Supplementary Materials for · 10/24/2016 · Published 28 October 2016, Sci. Adv. 2, e1601428 (2016) DOI: 10.1126/sciadv.1601428 ... CV spectra of 2PTPS, 3PTPS, and 4PTPS. fig

advances.sciencemag.org/cgi/content/full/2/10/e1601428/DC1

Supplementary Materials for

Ultrahigh-efficiency solution-processed simplified small-molecule

organic light-emitting diodes using universal host materials

Tae-Hee Han, Mi-Ri Choi, Chan-Woo Jeon, Yun-Hi Kim, Soon-Ki Kwon, Tae-Woo Lee

Published 28 October 2016, Sci. Adv. 2, e1601428 (2016)

DOI: 10.1126/sciadv.1601428

This PDF file includes:

Supplementary Materials and Methods

table S1. Physical properties of 2PTPS, 3PTPS, and 4PTPS.

table S2. Work functions with a function of PFI concentration in GraHIL

compositions measured by UV photoelectron spectroscopy in air (AC2, Riken

Keiki Co. Ltd.).

table S3. Calculated HOMO, LUMO, ET, and dipole moment.

fig. S1. CV spectra of 2PTPS, 3PTPS, and 4PTPS.

fig. S2. UV-vis absorption and photoluminescence of 2PTPS.



fig. S5. Phosphorescence spectra of 2PTPS, 3PTPS, and 4PTPS at 77 K.

fig. S6. Chemical structure of PFI.

fig. S7. X-ray photoelectron spectroscopy molecular depth profiles of the GraHIL.

fig. S8. Angular EL distributions according to viewing angles of solution-

processed OLEDs.

fig. S9. Normalized EL spectra according to viewing angles of solution-processed

OLEDs.

fig. S10. CEs 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. Capacitance versus voltage characteristics of mixed-host EMLs.

fig. S14. Current density versus voltage of OLEDs using TCTA/2PTPS EML

according to phosphorescent dopants.

fig. S15. Schematic illustrations of device structure for solution-processed single-

carrier devices.

fig. S16. Current density versus voltage of single-carrier devices according to

phosphorescent dopants.

fig. S17. Negative differential susceptance versus frequency of EODs.

fig. S18. Calculated electron mobilities of 2PTPS, 3PTPS, 4PTPS, and TPBI.

fig. S19. Density functional theory calculations of 2PTPS, 3PTPS, and 4PTPS.

Supplementary Material

Supplementary Materials and Methods

Synthesis of 2-(tributylstannyl)pyridine: Preparation followed a procedure reported previously [J.

Organomet. Chem. 11, 499-502 (1968)]. Yield: 73% (51.2 g). 1H-NMR (300 MHz, CDCl3) [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 °C. The mixture was stirred at room temperature (RT) for 1 h. The

reaction mixture was cooled to -78 °C, then dichlorodiphenylsilane (9.66 g, 38.152 mmol) was added

to the reaction mixture and stirred for 12 h at RT. Finally, 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, CDCl3) [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 (2PTPS): 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 (0.23 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

HCl. 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, CDCl3) [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); 13C-NMR

(300 MHz, CDCl3) 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, 120.8; HRMS (FAB+)m/z for C34H27N2Si (M+): 491.1899, .0, found 491.1941.

Synthesis of diphenylbis(3-(pyridine-3-yl)phenyl)silane (3PTPS): 3PTPS 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 (THF); 20 mL of aqueous 2 M

K2CO3 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, 0.4 mmol) was added and the result

solution was refluxed for 24 h at 110 °C. After reaction was completed, the crude product was

worked up with 2-N HCl 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, CDCl3) [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); 13C-NMR (300 MHz, CDCl3) δ: 148.4, 148.3, 137.3,

136.7, 136.4, 136.2, 135.2, 134.9, 134.5, 133.4, 130.0, 128.7, 128.6, 128.1, 123.6; C34H27N2Si (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 3PTPS. Yield: 36% (1.07 g). 1H-NMR (300 MHz, CDCl3) [ppm] δ

8.63(d, 4H), 7.88(s, 2H), 7.74-7.63(m, 8H), 7.58-7.43(m, 12H); 13C-NMR (300MHz, CDCl3) 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

(FAB+); C34H27N2Si (M+) : 491.1899, found 491.1942

Thermal properties

The thermal properties of 2PTPS, 3PTPS and 4PTPS were evaluated using thermogravimetric

analysis (TGA) and differential scanning calorimetry (DSC) in an N2 atmosphere. Although 2PTPS

and 3PTPS 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 2PTPS, 3PTPS, and 4PTPS.

Compound λabs

(nm) λem

(nm) Tg

(ºC) Tm

(ºC) Td

(ºC) HOMO

(eV) ΔE

(eV) LUMO

(eV) ET

(eV)

2PTPS 252 324 175 - 314 -6.47 4.06 -2.41 2.82

3PTPS 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 (CV)

measurements were performed in a conventional three-electrode configuration in acetonitrile

containing 0.1 M tetrabutylammonium perchlorate (Bu4NClO4) as the supporting electrolyte versus

an Ag/AgCl 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 2PTPS, 2.09 V for 3PTPS, and 2.14 Vfor 4PTPS.

The reduction processes (n-doping) started at -2.0 V for 2PTPS, -2.11 V for 3PTPS, 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 (EFOC) as the internal standard (which has a value of -4.8 eV relative to the

vacuum level), and the EFOC was calibrated to be 0.39 V versus the Ag/AgCl 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 2PTPS, -6.50 eV for 3PTPS, and -6.55 eV for 4PTPS. The LUMO

levels calculated from optical onset energy and HOMO level, were -2.41 eV for 2PTPS, -2.30 eV for

3PTPS, and -2.27 eV for 4PTPS.

fig. S1. CV spectra of 2PTPS, 3PTPS, and 4PTPS.

fig. S2. UV-vis absorption and photoluminescence of 2PTPS. UV-vis absorption (red) and

photoluminescence spectra (blue) of 2PTPS in 10-6 M CHCl3 solution at room temperature and the

absorption spectrum of FIrpic (black).

3 2 1 0 -1 -2 -3

Cu

rre

nt (a

.u.)

Potential (V) vs. Ag/AgCl

4-DPPS

3-DPPS

2-DPPS

250 300 350 400 450

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

PL

In

ten

sity (

a.u

.)

Wavelength (nm)

UV

Ab

so

rba

nce

(a

.u.)







Room temperature UV-vis absorption and photoluminescence (PL) spectra of 2PTPS (fig. S2),

3PTPS (fig. S3) and 4PTPS (fig. S4) were measured in CHCl3 solution and displayed with the

absorption spectrum of bis[2-(4,6-difluorophenyl)pyridinato-N,C2] (picolinato)iridium. The

absorption spectra peaks at 251 nm for 2PTPS, 250 nm for 3PTPS and 257 nm for 4PTPS can be

attributed to their π-π* transitions. Upon UV excitation, the fluorescent spectra peaked at 324 mn for

2PTPS, 316 nm for 3PTPS 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 2PTPS and 3PTPS [Adv. Funct. Mater. 19, 1260-1267 (2009)]. From

absorption edges, the energy band gaps can be estimated to be 4.06 eV for 2PTPS, 4.20 eV for

3PTPS and 4.28 eV for 4PTPS.

250 300 350 400 450

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

Wavelength (nm)

PL

In

ten

sity (

a.u

.)

UV

Ab

so

rba

nce

(a

.u.)

250 300 350 400 450

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

Wavelength (nm)

PL

In

ten

sity (

a.u

.)

UV

Ab

so

rba

nce

(a

.u.)

Photophysical Properties

Phosphorescence of 2PTPS, 3PTPS and 4PTPS was measured in a CHCl3 matrix at 77 K (fig. S5).

The highest-energy 0-0 phosphorescent emissions located at 2.82 eV for 2PTPS, 2.82 eV for 3PTPS

and 2.90 eV for 4PTPS were used to calculate their triplet energy (ET) gaps.

fig. S5. Phosphorescence spectra of 2PTPS, 3PTPS, and 4PTPS at 77 K.

Hole injection layer

Our HILs are composed of PEDOT:PSS and a perfluorinated ionomers (PFI), tetrafluoroethylene-

perfluoro-3,6-dioxa-4-methyl-7-octene-sulfonic acid copolymer (fig. S6), which develops a gradient

work function (WF) by self-organization (We call “GraHIL”).

C

C

C

C

F

F

F

F

F

F

F

O

C

m n

CF

FF

O

C

C

S

F

F

F

F

O

O

OH

CF3

fig. S6. Chemical structure of PFI.

Because PFI has lower surface energy (~20 mN/m) than PEDOT:PSS, PFI preferentially tends to be

located toward the surface of the HIL film. Because PFI has higher ionization potential energy than

PEDOT:PSS, the concentration gradient of PFI from the bottom to the top through the film generates

a gradient work function in the HIL film [Adv. Funct. Mater. 17, 390 (2007)].

400 420 440 460 480

0.0

0.2

0.4

0.6

0.8

1.0P

L I

nte

nsity (

a.

u.)

Wavelength (nm)

2PTPS

3PTPS

4PTPS

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 C1s peak at 291.6 eV for the PFI concentration were used [Adv. Funct. Mater. 17, 390 (2007)].

We used the C1s peak at 292 eV to calculate the PFI concentration in the GraHIL. This peak can be

assigned to CF2, which is evidently a component of PFI. In the GraHIL, the measured molecular

concentration of PFI 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 PFI increases the ionization

potential in the composition, the gradient PFI concention implies formation of a gradient work

function in the GraHIL film.

table S2. Work functions with a function of PFI concentration in GraHIL compositions

measured by UV photoelectron spectroscopy in air (AC2, Riken Keiki Co. Ltd.) (31).

Sample code PEDOT/PSS/PFI Work function

(eV) (AC2)

AI4083 1 / 6 / 0 5.20

GraHIL 1 / 6 / 25.4 5.95

0 20 40 60 80

0

10

20

30

40

50

60

70 PEDOT

Sulfonic acid

Sulfone

Sulfide

CF3

Mo

lec

ula

r c

on

ce

ntr

ati

on

(%

)

Sputter time (sec)

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).

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 (FIrpic), and white

(FIrpic/Bt2Ir(acac)) OLEDs that use TCTA/2PTPS-host EML, (B) orange-red (Bt2Ir(acac)), and (C)

green (Ir(ppy)3) OLEDs that use TCTA/ 2PTPS-host, TCTA/3PTPS-host, TCTA/4PTPS-host and

TCTA/TPBI-host EMLs.

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), (C) blue (FIrpic), and (D)

white (FIrpic/Bt2Ir(acac)) OLEDs that use TCTA/2PTPS-host EML.

0

30

60

90

Lambertian

2PTPS

3PTPS

4PTPS

TPBI

1.0

0.8

0.6

0.4

0.2

0

Inte

nsity (

a.u

.)

0

30

60

90

Lambertian

2PTPS

3PTPS

TPBI

1.0

0.8

0.6

0.4

0.2

0

Inte

nsity (

a.u

.)

0

30

60

90

Lambertian

Bt2Ir(acac)

Ir(ppy)3

FIrpic

FIrpic:Bt2Ir(acac)

1.0

0.8

0.6

0.4

0.2

0

Inte

nsity (

a.u

.)

CBA

400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

Spectr

al ra

dia

nce (

W/s

r nm

m2)

Wavelength (nm)

0

10

20

30

40

50

60

70

500 600 700 8000.0

0.2

0.4

0.6

0.8

1.0

Spectr

al ra

dia

nce (

W/s

r nm

m2)

Wavelength (nm)

0

10

20

30

40

50

60

70

400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

Spectr

al ra

dia

nce (

W/s

r nm

m2)

Wavelength (nm)

0

10

20

30

40

50

60

70

400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

Spectr

al ra

dia

nce (

W/s

r nm

m2)

Wavelength (nm)

0

10

20

30

40

50

60

70

BA

DC

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)].

fig. S10. CEs of solution-processed OLEDs. Current efficiencies of (A) orange-red (Bt2Ir(acac)),

(B) green (Ir(ppy)3), (C) blue (FIrpic), and (D) white (FIrpic/Bt2Ir(acac)) OLEDs that use TCTA:

2PTPS, TCTA/3PTPS, TCTA/4PTPS, and TCTA/TPBI EML (insets: optical images of solution-

processed OLEDs).

10-3

10-2

10-1

100

101

10-1

100

101

102

2PTPS

3PTPS

4PTPS

TPBICurr

ent effic

iency (

cd/A

)

Current density / mA cm-2

10-3

10-2

10-1

100

101

10-1

100

101

102

2PTPS

3PTPS

4PTPS

TPBICurr

ent effic

iency (

cd/A

)

Current density (mA/cm2)

D

10-3

10-2

10-1

100

101

10-1

100

101

102

2PTPS

3PTPS

TPBICurr

ent effic

iency (

cd/A

)


10-3

10-2

10-1

100

101

10-1

100

101

102

2PTPS

3PTPS

TPBICurr

ent effic

iency (

cd/A

)


BA

C

Exciton generation mechanism

fig. S11. Photoluminescence of mixed-host EMLs and UV-vis absorption of phosphorescent

dopants. Normalized photoluminescence spectra of solution-processed film of (A) TCTA, TPBI,

TCTA:TPBI, (B) TCTA, 2PTPS, TCTA/2PTPS, and (C) Absorption spectra of phosphorescent

dopants, and normalized photoluminescence spectra of solution-processed mixed-host films.

fig. S12. Photoluminescence of mixed-host EMLs according to concentration of phosphorescent

dopant. Normalized photoluminescence spectra of solution-processed film of (A)

TCTA/2PTPS/Bt2Ir(acac), and (B) TCTA/TPBIBt2Ir(acac) according to concentration of

phosphorescent dopant.

fig. S13. Capacitance versus voltage characteristics of mixed-host EMLs. Capacitance versus

voltage characteristics of solution-processed OLEDs that use (A) TCTA/TPBI/Bt2Ir(acac) and (B)

TCTA/2PTPS/Bt2Ir(acac) according to concentration of phosphorescent dopant.

300 350 400 450 500

PL

inte

nsity

(a.u

.)

Ab

so

rptio

n (

a.u

.)

Wavelength (nm)

Bt2Ir(acac)

Ir(ppy)3

FIrpic

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

TCTA:2PTPS

TCTA:TPBI

350 400 450 5000.0

0.2

0.4

0.6

0.8

1.0

1.2

PL

in

ten

sity (

a.u

.)

Wavelength (nm)

PL_TCTA

PL_TPBI

PL_TCTA:TPBI

350 400 450 5000.0

0.2

0.4

0.6

0.8

1.0

1.2

PL

in

ten

sity (

a.u

.)

Wavelength (nm)

PL_TCTA

PL_2PTPS

PL_TCTA:2PTPS

A B C

350 400 450 500 550 600 650 700

0.0

0.2

0.4

0.6

0.8

1.0

Wavelength (nm)

PL

in

ten

sity (

a.u

.)

0%0.1%

0.3%0.5%

1%5%

15%

350 400 450 500 550 600 650 700

0.0

0.2

0.4

0.6

0.8

1.0

Wavelength (nm)

PL inte

nsity (

a.u

.)

0%0.1%

0.3%0.5%

1%5%

15%

A B

0 2 4 6 8 10

0.6

0.8

1.0

1.2

1.4

1.6

No

rma

lize

d c

ap

acita

nce

(C

p/C

0)

Voltage (V)

TCTA:TPBI

0.3%

5%

15%

A B

0 2 4 6 8

0.6

0.8

1.0

1.2

1.4

1.6

No

rma

lize

d c

ap

acita

nce

(C

p/C

0)

Voltage (V)

TCTA:2PTPS

0.3%

1%

5%

15%

0 5 10 155

6

7

8

Vm

ax

Dopant concentration (%)

The TCTA/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 TCTA/TPBI mixed-host EML, comparison of the capacitance versus voltage

characteristics with those of TCTA/2PTPS (fig. S13A, B) suggests that the dominant exciton

generation and recombination mechanism can be regarded as energy transfer from exciplexes.

In contrast, in the TCTA:2PTPS mixed host that does not form exciplexes, most of the holes are

readily trapped or directly injected into phosphorescent dopants, because TCTA 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. Funct. Mater. 13, 439 (2003)]. Because 2PTPS 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 TCTA/2PTPS 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 TCTA/2PTPS 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)]. Furthermore, 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

TCTA/2PTPS 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

TCTA/2PTPS 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 (FIrpic), white (Bt2Ir(acac)/FIrpic) OLEDs using TCTA/2PTPS 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 FIrpic) increased the

current density of white OLED.

fig. S14. Current density versus voltage of OLEDs using TCTA/2PTPS EML according to

phosphorescent dopants. Current densities of orange-red (Bt2Ir(acac)), green (Ir(ppy)2(acac)), blue

(FIrpic), and White (Bt2Ir(acac)/FIrpic) solution-processed OLEDs that use TCTA/2PTPS host.

0 4 8 12 16

2

4

6

8

10

Curr

ent den

sity (

mA

/cm

2)

Voltage (V)

Bt2Ir(acac)

Ir(ppy)3

FIrpic

Bt2Ir(acac):FIrpic

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 solution-

processed EML (~100 nm) that consists of TCTA/2PTPS mixed host with various dopant materials

(fig. S15). To fabricate HODs, the GraHIL was used to facilitate hole injection from indium-tin-

oxide (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-nm-

thick LiF/ 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.

fig. S15. Schematic illustrations of device structure for solution-processed single-carrier

devices. (A) hole-only-devices, and (B) electron-only-devices that use TCTA/2PTPS 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 TCTA/2PTPS mixed-host film (fig. S16A). Although FIrpic 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 TCTA (~5.7 eV) [Appl. Phys. Lett. 91, 263503 (2017)] and 2PTPS (~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, FIrpic

has deeper HOMO energy level of ~5.9 eV [J. Mater. Chem. 17, 1692 (2007)] than do Ir(ppy)3 and

Bt2Ir(acac). Because addition of 15 wt% FIrpic can reduce the hole injection energy barrier between

2PTPS (HOMO: ~6.47 eV) and HIL, hole current density increased as FIrpic concentration increased.

However, the deeper HOMO energy level of FIrpic than that of TCTA 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 TCTA (~2.3 eV) and 2PTPS (~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

A B

compared with the others. In contrast, FIrpic is an electron-transporting emitter [Adv. Mater. 20,

4189 (2008)], and its device showed the highest electron current density.

fig. S16. Current density versus voltage of single-carrier devices according to phosphorescent

dopants. (A) hole-only-devices, and (B) electron-only-devices that use TCTA/2PTPS mixed-host

with various dopant materials.

Doping of phosphorescent emitter molecules in the solution-processed TCTA/2PTPS 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 FIrpic 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

TCTA/2PTPS has the best-balanced charge injection and transport, relatively unfavorable hole

injection into the FIrpic-doped TCTA/2PTPS 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 FIrpic 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 2PTPS, 3PTPS, 4PTPS and TPBI: [Al (100 nm)/ electron transporting layer (200 nm)/ TPBI

(10 nm)/ LiF (1 nm)/ Al (100 nm)]. The capacitance of the device changed drastically at ω𝑡𝑡 ~1

where is angular frequency and 𝑡𝑡 is transit time because the current induced by carriers lags

behind AC voltage when ω𝑡𝑡 > 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 DC bias. Transit time is

observed in the plot of negative differential susceptance (−∆B = C − 𝐶𝑔𝑒𝑜), where Cgeo is

geometrical capacitance according to frequency. The maximum frequency of EODs gradually

increased as applied DC voltage was increased (fig. S17A). Comparison 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 < 3PTPS < 2PTPS.

BA

0 10 20 300

5

10

15

20

Cu

rren

t d

en

sity (

mA

/cm

2)

Voltage (V)

undoped

Bt2Ir(acac)

Ir(ppy)3

FIrpic

0 5 10 15 20 250.0

0.2

0.4

0.6

0.8

1.0

Curr

ent den

sity (

mA

/cm

2)

Voltage (V)

undoped

Bt2Ir(acac)

Ir(ppy)3

FIrpic

fig. S17. Negative differential susceptance versus frequency of EODs. Negative differential

susceptance versus frequency characteristics of electron-only-devices with (A) TPBI varying DC

applied bias, and (B) 2PTPS, 3PTPS, 4PTPS, and TPBI at 8 V.

By using frequency (fmax) at the maximum magnitude of −∆B, 𝑡𝑡 can be determined as 𝑡𝑡 ≈0.72/𝑓𝑚𝑎𝑥

[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

𝑑2

𝑡𝑡(𝑉𝑑𝑐−𝑉𝑏𝑖) (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). Calculated e of

TPBI was ~10-5 cm2/(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 2PTPS according to electric field was ~

10-4 cm2/(V∙s), which is one order of magnitude higher than that of TPBI. e obviously increases in

the order TPBI < 4PTPS < 3PTPS < 2PTPS (fig. S18); this order is identical with those of current

density in solution-processed EODs and OLEDs described in the manuscript.

fig. S18. Calculated electron mobilities of 2PTPS, 3PTPS, 4PTPS, and TPBI.

105

106

10-7

10-6

10-5

10-4

10-3

2PTPS

3PTPS

4PTPS

TPBI

- d

elta

B (

s)

Frequency (Hz)

104

105

106

10-8

10-7

10-6

10-5

- d

elta

B (

s)

Frequency (Hz)

4 V

5 V

6 V

7 V

8 V

9 V

10 V

A B

400 450 500 550 600 650 70010

-6

10-5

10-4

10-3

(

cm

2/(

Vs

))

E1/2

(V/cm)1/2

2PTPS

3PTPS

4PTPS

TPBI

DFT calculation

fig. S19. Density functional theory calculations of 2PTPS, 3PTPS, and 4PTPS.

Density function theory (DFT) 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 2PTPS, 3PTPS, and 4PTPS

as shown in fig. S19. 2PTPS, 3PTPS, 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. Calculated HOMO, LUMO, ET, and dipole moment.

HOMO

(eV)

LUMO

(eV)

Et

(eV)

Dipole Moment

(Debye)

2PTPS -8.63 -5.02 3.06 3.30

3PTPS -8.64 -4.84 3.26 3.47

4PTPS -8.96 -4.98 3.35 4.84

HOMO: -8.63 LUMO: -5.02

µ = 3.47 DebyeLUMO: -4.84HOMO: -8.64

µ = 3.30 Debye

HOMO: -8.96 LUMO: -4.98 µ = 4.84 Debye

Si

N

N

Si

N

N

Si

N

N

2PTPS

3PTPS

4PTPS

Documents

Supplementary Materials for · 10/24/2016 · Published 28 October 2016, Sci. Adv. 2, e1601428 (2016) DOI: 10.1126/sciadv.1601428 ... CV spectra of 2PTPS, 3PTPS, and 4PTPS. fig