Abstract
A self-assembled monolayer (SAM), 4-fluorothiophenol (4-FTP), is employed to modify the Ag anode of a top-emitting polymer light-emitting diode (T-PLED) to enhance the hole injection and the performance of a T-PLED device. The results show that the reflectivity of the Ag anode does not decrease due to the formation of a SAM. A brightness of 68 981 cd m2 and a luminous efficiency of 10.3 cdA have been achieved for the 4-FTP-modified device. The improved performance is attributed to the work function increase of the Ag/4-FTP anode due to the presence of fluorine atoms at the outer surface of the modified anode.
| Original language | English |
|---|---|
| Article number | 233513 |
| Journal | Applied Physics Letters |
| Volume | 89 |
| Issue number | 23 |
| DOIs | |
| Publication status | Published - 2006 |
All Science Journal Classification (ASJC) codes
- Physics and Astronomy (miscellaneous)
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In: Applied Physics Letters, Vol. 89, No. 23, 233513, 2006.
Research output: Contribution to journal › Article › peer-review
TY - JOUR
T1 - Self-assembled monolayer-modified Ag anode for top-emitting polymer light-emitting diodes
AU - Chong, Lai Wan
AU - Lee, Yuh Lang
AU - Wen, Ten Chin
AU - Guo, Tzung Fang
N1 - Funding Information: Chong Lai-Wan Lee Yuh-Lang a) Wen Ten-Chin b) Department of Chemical Engineering, National Cheng Kung University , Tainan, Taiwan 701, Republic of China and Advanced Optoelectronic Technology Center, National Cheng Kung University , Tainan, Taiwan 701, Republic of China Guo Tzung-Fang Institute of Electro-Optical Science and Engineering, National Cheng Kung University , Tainan, Taiwan 701, Republic of China a) Electronic mail: [email protected] b) Electronic mail: [email protected] 04 12 2006 89 23 233513 28 08 2006 06 11 2006 07 12 2006 2006-12-07T15:21:31 2006 American Institute of Physics 0003-6951/2006/89(23)/233513/3/ $23.00 A self-assembled monolayer (SAM), 4-fluorothiophenol (4-FTP), is employed to modify the Ag anode of a top-emitting polymer light-emitting diode (T-PLED) to enhance the hole injection and the performance of a T-PLED device. The results show that the reflectivity of the Ag anode does not decrease due to the formation of a SAM. A brightness of 68 981 cd ∕ m 2 and a luminous efficiency of 10.3 cd ∕ A have been achieved for the 4-FTP-modified device. The improved performance is attributed to the work function increase of the Ag/4-FTP anode due to the presence of fluorine atoms at the outer surface of the modified anode. NSCT NSC95-2221-E-006-324 NSCT NSC95-2221-E-006-409-MY3 Top-emissive organic and polymer light-emitting devices (T-OLEDs and T-PLEDs) have attracted increasing attention in recent years due to their potential in the fabrication of flat panel displays with high-contrast, full-color, low drive voltage and head up displays. Because light is emitted from the semitransparent top cathode, T-O/PLEDs provide a feasible fabrication of organic light-emitting diode (OLED) displays on opaque substrates (such as silicon wafer) with pixel circuits of thin-film transistors. Therefore, T-O/PLEDs are desirable to assure a high aperture ratio in active-matrix OLED displays. In order to increase the luminous efficiency (LE) in T-O/PLEDs, an anode with a high reflectivity and a low electrical resistivity is necessary. Ag is a suitable material to fit these requirements due to its high reflectivity ( ∼ 94 % at 520 nm ) and low electrical resistivity ( 1.47 μ Ω at 298 K ). 1 However, Ag is not considered an ideal anode for OLEDs due to the poor hole-injection efficiency from the Ag anode to the organic layer. This result is caused by the low work function of Ag ( ∼ 4.3 eV ) (Refs. 1 and 2 ) and the mismatching between this work function and the ionization potential of organic materials commonly used in OLEDs. In previous studies, a thin silver oxide ( Ag 2 O ) inserted between the Ag anode and organic layer was used to increase the Ag work function and to enhance hole-injection efficiency. However, the reflectivity of the Ag anode decreased due to the formation of a silver oxide layer, which is known to be disadvantageous to the efficiency of a T-OLED device. 1,2 Here, we report a facile method, self-assembled monolayer (SAM), to modify the Ag anode without reducing its reflectivity. Moreover, the performance of T-PLEDs is dramatically enhanced by using a suitable SAM. SAMs have been applied to modify the anode surface of traditional OLEDs. The functions of SAMs reported in the literature for OLED applications include enhancing the interfacial compatibility between indium tin oxide (ITO) anode and the hole transport layer (HTL), enhancing adhesion and stability of HTL, acting as a surface dipolar layer to enhance charge injection, 3–8 changing the work function of anode, 9 acting as a current blocking layer, 10,11 and acting as a moisture penetration blocking layer. 12,13 Silane derivatives were always employed to form the SAMs on ITO in bottom-emitting OLEDs. For T-O/PLEDs with a Ag anode, thiol derivatives should be used as surface modifiers. In this letter, two kinds of benzene thiols, 4-fluorothiophenol (4-FTP) and thiophenol (TP), are used to modify the Ag anode. The presence of a fluorine atom at the para position of the 4-FTP molecule is expected to induce a strong dipole due to its high electronegativity, leading to a higher work function of the Ag anode. 9 The performance of the 4-FTP modified T-PLEDs should be different from a device modified by TP, which does not have a fluorine atom. The performance of the devices was found to be significantly affected by the introduction of SAMs. Two T-PLEDs ( Ag ∕ HY - PPV ∕ Ca ∕ Ag and Ag ∕ SAM ∕ HY - PPV ∕ Ca ∕ Ag ) were prepared in this study. A silver film of 150 nm thickness was deposited on a glass substrate by thermal evaporation and used as an anode. The deposition was performed under a base pressure of 10 − 6 torr . After the formation of a silver film, the Ag anode was immediately surface modified by immersing in 1 mM 4-FTP (98%, Aldrich) or TP (98%, Fluka) ethanol solution for 30 min . After the reaction, the substrates were rinsed with ethanol and dried in a nitrogen steam. “High-yellow” phenyl-substituted poly( para -phenylenevinylene) copolymer (HY-PPV) was spin coated onto the Ag anodes and used as the light-emissive layer. Finally, a semitransparent top cathode, Ca ( 12 nm ) ∕ Ag ( 17 nm ) , was vapor deposited under 10 − 6 torr . The active pixel area of the device was 6 mm 2 . The current-voltage ( I - V ) and luminance-voltage ( L - V ) measurements were carried out by a current/voltage source measurement unit (Keithley 2400) and a calibrated silicon photodiode driven by a Keithley source. All the measurements were performed in a nitrogen-filled glovebox. X-ray photoelectron spectroscopy (XPS) (VG ESCA-210) equipped with a Mg K α source ( 1253.6 eV ) was used to analyze the surface composition of the modified Ag substrates. The reflectivity of the Ag surfaces was measured by a Hitachi U-4100 UV-Vis-NIR spectrometer. The work functions of the anode electrodes were measured by a Riken Keiki AC-2 photoelectron spectroscopy in air. The Ag anodes modified by the SAM were first analyzed by XPS. High resolution XPS scans over the S 2 p and F 1 s binding energy regions of the SAM-modified Ag anodes are presented in Fig. 1 , as well as the result recorded for a bare Ag surface. In Fig. 1(a) , the feature peak of S 2 p has a doublet structure at 161.9 ∕ 163.2 eV , corresponding to the S 2 p 3 ∕ 2 and S 2 p 1 ∕ 2 peaks of silver thiolate sulfur species (Ag–S–R). This result is consistent with the result reported for a thiolate on a gold surface. 14,15 The S 2 p peaks appear only on 4-FTP- and TP-modified anodes, demonstrating that the two thiol molecules have bound to the Ag surface. In Fig. 1(b) , the peak at 686.5 eV appearing on the 4-FTP-modified anode is consistent with the binding energy of a fluorine atom bonded to an aromatic ring. 16 It is reasonable that the F 1 s peak does not appear on the TP-modified and unmodified Ag anodes. The reflectivity measurement reveals that the reflectivity of the Ag anode did not decrease after the modification of SAMs. The measured reflectivities on Ag/4FTP and Ag/TP surfaces were 97.5% at 541 nm , which is slightly higher than the reflectivity measured on a bare Ag surface (96.9%). It is inferred that the SAM formed on the Ag surface can prevent the oxidation of a Ag surface and is thus responsible for the slight increase in reflectivity. The effect of the SAMs on hole injection was studied first by using aluminum as a cathode. Due to the high work function of aluminum, the electron injection was inhibited and the devices should be hole dominated. The I - V characteristics of Ag ∕ HY - PPV ∕ Al and Ag ∕ SAM ∕ HY - PPV ∕ Al devices are shown in Fig. 2(a) . The threshold voltage of the charge injection of the 4-FTP-modified device is greatly reduced compared with the base one. However, an increase of the threshold voltage is observed for the TP-modified device. This result is attributable to the change in work function due to the formation of SAM. The work functions of the three anodes measured by AC-2 photoelectron spectroscopy are 4.6 eV for the bare Ag, 5.2 eV for Ag/4FTP, and 4.1 eV for Ag/TP. The higher work function of the Ag/4-FTP anode is attributed to the presence of the fluorine at the para position of the benzene ring. The high electronegativity of fluorine atoms leads to a polarization of electrons toward the fluorine atom, which also increases the work function of the Ag/4-FTP anode. Therefore, the hole-injection barrier between the Ag/4-FTP anode and HY-PPV is greatly decreased. This result is consistent with the effect of perfluorinated alkanethiol SAMs reported by de Boer et al. 9 For the Ag/TP anode, without the presence of fluorine on the benzene ring, the polarization of electrons have an opposite direction to that of Ag/4-FTP. Therefore, the TP SAM demonstrates the opposite effect on the work function and hole injection. The hole injection of these devices was also examined by the Fowler-Nordheim (FN) tunneling theory J ∝ F 2 exp ( − κ ∕ F ) , where J is the current density of device, F is the electric field under bias, and κ is a parameter related to the shape of the injection barrier height ( φ ) . Figure 2(b) shows the plots of ln ( J ∕ F 2 ) vs 1 ∕ F for the hole-only devices. The straight-line FN curves shown for these devices indicate that the holes are injected through the tunneling process and are the dominant charge carriers. It has been reported that the slope of the line is proportional to φ 3 ∕ 2 . 11,17 The slope of these curves indicates that the hole-injection barrier for these devices increase in the order Ag ∕ 4 - FTP < Ag < Ag ∕ TP . This result is consistent with the barrier height evaluated from the direct measurement of the work function. The higher work function of the Ag/4-FTP anode leads to a lower barrier for the injection of holes, resulting in a lower onset potential and higher current density of the related device. To enhance the electron injection and the device efficiency of the top-emission device, a semitransparent Ca ∕ Ag layer was used as a cathode. The I - V characteristics shown in Fig. 3(a) demonstrate that the effect of the SAMs on the onset potential is similar to the hole-dominant devices shown in Fig. 2(a) . The voltages required to obtain a current density of 1 mA ∕ cm 2 are 3.2, 5.4, and 7.5 V for Ag/4-FTP, base Ag, and Ag/TP anodes, respectively. The effect of the SAMs on luminance performance is very significant, as shown in Fig. 3(b) . For the Ag/4-FTP anode, the maximum brightness is 68 981 cd ∕ m 2 biased at 9 V . However, the brightness for the unmodified Ag and Ag/TP anodes are only 1006 and 65 cd ∕ m 2 , respectively, at the same bias. The luminous efficiencies of the devices are shown in the inset of Fig. 3(b) . At a light intensity of 1000 cd ∕ m 2 , the LE of the Ag/4-FTP device is as high as 9.9 cd ∕ A , which is fivefold higher than the efficiency of a base device ( 1.7 cd ∕ A ) . Although the luminous efficiency decreases slightly with an increase of current density, the efficiency remains high in the higher brightness regime. For the Ag/TP anode, the efficiency is only 0.8 cd ∕ A at 1000 cd ∕ cm 2 , attributable to the blocking effect on hole injection. In summary, SAM is found to be a facile method to modify the Ag anodes for application in T-PLED. A 4-FTP-modified Ag electrode can be utilized as an effective anode to improve the emitting characteristic of a T-PLED. The Ag/4-FTP anode can enhance the hole injection, reduce the operation voltage, and significantly increase the current intensity and luminous efficiency of the device, without decreasing the reflectivity of the Ag anode. The improved performance of the device is mainly attributed to the higher work function of the Ag/4-FTP anode induced by the presence of fluorine atoms at the outer surface of the modified anode. For the TP-modified device, a contrary result is obtained due to the absence of fluorine atoms. The authors would like to thank the National Science Council (NSC) of Taiwan (NSC95-2221-E-006-324 and NSC95-2221-E-006-409-MY3). Sung-Nien Hsieh is gratefully acknowledged for his helpful discussion. Ruei-Tang Chen from Eternal Chemical Co., Ltd., is appreciated for providing the HY-PPV polymer. Ching-Chang Tseng from Chi Mel EL Corporation is thanked for detecting the work functions of the anode electrodes. FIG. 1. High resolution XPS scans of S 2 p (a) and F 1 s (b) regions for the Ag anodes with or without the modification of SAM. FIG. 2. I - V characteristics (a) and Fowler-Nordheim plots (b) for the Ag ∕ HY - PPV ∕ Al and Ag ∕ SAM ∕ HY - PPV ∕ Al devices. FIG. 3. I - V (a) and L - V (b) characteristics for the Ag ∕ HY - PPV ∕ Ca ( 12 nm ) ∕ Ag ( 17 nm ) and Ag ∕ SAM ∕ HY - PV ∕ Ca ( 12 nm ) ∕ Ag ( 17 nm ) devices. The inset is the luminous efficiency (LE) characteristics of the top-emitting devices.
PY - 2006
Y1 - 2006
N2 - A self-assembled monolayer (SAM), 4-fluorothiophenol (4-FTP), is employed to modify the Ag anode of a top-emitting polymer light-emitting diode (T-PLED) to enhance the hole injection and the performance of a T-PLED device. The results show that the reflectivity of the Ag anode does not decrease due to the formation of a SAM. A brightness of 68 981 cd m2 and a luminous efficiency of 10.3 cdA have been achieved for the 4-FTP-modified device. The improved performance is attributed to the work function increase of the Ag/4-FTP anode due to the presence of fluorine atoms at the outer surface of the modified anode.
AB - A self-assembled monolayer (SAM), 4-fluorothiophenol (4-FTP), is employed to modify the Ag anode of a top-emitting polymer light-emitting diode (T-PLED) to enhance the hole injection and the performance of a T-PLED device. The results show that the reflectivity of the Ag anode does not decrease due to the formation of a SAM. A brightness of 68 981 cd m2 and a luminous efficiency of 10.3 cdA have been achieved for the 4-FTP-modified device. The improved performance is attributed to the work function increase of the Ag/4-FTP anode due to the presence of fluorine atoms at the outer surface of the modified anode.
UR - http://www.scopus.com/inward/record.url?scp=33845438159&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=33845438159&partnerID=8YFLogxK
U2 - 10.1063/1.2404589
DO - 10.1063/1.2404589
M3 - Article
AN - SCOPUS:33845438159
SN - 0003-6951
VL - 89
JO - Applied Physics Letters
JF - Applied Physics Letters
IS - 23
M1 - 233513
ER -