chapter is an effort to outline fabrication processes and manufacturing methodologies for commercial production of large area PV modules as an alternative green source of energy.
Keywords: Solar cell, photovoltaics, p-n junction, photovoltaic panels, crystalline silicon solar cell, renewable energy, physics of solar cell, fabrication of solar cell
1.1 Introduction
There has always been a surge to discover newer sources of energy which can be effective alternatives for the orthodox sources of energy, such as, petrol, kerosene, wind energy, thermal power generators [1,2]. In this quest, the sun is a natural huge source of renewable green energy. It is noteworthy that the terrestrial soil is exposed to an enormous amount of solar energy as large as about ten thousand times of all the energy used around the globe. The terrestrial hemisphere facing the sun receives power in excess of 50,000 terawatt (TW) in each instance, which makes reception of an enormous amount of energy possible [3]. Photovoltaics (PV) technology is a technology that relies on this infinite source of sunlight and possesses inherent qualities of highly reduced service costs since the sun provides free energy, reliability, noiseless, minimum maintenance costs and readily installation features [4, 5].
As a matter of fact, thermonuclear fusion reactions happen non-stop at a temperature of millions of degrees to generate huge energy in the form of electromagnetic radiation of sunlight [5,6]. The outer layer of the earth’s atmosphere receives partial energy of the total energy from the sun with a solar constant or an average irradiance of approximately 1367 Wm-2 with a variation of ±3% [8]. This value of solar constant is dependent on the earth-to-sun distance and on the solar activity. The solar constant is defined as the intensity of solar electromagnetic radiation impinging on a unit surface area and is expressed in units of kWm-2 and is equal to the integral of the power of the individual frequencies in the spectrum of solar radiation. The geometry of the sun-to-earth distance is displayed in Figure 1.1 given below.
Solar irradiation is the integral of solar irradiance over a particular period of time depicted by kWhm-2 and the radiation falling on the surface of the earth is actually diffuse radiation [8]. Diffuse radiation is that part of light radiation striking the surface from whole of the sky, while other radiations are the part reflected from the ground, and by surrounding atmosphere. Different types of radiation received by a solar panel [9] are displayed in Figure 1.2 as shown below.
1.1.1 Introduction to Si-Based Fabrication Technology
Photovoltaics technology is a green method of energy production which is based on fabrication and manufacturing of solar cells on platform of Si wafers [9]. In this regard, it is mandatory to know about the Si wafers. So the silicon and its geometry as an integral component of the solar cell technology will be discussed first.
Figure 1.1 The above schematic shows the sun-earth geometry portraying distance between the two celestial objects, diameters and the value of solar constant.
Figure 1.2 The above figure portrays different radiations occurring from the sun which consists of direct, diffuse and reflected radiations.
1.1.2 Introduction to Si Wafer
Silicon is a member of group 14 in the periodic table and is tetravalent metalloid, semiconductor and brittle crystalline solid [10-12]. In 1906, a silicon radio crystal detector was developed as the first silicon-based semiconductor device by Greenleaf Whittier Pickard [13]. Russell Ohl discovered the nonlinear semiconductor devices, p-n junction, and photovoltaic effect in the metalloid Si in 1940 [14]. In 1941, during the Second World War, radar microwave detectors were invented by developing techniques for production of high quality germanium (Ge) and Si crystals [15]. William Shockley proposed a field-effect amplifier based on Ge and Si in 1947, but could not demonstrate the prototype practically [16]. John Bardeen and Walter Brattain built the first working device, point-contact transistor, in 1947 under the direction of Shockley only [17]. The first Si-based junction transistor was fabricated by the physical chemist Morris Tanenbaum in 1954 at Bell Labs [18]. At Bell Labs in 1954, Carl Frosch and Lincoln Derick found out by accident that it is possible to grow silicon-di-oxide (SiO2) on Si wafers [19]. Later on, in 1958, they discovered that this as-grown SiO2 could be used to mask Si surfaces during diffusion processes [19].
Si atom has fourteen electrons with electronic configuration 2,8,4 [1s2, 2s2, 2p6, 3s2, 3p2] specifying that the number of valence electrons is 4 [10,11]. These valence electrons occupy the 3s orbital and two 3p orbitals. In order to complete its octet and attain the stable noble gas configuration of Argon (Ar), it can combine with other elements to form SiX4 derivatives by forming sp3 hybrid orbitals. In this case, the central Si atom taking part in the bonding with other element shares an electron pair with each of the four atoms of the bonding element.
Si and Ge crystallize in a diamond-type cubic lattice structure which has the space-lattice of face-centered cubic (fcc) [20, 21]. The atomic positions in the diamond-type cubic lattice projected on a cubic platform are shown in Figure 1.3. In a space-lattice of fcc-type, two identical atoms at 000 and
Figure 1.3 Schematic to show atomic positions in diamond-type cubic lattice.
Si is tetravalent and can be made p-type by adding dopants of boron (B), aluminium (Al) & gallium (Ga), and addition of antimony (Sb), phosphorous (P) & arsenic (As) generates n-type semiconductor material [10,11,20,21]. B and Ga possess only three valence electrons and when they are mixed into the Si lattice, deficiency of an electron is created which is termed as positively charged “vacancy” or “hole”. Holes take part in conduction accepting an electron from the neighbour and transitioning over the atoms. For making Si an n-type semiconductor, Sb, P and As are added into the Si lattice in
