Chapter 1 Introduction
1.3 Basic information about P3HT
Poly(3-hexylthiophene), P3HT, is a conjugate polymer and is used as the active layer of organic thin-film transistors. Four carbon atoms and one sulfur atom compose a thiophene. The thiophene is the main chain structure of the P3HT. The side chain of P3HT is a hexyl group. Depending on the position of the alkyl chain to the main chain, there are two different regioregularities: head-to-tail and head-to-head, respectively. Fig. 1-3 shows the chemical structure of the P3HT and the different structure between the head-to-tail and the head-to-head.
Fig. 1-3 The chemical structure of the P3HT [22]
According to the research of H. Sirringhaus et al., in solution processes, the self-organization of conjugated polymers forms ordered microstructures, in which these micro-size domains are embedded in an amorphous matrix. In P3HT, the self-organization can result in lamellaes with two-dimensional conjugated sheets formed by interchain stacking as shown in Fig. 1-4 (a) [23]. The two-dimensional conjugated sheets may form a narowire-like structures (Fig. 1-4 (b)) when strong self-organization process occurs. As the P3HT was spin-coated on the substrate, the lamellae of P3HT have two orientations related to the substrate: normal and parallel to the substrate. Fig. 1-5 illustrates the two different orientations of P3HT. [24]
Fig. 1-4 (a) 2D conjugated P3HT lamellae (b) nanowire-like structures form by self-organization [23]
Fig. 1-5 Orientations of P3HT on the substrate (a) P3HT lamellae normal to substrate (b) P3HT lamellae parallel to substrate [24]
The P3HT is a semiconducting material. Because of the overlaps of the electron orbitals, the energies of excitation states and the steady states of π electrons split into the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The HOMO and the LUMO are a concept that is similar to the energy band in inorganic semiconductors. However, the HOMO is analog to the valence band and the LUMO is analog to the conduction band. The HOMO of P3HT is between -4.8 and -5.2 eV and the LUMO of P3HT is about from -2.7 to -3.0 eV according to different literature. The band gap of P3HT is about 2.0 eV.
(a) (b)
(a) (b)
1.4 Basic information about PCBM
It have been reported that solution- processed [6,6]-phenyl C61-butyric acid methyl ester (PCBM),shows high field-effect electron mobility 0.004–0.01 cm2/V s. [25-26] And its structure is showed in Fig 1-6. The HOMO of PCBM is about -6.1 eV and the LUMO of PCBM is about -4.2 eV according to different literature. The band gap of PCBM is about 2.0 eV.
Fig. 1-6 The chemical structure of [6,6]-phenyl-C61-butyric acid methyl ester[25-26]
Besides, no diffraction peak was observed in the PCBM film from the XRD measurement in FIG. 1-7. The AFM image of the PCBM film demonstrates a homogeneous morphology without large crystalline domains FIG. 1-7.These results indicate that the PCBM film takes an amorphous-like structure or is composed of homogeneously distributed small nanocrystals. These findings agree with the results of transmission electron microscopy observation and electron diffraction measurement of spin-coated PCBM film reported by Yang et al.[27]
Fig. 1-7 Out-of-plane XRD patterns of spin-coated PCBM films.
Insets show AFM images of PCBM films .[27]
1.5 Top Contact Structure of Field-Effect Transistors
According to the position of source-drain electrode, organic field-effect transistors (FET) are divided into two structures: top contact and bottom-contact. In Fig. 1-8, the advantage of top contact structures is that there is a smaller contact resistance between the active layer and electrodes and the performance is better generally. But the channel length defined by the shadow mask can hardly decrease. The FETs with top contact structures has relative high off current because the back channel current flow near the source drain electrode.
Fig. 1-8 The field effect transistor structures Top contact
1.6 Motivations
Polymer field-effect transistors (FETs) show promise as the critical components for low-cost, flexible electronics with various applications. And it is reasonable for us to adopt the organic material P3HT and PCBM which have better solubility for solution-process step.
These applications typically require complementary logic elements, which require both p-type and n-type transistors. However, it is beneficial to utilize “ambipolar” FETs, which
function both as p-channel and n-channel transistors by using the composite film of P3HT and PCBM. Therefore, it is our important aim to practice ambipolar complementary field-effect transistors inverters in this thesis.
Besides, we want to investigate that the characteristics of OFETs made of polymer
blends consisting of P3HT and PCBM by varing the composition ratio. Furthermore, the correlation between the contact metal and work function needed to be realized clearly. Since P3HT and PCBM were studied in detail in organic solarcells, the condition for solvent annealing and proper thermal annealing may exist the condition for optimization and is needed to consider in OFET . Finally, because the FETs based on photosensitive composite films, we want to check how the device can tune its electrical characteristic under illumination.
And the mechanism of photoelectric effect are tried to observed and explained.
1.7 Thesis Organization
This thesis is organized as following. In this chapter, the background of this study is described briefly:
Chapter 1 introduces the background knowledge of materials and FFT structures
Chapter 2 explains the mechanism of the charge transportation in organic materials and the method of the parameter extraction in this article.
Chapter 3 describes the experiments and the measurement instruments used in this study.
Chapter 4 describes the results of characteristics from measurement.
Finally, chapter 5 gives the conclusions of our experiments.
Chapter 2
Mechanism and Operation
2.1 The Charge Carrier Transportation in Organic Semiconductors
The interaction between molecules in organic materials is van der Waals force. The van der Waals force is relative weaker than the covalent bonding between the atoms in inorganic materials. Hence the charge carrier transportation is quite different between organic and inorganic materials.The π-bonding electron cloud has two states: localized states and delocalized states.
The π-bonding electron is localized if the electron bound to particular atom. The localized π-bonding electron can not contribute to the carrier transportation. Fig. 2-1 shows the π-bonding electron states of benzene.
Fig. 2-1 π electron states (a),(b): localized states (c): delocalized states [28]
Generally, there are two models to describe the delocalized electrons transportation in organic materials, hopping model [29] and multiple trapping and release model (MTR) [30]
(a) (b) (c)
2.2 Hopping Model
In organic materials, the intermolecular transportation of charge carrier depends on hopping as shown in Fig.2-2. This is the limitation of the charge carrier mobility in organic materials. Because the phonons would assist the hopping of carriers, the mobility increases with the increasing temperature. The relation between the mobility of the hopping and the temperature can be described as the following equation:
1/
0exp[ ( / ) ]T T0 α
μ μ= −
where the α is ranged from 1 to 4.
Fig.2-2 Charge carrier hopping
2.3 Multiple Trapping and Release (MTR)
The MTR model is widely used in a-Si semiconductors. In MTR model, it assumes that there exist localized levels in the band gap due to defects. These localized levels are like traps for charge carriers. These levels would form a narrow band with high concentration of trap levels. When the charge carriers transport to the levels, the carriers would be trapped with the probability near 100%. On the other hand, the activation energy of the carrier determines the release of the carrier. The released carrier would contribute to the transportation and the drift mobility is given as following:
0 exp( / )
D E kTt
μ =μ α −
where Et is the energy level of the defects, α is the ratio between the density of states at the bottom of the band and the density of traps.
2.4 The Operation of Organic Field Effect Transistor
The organic thin-film transistors are operated in accumulation mode. When the gate biases a negative voltage, the holes would accumulate in the interface between the active layer and the insulator. If there are enough holes to accumulate in the interface, the channel is formed and the transistor is in ON state. When the drain is biased at a negative voltage, the drain current would flow from the source to the drain. The concept is also the same for a n-type OFET while the gate biases a positive voltage, the electrons would accumulate in the interface between the active layer and the insulator.
Fig. 2-3 illustrates the operation of the (a) p-type and (b) n-type organic transistors.
Fig. 2-3 Operation of a)p-type and b)n-type OFET
The transistor is a three-terminal device. There are two transfer characteristics plots.
The first plot is ID-VD characteristics as shown in Fig. 2-4. The ID versus VD relation is measured under several constant VG. The transfer characteristics are separate into two regimes. The linear regime occurs when VD approaches to zero. The transistor behaves as a resistance. When VD is increasing, the transistor would be in the saturation regime. In saturation regime, the ID is governed by VG.. The second plot is ID-VG transfer characteristics as shown in Fig. 2-5. From the ID-VG plot, many parameters like the mobility and threshold voltage can be extracted.
(a) (b)
2.5 The Parameters Extraction of Organic Thin Film Transistors
Although the transportation mechanism in organic materials is different from that of inorganic materials, the transfer characteristics are similar. The formula derived from the inorganic semiconductors was adopted to calculate the parameters like the field-effect mobility and threshold voltage in this article.0 -10 -20 -30 -40 -50 -60
Fig. 2-4 ID-VD output characteristics plot of the p-type transistor
The typical p-type ID-VD transfer characteristics is shown Fig. 2-4. When the TFT is operated at linear regime, the drain current is governed by the equation 2-1.
D capacitance per unit area in F/cm2, μ is the field-effect mobility in cm2/Vs, the Vt is the threshold voltage in Volt.
When VD>VG-VT, the TFT is operated at saturation regime. The drain current is given by the equation 2-2.
The μ can be calculated by differentiating the square root of ID in saturation regime
L
μ
The slope can be obtained from the ID-VG transfer characteristics plot shown in Fig.
2-5. The fitting line of the square root of ID in ID1/2-VG plot in the saturation regime, the intersect of the line and the x-axis is the VT. The turn on voltage Von and the on/off ratio can be obtained form the ID-VG transfer characteristics plot.
The subthreshold swing is a parameter to determine the switch speed of a transistor. It can be calculated by equation 2-4.
]
The dimension of subthreshold swing is V/decade. The subthreshold swing gives the degree of the switch property of a transistor.
10-9
Fig. 2-5 ID-VG transfer characteristics plot of the p-type transistor
2.6 The Principle for the Operation of Complementary-like Inverter
Complementary metal–oxide–semiconductor (CMOS) is a major class of integrated circuits used in digital logic circuits and for a wide variety of analog circuits such as image sensors, data converters, and highly integrated transceivers for many types of communication. A CMOS inverter contains a PMOS and a NMOS transistor connected at the drain and gate terminals, a supply voltage VDD at the PMOS source terminal, and a ground connected at the NMOS source terminal, were VIN is connected to the gate terminals and VOUT is connected to the drain terminals. It is important to notice that the CMOS does not contain any resistors. Two important characteristics of CMOS devices are high noise immunity and low static power consumption. Significant power is only drawn when the transistors in the CMOS device are switching between on and off states.
Three potentials in the model would simply be VG, VD, and VS. We use the source/drain-symmetric model to show that two ambipolar transistors with the same gate voltage connected in parallel, as shown in Fig.2-6 , behave as a NMOS and a PMOS transistor with twice the channel current of either single device for any combination of VG, VS, and VD.
Fig. 2-6 ID-VG transfer characteristics plot of the ambipolar transistor
The static characteristics of an inverter are usually described by a voltage transfer characteristic (VTC), which is sometimes also called a DC transfer characteristic. The VTC
is essentially a plot of the inverter’s output voltage as a function of its input voltage. A typical inverter VTC is shown in Fig.2-7 .
Fig. 2-7 Vout-Vin transfer characteristics plot of the ambipolar FET
When the inverter’s input voltage is low, its output voltage is high. When the inverter’s input voltage is high, it’s output voltage is low. For each input voltage, we define an incremental voltage gain of the inverter as the slope of the VTC at that point. The incremental
voltage gain basically tells us by how much the output voltage will change for a given small change in the input voltage at any point along the curve. Note that the VTC has a negative slope everywhere, which implies that the inverter’s output voltage decreases as its input voltage increases and vice versa. The VTC has three distinct regions: two in which the curve is relatively flat and one in which the curve is quite steep. We normally operate the inverter in one of the two flat parts of the curve when we use it as a logic gate.
By keeping the inverter biased in the steep part of the VTC, we can also use it to linearly amplify small signals. The logic high (VIH) and low (VIL) threshold voltages are defined by where the slope of the VTC is negative one. Input voltages below VIL are
considered to be logical 0s and input voltages above VIH are considered to be logical 1s. The range of input voltages between VIL and VIH is called the transition region. The point in the transition region where the inverter’s input voltage is equal to its output voltage is called the transition threshold (VM). The incremental gain of the inverter attains its maximum value at
the transition threshold, so we would like to operate the inverter at or near this point when using it as an amplifier.
The general shape of the VTC of the inverter is critical to the reliable operation of digital circuits. Because the magnitude of slope of the VTC is less than unity on the ends of the VTC (i.e., in the logic high and low parts of the curve) and it is larger than unity in the transition region, the inverter has two very important properties that make digital circuits robust.
In essence, an inverter can take a weak 0 and turn it into a strong 1 or it can take a weak 1 and turn it into a strong 0. We simply could not build digital circuits with many stages of processing if it weren’t for this restoring property.
In Table 2-1,the switch model of the traditional MOSFET transistor is defined as follows:
Table 2-1 Transistor "switch model"
MOSFET Condition on MOSFET State of MOSFET
NMOS Vgs<Vtn OFF
NMOS Vgs>Vtn ON
PMOS Vsg<Vtp OFF
PMOS Vsg>Vtp ON
When VIN is low, the NMOS is "off", while the PMOS stays "on": instantly charging V to logic high. When Vin is high, the NMOS is "on and the PMOS is "on: draining the voltage at V to logic low.
Chapter 3
Experiments
3.1 The Materials
The head-to-tail regioregular poly(3-hexylthiophene) (or as called HT rr-P3HT) bought from Rieke Metals.Inc。and [6,6]-phenyl-C61-butyric methyl ester (PCBM) which is shown in Fig. 3-1(a) (b) were used in the composite active layer of organic ambipolar field-effect transistors. We dissolved the P3HT and PCBM in the chlorobenzene (CB) . In order to make better ambipolar OFET, PTCDI-C8 is the candidate to replace the PCBM and serves as the role to study the OFET with bi-layer structure. Besides, wafers with 200 nm SiO2 deposited , calcium, aluminum, gold, silver ,Molybdenum oxide (MoO3 :Sigma-Aldrich, 99.99% purity) are used in experiment.
The chemical structures of materials mentioned are shown in Fig. 3-1.
Fig. 3-1 The structure of materials that used in experiments.
(d) (b)
(c) (a)
3.2 The Device Fabrication
As the wafers were prepared, fig. 3-2 show the flow chart of experimental procedure and the devices were fabricated in N2-filled environment
by following steps:
1. The surface treatment of the gate/active layer interface.
2. To spin coat the P3HT and PCBM as the active layer.
3. To treat the devices with the solvent annealing.
4. To treat the devices with the thermal annealing.
5. To evaporate wide bandgap material as buffer layer only in special cases . 6. To evaporate metal electrode as the electrode.
3.2.1 Substrates Preparation
The substrates were heavily doped n-type highly silicon wafers and resistance was about 0.001~0.003 ohm-cm. The conductance of the wafers is high enough to serve as the gate electrode. By using thermal growth, about 200 nm SiO2 deposited on the wafers is as the gate insulator. The measured capacitance per unit area is about 14.1 nF/cm2.Our substrates are rinse with DI (de-ionized) water for 5 minute to remove large particles. And then the substrates are rinsed with the mixture of H2SO4 and H2O2 for 15 minute. The mixed ratio is H2SO4:H2O2=3:1. H2SO4 and H2O2 are strong oxidant and capable of removing chemical substances. To remove the residual mixture H2SO4 and H2O2, the wafers are rinsed with DI water for 5 minute. Finally, the nitrogen gun is uesd to blow our wafers to remove the residual water in sequence. After the first steps of RCA cleaning, the wafers are placed into an hot plate which the temperature is set to 110℃ for over 30 mins to remove the crystal water. After cleaning the substrate, the substrate was also processed with the UV/ozone dry-cleaning technology .
3.2.2 The Surface Treatment of the Gate/Active Layer Interface
Generally , the defects between the interface of inorganic material and the organic material come from the different surface properties. Before the deposition of the active layer, the surface of the wafers are treated with UV-ozone for 15 minute to increase –OH groups on the wafers and the organic material in some cases.
3.2.3 Spin Coating of the P3HT/PCBM As the Active Layer
The rr-P3HT was obtained from the Rieke Metals and the PCBM was obtained from the Rieke Metals, Inc. The composite films of active layer of P3HT and PCBM were spin coated on the surface-treated substrate in the glove box which filled with nitrogen. The spin rate was set to 3000 rpm for 60 seconds to obtain uniform films.
3.2.4 Treat with Solvent Annealing and Thermal Annealing
In spite of high spin rate, there was still some solvent left on the device . To controll the evaporation rate of the residual solvent to be slower than usual . The device is placed in a smaller oven and then the evaporation rate can decrease in saturation solvent vapor pressure .All devices were placed in small Petri dishes until the residual solution dried for 30mins.For our OFETs, the proper annealing temperature is an important factor to optimize The Electrical characterization of ambipolar OFET. After solvent annealing, we placed devices on a 148℃ pre-heated hotplate for 20 minutes. Then all devices were placed in a room temperature environment to cool down.
3.2.5 Ultra Thin Metal oxide MoO3 Deposited
Since the MoO3 are wide band-gap materials . Thus, they can serve as buffer layer between electrode and active layer and for further comparison with standard device.
3.2.6 Evaporation Metal Electrode on Semiconductor Layer as the Electrodes After the composite film of P3HT-PCBM employed as the active layer, different metals Ca, Al, Ag and Au should be chosen as the main electrode by thermal evaporation to form the source and drain electrodes called top-contact structure in the N2-filled glove box. It was sublimated by thermal coater under a back pressure below 2×10-3 Pa. As for deposition rate, it was controlled at a rate of 2.0~2.5 Å/sec by a quartz oscillator during the electrode formation until the total thickness approached 50 nm. However, the channel length and
3.2.6 Evaporation Metal Electrode on Semiconductor Layer as the Electrodes After the composite film of P3HT-PCBM employed as the active layer, different metals Ca, Al, Ag and Au should be chosen as the main electrode by thermal evaporation to form the source and drain electrodes called top-contact structure in the N2-filled glove box. It was sublimated by thermal coater under a back pressure below 2×10-3 Pa. As for deposition rate, it was controlled at a rate of 2.0~2.5 Å/sec by a quartz oscillator during the electrode formation until the total thickness approached 50 nm. However, the channel length and