Types of solar cell [1] 

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... photovoltaic, solar thermal electricity, solar archi- tecture and solar heating. Due to the climbing efficiencies in Table 1 coupled with lower PV module prices in Fig. 1, the production of photovoltaic cells and arrays have advanced greatly and penetration in photovoltaic system has observed to be growing. Table 1 and Fig. 1 is taken from reference [3], it can be seen that how efficiency for these different technologies varies with the surface area (to get 1 kwp) and it can also be seen that the price variations in all the technologies as modules have approximately similar downhill slope. Photovoltaic system is comprised of PV cells assembled into modules, which are connected into arrays, and the so called balance of system (BOS) components [2]. A PV system can produce outputs from microwatt to megawatt. Global focus has increasingly shifted from using conventional methods of generating electricity to photovoltaic system and large investments have been made with the testing and installation of large PV power plants [3]. Currently, the use of photovoltaic system placed after the use of wind and hydro power, as the biggest renewable power source with regards to globally installed capacity. Considering the cost and the fact that PV systems is limited in generating any power during night periods demands that the highest possible efficiency during day light hours should be achieved which is very critical for solar power viability. So, the trend is going in a right direction, so that the reduction in the cost of PV systems will be accomplished through improved materials usage and higher efficiencies. In this context the paper reviews the current status of three generations of photovoltaic technologies and investigates their reliability, efficiency and cost. Furthermore, developments and future potentials with these systems will also be summarized. The Sun is the main source (directly or indirectly) of almost all power, used on earth. The Sun is center of our solar system and 150 million kilometers away from the earth [4]. Solar energy is the radiant heat and light from the sun. This energy supports life and can be converted to other forms of energy that are useful to humans. The radiation emitted by the sun is estimated to be about 63 MW / m 2 of this huge solar potential, the energy that reaches the earths surface without any significant absorption is measured to be 1353–1366 W / m 2 ; this is referred as the solar constant [5]. A normal quantity of solar radiation reaching the surface of Earth is 1 . 05 × 10 5 terawatts (TW) in a year, even though global electricity need averages 1.99 (TW) in a year [6]. Solar energy has been used to produce electrical power for a couple of centuries using a range of constantly evolving technologies. The Table 2 shows a summary on the progress of the PV cells with time [7]. Solar cells were developed during the 1950s, primarily at the Bell Telephone laboratories. These cells proved to be the best power sources for extra-terrestrial missions, and more than 1000 satellites using solar cells were launched between 1960 and 1970. In mid-seventies, efforts were initiated to make solar cells for terrestrial applications. Last three decades saw newer device technologies enabling reduction in cost and hence opening new horizons for commercial applications of solar cells. Photovoltaic effect can be observed when certain semiconductor materials exposed to sunlight. Using this technique, semiconductors which have the photovoltaic effect can then be used to convert solar rays into direct current electricity. A solar cell is a semiconductor electrical junction device which is usually made up of element silicon. Sunlight is composed of energy packets called photons. These photons consist of energy packets corresponding to the different wavelengths associated with light [8]. When a solar cell is struck by photons with appropriate energy, the photons are consumed through the space charge region of the p-n junction of cell, this leads to transfer of the photons energy to an electron. The electron thus becomes excited to knock free from its atoms and electron hole pairs formed in the junction. In order to generate electricity, the electrons and holes need to be separated. When a load is connected at the terminals that causes an electron current flow and electrical power is available at the load. The free electron movement from one layer to the next generates electricity. Due to well-designed structure of photovoltaic cell, the electrons are permitted to go in one way. An array of solar cells converts solar power into direct current (DC) electrical power. Photovoltaic power generation uses solar panels and these panels are composed of an array of packaged solar cells, constructed from semiconductor material. At present, materials used for photovoltaic cells are consists of monocrystalline silicon, polycrystalline silicon, amorphous silicon cadmium telluride and copper indium gallium selenide. Figure 2 provides how different types of solar cells are constructed; this figure has been taken from reference [1]. The following subsec- tions will provide the brief description of these materials. Solar cells constructed with crystalline silicon (Si) semiconductor are most efficient [1]. Silicon is currently predominating solar cell material and is expected to remain dominant until a more inexpensive material and higher efficiency PV technologies are developed [7]. As outlined by NREL, throughout 2011, 90 % of market sales were from Silicon based photovoltaic (PV) products and the annual production of Si-based PV was reported to reach 15 GW [9]. However, crystalline silicon solar cells achieved the highest efficiency on the expense of high manufacturing cost. A crystalline silicon solar cell could be designed with distinct techniques such as the industry dominating single-crystalline/monocrystalline and polycrystalline/ multi-crystalline techniques. Commercial manufacture of standard monocrystalline silicon cells has obtained a good efficiency of 17–18 % [1], although multicrystalline silicon cells currently have achieved 16–17 % efficiencies. Tech- nological developments are currently in progress to develop cells with higher efficiencies [1]. Crystalline cells are made from silicon wafers simply by cleaning as well as doping the particular wafer and in a separate manufacturing process; several cells are then wired up to form a module. HIT (Hetrojunction with Intrinsic Thin-layer) solar cells are attracting a growing number of interests after appearing in 1992. When compared to a-Si solar cells, the efficiency of HIT solar cells is much higher [10]. On the other hand, the low temperature technology of HIT solar cells makes it possible to reduce cost by the application of low quality Si materials and high temperature performance is improved a lot compared to the c-Si solar cells. HIT Cell design processes involve an ultra-thin layer of amorphous silicon that is deposited on both faces of textured or thin single-crystal wafer [1]. Efficiency of HIT cells can be improved using both crystalline and amorphous silicon layers to efficiencies over 22 %. Silicon as a solar cell material has many advantages; however, it also conveys disadvantages. Table 3 from reference [13] describes that silicon is an indirect semiconductor and its absorption coefficient near to band edge is low. Thus, a fairly thick substrate is required for crystalline silicon cell manufacturing. This leads to substantial material and mechanical processing costs. Thin-film technology is an attempt to reduce the cost and maintain the efficiency of crystalline solar cells. Thin-film technology uses direct semiconductor materials which has absorption coefficient higher than silicon. This means that fewer micrometers of thickness of semiconductor material are sufficient for the development of solar cells. Thin film solar cells could therefore be manufactured with small amount of semiconductor material leading to decreases in price ranges. The cost of raw material is much lower than the capital equipment and processing since thin film production unit requires more space. Thin film cells are prepared by depositing layers of semiconductor material barely 0.3–2 μ m thick onto glass or stainless steel substrates [11]. This provides roll-to-roll layer that gives positive aspects with reference to manufacturing and current carrying capability. Efficiencies of 11–14 % have been achieved with this construction [12]. Thin film technologies including amorphous silicon/microcrystalline silicon (a-Si/c-Si), Copper Indium Selenide (CIS), Cadmium telluride (CdTe) absorb the solar spectrum and are much more efficiently than c-Si or mc-Si [14]. Presently, the thin film technologies a-Si/c-Si, CIS, CdTe have included integrated adjust- ments to help size fabrication and enjoy good commercial results. Overall, thin film solar panels are less efficient but widely used in PV industry due to its afford- ability. Figures 3 and 4 shows the relative efficiency of these technologies and the market share by them respectively [15]. Amorphous silicon was developed in the early days of thin film technologies. It is a non-crystalline form of silicon. It requires small quantity of active material and has considerably better light absorption capability i.e. 1 μ m thick film will absorb light significantly better when compared with crystalline silicon. Thin film a-Si solar cell provides an advantage since cells can be manufactured onto rigid (glass) or flexible substrates and could potentially offer lower costs. However, the disordered nature of the amorphous silicon results in dangling bonds and lattice defects. Consequently this produces less charge carriers as a result of the lower efficiency i.e. 4–8 %. In spite of low reported efficiencies, amorphous silicon is mass produced for applications where efficiency is not crucial and can be tolerated. Copper Indium Selenium ...