Synthesis and characterization of spin-coatable tert-amine molecules for hole-transport in organic light-emitting diodes

Article abstract of DOI:10.1016/j.tetlet.2006.04.117

The synthesis of two tert-amine-based, non-fluorescent, hole-transport molecules (4,4′-[bis-{(4-di-n-hexylamino)benzylideneamino)]stilbene (DHABS) and 4,4′-[bis-{(4-diphenylamino)benzylideneamino}]stilbene) (DPABS) that are suitable for spin coating on in

Full text of DOI:10.1016/j.tetlet.2006.04.117

Tetrahedron Letters 47 (2006) 4715–4719
Synthesis and characterization of spin-coatable
tert-amine molecules for hole-transport
in organic light-emitting diodes
A. Mishra, P. K. Nayak, D. Ray, M. P. Patankar, K. L. Narasimhan and N. Periasamy*
*
Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400 005, India
Received 13 December 2005; revised 18 April 2006; accepted 26 April 2006
Available online 19 May 2006
0
Abstract—The synthesis of two tert-amine-based, non-fluorescent, hole-transport molecules (4,4 -[bis-{(4-di-n-hexylamino)benzyl-
0
ideneamino)]stilbene (DHABS) and 4,4 -[bis-{(4-diphenylamino)benzylideneamino}]stilbene) (DPABS) that are suitable for spin
coating on indium tin oxide (ITO) for electronic device fabrication is described and compared with the widely used TPD. Electro-
luminescence occurred at a turn-on voltage of 7–8 V in ITO/hole-transport layer (HTL, spin coated)/Alq
2006 Elsevier Ltd. All rights reserved.
3
/Al devices.
ꢀ
Thin films of organic molecules and polymers can serve
as semiconductors and are used in new generations of
electronic devices such as field effect transistors and
LEDs. Several tert-amine-based small molecules and
polymers have been investigated for applications such
as xerography and organic light-emitting diodes
ine.7,8 These high-mass molecules were not sublimable
but were soluble in organic solvents such as xylene.
Organic devices can be made with these molecules only
by using the technique of spin coating. Hole-transport
molecules specifically suitable for spin coating have been
reported, but are not yet commercially available. In this
letter, we describe two hole-transport molecules that are
suitable for spin coating for organic electronics.
1
–3
9
4
,5
(
OLEDs).
For OLED devices, a few ‘tert-amine’
0 0
(N,N -diphenyl-N,N -bis(3-methylphenyl)-
molecules
(
0
0
0
1,1 -biphenyl)-4, 4 -diamine (TPD) and N,N -diphen-
yl-N,N -bis(1-naphthyl)-(1,1 -biphenyl)-4,4 -diamine
NPB)) were found to be suitable for hole injection at
0
0
0
N,N-Di-n-hexylaniline 1 prepared by reaction of aniline
with 1-hexyl bromide, was formylated via a standard
Vilsmeier reaction using DMF and phosphoryl chloride
to give the aldehyde 2 as a colourless viscous liquid.
4-(Diphenylamino)benzaldehyde 3 was prepared via
formylation of triphenylamine as a yellow solid in 72%
(
the indium tin oxide electrode and efficient hole-trans-
port with a field dependent hole mobility of 1–2 ·
ꢀ
3
2
6
1
0
cm /V s, and hence are widely used. The success
stories of a few molecules have spurred synthetic activity
in designing molecules which mimic the chemical
structure of well-known molecules such as TPD and
NPB, whilst expecting enhancement in desirable
physical properties. TPD and NPB are sublimable and
readily used to make devices by vapour deposition in
conjunction with other small molecules such as,
for example, tris(8-hydroxyquinolinato)aluminium(III)
1
0
1
yield. The compounds were characterized by
H
1
3
11
NMR, C NMR, ESI-MS, and elemental analysis.
Condensation of aldehydes 2 and 3 with 4,4 -diamino-
stilbene dihydrochloride afforded 4,4 -[bis-{(4-di-n-hexyl-
amino)benzylideneamino)]stilbene (DHABS) and 4,4 -
0
0
0
[bis-{(4-diphenylamino)benzylideneamino}]stilbene
1
2
(DPABS) as yellow crystalline solids (Scheme 1).
(
Alq ). Recently, we synthesized a series of derivatives
3
of Alq , which were based on modified 8-hydroxyquinol-
Thin films of DHABS and DPABS on glass and UV-
treated ITO substrates were prepared by spin coating
using, after filtering through a 0.2 lm filter, solutions
of 20 mg/mL of DHABS in xylene or 3.3 mg/mL of
DPABS in chloroform, at 3000 rpm under a nitrogen
atmosphere. The film thickness was directly determined
by atomic force microscopy (AFM) after making a
3
Keywords: tert-Amine; OLED; Hole-transport layer (HTL); Electro-
luminescence; Spin coating.
*
Corresponding authors. Fax: +91 22 22804610; e-mail addresses:
peri@tifr.res.in; amaresh.mishra@rediffmail.com
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.04.117

4716
A. Mishra et al. / Tetrahedron Letters 47 (2006) 4715–4719
layers was monitored using a quartz crystal balance.
The LiF layer was used to facilitate electron injection
from the aluminium cathode. Some devices were made
with PEDOT:PSS (Aldrich,USA) spin coated on ITO
prior to spin coating the hole-transport layer. The typi-
N
N
N
N
DHABS
2
cal device area was 1 mm . Current–Voltage–Light (I–
V–L) characteristics were measured in vacuum. Light
measurements were made using a large area calibrated
silicon photodetector placed in close proximity at a fixed
geometry to the sample to collect the light through the
substrate. The photodetector/geometry arrangement
was calibrated to measure light emission from the device
in Cd/m . The thickness of the spin coated layers was
measured using both optical absorption and capacitance
measurements on the actual devices.
POCl /DMF
3
N
EtOH,
O
N
0 oC, 3 h
reflux, 3 h
8
2
1
2
HCl H N
2
NH2 HCl
POCl /DMF
EtOH,
reflux, 4 h
Figure 1 shows the absorption spectra of DHABS and
DPABS in chloroform with kmax at 410 (DHABS) and
3
N
N
0 oC, 3 h
O
8
4
08 nm (DPABS). The spectra in thin film are similar,
but the kmax is blue shifted by 19 nm (DHABS) and
2 nm (DPABS), respectively. The 0–0 transition corre-
3
1
sponds to the crossing of the normalized absorption and
emission spectra, which is 436 nm (2.84 eV) for DHABS
and 439 nm (2.82 eV) for DPABS. Absorption spectra
N
N
N
N
(transition energies) of DHABS and DPABS were also
calculated by the quantum chemical method (MNDO/
D) using HYPERCHEM software for a singly excited
state up to 12 eV. The singlet excited state was calcu-
lated to be at 3.07 eV (401.5 nm) (DHABS) and
DPABS
Scheme 1. Synthesis of DHABS and DPABS.
3
.03 eV (406.9 nm) (DPABS), which did not match with
scratch mark using a sharp blade which exposed the
substrate. Using the absorbance of the film at kmax
the absorption coefficient was determined to be
the 0–0 transition. For comparison, the values for TPD
were also determined by the same theoretical method
which placed the singlet excited state at 3.2 eV
(385.5 nm) which matched well with the 0–0 transition
at 3.2 eV. We used the experimental values of the 0–0
transition energy for the placement of the lowest unoc-
cupied molecular orbital (LUMO) level (see below) of
DHABS and DPABS.
,
5
ꢀ1
5
1
.56 · 10 cm at 391 nm for DHABS and 1.24 · 10
ꢀ
1
cm at 398 nm for DPABS. In a separate experiment,
a film of thickness (t in A) on the substrate of area (A
in cm ) was redissolved in chloroform (v in mL) and
˚
2
the absorbance (OD) of the solution was measured at kmax
.
The densities of the solid films of DHABS and DPABS
were determined using the formula, d (g/mL) =
DHABS and DPABS are weakly fluorescent. The fluo-
rescence quantum yields of DHABS and DPABS in
chloroform were determined to be low with respect to
5
1
0 ODvM/eLAt, where M is the molecular mass, e is
ꢀ
1
ꢀ1
the molar extinction coefficient (M cm ) at kmax, L
cm) is the optical path length in the absorption mea-
(
4
ꢀ1
ꢀ1
surement. The value of e was 9.7 · 10 M cm
at
4
4
10 nm for DHABS in chloroform and 5.6 · 10
ꢀ
1
ꢀ1
M
cm at 408 nm in chloroform for DPABS. The
calculated density was 1.04 g/cc for DHABS and
.42 g/cc for DPABS. The lower density is expected
1
5
for DHABS because of the presence of an alkyl chain
in the structure. The thickness of the spin coated layers
of DHABS and DPABS on ITO was determined by
capacitance measurements of ITO/DHABS (or
DPABS)/Al devices, assuming that the dielectric con-
stant of the organic materials was similar. The thickness
measurement by capacitance was in agreement with the
optical absorbance.
1
4
0
.5
OLED devices were made by spin coating DHABS (or
DPABS) on a pre-cleaned, pre-patterned ITO substrate
0
350
450
550
650
˚
˚
followed by vacuum deposition of 600 A Alq , 10 A LiF
3
wavelength/nm
and an aluminium top contact through a metal mask.
The depositions were carried out sequentially without
breaking the vacuum. The thickness of the deposited
Figure 1. Absorption and emission spectra of DHABS (solid line) and
DPABS (dashed line) in chloroform.

A. Mishra et al. / Tetrahedron Letters 47 (2006) 4715–4719
4717
TPD in the same solvent. The ratio of the quantum
yields is equal to the ratio of the area of the fluorescence
spectra, corrected for the wavelength sensitivity of the
photomultiplier, for samples of equal absorbance at
the excitation wavelength. The quantum yield ratio
was 0.036 for DHABS and 0.063 for DPABS, with
respect to TPD.
are used in the OLED structure. The HOMO level of
TPD is at ꢀ5.4 eV, which means there is a barrier
(0.6 eV) for hole injection from ITO. The HOMO levels
of DHABS and DPABS indicate that the barrier is more
(0.8 eV) and less (0.4 eV) by 0.2 eV for these molecules
compared to TPD. OLED structures were made using
Alq as the electron transport layer (ETL). The existence
3
of barriers for electrons and holes at the organic–organ-
ic interface has been shown to result in increasing of the
efficiency of OLED devices. This condition is satisfied
The highest occupied molecular orbital (HOMO) levels
of TPD, DHABS and DPABS were determined using
cyclic voltammetry as follows. Current–voltage curves
using Pt wire electrodes were obtained from solutions
of the amine (TPD, DHABS or DPABS) and chloranil
in dichloromethane with tetra-n-butylammonium
fluoroborate (0.1 M) as the conducting salt. A complete
cycle of potential sweep generates the reduction
1
5
for TPD, DHABS or DPABS as HTL and Alq as ETL.
3
The HOMO and LUMO levels of Alq are at ꢀ5.8 eV
3
and ꢀ3.1 eV (see Fig. 2), which constitute an effective
electron barrier from Alq and hole barrier from the
3
amine at the interface. Thus, based on energy level con-
siderations, DHABS and DPABS, are expected to be
suitable replacements for TPD.
ꢀ
+
(
M ! M ) peak of chloranil and oxidation (R ! R )
peak of the amine. The oxidation peaks of TPD,
DHABS and DPABS were determined to be +0.70 V
The uniformity of the films of DHABS and DPABS on
ITO was checked by AFM. The surface roughness (rms
value) was estimated to be 4.7 nm (DHABS) and 1 nm
(DPABS).
(
TPD), +0.90 V (DHABS) and +0.54 V (DPABS) with
respect to the reduction peak of chloranil. The reduction
peak of chloranil with respect to the normal hydrogen
electrode (NHE) occurs at +0.17 V. Thus, the oxidation
potentials of the amines with respect to the NHE are
Four types of OLED devices were prepared, with and
without PEDOT:PSS as a contact layer on ITO. These
+
0.87 V (TPD), +1.07 V (DHABS) and +0.71 V
(
DPABS). The NHE level with respect to a vacuum is
Thus, the HOMO levels
i.e., the hole-transport levels) of the amines are located
were ITO/DHABS/Alq /Al, ITO/PEDOT/DHABS/
3
1
,13
approximately ꢀ4.5 eV.
Alq /Al, ITO/DPABS/Alq /Al, and ITO/PEDOT/
3
3
(
DPABS/Alq /Al. The thickness of the amine and Alq
3
3
˚
˚
at ꢀ5.37 eV (TPD), ꢀ5.57 eV (DHABS) and ꢀ5.21 eV
layers were, 480 A (DHABS) and 280 A (DPABS) and
˚
(
DPABS). The LUMO levels (electron transport) are
600–700 A (Alq ), respectively. The spin coating and
3
above the HOMO levels by the band gap energies which
are the 0–0 transition energies determined from the
absorption and emission spectra as above, which are
other details (evaporation of Alq film and metal film,
3
encapsulation) are described above. The devices without
PEDOT films gave better and more stable performance
(current–voltage curves) than the devices containing
PEDOT films. The results given below are for the
devices without PEDOT films.
3
.2 eV (TPD), 2.84 eV (DHABS) and 2.82 eV (DPABS).
Thus the LUMO levels are at ꢀ2.17 eV (TPD),
ꢀ
2.73 eV (DHABS) and ꢀ2.39 eV (DPABS). It is to
be noted that the HOMO–LUMO levels determined
above are consistent with literature values for TPD
Figure 3 shows current as a function of applied bias
voltage for the two devices. The figure shows that the
current for the DPABS device is larger than that for
DHABS. Figure 3 (middle panel) shows the EL–voltage
characteristics for the same two devices. It is seen that
the DPABS device exhibits better EL–voltage character-
istics than the DHABS device. The voltage that marks
the onset of EL for the DPABS device is lower (ꢁ7 V)
than that for the DHABS device (ꢁ8 V). Figure 3 (bot-
tom panel) shows the EL versus current (L–I) for the
two devices. From the figure, the slope of the L–I curve
is similar for the two devices at low current. Above
1
4
within ± 0.2 eV. Thus, the values for DHABS and
DPABS are expected to have the same uncertainty as
that of TPD.
Figure 2 shows the energy levels of TPD, DHABS and
DPABS together with those of ITO, Alq and Al which
3
-2.2
-2.4
-2.7
20 lA, the DPABS device is much better.
-3.1
-
4.2
As can be seen in Figure 2, the offset between the
HOMO and the Fermi level of the ITO is smaller for
DPABS (0.4 eV) than for DHABS (0.8 eV). The smaller
barrier for hole injection should result in increased hole
injection efficiency for the DPABS device. In fact, this
barrier is even smaller than that for TPD (0.6 eV). We
also see from the figure that the offset between the
-
4.8
Al
ITO
-5.2
DPABS
-5.4
-5.6
-
5.8
TPD
DHABS
Alq3
HOMO of Alq and the HOMO of DPABS (0.6 eV) is
larger than that for DHABS (0.2 eV). A large off-
3
Figure 2. The Fermi level of ITO and aluminium electrodes and the
HOMO–LUMO levels of the hole-transport tert-amine molecules
(
TPD, DHABS and DPABS) and the electron transport and lumines-
cent molecule, Alq . The energy levels are given in eV with respect to
the vacuum level as zero.
set allows for holes to accumulate at the DPABS/Alq3
interface and promote electron injection from the cath-
ode. Both these features in the electronic energy levels
3

4718
A. Mishra et al. / Tetrahedron Letters 47 (2006) 4715–4719
1
1
1
.E-04
.E-06
.E-08
5
4
1
1
.E-10
.E-12
0
5
10
15
20
voltage (V)
8
6
.E+01
.E+01
5
5
4
4.E+01
5
10
15
4
2
0
.E+01
.E+00
0
5
10
15
20
voltage (V)
8
6
.E+01
.E+01
5
4
2
0
.E+01
.E+01
.E+00
4
0.E+00
2.E-05
4.E-05
6.E-05
Current (A)
Figure 3. Current–voltage (top panel), EL intensity–voltage (middle) and EL–current (bottom) data for the OLED devices, ITO/DHABS (m)/Alq
3
/
Al and ITO/DPABS (j)/Alq /Al. Inset in the middle panel shows EL versus voltage in log-linear scale, which shows the turn-on voltage at 8 and 7 V
3
for DHABS and DPABS, respectively.
of DPABS vis-a-vis DHABS result in a lower turn-on
voltage for the DPABS device and a larger forward bias
current at a given voltage bias. It is interesting that the
L–I characteristic is similar for the two devices at low
current. At larger current (or voltage), the smaller L–I
slope for DHABS suggests that the hole current leaks
voltage of 7–8 V. The electroluminescence of the devices
using spin-coated DHABS or DPABS as hole-transport-
ers is weak compared to the standard devices using vac-
uum evaporated TPD as the hole-transporter. Spin
coating and further processing of EL devices are still
underway to improve the EL and current efficiency of
these new compounds.
into the Alq layer due to the low barrier of 0.2 eV
3
and dominates the I–V characteristic. Thus, the L–I–V
characteristics can be satisfactorily explained using the
electronic levels of the molecules and the electrodes in
OLED.
References and notes
1
. Pope, M.; Swenberg, C. E. Electronic Processes in Organic
Crystals and Polymers; Oxford University Press, 1999.
. Kelley, T. W.; Baude, P. F.; Gerlach, C.; Ender, D. E.;
Muyres, D.; Haase, M. A.; Vogel, D. E.; Theiss, S. D.
Chem. Mater. 2004, 16, 4413–4422.
. Adachi, C.; Tsutsui, T. In Organic Light Emitting
Devices—A Survey; Shinar, J., Ed.; Springer, 2002;
Chapter 2; pp 43–69.
In conclusion, two spin-coatable hole-transport mole-
cules (DHABS and DPABS) were synthesized which
have HOMO levels comparable to the widely used
TPD. OLED devices were made using a spin coated
hole-transport layer and a vacuum evaporated electron
2
3
transport layer (Alq ) which showed EL at a turn-on
3

A. Mishra et al. / Tetrahedron Letters 47 (2006) 4715–4719
4719
0
4
5
. Tsutsui, T. MRS Bull. 1997, 22, 39–45.
. Liu, S.; Jiang, X.; Ma, H.; Liu, M. S.; Jen, A. K. Y.
Macromolecules 2000, 33, 3514–3517.
12. General procedure for the synthesis of 4,4 -[bis-{(4-di-n-
hexylamino)benzylideneamino}]stilbene (DHABS): To a
0
solution of 4,4 -diaminostilbene dihydrochloride (200 mg,
6
7
8
9
. Stolka, M.; Yanus, J. F.; Pai, D. M. J. Phys. Chem. 1984,
3
0.7 mmol) in ethanol (10 mL) were added Et N (85.6 mg,
8
8, 4707–4714.
0.85 mmol) and 2 (430 mg, 1.49 mmol). The resulting
mixture was stirred at reflux for 4 h. After cooling in an ice
bath, the yellow precipitate formed was filtered and
recrystallized from ethanol to afford DHABS in 92%
. Mishra, A.; Periasamy, N.; Patankar, M. P.; Narasimhan,
K. L. Dyes Pigm. 2005, 66, 89–97.
. Mishra, A.; Nayak, P. K.; Periasamy, N. Tetrahedron
Lett. 2004, 45, 6265–6268.
1
yield, as a yellow solid: 489 mg; mp 160 ꢁC. H NMR
. Shirota, Y. J. Mater. Chem. 2000, 10, 1–25.
(500 MHz, CDCl ) d 0.91 (12H, t, J = 6.5 Hz, CH ), 1.31–
3
3
1
0. Baker, T. N.; Doherty, W. P.; Kelley, W. S.; Newmeyer,
W.; Rogers, J. E.; Spalding, R. E. R.; Walter, I. J. Org.
Chem. 1965, 30, 3714–3718.
1.40 (24H, m, CH
t, J = 7.5 Hz, CH ), 6.66 (4H, d, J = 8.5 Hz, ArH), 7.09
2
(2H, s, CH@CH), 7.19 (4H, d, J = 8 Hz, ArH), 7.52
(4H, d, J = 8.5 Hz, ArH), 7.73 (4H, d, J = 8.5 Hz), 8.33
2
), 1.51–1.70 (8H, m, CH
2
), 3.33 (8H,
1
1. N,N-Di-n-hexylaniline 1 was synthesized in 75% as a
1
3
+
colourless viscous liquid; C NMR (125 MHz, CDCl
3
) d
(2H, s, CH@N): ESI-MS m/z 753.54 (M+H) , (Calcd for
1
5
4.20 (CH
1.38 (NCH
3
), 22.80 (CH
3
CH
2
), 27.35, 28.20, 31.46 (CH ),
2
C
52
H
72
N
4
; 752.58); Anal Calcd for C52
H
72
N
4
: C, 82.93; H,
1
0
2
), 114.29, 118.32, 129.70, 149.53 (CAr);
H
9.64; N, 7.44; found: C, 83.05; H, 9.62; N, 7.58. 4,4 -[bis-
{(4-diphenylamino)benzylideneamino}]stilbene (DPABS)
NMR (500 MHz, CDCl ) d 0.94 (6H, t, J = 6.4 Hz, CH ),
3
3
1
.25–1.34 (12H, m, CH
2
), 1.54 (4H, m, CH
2
) 3.33 (4H, t,
was obtained in 92% yield as a yellow solid: mp 162 ꢁC.
1
J = 6.5 Hz, CH
2
), 6.60 (3H, m, ArH), 7.07 (2H, t,
3
H NMR (500 MHz, CDCl ) d 6.67 (4H, d, J = 8.5 Hz,
J = 8.5 Hz, ArH). Synthetic procedure for 4-(N,N-n-
dihexylamino)benzaldehyde 2: To a cooled (5 ꢁC) solution
of freshly distilled anhydrous DMF (20 g), was added
ArH), 6.84 (2H, s, CH@CH), 7.08–7.35 (24H, m, ArH),
7.33 (4H, d, J = 8.5 Hz, ArH), 7.74 (4H, d, J = 8.5 Hz),
+
8.40 (2H, s, CH@N): ESI-MS m/z 721.3 (M+H) , (Calcd
POCl
3
(615 mg, 4 mmol) within 5 min. The mixture was
52 40 4 40 4
C H N ; 720.33); Anal Calcd for C52H N : C, 86.64; H,
stirred for 30 min, then 1 (1 g, 3.8 mmol) was added and
the resulting mixture was heated for 3 h at 80 ꢁC. The
mixture was hydrolyzed by slow addition of ice-cold water
and then neutralized with 5 M NaOH. The product was
extracted with diethyl ether and washed with water and
5.59; N, 7.77; found: C, 86.73; H, 5.54; N, 7.66.
13. This value of ꢁ4.5 eV can be obtained using the thermo-
dynamic data:
4
dried over MgSO . After evaporation of the solvent in
½
H
2
(g) → H(g),
ΔH = 2.25 eV,
vacuo, the product was purified by column chromatogra-
phy eluting with hexane–ethyl acetate (95:5) to afford the
+
H(g)
→ H (g) + e(g), ΔH = 13.6 eV,
+
+
1
3
H (g) → H (aq),
ΔH = –11.3 eV,
aldehyde 2 in 80% yield, as a viscous liquid: 885 mg;
NMR (125 MHz, CDCl ) d 14.14 (CH ), 22.80, 27.13,
), 51.39 (NCH ), 114.79, 126.40, 130.78,
55.3 (CAr), 190.07 (CHO); H NMR (500 MHz, CDCl ) d
C
+
2
which give, ½H (g)
→
H (aq) + e(g), ΔH = 4.55 eV.
3
3
2
1
0
1
8.10, 31.50 (CH
2
2
1
3
.90 (6H, t, J = 6.43 Hz, CH
.54 (4H, m, CH ), 3.34 (4H, t, J = 6.4 Hz, CH
3
), 1.24–1.38 (12H, m, CH
2
),
14. Shinar, J.; Savvateev, V. In Organic Light Emitting
Devices; Shinar, J., Ed.; Springer: New York, 2004; p 15.
15. Brown, A. R.; Bradley, D. D. C.; Burroughes, J. H.;
Friend, R. H.; Greenham, N. C.; Burn, P. L.; Holmes, A.
B.; Kraft, A. Appl. Phys. Lett. 1992, 61, 2793–2795.
2
2
), 6.76
(
(
(
2H, d, J = 9 Hz, ArH) 7.62 (2H, d, J = 9 Hz, ArH), 9.85
+
1H, s, CHO); ESI-MS m/z 290.3 (M+H) , 261.4
+
M ꢀCO), (Calcd for C19
H31NO; 289.24).