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Boost the solar cell performance by nanotechnology
The nano in ‘Nanotechnology’ refers to nanometer scale. 1 nanometer is 109 times smaller than 1 meter. How small is 1 nanometer? The difference between 1 nanometer and 1 meter is same as the difference between the smallest particle we can see by our naked eye (~100 µm) and the size of the Paris region (~100 Km). Nanometer scale is just one of the plenty length scales of space, as shown in figure 1.5. Modern physics suggest that length can range from Planck length, which is around 1.6 x 10-35 m, to the size of the observable universe, which is around 8.8 x 1026 m. Theoretically, the size of the object can be any value between the two size limits. We know that all the objects are composed of elementary particles. If we say the ultimate of the science and technology is to understand, identify and modify each elementary element up to Planck length scale, then we can say that the nanoscience and nanotechnology is to understand and modify the elementary particles or cluster of elementary particles at nanometer scale. This is an attractive ability. However, we human do not have this kind of ability naturally, because the smallest object our naked eye can see is around 100 µm. So naturally we do not manipulate with very small stuff, or we do not pay attention to them. Since thousands of years ago, people have invented lens which can zoom the image of the object. But till now, the best lens can only allow us to see objects with µm scale, not nm scale. In 1895, Wilhelm Röntgen discovered the X-ray, and in 1924 Louis de Broglie discovered the wave property of electron. These two discoveries allow the invention of X-ray tools and electron beam tools. These tools bring our vision from µm scale to nm scale. The concepts of nanotechnology came up soon after the advance of the microscope tools. In 1959, famous physicist Richard Feynnam considered the possibility of manufacture things atom by atom30. Generally, people consider that the starting of nanotechnology age is the beginning of 1980s, when scanning tunneling microscope and atomic-force microscopy was invented. These tools allow us to modify material at nanoscale. Nowadays, nanotechnology is familiar to society. It is studied in various domains such as physics, chemistry, and biology. It is also commonly used in semiconductor industry. Since 1989, the semiconductor manufacturing processes enter nanometer scale31. Among the nanotechnology research teams, there is a group of researchers who study nanowires. Nanowire is a wire like object with diameter in the nanometer scale. The research on nanowires was started in 1964 by Ellis and Wagner, they have observed the catalyst induced nanowire growth33. Since 199834, the nanowire research gets increased interest, and from 200535, researchers start to use nanowires to fabricate solar cells. Figure 1.6 shows the image of a germanium nanowire acquired by scanning electron microscope. This nanowire has a diameter of 60 nm and a length of 1600 nm. Nanowire structure has many advantages for solar cells application. Firstly, in nanowire solar cells, the light absorption direction is along axial direction, while the electrons and holes generated in the solar cell are collected in the radial direction. Since the distance in the radial direction is nanometer scale, the carriers can be collected efficiently even with low quality materials. This means that high efficiency solar cells can be made with low cost materials and low cost fabrication process. Secondly, the size of the nanowire solar cells is in the similar length scale with the wavelength of the visible light, which contains the majority part of the energy of sunlight. The similar length scale makes the nanowire structure to interact strongly with light. Thus only small amount of material is needed to absorb the incoming light. Lastly, at nanometer scale, the materials can exhibit novel properties such as discrete band structure, ballistic transport, quantum confinement and novel crystalline structures.
Characterization of the n and k of the material
Spectroscopic Ellipsometry measurement is a way to obtain and of a material. In an ellipsometry experiment, the objective is to measure the complex ratio of Fresnel coefficients, which is given by19 = = ∆ (2.20) Where is the complex Fresnel reflection coefficient for light polarized parallel to the incident plane, is the complex Fresnel reflection coefficient for light polarized perpendicular to the incident plane, and and ∆ are ellipsometry angles.
In our experiments, we use a phase-Modulated Ellipsometer UVISEL with a PSMA configuration which is shown in figure 2.2 b). Here P, S, M, and A stand for fixed polarizer, sample, modulator and fixed analyzer, respectively20. The signal we measured is: ( ) = 0{1 + sin( [ ]) + cos( [ ])} (2.21).
With:
= sin[2( − )] [ 2 (2 ( 2 − 2 ) + 2 2 2 ∆] (2.22).
= sin[2( − )] 2 2 ∆ (2.23).
Where , , and are the azimuth of the polarizer, the photo-elastic modulator and the linear analyzer with respect to the plane of incidence, respectively. ( ) = sin( ), is the angular rotation speed of the phase-modulator, is the acquisition time. For our measurements, the configuration is = 0°, = 45°, and = 45°. This is known as configuration II, and it gives: = 2 ∆ (2.24) = 2 ∆ (2.25)
However, when the measured is bigger than 45°, the sample will be also measured with configuration III, with = 45°, = 90°, and = 45°. In this case, will be taken from the.
The structure of the solar cells for optical simulation
In this study, the main purpose the of the optical modeling is to achieve a good understanding of the interaction of the light and NW solar cells, and also to get information for the solar cell fabrication processes optimization. With such a goal, we have built optical models based on the solar cells structure we have fabricated. A top view SEM image of our solar cell is shown in figure 2.4 a). In this image, each wire is a solar cell which is composed of a p-type NW core, intrinsic absorber shell and a n-type shell. The NWs in this image are randomly oriented, and this brings a big challenge for the modeling. In order to simplify the problem, it is considered that the NWs are perpendicular to the substrate and have a square array arrangement. Figure 2.4 b) shows the top view of the simplified NW array, and figure 2.4 c) shows the detailed structure of each unite in figure 2.4 b). The pitch of NW array is usually around 1 µm. In figure 2.4 c), the axial direction of the NW solar cell is perpendicular to the substrate. The light is incident from the top along the normal direction. The boundary condition for the sidewall is Floquet periodicity. This cell has core multi-shell structure and the shells are layers of different materials and thicknesses. The boundary condition for the bottom surface is a perfect reflector. The space between NW solar cells is filled by air. For all the simulation of NW solar cells, we always use an infinite periodic array configuration, the axial direction of the NW is perpendicular to the substrate, the light is incident from top plane along normal direction and with a power of 1 w for one cell. Floquet periodicity is used for four sidewalls.
Optical modeling with Comsol multiphysics
As described before, optical modeling is used to study the interaction between the light and the materials. Since the structure of the material also plays an important role in the light matter interaction, there are three main elements in the optical simulation: light, material and structure. These three elements have been used as inputs of a commercial software (Comsol Multiphysics) to calculate the light absorption. During the simulation, the main function of Comsol software is to solve the partial differential equations (PDE) which describe the light propagation in the material. In order to use finite element methods to solve PDE, the software firstly generates the mesh of the geometry and the weak form of the PDE. With the weak formulation, it is possible to discretize the mathematical model equations to obtain the numerical model equations. Then the software transfers the weak formulation, the boundary conditions and the meshed structure to matrix equations. After solving the matrix equations, some results can be visualized directly, such as the electric field. While some other results need post-processing. The calculation process is shown in figure 2.5 a). Figure 2.5 b) shows the mesh of a NW solar cell.
Table of contents :
Chapter 1 Introduction
1.1 Let there be light
1.2 A new era of using light
1.3 Boost the solar cell performance by nanotechnology
1.4 Outline of this thesis
References
Charter 2 Optical modeling of nanowire radial junction solar cells
2.1 Introduction
2.2 Theoretical back ground of optical modeling
2.2.1 The light
2.2.2 The material
2.2.3 Characterization of the n and k of the material
2.2.4 The structure of the solar cells for optical simulation
2.2.5 Optical modeling with Comsol multiphysics
2.3 Modeling of NW solar cells with different configurations
2.3.1 cSiNW on a Ag layer
2.3.2 Comparison of NW solar cells and planar solar cells
2.3.3 a-Si:H/μc-Si:H tandem solar cells
2.4 Summary
2.5 References
Chapter 3 Silicon nanowire growth
3.1 Introduction
3.2 Experimental setups
3.2.1 Thermal evaporator
3.2.2 PECVD reactor
3.2.3 Other experimental tools and the SiNW growth processes
3.3 Experimental results
3.3.1. Droplets engineering
3.3.2 NW growth process
3.3.3 Hexagonal diamond crystalline SiNW
3.4 Summary
Chapter 4 SiGeNW and GeNW growth
4.1 Introduction
4.1.1 Why we study SiGeNWs
4.1.2 The state of the art of the SiGeNW and GeNW synthesis
4.2 SiGeNW and GeNW growth
4.2.1 Growth of SiGeNWs at 400°C
4.2.2 Increasing the Ge content of the SiGeNWs
4.2.3 GeNW growth at high temperature
4.3 Properties of SiGeNWs and GeNWs
4.3.1 Ge content studied by Raman and EDX
4.3.2 Crystallinity and chemical composition studied by TEM
4.3.3 Electrical and Optical properties studied by photoluminescence and absorptance measurements
4.4 Summary
Chapter 5 Towards low cost NW based radial junction solar cells
5.1 Introduction
5.2 Fabrication and characterization of NW radial junction solar cells
5.3 The performance of NW radial junction solar cells
5.3.1 SiNW based NW radial junction solar cells
5.3.2 SiGeNW based NW radial junction solar cells
5.4 Summary
References
