In isolation, a p type semiconductor has a large number of holes as compared to electrons and a N- type semiconductor has a large number of electrons as compared to holes. When two materials come in contact with each other, there is a difference in both the types (electrons and holes) of carrier concentration from one side to the other. In this situation, diffusion of electrons from N-side to P side and diffusion of holes from p- Side to N-side take place. This diffusion of carriers does not occur indefinitely. As holes diffuse from p- side to nside, it leaves behind a fixed negative charge in the form of ionized acceptor impurity atom. Acceptor impurities become negatively charged. Similarly, electrons diffuse from N-side to P-side and leaves behind fixed +ve charge in the form of ionized donor impurities. In this way, a layer of +ve charge, and a layer of –ve charge appear where P-N junction forms. This charged region in a P-N junction is also called space charge region or depletion region. Due to the separation of +ve and –ve charges in the space charge region, there exists an electric field. The presence of electric field causes drift current to flow. At equilibrium, the drift current will be such that it is equal and opposite to the diffusion current Region outside the space charge region is electrically neutral at both P- side and N-side, implying that the electric field is zero (also known as Quassi neutral region) Thus, in space charge region, there exists net charge density, but in quassi neutral region, charge density is less. P- N junction under illumination: Solar Cell (Photovoltaic Effect) Band diagram of a P-N junction, there should be a straight line for the Fermi level. When P- type and N- type materials are brought in contact with each other, space charge region is developed, which gives rise to electric field. In the presence of electric field, bands bend Under illumination, generation of carrier will occur in the space charge region as well as in quassi neutral region. The carriers that are generated in the space charge region gets immediately swept away due to electric field (electrons towards N-side and holes towards P-side) Due to electric field, chances of recombination of these electron hole pairs are quite less. The electron hole pairs which are generated in quassi neutral region, will wander around randomly due to absence of electric force. In this random motion, some of the generated minority carriers will come near the spce charge edge, where they will experience a force due to electric field and will be pulled at the other side. Only minority carriers will cross the junction, as they have to go downhill in terms of energy. In this way minority electrons from P-side will come to N-side (leaving behind their +ve charge, holes) and minority holes will come from N-side to P-side (leaving behind their –vely charged partners, electrons) Net increase in the +ve charges at P side and a net increase in the –ve charges at the N side This build up of +ve and –ve charge causes a potential difference to appear across the P-N junction due to light falling on it. This generation of photovoltage is known as photovoltaic effect. Not all minority carriers that are generated in quassi neutral region will cross the junction Minority carrier after being generated will travel an average distance of Xp or Xn (refer diag) also known as diffusion lengths of holes and electrons respectively. Therefore minority carriers that are generated within the diffusion length are able to reach the space charge region edge to get sucked to the other side. The minority carriers that are generated further away will recombine or die before reaching the space charge region and do not contribute to the photovoltage. Contribution of photovoltage comes only from the carriers that are generated within the width Xp+0+Xn Thin Fim Solar Cell Technologies The solar cell technologies based on Si wafer are generally referred as Ist generation technologies, while the cell technologies based on thin film are referred to as 2nd generation technologies The primary objective of development of thin film technologies is to reduce the cost of PV modules significantly lower than the cost of PV modules obtained from wafer based solar PV modules Thin film is referred to ‘random nucleation and growth process of individually condensing/reacting/atomic/ionic/molecular species on a substrate’ The structural, chemical, metallurgical, and physical properties of such film strongly depend on the deposition parameters of the film. These properties may also depend on the thickness of the film. Thus it is best to define film in terms of production process rather than film thickness. Advantages of Thin Film Technologies Low material consumption: Fabrication of solar cells is based on thin films (1µm to 5µm, upto 30 µm in case of polycrystalline Si thin films), the amount of material consumed per unit solar cell of given area is significantly lower than the wafer based solar cell technology (which are 200µm to 300µm thick). The lower material consumption leads to lower production cost and hence low cost of PV modules. Shorter energy pay back period: The energy pay back period of a solar cell is referred to the time of cell operation in the field during which it will generate the amount of energy required for its production. The energy pay back period of thin film technology is better by a factor of 2. Monolithic integration: It is referred to a process in which the solar cells are connected in series during their fabrication to make a module. Most thin film technologies offers the possibilities of monolithic integration. The integration of cells in module, in case of wafer based solar cells is a labour intensive task and adds to the module cost. Thus this feature of thin film provides additional cost benefit. Large area modules: The size of a wafer based solar cell depends on the wafer size, which in turn depends on the production capability of Si ingots and wafers. 6 Tuneable material properties: The optical and electrical properties of thin films depend on the structure and the composition of the films. For most of the thin film technologies, both of these parameters can be controlled at the time of deposition by controlling the deposition parameters. Low temperature processes: (<500degC, it can be as low as 200deg C) as against about 800 degC to 1000 degC used for wafer based Si cell manufacturing. The overall thermal budget of thin film technologies is low. The low temperature processes allows the use of cheap substrate such as glass, plastic and stainless steel foils. Transparent modules can be made: Thin film materials can be deposited on the glass substrate and the thickness of the films can be controlled. The film thickness less than the optimum required thickness to absorb full spectrum will be transparent in nature. Disadvantages: Low solar cell efficiencies. Due to this, large area thin film PV modules are required in order to generate a given amount of power. The low cell efficiency results from the structural deviation of thin films (which results in defects) from their bulk mono crystalline material The defected material gives poorer electrical properties of the film, such as high carrier recombination rates and low diffusion lengths. The structural defects in the film can affect the stability of the material as well, which results in the decrease in solar cell efficiency over the period. 7 Materials for Thin Film Technology: In principle any semiconductor material is suitable for making a solar cell. But there are only few materials which can provide reasonable efficiency for conversion of light into electricity. The following requirements should be fulfilled by a semiconductor material for suitability as thin film solar cell material: It should have a band gap between 1 eV and 1.5 eV The absorption coefficient of the material should be high Recombination rate of generated charge carriers should be low Diffusion length of generated charge carriers should be high Materials should be widely available, reproducible, and non toxic Si satisfies the above requirements except that is is an indirect band gap material. It has band gap of 1.12 eV, and abundantly available. Si can be deposited in several forms as a thin film absorber, in forms of amorphous Si, monocrystalline Si (size smaller than 0.1µm) and polycrystalline Si (size larger than 0.1µm and smaller than 1 mm) 8 Crystal Structure of Silicon The crystal structure, or atomic arrangement, of any material has a great deal to do with its electrical properties. When analyzing the physical structure of any material, we can speak of its existing in a crystalline, a polycrystalline or an amorphous form. In its crystalline form, a material is characterized by an ordered array of component atoms. This array is repetitive with displacement through the material sample. The most widely used technique for making single crystal silicon is Czochralski process. Where a polycrystalline material is concerned, the object is composed of a number of sub-sections, each of which is crystalline in form. These subsections, however, are independently oriented so that at their interfaces the atomic order and regularity undergo sharp discontinuities. They are less efficient than those of single crystal silicon, but they can be less expensive to produce. It is produced by casting method. The final category, the amorphous material, like common glass, displays no atomic regularity of arrangement on any macroscopic scale. They do not form crystalline structures at all and they contain large numbers of structural and bonding defects. But, they have some economic advantages over other materials that make them appealing for use in solar electric or PV systems. It s common in solar powered consumer devices that have low power requirements such as wristwatches and calculators. Amorphous silicon absorbs solar radiation 40 times more efficiently than does single crystal silicon, so a film only about 1micrometer thick can absorb 90% of the usable light energy shining on it. This is one of the chief reasons that amorphous silicon could reduce the cost of photovoltaics. Other economic advantages are that it can be produced at lower temperatures and can be deposited on low cost substrates such as glass, plastic and metal. These characteristics make amorphous silicon the leading thin film PV material. Photovoltaic cells made from polycrystalline materials are less expensive to construct per unit area than single crystal solar cells (both in terms of finance and in terms of energy). However, they are less efficient and often more sensitive to changes in ambient conditions-both undesirable attributes.
