Chapter 2 Literature Reviews
2.4 Atmospheric Pressure Plasma System
The operation of arc plasma is similar to an arc-welding machine, where an electrical arc is struck between two electrodes. The high energy of arc creates high temperatures ranging from 3000℃ to 7000℃. The plasma is highly ionized gas which is enclosed in a chamber. The waste material is fed into the chamber and the intense heat of the plasma break down organic molecules into their elemental atoms. In the strict control of the process, these atoms recombine into harmless gases, such as carbon dioxide. Solids such as glass and metals are melted to form materials, similar to hardened lava, in which toxic metals are encapsulated. There is no burning or incineration and no formation of ash with plasma arc technology. There are two main types of plasma arc processes: plasma arc melter and plasma torch.
Plasma arc melters have very high destruction efficiency. They are very robust; they can treat any waste with minimal or no pretreatment;
and they produce a stable waste form. The arc melter uses carbon electrodes to strike an arc in a bath of molten slag. The consumable 28 carbon electrodes are continuously inserted into the chamber, eliminating the need to shut down for electrode replacement or maintenance. The high temperatures produced by the arc convert the organic waste into light organics and primary elements.
Combustible gas is cleaned in the off-gas system and oxidized to CO2
and H2O in ceramic bed oxidizers. Due to the use of electrical heating in the absence of free oxygen, the potential for air pollution is low. The
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inorganic portion of the waste is retained in a stable, leach-resistant slag.
In plasma torch systems, an arc is struck between a copper electrode and either a bath of molten slag or another electrode of opposite polarity. Plasma torch systems have very high destruction efficiency with plasma arc systems; they are very robust; and they can treat any waste or medium with minimal or no pre-treatment. The inorganic portion of the waste is retained in a stable, leach-resistant slag. The air pollution control system is larger than for the plasma arc system, due to the need to stabilize torch gas.
2.4.2 Atmospheric Pressure Plasma Jet
Atmospheric pressure plasma jet is meaning that operating at atmospheric pressure and it is non-thermal glow discharge plasma system.
The non-thermal plasma generates highly reactive ions, electrons and free radicals. The reactive species are directed onto a surface where the desired chemistry occurs. However the overall gas temperature remains quite cold, but the electrons are quite hot, typically 50-300℃.
2.4.3 Corona Discharge
A corona is a process by which a current develops between two high-potential electrodes in air, by ionizing that fluid to create a plasma around one electrode, and by using the ions generated in plasma processes as the charge carriers to the other electrode.
Corona discharge usually involves two asymmetric electrodes, one highly curved such as the tip of a needle or a narrow wire, and another
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one of low curvature such as a plate or the ground. The high curvature assures a high potential gradient around one electrode, for the generation of the plasma.
Coronas may be positive, or negative. This is calculated by the polarity of the voltage on the high curvature electrode. If the curved electrode is positive associated to the flat electrode, it will have a positive corona, and vice visa. The physics of positive and negative coronas are obviously different. This asymmetry structure is a result of the great difference in mass between electrons and positively charged ions, and so only the electron having the ability to undergo a significant degree of ionizing inelastic collision at common temperatures and pressures.
2.4.4 Dielectric Barrier Discharge (DBD)
Dielectric barrier discharges involve a specific class of high voltage, ac, gaseous discharges that typically operate in the near atmospheric pressure range. Their defining feature is the presence of dielectric layers that make it impossible for charges generated in the gas to reach the conducting electrode surfaces. With each half cycle of the driving oscillation, the voltage applied across the gas exceeds that required for breakdown, and the formation of narrow discharge filaments initiates the conduction of electrons toward the more positive electrode. As charge accumulates on the dielectric layer at the end of each filament, the voltage drop across the filament is reduced until it falls below the discharge sustaining level, therefore the discharge is quenched. The low charge mobility on the dielectric not only contributes to this self-arresting
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of filaments but also limits the lateral region over which the gap voltage is diminished, thereby allowing parallel filaments to form in close proximity to one another. Thus, the entire gas filled space between parallel electrodes can become, on average, uniformly covered by transient discharge filaments, each roughly 0.1mm in diameter and lasting only about 10ns.
The DBD’s unique combination of non-equilibrium and quasi-continuous behavior has provided the basis for a broad range of applications and fundamental studies. Its use in industrial ozone reactors has generated interest in optimizing conditions for specific chemical reactions. To this end, experimental DBD studies have explored different gas mixtures, electrical characteristics, and geometries. Related work has focused on maximizing the ultraviolet radiation from excimer molecules produced in DBD’s. Several researchers have modeled single filament dynamics in order to account for the many reactions involving electrons, ions, neutral atoms, and photons. These efforts have been moderately successful in explaining and predicting the chemical and radiative properties of various DBD systems. On another research effort, it has been seen that the transverse spatial distribution of discharge filaments in 2D, parallel plate DBD’s can take the form of stable, large-scale patterns reminiscent of those associated with magnetic domains. These patterns have been modeled with some success using methods that apply generally to pattern formation in nonlinear dynamical systems. Thus, the dynamical interactions between filaments, as well as the chemical and electronic interactions within filaments have proven interesting.
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Figure 2-1 Schematic Diagram of Co-evaporation System.[46]
Figure 2-2 Schematic Diagram of RF Sputtering System.[47]
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Figure 2-3 Schematic Diagram of Electro-Deposition System.[48]
Figure 2-4 Schematic Illustration of the Arc Plasma.[49]
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Figure 2-5 Schematic Illustration of the AP Plasma Jet structures.[50]
Figure 2-6 Schematic the Principle of Corona Discharge.[51]
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Figure 2-7 Schematic the Principle of Dielectric Barrier Discharge.[52]
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Table 2-1 The Properties of Selenium.
Physic Properties
Electron affinity 194.97 KJ/mol (2 eV) Dissociation of the
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Table 2-2 Amorphous and Crystalline Selenium.
Form Structure Appearance Characteristic
Amorphous
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Table 2-3 Density of Charge Species in the Plasma Discharge.
Source Plasma Density (cm-3)
Low Pressure Discharge 108-1013 Arc and Plasma Torch 1016-1019
Corona Discharge 109-1013
Dielectric Barrier Discharge 1012-1015 Capacitive Discharge 1011-1012
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Chapter 3 Experiments
3.1 Experimental Procedures
3.1.1 Optimize the Fabrication Parameters
Figure 3-1 Schematic Illustration of Optimize the Fabrication Parameters.
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3.1.2 Investigation of Selenium Films by APPECVD
Figure 3-2 Schematic Illustration of Investigation of Selenium Films by APPECVD.
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3.1.3 Investigation of Device Characterization
Figure 3-3 Schematic Illustration of Investigation of Device Characterization.
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3.2 Experimental Equipment and Parameters 3.2.1 Experimental Equipment
The equipment of amorphous pressure plasma chemical vapor deposition is show in Figure 3-4. This technique uses the element Se vapor to deposit Se film. The source of selenium vapor is selenium pellet heated by the heater to higher than 220℃ and the selenium vapor fill in the quartz holder. The equipment use the Argon gas to be the carrier gas and the main gas, and carry the Se vapor to pass through the electrode of applied high voltage. Therefore, the plasma will dissociate the ring structures of Sen, where n varies from 2<n<8, and become the Se element.
And then the Se element cause by the carrier gas to deposit on the substrate. Gap is between quartz holder and the substrate, which affects the quality and deposition rate of the selenium film. The substrate temperature can control by the hot plant in order to benefit deposition the film. In this equipment, the substrate is immovable, but the quartz holder and electrode is movable. The scan area is show in Figure 3-5, which is 10* cm2, where is the moving distance of quartz holder.
3.2.2 Experimental Parameters
The standard glass clean procedure is follow:
(1) Ultrasonic clean in Acetone 10min (2) Ultrasonic clean in Isopropanol 10min (3) Ultrasonic clean in DI water 10min (4) Nitrogen Drying
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The APPECVD systems have lots of parameters including power, substrate temperature, gap distance, scan times, scan speed, carrier gas flow rate, background pressure, selenium temperature. The experimental of optimize the fabrication parameters in order to find the optimal fabrication parameters for the APPECVD system. Use change the different parameters, i.e. selenium temperature, carrier gas flow rate, background, power, to investigate quality, surface roughness and deposition rate of selenium film on the glass. The parameters are show in Table 3-1.
The parameters of investigation of selenium films by APPECVD are show in Table 3-2. Use change the different plasma power and substrate temperature to investigate quality, surface roughness, crystalline, adhesion of selenium film on the CIG recourse layer. In this essay, the Se/In/CuGa/Mo/Glass samples will sequentially run the selenization in 550℃ by the RTP process, the temperature curve of RTP process was show in Figure 3-6. And then investigate quality, surface roughness, crystalline, compound, composition of Cu(In,Ga)Se2 film on coated-Mo glass. The cell completions of Al/ITO/ZnO/CdS/CIGS grown on Mo-coated substrate follow by the standard process. The current density-voltage (J-V) measurement was performed under the standard AM1.5G spectrum for 100mW/cm2 irradiance at room temperature. The optical loss mechanism of the device can measure by external quantum effect (EQE).
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3.3 Characterization Analysis and Measurement Equipment 3.3.1 Scanning Electron Microscopy (SEM)
SEM stands for scanning electron microscope. The SEM is a microscope that substitute electron for light to form an image. Scanning electron microscopes have developed new areas of study in the medical and physical science communities since their development in the 1950.
SEM uses a focused beam of high energy electrons to generate a variety of signals at the surface of solid samples. The signals that derive from electron and sample interactions reveal information about the sample including external morphology, chemical composition, and crystalline structure and orientation of materials making up the sample. The researchers can inspect a much larger variety of samples by SEM.
Accelerated electrons carry significant amounts of kinetic energy in the SEM. When the incident electrons are decelerated in the solid sample, this energy is dissipated as a variety of signals produced by electron and sample interactions. These signals include secondary electrons, backscattered electrons, diffracted backscattered electrons, photons and heat. Secondary electron and backscattered electrons are commonly used for imaging samples. Secondary electrons are most valuable for showing morphology and topography on samples and backscattered electrons are most valuable for illustrating contrasts in composition in multiphase sample. X-ray generation is produced by inelastic collisions of the incident electrons with electrons in discrete orbital of atoms in the sample.
X-rays are fixed wavelength as the excited electrons return to lower energy states. Thus, characteristic X-rays are produced for each element
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in a mineral that is excited by the electron beam. SEM analysis is considered to be non-destructive; that is, X-rays generated by electron interactions do not lead to volume loss of the sample, so it is possible to analyze the same materials repeatedly.
The SEM has many advantages over traditional microscopes. We describe it following:
(1) It has large depth of field, which allows more of a specimen to be in focus at one time.
(2) SEM has much higher resolution so closely spaced specimens can be magnified much higher levels.
(3) Researcher has more control in the degree of magnification, because of the SEM uses electromagnets rather than lenses.
3.3.2 Atomic Force Microscopy (AFM)
Atomic force microscopy is a manner of measuring surface morphology on a scale from angstroms to 100 microns. The technique involves imaging a sample through the use of a probe or tip, with a radius of 20 nm. The tip is held several nanometers above the surface using a feedback mechanism that measures surface tip interactions. Variations in tip height are recorded while the tip is scanned repeatedly across the sample, producing a topographic image of the surface.
In addition to basic AFM, the instrument in the Microscopy Suite is capable of producing images in a number of other modes, including tapping, magnetic force, electrical force and pulsed force. In tapping mode, the tip is oscillated above the sample surface, and data may be
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collected from interactions with surface morphology, stiffness and adhesion. This result in an expanded number of image contrast methods compared to basic AFM. Magnetic force mode imaging utilizes a magnetic tip to enable the visualization of magnetic domains on the sample. In electrical force mode imaging a charged tip is used to locate and record variations in surface charge. In pulsed force mode, the sample is oscillated beneath the tip, and a series of pseudo force distance curves are generated. This permits the separation of sample topography, stiffness, and adhesion values, producing three independent images, or three individual sets of data, simultaneously.
3.3.3 X-Ray Diffraction (XRD)
X-ray diffraction (XRD) is one of the most important techniques for qualitative and quantitative analysis of crystalline compounds. The XRD technique provides information includes types and nature of crystalline phase present structural makeup of phase, degree of crystallinity, amount of amorphous content which microstrain and size and orientation of crystallites.
When a sample is irradiated with a parallel beam of monochromatic X-ray, the atomic lattice of the sample acts as a three dimensional diffraction grating causing the X-ray beam to be diffracted to specific angles. The diffraction pattern that includes position (angles) and intensities of the diffracted beam that provides several types of information about the sample which are discussed below:
Angles are used to calculate the interplanar atomic spacing
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(d-spacing). Because every crystalline material will give a characteristic diffraction pattern and can act as a unique “fingerprint”, the position “d”
and intensity “I” information are used to identify the type of material by comparing them with patterns for over 80,000 data entries in the International Powder Diffraction File (PDF) database. Hence, identification of any crystalline compounds, even in a complex sample by XRD.
The position “d” of diffracted peaks also provides information about how the atoms are arranged within the crystalline compound (unit cell size or lattice parameter). The intensity information is used to assess the type and nature of atoms. Determination of lattice parameter helps understand extent of solid solution (complete or partial substitution of one element for another, as in some alloys) in a sample.
Width of the diffracted peaks is used to determine crystallite size and microstrain in the sample. The “d” and “I” from a phase can also be used to quantitatively estimate the amount of that phase in a multicomponent mixture. As mentioned earlier, XRD can be used not only for qualitative identification but also for quantitative estimation of various crystalline phases. This is one of the important advantages of the X-ray diffraction technique.
3.3.4 Photoluminescence (PL)
Photoluminescence spectroscopy is a contactless, nondestructive method of probing the electronic structure of materials. Light is directed onto a specimen, where it is absorbed and imparts excess energy into the
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material in a process called photo-excitation. One manner this excess energy can be dissipated by the sample is through the emission of light, or luminescence. In the case of photo-excitation, this luminescence is called photoluminescence. The intensity and spectral content of this photoluminescence is a direct measure of various important material properties.
Photo-excitation causes electrons within the material to move into permissible excited states. When these electrons return to their equilibrium states, the excess energy is released and may include the emission of light or may not. The energy of the emitted light relates to the difference in energy levels between the two electron states involved in the transition between the excited state and the equilibrium state. The particular wavelength. If the cell’s quantum efficiency is integrated over the whole solar electromagnetic spectrum, one can evaluate the amount of current that the cell will produce when exposed to sunlight. The external quantum efficiency is the ratio of the number of charge carriers collected by the solar cell to the number of photons of a given energy incident photons. The external quantum efficiency therefore depends on both the absorption of light and the collection of charges. Once a photon has been
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absorbed and has generated an electron-hole pair, these charges must be separated and collected at the junction. The ideal quantum efficiency graph has a square shape, where the QE value is fairly constant across the entire spectrum of wavelengths measured. The QE for most solar cells is reduced because of the effects of recombination, where caharge carriers are not able to move into an external circuit. The quantum efficiency of solar cells can be seen as the absorptive ability of solar cells on a single wavelength of light.
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Figure 3-4 Schematic Illustration of Se Film Prepared with APPECVD.
Figure 3-5 Schematic Illustration of Movable Mechanism of APPECVD.
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Figure 3-6 The Temperature Curve of RTP Process.
Figure 3-7 Schematic Illustration of SEM Instrument.[55]
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Figure 3-8 Schematic Illustration of AFM instrument.[56]
Figure 3-9 Schematic Illustration of XRD Instrument.[57]
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Figure 3-10 Schematic Illustration of PL Instrument.[58]
Figure 3-11 Schematic Illustration of EQE Instrument.
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Table 3-1 Optimize the Fabrication Parameters for APPECVD.
Parameter Value
Power (Watt) 0 50 50, 60, 70
Selenium Temperature(℃) 325, 335, 345 345, 355, 365 355 Carrier Gas Flow Rate (SLM) 10, 15, 20 15, 20, 25 20
Background Pressure (Torr) 150, 350, 550
Substrate Temperature (℃) 45
Gap Distance (mm) 5
Scan Times 50
Scan Speed (mm/s) 10
Table 3-2 Parameters of Selenium Film by APPECVD.
Parameter Value
Power (Watt) 0, 50, 60 and 70
Substrate Temperature (℃) 45, 85 and 125
Gap Distance (mm) 5
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Chapter 4
Result and Discussion
In this chapter, we will discuss and explain the characterizations of selenium films in the different experiment, which include the selenium deposition on the glass, selenium deposition on the CIG precursor layer, and the CIGS films after selenization. The characterization of selenium films on the glass use the SEM to analysis surface morphology and film thickness in order to optimize the fabrication parameters. We use the SEM to analysis surface morphology and films thickness, and XRD to analysis degree of crystalline selenium for the selenium films on the CIG precursor layer. After selenization process, we use the XRD, Raman spectroscopy, Photoluminescence, SEM and AFM to analysis the characterization of CIGS films, which include surface morphology, films thickness, surface roughness, degree of crystalline CIGS chalcopyrite, secondary phase and band gap. Finally, the completion of the CIGS solar cell devices measure the I-V curve and Extra Quantum Efficiency in order to discuss the films quality affect device performance.
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4.1 Optimize the Fabrication Parameters 4.1.1 Evaporation Deposition Selenium Films
The evaporation deposition selenium films represent “without plasma”, so we discuss three different parameters, included Se source temperature, main gas flow rate and background pressure in order to find the optimal parameter for this equipment.
The different Se source temperatures are 325℃, 335℃ and 345℃, respectively. The Se source temperature is more and higher and the selenium film is more and more thick show in SEM top-view images of Table 4-1 due to increase the vapor pressure of Se. Though the deposition rate is more and higher, but surface become more and more rough show
The different Se source temperatures are 325℃, 335℃ and 345℃, respectively. The Se source temperature is more and higher and the selenium film is more and more thick show in SEM top-view images of Table 4-1 due to increase the vapor pressure of Se. Though the deposition rate is more and higher, but surface become more and more rough show