advances.sciencemag.org/cgi/content/full/2/10/e1601428/d1 Supplementary Materials for Ultrahigh-efficiency solution-processed simplified small-molecule organic light-emitting diodes using universal host materials Tae-Hee Han, Mi-Ri hoi, han-woo Jeon, Yun-Hi Kim, Soon-Ki Kwon, Tae-Woo Lee This PD file includes: Published 28 October 2016, Sci. Adv. 2, e1601428 (2016) DOI: 10.1126/sciadv.1601428 Supplementary Materials and Methods table S1. Physical properties of,, and 4PTPS. table S2. Work functions with a function of PI concentration in GraHIL compositions measured by UV photoelectron spectroscopy in air (A2, Riken Keiki o. Ltd.). table S3. alculated HOMO, LUMO, ET, and dipole moment. fig. S1. V spectra of,, and 4PTPS. fig. S2. UV-vis absorption and photoluminescence of. fig. S3. UV-vis absorption and photoluminescence of. fig. S4. UV-vis absorption and photoluminescence of 4PTPS. fig. S5. Phosphorescence spectra of,, and 4PTPS at 77 K. fig. S6. hemical structure of PI. fig. S7. X-ray photoelectron spectroscopy molecular depth profiles of the GraHIL. fig. S8. Angular EL distributions according to viewing angles of solutionprocessed OLEDs. fig. S9. Normalized EL spectra according to viewing angles of solution-processed OLEDs. fig. S10. Es of solution-processed OLEDs. fig. S11. Photoluminescence of mixed-host EMLs and UV-vis absorption of phosphorescent dopants. fig. S12. Photoluminescence of mixed-host EMLs according to concentration of phosphorescent dopant. fig. S13. apacitance versus voltage characteristics of mixed-host EMLs. fig. S14. urrent density versus voltage of OLEDs using TTA/ EML according to phosphorescent dopants.
fig. S15. Schematic illustrations of device structure for solution-processed singlecarrier devices. fig. S16. urrent density versus voltage of single-carrier devices according to phosphorescent dopants. fig. S17. Negative differential susceptance versus frequency of EODs. fig. S18. alculated electron mobilities of,, 4PTPS, and TPBI. fig. S19. Density functional theory calculations of,, and 4PTPS.
Supplementary Material Supplementary Materials and Methods Synthesis of 2-(tributylstannyl)pyridine: Preparation followed a procedure reported previously [J. Organomet. hem. 11, 499-502 (1968)]. Yield: 73% (51.2 g). 1 H-NMR (300 MHz, Dl3) [ppm] δ 8.75(d, 1H), 7.49(t, 1H), 7.42(d, 1H), 7.14(t, 1H), 1.55(m, 12H), 1.13(m, 6H), 0.92(m, 9H). Synthesis of bis(3-bromophenyl)diphenylsilane: n-butyllithium (1.6 M in hexane, 58.3 ml, 93.260 mmol) was added to a solution of 1,3-dibromobenzene (20 g, 84.782 mmol) in dehydrated diethyl ether (200 ml) at -78. The mixture was stirred at room temperature (RT) for 1 h. The reaction mixture was cooled to -78, then dichlorodiphenylsilane (9.66 g, 38.152 mmol) was added to the reaction mixture and stirred for 12 h at RT. inally, water was added to quench the reaction. The product was extracted with diethyl ether, then dried with MgSO4. The solvent was evaporated, then the crude product was purified by column chromatography (eluent: n-hexane / methylene chloride = 10 / 1). Yield: 38.7% (16.20 g). 1H-NMR (300 MHz, Dl3) [ppm] δ 7.62-7.63(m, 4H), 7.53(t, 2H), 7.36-7.46(m, 12H). Synthesis of diphenylbis(3-(pyridine-2-yl)phenyl)silane (): Bis(3-bromophenyl) diphenylsilane (2 g, 4.046 mmol) and 2-(tributylstannyl)pyridine (3.43 g, 9.306 mmol) were mixed in 40 ml dehydrated toluene. The mixture was degassed and tetrakis(triphenylphosphine)palladium (3 g, 5 mol%) was added in one portion under an atmosphere of N2. The solution was then heated under reflux for 72 h under N2. The reaction mixture was cooled and added to a 2-N aqueous solution of Hl. The resulting mixture was extracted with chloroform, then dried with MgSO4. After the solvent was evaporated, the crude product was purified by column chromatography (eluent: n-hexane / ethyl acetate (EA) = 5 / 1). Yield: 86% (1.71 g). 1H-NMR (300 MHz, Dl3) [ppm] δ 8.68(d, 2H), 8.28 (s, 2H), 8.28(s, 2H), 7.69-7.73(m, 10H), 7.65-7.67(m, 2H), 7.44-7.56(m, 6H), 7.20(m, 2H); 13 -NMR (300 MHz, Dl3) δ: 157.5, 149.5, 138.7, 137.1, 136.8, 136.4, 134.7, 134.5, 133.9, 129.7, 128.6, 128.4, 127.9, 122.1, 12; HRMS (AB+)m/z for 34H27N2Si (M+): 491.1899,.0, found 491.1941. Synthesis of diphenylbis(3-(pyridine-3-yl)phenyl)silane (): was obtained using the Suzuki coupling reaction. Bis(3-bromophenyl)diphenylsilane (2 g, 4.046 mmol) and 3-pyridine boronic acid (1.14 g, 9.306 mmol) were mixed in tetrahydrofuran (TH); 20 ml of aqueous 2 M K2O3 solution was added to the mixture. The mixture was then left under an N2 stream for 15 min. Tetrakis(triphenylphosphine)palladium (Pd(PPh3)4) (0.5 g, mmol) was added and the result solution was refluxed for 24 h at 110. After reaction was completed, the crude product was worked up with 2-N Hl aqueous solution. The solvent was evaporated, then the crude product was purified by column chromatography (eluent: n-hexane / ethyl acetate (EA) = 5 / 1). Yield: 61.4% (2.44 g). 1H-NMR (300 MHz, Dl3) [ppm] δ 8.82(s, 2H), 8.57(d, 2H), 7.85-7.80(m, 4H), 7.71-7.66(m, 8H), 7.57-7.44(m, 8H), 7.33(m, 2H); 13 -NMR (300 MHz, Dl3) δ: 148.4, 148.3, 137.3, 136.7, 136.4, 136.2, 135.2, 134.9, 134.5, 133.4, 13, 128.7, 128.6, 128.1, 123.6; 34H27N2Si (M+): 491.1899, found 491.1942 Synthesis of diphenylbis(3-(pyridine-4-yl)phenyl)silane (4PTPS): 4PTPS was obtained using the same procedure used to produce. Yield: 36% (7 g). 1H-NMR (300 MHz, Dl3) [ppm] δ 8.63(d, 4H), 7.88(s, 2H), 7.74-7.63(m, 8H), 7.58-7.43(m, 12H); 13-NMR (300MHz, Dl3) δ: 150.1, 148.4, 137.7, 137.1, 136.4, 135.2, 134.7, 133.2, 130.1, 128.8, 128.5, 128.2, 121.7; HRMS (AB+); 34H27N2Si (M+) : 491.1899, found 491.1942 Thermal properties
The thermal properties of, and 4PTPS were evaluated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DS) in an N2 atmosphere. Although and are small molecules, they can form stable amorphous film, which is a basic requirement for materials to be used as hosts in OLEDs. 4PTPS has symmetric structure and therefore does not show Tg. table S1. Physical properties of,, and 4PTPS. ompound λabs (nm) λem (nm) Tg (º) Tm (º) Td (º) HOMO (ev) ΔE (ev) LUMO (ev) ET (ev) 252 324 175-314 -6.47 4.06-2.41 2.82 250 316 190-340 -6.50 4.20-2.30 2.82 4PTPS 257 355-193 348-6.55 4.28-2.27 2.90 Electrochemical Properties To investigate the electrochemical properties and energy levels, cyclic voltammetry (V) measurements were performed in a conventional three-electrode configuration in acetonitrile containing 0.1 M tetrabutylammonium perchlorate (Bu4NlO4) as the supporting electrolyte versus an Ag/Agl platinum disk as the working electrode and platinum wire as the counter electrode. All three new phosphorescent host materials exhibited irreversible p-doping and n-doping processes (fig. S1). The oxidation (p-doping) started at 2.06 V for, 2.09 V for, and 2.14 Vfor 4PTPS. The reduction processes (n-doping) started at -2.0 V for, -2.11 V for, and -2.14 V for 4PTPS. The highest ocuppied molecular orbital (HOMO) and lowest unocuppied molecular orbital (LUMO) levels depend on the electrochemical or redox properties of each individual component in the system. The HOMO (ionization potential, IP) and LUMO (activation energy EA) values were calculated using ferrocene (EO) as the internal standard (which has a value of -4.8 ev relative to the vacuum level), and the EO was calibrated to be 0.39 V versus the Ag/Agl electrode. According to the equation IP = - ([Eonset] ox + 4.41) ev [Synth. Met. 87, 53-59 (1997)], the HOMO energy levels were estimated, and the band gaps were calculated from the onset energy of optical absorption. The HOMO levels were -6.47 ev for, -6.50 ev for, and -6.55 ev for 4PTPS. The LUMO levels calculated from optical onset energy and HOMO level, were -2.41 ev for, -2.30 ev for, and -2.27 ev for 4PTPS.
4-DPPS 3-DPPS 2-DPPS urrent (a.u.) 3 2 1 0-1 -2-3 Potential (V) vs. Ag/Agl fig. S1. V spectra of,, and 4PTPS. UV Absorbance (a.u.) PL Intensity (a.u.) 250 300 350 400 450 fig. S2. UV-vis absorption and photoluminescence of. UV-vis absorption (red) and photoluminescence spectra (blue) of in 10-6 M Hl3 solution at room temperature and the absorption spectrum of Irpic (black).
UV Absorbance (a.u.) PL Intensity (a.u.) 250 300 350 400 450 fig. S3. UV-vis absorption and photoluminescence of. UV-vis absorption (red) and photoluminescence spectra (blue) of in 10-6 M Hl3 solution at room temperature and the absorption spectrum of Irpic (black). UV Absorbance (a.u.) PL Intensity (a.u.) 250 300 350 400 450 fig. S4. UV-vis absorption and photoluminescence of 4PTPS. UV-vis absorption (red) and photoluminescence spectra (blue) of 4PTPS in 10-6 M Hl3 solution at room temperature and the absorption spectrum of Irpic (black). Room temperature UV-vis absorption and photoluminescence (PL) spectra of (fig. S2), (fig. S3) and 4PTPS (fig. S4) were measured in Hl3 solution and displayed with the absorption spectrum of bis[2-(4,6-difluorophenyl)pyridinato-n,2] (picolinato)iridium. The absorption spectra peaks at 251 nm for, 250 nm for and 257 nm for 4PTPS can be attributed to their π-π * transitions. Upon UV excitation, the fluorescent spectra peaked at 324 mn for, 316 nm for and 355 nm for 4PTPS. The slightly red-shifted absorption and fluorescence of 4PTPS may be attributed to its higher polarity and thus stronger molecular interaction, compared to and [Adv. unct. Mater. 19, 1260-1267 (2009)]. rom absorption edges, the energy band gaps can be estimated to be 4.06 ev for, 4.20 ev for and 4.28 ev for 4PTPS.
Photophysical Properties Phosphorescence of, and 4PTPS was measured in a Hl3 matrix at 77 K (fig. S5). The highest-energy 0-0 phosphorescent emissions located at 2.82 ev for, 2.82 ev for and 2.90 ev for 4PTPS were used to calculate their triplet energy (ET) gaps. PL Intensity (a. u.) 4PTPS 400 420 440 460 480 fig. S5. Phosphorescence spectra of,, and 4PTPS at 77 K. Hole injection layer Our HILs are composed of PEDOT:PSS and a perfluorinated ionomers (PI), tetrafluoroethyleneperfluoro-3,6-dioxa-4-methyl-7-octene-sulfonic acid copolymer (fig. S6), which develops a gradient work function (W) by self-organization (We call GraHIL ). m O n 3 O O S O OH fig. S6. hemical structure of PI. Because PI has lower surface energy (~20 mn/m) than PEDOT:PSS, PI preferentially tends to be located toward the surface of the HIL film. Because PI has higher ionization potential energy than PEDOT:PSS, the concentration gradient of PI from the bottom to the top through the film generates a gradient work function in the HIL film [Adv. unct. Mater. 17, 390 (2007)].
Molecular concentration (%) 70 PEDOT Sulfonic acid 60 Sulfone Sulfide 50 3 40 30 20 10 0 0 20 40 60 80 Sputter time (sec) fig. S7. X-ray photoelectron spectroscopy molecular depth profiles of the GraHIL (4). To determine the surface composition and molecular distribution according to sputter time (i.e., film depth), X-ray photoelectron spectroscopy (XPS) was used. Deconvoluted S2p peaks for PEDOT (164.5 ev), sulfonic acid (168.4, 168.9 ev), sulfide (162 ev), and sulfone (166.6 ev) concentrations and 1s peak at 291.6 ev for the PI concentration were used [Adv. unct. Mater. 17, 390 (2007)]. We used the 1s peak at 292 ev to calculate the PI concentration in the GraHIL. This peak can be assigned to 2, which is evidently a component of PI. In the GraHIL, the measured molecular concentration of PI was rich at the surface, but gradually decreased with a depth; this trend is evidence of a gradient chain morphology in the film. Because the PI increases the ionization potential in the composition, the gradient PI concention implies formation of a gradient work function in the GraHIL film. table S2. Work functions with a function of PI concentration in GraHIL compositions measured by UV photoelectron spectroscopy in air (A2, Riken Keiki o. Ltd.) (31). Sample code PEDOT/PSS/PI Work function (ev) (A2) AI4083 1 / 6 / 0 5.20 GraHIL 1 / 6 / 25.4 5.95
External quantum efficiencies By measuring the spectral radiances according to the viewing angle, we calculated external quantum efficiencies (EQEs) of solution-processed OLEDs (figs. S8 10). A Intensity (a.u.) 0 0 30 Lambertian Bt 2 Ir(acac) 60 90 Ir(ppy) 3 Irpic Irpic:Bt 2 Ir(acac) B Intensity (a.u.) 0 0 30 Lambertian 4PTPS TPBI 60 90 Intensity (a.u.) 0 0 30 Lambertian TPBI 60 90 fig. S8. Angular EL distributions according to viewing angles of solution-processed OLEDs. Angular EL distributions of (A) orange-red (Bt2Ir(acac)), green (Ir(ppy)3), blue (Irpic), and white (Irpic/Bt2Ir(acac)) OLEDs that use TTA/-host EML, (B) orange-red (Bt2Ir(acac)), and () green (Ir(ppy)3) OLEDs that use TTA/ -host, TTA/-host, TTA/4PTPS-host and TTA/TPBI-host EMLs. A Spectral radiance (W/sr nm m 2 ) 500 600 700 800 0 10 20 30 40 50 60 70 B Spectral radiance (W/sr nm m 2 ) 400 500 600 700 0 10 20 30 40 50 60 70 Spectral radiance (W/sr nm m 2 ) 400 500 600 700 0 10 20 30 40 50 60 70 D Spectral radiance (W/sr nm m 2 ) 400 500 600 700 0 10 20 30 40 50 60 70 fig. S9. Normalized EL spectra according to viewing angles of solution-processed OLEDs. Normalized EL spectra (A) orange-red (Bt2Ir(acac)), (B) green (Ir(ppy)3), () blue (Irpic), and (D) white (Irpic/Bt2Ir(acac)) OLEDs that use TTA/-host EML.
Relatively large variation of EL spectrum in WOLEDs according to viewing angles could originate from the variation in the cavity effect according to the wavelength of emitted light. Extraction efficiency of OLEDs increases with wavelength, so more blue light than orange-red light could be trapped by a glass substrate. Therefore, in WOLEDs in which geometry causes a cavity effect (GraHIL with low refractive index (n) ~1.4 and ITO with high n ~1.9), the slight difference of extraction efficiency between blue and orange-red light can cause EL spectral change according to viewing angles because trapping of short-wavelength light causes edge-emitted light to be relatively bluish [Appl. Phys. Lett. 92, 033303, (2008)]. A urrent efficiency (cd/a) 10 2 10 1 10 0 10-1 10 2 4PTPS TPBI 10-3 10-2 10-1 10 0 10 1 urrent density (ma/cm 2 ) B urrent efficiency (cd/a) D 10 2 10 1 10 0 10-1 10-3 10-2 10-1 10 0 10 1 10 2 TPBI urrent density (ma/cm 2 ) urrent efficiency (cd/a) 10 1 10 0 10-1 4PTPS TPBI 10-3 10-2 10-1 10 0 10 1 urrent density / ma cm -2 urrent efficiency (cd/a) 10 1 10 0 TPBI 10-1 10-3 10-2 10-1 10 0 10 1 urrent density (ma/cm 2 ) fig. S10. Es of solution-processed OLEDs. urrent efficiencies of (A) orange-red (Bt2Ir(acac)), (B) green (Ir(ppy)3), () blue (Irpic), and (D) white (Irpic/Bt2Ir(acac)) OLEDs that use TTA:, TTA/, TTA/4PTPS, and TTA/TPBI EML (insets: optical images of solutionprocessed OLEDs).
PL intensity (a.u.) Exciton generation mechanism A B PL intensity (a.u.) 1.2 PL_TTA PL_TPBI PL_TTA:TPBI PL intensity (a.u.) 1.2 PL_TTA PL_ PL_TTA: Absorption (a.u.) Bt 2 Ir(acac) Ir(ppy) 3 Irpic TTA: TTA:TPBI 1.4 1.2 350 400 450 500 350 400 450 500 300 350 400 450 500 fig. S11. Photoluminescence of mixed-host EMLs and UV-vis absorption of phosphorescent dopants. Normalized photoluminescence spectra of solution-processed film of (A) TTA, TPBI, TTA:TPBI, (B) TTA,, TTA/, and () Absorption spectra of phosphorescent dopants, and normalized photoluminescence spectra of solution-processed mixed-host films. A PL intensity (a.u.) B PL intensity (a.u.) 0.5% 1%5%15% 350 400 450 500 550 600 650 700 0% 0.1%0.3% 0.5% 1%5%15% 350 400 450 500 550 600 650 700 0% 0.1%0.3% fig. S12. Photoluminescence of mixed-host EMLs according to concentration of phosphorescent dopant. Normalized photoluminescence spectra of solution-processed film of (A) TTA//Bt2Ir(acac), and (B) TTA/TPBIBt2Ir(acac) according to concentration of phosphorescent dopant. A Normalized capacitance (p/0) 1.6 1.4 1.2 TTA:TPBI 0.3% 5% 15% 0 2 4 6 8 10 Voltage (V) fig. S13. apacitance versus voltage characteristics of mixed-host EMLs. apacitance versus voltage characteristics of solution-processed OLEDs that use (A) TTA/TPBI/Bt2Ir(acac) and (B) TTA//Bt2Ir(acac) according to concentration of phosphorescent dopant. B Normalized capacitance (p/0) 1.6 1.4 V max 8 7 6 1.2 5 0 5 10 15 Dopant concentration (%) TTA: 0.3% 1% 5% 15% 0 2 4 6 8 Voltage (V)
The TTA/TPBI mixed host forms exciplexes that recombine in the EML (fig. S11). Light emitted from exciplexes is absorbed by phosphorescent dopants via energy transfer processes (fig. S12). Although charge carriers are partially trapped or directly injected into phosphorescent dopants in the exciplex-forming TTA/TPBI mixed-host EML, comparison of the capacitance versus voltage characteristics with those of TTA/ (fig. S13A, B) suggests that the dominant exciton generation and recombination mechanism can be regarded as energy transfer from exciplexes. In contrast, in the TTA: mixed host that does not form exciplexes, most of the holes are readily trapped or directly injected into phosphorescent dopants, because TTA has deeper HOMO energy level (5.7-5.9 ev) than do phosphorescent dopants such as Ir(ppy)3 and Bt2Ir(acac) (~5.2-5.6 ev) (fig. S11) [J. Appl. Phys. 92, 1598 (2002),.Appl. Phys. Lett. 85, 17, (2004), Appl. Phys. Lett. 106, 123306 (2015)]. The cationic excited state of the dopant formed by hole trapping provides an electron trap in the EML [Adv. unct. Mater. 13, 439 (2003)]. Because has much shallower LUMO energy level (~2.41 ev) than does phosphorescent dopant (e.g., Bt2Ir(acac): ~3.4 ev), dopants provide deep traps for electrons in the TTA/ mixed host, and charge transport is influenced by slow release of electrons from dopant [IEEE J. Sel. Topics Quantum Electron. 4, 119, (1998)]. Therefore, in devices that use a TTA/ EML, low concentration of phosphorescent dopants reduces charge-carrier transport and increase accumulated charges (fig. S13B). However, high concentration of phosphorescent dopants can provide favorable charge carrier hopping sites between dopants for conduction, thereby leading more efficient charge transfer and direct recombination in phosphorescent dopants [J. Appl. Phys. 92, 1598 (2002)]. urthermore, direct injection from a hole injection layer can improve the charge balance in the EML that has high dopant concentration. Therefore, the dominant exciton generation mechanism of exciplex-free type TTA/ can be considered to be direct recombination in phosphorescent dopants by charge trapping on dopants; the direct charge injection and trapping on phosphorescent dopant in TTA/ facilitate balanced charge transport and efficient direct recombination. When we compare the current densities of the solution-processed orange-red (Bt2Ir(acac)), green (Ir(ppy)2(acac)), blue (Irpic), white (Bt2Ir(acac)/Irpic) OLEDs using TTA/ mixed host used in our manuscript, orange-red and green emitting devices had much higher current densities than did the blue-emitting device (fig. S14). The white OLED showed slightly higher current density than the blue OLED; i.e., addition of a small amount of Bt2Ir(acac) (4.5 wt% to Irpic) increased the current density of white OLED. urrent density (ma/cm 2 ) 10 8 6 4 2 Bt 2 Ir(acac) Ir(ppy) 3 Irpic Bt 2 Ir(acac):Irpic 0 4 8 12 16 Voltage (V) fig. S14. urrent density versus voltage of OLEDs using TTA/ EML according to phosphorescent dopants. urrent densities of orange-red (Bt2Ir(acac)), green (Ir(ppy)2(acac)), blue (Irpic), and White (Bt2Ir(acac)/Irpic) solution-processed OLEDs that use TTA/ host.
To investigate influences of dopant materials on charge injection and transport characteristics, we additionally fabricated hole-only devices (HODs) and electron-only devices (EODs) using solutionprocessed EML (~100 nm) that consists of TTA/ mixed host with various dopant materials (fig. S15). To fabricate HODs, the GraHIL was used to facilitate hole injection from indium-tinoxide (ITO) anode to the EML, and a 5-nm-thick MoO3 / Al cathode was used to block electron injection from the cathode (fig. S15A), and a 10-nm-thick branched polyethylenimine (PEI) interfacial layer was used on top of the ITO anode to block hole injection from the anode, and 1-nmthick Li/ Al cathode and TPBI electron transport layer were used to allow favorable electron injection and transport into the solution-processed EML from the cathode (fig. S15B). To quantify the effect of dopant materials on charge injection and transport in OLEDs, each host material was doped with 15 wt% phosphorescent dopant. A B fig. S15. Schematic illustrations of device structure for solution-processed single-carrier devices. (A) hole-only-devices, and (B) electron-only-devices that use TTA/ mixed-host with various dopant materials. Both addition of Ir(ppy)3 and Bt2Ir(acac) greatly improved hole current density of HODs compared with undoped TTA/ mixed-host film (fig. S16A). Although Irpic also showed increased hole current density compared with that of undoped film, the value was much lower than those with Ir(ppy)3 or Bt2Ir(acac). Ir(ppy)3 has HOMO energy level of ~5.2 ev and Bt2Ir(acac) has HOMO energy level of ~5.6 ev [Appl. Phys. Lett. 94, 193305 (2009), Nature Mater.12, 652 (2013)], which are shallower than those of TTA (~5.7 ev) [Appl. Phys. Lett. 91, 263503 (2017)] and (~6.47 ev). Therefore, hole injection can be increased by direct charge injection into dopant materials from HIL; this injection reduces the energy barrier to hole injection into host materials. However, Irpic has deeper HOMO energy level of ~5.9 ev [J. Mater. hem. 17, 1692 (2007)] than do Ir(ppy)3 and Bt2Ir(acac). Because addition of 15 wt% Irpic can reduce the hole injection energy barrier between (HOMO: ~6.47 ev) and HIL, hole current density increased as Irpic concentration increased. However, the deeper HOMO energy level of Irpic than that of TTA hole transporting host did not yield dramatic increase of hole injection in HODs compared to those with the other dopants. All the phosphorescent dopant materials also increased electron current density of each EOD because LUMO energy levels of TTA (~2.3 ev) and (~2.41 ev) in the EML are both much shallower than that of the TPBI electron transporting layer (~ 2.7 ev) [Appl. Phys. Lett. 91, 263503 (2007)]. Addition of dopant materials that have deeper LUMO energy level than that of TPBI reduces the energy barrier for electron injection by directly injecting electrons from the overlying ETL into the dopants. Relatively lower electron current density of EOD that uses Bt2Ir(acac) doped EML may be attributed to relatively deep trapping of electrons during electron transport in the EML
compared with the others. In contrast, Irpic is an electron-transporting emitter [Adv. Mater. 20, 4189 (2008)], and its device showed the highest electron current density. A urrent density (ma/cm 2 ) 20 15 10 5 undoped Bt 2 Ir(acac) Ir(ppy) 3 Irpic B urrent density (ma/cm 2 ) undoped Bt 2 Ir(acac) Ir(ppy) 3 Irpic 0 0 10 20 30 Voltage (V) 0 5 10 15 20 25 Voltage (V) fig. S16. urrent density versus voltage of single-carrier devices according to phosphorescent dopants. (A) hole-only-devices, and (B) electron-only-devices that use TTA/ mixed-host with various dopant materials. Doping of phosphorescent emitter molecules in the solution-processed TTA/ mixed-host structured EML increased both hole and electron injection of single-carrier-devices. However, the difference in electron current densities of devices with different kinds of phosphorescent dopants was smaller in EODs than in HODs. Therefore, overall current density in blue OLEDs was lowest because doping of Irpic showed the poorest hole injection among devices, and white OLED had slightly increased current density compared with that of the blue OLED due to increased hole injection by Bt2Ir(acac) addition in the white OLED. Because the capacitance-voltage characteristics of solution-processed orange-red OLEDs in the manuscript proved that Bt2Ir(acac) doped TTA/ has the best-balanced charge injection and transport, relatively unfavorable hole injection into the Irpic-doped TTA/ compared with the other devices can disrupt the balance of charge injection in the EML. In this regard, unbalanced charge injection and transport in the Irpic doped EML can decrease the luminous efficiency of the solution-processed blue OLEDs. Electron mobilities We performed impedance spectroscopy measurement to determine charge carrier mobility in our electron-transporting materials. We fabricated electron-only-devices to measure electron mobility ( e) of,, 4PTPS and TPBI: [Al (100 nm)/ electron transporting layer (200 nm)/ TPBI (10 nm)/ Li (1 nm)/ Al (100 nm)]. The capacitance of the device changed drastically at ωt t ~1 where is angular frequency and t t is transit time because the current induced by carriers lags behind A voltage when ωt t > 1 [Jpn. J. Appl. Phys. 2014, 53, 02BE02, Phys. Rev. B, 2001, 63, 125328, Eur. Phys. J. Appl. Phys. 2014, 68, 30202]. Therefore, we can determine the charge carrier mobility of materials by obtaining the frequency at which transit time effect occurs. We measured frequency dependences of capacitance in EODs under various applied D bias. Transit time is observed in the plot of negative differential susceptance ( B = geo ), where geo is geometrical capacitance according to frequency. The maximum frequency of EODs gradually increased as applied D voltage was increased (fig. S17A). omparison of the negative differential susceptances of EODs with various electron host materials revealed that the maximum frequency induced by transit time effect increased in the order TPBI < 4PTPS < <.
A B 10-3 10-5 10-4 - delta B (s) 10-6 10-7 10-8 10 4 10 5 10 6 requency (Hz) 4 V 5 V 6 V 7 V 8 V 9 V 10 V - delta B (s) 10-5 10-6 10-7 10 5 10 6 requency (Hz) 4PTPS TPBI fig. S17. Negative differential susceptance versus frequency of EODs. Negative differential susceptance versus frequency characteristics of electron-only-devices with (A) TPBI varying D applied bias, and (B),, 4PTPS, and TPBI at 8 V. By using frequency (fmax) at the maximum magnitude of B, t t can be determined as t t 0.72/f max [Jpn. J. Appl. Phys. 2014, 53, 02BE02, Phys. Rev. B, 2001, 63, 125328, Eur. Phys. J. Appl. Phys. 2014, 68, 30202]. Therefore, charge carrier mobility in a diode can be calculated as μ = 4 3 d 2 t t (V dc V bi ) (S1) We also calculated e of electron transporting host materials by using eq. S1 and transit time which were determined in the plot of negative susceptance versus frequency (fig. S17B). alculated e of TPBI was ~10-5 cm 2 /(V s), which concurs with that measured by using time-of-flight in a previous report (fig. S18) [Appl. Phys. Lett. 2006, 88, 064102]. e of according to electric field was ~ 10-4 cm 2 /(V s), which is one order of magnitude higher than that of TPBI. e obviously increases in the order TPBI < 4PTPS < < (fig. S18); this order is identical with those of current density in solution-processed EODs and OLEDs described in the manuscript. 10-3 4PTPS TPBI (cm 2 /(V s)) 10-4 10-5 10-6 400 450 500 550 600 650 700 E 1/2 (V/cm) 1/2 fig. S18. alculated electron mobilities of,, 4PTPS, and TPBI.
DT calculation N Si N HOMO: -8.63 LUMO: -5.02 µ = 3.30 Debye N Si N HOMO: -8.64 LUMO: -4.84 µ = 3.47 Debye N Si 4PTPS N HOMO: -8.96 LUMO: -4.98 µ = 4.84 Debye fig. S19. Density functional theory calculations of,, and 4PTPS. Density function theory (DT) calculations were performed using B3LYP 6-311+G(d,p)//B3LYP 6-31G as a base set, and electron-density distributions of the orbitals of the,, and 4PTPS as shown in fig. S19.,, and 4PTPS have highly-twisted tetrahedral structure. The electron density distributions of HOMO and LUMO are both located on pyridine-substituted phenyl groups. The calculated HOMO levels, band gap, triplet energy and dipole moment increased as the substituted position of pyridine was changed from the 2 position to the 4 position. Especially, 4PTPS has drastically increased dipole moment (table S3). table S3. alculated HOMO, LUMO, ET, and dipole moment. HOMO (ev) LUMO (ev) Et (ev) Dipole Moment (Debye) -8.63-5.02 3.06 3.30-8.64-4.84 3.26 3.47 4PTPS -8.96-4.98 3.35 4.84