Post Doping by Wet Deposition Process in Polymer Light-Emitting

May 5, 2009 - Post Doping by Wet Deposition Process in Polymer Light-Emitting Diode ... Chemical Engineering Department, National Tsing-Hua University...
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J. Phys. Chem. C 2009, 113, 9398–9405

Post Doping by Wet Deposition Process in Polymer Light-Emitting Diode Fabrication for Color Tuning and Performance Improving Hsin-Hung Lu, Chih-Hao Chang, and Show-An Chen* Chemical Engineering Department, National Tsing-Hua UniVersity, Hsinchu, 30041 Taiwan, R.O.C. ReceiVed: February 13, 2009; ReVised Manuscript ReceiVed: April 9, 2009

We present a wet deposition process for doping of fluorescent dye and then metal salt by diffusion into a single emitting polymer layer of triazole-end-capped polyfluorene (TazPFO), which can provide color tuning and performance improvement for polymer light-emitting diode fabrication. The original blue emission from TazPFO can be tuned into sky-blue, green, and orange-yellow after its thin film is dipped in the solutions of the fluorescent dyes 2,5,8,11-tetra-tert-butylperylene (TBPe), 10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7tetrahydro-1H,5H,11H-benzo[l]-pyrano[6,7,8-ij]quinoli-zin-11-one (C545T), and 4-(dicyanomethylene)-2-tertbutyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB), respectively. Further dipping the dye-doped polymer films into a solution of cesium carbonate to deposit cesium carbonate in the polymer surface region results in an enhancement of maximum current efficiency (and brightness) by a factor of 5.9 (13.5), 4.0 (17), and 1.4 (3.6) for TBPe-, C545T-, and DCJTB-doped devices respectively as compared to those of the corresponding dye-doped devices without the further treatment. 1. Introduction Solution processing allows polymer light-emitting diode (PLED) to achieve the aim of large-area and low-cost display fabrication1 in comparison with small-molecule organic lightemitting diode (OLED), which needs an elaborated vacuum system.2,3 For color-tuning in PLED, physical blending of one polymer with fluorescent dyes,4 phosphorescent dyes,5,6 or other conjugated polymers7,8 in solutions has been extensively used. In addition, spin-coating a polymer film followed by the subsequent dopant-diffusion procedures has been reported recently for tuning emission colors such as dye-thermal-diffusion treatments at elevated temperature in an oven9 or by the use of joule-heating through applying an electric field on patterned ITO lines10 and the inkjet-printing-assisted polymer-diffusion process.11 However, these dye-thermal-diffusion methods needed an elevation of temperature to start the diffusion process from dye-reservoir to dye-acceptor (i.e., active emitting polymer), not only making the process time-consuming but also possibly imposing chemical damage on the emitting polymer layer. Also, the inkjet-printing-assisted polymer-diffusion process required a water-soluble dopant and a polymer host slightly soluble in water to form an intermixed layer for color tuning; the resulting device gave a performance with the maximum brightness for red emission at only about 7 cd/m2 at the high applied voltage of 22 V. Thus, it is necessary to develop a more practical diffusion process for tuning emission color. For enhancing device performance, a choice of cathode material of low work function metal (Ca or Ba)12 or alkali metal halides (LiF or CsF)13 for better electron injection and an insertion of inorganic metal salt (such as CH3COOCs, NaOH, and Cs2CO3)14-17 between the cathode and the emitting polymer layer as the electron injection layer are two widely used approaches. These materials are usually deposited on the emitting polymer by the thermal deposition process but some reports have demonstrated that this thermal process can cause * To whom correspondence should be addressed. E-mail: sachen@ che.nthu.edu.tw.

damage to the chemical structure of the polymer layer.18,19 For example, a chemical defect (probably 9,9′-bisfluorenyl linkage) on the surface of spiro-polyfluorene (spiro-PF), which resulted in an undesired emission band (ranging from 478 to 507 nm), was found while aluminum was thermally evaporated on top of this spiro-PF film.18 In addition, the formation of carbonyl group and scissoring of vinyl double bond in the poly(2methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene) (MEHPPV) backbone due to an occurrence of oxidation by oxygen released from a decomposition of Cs2CO3 during thermal deposition was demonstrated by us.19 Therefore, it is important to develop a process to replace the conventional thermal deposition process. Recently, Huang et al.17 have used the spincoating method to form a Cs2CO3 layer (about several nanometers) on top of poly(9,9-di-n-octylfluorene) (PFO) from its dilute solution in 2-ethoxyethanol. However, the thin Cs2CO3 layer formed by spin-coating was demonstrated to appear as particles with size of about 400 × 200 nm rather than uniform film (as revealed by the corresponding surface topological image using atomic force microscopy), resulting in that the active area of the corresponding PFO-based device showed poor uniformity of luminescence.19 Here, we demonstrate a new process that can finely tune the electroluminescence (EL) color of a spin-coated conjugated polymer film by dipping it into a solution of a chosen fluorescent dye. This process is termed wet deposition to distinguish it from the conventional vacuum deposition process widely used in OLED or the diffusion methods used in PLED as discussed above. We use triazole-end-capped polyfluorene (TazPFO, Figure 1 for its chemical structure) as the polymer host due to its blue emission, high solid-state photoluminescence quantum efficiency (47%), and promising device performance.20 For the fluorescent dyes, we use sky-blue-emitting 2,5,8,11-tetra-tertbutylperylene (TBPe),21 green-emitting 10-(2-benzothiazolyl)1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-benzo[l]-pyrano[6,7,8-ij]quinolizin-11-one (C545T),22 and orange-redemitting 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB)23 (Figure 1 for their

10.1021/jp901334u CCC: $40.75  2009 American Chemical Society Published on Web 05/05/2009

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Figure 1. Chemical structures and emission colors of TazPFO, TBPe, C545T, and DCJTB.

chemical structures) because of their good thermal stability and lower tendency toward aggregation. Furthermore, this wet deposition process is also used to improve device performance by further dipping the dye-doped polymer films into a solution of cesium carbonate (Cs2CO3, an electron-injection and holeblocking material)17 to deposit cesium carbonate in the polymer surface region, resulting in the enhancement of maximum current efficiency (and brightness) by a factor of 5.9 (13.5), 4.0 (17), and 1.4 (3.6) for TBPe-, C545T-, and DCJTB-doped devices respectively as compared to those of the corresponding dye-doped devices without further dipping in the cesium carbonate solution. The external quantum efficiencies of the devices so treated are much better than those of the devices fabricated by the above-mentioned dye-thermal-diffusion process reported in the literature.9 To the best of our knowledge, this is the first wet process that can efficiently and conveniently provide both color tuning and performance improvement for PLED fabrication based on the concept of post doping via diffusion mechanism. 2. Experimental Methods 2.1. Materials. The synthetic procedures for TazPFO were the same as those reported in our previous work.20 The molecular weight (Mw) and polydispersity index of TazPFO are 268 500 Da and 1.82 respectively as determined by gel permeation chromatography using polystyrenes as standards. Fluorescent dyes (TBPe, C545T, and DCJTB) were purchased from Luminescence Technology Corporation (in Taiwan) and used without further purification. 2.2. Wet Deposition Process. Details of the present process are depicted as follows. A TazPFO film spin-coated on an indium-tin oxide (ITO) glass substrate from its polymer solution in tetrahydrofuran (THF) (7.5 mg/mL) was dipped in a mixed solvent/nonsolvent, THF/methanol (MeOH) at the volume ratio 1/1, for 1 min to obtain β-phase-containing TazPFO (termed as β-TazPFO). In addition, three spin-coated TazPFO films were separately dipped in three dye solutions

(each contains only one dye of TBPe, C545T, and DCJTB with a concentration of 1 or 5 mg/mL in mixed THF/MeOH solution with volume ratio of 1/1) for 1 min. After dipping in the dye solution, the postdoped β-TazPFO film was further dipped in MeOH, THF/MeOH (volume ratio ) 1/10), and MeOH for 10 s in sequence to remove the residual dye molecules on the polymer surface. For further wet-deposition with Cs2CO3, the postdoped β-TazPFO film was additionally immersed in Cs2CO3 solution (5 mg/mL in methanol) for 10 s without further rinsing with solvent. 2.3. Device Fabrication. For a typical bipolar device, an ITO glass substrate was exposed to oxygen plasma at a power of 30 W under a pressure of 193 mTorr for 5 min. A thin hole injection layer (30 nm) of poly(styrene sulfonic acid)-doped poly(3,4ethylenedioxythiophene) (PEDOT) (Baytron P CH 8000 from Bayer, its conductivity is 10-5 S/cm) was spin-coated on the treated ITO. On top of it, a thin layer (100 nm) of TazPFO film was spin-coated from its solution in THF (7.5 mg/mL). The polymer film was dipped in the THF/MeOH mixed solution (volume ratio ) 1/1) or dye solutions as described in the wet deposition process. For bipolar devices treated with CFx and Cs2CO3, all fabrication procedures were the same as those for a typical bipolar device except that PEDOT was replaced by a plasma-polymerized CFx film (at a power of 30 W under a CHF3 pressure of 130 mTorr for 20 s) and the postdoped β-TazPFO film was additionally dipped in Cs2CO3 solution as described in the wet deposition process. Finally, a thin layer of calcium (about 3 nm) covered with a layer of aluminum (about 100 nm) as a protective layer was deposited in a vacuum thermal evaporator below 10-6 Torr through a shadow mask to form a bipolar device. To fabricate a hole-only device, the procedures were the same as those for a typical bipolar device except that a layer of gold (40 nm) instead of calcium was thermally deposited on top of polymer film without a protective aluminum layer. For an electron-only device, an oxygen-plasma-treated ITO glass (experimental parameters are the same as those used for a typical bipolar device) was deposited with a layer of

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aluminum (50 nm) followed by calcium (25 nm) to replace a PEDOT film, and the residual procedures were the same as those for a typical bipolar device. The active area of the diode is about 8-10 mm2. The electric characteristics and luminance of the device were measured by a Keithley power supply (Model 238) and a luminance meter (BM8 from TOPCON), respectively. The thickness of a polymer film was measured by a surface profiler (Tencor P-10). 2.4. UV-vis (UV) Absorption, Photoluminescence (PL), and Electroluminescence (EL) Spectroscopic Measurements. TazPFO film (100 nm) used for measuring its PL spectrum was obtained by spin-coating from its solution in THF (7.5 mg/mL) on an ITO glass substrate. β-TazPFO film for its UV absorption and PL spectroscopic measurements or postdoped β-TazPFO films for the PL measurements were obtained as described in wet deposition process mentioned above. For the UV absorption spectrum of a TBPe film, the film was spin-coated from TBPe solution (5 mg/mL in THF) on an ITO glass substrate. A quartz cell was used for UV absorption and PL measurements of samples in solution state. UV absorption spectra were collected by a UV-vis-near IR spectrometer (PerkinElmer, Lambda 19). PL and EL spectra were measured by a fluorescence spectrometer (FluoroMAX-3 from Jobin Yvon). All of the measurements of EL spectra were kept in a vacuum environment. 2.5. X-ray Photoelectron Spectroscopy (XPS) Measurements. Films were obtained by spin-coating from TazPFO solution in THF (7.5 mg/mL) and then being dipped in C545T or DCJTB solution for wet deposition of the dye or by spincoating from TazPFO solution (containing 1 or 5 wt % DCJTB) in THF (7.5 mg/mL). XPS spectra were obtained with a photoelectron spectroscopy system (VG, MULTILAB 2000) under a base pressure of 1 × 10-9 mbar by using monochromatized Mg (KR) X-rays (hν ) 1254.6 eV). 2.6. Cyclic Voltammetry (CV) Measurements. Cyclic voltammetry measurements were performed with a potentiostat (from Autolab, Eco Chemie BV) and a one-component threeelectrode electrochemical cell with a 0.1 M tetrabutylammonium percolate (Bu4NClO4) solution in acetonitrile (designated as electrolyte solution) at room temperature under ambient conditions. An ITO glass was used as a working electrode, and a platinum plate and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. While collecting the cyclic voltammogram for β-TazPFO, an ITO glass with a β-TazPFO film was immersed in an electrolyte solution containing ferrocene (about 5 mg), which was used as an internal standard and also as a basis to calculate the highest occupied molecular orbital (HOMO) level of the sample. For TBPe, an ITO glass was dipped in an electrolyte solution containing TBPe and ferrocene (each 5 mg) and then the cyclic voltammogram was recorded. The scanning rate was set at 100 mV/s. 3. Results and Discussion 3.1. Optical and EL Properties. Part a of Figure 2 shows the PL spectra from spin-coated TazPFO films dipped in mixed THF/MeOH solution (its volume ratio is 1/1) or dye solutions (volume ratio of THF/MeOH is also 1/1) with a dye concentration of 5 mg/mL for 1 min. THF and MeOH are solvent and nonsolvent to TazPFO respectively, and their equal volume mixture does not dissolve this polymer (however, a swelling of the polymer film is expected to allow a diffusion of dye into the film as to be revealed below). After dipping in the THF/ MeOH mixed solution, the PL spectrum contains three peaks at 439, 465, and 496 nm, which are characteristic emissions of the β-phase.24 The presence of the β-phase is further demonstrated by the existence of a β-phase absorption peak at 430

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Figure 2. (a) PL spectra (excited at 380 nm) from spin-coated TazPFO films dipped in THF/MeOH mixed solution (volume ratio ) 1/1) or three different dye solutions (TBPe, C545T, and DCJTB) for 1 min. “β-TazPFO” in the notation represents the case in which TazPFO dipped in THF/MeOH mixed solution (volume ratio ) 1/1) for 1 min. Dyes were dissolved in THF/MeOH mixed solvent (its volume ratio is 1/1) with a concentration of 5 mg/mL. (b) UV absorption spectrum of β-TazPFO film. (c) PL spectra of three organic dye solutions in toluene (excited at 390, 445, and 485 nm for TBPe, C545T, and DCJTB, respectively).

nm as shown in part b of Figure 2.25 Thus, the TazPFO film so treated is designated as β-TazPFO hereafter, and TazPFO film denotes the polymer film spin-coated from TazPFO solution (7.5 mg/mL in THF) without further dipping treatment. In addition, after the deconvolution of the UV absorption spectrum of β-TazPFO, the amount of β-phase in β-TazPFO is calculated based on its area of absorption spectrum relative to the area of the entire spectrum to be 1.5% and the other part (98.5%) is amorphous.25 On the other hand, after TazPFO films were dipped in dye solutions, their PL spectra are characterized as follows: three peaks at 439, 458, and 487 nm with a shoulder at 520 nm for TazPFO film dipped in TBPe solution; two peaks at 439 and 489 nm with a shoulder at 520 nm for TazPFO film dipped in C545T solution; three peaks at 439, 465, and 566 nm with a shoulder at 494 nm for TazPFO film dipped in DCJTB solution (part a of Figure 2). Obviously, there are other emission components existing in each PL spectrum in addition to the β-phase emission peaks (the β-phase formed with this treatment is attributed to the fact that the mixed solvent of dye solution

Post Doping by Wet Deposition Process

Figure 3. UV absorption spectra of the three organic dye solutions (in toluene) and PL spectra (excited at 380 nm) from TazPFO and β-TazPFO films.

is THF/MeOH with a volume ratio 1/1 also). To demonstrate that these additional emission components come from dye emissions, PL spectra of TBPe, C545T, and DCJTB solutions in toluene with concentrations of 10-3, 10-3, and 10-2 mg/mL respectively are measured (part c of Figure 2). For TBPe solution, its PL spectrum is characteristic of two peaks at 456 and 485 nm with a shoulder at 520 nm; for C545T solution, its PL emission peak is at 491 nm with a shoulder at 520 nm; and for DCJTB solution, the PL emission peak is located at 564 nm with a weak shoulder at 600 nm. Undoubtedly, the wellmatching of dye emissions with the above-mentioned additional emission components indicates that these additional emission components come from the emission of the corresponding dye molecule, and dye emissions actually dominate each PL spectrum due to their higher intensities than β-phase emission. In addition, it is reasonable to infer that dye molecules resided in a polymer film should result from the diffusion of dye molecules from its solution into the β-TazPFO film during this wet deposition process. We must emphasize that emissions from the dyes are mainly contributed from an energy transfer from the amorphous phase (and β-phase) to these dyes. This is because, as shown in Figure 3, there is a large spectral overlap between the UV absorption spectrum of each dye and the PL spectrum of TazPFO film (which mainly contributes to the amorphous phase due to the appearance of the characteristic amorphous phase emissions with a peak at 422 nm and a shoulder at 449 nm even though a negligible amount of β-phase (as revealed by the peak at 438 nm) still exists due from the use of THF as solvent for the preparation of TazPFO solution)24 or β-TazPFO.26 After understanding PL behaviors of TazPFO films dipping in THF/MeOH mixed solution or dye solutions, we further examine the feasibility to tune the EL color of TazPFO film by this wet process. The EL spectrum of TazPFO film on a substrate after dipping in THF/MeOH mixed solution (volume ratio 1/1) for 1 min exhibits only characteristic β-phase emissions peaked at 439, 465, and 496 nm (Figure 4). For those TazPFO films dipped in the THF/MeOH mixed solution (volume ratio 1/1) with the dyes (TBPe, C545T, or DCJTB with a concentration of 5 mg/mL) for 1 min, the emissions from the dyes dominate their corresponding EL spectra, whose main peaks are located at 461, 500 (and 525), and 575 nm for those with TBPe, C545T, and DCJTB, respectively (Figure 4). The additional weak peak at 439 nm for that with TBPe (or C545T) and two weak peaks at 439 and 465 nm for that with DCJTB are resulting from residual β-phase emissions due to an incomplete energy transfer to the dye dopant. However, it is evident that this wet deposition process can successfully tune the EL color from the blue of β-TazPFO to sky-blue (TBPe), green (C545T), and orange-yellow (DCJTB) without a dissolution of TazPFO film

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Figure 4. EL spectra measured from devices based on TazPFO films dipped in THF/MeOH mixed solution (volume ratio ) 1/1) or dye solutions (5 mg/mL, volume ratio of THF/MeOH ) 1/1) for 1 min. “6 V” and “12 V” in parentheses of notation represent the voltage applied to devices for collecting EL spectra. Device structure is ITO/PEDOT/ polymers/Ca/Al.

due to the presence of the nonsolvent, MeOH, in the THF/ MeOH mixed solution.24 On the other hand, EL characteristics of these cases are similar to their corresponding PL spectral features as illustrated above, except that the contributions of dye emissions in the EL spectra are larger than those in the PL spectra and that the intensity of β-phase emission in the former is much lower than that in the latter. These results indicate that the PL behaviors are mainly controlled by energy transfer mechanism and the EL behaviors are contributed mainly by charge-trapping and weakly by Fo¨rster energy transfer mechanisms.27 3.2. Device Performance. Current density-voltage-brightness (J-V-B) curves of the devices based on β-TazPFO and TazPFO films after dipping in dye solutions are illustrated in part a of Figure 5, and their corresponding current efficiencies versus voltage are shown in part b of Figure 5. For β-TazPFO, light is turned on with a measurable brightness of 2 cd/m2 at 4.5 V and arrives at the maximum 5884 cd/m2 at 11 V; the maximum current efficiency is 2.2 cd/A at the luminance of 1489 cd/m2 (7.5 V). After doping with the dyes, the current density decreases remarkably, resulting in the higher turn-on voltage, 6, 12.5, and 11.5 V, lower maximum brightness, 365, 879, and 1428 cd/m2, and maximum current efficiencies 0.14, 0.96, and 2.5 cd/A for the devices with TBPe, C545T, and DCJTB, respectively. Obviously, the doping with the dye reduces both hole and electron current densities tremendously as shown in J-V curves of the single carrier devices (part a of Figure 6). For example, at the typical field 5 × 105 V/cm, hole (and electron) current densities of β-TazPFO doped with TBPe, C545T, and DCJTB are lower than that of β-TazPFO by factors of 2.8 (19.7), 10.2 (127.5), and 49.2 (407.3), respectively. In addition, the hole current density is higher than the electron current density for all of the cases with doping, in contrast to the more balanced charge fluxes in the case without doping. The occurrence of current density reduction is explained as follows. For β-TazPFO with C545T and DCJTB, the decreases of hole and electron current densities result from hole and electron trapping in C545T and DCJTB molecules because the HOMO and the lowest unoccupied molecular orbital (LUMO) levels for these two dyes (5.3 and 2.9 eV for C545T,22 5.28 and 3.15 eV for DCJTB)23 are located in between those of the amorphous phase in β-TazPFO (5.76 and 2.82 eV as revealed in Section S1 and part a of Figure S1 of the Supporting Information) as shown in part b of Figure 6. We must emphasize that it is more reasonable to use the energy levels of the amorphous phase of β-TazPFO in the explanation of charge

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Figure 5. (a) Characteristics of current density (J) and brightness (B) versus voltage for devices based on TazPFO films dipped in THF/ MeOH mixed solution (volume ratio ) 1/1) or dye solutions (5 mg/ mL, volume ratio of THF/MeOH ) 1/1) for 1 min. (b) The corresponding variations of efficiency with voltage for these devices in (a). The device structure is ITO/PEDOT/polymers/Ca/Al.

Figure 6. (a) Current densities from hole-only (h) and electron-only (e) devices based on β-TazPFO as well as TazPFO films dipped in dye solutions (5 mg/mL, volume ratio of THF/MeOH ) 1/1) for 1 min. Structures of hole- and electron-only devices are ITO/PEDOT/ polymers/Au and ITO/Al/Ca/polymers/Ca/Al, respectively. (b) The energy level diagram of the materials used in the device structure.

injection and transport behaviors due to its major content in β-TazPFO (98.5% as revealed in section 3.1). On the other hand, the TBPe molecule (HOMO ) 5.36 eV and LUMO ) 2.69 eV as revealed in Section S1 and part b of Figure S1 of the

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Figure 7. (a) Characteristics of current density (J) and brightness (B) versus voltage for devices based on TazPFO films dipped in three dye solutions (5 mg/mL, volume ratio of THF/MeOH ) 1/1) for 1 min and then Cs2CO3 solutions (5 mg/mL in methanol) for 10 s. The device structure is ITO/CFx (30 W)/polymers/Cs2CO3/Ca/Al. (b) The corresponding variations of efficiency versus voltage for the three devices in (a).

Supporting Information) can also serve as a hole trap because its HOMO level is closer to the vacuum level than that of the amorphous phase in β-TazPFO, whereas the decrease of electron current density should result from the fact that the barrier of electron injection from the calcium cathode (its work function is 2.9 eV)28 to the TBPe molecules on the film surface is higher than that of β-TazPFO because the LUMO level of TBPe (2.69 eV) is much closer to the vacuum level than that of the amorphous phase in β-TazPFO (2.82 eV). For improving device performance based on β-TazPFO postdoped with the dyes, we must try to promote electron current density and chance for hole-electron recombination. We introduce Cs2CO3 (an electron-injection and hole-blocking material)17 at the interface between the emitting polymer and the calcium cathode by the present wet deposition process, that is to immerse the β-TazPFO postdoped with the dye in Cs2CO3 solution in MeOH (a nonsolvent for β-TazPFO) with a concentration of 5 mg/mL for 10 s and the substrate used is ITO treated by CFx-plasma (this is to increase hole current density for balancing hole and electron current densities because the use of Cs2CO3 can increase electron current density). The devices so prepared have the significantly lower turn-on voltages (4.2, 5.8, and 6.2 V) and the improved maximum brightnesses (4931, 14 966, and 5175 cd/m2) for the cases of TBPe, C545T, and DCJTB, respectively (part a of Figure 7). In addition, the maximum current efficiencies are improved also, being: 0.83 cd/A (TBPe) and 3.85 cd/A (C545T), and 3.6 cd/A (DCJTB) (part b of Figure 7); and their corresponding external quantum efficiencies are 0.52%, 1.15%, and 1.1%, respectively, which are much better than those of the devices fabricated by the above-mentioned dye-thermal-diffusion process reported in the literature (0.45% and 0.13% for the green and orange-red

Post Doping by Wet Deposition Process

Figure 8. (a) C545T concentration distribution along the direction of film thickness for a spin-coated TazPFO film dipped in C545T solution (5 mg/mL, volume ratio of THF/MeOH ) 1/1) for 1 min. For depth larger than 6 nm, the concentration of C545T is too low to be detected by XPS. (b) DCJTB concentration distribution along the direction of film thickness for a TazPFO film containing 5 wt % DCJTB formed by spin-coating from the TazPFO solution with 5 wt % DCJTB.

emissions respectively but no data for the blue emission).9 The improved performance could have resulted from an enhanced hole current density due to reduced hole-injection barrier by introducing the CFx thin layer with the specific composition (F/C ratio 1.47-1.51) and ionization potential (5.6-5.7 eV),29 which is much closer to the HOMO level of β-TazPFO (5.76 eV) as compared with that of PEDOT (its work function is 5.1 eV)29 as shown in part b of Figure 6; and resulting from a promoted electron injection number and confined holes in the emitting layer to increase a chance of charge recombination by the insertion of Cs2CO3 at the interface with the Ca cathode.17 On the other hand, the maximum brightness of the device thus prepared for C545T-postdoped β-TazPFO is better than that (5900 cd/m2) of a device based on PFO blending with 0.1% C545T (and these two devices exhibit the comparable maximum current efficiencies, 3.85 cd/A and 4.6 cd/A for the Cs2CO3treated and PFO/C545T blend devices, respectively).30 Moreover, the device performances (maximum current efficiency and brightness) of DCJTB-postdoped β-TazPFO so prepared are better than those (3.0 cd/A and 4720 cd/m2) of PFO blending with 0.7% 4-(dicyanomethylene)-2-i-propyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (an analogue of DCJTB except that the tert-butyl group at 2-position of DCJTB is replaced by an isopropyl group).31 Although the device performance of TBPe-postdoped β-TazPFO thus prepared is somewhat lower than that (1.7 cd/A and 16 400 cd/m2) of the blend consisting of carbazole- and triphenylamine-grafted polyspirofluorene (Cz-TPA-sPF) and 2 wt % TBPe,32 this process does not need to synthesize the complicated structure Cz-TPA-sPF. These results demonstrate the applicability of this wet deposition process for the performance improvements of PLEDs. 3.3. Determination of Penetration Depth and Concentration Distribution of the Dye Molecule in Polymer Film. To investigate the penetration depth and concentration distribution

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Figure 9. (a) Characteristics of brightness versus voltage for devices based on TazPFO film and TazPFO films dipped in dye solutions (1 mg/mL, volume ratio of THF/MeOH ) 1/1) for 1 min. (b) EL spectra from devices based on TazPFO film and TazPFO films dipped in three dye solutions (1 mg/mL, volume ratio of THF/MeOH ) 1/1) for 1 min. “6 V”, “8 V”, and “9 V” in parentheses of notation represent the voltage applied to devices for collecting EL spectra. The device structure is ITO/PEDOT/polymers/Ca/Al.

of the dye molecule in β-TazPFO film, we have performed XPS measurements with the aid of argon-ion etching33 on β-TazPFO films postdoped with the dyes. For a TazPFO film dipped in C545T solution (5 mg/mL) for 1 min, we found that C545T molecules diffuse into the β-TazPFO film by a depth of about 6 nm with the maximum concentration 3.2 wt % at the film surface (part a of Figure 8), which were determined by calculating the weight ratio of the dye molecules and fluorene repeat units from the atom ratio of oxygen to carbon at each etching time (which then converts to corresponding etching depth) acquired by XPS measurements. As the etching depth is larger than 6 nm, no appreciable signal of O 1s standing for C545T is observed. For the case with DCJTB, however, signals of oxygen or nitrogen atoms are weak due to the low concentration of DCJTB in β-TazPFO. To determine the concentration profile of DCJTB is difficult. Therefore, we turned to perform XPS measurements with argon-ion etching on a film of TazPFO blended with a specific amount of DCJTB to evaluate the approximate DCJTB concentration in postdoped β-TazPFO. For the film with 5 wt % DCJTB, we found that the concentration distribution along the film-thickness direction is in between 4.6-5.9 wt % with an average value of 5.3 wt % (part b of Figure 8), implying that, at least, TazPFO with 5 wt % DCJTB can be detected. For the film with 1 wt % DCJTB, no O 1s signals were observed. Hence, we can reasonably infer that, for a TazPFO film dipped in the DCJTB solution (5 mg/mL) for 1 min, the concentration of DCJTB everywhere in the β-TazPFO film is less than 5 wt %. For the case with TBPe, it is impossible to measure the concentration profile by XPS because all of the atoms in a TBPe and an overwhelming majority atoms in TazPFO are carbon.

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Figure 10. Photograph from a passive dot-matrix three-color device (4096 pixels) fabricated by dipping one area of a spin-coated TazPFO film into C545T solution (1 mg/mL, volume ratio of THF/MeOH ) 1/1) for 1 min followed by another region in DCJTB solution (1 mg/mL, volume ratio of THF/MeOH ) 1/1) for 1 min and leaving the central area without doping. The device structure is ITO/PEDOT/polymer/Ca/Al.

However, the issue of penetration depth of the dyes in polymer films via diffusion mechanism needs to be further discussed. Taking the C545T as an example, because the exciton diffusion length34 and Fo¨rster energy transfer distance35 is about 10 nm, the emission from host polymer (with a thickness of 100 nm) should dominate the PL spectrum in part a of Figure 2 if C545T molecules do not diffuse to a depth larger than 6 nm. However, C545T emission actually dominates PL spectra as we can see in part a of Figure 2, implying that C545T molecules must diffuse into the polymer film with a deeper depth larger than 6 nm even though we cannot determine this penetration depth by XPS measurements. Furthermore, the acceptable device performances of β-TazPFO postdoped with the dyes further support this inference because the device performances should be poor if the emission zone was close to the cathode (i.e., dyes are only within the region of 6 nm from the interface of the emitting layer and cathode).36However, according to the inference given in our previous work, the distribution of β-phase formation should occur homogeneously throughout a PFO film when the PFO film is dipped in the solvent (THF)/nonsolvent (MeOH) mixture.24 It follows that the dyes (TBPe, C545T, and DCJTB) might diffuse throughout the whole TazPFO film because the same solvent/ nonsolvent mixture is used. 3.4. Demonstration of Strip-Shape Three-Color PLED Fabrication. To examine the feasibility of this wet deposition process on a fabrication of a simple three-color display, a passive dot-matrix strip-shape three-color display (4096 pixels) was fabricated by dipping top 1/3 part of a spin-coated TazPFO film into C545T solution of 1 mg/mL followed by dipping bottom 1 /3 part in DCJTB solution of 1 mg/mL (each for 1 min period) and leaving the central part without doping. The use of C545T and DCJTB solutions with the lower concentration (1 mg/mL) is for obtaining more equivalent brightness emitted from the three parts under the same applied voltage. Although the brightness-voltage curves of the TazPFO, β-TazPFO postdoped with C545T (1 mg/mL), and β-TazPFO postdoped with DCJTB (1 mg/mL) vary from each other (part a of Figure 9), the situation is much better than the case in which β-TazPFO, β-TazPFO postdoped with C545T (5 mg/mL), and β-TazPFO postdoped with DCJTB (5 mg/mL) were used for the three parts

of the device (part a of Figure 5). In addition, the EL spectra of TazPFO film and TazPFO films dipped in dye solutions (TBPe, C545T, and DCJTB with a concentration of 1 mg/mL) are shown in part b of Figure 9 (the inclusion of the EL spectrum of TazPFO film dipped in TBPe solution is only for reference as it does not participate in any part of this three-color device). Obviously, these EL spectra of β-TazPFO postdoped with dyes resemble to those of TazPFO dipped in dye solutions with a higher concentration of 5 mg/mL (Figure 4) except that the larger intensities of β-phase emission at 439 nm happen to TazPFO dipped in dye solutions of 1 mg/mL due to the less dye molecules diffused into TazPFO. The EL spectrum of TazPFO film exhibits the characteristic amorphous phase emissions peaked at 421 and 445 nm, and the long-wavelength component (470-650 nm) is attributed to the formation of fieldinduction excimers.37 (Note that other information including the PL and electrical characteristics of TazPFO film dipped in the dye solutions with 1 mg/mL concentration are illustrated in Section S2 and Figure S2 of the Supporting Information.) We must emphasize that the exact control of dye doping level can be rapidly achieved by choosing various concentrations of dye solutions and keeping other experimental parameters (such as volume ratio of the mixed solvent/nonsolvent and dipping time) the same, resulting in device performance with high reproducibility. As shown in Figure 10, the photograph of this display exhibits a clear image with the colors of blue (TazPFO), green (C545T), and orange-yellow (DCJTB), demonstrating the potentiality of this process for color tuning of a blue-emitting polymer layer to various emission colors. This process can be extended to fabricate displays with multistrip area color by screen printing, with area-color by soft-lithography technique, and with multicolor (or full-color) by inkjet printing technique with the ink solution containing dye only. 4. Conclusions We found that the present wet deposition process for dyeand then metal salt-doping by diffusion in single emitting polymer layer provides two functionalities for PLED fabrication, that are color tuning and performance improvement, which cannot be achieved by thermal vacuum deposition generally used in OLED fabrication.

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