driving characteristics of poly(n-vinylcarbazole) and...

Electron. Mater. Lett., Vol. 9, No. 5 (2013), pp. 663-668

Driving Characteristics of Poly(N-vinylcarbazole) and 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3-4-oxadiazole-Based

Polymer Light Emitting Diodes

Dae-Yeol Lee,1 Myong-Hoon Lee,

1 Chan-Jae Lee,

2 and Sung-Kyu Park

3,*

1Graduate School of Flexible and Printable Electronics, Chonbuk National University, Jeonju, Chonbuk 561-756, Korea

2Display Components and Materials Research Center, Korea Electronics Technology Institute, Seongnam, Gyeonggi 463-816, Korea

3School of Electrical and Electronics Engineering, Chung-Ang University, Seoul 156-756, Korea

(received date: 17 March 2013 / accepted date: 11 April 2013 / published date: 10 September 2013)

We studied the driving characteristics of Poly(N-vinylcarbazole) (PVK)-based polymer light emitting diodes(PLEDs) by incorporating various 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3-4-oxadiazole (PBD) concentra-tions. PVK (Mn= 25,000~50,000 g/mol) and tris(2-(4-tolyl)phenylpyridine)iridium (Ir(mppy)3) were used asthe host and dopant materials, respectively. The transient electroluminescence (EL) of the PLED measurementwas used to analyze the characteristics of the carrier transport by synchronizing the DC-pulse generator (10 V,70 Hz) and photodiode. Among the fabricated PLEDs including those with different PBD concentrationsfrom 0% to 60%, the devices with 60% PBD concentration showed the highest efficiency (15.5 cd/A) at1000 cd/m2. By increasing the PBD concentration, a shorter rising time of the PLEDs was achieved possiblydue to improved transport behavior of the electrons by the PBD incorporation. This result is well agreementwith the increased device efficiency by increasing the PBD concentration, which shows that the transientEL measurement can be a good method for analyzing PLED performance and charge balancing.

Keywords: PLED, PVK, PBD, Ir(mppy)3, transient electroluminescence (EL)

1. INTRODUCTION

Solution-processed polymer light emitting diodes (PLEDs)

have been of great interest in displays and lighting applications

due to their simple and low cost fabrication processes.[1,2]

Although device performance has increased in PLEDs,

several crucial issues still remain such as high driving

voltage, low efficiency and short lifetime. For high performance

solution-processed PLEDs, the balance between the hole and

electron within the emitting layer (EML) has been one of the

problematic issues.[3]

In the study of phosphorescent polymer light emitting

diodes (Ph-PLEDs), the commonly used concept is to blend

low molecular weight phosphorescent dyes with proper

polymer hosts.[4-9] Generally large band gap polymers such

as PVK are used for the host material due to its high triplet

state energy over phosphorescent dyes, which confines the

triplet excited state to the Ph-PLEDs. However, PVK shows

poor electron transporting properties, so electron transporting

materials such as 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-

1,3-4-oxadiazole (PBD) have been blended with PVK to

achieve high efficiencies and low driving voltages.[7-9]

Indeed, it is expected that the control of the recombination

zone position and carrier balance in the EML can be adjusted

by changing the transfer property of holes and electrons in

devices.[10-13] In this paper, an analysis of the charge-carrier

transport in the PVK and PBD system was carried out by

using transient electroluminescence (EL) in addition to

analyzing I-V characteristics. Driving characteristics such as

the transient EL of phosphorescence-based devices have

been widely used for analyzing the emission mechanism in

the devices.[10-13] In the analysis, the time difference between

addressing a voltage input and the appearance of EL is

mainly induced by superposition of various carrier behaviors

such as charge-carrier injection, charge-carrier transport,

build-up of space charges, formation of excited state, and

radiative decay of the excited state.[11-13] Although a con-

siderable body of literature has reported on the transient EL

characteristics of Ph-PLEDs, the effect of charge balancing

in the EML by incorporating PBD has not been clearly

studied by driving characteristics of Ph-PLED systems. In

this report, we investigated the effects of PBD incorporation

in the EML of the Ph-PLED and the correlation between

enhanced device performance and transient EL characteristics.

DOI: 10.1007/s13391-013-3061-y

*Corresponding author: [email protected]©KIM and Springer

664 D.-Y. Lee et al.

Electron. Mater. Lett. Vol. 9, No. 5 (2013)

2. EXPERIMENTAL PROCEDURE

Single layer Ph-PLEDs of the structure ITO/poly(3,4-

ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/

EML/2,2',2"-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimida-

zole (TPBi)/LiF/Al were fabricated. Figure 1 shows the

device structure and a schematic energy diagram of the

fabricated device. PVK, PBD and Ir(mppy)3 were used as

the co-hosts and dopant in the EML, respectively. For the

PVK and Ir(mppy)3 EML solution, pristine PVK (Sigma-

Aldrich) solution (1 wt. % in chlorobenzene) and Ir(mppy)3

(WithEL Inc.) were mixed to formulate the EML with a ratio

of 1:0.08. To study the effects of PBD incorporation on the

electrical and optical properties of the EML, PBD (WithEL

Inc.) was incorporated into the EML solution with a con-

centration of 0, 10, 30, and 60%.

For the evaluation of the electro-optical properties of the

PBD incorporated EML system, Ph-PLEDs were fabricated

on glass substrates with a 150-nm thick patterned indium-

tin-oxide (ITO) anode (10 Ω/sq.). Prior to building the

devices, the ITO anode were cleaned by ultrasonication and

oxygen plasma. A 50-nm thick PEDOT:PSS was spun over

the anode and baked at 120°C for 20 min. Subsequently, the

EML with various concentrations of PBD was spin-coated

on the PEDOT:PSS film and baked at 120°C for 20 min.

TPBi, an electron transporting layer (ETL) (20 nm), was

deposited by thermal evaporation under the vacuum pressure

of less than 2 × 10−6 torr with an emitting area of 4 × 6 mm2.

Subsequently, LiF and Al cathode layers (0.5 nm and 120 nm)

were deposited without breaking the vacuum. After the

device fabrication, an encapsulation process was conducted

in a nitrogen-filled glove box with an oxygen/humidity level

of less than 10 ppm. The morphological and photophysical

properties of the solution-processed films were investigated

by atomic force microscopy (AFM, XE-100, Park Systems)

and photoluminescence (PL) spectroscopy (ENF-260C,

Spectroline). All the electrical and optical measurements were

carried out in a dark and ambient condition using a Keithley

2400 electrometer and Minolta LS-100/CS-1000 luminance

meter, respectively. The luminance of the device was

determined by measuring the photocurrent induced by the

light emission of the devices. Additionally, the device lifetime

was recorded starting at an initial luminance of 1000 cd/m2

by using a McScience Polaronix OLED Lifetime Test System.

The transient EL measurements were conducted by providing

a rectangular voltage pulse to the Ph-PLEDs and detecting

the signal of silicon photodiodes (FDS100, Thorlabs) from

the emitted light of the Ph-PLED. An Agilent 33120A pulse/

function generator (15 MHz) was used to generate the voltage

pulse with a frequency of 70 Hz and a duty cycle of 30%,

and the resultant photo diode was analyzed with a Tetronix™

TDS 5054 oscilloscope with 50 Ω input resistance. All the

measurements were carried out at room temperature in dark

and ambient conditions. The detailed experimental set-up for

the transient EL measurements is demonstrated as a block

diagram in Fig. 2.

Fig. 1. (a) Schematic energy profile of the multilayer device structuredesign. (b) Energy level of the materials.

Fig. 2. Schematic diagram of the experimental setup for the transientEL measurements.

D.-Y. Lee et al. 665


3. RESULTS AND DISCUSSION

In order to fabricate stable OLEDs, an organic layer must

become amorphous thin film. So, from the AFM study, the

morphology and crystallization of the EML are analyzed as

PBD concentration. The blended solution as PBD concentration

is coated on bare glass substrates. Figure 3 shows AFM

images of spun PVK films including various concentrations

of PBD over bare glass substrates. The ranges of root-mean-

square (RMS) roughness values are from 0.155 nm to

0.432 nm for the solution-processed PVK, PVK:Ir(mppy)3,

and PVK:Ir(mppy)3 with PBD. The higher roughness of

PVK:Ir(mppy)3 film may be attributed to the low solubility

of Ir(mppy)3 in common organic solvents such as toluene

and chlorobenzene, which are used for processing polymer

hosts.[14] As a result of increasing the PBD concentration, the

PBD-mixed PVK:Ir(mppy)3 films show a smooth surface

morphology while the PBD-only film demonstrates a much

higher roughness possibly due to the PBD crystallization.

Jiang et al.[15] reported that PBD, oxadiazole film, is amorphous

but has a strong tendency to be recrystallized, resulting in a

much rougher surface. To prevent the formation of a rough

crystallized surface, a guest-host system that employs

embedded oxadiazoles in a polymer matrix was suggested

to prevent the recrystallization of the PBD molecules while

avoiding aggregation of the small molecules. In these studies,

it is likely that the PVK prevents the re-arrangement of PBD

molecules possibly due to the molecular structure and molecular

weight of the polymer host, making the PBD-mixed PVK:

Ir(mppy)3 films amorphous and smooth.

To analyze the effect of emission in the PBD-mixed

PVK:Ir(mppy)3 Ph-PLED system, the PL spectra of the EML,

which has the same thickness but various concentrations of

PBD over bare glass substrates, were investigated as shown

in Fig. 4. An ultraviolet with a wavelength of 365 nm is used

as the excitation source for the photoluminescence measure-

Fig. 3. Surface morphology of spin-coated films with various concentration of PBD in PVK, (a) PVK (RMS : 0.155 nm), (b) PVK:Ir(mppy)3(RMS : 0.432 nm), (c) PVK:Ir(mppy)3:PBD 10% (RMS : 0.427 nm), (d) PVK:Ir(mppy)3:PBD 30% (RMS : 0.182 nm), (e) PVK:Ir(mppy)3:PBD60% (RMS : 0.185 nm), (f) PBD (RMS : 34.567 nm). Here, Ir(mppy)3 concentration is fixed as 8% in weight of PVK and PBD.

Fig. 4. PL spectra of PVK:Ir(mppy)3:PBD films fabricated by differ-ent PBD concentration in EML.



ments. The films exhibit the PL emissive property of Ir(mppy)3

and the peak wavelength is 512 nm with a shoulder at

approximately 540 nm. As shown in Fig. 4, PL intensity is

sharply increased by adding the PBD concentration, which

suggests that the PBD may have contributed to the efficient

energy transference between the host (PVK and PBD) and

dopant (Ir(mppy)3) materials.

We also analyzed the current-voltage-luminance (I-V-L)

characteristics and transient electroluminescence (EL) of the

Ph-PLED systems. Figure 5 shows current density (Fig.

5(a)) and luminance (Fig. 5(b)), and efficiency (Fig. 5(c)) of

devices with PBD concentrations of 0, 10, 30, and 60% as a

function of supplied voltage and current density, respectively.

The Ph-PLED system with a higher PBD concentration

typically has shown higher luminance and efficiency. Although

the operating voltages became lower while increasing the

PBD concentration, current density at the same voltage

(10 V) is lowered from 5.815 to 1.403 mA/cm2 while

increasing PBD concentration. These decreases in current

density are possibly due to poor electron transport behaviors

of PBD compared with those of the hole of PVK. In the

previous studies, it is noted that electron mobility of PBD

(2 × 10−5 cm2 · V−1· s−1) is slower than hole mobility of PVK

(7.8 - 8.2 × 10−4 cm2V−1 · s−1)[16,17] and in the case of out of

balance between holes and electrons, the excess of holes

does not contribute to light emission. Therefore, it is believed

that the devices with a small ratio or absent of PBD show

high current density but low efficiency possibly due to low

carrier balance. From these results, it is noted that the

incorporation of PBD in the EML may enhance the device

performance of the co-host system by improving electron

transport and charge balance in the EML, which is consistent

with the band-off diagram of Fig. 1 and experiemental

results of Fig. 4.

We observed the transient EL response of the Ph-PLEDs

by using a 10 V voltage pulse and 70 Hz frequency with a

30% duty cycle. The rising time was defined as the time

required to change the luminance intensity for a leading edge

from 10 to 90% of its total intensity change. We also defined

the delay time as the time difference between voltage onset

and the apperance of the EL signal. As shown in Figure 6,

Fig. 5. Comparison of device characteristics (a) Current density vs.voltage, (b) Luminance vs. voltage (c) Current efficiency vs. voltagewith PBD concentration of 0% (square), 10% (circle), 30% (up trian-gle) and 60% (down triangle).

Fig. 6. Transient electroluminescence characteristics of Ph-PLEDswith different PBD concentration in EML (rising time : 581, 330,238, 330 µs of PBD concentration 0% - 60 %).

D.-Y. Lee et al. 667


the rising time of the devices strongly depends on the PBD

concentrations in EML, whereas the delay time was not

dominated by the composition of EML. For analyzing the

transient EL characteristics, we investigated charge carrier

balance and the recombination zone (RZ) shift by the

transport behavior of the holes and electrons in EML.

Generally, PVK is known as hole dominant (p-type) material

with high hole mobility and PBD is known as electon

dominant (n-type) material with electron mobility of 2 × 10−5

cm2 · V−1 · s−1.[16,17] Considering the charge carrier transport

in the EML, the EML with a 0% PBD layer may be a hole-

dominant region that can conduct hole-only transferring and

hole accumulation at the EML/TPBi interface. In this case,

the hole-electron recombination will take place by the

arrived electrons at the EML/TPBi interface, resulting in low

current efficiency and a long rising time due to the poor

charge balance in the EML.[18] In contrast, the EML

incorporated with PBD increases electron concentration and

movement, resulting in a shift of the recombination zone

toward the anode direction, improving charge carrier balance

and matching the traveling distance of the hole and electron

in the EML. However, because the increased electron mobility

is not reached to the same level of the hole mobility of PVK,

the PVK-only system typically shows higher current density

at a specified voltage as shown in (Fig. 5(a)). Therefore, the

devices with PBD incorporation typically show improved

current efficiency and shorter rising time but lower current

density with increasing PBD concentration.

We also analyze lifetime until 80% of initial luminance

1000 cd/m2 in order to examine the introduced PBD in the

formation of the EML (see Fig. 7). With the PBD increase,

the lifetime of Device 4 is increased more than that of

Device 1-3. This result indicates that the addition of PBD

improves lifetime due to a good charge carrier balance. We

supposed that the lifetime is also related to the current

efficiency and the transient EL. Therefore, the charge carrier

balance and recombination zone shift is influenced by the

behavior between the hole and electron in the EML. In this

study, it is found that the transient EL behavior and lifetime

of PBD-mixed PVK:Ir(mppy)3 Ph-PLED system can be

explained by the difference in the electron transport ability of

the EML, which is closely related to the charge balance in

the region.[19,20] As a result, we suppose the oxadiazole

groups in the polymer will increase the electron transport

ability, improving charge balance in the EML.

4. CONCLUSIONS

By adding PBD into the EML, improved charge balance

and enlongated lifetime were obtained in the PVK-PBD-

based Ph-PLED system. Typically, the devices incorporated

with PBD show shorter transient EL time than those with no

PBD addition, which induces the faster recombination of

holes and electrons in the EML. Through the measurement

of transient EL, it is found that the overall transient EL

depends on the charge carrier balance and recombination

zone of the EML. In conclusion, it is noted that PBD, the

electron transporting material, has contributed to an increase

in the movement of electrons, resulting in a charge carrier

balance in the recombination region of the EML and

increased current efficiency.

ACKNOWLEDGEMENTS

This research was partially supported by the Chung-Ang

University Research Grants in 2011, the IT R&D Program of

MKE/KEIT (10041957, Design and Development of fiber-

based flexible display), and the Inter-ER Cooperation

Projects of MKE/KIAT.

Fig. 7. Lifetime until 80% of Ph-PLEDs with different PBD concen-tration in EML (lifetime : 0.02, 0.06, 0.12, 0.18 h of PBD concentra-tion 0% - 60%).

Table 1. Device Performance of Ph-PLEDs with different PBD concentrations in EML.

Device

PBD

(concentration

vesus PVK)

Operating

Voltage

(V)

Voltage

(V)

Current

Density

(mA/cm2)

Current

Efficiency

(cd/A)

Power

Efficiency

(lm/W)

Lifetime (h)

(until 80%)

Rising Time

(µs)

1 - 6.5 11.7 33.8 3.0 0.8 0.02 581

2 10% 6.0 11.3 17.3 5.7 1.5 0.06 330

3 30% 5.8 11.0 8.2 12.1 3.4 0.12 238

4 60% 6.4 11.3 6.3 15.5 4.3 0.18 330



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