use is appropriate.


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Figure 4. Normalized PLE (monitored at 525 nm and normalized at 360 nm) and PL (excited at 382 nm and normalized at 439 nm) spectra of β-PFO-based device: (a) With PFCn6:K+ as EIL, (b) Without EIL at initial and after use for 120 minutes operation. The inset in (a) indicates the difference in PL intensities for the device at initial and after use for 120 minutes operation. The indirect evidence of excimer formation could also be observed from the corresponding luminous efficiency verse operation time for β-PFO-based device with PFCn6:K+ as EIL. As shown in Figure S5 in Supporting Information, the brightness and current density of this device gradually decay to 82.6 % and 94.3 % of their initial values, respectively. As a result, the luminous efficiency (it is defined as the brightness divides by the current density) gradually decays with the operation time. The decay of luminous efficiency with operation time could also be ascribed to increase of the excimer content during the operation, which results in a lower probability for electron trapping into β-PFO segment and therefore lower EL efficiency. Besides, the J-V characteristics of β-PFO-based device with PFCn6:K+ as EIL before and after 120 minutes operation (Figure S6, Supporting Information) indicates slight decay in current density


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as voltage is greater than 4.5 V, which might be resulted from slight change in electric property at the interface of EML/EIL after the prolonger operation. 2.3. Thermal Relaxation Motion of High-Polarity Side Chain in Polyelectrolyte or Its Precursor Leading to Excimer Formation at EML/EIL Interface As discussed above, we suggest that excimer formation in the EML at the interface is caused by the thermal relaxation motion of high-polarity side chain in the polyelectrolyte under electric field induction. To verify this consideration of the side chain thermal motion effect, the device with the polyelectrolyte precursor PFCn6 as EML (ITO/PEDOT:PSS/PFCn6 (70 nm)/Al) is fabricated. At the initial period, its normalized EL spectra (Figure S7, Supporting Information) are similar to that of the device with pristine PFO as EML54, then after 10 minutes operation, the intensity of green component (470-650 nm) in the EL spectra grows rapidly with the peak at 490 nm reaching to the same level of the initial blue emission peak at 434 nm. Further operation for 20 minutes, the EL turns into a featureless broad peak ranging in 400-600 nm with a peak at 460 nm giving sky-blue emission with the CIEx,y (0.174, 0.235), which is highly deviated from the initial blue emission with the three peaks at 434, 458, and 489 nm having the CIEx,y (0.167, 0.163). In our previous study52, we have observed the presence of green emission component from the blue emission PFO derivative containing polar oxetane-containing side chains, poly(9,9-di(6(2-(3-oxetanyl)butoxyl)hexyl)-2,7-fluorene) (POBOHF), in which the green component (peaked at 487 nm and shoulder at 520 nm) grows and even dominates in EL spectrum after a few cycles operation at room temperature due to the side-chain thermal motion in POBOHF resulting in a formation of excimer. The above reasoning is supported by correlation between the evolution of EL spectra with temperature and the locations of side chain relaxation temperature, in which the


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EL spectrum contains only the green emission component at or above the temperature of side chain motion (β relaxation)52. Since the chemical structure of the side chain of PFCn6 is highly close to that of POBOHF as both containing an alkoxy group and a cyclic ether group, we may expect a side chain relaxation temperature of PFCn6 close to that of POBOHF. Therefore, the strong electric field induction and side chain motion assistance lead to excimer emission during 20 minutes operation in the device with PFCn6 as EML is also confirmed as shown in Figure S7. Note that the main chain relaxation temperature of POBOHF (307 K)52 is higher than the room temperature and well above the temperature at which the green emission from the excimer formation starts to dominate the entire emission at 240 K (β relaxation), indicating that main chain relaxation is not the main factor for excimer formation in the case of PFCn6 as its thermal relaxation temperature is close to POBOHF. Such thermal relaxation motion of side chain in the polyelectrolyte layer will induce the excimer formation at EML/EIL interface of the present system. One might wonder if the excimer formation is due to the generated heat during the operation. However, in our previous study52, the UV and PL spectra of POBOHF show no apparent variation in the intensity at the long-wavelength tail (UV: 400-475 nm; PL: 470-650 nm) after the film was thermally annealed at 393 and 413 K for 14 and 1.5 h, respectively. These temperatures are above its side chain and main chain relaxation temperatures of 240 K and 307 K, respectively. Therefore, a formation of excimer in POBOHF cannot be resulted from heat generation during the operation. Similar to the POBOHF polymer, the green-emission excimer observed in the β-PFO-based device with polyelectrolyte or its precursor, such as PFCn6:K+ and PFCn6, as EIL can be inferred that by heat generation alone during the operation can not generate excimer.


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The low-temperature EL spectra at 78 K for β-PFO-based devices with PFCn6:K+ and PFCn6 as EILs are also investigated. We observe a very stable deep-blue emission (the CIEx,y almost remains at (0.16, 0.10)) for as long as 180 minutes operation in both devices (Figure 5a and 5b). This stable deep-blue emission is also observed in the β-PFO-based device with PFN as EIL at 78 K (Figure S8, Supporting Information). The reason is that the polar side chains in the polyelectrolytes or their precursors are frozen at 78 K (as this temperature is well below the side chain relaxation temperature52) and are not able to induce the local segmental alignment of PFO main chains at the EML/EIL interface under electrical field. Therefore, stable deep-blue emission can be sustained. In other words, the electric field indeed facilitates the formation of excimer, and the thermal relaxation motion of polar side chains also plays an important role in the formation of the electric-field-induced excimer. To clarify the side chain relaxation effect on excimer formation, the EL spectra of β-PFObased devices with PFCn6:K+ and PSCn6:K+ as EILs after 20 minutes operation at 4.5 V at temperatures ranging from 210 K to 270 K are measured (Figure S9, Supporting Information). We can clearly observe that an increase in green EL band starts at 250 and 270 K for PFCn6:K+ and PSCn6:K+ cases, respectively, and thus those temperatures can be reasonably assigned as side chain relaxation temperatures of PFCn6 and PSCn6, respectively. However, the very similar CIE values of x+y = 0.500 and 0.493 of β-PFO-based devices with PFCn6:K+ and PSCn6:K+ as EIL are observed after 120 minutes operation at room temperature, indicating that the dependence of side-chain length on the excimer formation is not significant as long as the operation temperature is higher than the side chain relaxation temperature. According to above observations, the excimer formation in the PFO EML at the interface with EIL resulting from electric field induced reorientation of polar moieties of polyelectrolyte


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or its precursor in EIL toward the interface that leads to local segmental alignment of PFO main chains near the interface can be schematically illustrated as shown in Figure 5c.

Figure 5. Normalized EL spectra (normalized at 442 nm) of β-PFO-based devices at low temperature 78 K with: (a) PFCn6:K+, and (b) PFCn6 as EIL operated at 4.5 V from 0 to180 minutes. The β-PFO-based device configuration is ITO/PEDOT:PSS (25 nm)/β-PFO (100 nm)/EILs (20 nm)/Al (70 nm). (c) The sketch illustrates the thermal relaxation motion of polar moiety in EIL resulting in the local segmental alignment of PFO main chains in EML at the interface under a fixed electric field and thus excimer formation.


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2.4. Prevention of Excimer Formation at EML/EIL Interface by Inserting Buffer Layer (TPBi) in between EML and EIL and Its Effect on Device Performance As the excimer formation is due to alignment of PFO main chains at the EML/EIL interface caused by electric-field-induced reorientation of high polar side chains in the polyelectrolyte or its precursor, we accordingly insert a buffer layer in between EML and EIL attempting to suppress the formation of excimer. As judged from the band diagram of the materials investigated (Figure 6a), in which the energy levels of PEDOT:PSS, β-PFO, PFCn6:K+, PSCn6:K+ and PFN are taken from the previous reports36,48, we use 2,2',2"-(1,3,5phenylbenzenetriyl)tris[1-phenyl-1H-benzimidazole] (TPBi) (its structure and energy level are also given in Figure 6a) as the buffer layer for two reasons. Firstly, its deep-lower HOMO level (6.7 eV) relative to EML (5.8 eV) can act a hole blocking layer to reduce the tendency of holes from passing through the EML without recombination. Secondly, the LUMO level of TPBi (2.7 eV) is close to that of EML (2.86 eV) and EIL (2.86 eV), therefore, the energy barrier for electron transport from EIL to EML can be minimum. The time-dependent normalized EL spectra from the devices ITO/PEDOT:PSS (25 nm)/β-PFO (100 nm)/TPBi (50 nm)/EILs (20 nm)/Al (70 nm) with PFCn6, PFCn6:K+ and PFN as EILs, respectively, at 4.5 V for 120 minutes are shown in Figure 6b-d. The growth of green component emission in EL is completely suppressed, similar to the EL spectra from the β-PFO-based device without PFCn6:K+ layer. Although parts of TPBi layer could be dissolved70 when EIL from its solution in polar mixed solvent methanol/water (19:1, v/v) is coated atop it, the residual TPBi layer (remaining about 60 % relative to the initial thickness 50 nm as measured by surface profiler) can still successfully prevent the polyelectrolytes or their precursors from permeating into the EML surface. The EL emission colors and their corresponding CIEx,y coordinates of the devices (PFCn6:K+ is used as


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EIL) without/with TPBi as buffer layer after 120 minute operation are shown in Figure 6e for comparison.


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Figure 6. (a) Energy diagram and chemical structures of the materials used. Normalized EL spectra (normalized at 439 nm) of β-PFO-based devices with TPBi as buffer layer and (b) PFCn6, (c) PFCn6:K+ and (d) PFN as EILs operated at 4.5 V for 120 minutes. The β-PFO-based device configuration is ITO/PEDOT:PSS (25 nm)/β-PFO (100 nm)/TPBi (~30 nm)/EILs (20 nm)/Al (70 nm). (e) CIE coordinates of β-PFO-based devices with PFCn6:K+, without and with TPBi as buffer layer after 120 minutes operation on the CIE 1931 chromaticity diagram, and their corresponding emission colors, respectively. Apart from introducing the buffer layer between EML and EIL to prevent the excimer formation, we also perform an experiment of applying a negative bias (-4.5 V) for 20 minutes to the used β-PFO-based device with PFCn6:K+ as EIL (operation condition: 4.5 V, 120 minutes) to see if the excimer can be eliminated. However, the CIEx,y (0.201, 0.299) of used β-PFO-based device does not show a significant change after the negative bias is applied CIEx,y (0.200, 0.293) (Figure S10, Supporting Information). Therefore, it indicates that the excimer cannot be eliminated by applying a negative bias with same magnitude. The performances of β-PFO-based devices without and with TPBi layer are summarized in Table 1, and the corresponding current density versus applied voltage (J-V), brightness versus applied voltage (B-V) as well as the current density dependent LE (LE-J) are also shown in Figure 7. After inserting TPBi layer, the maximum LE of devices with PFCn6, PFCn6:K+ and PFN as EIL have been improved by a factor of 2.07, 2.13 and 2.05, respectively. Among these EILs, the device with PFCn6:K+ as EIL and TPBi as buffer layer gives the best performance, exhibiting low turn-on voltage at 3.2 V, maximum LE of 4.54 cd/A, Bmax of 22300 cd/m2 and deep-blue emission CIEx,y (0.170, 0.111) at 4.5 V for the entire period of 120 minutes (the luminance is 3000 cd/m2 at initial). This experimental result also confirms that our consideration


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of excimer formation at EML/EIL interface induced by the reorientation of highly-polar groups grafted at polyelectrolyte side chains that induces a formation of excimer in the PFO chains at the interface. The strategy of inserting a buffer layer between EML and EIL provides an effective way for improvement in stability of the EL spectra during continuously prolonged operation.

Figure 7. (a) Current density, (b) Brightness vs voltage and (c) Luminous efficiency vs current density for β-PFO-based devices without or with TPBi layer. The β-PFO-based device configuration is ITO/PEDOT:PSS (25 nm)/β-PFO (100 nm)/(without/with TPBi)/EILs (20 nm)/Al (70 nm).


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Table 1. Characteristic parameters of the device performance for β-PFO-based devicesa) without and with buffer layer TPBi. Vonb)

Buffer layer

EIL

Without

Bmax 2

LEmax

Brightness at 4.5 V 2

CIEx,yc)

[V]

[cd/m ]

[cd/A]

[cd/m ]

PFCn6

4.4

3500

0.71

20

(0.203, 0.295)

Without

PFCn6:K+

3.4

26200

2.13

1500

(0.201, 0.299)

Without

PFN

4.4

6200

1.09

40

(0.234, 0.315)

TPBi

PFCn6

4.0

4200

1.47

70

(0.168, 0.109)

TPBi

PFCn6:K+

3.2

22300

4.54

3000

(0.170, 0.111)

TPBi

PFN

3.8

8100

2.24

1200

(0.175, 0.113)

a)

The devices configuration is ITO/PEDOT:PSS/β-PFO/(without/with TPBi)/EILs/Al;

b)

Brightness at 2 cd/m2;

c)

CIEx,y coordinates of the device after 120 minutes operation.

3. Conclusion In conclusion, we found for the first time that the emission shifting from deep-blue to skyblue light region occurs during continuous operation in the β-PFO-based PLEDs with various polyelectrolytes and their precursors as EILs. The origin of undesirable green emission component is originated from the excimer formation at the EML/EIL interface, which can be ascribed to reorientation of high-polarity side chains in polyelectrolytes and their precursors under electric field leading to local segmental alignment of PFO main chains at EML/EIL interface and thus excimer formation. The variation in EL spectra can be completely suppressed via inserting a buffer layer between EML and EIL. The insertion of buffer layer also leads to an improvement in device performance.


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EXPERIMENTAL SECTION Materials. PFO was synthesized in accordance with the procedure in our previous works54, poly(acrylic acid) (PAA) was from sigma-aldrich, KPAA was synthesized from PAA neutralized to PH=7 by KOH, TPBi was from Lumtec, KOH was from sigma-aldrich, tetrahydrofuran and methanol were from Merck. The procedures of chelation of PFCn6 to K+ (from K2CO3, KF, KCl, KBr and KPAA) and PSCn6 to K+ (from K2CO3) were followed our previous report48. Synthesis of Poly[p-chloromethylstyrene], (PS-Cl). p-Chloromethylstyrene (5 g, 32.7 mmol) in chlorobenzene (3 mL) was polymerized in the presence of the initiator 2,2'-Azobis(2methylpropionitrile) (AIBN) (26.9 mg, 0.164 mmol) at 70 ℃ under inert gas surrounding and rigorous agitation for 12 h. The polymer was obtained by precipitation in methanol. The precipitated polymer (3.83 g) was dissolved in 7 ml tetrahydrofuran and then re-precipitated in methanol again. The resulting polymer of PS-Cl (3.75 g, 75 % yield) was obtained as white powder. 1H NMR (500 MHz, CDCl3). δ (ppm): 6.82-7.16 (b, 2H), 6.24-6.56 (b, 2H), 4.41-4.62 (b,2H), 1.25-1.82 (b, 3H). Gel permeation chromatography (GPC) analysis showed its weightaverage molecular weight (Mw) and polydispersity index of 82000 Dalton and 1.45, respectively, relative to polystyrene standards. Synthesis of Poly[p-((18-crown-6)methoxy)methyl styrene], (PSCn6). To a solution of 2hydroxymethyl-18-crown-6 (1.5 g, 5.1 mmol) in 5 ml dry tetrahydrofuran, sodium hydride (60 wt%) was added in several portions at room temperature, and the mixture was refluxed for 3 h under nitrogen atmosphere. A solution of PS-Cl (0.72 g, 4.6 mmol) in 5 ml dry tetrahydrofuran was added dropwisely into the mixture. The mixture was then refluxed for 36 h. After the completion of the reaction, the reaction mixture was filtrated to remove the solid precipitate, and the filtrate was concentrated and poured into water and extracted with chloroform. The separated


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organic layer was dried over magnesium sulfate, and the solvent was evaporated. The resulting polymer was precipitated in hexane. After that, the polymer was dissolved in chloroform and then precipitated in hexane to remove the unreacted reactant. Finally, the resulting polymer was dried under vacuum for 24 h to obtain yellow fiber-like solid (1.73 g). Elemental Analysis shows that the weight fractions of C, H, and O atoms were 64.7 %, 8.17 % and 25.93 %, respectively, which indicated that the graft ratio in PSCn6 is 88.3 %. Device Fabrication and measurements. The cleaned indium tin oxide (ITO) substrate was exposed to oxygen plasma (50 W, 5 min) at 193 mTorr. A thin hole-injection layer (25 nm) of poly(styrene sulfonic acid)-doped poly(3,4-ethylenedioxythiophene) (PEDOT:PSS) (Baytron PVP. AI 4083 from Heraeus, with a conductivity of 2×10-4 - 2×10-3 S/cm) was spin-coated on the treated ITO substrate. After baking for 1 h at 140 ℃ under the vacuum, a thin layer (100 nm) of PFO was spin-coated over PEDOT:PSS layer from its solution (7 mg/mL in tetrahydrofuran). The PFO film was treated to form β-phase by spin-coating ethyl acetate atop it48. Finally, PFCn6:K+ (20 nm) (based on PFCn6 1 mg/ml in the mixed polar solvent of methanol/water (19: 1, v/v) with Cn6:K+ =1: 3 (by mole ratio, and K2CO3 was used in the intercalation)) as EIL was successively spin-coated on top of β-phase PFO and then thermally deposited with a layer of Al (70 nm) as cathode under the vacuum below 10-6 Torr through a shadow mask. In addition, the PFCn6, PFN, PSCn6:K+ and different kinds PFCn6:K+X- which chelating from various potassium salt (X: F, Cl, Br and PAA) were spin-coated on top of β-PFO under the same condition like PFCn6:K+. For the case of inserting TPBi as buffer layer between EML and EIL, the TPBi layer (50 nm) was thermally deposited in a vacuum thermal evaporator below 10–6 Torr through a shadow mask. The active area of the diode was about 8–10 mm2. The active area and thickness of polymers and TPBi films were measured using a surface profiler (Tencor P-10). The


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electrical characteristics of the devices were measured using a Keithley power supply (model 238) computer-controlled with a Labview program. The PL, EL and PLE spectra were measured using a fluorescence spectrometer (FluoroMAX-3 from Jobin Yvon). In the case of EL spectra measured at low temperature (78 K), the device was first mounted in a cryostat, which was cooled with liquid nitrogen and maintained at a vacuum less than 10-5 Torr, and then installed into the fluorescence spectrometer mentioned above for measurements. Open-circuit voltage measurements were performed using a Keithley 2400 Source Meter under the illumination of a white light of 100 mW/cm2 onto β-PFO-based devices (from the ITO glass side) from a simulated AM 1.5 light source (Oriel Co.). VOC were determined at the zero-photocurrent points on curves of photocurrent density versus voltage.

ASSOCIATED CONTENT Supporting Information. EL spectra of β-PFO-based device with the PFCn6 chelating to K+ from various salts as EIL, open-circuit voltage measurements, EL spectra of β-PFO-based device without EIL but with a treatment of mixed polar solvent, EL spectra of device with PFCn6 as EML, and EL spectra of βPFO-based device operated at low temperature 78 K with PFN as EIL (PDF)

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We thank the Ministry of Education and the Ministry of Science and Technology for financial support through project MOE106N502CE1, MOST-105-2633-M-007–003, MOST-105-2119-M007-017, MOST-105-2221-E-007-134 and MOST-106-2221-E-007-104. REFERENCES (1) Gather, M. C.; Köhnen, A.; Falcou, A.; Becker, H.; Meerholz, K. Solution-Processed FullColor Polymer Organic Light-Emitting Diode Displays Fabricated by Direct Photolithography. Adv. Funct. Mater. 2007, 17, 191-200. (2) D'Andrade, B. W.; Forrest, S. R. White Organic Light-Emitting Devices for Solid-State Lighting. Adv. Mater. 2004, 16, 1585-1595. (3) So, F.; Kido, J.; Burrows, P. Organic Light-Emitting Devices for Solid-State Lighting. MRS Bull. 2008, 33, 663-669. (4)

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Electric-Field-Induced Excimer Formation at Interface of Deep-Blue

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