Chapter 2 Theoretical Background and Device Fabrication
3.3 Discussion
To improve output current, pentacene and fullerene (C60) are used to fabricate as SCLTs. The J-V curves for emitter-collector diodes with different organic materials are compared in the Figure 3.9. Typical parameters are listed in Table I. Pentacene and C60 provide higher output current than P3HT for SCLTs from Table I. However, pentacene SCLTs and C60 SCLTs suffer from high leakage current even as base leakage is as low as in P3HT SCLTs. New leakage phenomena are proposed and discussed.
Morphology effect:
Leakage phenomenon is the morphology effect. AFM images of pentacene, C60, and P3HT above grid structure are compared in Figure 3.10(a), 3.10 (b) and 3.10(c).
Surface profile reveals that the grain structure in pentacene and C60 produce holes while P3HT have rather smooth and dense morphology. Holes in pentacene/C60 make the top collector metal penetrates into pentacene/C60 film and shield the base electric
field. As a result, base metal looses ability to turn off the channel.
-1.0 -0.5 0.0
negatively biased at VC with respect to Au. The negative collector current IC means the holes are collected by the Al collector and flows out from the transistor. The gate current IG is no more than a few nA for all measurements. (a) The collector current as a function of the collector voltage various by step VG. (b) The grid current as a function of the collector voltage of the transistor with same fabrication procedure described in (a).Figure 3.2 The electric characteristics of the P3HT-based SCLT in double logarithmic scale with fixed VG.
0.1 1
10-8 10-6 10-4 10-2 100 102
slope=1
slope=2 -3V, 17mA/cm2
-VC(V) J C(mA/cm2 )
-1 0
Figure 3.3 The electric characteristics of the Pentacene-based SCLT with various grid voltages applied. The Au electrode is commonly grounded and the Al collector is negatively biased at VC with respect to Au. The negative collector current IC means the holes are collected by the Al collector and flows out from the transistor. (a) The collector current as a function of the collector voltage various by step VG. (b) The grid current as a function of the collector voltage of the transistor with same fabrication
Figure 3.4 The electric characteristics of the Pentacene-based SCLT in double logarithmic scale with fixed VG.
0.1 1
10
-610
-410
-210
010
2slope=1
slope=2
J
C(m A /c m
2)
-V
C(V)
- 4V, 186.35mA/cm
20.0 0.5 1.0 positive biased at VC with respect to Au. The collector current IC means the electrons are collected by the Au collector. (a) The collector current as a function of the collector voltage various by step VG. (b) The grid current as a function of the collector voltage of the transistor with same fabrication procedure described in (a).
0.0 0.5 1.0
Figure 3.6 The electric characteristics of the C60-based SCLT in double logarithmic scale with fixed VG.
0.01 0.1 1
10-3 10-2
10-1 0.8V, 0.1995mA/cm2
slope=2
slope=1
VC(V)
J C(mA/cm2 )
A B
Emitter (Au)
SIO Al SIO Al
SIO SIO
Collector (Al)
50nm
A B
Emitter (Au)
SIO Al SIO Al
SIO SIO
Collector (Al)
50nm
(a)
(b)
(c)
Figure 3.7 (a) The device structure near one opening of polymer SCLT. Position A is at the center of the opening, while position B is near the grid. (b) The potential profile along the emitter–collector path through the opening when VC is fixed at a negative value.(x), (y), (z) are the potential profile along the path for various conditions. (c) The schematic current–voltage curve of EC diode with the structure Au/P-type/Al. The path through position A in on or off state are denote as AON or AOFF in the diode IV curve. The state of the path through B is also shown. Because of the proximity to the positive biased grid B path can never be fully turned on as A.
A B
Collector (Au)
SIO Al SIO Al
SIO SIO
Emitter (Al)
50nm
A B
Collector (Au)
SIO Al SIO Al
SIO SIO
Emitter (Al)
50nm
(a)
(b)
(C)
Figure 3.8 (a) The device structure near one opening of polymer SCLT. Position A is at the center of the opening, while position B is near the grid. (b) The potential profile along the emitter–collector path through the opening when VC is fixed at a positive value. (x), (y), (z) are the potential profile along the path for various conditions. (c) The schematic current–voltage curve of EC diode with the structure Au/C60/Al. The path through position A in on or off state are denote as AON or AOFF in the diode IV curve. The state of the path through B is also shown. Because of the proximity to the negative biased grid B path can never be fully turned on as A.
-1 0 1 2 structure. The dimension of these images is 3um×3um.
Chapter 4
Self-Assembled Monolayer
4.1 Motivation
Structure of two-tier silicon oxide provides low leakage current, however the deposition of silicon oxide is very time-consuming. Section 4.2.1 will introduce two kinds self-assembled-monolayer (SAM) to replace the top silicon oxide. The benefits of SAM are fast fabrication, good coverage, and can reach the ideal order of leakage current. The metal (base) in chapter 3 is Aluminum (Al), therefore the binding force between Al and SAM is the key point to achieve good coverage. There is a thin film of alumina on Al surface, and alumina has good interaction with acid-based groups.
Therefore, compound I, 4-hexadecyloxybenzoic acid, and Compound , n-octadecylphosphonic acid, are chosen in this chapter.
The metal (emitter) in chapter 3 is Gold (Au), but Indium Tin Oxide (ITO) is the most widely used as a transparent anode in organic electroluminescent (EL) devices due to its high conductivity, work function, and transparency in the visible spectral range. Therefore, in this chapter ITO is used as the follow-up study. Because the work function of ITO is generally not sufficiently large for the contact to be ohmic, there is a barrier to holes injection. Thus, various surface treatments of ITO have been attempted to change the work function of ITO in order to reduce the holes injection barrier height. The work function controlled by chemical modification has recently been applied to enhance holes injection at ITO. The compound , p-chlorobenzoyl chloride dichloride with –COCl binding groups are chosen in this work.
4.2 Materials
4.2.1 SAM as Dielectric
Compound :
Figure 4.1(a) is the chemical structure of compound I, 4-hexadecyloxybenzoic acid. The series of compound I was prepared from 4-hexadecyloxybenzoic acid (obtained from Tokyo Chemical Inc.) via alkylation of the hydroxy group with alkyl bromides according to a literature procedure. Compound Iwas dissolved in a mixture of n-hexadecane and THF (1:1, v/v) at a concentration of 0.25 mM and kept at 25 .
Compound :
Figure 4.1 (b) shows the chemical structure of n-octadecylphosphonic acid.
SAMs were prepared in a solution of 2-propanol at room temperature. Compound was dissolved in isopropanol at a concentration of 5 mM.
4.2.2 SAM as improving holes injection of Indium Tin Oxide
As we know, the work function of ITO is generally not sufficiently large for the contact to be ohmic and so there is a barrier for holes injection. Thus, various surface treatments of ITO have been attempted to change the work function of ITO to reduce the holes injection barrier height. The work function controlled by chemical modification has recently been applied to enhance holes injection at ITO. Figure 4.1 (c) shows the chemical structure of Compound , p-chlorobenzoyl chloride dichloride. It was dissolved in absolute alcohol at a concentration of 5 mM in this work.Figure 4.2 shows the schematic energy diagrams for ITO-treated/Pentacene/Al hole-only single-carrier devices.
4.3 Diode Fabrication
4.3.1 Metal (Al)/Insulator (SAM)/Metal (Al)
Figure 4.3 shows the structure of the diode (Al/SAM/Al) and diode.
Glass substrate clean
Glass substrate (CORNING Eagle 2000) must keep clean or films may become rough. The rough surface would cause point discharge between the insulator and
metal. The steps of clean glass substrate are shown as.
Steps:
(1) De Ion (DI) water current flows for 5 minutes in order to remove the particles.
(2) The substrates should be placed into the in the acetone and under the ultrasonic resonance for 5 minutes in order to remove the organic pollution.
Then, the substrates have to put under the DI water current flow for 5 minutes in order to remove the solvent.
(4) The substrates were put in the KG detergent bath with ultrasonic resonance for 5 minutes in order to remove the particles, fingerprint, and ionic.
(5) The substrates were put under the DI water current flow for 5 minutes in order to remove the solvent.
(6) Finally, the substrates would be fried with dry N2 flow to blow off the water on the substrates.
Bottom and Top metal deposition:
The deposition was started at the pressure around 5×10-6 torr. The 50-nm-thick Al was deposited by thermal evaporation at a deposition rate of 5Ǻ/s. The region was defined by shadow mask.
Compound :
The substrate was exposed under the ultra-violate light for 15 mins to keep surface clean first. It was immersed in the solution containing compound I for four minutes. Then, it was cleaned surface by hexane-soaked tissue.
Compound :
The substrate was exposed under the ultra-violate light for 15 mins to keep
surface clean first. It was immersed in the solution containing compound for four minutes. Then, it was cleaned surface by isopropanol.
4.3.2 Al/SAM/Organic/Al
Figure 4.4 shows the structure of the diode (Al/SAM/organic/Al) in this work.
This section just introduced the organic layer deposition and other processes were same as section 4.2.1. From the result of section 4.3 compound was be used in this section.
Organic active layer fabrication:
4. P3HT:
P3HT was spin coated from chlorobenzene solution (2.5 wt% 1000rpm) on the Au layer, and baked at 200 °C for 10min in vacuum. After we spin coated the P3HT film, we use acetone to clean the unnecessary area. Then, a thin P3HT layer of about 1338 Å was obtained.
5. Pentacene
The pentacene material obtained from Aldrich without any purification was directly placed in the thermal coater for the deposition. The deposition was started at the pressure around 3×10-6torr. The 500Å -thick pentacene was deposited by thermal evaporation at a deposition rate of 1Ǻ/s. The active region was defined by shadow mask.
6. C60
The substrate was exposed under the ultra-violate light for 15min to keep surface clean. The C60 was directly placed in the thermal coater for the deposition. The deposition was started at the pressure around 3×10-6torr. The 200-nm-thick C60 was deposited by thermal evaporation at a deposition rate of 1Ǻ/s. The active region was defined by shadow mask.
4.3.3 ITO/SAM/Organic/Al
Figure 4.5 shows structure of the diode (ITO/SAM/pentacene/Al) and the diode in this work. ITO glass was cleaned with acetone and isopropanol first. The sample was immersed in the solution containing compound for forty minutes. Then it was cleaned surface by absolute alcohol.
Organic active layer fabrication:
Pentacene
The pentacene material obtained from Aldrich without any purification was directly placed in the thermal coater for the deposition. The deposition was started at the pressure around 3×10-6torr. The 180-nm-thick pentacene was deposited by thermal evaporation at a deposition rate of 1Ǻ/s. The active region was defined by shadow mask.
Top metal deposition
The deposition was started at the pressure around 5×10-6torr. The 50-nm-thick Al was deposited by thermal evaporation at a deposition rate of 5Ǻ/s. The region was defined by shadow mask.
4.4 Diode characteristics
4.4.1 SAM as DielectricFigure 4.6 shows the insulator characteristic of different SAM. Figure 4.7 shows the insulator characteristics of Al/SAM/organic/Al diodes. According to the structures of SAMs, the number of OH groups of compoundis more than that of compound.
Therefore, compound has better binding with Aluminum than compound. From the difference, the characteristic of compound is better than that of compound .
Compound is chosen as insulator in this work.
4.4.2 SAM as improving holes injection of Indium Tin Oxide
Figure 4.8 shows the J-V characteristics of diode with ITO chemically modified with –COCl binding groups of p-chlorobenzene derivatives. In Figure 4.8, J-V
characteristics of hole-only single-layer devices with ITO modified with p-chlorobenzene derivatives with –COCl binding groups are compared with those of the device with as-cleaned ITO. While ITO treated with –COCl gave the intermediate J-V characteristics that are much better than those of as-cleaned ITO. These results suggest that the efficient hole injection into Pentacene. It is because that the increase in the work function of ITO covered with the two dipole layers introduced by the surface modification. Therefore, the device characteristics are strongly correlated with the change in the observed work functions of various modified ITO.
4.5 Discussion
1. The characteristics of Al/SAM/Al and Al/SIO/Al.
Figure 4.9 shows the characteristics of Al/SAM/Al and Al/SIO/Al. When the operating voltage is 1V the leakage with thickness of SIO is 30-nm, the leakage current level is 10-2 to 10-3mA/cm2. When the operating voltage is 1V the leakage with n-octadecylphosphonic acid, the leakage current level is 10-3 to 10-4mA/cm2. The result of the insulating properties of n-octadecylphosphonic acid is superior to SIO. It is because that n-octadecylphosphonic acid uses functional groups to attach with alumina on aluminum surface. It can reduce the leakage current from the side of aluminum. Therefore, the benefits of n-octadecylphosphonic acid are fast fabrication, good coverage, and can reach the lower leakage current than SIO.
2. The difference of Au/pentacene/Al and ITO/pentacene/Al
Figure 4.10 shows the diode characteristics of different bottom metal. When the operating voltage is 1V the characteristic of ITO-based diode is better than Au-based diode. For our device operating voltage is no more than 1V, therefore under the condition ITO-based is better than Au-based for Pentacene SCLT.
Chapter 4
(a) (b) (c)
Figure 4.1 Self-assembled monolayer dielectrics (a) Chemical structure of 4-hexadecyloxybenzoic acid. (b) Chemical structure of n-octadecylphosphonic acid.
(C) Chemical structure of p-chlorobenzoyl chloride dichloride.
Figure 4.2 Schematic of energy diagrams for ITO treated/Pentacene/Al hole-only single-carrier devices.
Glass Al Al Glass
Al Al
organic
Glass Al Al organic
Glass Al Al
Figure 4.3 Structure of the diode (Al/SAM/Al) and diode.
Figure 4.4 Structure of the diode (Al/SAM/organic/Al) in this work.
o rg a n ic
G la s s IT O
A l o rg a n ic
G la s s IT O
A l
Figure 4.5 Structure of the diode (ITO/SAM /Al) and the diode in this work.
-4 -2 0 2 4 10
-410
-310
-210
-110
010
110
210
310
4Al/Al
Al/SAM 1/Al Al/SAM 2/Al
J( m A /c m
2 )
V(V)
Figure 4.6 The insulator characteristic of different SAM.
-4 -2 0 2 4
-4 -2 0 2 4 10
-610
-510
-410
-310
-210
-110
010
110
2V(V)
J( m A /c m
2 )
Al/C60/Al
Al/SAM/C60/Al
(C)
Figure 4.7 The insulator characteristics of Al/SAM/organic/Al diodes. (a) Comparison of Al/P3HT/Al and Al/SAM/P3HT/Al, (b) Comparison of Al/Pentacene/Al and Al/SAM/Pentacene/Al and (c) Comparison of Al/C60/Al and Al/SAM/C60/Al
-2 0 2 4 10
-710
-510
-310
-110
110
310
5V(V)
J( m A /c m
2 )
ITO/Pentacene/Al ITO/SAM/Pentacene/Al
-4 -2 0 2 4
10
-510
-410
-3V(V)
J( m A /c m
2 )
Al/SIO/Al Al/SAM/Al
Figure 4.8 The J-V characteristics of diode with ITO chemically modified with –COCl binding groups of p-chlorobenzene derivatives
Figure 4.9 shows the characteristics of Al/SAM/Al and Al/SIO/Al.
Figure 4.10 shows the diode characteristics of different bottom metal.
-2 0 2 4 6 8 10
10
-910
-210
5J( m A /c m
2 )
V(V)
Au/Pentacene/Al
ITO/SAM/Pentacene/Al
Chapter 5
Conclusions
We try to fabricate the low operation voltage space-charge-limited transistor (SCLT) with new structure and investigate the effects of self-assembled-monolayer (SAM) on Al or ITO surface. A 1-V P3HT-based SCLT with on/off current ratio 24310 is firstly demonstrated. Significant impacts of thin film morphology on the leakage current of organic SCLTs are firstly observed and recognized as new leakage phenomena. Surface profile reveals that the grain structure in pentacene/C60 produces holes make the top metal penetrates into pentacene/C60 film and shield the base electric field while P3HT have rather smooth and dense morphology. Therefore, pentacene SCLTs and C60 SCLTs suffer from high leakage current even when base leakage is as low as in P3HT SCLTs.
We tried to reduce the production time and increase output current by using SAMs. N-octadecylphosphonic acid using OH groups to attach with alumina on Al surface to reduce the leakage current from the side of Al. Therefore, it is faster fabrication, better coverage, andlower leakage current than SIO. P-chlorobenzene derivative with –COCl binding groups is used to increase the work function of ITO.
For our device operating voltage is no more than 1V, therefore under the condition ITO-based is better than Au-based for Pentacene SCLT.
In future work, we will try to solve the morphology effect by spin solution-processed polymer on pentacene. We will fabricate SCLTs with various bottom metal based on this thesis results.
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