Article pubs.acs.org/Macromolecules
Design of Deep Blue Electroluminescent Spiro-Polyfluorenes with High Efficiency by Facilitating the Injection of Charge Carriers through Incorporation of Multiple Charge Transport Moieties Chih-Wei Huang, Chia-Lin Tsai, Ching-Yang Liu, Tzu-Hao Jen, Neng-Jye Yang, and Show-An Chen* Chemical Engineering Department and Frontier Research Center on Fundamental and Applied Sciences of Matters, National Tsing-Hua University, No. 101, Section 2, Kuang-Fu Road, Hsinchu, Taiwan 30013, Republic of China S Supporting Information *
ABSTRACT: For polymer light-emitting diodes, developing highly efficient, stable, and saturated blue emitting polymer is essential in display and lighting applications and has long been a challenge. Here we report a concept for designing highly efficient electroluminescent polymers by introducing multiple charge transport moieties for efficient injection of charge carriers into spiro-polyfluorene (sPF). We integrate the triphenylamine (TPA) and carbazole (Cz) in the same side chain of sPF with logical spatial and energetic sequence of these moieties to establish graded route for more efficient hole injection and incorporate the electron transport moiety with strong electron-withdrawing capability, triazole (TAZ), on both chain ends to give favorable arrangement in space and energy for electron injection. These two factors allow the corresponding single layer device to exhibit deep blue (db) emission with external quantum efficiency (ηext) of 7.28%, which is the highest value among the db polymer fluorescent diodes ever documented.
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INTRODUCTION For promoting the performance of polymer light-emitting diode (PLED), the key factor is to design structure of the emitting layer, which can be made by the physical method1,2 and chemical structure tuning.3−7 The emitting color “deep blue (db)” of interest here is defined as the summation of the x and y values in the Commission Internationale d’Enclairage (CIExy) coordinates smaller than 0.3. For the former by physical method, creating β phase (chain with extended conjugation length) via the dipping method1 and inserting a hole blocking/electron transport layer2 have been proposed for the poly(9,9-di-n-octylfluorene) (PFO)-based device, leading to the maximum luminous efficiencies (LEmax) 3.85 cd A−1 (external quantum efficiency (ηext) 3.33%) and 3.5 cd A−1, respectively. For the latter by chemical structure tuning, incorporating electron-deficient moieties as the end-cappers4 and integrating bipolar transporting moieties on the structures (oxadiazole (OXD) as side chain along with triphenylamine (TPA) in main chain)5 have been proposed on polyfluorenes (PFs), exhibiting the LEmax 1.67 and 2.07 cd A−1 (ηext 1.59%), respectively. Furthermore, db PFs can also be achieved by utilizing donor−acceptor structure6 or introducing silafluorene derivatives,7 showing the LEmax 2.5 cd A−1 (ηext 2.83%) and 2.02 cd A−1 (ηext 3.34%), respectively. In spite of the above dbPLEDs achieving good blue color purity that is sufficient for full color displays, their device efficiencies remain ample room to be promoted. The reported ηext above are still low (3.34% being the highest), mainly due to their ineffective utilization of charge carriers. To overcome such deficiency, in addition to promotion © 2012 American Chemical Society
of charge balance and injection, suppressing of bypassed charge carriers through the emitting layers becomes a critical issue. Recently, via bringing in a concept of creating a molecularscale graded electronic profile in a single polymer, we have synthesized the sky blue and db emitting spiro-polyfluorene (sPF), TPA-carbazole (Cz)-sPF, and Cz-TPA-sPF,8 in which TPA moieties are located away from the main chain in TPACz-sPF and those in Cz-TPA-sPF are close to the main chain (in both cases, TPA and Cz moieties are on different side chains). The TPA and Cz moieties in sPFs along with main chain segments and anode notably form a downward grading sequence of HOMO levels (from anode to TPA, Cz, and finally to the main chain), leading to the promoted hole injection by factors of 1.2 × 104 (TPA-Cz-sPF) and 8.6 × 103 (Cz-TPAsPF) relative to that of the unmodified sPF at electric field of 5 × 105 V/cm. As such, the single layer PLED fabricated with TPA-Cz-sPF gives CIExy coordinates and ηext of (0.19, 0.20) and 7.53%, respectively, and those for Cz-TPA-sPF give (0.16, 0.10) and 4.54%, respectively, in which both PLEDs yield better performances over that of unmodified sPF (ηext 1%). Besides, the ηext of dbPLED based on Cz-TPA-sPF is superior to the best dbPLED stated above by a factor of 1.36. For further improving the device efficiency, here we report a remarkable improvement on designing db light-emitting polymers (dbLEPs) via further manipulating the spatial and Received: October 31, 2011 Revised: December 29, 2011 Published: January 23, 2012 1281
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Figure 1. Schematic illustration of molecular design concept and chemical structures as well as the energy levels diagram of the materials used in this study. The polymer end-capper for G-sPFs is R1 (tert-butylphenylene (TBP)), and that for G-sPFs-end-TAZ is R2. bipolar device were deposited sequentially in a vacuum thermal evaporator through a shadow mask at a pressure of less than 10−6 Torr. The active area of the device is about 15 mm2. Current−voltage− luminance characteristics of the devices were measured using a Keithley power supply (model 238) and luminance meter (BM8 from TOPCON), respectively; both are computer-controlled with a Labview program. The external quantum efficiencies of the devices were calculated from their corresponding current efficiencies (cd A−1) using the equations described in the literature.10 The HOMO level of PEDOT:PSS was determined by UV photoelectron spectroscopy from Thermo Electron Corp. The thickness of polymer film was measured by a Tencor P-10 surface profiler. Ultraviolet−visible (UV−vis) spectra and photoluminescent (PL) and electroluminescent (EL) spectra were measured using an UV−vis−near-IR spectrometer (Perkin-Elmer, Lambda 19) and a fluorescence spectrometer (Jobin Yvon Horiba, Fluoromax-3), respectively. No correction was made for reflection from the surface and absorption by the sample cell. For preparation of samples for time-of-flight (TOF) measurements, a thick polymer layer (thickness ca. 1.5 μm) was drop-cast on the pretreated ITO substrate from 15 mg mL−1 polymer solution at room temperature for 2 days in a glovebox with inert atmosphere and then baked at 50 °C under vacuum for 1 day, and an aluminum layer of 100 nm was then deposited under 10−6 Torr over the polymer layer through a shadow mask. The photocurrent was generated by a nitrogen-laser-pumped dye laser at 390 nm (near the absorption maxima of the polymers) with a pulse width of 500 ps through an ITO electrode. A transient current was measured across the load resistor using a 500 MHz digital oscilloscope. The carrier mobility μ is calculated from transient time tT by the relation μ = d/EtT, where d is film thickness and E is external electric field.
energetic sequence of hole transporting moieties along a side chain (i.e., integrating TPA and Cz in same side chain of sPF as TPA-flexible spacer-Cz-sPF, termed as G-sPF, as compared to the more randomly distributed arrangement in Cz-TPA-sPF or TPA-Cz-sPF8), giving downward grading HOMO levels along the side chain and additionally introducing the electron transport moiety (ETM) triazole (TAZ) on both chain ends to give G-sPF-end-TAZ for more effective electron injection, allowing the corresponding single layer PLED to exhibit db emission with CIExy coordinates of (0.16, 0.07) and ηext of 7.28%, which is the highest value among the db fluorescent PLEDs ever documented (see the Supporting Information).
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EXPERIMENTAL SECTION
Materials. The reaction routes for monomers and polymers as well as their detailed synthetic procedures and characterizations are depicted in the Supporting Information. The polymers (poly-spiros) were prepared by Yamamoto polymerization, and the energy levels are determined according to previous reports.2,9 The actual contents of monomers are denoted in the Supporting Information. Device Fabrication and General Instrumentation. An indium−tin oxide (ITO) glass plate was exposed on oxygen plasma at a power of 45 W for 5 min. A thin hole injection layer (25 nm) of poly(styrenesulfonic acid)-doped poly(ethylenedioxythiophene) (PEDOT:PSS, Baytron PVP. AI 4083 from Bayer, with a conductivity of 2 × 10−4−2 × 10−3 S cm−1) was spin-coated on the treated ITO as hole injection layer. On top of it, a thin layer (ca. 100 nm) of polymer was spin-cast from its solution (8 mg mL−1) in the mixed solvent, tetrahydrofuran (THF):chlorobenzene (3:1 in volume). Finally, a thin layer of CsF (about 1.5 nm) and a layer of aluminum (ca. 70 nm) for 1282
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Figure 2. (a) Hole fluxes and (b) hole mobilities of poly-spiros. The structure for hole-dominated device is ITO/PEDOT:PSS/poly-spiros/Au.
Figure 3. (a) Electron fluxes and (b) electron mobilities of poly-spiros. The structure for electron-dominated device is ITO/Al/poly-spiros/CsF/Al. Time-Resolved Photoluminescence (TRPL) Measurement. The photoluminescence decay curves of polymers were measured by a time-correlated single photon counting (TCSPC) system with a microchannel plate photomultiplier tube (Hamamatsu Photonics R3809U-50) and a spectrometer (Edinburgh, Lifespec-ps with TCC900 data acquisition card). Excitation pulse for the TCSPC experiments was provided by a frequency-doubled output (harmonic generator, Inrad 5-050) of a mode-locked Ti−sapphire laser (Coherent Mira-900) pumped by a diode laser (Coherent VerdiV10). The repetition rate was reduced to 4 MHz by a pulse picker (Coherent Mira 9200) between Mira-900 and In-rad 5050.
1), which allows holes injected from the anode to transport through the interface to the main chain by three descending HOMO levels. The abrupt change in HOMO level from the anode to backbone in sPF (from 5.3 to 5.7 eV, i.e., 0.4 eV) can be reduced to three smaller steps: the first step (from the anode to TPA) is nearly barrier free (0.0 eV) and the second (from TPA to Cz) and the third (Cz to the backbone) steps are both 0.2 eV, which allows an easier hopping of injected holes from the anode to the main chain. Besides, we have previously shown from the TRPL that TPA has the largest hole trapping capability in TPA-Cz-sPF, followed by Cz and then main chain.8 Therefore, similar result will be expected in the G-sPF. When hole is injected from PEDOT:PSS to G-sPF, it would hop to TPA much easier than to Cz or main chain (TPA exhibits the strongest hole trapping capability) if spatially allowed. Further hopping from TPA to Cz is much easier than to main chain due to stronger hole trapping capability of Cz. Finally, the hole in Cz would hop into the main chain. In addition to energetic considerations discussed above, G-sPF also gives favorable spatial arrangement for hole injection; that is, the TPA moieties have higher possibility to come in contact with the anode. This is because TPA moieties are located away from the main chain in both G-sPF and TPA-Cz-sPF and that more TPA moieties are on the interface in contact with the hole injection layer PEDOT:PSS than in the bulk as previously demonstrated in TPA-Cz-sPF by X-ray photoelectron spectroscopy (XPS) analysis with the aid of argon-ion etching.8 Therefore, the same result would also be expected in G-sPF. These factors lead to an improvement in hole injection. Note that even though the energetic arrangement, spatial arrangement, and TPA content of 50 G-sPF are the same as TPA-CzsPF, the carrier density ratio of 50 G-sPF above is higher than that of previously reported TPA-Cz-sPF with TPA and Cz in different side chains8 (3.57 × 103 at electric field of 2.5 × 105 V/cm) by a factor of about 1.7, implying more effective hole
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RESULTS AND DISCUSSION Hole Injection and Transport. The chemical structures and energy levels of the poly-spirofluorenes (poly-spiros) so designed as well as the design concept are shown in Figure 1. Detailed synthesis procedures and determination of energy levels are given in the Supporting Information. To prove the proposed concept, we fabricated hole-dominated ITO/ PEDOT:PSS/poly-spiros/Au) and electron-dominated (ITO/ Al/poly-spiros/CsF/Al) devices separately, and their current− voltage curves are given in Figures 2a and 3a, respectively. By applying the equation J = nqμE for the single carrier device (J is the current density, n is the carrier number, μ is the carrier mobility, and q is the charge), along with the values of charge carrier mobilities (μ) (in Figures 2b and 3b) determined by time-of-flight, the injected carrier density ratios of the modified sPFs relative to that of sPF at a specific electric field can be determined. For example, at 2.5 × 105 V cm−1, the injected hole carrier density ratios relative to that of sPF for 25 G-sPF, 50 GsPF, 50 G-sPF-end-TAZ, 75 G-sPF, and 100 G-sPF are 1.85 × 103, 6.13 × 103, 1.73 × 103, 3.09 × 103, and 2.48 × 103 (optimal at 50 G-sPF), respectively, suggesting that the designed structure can largely improve hole injection capability. The G-sPF possesses three steps of descending HOMO levels: 5.3 (for TPA), 5.5 (for Cz), and 5.7 eV (for the backbone) (Figure 1283
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Figure 4. Luminescence decays of emission (424 nm) from (a) 50 G-sPF and (b) 50 G-sPF-end-TAZ thin films. The solid lines are doubleexponential fits with fall times of 557 and 1446 ps for 50 G-sPF; those for 50 G-sPF-end-TAZ are 516 and 1116 ps, respectively.
Figure 5. Device performances and EL spectra of poly-spiros with the device structure ITO/PEDOT:PSS/poly-spiros/CsF/Al. (a) Luminance− voltage curves. (b) Current density−voltage curves. (c) Efficiency−current density curves. (d) EL spectra.
cm−1, the injected electron carrier density ratio (versus 50 GsPF) is 1.57 (as determined from Figure 3a,b by applying the applying the equation J = nqμE). The reason is that the TAZ can serve as an additional injection channel for electron (as compared to tert-butylphenylene (TBP) in 50 G-sPF). To support the above consideration, XPS analysis with the aid of argon-ion etching process was performed to investigate the distributions of moieties and main chain segments in 50 G-sPFend-TAZ film (see Supporting Information for details). The distributions of TAZ moieties and main chain segments in 50 G-sPF-end-TAZ on the free surface are quite different from those in the bulk. On the free surface, the determined contents of C, N, and O in terms of atom % are 94.87, 2.88, and 2.25, respectively, whereas in the bulk, the corresponding contents (atom %) are 94.82, 2.61, and 2.57, respectively. The content of nitrogen atoms (TAZ moiety can be an additional source of N atom; therefore, the investigating variation of N content can provide the TAZ distribution) increases from 2.61% in the bulk to 2.88% on the surface, indicating that there are more TAZ on the surface than in the bulk by 10%, and this is possibly due to higher degree of freedom of chain end in polymer. Therefore,
injection can be achieved in 50 G-sPF. This result could be explained as shorter distance between TPA and Cz moieties in G-sPF relative to TPA-Cz-sPF facilitating hopping of hole from TPA to Cz and thus injection process. As holes transport in the bulk, it is reasonable to conceive that holes prefer to hop to TPA or Cz moieties due to larger hole trapping capability and higher HOMO level as compared to backbone of G-sPF as mentioned above. In other words, TPA and Cz moieties can be considered as hole traps. Therefore, hole mobilities of G-sPFs are lower than that of unmodified sPF by a factor of 7−17.5 due to the trapping effect (Figure 2b). The higher mobility observed with the increasing content of hole transport moiety in G-sPFs can be explained as intermoiety transport toward counter electrode, in which holes at TPA or Cz moieties can hop to neighboring TPA or Cz sites and increase the hole mobility (Figure 2b). Electron Injection and Transport. Also, the incorporation of TAZ on chain ends leads to an enhancement in electron injection. Taking 50 G-sPF-end-TAZ (the fraction of polymer chain ends terminated with TAZ is about 89%; see Supporting Information for detail estimation) as an example, at 3 × 105 V 1284
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charge fluxes and thus a low efficiency (ηext 1%).8 After grafting TPA and Cz in same side chain of sPF here, the G-sPFs show promoted hole fluxes by enhanced hole injection. Besides, TPA possesses a higher LUMO level than the backbone by 0.7 eV (Figure 1), which causes more difficulty for electron injection from the cathode to the bulk polymer (note that there are more TPA moieties on the surface than in the bulk as mentioned previously) as supported by the decrease in the electron fluxes in the electron-dominated devices (Figure 3a). These factors lead to a better charge balance relative to sPF, and thus device efficiencies of G-sPFs based PLEDs are superior to that of sPF by a factor of 2−5. However, high content of hole transport moieties could degrade the performances of G-sPFs-based PLEDs. As shown in Table 1, 50 G-sPF gives the best performances (maximum luminance (Bmax) 12 100 cd m−2 and ηext 5.00%), rather than the PLED fabricated with 100 G-sPF (Bmax 6700 cd m−2 and ηext 3.75%). The reason is that high content of hole transport moieties can over suppress electron flux (Figure 3a), resulting in imbalance of hole and electron fluxes and thus degrading the efficiency. Furthermore, as discussed previously, as holes transport in the bulk, it is reasonable to conceive that holes prefer to hop to TPA or Cz moieties due to larger hole trapping capability and higher HOMO level as compared to backbone of G-sPF. In other words, TPA and Cz moieties can be considered as hole traps. When the content of hole transport moiety increases, holes at TPA or Cz moieties prefer to hop to neighboring TPA or Cz sites, and those located at the main chain also prefer to hop to TPA or Cz and then to neighboring TPA or Cz, which would lead to intermoiety transport toward the counter electrode. The increasing hole mobility (Figure 2b) at higher content of hole transport moiety in G-sPFs also supports an increase in possibility of the intermoiety hole transport. As a result, the chance of hole transport following the sequence of TPA, Cz, and finally main chain (where charge recombination takes place) due to the spatially and energetically favorable binding sequence is decreased, giving lower device performances. For the devices based on G-sPF-end-TAZs, they exhibit further remarkably enhanced performance as reflected in the increased ηext 7.28% (25 G-sPF-end-TAZ), 6.08% (50 G-sPFend-TAZ), and 4.91% (75 G-sPF-end-TAZ) as compared to those of the corresponding G-sPF (without TAZ) (Table 1), in which the ηext 7.28% is the highest value among the db fluorescent PLEDs ever documented (see the Supporting Information). The reason for improvement is that TPA moieties in G-sPFs might oversuppress the electron fluxes mentioned above, leading to an imbalance in hole and electron fluxes. The TAZ introduced can serve as an additional injection route for electron toward the main chain as discussed previously, thereby improving the device efficiency as compared to G-sPFs with merely TBP as end-capper. Besides, the hole blocking capability of TAZ moiety could reduce number of injected hole in G-sPF (as supported by smaller injected hole carrier density ratio of 50 G-sPF-end-TAZ (1.73 × 103) as compared to 50 G-sPF (6.13 × 103) mentioned previously and lower HOMO level of TAZ relative to polymer backbone), leading to more balance in charge fluxes and thus performances improvement. As discussed above, the TAZ moiety in the polymer chain ends can enhance the electron injection from the cathode, accompanying by reducing the hole injection due to its holeblocking capability. Therefore, it is reasonable to speculate that there will be an optimized TAZ composition to obtain high EL
TAZ can be a possible injection route for electron from the spatial consideration. The possibility of electron injection via TAZ moieties is also explored by TRPL. Figure 4 shows the time-resolved luminescence decay curves of 50 G-sPF and 50 G-sPF-endTAZ (excitation at 390 nm, i.e., main chain is excited; collected wavelength is 424 nm). The luminescence decays from 50 GsPF and 50 G-sPF-end-TAZ thin films can be fitted by a double-exponential function with fall times of 557 ps (91.13%) and 1446 ps (8.87%) for 50 G-sPF; those for 50 G-sPF-endTAZ are 516 ps (88.56%) and 1116 ps (11.44%), respectively. The major one could be attributed to exciton relaxation from the main chain and the minor one to exciton dissociation by TPA and then recombined in the main chain.8 As for 50 G-sPFend-TAZ (516 ps) and 50 G-sPF (557 ps), end-capping the TAZ moiety can increase the exciton dissociation rate (in other words, shorter exciton lifetime). That is, when exciton is formed on the main chain after excitation, TAZ can dissociate exciton (i.e., trap electron) in the main chain efficiently, which could be ascribed to strong electron-withdrawing capability of the TAZ moiety. Thus, the “dissociation capability” of the TAZ can be taken as its “electron trapping capability”. Therefore, from the energetic point of view, when electron injected from the cathode (CsF/Al) in the PLEDs, it would preferably hop to TAZ than to the main chain if spatially allowed. As electron injected to TAZ, further hopping from TAZ to the main chain would be favorable since very close spatial position and nearly isoenergetic LUMO levels (LUMO of TAZ is lower only by 0.1 eV than the main chain (Figure 1)) between TAZ and the main chain will facilitate electron hopping from TAZ to backbone of G-sPF and thus injection process. As electron transport in the bulk, the TAZ moieties on the polymer chain end could facilitate intrachain electron transport; that is, when electrons hop to TAZ moieties in the bulk film, they would prefer to hop to the main chain nearby, and their transport along the polymer chain would be favorable. Take 50GsPF for example; an enhancement in electron mobility by a factor of 4.8−8.4 in the whole range of electric field relative to that without such modification can be observed due to this reasoning (Figure 3b). Device Performances. The performances and EL spectra of the PLEDs with the device structure ITO/PEDOT:PSS/ poly-spiros/CsF/Al are shown in Figure 5, and their characteristic values are listed in Table 1. All the poly-spiros exhibit db Table 1. Electroluminescence Data for Poly-Spirosa polymer
Bmax [cd m−2]
25 G-sPF 25 G-sPF-end-TAZ 50 G-sPF 50 G-sPF-end-TAZ 75 G-sPF 75 G-sPF-end-TAZ 100 G-sPF
8100 14700 12100 10100 8300 9800 6700
a
LEmax; ηext [cd A−1]; [%] 1.81; 4.88; 3.01; 3.52; 1.98; 3.11; 2.46;
2.23 7.28 5.00 6.08 3.13 4.91 3.75
CIE(x,y) (0.16, (0.16, (0.16, (0.16, (0.16, (0.16, (0.16,
0.07) 0.07) 0.06) 0.05) 0.06) 0.06) 0.06)
Device configuration ITO/PEDOT:PSS/polymer/CsF/Al.
emission (Figure 5d and Table 1), which is extremely important for high degree of color saturation. As for the performances, it has been reported that the poly-spiro without any structure modification (sPF) exhibits considerably higher electron flux than hole flux, which leads to an imbalance in 1285
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Figure 6. PL spectra of poly-spiros before and after annealing at 180 °C in air for 2 h.
spiros. However, as hole transport in the bulk of G-sPFs, the chance for hole hopping from TPA to Cz and then to the main chain (where charge recombination takes place) would be higher than that of TPA-Cz-sPF due to shorter distance between TPA and Cz moieties in G-sPF relative to TPA-CzsPF facilitating hole hopping following the sequence of TPA, Cz and finally main chain. As for end-capping TAZ moiety in G-sPF, more importantly, we think that incorporation of TAZ moiety on the polymer chain end can facilitate intrachain electron transport; that is, when electrons hop to TAZ moieties in the bulk film, they would prefer to hop to the main chain nearby, and their transport along the polymer chain would be favorable as evidenced by an enhancement in electron mobility by a factor of 4.8−8.4 in the whole range of electric field
efficiency as hole transport moieties (TPA and Cz) in the side chain mentioned previously. In principle, balanced hole and electron fluxes are required to obtain a highly efficient PLED. Thus, the optimized amount of TAZ moiety capping on the polymer chain end should depend on the hole and electron fluxes in the investigated PLED. If the hole flux is far higher than the electron flux, more TAZ moieties should be capped on the chain end to reduce the hole flux and increase the electron flux so that more balanced current fluxes (and thus highly efficient PLED) can be achieved. A comparison between the performances of the present polyspiros and TPA-Cz-sPF is not attempted since a green tail appears (which might be attributed to exciplex or excimer emission)8 in TPA-Cz-sPF, relative to db emission in poly1286
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relative to that without such modification in 50 G-sPF (Figure 3b). This result would also promote the chance of charge recombination on the main chain. We also compare the performances of present poly-spiros with the dbPLEDs fabricated with Cz-TPA-sPF. As expected, the efficiencies of 50 G-sPF (ηext 5%) and 50 G-sPF-end-TAZ (ηext 6.08%) are higher than that of Cz-TPA-sPF (ηext 4.54%) by a factor of 1.1 and 1.34, respectively. However, the brightness of 50 G-sPF (Bmax 12 100 cd m−2) and 50 G-sPF-end-TAZ (Bmax 10 100 cd m−2) is lower than that of Cz-TPA-sPF (Bmax 22 000 cd m−2) by a factor of 1.8 and 2.17, respectively. The possible reason is that different cathodes were employed in the dbPLEDs (CsF/Ca/Al for Cz-TPA-sPF and CsF/Al for present poly-spiros). In comparison with CsF/Al, it has been indicated that higher electron current can be achieved as CsF/Ca/Al is utilized as a cathode.11 Therefore, we believe the performances (ηext and Bmax) of present poly-spiros can be further improved if more efficient cathode CsF/Ca/Al is employed. Furthermore, all of the EL devices exhibit rather stable emission color under various operating voltages from low to high. Taking 25 G-sPF-end-TAZ as an example, the CIE values at various current densities of 1, 50, and 200 mA cm−2 are all identical and close to the standard pure blue emission, being x = 0.16 and y = 0.07−0.08 (details are shown in the Supporting Information). In addition to the EL stability, the poly-spiros also exhibit excellent photoluminescence thermal stability (Figure 6). After annealing at 180 °C for 2 h in air, the PL spectra of these poly-spiros remain identical to their pristine state PL spectra, in which no green-blue band emission12 is observed after thermal treatment.
Article
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CONCLUSIONS In conclusion, we have developed the novel molecular design strategy by introducing multiple charge transport moieties for efficient injection of charge carriers, in which the hole transport moieties, TPA and Cz, in the same side chain of sPF and the electron transport moiety TAZ on both chain ends are integrated. The ultrahigh-efficiency (7.28%) dbPLEDs with CIExy coordinates (0.16, 0.07) can be achieved, which are far superior to any db electroluminescent polymers reported in the literature. The present designing concept reveals a major advancement in electroluminescent polymers as well as provides a broad avenue for further development of illumination devices.
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ASSOCIATED CONTENT
S Supporting Information *
Synthesis, cyclic voltammetry, X-ray photoelectron spectroscopy, EL stability at various current densities, and performances comparison of EL devices. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
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ACKNOWLEDGMENTS
The authors thank the Ministry of Education and the National Science Council (Project NSC 96-2752-E-007-008-PAE, NSC 96-2752-E-007-005-PAE, and NSC 99-2221-E-007-002-MY3) for financial support. 1287
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