Numerical simulation of solar cell based on graphene nano-strip using NEGF non-equilibrium Green's function method

Dissertation of Master of Electrical-Electronic Engineering

Abstract

In this thesis, for the first time, we have used graphene nano-ribbon as an active layer of a solar cell. To simulate this cell, the non-equilibrium Green's function method was used in the mode space, and the effect of electron-photon interactions was calculated by means of Born's self-consistent approximation. In order to increase the simulation speed, we used the potential profile obtained in the dark state for the simulations under irradiation, and thus we have avoided re-solving the Poisson equation in the form of a couple with the Schr?dinger equation. In addition, we have used the local (diagonal) approximation in the calculation of the self-energy resulting from the electron-photon interaction.

Key words: solar cell, graphene nanostrip, numerical simulation, non-equilibrium Green's function (NEGF) method.

1-1- Types of solar cells

1-1-1- First generation solar cells (silicon crystal cells)

In this category of solar cells, silicon wafers are used as active semiconductors. Silicon with an energy gap of ev1.12 is considered a very suitable material for absorbing the sun's spectrum. It is also the second most abundant element in nature. This means that obtaining raw silicon will not cost much and there is no concern about running out of its resources.

In order to achieve high conductivity, increase cell life and prevent loss of efficiency (due to recombination of carriers), silicon is used in single crystal form and of high quality. Sometimes, multi-crystal silicon is used to reduce costs.

1-1-1-1-         The growth process of semiconductor crystals

The growth conditions of semiconductor crystals (crystals) used to make electronic components are much more precise and difficult than the conditions of other materials. In addition to the fact that semiconductors must be available in crystalline form, their purity must also be controlled within a very fine range. For example, the density of most impurities used in today's Si crystals is less than 1 part in ten billion. Such degrees of purity require great precision in the use and application of materials at every stage of the manufacturing process[[i]]. After separating and performing the initial stages of purification, a semiconductor material is melted and made into ingots [1]. Si or Ge obtained after an annealing step [2] is polycrystalline.

If the cooling process is not controlled, the crystal areas will have completely random directions. For crystal growth in only one direction, it is necessary to have a precise control at the boundary between the molten and solid material during cooling [3]. A common method for growing single crystals is selective cooling of the molten material so that solidification is done along a specific crystal direction. For example, consider a container made of silica containing molten Ge; It can be taken out of the furnace in such a way that freezing starts from one end and gradually progresses to the other end. The quality of crystal growth can be increased by placing a small crystal grain [3] at the starting point of freezing. If the cooling rate is carefully controlled and the solid-melt interface is moved slowly along the melt, the germanium atoms will form a diamond lattice along with the cooling of the crystal. The shape of the obtained crystal is determined by the melting vessel. Ge, GaAs and other semiconductor crystals are usually grown by this method, which is called the horizontal Bridgman method [4]. In another form of this method, a small area of ??a crystalline substance is melted and then a molten area is moved to the other side in such a way that a crystal is formed behind the molten area during its movement [3]. One of the disadvantages of growing a crystal in a molten container is that a molten material comes into contact with the walls of the container, and as a result, during freezing, stresses are created that take the crystal out of the state of a complete network structure. This is a serious problem, especially in the case of Si, which has a high melting point and tends to stick to the materials of the melting pot. An alternative method, which overcomes this problem, involves pulling the crystal from the melt as it grows. In this method, a crystal grain is placed inside a molten material and is slowly pulled up, and the crystal is allowed to grow.In this method, a crystal grain is placed inside a molten material and is slowly pulled up, allowing the crystal to grow on the grain. Usually, during growth, the Ylor is rotated slowly to moderate any temperature changes (which lead to impossible freezing) in addition to gently stirring the melt. This method, which is called Chukralsky method, is widely used in the growth of Si, Ge and some compound semiconductors [3].

1-1-1-2-         Silicon crystal solar cells

These cells can be divided into two categories depending on the silicon crystal structure: single-crystal silicon solar cells and multi-crystal silicon solar cells. In the second category, polycrystalline silicon is used as an active semiconductor. In the first category, in order to achieve higher efficiency during an additional step, polycrystalline silicon is converted to single crystal. This will increase the construction cost. On the other hand, since the semiconductor must first be squared and then used, the material size in this category is more than that of multi-crystal cells (multi-crystal silicon can be grown in square molds).

1-1-2-         Second generation solar cells (thin layer cells)

Since the manufacturing cost of first generation solar cells is very high, a way to reduce costs must be found. For this, it is necessary to see what causes the high cost of production in those cells. By recalling the previous material, it is determined that by reducing the materials used and also by reducing the quality and purity of the crystalline structure, the costs can be reduced, even though the efficiency is reduced. In addition to these, thin-film semiconductors are also flexible, and this can provide new applications for them. In these cells, in order to further reduce the cost, it is possible to use semi-conductors as well.

In this chapter, the important types of thin film solar cells are briefly described. It should be noted that the criterion for placing these cells in the second generation is only the thinness of the semiconductor layer in them; While some of these cells can be included in the third generation cells because their efficiency can exceed the Shockley-Queiser limit. Figure 1-1- An example of a thin-film solar cell 1-1-2-1 Silicon thin-film solar cells This category of solar cells includes several types of cells, including thin-film cells with single-crystal, multi-crystal, and bicrystalline silicon. Nanocrystal pointed out. Multi-crystals are cheaper than single-crystals, but they are also less efficient. Bischel is cheaper than a few crystals, with the difference that it has a lower efficiency and a higher energy gap. Also, the energy gap of nanocrystal is equal to single crystal. These materials can also be used in the form of multiple bonds, an example of this work is shown in Figure 1-2 [[ii]].

Figure 1-2- Silicon thin film solar cell with multiple bonds

Despite all the advantages that silicon has, it has a major weakness: the indirect band structure causes the light absorption in silicon to be low. The result of this is that its thickness must be much greater than when we use a semiconductor with a straight band structure[[iii]], so when using a thin layer of silicon as a semiconductor, light absorption should be increased with light trapping techniques so that there is no need to have a high thickness. It is extraordinary, the absorption coefficient of this semiconductor reaches more than 3 to 104 (/cm) at wavelengths below 1000 nm, and its direct energy gap is between 0.95 and 1.04[[iv]].

The type of electrical conductivity in CuInSe2 is basically determined by deviations[7] from stoichiometry[8] and native defects[9]. Placing copper atoms in place of indium atoms and leaving indium atoms empty makes the copper-rich regions [10] have p-type hadide, while the indium-rich phases will be n-type [6]. Placing zinc or cadmium atoms in place of copper atoms changes the conductivity from p-type to n-type. Electrical conductivity also depends on the absorption or diffusion of oxygen at the boundaries [6].