Research article Received: 4 April 2014,
Accepted: 10 July 2014
Published online in Wiley Online Library: 14 August 2014
(wileyonlinelibrary.com) DOI 10.1002/bio.2753
Electrical conduction behavior of organic lightemitting diodes using fluorinated self-assembled monolayer with molybdenum oxide-doped hole transporting layer Sang-Geon Parka* and Tatsuo Morib ABSTRACT: The electrical conductivity behavior of a fluorinated self-assembled monolayer (FSAM) of a molybdenum oxide (MoOx)-doped α-naphthyl diamine derivative (α-NPD) in organic light-emitting diodes (OLEDs) was investigated. The current density of the MoOx-doped α-NPD/FSAM device was proportional to its voltage owing to smooth carrier injection through the FSAM and the high carrier density of its bulk. The temperature-dependent characteristics of this device were investigated. The current density–voltage characteristics at different temperatures were almost the same owing to its very low activation energy. The activation energy of the device was estimated to be 1.056 × 102 [eV] and was very low due to the inelastic electron tunneling of FSAM molecules. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: organic light-emitting diodes; self-assembled monolayer; molybdenum oxide; space charge limited current
Introduction
416
Organic light-emitting diodes (OLEDs) have emerged as nextgeneration flat panel displays and have a variety of advantages such as their improved range of colors and brightness. OLED technologies have undergone continuous development (1–4). Tang et al. reported the first efficient device with organic light-emitting material with a low-voltage below 10 V. This device consisted of a two-layer film of organic materials with a total thickness less than 150 nm (1). Van Slyke et al. reported on the characteristics of OLEDs made with an inserted carrier-injection layer between an anode and a charge transport layer (2). Carrier injection is a procedure that allows holes or electrons to move between electrodes with the highest occupied molecular orbit (HOMO) and lowest unoccupied molecular orbit (LUMO) in the organic layers of OLEDs (5). Ideally, the anode should have a higher work function than the HOMO of the hole injection layer (HIL) and the cathode should have a lower work function than the LUMO of the electron-injection layer (EIL) (5). In reality however, a barrier height of hole injection between the anode and the hole transport layer (HTL) exists. The barrier height of the hole injection from an anode to an organic material is related to the energy difference between the work function of the anode and the ionization potential of the organic material. The HIL can facilitate the hole injection from the anode to the HTL. The HIL acts to improve the smoothness of the electrode surface due to the bare indium-tin oxide (ITO) roughness. Also, the HIL reduces the probability of electrical shorts and becomes an important factor to fabricate high performance OLEDs (6). Electrical conduction in organic materials depends on carrier injection and charge transport. Carrier injection is important and is affected by proper design at the interface between the anode and organic materials (5). A charge transport mechanism
Luminescence 2015; 30: 416–419
is also an important factor that governs the performance of OLEDs. The primary factor in charge transport is carrier mobility. The usually mobility in organic materials used in OLEDs is between 103 and 107 cm2/V·s (5). Currents in OLED devices are limited by charge transport in the organic layers. This current limit is called as the space charge limited current (SCLC) (3). SCLC is behavior based on voltage–current characteristics caused by the accumulation of the injected charge. In this study, we investigated the temperature-dependent characteristics of a device that has MoOx-doped α-NPD as the carrier-transport layer and FSAM as the carrier-injection layer.
Materials and methods α-NPD used was from Nippon Steel Chemical Co. The ITO substrate for OLEDs used was from Geomatic Co. Acetone and Semico-clean were used to eliminate organic materials on ITO substrates. Pure water and 2-propanol were used to eradicate * Correspondence to: S.-G. Park, Department of Electrical Engineering and Computer Science, Nagoya University, Nagoya 464-8603, Japan. E-mail: [email protected] a
Department of Electrical Engineering and Computer Science, Nagoya University, Nagoya, 464-8603, Japan
b
Department of Electrical Engineering, Aichi Institute of Technology, Toyota, Aichi, 470-0392, Japan Abbreviations: α-NPD, α-naphthyl diamine; EIL, electron-injection layer; FSAM, fluorinated self-assembled monolayer; HIL, hole injection layer; HOMO, highest occupied molecular orbit; HTL, hole transport layer; ITO, indium-tin oxide; LED, light emitting diode; LUMO, lowest unoccupied molecular orbit; MoOx, molybdenum oxide; OLED, organic light-emitting diodes; SCLC, space charge limited current
Copyright © 2014 John Wiley & Sons, Ltd.
Electrical conduction behavior of organic LEDS
Results and Discussion Figure 1 shows the transmittance-wavelength characteristics of FSAM, MoOx (10 nm) and MoOx-doped α-NPD thin film (1:1, 10 nm). The transmittance of FSAM is the highest value due to the absence of absorption and very thin structure. FSAM can pass light through more efficaciously than MoOx or MoOxdoped α-NPD thin film. Figure 2 shows the work function of various HILs measured by photoemission yield spectroscopy in air. The work functions of ITO/FSAM, ITO/MoOx (10 nm) and ITO/ MoOx-doped α-NPD (1: 1, 10 nm) are estimated to be 5.45, 5.70 and 5.36 eV, respectively. Figure 3 shows the current density–voltage characteristics of the FSAM devices with α-NPD or MoOx-doped α-NPD thin film. The structure of the devices are ITO/FSAM/α-NPD (50 nm or 100 nm) or MoOx:α-NPD (1:1, 50 nm or 100 nm)/Al (100 nm). The plots of current density versus squared voltage (J–V2) for FSAM/α-NPD specimens were investigated. When a bare ITO is used as an anode, the J–V2 curve is not always linear. However, when the FSAM-modified ITO is used, the J–V2 curve shows the linear behavior. The mobility of an α-NPD thin film can be calculated by the SCLC model. If the current density characteristics satisfy the SCLC theory, the slope in the current density–voltage squared characteristics is given by 9 με 8 d3
Figure 2. Square root of thephotoemission yield-energy spectra.
100
Current density [A/cm2]
Semico-clean and moisture, respectively. Then, UV ozone treatments using an UV ozone cleaner (NL-UV253, Japan Laser Electronics – presently Filgen Co.) eradicated organic and residual organic solvents, respectively. After placing the ITO substrates into the chamber, UV light irradiation (10 min) removed the organic materials on the substrate surface. After UV ozone treatment, the substrate was quickly placed into a vacuum deposition chamber in order to prevent pollution of the substrate surface. FSAM, purchased from Gelest Co. FSAM, was made by a gas-phase method in an electric oven. FSAM-modified ITO substrates were cleaned by ultrasonic washing in 2-propanol to eradicate surplus FSAM molecules. The measurements of the luminance–voltage–current characteristic were performed using a GUI programming language (LabVIEW, National Instruments) under a pressure of 1 × 101 Pa at room temperature. The current was measured with lamp voltage (rising rate: 0.5 V/s) by a source measurement unit (2400 Source Meter, Keithley Instrument, Inc.). The currents and voltages were recorded with a personal computer through a GP-IB interface. The temperature operations were performed in a temperature-variable cryogenic probe station (Measure Jig Co., model WM-365B).
10-2 10-4 10-6 10-8 10-4
Transmittance [%T]
ITO/FSAM ITO/MoOx ITO/MoOx: -NPD
90 85 80 75 70 400
500
600
700
800
Wavelength [nm]
Luminescence 2015; 30: 416–419
10-1
100
mobility, d is the film thickness and V is the applied voltage. The slopes with FSAM/α-NPD (100 nm) and with FSAM/α-NPD (50 nm) were estimated to be approximately 350.677 and 620.41 [A/(V m)2], respectively: 9 V2 J ¼ εμ 3 8 d Slope ¼
(1)
9 με 8 d3
μ ¼ Slope
(2) 8 d3 9 ε
(3)
When the carrier mobility of α-NPD, calculated as the dielectric constant of α-NPD, is assumed to be 3, the hole mobilities of α-NPD thin films were estimated to be 2.60 × 109 m2/Vs (50 nm) and 1.17 × 108 m2/Vs (100 nm) from Equation (3). The carrier mobility of a MoOx-doped α-NPD thin film is predicted to be much larger than the hole mobility of α-NPD since the currents of MoOx-doped α-NPD thin films show ohmic behavior (JαV). The slopes of current density–average electric field curves are estimated to be approximately 1.03 × 104 S/m. Since the conductivity of non-doped trans-poly(acetylene) was almost 102 S/m, the conductivity of the MoOx-doped α-NPD thin film was not high. If the carrier mobility of the MoOx-doped α-NPD thin film is assumed to be 104 m2 V1 s1, which is the same as the minimum
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/luminescence
417
Figure 1. Transmittance–wavelength characteristics.
10-2
Figure 3. (a) Current density-voltage characteristics with log-log scale at different temperatures where the device structure is ITO/FSAM/MoOx:α-NPD(1:1,50nm or 100nm) or α-NPD (50nm or 100nm)/Al(100nm).
2
95
10-3
Voltage [V]
(∵J ¼ 98 ε0 εr μ Vd3 ), where ε is the permittivity, μ is the carrier 100
FSAM/MoOx:NPD50nm FSAM/MoOx:NPD100nm FSAM/NPD50nm FSAM/NPD100nm
S.-G. Park and T. Mori mobility of band conduction, the carrier density must be 1.60 × 1018 m3 (4). This calculation showed that the carrier density of the MoOx-doped α-NPD thin film was much lower than that of a conventional organic semiconductor, e.g. CuPc, pentacene: ~1024 m3. However, this value is higher than that of the carrier density of 1016 m3 found with an intrinsic semiconductor, e.g. Si: 1.5 × 1016 m3 but lower than that of the carrier density of 1020–1021 m3 found with an impurity semiconductor. The J–V proportional characteristics of the MoOx-doped α-NPD thin film may be caused by both easy carrier injection through the FSAM and the high carrier density of the bulk by charge transfer complex formation in the MoOx-doped α-NPD thin film. Figure 4 shows the current density–voltage characteristics of the FSAM device with MoOx-doped α-NPD at different temperatures. The device structure is ITO/FSAM/MoOx:α-NPD (1:1, 100 nm)/Al (100 nm). The current density–voltage curves are almost same at various temperatures. The FSAM device did not show temperature-dependent characteristics. Figure 5 shows the Arrhenius plot of different current densities. We calculated the activation energy based on the Arrhenius plot of the current densities and the Arrhenius formula (7,8): Ea J∝exp kT
(4)
10-1
Applied voltage [V]
Activation energy [eV]
0.1 0.4 0.7 1
0.0171 0.0158 0.00803 0.001289
device with MoOx-doped α-NPD showed very low temperature dependence owing to the inelastic electron tunneling characteristics of FSAM molecules (8,9). Therefore, this device in OLEDs is effective due to its robustness against temperature changes. We concluded that the conduction mechanism through FSAM is tunneling, either direct or Folwer–Nordheim, therefore we investigated the conduction mechanism of the tunneling through FSAM. Figure 6(a) shows characteristics of both current density–voltage and ln(J/V^2)–1/V at different temperatures (8–10). In the case of direct tunneling, current density–voltage characteristics indicated linear increases at different temperatures as shown in Fig. 6(b). However, in the case of Folwer– Nordheim tunneling, ln(J/V^2)–1/V characteristics indicated as a parabola curve at different temperatures. Therefore, this confirms the tunneling nature of the conduction mechanism through FSAM molecules due to its voltage dependence (J∝V). If the current is space charge limited, the J–V characteristics can be depicted from Equation (1). The field dependent mobility (μ) and the temperature (T) is given by:
(a) 0.5
10-2
0.0 10-3
-0.5
10-4
+25 -25 -50 -100
10-5 10-6 0.0
ln(J/V2)
Current density [A/cm2]
where J is the current density [A/cm2], Ea is the activation energy [eV], k is the Boltzman constant [eV/K], and T is the temperature [K]. The activation energies were very low. The average activation energy was estimated to be 1.056 × 102 [eV]. That is, the FSAM
Table 1. Equation calculated from the ln(J)–1/T curve
0.2
0.4
0.6
0.8
1.0
-1.0 -1.5 25 -25 -50 -100
-2.0 -2.5
1.2
Voltage [V]
-3.0 10
5
-1 -2
ln(J)
-3 -4
-7 3.0
20
(b) 100x10-3
+25 -25 -50 -100
80 60 40 20 0
-5 -6
15
1/V (1/V)
Current density [A/cm2]
Figure 4. (a) Current density-voltage characteristics with log-linear scale at different temperatures where the device structure is ITO/FSAM/MoOx-doped α-NPD (1:1,100nm)/Al(100nm).
0.0
0.1V 0.4V 0.7V 1V
3.5
0.2
0.4
0.6
0.8
1.0
Voltage [V] 4.0
4.5
5.0
6.0x10-3
1/T (1/K)
418
Figure 5. Arrhenius plot of the current densities at different temperatures.
wileyonlinelibrary.com/journal/luminescence
Figure 6. (a) Current density-voltage characteristics with ln(J/V^2)-1/V at different temperatures where the device structure is ITO/FSAM/MoOx-doped α-NPD (1:1,100nm)/Al(100nm). (b) Current density-voltage characteristics with linear-linear scale at different temperatures where the device structure is ITO/FSAM/ MoOx-doped α-NPD(1:1,100nm)/Al(100nm).
Copyright © 2014 John Wiley & Sons, Ltd.
Luminescence 2015; 30: 416–419
Electrical conduction behavior of organic LEDS
Current density [A/cm2]
100 10
with MoOx-doped α-NPD thin film was very small or almost zero due to the non-elastic characteristics of FSAM molecules. The FSAM device with MoOx-doped α-NPD thin film is expected to achieve high performances in this field.
-1
10-2 10-3
References
25 -25 -50 -100 Fitting Line
10-4 10-5 10-6 0.0
0.2
0.4
0.6
0.8
1.0
1.2
Voltage [V] Figure 7. (a) Current density-voltage characteristics with log-linear scale at different temperatures where the device structure is ITO/ MoOx-doped α-NPD (1:1,100nm)/Al(100nm). The line is fits to SCLC model with the Poole-Frenkel mobility.
pffiffiffi ΔE β E μ ¼ μ0 exp kT
(5)
where ΔE is the activation energy, β is the Poole-Frenkel factor, T is the temperature (in absolute degree Kelvins), k is the Boltzman constant, and E is the electric field (11). The field dependent mobility (μ) was 3.32 × 104 cm2/V s (μ0 = 5 × 104 cm2/ V s) β was 3 × 1010 and the average ΔE was 1.056 × 102 [eV] and k was 1.38 × 1023 JK1. The SCLC model calculated with the Poole–Frenkel mobility is in rough agreement with the J–V characteristics curve (Fig. 7).
Conclusions In this study, the FSAM device with a MoOx-doped α-NPD thin film showed non-sensitive characteristics with respect to temperature changes. The apparent activation energy of the current of FSAM
1. Tang CW, Van Slyke SA. Organic electroluminescent diodes. Appl Phys Lett 1987;51:913–5. 2. Van Slyke SA, Chen CH, Tang CW. Organic electroluminescent devices with improved stability. Appl Phys Lett 1996;69: 2160–2. 3. Mori T. What is space charge limited current. In: Mori T, editor. Guidebook of easy OLED. Tokyo: Nikkan Kogyo Shimbun, 2008:46–7 (in Japanese). 4. Park SG, Wang H, Mori T. Effects of using a Ag anode with a coevaporation thin film of MoOx and α-naphthyl diamine derivative in organic light-emitting diode. Jpn J Appl Phys 2013;52:052102-1-4. 5. Qiu J. High efficiency organic light emitting diodes with moo3 doped hole transport layer [dissertation]. Toronto: University of Toronto, 2012. 6. Kitai A. Hole injection layer. In: Kitai A, editor. Principle of solar cells, LEDs and diodes: the role of the PN junction. New York: John Wiley & Sons, Ltd, 2011:271. 7. Hu B, Chen XC, Huang W, Tanka H, Ogawa T, You XZ. Temperaturedependent I–V characteristics for the nanocomposite semiconducting films composed of a thiol end-capped dinuclear macrocyclic complex and Au-NPs bridging 1 μm gap gold electrodes. Dalton Trans 2012;47:14213–440. 8. Nijhuis CA, Reus WF, Barber JR, Dickey MD, Whitesides GM. Charge transport and rectification in arrays of SAM-based tunneling junctions. Nano Lett 2010;10:3611–9. 9. Wang W, Lee T, Reed MA. Characterization of electron tunneling in self-assembled alkanethiol monolayer devices. Physica E 2003;19:117–25. 10. Wang W, Lee T, Reed MA. Characterization of electron tunneling in self-assembled alkanethiol monolayer devices. Phys Rev B 2003;68:035416-1-7 11. Weisz SZ, Cobas A, Richardson PE, Szmant HZ, Trester S. Detection of radiation damage in anthracene crystals by space charge limited current measurements. J Chem Phys 1966;44:1364–8.
419
Luminescence 2015; 30: 416–419
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/luminescence
Accepted: 10 July 2014
Published online in Wiley Online Library: 14 August 2014
(wileyonlinelibrary.com) DOI 10.1002/bio.2753
Electrical conduction behavior of organic lightemitting diodes using fluorinated self-assembled monolayer with molybdenum oxide-doped hole transporting layer Sang-Geon Parka* and Tatsuo Morib ABSTRACT: The electrical conductivity behavior of a fluorinated self-assembled monolayer (FSAM) of a molybdenum oxide (MoOx)-doped α-naphthyl diamine derivative (α-NPD) in organic light-emitting diodes (OLEDs) was investigated. The current density of the MoOx-doped α-NPD/FSAM device was proportional to its voltage owing to smooth carrier injection through the FSAM and the high carrier density of its bulk. The temperature-dependent characteristics of this device were investigated. The current density–voltage characteristics at different temperatures were almost the same owing to its very low activation energy. The activation energy of the device was estimated to be 1.056 × 102 [eV] and was very low due to the inelastic electron tunneling of FSAM molecules. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: organic light-emitting diodes; self-assembled monolayer; molybdenum oxide; space charge limited current
Introduction
416
Organic light-emitting diodes (OLEDs) have emerged as nextgeneration flat panel displays and have a variety of advantages such as their improved range of colors and brightness. OLED technologies have undergone continuous development (1–4). Tang et al. reported the first efficient device with organic light-emitting material with a low-voltage below 10 V. This device consisted of a two-layer film of organic materials with a total thickness less than 150 nm (1). Van Slyke et al. reported on the characteristics of OLEDs made with an inserted carrier-injection layer between an anode and a charge transport layer (2). Carrier injection is a procedure that allows holes or electrons to move between electrodes with the highest occupied molecular orbit (HOMO) and lowest unoccupied molecular orbit (LUMO) in the organic layers of OLEDs (5). Ideally, the anode should have a higher work function than the HOMO of the hole injection layer (HIL) and the cathode should have a lower work function than the LUMO of the electron-injection layer (EIL) (5). In reality however, a barrier height of hole injection between the anode and the hole transport layer (HTL) exists. The barrier height of the hole injection from an anode to an organic material is related to the energy difference between the work function of the anode and the ionization potential of the organic material. The HIL can facilitate the hole injection from the anode to the HTL. The HIL acts to improve the smoothness of the electrode surface due to the bare indium-tin oxide (ITO) roughness. Also, the HIL reduces the probability of electrical shorts and becomes an important factor to fabricate high performance OLEDs (6). Electrical conduction in organic materials depends on carrier injection and charge transport. Carrier injection is important and is affected by proper design at the interface between the anode and organic materials (5). A charge transport mechanism
Luminescence 2015; 30: 416–419
is also an important factor that governs the performance of OLEDs. The primary factor in charge transport is carrier mobility. The usually mobility in organic materials used in OLEDs is between 103 and 107 cm2/V·s (5). Currents in OLED devices are limited by charge transport in the organic layers. This current limit is called as the space charge limited current (SCLC) (3). SCLC is behavior based on voltage–current characteristics caused by the accumulation of the injected charge. In this study, we investigated the temperature-dependent characteristics of a device that has MoOx-doped α-NPD as the carrier-transport layer and FSAM as the carrier-injection layer.
Materials and methods α-NPD used was from Nippon Steel Chemical Co. The ITO substrate for OLEDs used was from Geomatic Co. Acetone and Semico-clean were used to eliminate organic materials on ITO substrates. Pure water and 2-propanol were used to eradicate * Correspondence to: S.-G. Park, Department of Electrical Engineering and Computer Science, Nagoya University, Nagoya 464-8603, Japan. E-mail: [email protected] a
Department of Electrical Engineering and Computer Science, Nagoya University, Nagoya, 464-8603, Japan
b
Department of Electrical Engineering, Aichi Institute of Technology, Toyota, Aichi, 470-0392, Japan Abbreviations: α-NPD, α-naphthyl diamine; EIL, electron-injection layer; FSAM, fluorinated self-assembled monolayer; HIL, hole injection layer; HOMO, highest occupied molecular orbit; HTL, hole transport layer; ITO, indium-tin oxide; LED, light emitting diode; LUMO, lowest unoccupied molecular orbit; MoOx, molybdenum oxide; OLED, organic light-emitting diodes; SCLC, space charge limited current
Copyright © 2014 John Wiley & Sons, Ltd.
Electrical conduction behavior of organic LEDS
Results and Discussion Figure 1 shows the transmittance-wavelength characteristics of FSAM, MoOx (10 nm) and MoOx-doped α-NPD thin film (1:1, 10 nm). The transmittance of FSAM is the highest value due to the absence of absorption and very thin structure. FSAM can pass light through more efficaciously than MoOx or MoOxdoped α-NPD thin film. Figure 2 shows the work function of various HILs measured by photoemission yield spectroscopy in air. The work functions of ITO/FSAM, ITO/MoOx (10 nm) and ITO/ MoOx-doped α-NPD (1: 1, 10 nm) are estimated to be 5.45, 5.70 and 5.36 eV, respectively. Figure 3 shows the current density–voltage characteristics of the FSAM devices with α-NPD or MoOx-doped α-NPD thin film. The structure of the devices are ITO/FSAM/α-NPD (50 nm or 100 nm) or MoOx:α-NPD (1:1, 50 nm or 100 nm)/Al (100 nm). The plots of current density versus squared voltage (J–V2) for FSAM/α-NPD specimens were investigated. When a bare ITO is used as an anode, the J–V2 curve is not always linear. However, when the FSAM-modified ITO is used, the J–V2 curve shows the linear behavior. The mobility of an α-NPD thin film can be calculated by the SCLC model. If the current density characteristics satisfy the SCLC theory, the slope in the current density–voltage squared characteristics is given by 9 με 8 d3
Figure 2. Square root of thephotoemission yield-energy spectra.
100
Current density [A/cm2]
Semico-clean and moisture, respectively. Then, UV ozone treatments using an UV ozone cleaner (NL-UV253, Japan Laser Electronics – presently Filgen Co.) eradicated organic and residual organic solvents, respectively. After placing the ITO substrates into the chamber, UV light irradiation (10 min) removed the organic materials on the substrate surface. After UV ozone treatment, the substrate was quickly placed into a vacuum deposition chamber in order to prevent pollution of the substrate surface. FSAM, purchased from Gelest Co. FSAM, was made by a gas-phase method in an electric oven. FSAM-modified ITO substrates were cleaned by ultrasonic washing in 2-propanol to eradicate surplus FSAM molecules. The measurements of the luminance–voltage–current characteristic were performed using a GUI programming language (LabVIEW, National Instruments) under a pressure of 1 × 101 Pa at room temperature. The current was measured with lamp voltage (rising rate: 0.5 V/s) by a source measurement unit (2400 Source Meter, Keithley Instrument, Inc.). The currents and voltages were recorded with a personal computer through a GP-IB interface. The temperature operations were performed in a temperature-variable cryogenic probe station (Measure Jig Co., model WM-365B).
10-2 10-4 10-6 10-8 10-4
Transmittance [%T]
ITO/FSAM ITO/MoOx ITO/MoOx: -NPD
90 85 80 75 70 400
500
600
700
800
Wavelength [nm]
Luminescence 2015; 30: 416–419
10-1
100
mobility, d is the film thickness and V is the applied voltage. The slopes with FSAM/α-NPD (100 nm) and with FSAM/α-NPD (50 nm) were estimated to be approximately 350.677 and 620.41 [A/(V m)2], respectively: 9 V2 J ¼ εμ 3 8 d Slope ¼
(1)
9 με 8 d3
μ ¼ Slope
(2) 8 d3 9 ε
(3)
When the carrier mobility of α-NPD, calculated as the dielectric constant of α-NPD, is assumed to be 3, the hole mobilities of α-NPD thin films were estimated to be 2.60 × 109 m2/Vs (50 nm) and 1.17 × 108 m2/Vs (100 nm) from Equation (3). The carrier mobility of a MoOx-doped α-NPD thin film is predicted to be much larger than the hole mobility of α-NPD since the currents of MoOx-doped α-NPD thin films show ohmic behavior (JαV). The slopes of current density–average electric field curves are estimated to be approximately 1.03 × 104 S/m. Since the conductivity of non-doped trans-poly(acetylene) was almost 102 S/m, the conductivity of the MoOx-doped α-NPD thin film was not high. If the carrier mobility of the MoOx-doped α-NPD thin film is assumed to be 104 m2 V1 s1, which is the same as the minimum
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/luminescence
417
Figure 1. Transmittance–wavelength characteristics.
10-2
Figure 3. (a) Current density-voltage characteristics with log-log scale at different temperatures where the device structure is ITO/FSAM/MoOx:α-NPD(1:1,50nm or 100nm) or α-NPD (50nm or 100nm)/Al(100nm).
2
95
10-3
Voltage [V]
(∵J ¼ 98 ε0 εr μ Vd3 ), where ε is the permittivity, μ is the carrier 100
FSAM/MoOx:NPD50nm FSAM/MoOx:NPD100nm FSAM/NPD50nm FSAM/NPD100nm
S.-G. Park and T. Mori mobility of band conduction, the carrier density must be 1.60 × 1018 m3 (4). This calculation showed that the carrier density of the MoOx-doped α-NPD thin film was much lower than that of a conventional organic semiconductor, e.g. CuPc, pentacene: ~1024 m3. However, this value is higher than that of the carrier density of 1016 m3 found with an intrinsic semiconductor, e.g. Si: 1.5 × 1016 m3 but lower than that of the carrier density of 1020–1021 m3 found with an impurity semiconductor. The J–V proportional characteristics of the MoOx-doped α-NPD thin film may be caused by both easy carrier injection through the FSAM and the high carrier density of the bulk by charge transfer complex formation in the MoOx-doped α-NPD thin film. Figure 4 shows the current density–voltage characteristics of the FSAM device with MoOx-doped α-NPD at different temperatures. The device structure is ITO/FSAM/MoOx:α-NPD (1:1, 100 nm)/Al (100 nm). The current density–voltage curves are almost same at various temperatures. The FSAM device did not show temperature-dependent characteristics. Figure 5 shows the Arrhenius plot of different current densities. We calculated the activation energy based on the Arrhenius plot of the current densities and the Arrhenius formula (7,8): Ea J∝exp kT
(4)
10-1
Applied voltage [V]
Activation energy [eV]
0.1 0.4 0.7 1
0.0171 0.0158 0.00803 0.001289
device with MoOx-doped α-NPD showed very low temperature dependence owing to the inelastic electron tunneling characteristics of FSAM molecules (8,9). Therefore, this device in OLEDs is effective due to its robustness against temperature changes. We concluded that the conduction mechanism through FSAM is tunneling, either direct or Folwer–Nordheim, therefore we investigated the conduction mechanism of the tunneling through FSAM. Figure 6(a) shows characteristics of both current density–voltage and ln(J/V^2)–1/V at different temperatures (8–10). In the case of direct tunneling, current density–voltage characteristics indicated linear increases at different temperatures as shown in Fig. 6(b). However, in the case of Folwer– Nordheim tunneling, ln(J/V^2)–1/V characteristics indicated as a parabola curve at different temperatures. Therefore, this confirms the tunneling nature of the conduction mechanism through FSAM molecules due to its voltage dependence (J∝V). If the current is space charge limited, the J–V characteristics can be depicted from Equation (1). The field dependent mobility (μ) and the temperature (T) is given by:
(a) 0.5
10-2
0.0 10-3
-0.5
10-4
+25 -25 -50 -100
10-5 10-6 0.0
ln(J/V2)
Current density [A/cm2]
where J is the current density [A/cm2], Ea is the activation energy [eV], k is the Boltzman constant [eV/K], and T is the temperature [K]. The activation energies were very low. The average activation energy was estimated to be 1.056 × 102 [eV]. That is, the FSAM
Table 1. Equation calculated from the ln(J)–1/T curve
0.2
0.4
0.6
0.8
1.0
-1.0 -1.5 25 -25 -50 -100
-2.0 -2.5
1.2
Voltage [V]
-3.0 10
5
-1 -2
ln(J)
-3 -4
-7 3.0
20
(b) 100x10-3
+25 -25 -50 -100
80 60 40 20 0
-5 -6
15
1/V (1/V)
Current density [A/cm2]
Figure 4. (a) Current density-voltage characteristics with log-linear scale at different temperatures where the device structure is ITO/FSAM/MoOx-doped α-NPD (1:1,100nm)/Al(100nm).
0.0
0.1V 0.4V 0.7V 1V
3.5
0.2
0.4
0.6
0.8
1.0
Voltage [V] 4.0
4.5
5.0
6.0x10-3
1/T (1/K)
418
Figure 5. Arrhenius plot of the current densities at different temperatures.
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Figure 6. (a) Current density-voltage characteristics with ln(J/V^2)-1/V at different temperatures where the device structure is ITO/FSAM/MoOx-doped α-NPD (1:1,100nm)/Al(100nm). (b) Current density-voltage characteristics with linear-linear scale at different temperatures where the device structure is ITO/FSAM/ MoOx-doped α-NPD(1:1,100nm)/Al(100nm).
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Luminescence 2015; 30: 416–419
Electrical conduction behavior of organic LEDS
Current density [A/cm2]
100 10
with MoOx-doped α-NPD thin film was very small or almost zero due to the non-elastic characteristics of FSAM molecules. The FSAM device with MoOx-doped α-NPD thin film is expected to achieve high performances in this field.
-1
10-2 10-3
References
25 -25 -50 -100 Fitting Line
10-4 10-5 10-6 0.0
0.2
0.4
0.6
0.8
1.0
1.2
Voltage [V] Figure 7. (a) Current density-voltage characteristics with log-linear scale at different temperatures where the device structure is ITO/ MoOx-doped α-NPD (1:1,100nm)/Al(100nm). The line is fits to SCLC model with the Poole-Frenkel mobility.
pffiffiffi ΔE β E μ ¼ μ0 exp kT
(5)
where ΔE is the activation energy, β is the Poole-Frenkel factor, T is the temperature (in absolute degree Kelvins), k is the Boltzman constant, and E is the electric field (11). The field dependent mobility (μ) was 3.32 × 104 cm2/V s (μ0 = 5 × 104 cm2/ V s) β was 3 × 1010 and the average ΔE was 1.056 × 102 [eV] and k was 1.38 × 1023 JK1. The SCLC model calculated with the Poole–Frenkel mobility is in rough agreement with the J–V characteristics curve (Fig. 7).
Conclusions In this study, the FSAM device with a MoOx-doped α-NPD thin film showed non-sensitive characteristics with respect to temperature changes. The apparent activation energy of the current of FSAM
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