Highly Promoting the Performances of Polymer Light-Emitting Diodes

Letter

Highly promoting the performances of polymer light-emitting diodes via controlling the residue of polar solvent on emissive layer Qiaoli Niu, Hengsheng Wu, Wentao Huang, Jing Tong, Fengye Ye, Yong Zhang, Wenjin Zeng, Ruidong Xia, and Yonggang Min ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 18 May 2017 Downloaded from http://pubs.acs.org on May 18, 2017

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ACS Applied Materials & Interfaces

Highly promoting the performances of polymer light-emitting diodes via controlling the residue of polar solvent on emissive layer Qiaoli niu,a Hengsheng Wu,a Wentao Huang,a Jing Tong,a Fengye Ye,b Yong Zhang,c Wenjin Zeng,a Ruidong Xia, a* Yonggang Min a* a. Key Laboratory for Organic Electronics and Information Displays & Institute of Advanced Materials, Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts and Telecommunications, 9 Wenyuan Road, Nanjing 210023, PR China b. I-Lab, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, Jiangsu 215123, China c. School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China

ACS Paragon Plus Environment


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ABSTRACT ： Polar solvent dimethyl formamide (DMF) was used to treat the emissive layer (EML) of polymer light-emitting diode (PLED). The formation of dipole layer at the EML/cathode interface after DMF treatment was proved, which led to a reduction of electron injection barrier. The dipole layer was formed mainly because of the intrinsic polarity of DMF. By controlling the residue of DMF on the EML, a maximum enhancement of peak luminous efficiency from 5.33±0.57 to 12.05±1.2 cd/A was achieved. This study suggests that solvent treatment is a simple and efficient approach to realize highly efficient PLEDs with high-work-function metal cathode.

KEYWORDS: solvent treatment, DMF, polymer light-emitting diodes, cathode modification, polar solvent


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Polymer light-emitting diodes (PLEDs) are “dual-injection” devices, with electron injected from the cathode and hole from the anode, respectively. 1-3 Therefore, ohmic contact is required at both the anode and cathode in order to realize effective carrier injection, therefore highly efficient electroluminescence.3,

4

To match the lowest

unoccupied molecular orbital (LUMO) of polymer emissive layer (EML), low-work-function metal or alkali or alkaline earth metals halides has to be used.5, 6 However, the above materials are all very sensitive to oxygen or water, leading to the quick degradation of devices. Thus, numerous research has been focused on exploiting electron injection materials during the past thirty years.7-14 Various materials have been used as electron injection layer, such as conjugated polyelectrolyte,7-9 ion liquid molecules,10 amine based molecular or polymer11, 12 and surfactant.13, 14 But, the preparation of electron injection materials requires expensive and complex synthesis processing. Recently, it has been found that commonly used alcohol solvents treatment to the EML can enhance the electron injection of PLEDs.15-18 For example, a 58 % enhancement of luminous efficiency (LE) was achieved by treating the EML poly(2-(4-(3,7,-dimethyloctyloxy)-phenyl)-p-phenylenevinylene)

(P-PPV)

using

ethanol.17 2,2,3,3,4,4,5,5-octafluoro-1-pentanol (F-alcohol) treatment to the EML super-yellow PPV (SY-PPV) led to an enhancement of the peak LE of PLEDs from 14 to 20.9 cd/A.18 The solvent treatment method was also used on other optoelectronic devices, such as polymer solar cell,19-21 and great performance enhancement has been achieved. Compared with the deposition of the electron



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injection materials, the solvent treatment method is not only effective but also easy-to-get. In addition, its wet-process technique is compatible with the fabricating technique of flexible device. However, to the best of our knowledge, only alcohol solvents were involved to perform solvent treatment to the EML of PLEDs. Besides, whilst performance enhancement has been demonstrated by solvent treatment, the control of residue of solvent was rarely addressed, which is crucial to the device performances. In this work, polar solvent dimethyl formamide (DMF) was used to treat the EML of PLED. Electron current and built-in potential data of PLEDs confirmed that electron injection was enhanced after solvent treatment due to the lowered electron injection barrier, leading to a maximum enhancement of peak luminous efficiency (LE) from 5.33±0.57 to 12.05±1.2 cd/A. AFM images, Ultraviolet–visible (UV-Vis) spectra and electroluminescence (EL) spectra data showed that solvent treatment did not change the optoelectronic properties of the EML. Detailed studies by employing Kelvin probe force microscope (KPFM) and Ultraviolet photoelectron spectroscopy (UPS) were carried out to understand the improvement of the electron injection. The PLED performance as a function of the nominal content of DMF on the EML was also investigated, which showed high nominal content of DMF could lead to the drop of efficiency due to the insulating character of DMF. Therefore, the residue of DMF has to be carefully controlled for optimized device performance.


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OC10H21 OC10H21

OC10H21

x

C10H21O

y z

x=0.04mol% y=0.48mol%

z=0.48mol%

Figure 1. The molecular structure of P-PPV.

PLED

with

the

device

configuration

of

indium

tin

oxide

(ITO)/

poly(ethylendioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) (30 nm) / P-PPV (67 nm)/LiF (1 nm)/Al (150 nm) was fabricated. P-PPV was synthesized via conventional Suzuki coupling method. 22 Its chemical structure was depicted in Figure 1. Solvent treatment was carried out by spin-coating solvent on top of P-PPV at 2000 rpm for 60 s. In order to control the residue of DMF on P-PPV, DMF was doped into methanol with different volume ratios, which is referred to as the nominal content of DMF. Though, the residual volume of DMF on P-PPV does not correspond to the nominal content of DMF. It is believed that decreasing the nominal content of DMF will reduce the residue of DMF on P-PPV. The device fabrication process and measurement details can be found in supporting information. Figure 2 shows the current density - voltage - luminance (J-V-L) and luminous efficiency - current density (LE-J) characteristics of PLEDs with EML treated by DMF, methanol or mixed solvents of methanol doped with 5 vol.%, 10 vol.%, 20 vol.% or 40 vol.% DMF, as well as the control device without solvent treatment. Detailed performance parameters of PLEDs are summarized in Table 1. The control



device, without solvent treatment, had a maximum external quantum efficiency (EQE) of 2.14%, LE of 5.33±0.57 cd/A and brightness of 20723 cd/m2, which increased to 2.49%, 6.22±0.95 cd/A and 25812 cd/m2 after methanol treatment and 3.59%, 8.97±0.73 cd/A and 23812 cd/m2 with DMF treatment. That is to say, the peak LE increased by 17% and 68% after methanol or DMF treatment, indicating that DMF treatment is more effective. For mixed solvents treatment, we can see from Figure 2 and Table 1 that the addition of only 5 vol.% DMF in methanol greatly increased the peak EQE, LE and brightness to 4.11%, 10.29 ± 0.5 cd/A and 30382 cd/m2, respectively, which were much higher than that of the pure DMF treated device. The optimal peak LE of 12.05±1.2 cd/A was obtained by methanol: 20 vol.% DMF treatment, which was only slightly higher than the 11.97±1.1 cd/A of methanol: 10 vol.% DMF treated device, showing that the PLED performance could not be further improved with increasing DMF ratio in methanol from 10 vol.% to 20 vol.%. For the case of methanol: 40 vol.% DMF treatment, the peak LE and brightness decreased to 10.47±1.1 cd/A and 29197 cd/m2, respectively, but still much higher than that of the pure DMF treated device. (a)

4

Untreated Methanol Methanol:5%DMF Methanol:10%DMF Methanol:20%DMF Methanol:40%DMF DMF

2

Current Density (mA/cm )

10

1000 100 10 1

5 10

4

4 10

4

3 10

4

2 10

4

1 10

4

0.1

2

0.01

Luminescence (cd/m )

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0.001

0 0

2

4

6

8

Voltage(V)


10

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(b)

Luminous Efficiency (cd/A)

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14 12 10 8 6 4 0

50

100

150

200

250

300 2


Figure 2. the current density-voltage -luminescence (J-V-L) (a) and

luminous

efficiency - current density (LE-J) (b) characteristics of PLEDs with EML treated by DMF, methanol or methanol doped with 5 vol.%, 10 vol.%, 20 vol.% and 40 vol.% DMF, as well as the control device without solvent treatment.

Table 1. Detailed performance parameters of PLEDs. Solvent treatment to the EML

Vtha (V)

Peak EQEb (%)

Peak LEc (cd/A)

Peak LE

Peak Luminance

@V

@J

@L

(cd/m2)

(V)

(mA/cm2)

(cd/m2)

Vbid (V)

Untreated

3.17

2.14 5.33±0.57

20723

5.2

5.07

270.2

1.23±0.01

Methanol

2.74

2.49 6.22±0.95

25812

6.36

29.29

1822.8

1.32±0.03

Methanol: 5vol.% DMF

2.6

4.11 10.29±0.5

30382

5.2

17.77

1828.6

1.45±0.01

Methanol: 10vol.%DMF

2.6

4.78 11.97±1.1

37464

4.9

9.61

1150.5

1.66±0.01


2.6

4.82 12.05±1.2

33611

5.2

13.91

1675.4

1.6±0.02


2.6

4.19 10.47±1.1

29197

5.6

24.13

2526.7

1.54±0.01

DMF

2.6

3.59 8.97±0.73

23812

4.9

6.20

556.27

1.48±0.03

a

Turn-on voltage, defined as the voltage with a luminescence of about 1 cd/m2;bExternal

quantum efficiency; cLuminous efficiency; dBuilt-in potential;



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We note that the current density of PLEDs under the same voltage increased after solvent treatment, as shown in Figure 2. It is likely that the enhanced current was originated from the increase of electron current after the modification of cathode interface. The electron current densities of PLEDs were detected by fabricating electron-only device with a structure of ITO/ P-PPV (67 nm)/LiF (1 nm)/Al (150 nm). Figure 3(a) shows the J-V characteristics of electron-only devices without or with solvent treatment by methanol: x vol.% DMF (x=0, 5, 10, 20, 40). J-V curves including other kinds of DMF concentrations, for example, 1 vol.%, 60 vol.% and 80 vol.%, were depicted in Figure S1. Under the same operation voltage, all solvent treated devices exhibit higher current density than the control device. Among them, the smallest and largest current density under the same bias resulted from pure methanol and methanol: 10 vol.% DMF treatment, respectively. There is only slight difference between the methanol: 10 vol.% DMF (88mA/cm2@12 V) and methanol: 20 vol.% DMF (85mA/cm2@12 V) treated device. The results indicated that the electron injection was improved by solvent treatment, resulting in more balanced electron and hole charge densities inside the EML. As a result, the performances of PLEDs were improved significantly. For PLEDs in this report, electron is the minority charge carrier. The improvement of electron injection will reduce the turn-on voltage (Vth) of PLEDs. From Figure 2 and Table 1, we can see that the Vth decreased from 3.17 V of the control device to 2.74 V and 2.6 V for the methanol treated and DMF treated device, respectively. The devices treated by methanol doped with DMF have


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similar Vth to pure DMF treated device because of the slight electron current difference between them, as shown in Figure 3(a). (a)

(b) 180 160 140 120

2

200 Untreated Methanol Methanol:5 Vol.%DMF Methanol:10 Vol.%DMF Methanol:20 Vol.%DMF Methanol:40 Vol.%DMF

100 80 60 40 11.5


2


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12

12.5

13

10

10

-6


-7

0

0.5

Voltage(V)

1

1.5

2

Voltage (V)

Figure 3. J-V curves of electron-only devices (a) and PLEDs under white-light illumination (100mWcm-2) (b), with EML treated by DMF, methanol or methanol doped with 5 vol.%, 10 vol.%, 20 vol.% and 40 vol.% DMF, as well as the control device without solvent treatment.

The injection of electron of PLEDs can be improved by reducing the electron injection barrier 3, which is directly related to the built-in electric field of PLEDs 23-26. Thus, the change of electron injection barrier can be determined by the variation of built-in potential (Vbi) when the anode and the organic layers are identical for all devices. The Vbi data were tested by measuring the J-V characteristics of the PLEDs with or without solvent treatment under white-light illumination (100mWcm-2), as demonstrated in Figure 3(b). The detailed values of Vbi are summarized in Table 1. Compared with the control device, methanol or DMF treatment increased the Vbi of PLEDs from 1.23±0.01 V to 1.32±0.03 V and 1.48±0.03 V, respectively. The Vbi further increased with methanol: DMF treatment. The maximum value of 1.66±0.01



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V was obtained with methanol: 10 vol.% DMF treatment, which was slight higher than the 1.6±0.02 V of methanol: 20 vol.% DMF treated device. The significant increase of Vbi (0.09 V from methanol and 0.43 V from methanol: 10 vol.% DMF) clearly confirms that the electron injection barrier was lowered.19 It facilitated the injection of electron, led to the decrease of Vth of PLEDs. As a result, the balance of electron and hole current was improved, and the performances of PLEDs was enhanced as evidenced in Figure 2 and Table 1.

Figure 4. Ultraviolet photoelectron spectroscopy data (a) and Contact potential difference (CPD) images of the EML, without solvent treatment (b), with methanol treatment (c) and with DMF treatment (d).

To further investigate the mechanism of solvent treatment, the binding energy and contact potential difference (CPD) of the surface of EML were measured by UPS and KPFM, respectively. The UPS data of P-PPV before and after methanol:10 vol.%


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DMF treatment were shown in Figure 4(a). After solvent treatment, the secondary electron cutoff (ESE) in the high binding energy region shifted from 14 eV to 14.4 eV, indicating a vacuum level elevation of 0.4 eV at the surface, which is in accordance with the variation of Vbi as shown in Table 1. Besides, KPFM measurement of CPD between the sample and the tip was studied, which is used extensively to measure the surface potential of various materials.27, 28 The KPFM images of pristine P-PPV and P-PPV treated by methanol or DMF on a 2×2 µm area are shown in Figure 4 (b-d) (more repeated CPD images can be found in Figure S2). The CPD values increased from -517±5 mV of pristine P-PPV to -448±5 mV and -389±5 mV, respectively, after methanol or DMF treatment, indicating the existence of dipole layer at the cathode interface. The lower surface potential of sample than tip suggested the direction of dipole layer pointing from polymer to cathode. Hence, the vacuum level of cathode was elevated 0.069 and 0.13 eV by methanol or DMF treatment, respectively. The change of surface potential is in agreement with the corresponding change tendency of Vbi values and ESE values (from UPS measurement). To well understand the formation mechanism of dipole layer after solvent treatment, the influences of solvent treatment on the EML were studied. The UV-Vis and EL spectra of the EML are identical before and after solvent treatment (Figure S3). The thickness (as summarized in Table 1) and surface roughness of the EML (Figure S4) remain unchanged. These results implied that solvent treatment did not change the photoelectric characteristics and morphology of the EML, which are in agreement with the previous reported works.17,18 Thus, the dipole layer was mainly



formed from the remained polar solvent on the surface of the EML.17, 18 Previous works have proved that polar solvent remained on the surface of polymer and will not be completely removed during the following routine device fabrication process.17-19 Both methanol and DMF are strong polar solvents. Their dipole moments are 5.67 ×10-30 C·m and 12.88 ×10-30 C·m, respectively.29 High dipole moment indicates the strong polarity of molecular. Therefore, DMF is more effective than methanol on treating the EML to improve the performances of PLEDs, as demonstrated in Figure 2. However, the Vbi of methanol: x vol.% DMF treated PLED will decrease if x is higher than 20 vol.% due to the insulating character of DMF, as also observed in LiF/Al and PFN/Al devices30. Hence, the ratio of DMF in methanol has to be well controlled to less than 20 vol.%. With methanol: 10 vol.% DMF treatment, the PLED demonstrated the highest Vbi and peak brightness. Luminous Efficiency (cd/A)

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12 Untreated Methanol Methanol:10% DMF Methanol:20% DMF Methanol:40% DMF Methanol:60% DMF DMF

10

8

6

4 0

50

100

150

200

250

300 2

350


Figure 5. LE-J curves of PLEDs pre-annealed at temperature of 50 oC for 10 mins with or without solvent treatment. Another commonly used method to reduce the residue of solvent is thermal annealing. Therefore, the influence of thermal annealing on the effect of solvent


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treatment was also studied. All samples with or without solvent treatment were pre-annealed at 50oC for 10 mins before the evaporation of cathode, LE-J curves of the corresponding PLEDs are presented in Figure 5. We can see that the control device had a peak LE of 4.91 cd/A, which increased to 5.75 cd/A and 8.32 cd/A after methanol or DMF treatment, respectively. The peak LE of PLEDs was also further promoted to 10 cd/A by methanol: DMF treatment. But the optimal DMF concentration in methanol was 40 vol.%, rather than 20 vol.% of the unannealed devices. It indicated that the residue of DMF was reduced by thermal annealing. We note that the peak LE of control device decreased after thermal annealing. Hence, the way of thermal annealing to control the residue of DMF had adverse effect on the device performance. Luminous Efficiency (cd/A)

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14 untreated methanol:40%DMF

12 Unannealed 10 8 6 4 2 0

0

50

120 o

Annealing temperature ( C)

Figure 6. The peak LE of PLEDs pre-annealed at temperature of 50 oC and 120 oC, with or without methanol: 40 vol. % DMF treatment. To examine the existence of DMF after high temperature annealing, 120 oC pre-annealing for 10 mins was applied to the sample before the evaporation of cathode. Figure 6 compares the peak LE of PLEDs pre-annealed at temperatures of 50 o

C and 120 oC, without or with methanol: 40 vol.% DMF treatment to the EML. With



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pre-annealed at 120 oC, the device still displayed a peak LE increment of 50% with solvent treatment, although 40 vol.% was probably no longer the optimal doping concentration of DMF. The J-V-L and LE-J curves of PLED pre-annealed at temperature of 120 oC are shown in Figure S5. These results show that it is difficult to remove DMF completely after the common heat treatment during the fabrication of PLEDs. It proved that solvent treatment is a reliable method. In conclusion, the efficiency of PLED was improved significantly by treating the surface of P-PPV with DMF, and even greater performance enhancement was achieved by controlling the residue of DMF. Photovoltaic measurement determined the increase of Vbi after solvent treatment, indicating the lowered electron injection barrier. The existence of dipole layer at the EML/cathode interface was proved by UPS data and KPFM measurement. Detailed studies show that the dipole layer was mainly formed because of the intrinsic polarity of DMF. As a result, the electron and hole current balance was improved, and the performances of PLEDs were improved significantly.

ASSOCIATED CONTENT

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI:xxx/xxx. Device fabrication and measurement details; The J-V curves of electron-only devices (Figure S1); CPD images (Figure S2); UV-Vis and EL spectra of EML (Figure S3); AFM images of the EML (Figure S4); J-V-L and LE-J curves of


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PLED pre-annealed at 120 oC with EML treated by methanol:40 vol.% DMF (Figure S5).

AUTHOR INFORMATION

Corresponding Authors *E-mail: [email protected]; *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS

We express our gratitude to the National Key Basic Research Program of China (973 Program, 2015CB932203), the National Natural Science Foundation of China (Grants 61376023 and 61504066), A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), Natural Science Foundation of Jiangsu Higher Education Institutions of China (15KJB430024), and Natural Science Foundation of Jiangsu Province, China (BK20150838), and the Natural Science Foundation of Nanjing University of Posts and Telecommunications (NUPTSF Grants NY212013 and NY213044).

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Table of Contents Graphic

Luminous Efficiency (Cd/A)

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Untreated Methanol Methanol:20 vol.%DMF DMF

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Highly Promoting the Performances of Polymer Light-Emitting Diodes

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