High Sensitivity Tuning of Work Function of Self-Assembled

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High Sensitivity Tuning of Work Function of Self-Assembled Monolayers Modified Electrodes Using Vacuum Ultraviolet Treatment Porraphon Tantitarntong, Peter Zalar, Naoji Matsuhisa, Kyohei Nakano, sunghoon lee, Tomoyuki Yokota, Keisuke Tajima, and Takao Someya ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09756 • Publication Date (Web): 15 Aug 2017 Downloaded from http://pubs.acs.org on August 17, 2017

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ACS Applied Materials & Interfaces

High Sensitivity Tuning of Work Function of SelfAssembled Monolayers Modified Electrodes Using Vacuum Ultraviolet Treatment Porraphon Tantitarntonga, Peter Zalara,b,†, Naoji Matsuhisaa,§, Kyohei Nakanoc, Sunghoon Leea, Tomoyuki Yokotaa,b, Keisuke Tajimac, Takao Someyaa,b,c,* a

Department of Electrical Engineering and Information Systems, Graduate School of

Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan b

Exploratory Research for Advanced Technology (ERATO), Japan Science and Technology

Agency (JST), 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan c

RIKEN Center for Emergent Matter Science (CEMS), 2-1 Hirosawa, Wako, Saitama 351-0198,

Japan KEYWORDS work function tuning, self-assembled monolayers, vacuum ultraviolet, organic semiconductors, UPS

ACS Paragon Plus Environment

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ABSTRACT

We demonstrate systematic work function tuning of thiol-based SAM-modified gold electrodes with high controllability and sensitivity as high as 0.05 eV using vacuum ultraviolet technique (VUV). Under different irradiation times, both work function and wettability of the metal surface is modified. Fine tuning of the electrode work function is demonstrated by observable changes in the reverse current of a polymer Schottky diode. Additionally, the change in SAM chemical functionality validates the work function changes of VUV-irradiated electrodes. Our selective work function patterning on a single Au electrode via VUV could also reduce the required fabrication steps for more complex circuits.


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Work function control of metal electrodes is critical for determining the performance of organic electronic devices such as organic light-emitting diodes (OLEDs), organic photovoltaics (OPVs), and organic field effect transistors (OFETs). Generally, due to the energy misalignment of energy level between organic semiconducting layer and metal, a Schottky barrier is formed.1–3 This energy barrier existing between the interface limits the charge injection from the metal electrodes to the organic semiconductors and vice versa. In order to realize optimal device performance, an ohmic contact should be formed between the electrode and the highest occupied molecular orbital (HOMO) or the lowest unoccupied molecular orbital (LUMO) of organic semiconductor. Hence, Manipulation of the metal work function is of great importance for all classes of organic electronic devices; for example, in improving efficiencies for light-emitting diodes, or reducing a contact resistances in field-effect transistors.4 As has been demonstrated previously, systematic work function tuning can be achieved in various ways.2,5,6 Examples include chemical modification of organic semiconductor materials to alter HOMO and LUMO energy levels, careful electrode selection, and modification of electrodes using chemical or physical methods. The latter method works by modifying the charge distribution on a metal surface, thereby modulating the effective work function of the electrode. Conventionally, self-assembled monolayers (SAMs) have been used to alter the surface properties of the metal electrodes.7 Most commonly, alkanethiols, perfluorinated alkanethiols, and phenyl thiols have been used to form monolayers atop Au or Ag electrodes.8 In addition to the work function, SAMs have been shown to alter the physical and chemical properties of the electrode such as surface energy, which affects the molecular alignment of deposited organic semiconductors and film quality.3,6 Systematic tuning of the work function can be realized through several techniques, such as the mixing of various ratios of different SAMs.9–11


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Alternatively, the chemical structure of SAMs can be altered to modify the resulting work function.5,11 However, the chemical synthesis of new SAM molecules requires complicated processes to achieve high tuning sensitivity. Additionally, the mixed SAM method requires precise control of the mixing ratio in order to obtain proper coverage, while only resulting in a work function tuning sensitivity of ~ 0.1 - 0.2 eV.9,10,12,13 In order to improve work function controllability, here we introduce a simpler SAM modification process featuring both higher sensitivity and compatibility with large-area patterning. With better controllability, a deeper understanding of device physics (particularly charge injection) is a new possibility. Our technique also presents an alternative device fabrication process, which is especially beneficial for ambipolar transistors.14 In this study, the concept of fine work function tuning of SAM-modified electrodes by photoirradiation with a tuning sensitivity as high as 0.05 eV is established using vacuum ultraviolet (VUV). In the VUV technique, light with wavelength of 172 nm is irradiated on the sample in the presence of oxygen, resulting in controllable oxidation of the surface.15,16 As a result, fine tuning of surface energies is demonstrated. Taking advantage of this approach, fine work function tuning is demonstrated via the fabrication of polymer Schottky diodes employing VUV modified SAM electrodes. Furthermore, a spatial patterning of work function is fabricated through a shadow mask to show the feasibility of work function patterning using VUV in future applications. The effect of VUV in work function was investigated using an Au electrode modified by 1dodecanethiol (C12) (Figure 1a). Alkanethiol SAMs have been widely used in the study of work function modification due to its advantages such as reproducibility, processability, and stability on Au electrodes.88 In previous studies, alkyl-based SAMs have been shown to be susceptible to


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modification under UV light, with clear alteration of surface properties and chemical properties.16,17

Figure 1. a) Schematic of the VUV irradiation process on a C12-modified Au electrode. b) Work function variation (black circles and red triangles) of C12-modified Au electrode under 0 sec to 70 sec VUV irradiation time measured by AC-2 and UPS respectively and wettability change (blue squares). The work function of Au and Au/SAM surfaces are measured using two systems: photoelectron spectroscopy in ambient conditions (AC-2, Riken Keiki) and ultraviolet photoelectron spectroscopy (UPS). Generally, the values measured under ambient environment and high vacuum have small deviations due to oxide films covering the metal surface.18,19 The unmodified Au work function was measured to be 4.74 eV and 4.92 eV using AC-2 and UPS respectively. When the C12 SAM was chemisorbed onto the Au surface, the work function showed a decrease to 4.29 eV and 4.34 eV, as measured by the AC-2 and UPS, respectively. Although other SAM-modified Au with shorter alkyl chain also can modify the work function of


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Au,20 C12 SAMs were selected for VUV modification purposes due to the longer alkyl chain length and the evident change of the Au work function after SAM modification. (Table S1). Fine work function control of C12-modified Au electrodes was demonstrated by subjecting the sample to varying VUV irradiation times (0, 10, 15, 20, 25, 30, 40, 50, 60, and 70 sec). Figure S1 in supporting information shows a schematic of the VUV setup. The VUV-treated C12modified Au electrode resulted in a work function change varying almost linearly with irradiation time from 4.29 to 4.69 eV and 4.34 to 4.72 eV, as measured under ambient environment and UPS, respectively. The tuning sensitivity is as small as 0.05 eV within as little as a 10 sec irradiation interval. As the VUV irradiation time increases, the changes in work function values measured by both systems is visually summarized in Figure 1b. This result indicates that VUV has the ability to systematically and finely control the work function of SAM-modified Au electrodes after exposure for particular irradiation times. In addition to the work function change, the electrode’s surface energy is also a crucial factor which can have effect on the processability and performance of layers deposited atop it. This was determined using water contact angle measurements. Unmodified and C12-modified Au electrodes showed a water contact angle value of 85.2° and 103.3°, respectively. This corresponds to the fact that alkanethiol SAMs with long alkyl chains make the Au surface more hydrophobic resulting in a lower wettability. On the contrary, unmodified Au surface showed a more hydrophilic surface with higher wettability. After VUV was treated on the sample with the same time range, water contact angle yielded a starting value of 103.3° and steadily dropped to 88.1° after 70 sec of VUV irradiation. The water contact angle results shown in Figure 1b indicates the change of surface wettability from low to high after increasing VUV exposure time on the surface of C12-modified Au. This result is in accordance with previous reports stated that


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the water contact angle steadily decreases over VUV irradiation time indicating a decrease in hydrophobicity of surface with a forming of new chemical functional hydrophilic groups including hydroxyl (OH), aldehydes (CHO) and carboxyl groups (COOH) on the surface of Au electrode resulting incomplete or partial cleavage of alkyl chain of SAMs, as accompanied by the simultaneous change of surface wettability.16 This property has been exploited to readily pattern inks in large area printing processes and high resolution patterning of electrodes.21 The fine work function tuning can be as a result of the change in dipole moment on the surface of SAMs owing to the oxidation and shortening process of the alkyl chain under VUV treatment.4,16 Therefore, the similar methodology of VUV is expected be applicable to other electrodes22 such as Ag and Cu since VUV treatment is specific to the alkyl chain modification. Table 1. Surface atomic percentage of carbon, oxygen, and gold of C12-modified Au electrode as a function of VUV irradiation time measured and calculated by XPS measurement.

VUV irradiation time [sec]

a

Surface Atomic Percentage [%]

0 10

Carbon (C1s)a 32.12 40.63

Oxygen (O1s) 0.35 0.00

Gold (Au4f5) 67.53 59.37

20

40.17

0.54

30 40 50 60

41.04 40.84 33.43 29.53

70 31.73 XPS measurement peak

Work Function [eV] AC-II

UPS

4.29 4.36

4.34 4.35

59.29

4.43

4.39

0.00 1.02 2.04 3.68

58.96 58.15 64.55 66.79

4.47 4.51 4.59 4.63

4.43 4.49 4.57 4.66

4.28

63.99

4.69

4.72

Chemical changes over the course of VUV irradiation were verified by X-ray photoelectron spectroscopy (XPS) investigation of the VUV-treated samples. Firstly, the carbon peak (C1s) at a binding energy of 285 eV was scanned to demonstrate the change of carbon percentage on the


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surface of treated samples. (Figure S2, Supporting Information). It can be seen that the intensity carbon peak reduces after being treated by VUV. C1s peak intensity decrease is clearly reflected in the calculation of the atomic percentage at the surface of the treated-sample as well. Secondly, as the irradiation time increases to 40 sec, O1s peak increases indicating the presence of oxygen on the surface of the treated samples as a result of the surface oxidation process. (Figure S3, Supporting Information). The surface atomic percentage of treated and untreated VUV samples is summarized in Table 1. The atomic percentage of oxygen clearly increases as the irradiation time surpasses 40 s. As the irradiation time is increased to 50 s, the carbon peak decreases, signifying partial cleavage of the alkyl chain. Finally, the intensity of the Au peak is stable during short irradiation time and increases after longer exposure, indicating removal of the SAM layer.16

Figure 2. a) Chemical structure of MEH-PPV b) Schottky diode test structures with C12modified Au as bottom electrode c) J-V curve of Schottky diodes with VUV-treated C12modified Au electrodes: 0 sec (red diamonds), 10 sec (blue squares), 15 sec (green triangles), 25


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sec (purple upside-down triangles) and a reference unmodified Au electrode (black circles). Modulated work function of each VUV-treated electrode is shown in parenthesis. In order to demonstrate that the observed fine work function change of VUV-treated electrode can be extended to functional electronic devices, a polymer Schottky diode of the structure, Au/SAM/poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]

(MEH-PPV)/MoO3/Au

was fabricated, as shown schematically in Figures 2a, b. Typically, when bottom Au electrode is used as hole-injection contact into active layer (MEH-PPV), a small energy barrier of ~ 0.6 eV is present owning to a difference between the work function of Au (4.7 eV) and highest occupied molecular orbital (HOMO) of MEH-PPV (5.3 eV).1,23 When Au is treated using the C12 SAM, the work function of the electrode is changed to 4.2 eV. As a result, the hole injection barrier increases to ~1.1 eV, giving a highly injection limited contact24. In our device configuration, MoO3/Au were utilized as top contact due to its ability to form an ohmic contact for hole injection with MEH-PPV.25,26 In Figure 2c, J-V plots of Schottky diodes (effectively, hole-only diodes) fabricated with varying VUV treatments on SAMs is shown. As expected from the work function measurements, an increase in the hole injection barrier from the C12-modified Au yielded a reduction in reverse current density of Schottky diode by 2-orders of magnitude, when compared with the bare Au electrode configuration. However, because of the ohmic MoO3/Au top contact, the forward, holedominated, current density of all devices remained unchanged. As shown above, high sensitivity tuning of the electrode work function was achieved in short VUV irradiation times. A VUV-treated C12-modified electrode with irradiation times of 10, 15, and 25 sec is fabricated and used in same diode configuration. With 10 and 15 sec VUV-treated Au, an incremental step change of reverse current can clearly be seen due to the reduction in


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hole-injection barrier because of the small change in the work function by a value of 0.05~0.1 eV. When sample is treated with 25 sec (4.47 eV), the reverse current went back to the same order as the unmodified Au electrode. The modulation reduced the hole injection barrier, giving a small incremental increase in reverse current as shown in Figure 2c. These changes in the reverse current correlate with the changes in the work function of the bottom electrode with and without VUV treatment. In addition, the same trend was demonstrated in a different polymer system with a similar HOMO level, namely Poly(3-hexylthiophene-2,5-diyl) (P3HT). (Figure S4, Supporting Information) To verify the quality the Schottky diodes, the hole mobility of MEH-PPV was calculated by fitting the forward current of the J-V curves to the space-charged limited current model, given by Equation (1):

9 = (1) 8 where ε0 is the vacuum permittivity, εr is the relative dielectric constant of the polymer, V is the applied voltage, and L is the active layer thickness. In our case, the average hole mobility was 5.1 × 10-6 cm2/Vs without much variation in each device. This value is comparable with previous reports.23,27 The reverse current change was confirmed to be caused by work function change, not by morphology change. Since the J-V characteristics are highly influenced by the thickness and topology of the semiconductor, the thickness and surface roughness was evaluated using a profilometer (DektakXT) and atomic force microscope (AFM), respectively.28 The active layer thickness on top of bare Au was measured to be 580 nm, while thickness on top of VUV-treated and untreated C12-modified Au yielded a similar value of 550 nm. The root-mean square (RMS) surface roughness of MEH-PPV, both VUV-treated and untreated samples were measured to be


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0.5 nm. (See Supporting Information, Figure S5) In spite of the surface wettability change, the film morphology and device hole mobility was not affected as stated above. Moreover, the J-V diode characteristics measured in the forward and reverse directions also showed negligible hysteresis indicating the absence of hole trapping both at the contact and in the bulk. Hence, the change in reverse current is clearly dominated by the modification of electrode work function.

Figure 3. Demonstration of spatial patterning of VUV modulating the electrode work function. a) Single Au electrode (top) irradiated with VUV (0, 15, 60 sec) and diode device (bottom) with structure of Au/SAM/MEH-PPV/MoO3/Au fabricated on top of VUV-modified electrodes. b) JV curves of VUV patterned devices: 0 sec (blue diamonds), 15 sec (red triangles), 60 sec (black squares) In addition, surface treatment with VUV hold advantages over other techniques with the ability to be used as a patterning tool in large-area electronics. Using VUV, the spatial patterning of work function was achieved. Figure 3a shows a single cathode of diode devices with different work function values at each spot patterned by VUV. Irradiation times of 0, 15, 60 sec was used. VUV irradiated area is 1.0 mm2 with a distance of 2.8 cm between each device. The device


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dimension is 200 µm in width and length. The measured J-V curves show a change in reverse current due to the modulation of the cathode’s work function as a result of VUV treatment of the devices’ cathode, consistent with the data in Figure 2c (Figure 3b). The VUV technique shown here provides a greatly simplified and faster method for patterning SAMs when compared with other available techniques. Related previous research29 were able to demonstrate patterning with VUV with resolutions as high as 1 µm. This unique patterning approach could be beneficial in fabricating potential devices which benefit from the tunability of work function, for example in ambipolar transistors.14 The modification of metal work function is the focus of contact engineering. As it stands, the tunability of the work function value is restricted to the careful selection and use of different types of SAMs to modify a metal’s surface. Utilizing VUV, the work function of C12-modified Au electrodes can be linearly tuned in a range from 4.2 and 4.72 eV with a step as small as 0.05 eV. By using SAMs with different chemical structures, this window can conceivably be widened. In addition, the surface wettability is also altered without affecting the film quality of spin coated polymer films. As a result of our understanding, the fine modulation of work function could prove to be a unique method for researchers to further develop a profound understanding of charge-injection in organic devices. Additionally, since VUV is an optical method, rapid work function patterning of large area organic electronic devices is also feasible. For example, for integrated devices, we envision the patterning of diodes and resistors on a single foil, reducing the number of required fabrication steps. Similarly, one can imagine organic transistors made with source and drain electrodes with differential work functions; especially useful for ambipolar transistors.14,30 Research into these industrially relevant applications is especially important for the scalable production of high quality organic electronic circuits.


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AUTHOR INFORMATION Corresponding Author Prof. Takao Someya: *Department of Electrical Engineering and Information Systems, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan E-mail: [email protected] Present Addresses Peter Zalar: †

Holst Centre / TNO, High Tech Campus 31, 5656 AE Eindhoven, The Netherlands

Naoji Matsuhisa: §

School of Materials Science and Engineering, Nanyang Technological University, Block N4.1,

50 Nanyang Avenue, 639798, Singapore Funding Sources This work was financially supported by JST ERATO Grant Number JPMJER1105, Japan.

ACKNOWLEDGMENT The author would like to thank the School of Engineering, University of Tokyo for their financial support. N.M. is supported by Advanced Leading Graduate Course for Photon Science (ALPS) and the JSPS research fellowship for young scientists. There are no competing financial


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interests. The author is grateful to Dr. David Ordinario and Hiroaki Jinno for their helps with English correction and fruitful discussions. ABBREVIATIONS vacuum ultraviolet technique (VUV); highest occupied molecular orbital (HOMO); lowest unoccupied molecular orbital (LUMO); self-assembled monolayers (SAMs); dodecanethiol SAM (C12); ultraviolet photoelectron spectroscopy (UPS); X-ray photoelectron spectroscopy (XPS); poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]

(MEH-PPV);

Poly(3-

hexylthiophene-2,5-diyl) (P3HT); atomic force microscope (AFM)

Supporting Information Experimental section, Work function and water contact angle value of other alkanethiol SAMs (Table S1), Schematic of VUV aligner machine (Figure S1), XPS measurement of carbon and oxygen peak (Figure S2 and S3), A P3HT Schottky diode result (Figure S4), AFM images of MEH-PPV surface (Figure S5).


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ABSTRACT GRAPHIC


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Figure 1. a) Schematic of the VUV irradiation process on a C12-modified Au electrode. b) Work function variation (black circles and red triangles) of C12-modified Au electrode under 0 sec to 70 sec VUV irradiation time measured by AC-2 and UPS respectively and wettability change (blue squares). 317x152mm (150 x 150 DPI)



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Figure 2. a) Chemical structure of MEH-PPV b) Schottky diode test structures with C12-modified Au as bottom electrode c) J-V curve of Schottky diodes with VUV-treated C12-modified Au electrodes: 0 sec (red diamonds), 10 sec (blue squares), 15 sec (green triangles), 25 sec (purple upside-down triangles) and a reference unmodified Au electrode (black circles). Modulated work function of each VUV-treated electrode is shown in parenthesis. 333x166mm (150 x 150 DPI)


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Figure 3. Demonstration of spatial patterning of VUV modulating the electrode work function. a) Single Au electrode (top) irradiated with VUV (0, 15, 60 sec) and diode device (bottom) with structure of Au/SAM/MEH-PPV/MoO3/Au fabricated on top of VUV-modified electrodes. b) J-V curves of VUV patterned devices: 0 sec (blue diamonds), 15 sec (red triangles), 60 sec (black squares) 341x162mm (150 x 150 DPI)



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218x73mm (150 x 150 DPI)


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High Sensitivity Tuning of Work Function of Self-Assembled

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