Australian Journal of Basic and Applied Sciences, 8(6) April 2014, Pages: 455-468

AENSI Journals Australian Journal of Basic and Applied Sciences ISSN:1991-8178

Journal home page: www.ajbasweb.com

Current State-of-the-Art Solar Photovoltaic (PV) Technologies

Banupriya Balasubramanian and A. Mohd Ariffin

Center of Renewable Energy (CRE), Universiti Tenaga Nasional (UNITEN), Jalan IKRAM – UNITEN, Kajang, Selangor, Malaysia

ARTICLE INFO ABSTRACT Article history: One of the most rapidly emerging renewable energy technologies is solar photovoltaic Received 25 January 2014 (SPV) cell. SPV cell is a specialized device that converts visible light or Received in revised form 12 light energy directly into useful electrical energy. This paper work aims to provide a March 2014 comprehensive overview on the current state-of-the-art solar PV technologies. Each Accepted 14 April 2014 technology is reviewed in detail particularly on its use of light absorbing materials Available online 25 April 2014 together with its structure and deposition processes. Furthermore, conversion efficiency, advantages and disadvantages of these technologies are also discussed. This Keywords: comprehensive review is hoped encourage more participation from various parties in Solar PV technologies, Review, SPV, advancing the use of SPV as an alternative to generate electricity. , Thin film

© 2014 AENSI Publisher All rights reserved. To Cite This Article: Banupriya Balasubramanian and A. Mohd Ariffin., Current State-of-the-Art Solar Photovoltaic (PV) Technologies. Aust. J. Basic & Appl. Sci., 8(6): 455-468, 2014

INTRODUCTION

Renewable energy is a naturally available energy that replenishes rapidly for instance solar, wind, hydro and biomass. Solar energy is a form of renewable energy that has zero carbon emission and can be produced almost at any time, as long as sunlight is present. A small conductive device converting photons in solar rays to direct-current and voltage and the associated technology are termed as Solar Photovoltaic (SPV) (Jayakumar, 2009). In 1839, a French physicist, Edmund Becquerel at his age of 19 discovered the world‟s first PV effect which is the physical phenomenon accountable for transforming light into electricity. He observed that voltage may appear by illuminating electrolytic cell made up of two metal electrodes in a weak conducting solution with different types of light, including sunlight. The basic physical process of the PV effect is that when photons which contain innumerable amounts of energy striking a PV cell may be reflected or absorbed or passes right through. The absorbed photon then transfers sufficient energy to release electrons; creating the photo voltage which can be utilized to drive a current through an electrical circuit (Friedrich Sick and Thomas Erge, 1996; Basic Photovoltaic Principles and Methods, 1982). During the last century, the first had been made with selenium as the semiconductor and it was very inefficient with 1-2% efficiency only. Since then, substantial research has been done in semiconducting materials by various scientists. During the 1920s and 1930s, advancements in the era of quantum mechanics provided the theoretical foundation on the quantum nature of light and electrons which subsequently made the research on PV more attractive. Yet, a breakthrough happened during the 1940s and early 1950s when Jan Czochralski, a Polish chemist, developed a technique to create highly pure crystalline or single- silicon named as Czochralski method. In the 1950s solar cells had been produced for the space activities and also the transistor industries evolution which triggers the solar PV industry. Similar materials used to produce transistors and PV cells and as well identical physical mechanisms encouraged many of their working principles (Friedrich Sick and Thomas Erge, 1996). The fast evolution of the global PV market in recent years leads to a cutting-edge research field at a tremendous rate in order to produce the most reliable solar cells which comprises of several parameters such as a good overall efficiency, a reasonable production cost and the possibility to be produced on an industrial scale (Green Technologies Research, 2014). New era of solar PV cells has been produced and this review paper provides an in-depth insight into those available technologies with its strengths and weaknesses.

Solar Photovoltaic Technologies: Solar PV cells use light-sensitive semiconductor materials that exhibit the photovoltaic effect that absorbs photons and emits electrons which can be channeled into an electrical current. Material substitution possibilities are very limited in manufacturing PV cells due to its electrical properties. Majority of the solar cells are made from silicon, however many manufacturers are looking at cadmium telluride and copper indium () di-

Corresponding Author: Banupriya Balasubramanian, Center of Renewable Energy (CRE), Universiti Tenaga Nasional (UNITEN), Jalan IKRAM – UNITEN, Kajang, Selangor, Malaysia. E-mail: [email protected] 456 Banupriya Balasubramanian and A. Mohd Ariffin, 2014 Australian Journal of Basic and Applied Sciences, 8(6) April 2014, Pages: 455-468 selenide. All of these materials have their own electrical properties and traits which will determine the cell‟s performance and efficiency, manufacturing method and cost. The semiconducting material alternatives characterize the two prime categories of solar PV technologies as either silicon in the form of wafers (wafers will be sliced from a solid block of silicon) or thin films of other materials (low-cost substrates will be deposited by a very thin coating of a semiconductor material). Some of the categories of the PV cells are mono- crystalline, polycrystalline or multi-crystalline or multi-silicon and amorphous or thin films essentially based on the type of photovoltaic materials used (Utility Scale Solar Power Plants, 2012). Currently existing solar PV technologies are determined in the form of family tree in Fig. 1. Solar panel is the basic and most crucial unit of the solar PV power generation system. The following are the three primary steps summarizing the photovoltaic effect (Jayakumar, 2009):  The light-sensitive semiconducting material for absorbing light or photons and converting it into electron- hole pairs.  The p-n junction and barrier potential appearing across the depletion region within the semiconductor, which separates and prevents further movement of the photo-generated carriers (holes and electrons).  The connection of top layer and the bottom layer of the cell which allows for free flow of electrons and so the current to the external load.

2.1 (c-Si): c-Si lays the basis for all the industry advancements in semiconductor technology and thus becomes the most preferable PV semiconducting material. Innovations developed in solar cell technologies results in a production of highly efficient solar cells with efficiency reaching up to 20% (Jayakumar, 2009). As such in solid semiconductor chips, very expensive ultrapure silicon raw material is manufactured. Normally 150-200 microns width (that is one fifth of a millimeter) silicon wafers are utilized (Handbook for Solar Photovoltaic (PV) Systems, 2011). c-Si cell (typically between 12.5 cm2 and 20 cm2) circuits are sealed in an environmentally protective lamination to form crystalline silicon modules. Encapsulation of the c-Si modules has been done between the front glass of the panel (or the transparent rigid outer layer) and PV back sheet or a backing material which is typically made from plastic or glass (Utility Scale Solar Power Plants, 2012). The most common solar cells nowadays are -based c-Si and its global market share is estimated to be 85% to 90%. Market share of c-Si PV modules is anticipated to be about 50% by 2020 and so until that time, c-Si PV modules will remain as the t leading PV technology (IEA‟s Energy Technology Perspectives, 2013). Durability, longevity, weather resistance and abundant primary resources are the reasons for c-Si technology popularity. However, increasing its efficiency, improving the cell concepts and, automation in the manufacturing process would still be the main challenges for c-Si modules. The following section discusses the different types of c-Si as shown in Fig. 1 employed by various solar PV manufacturing industries.

2.1.1 Mono Crystalline Silicon / Single Crystalline: Mono-crystalline cells are sawn into thin wafers from a singular continuous crystal of silicon which is quite an expensive process (Utility Scale Solar Power Plants, 2012). Highly pure single-crystal silicon rods will be developed and sliced in to tinny wafers to form mono-crystalline silicon cells. Single crystalline wafer cells tend to be more expensive since it has been sliced from ultra-pure silicon cylindrical . Substantial waste of refined silicon cannot be avoided since it couldn‟t able to cover the square solar module completely. Due to its purity level, mono-crystalline silicon cells have reached efficiencies between 17-18% (Jayakumar, 2009). Laboratory experimental PV cells made of single-crystal have achieved efficiencies as high as 29%. In the solar PV market, PV cells with efficiencies close to 20% can be found, which is made of single-crystal silicon more often also called as mono crystalline cells (Fig. 2 (a)) (Friedrich Sick and Thomas Erge, 1996).

2.1.2 Poly Crystalline / Multi Crystalline Silicon: For this type of PV cell, a cast block of silicon is sawn into bars and then to wafers resulting in cells and also sometimes referred to as multi crystalline cell (Fig. 2 (b)). Poly-Si cells are simpler and cheaper to manufacture than single crystal silicon cells since there is no need for the energy- intensive traditional processes for the silicon purification. Poly-Si cells are not quite as efficient as mono- crystalline cells. The efficiency of polycrystalline-based solar panels is typically around 13-14% (Jayakumar, 2009).

2.1.3 Spheral Crystalline Cell: It is essential to develop innovative technologies in PV due to the usage of excessive quantities of silicon. The main purpose is to control expensive raw material consumption but at the same time to improve the reliability in PV technology. One such innovative hi-tech development is spheral solar and its main advantages are that it is inexpensive, extremely versatile and adaptable, light weight and durable with smaller amount of silicon used. Spheral solar can be integrated in a vast array of new applications since it makes use of thin pliable 457 Banupriya Balasubramanian and A. Mohd Ariffin, 2014 Australian Journal of Basic and Applied Sciences, 8(6) April 2014, Pages: 455-468 aluminum foil substrates. This revolutionary PV technology consists of silicon spheres embedded in thin layer of inexpensive substrates typically aluminum foils. Positive and negative contacts are shaped on one side and the opposite front foil serves as the electrically negative contact to the n-layer which sets the spacing of the spheres. The back foil on the other hand acts as the electrically positive contact to the p-type core of the spheres. Unlike conventional PV cells, this PV system fabrication process involves sphere fabrication, forming the sphere p-n junction and finishing, cell fabrication and assemblage of the modules. Furthermore, these solar cells use a smaller amount of silicon and eventually cost less than the conventional cells. In addition they could also convert solar energy in to electrical power over longer period of time. Power generation efficiency of this cell ranges between 11-15%. Some laboratory tests had revealed that the spheral solar cell has efficiency as high as 20% and above; however the estimated efficiency of the commercial cells is around 11% only. These tiny spherical solar cells can aid to achieve greater efficiencies due to its design which can absorb direct, diffused and reflected sunlight from different angles (Spheral Technology in the Photovoltaic Industry, 2005). Fig. 3 (a) and (b) represent the spherical crystalline solar cell and cross sectional view of the spheral solar cell.

Fig. 1: PV technology family tree.

Fig. 2: (a) Mono crystalline solar cell, (b) Poly crystalline solar cell (Courtesy: SolarVis Energy Limited).

Fig. 3: (a) Spherical solar cell, (b) Spheral cell cross section (Courtesy: Pure Point Energy; Emerging Tech from Japan). 458 Banupriya Balasubramanian and A. Mohd Ariffin, 2014 Australian Journal of Basic and Applied Sciences, 8(6) April 2014, Pages: 455-468

2.1.4 Edge defined Film-fed Growth (EFG): EFG method has not been widely used in producing silicon solar cells. EFG mono crystalline solar cells were formed by sawing directly the silicon melt to wafers. This EFG technique does not waste much material due to cutting and this can lower the production cost. EFG process involves melting silicon pellets in large furnaces and thin-walled octagonal tube is drawn directly from the molten silicon. The hollow tube extends up to certain height may be about 5 meters. The long, octagonal tubes (Fig. 4 (a)) are sent to a laser station in order to cut into square shaped wafers with typical dimension of 100 mm x 100 mm. Modules that are developed using EFG technique offer greater power with lesser surface than the square shaped crystal cells with truncated sides. Copper bands are used to make them electrically active. Schott Solar employed the EFG process to produce solar cells from EFG silicon (PVResources, 2014).

2.1.5 Ribbon Silicon: Ribbon silicon can be attained by pulling the thin ribbons or sheets of multi crystalline silicon from the poly silicon melt but this can be an endless process. This successive sawing into thin wafers directly however, does not waste much material. Silicon feedstock requirements for ribbon silicon technology are around 5g of silicon per Watt, which is lesser than crystalline wafers that require 8g of silicon per Watt (Green Rhino Energy, 2014). It is formed by drawing flat thin films from molten silicon resulting in a multi-crystalline structure. Due to the enormous reduction of material waste, production costs are significantly inexpensive but the main drawback is the lower cell efficiency. Tabbing ribbon sometimes called PV ribbon is shown in Fig. 4 (b) (Renesis, 2014).

Fig. 4: (a) EFG growth equipment and substrate manufactured by octagonal prism, (b) PV ribbon, (c) Thin film cell (Courtesy: SNe Research; EXPO Solar; Indium Corporation Blogs).

2.2 Thin Film (TF) Technology: Thin films were produced by depositing very thin layers of photosensitive materials over low cost substrate usually made of glass, stainless steel or plastic. The thickness of these extremely thin layers ranges in micrometre (m). Amorphous silicon (a-Si) is the first thin film solar cell that has been tested. Amorphous tandem and triple cell configuration have been established from the basic a-Si single junction cells. A combination of thin amorphous and microcrystalline silicon cells have been developed to form micromorph cells (or thin hybrid silicon cells) mainly for increasing efficiency. a-Si, copper indium diselenide (CIS, CIGS) and cadmium telluride (CdTe) are the typical examples of photo-sensitive materials. An example of thin film solar cell is shown in Fig. 4 (c). Less material used, superior installation flexibility, better performance in hot weather, greater cell efficiency, high level of automation, and elevated ambient temperature resistance are the significant benefits of TF solar cells. Less efficiency and unproven durability are the limitations associated with TF cells (Technology Roadmap, 2010). TF PV cell can be categorized further into inorganic and organic cell type (Fig. 1). Organic type of solar cell can simply be integrated with the construction material and building structures such that it can result in better featured size and decoration (Samsung SDI, 2014).

2.2.1 Amorphous Silicon Thin Film: Plasma enhanced chemical vapor deposition (PECVD) method is applied with hydrogen-diluted silane as a process gas for silicon thin-film cells. Depending on the deposition parameters applied, it is possible to obtain the following: Amorphous silicon (a-Si or a-Si:H), silicon and Nanocrystalline silicon (nc-Si or nc-Si:H), also called microcrystalline silicon. For high open circuit voltage, mixed phase of proto-crystalline silicon with low volume fraction of nano is preferable as this combination results in low production costs. Energy conversion efficiency is also lower when compared to the bulk silicon due to the reduction in the ratio of the number of charge carriers per incident photons (Jayakumar, 2009). Tetrahedral structure continues over a large range in c-Si and thus forms a well-ordered crystal lattice. This long range order of crystalline silicon is not present in amorphous silicon technologies. Furthermore, a-Si can absorb much more light than crystalline silicon so the cells can be thinner and can be readily deposited onto any sort of low cost substrates. 459 Banupriya Balasubramanian and A. Mohd Ariffin, 2014 Australian Journal of Basic and Applied Sciences, 8(6) April 2014, Pages: 455-468

There are many applications for a-Si due to its low cost especially where high efficiency is not required and low cost is imperative (Utility Scale Solar Power Plants, 2012).

2.2.2 Amorphous Thin Film Triple: Amorphous thin film triple junction solar cells are designed as a three intrinsic absorber silicon layers sandwich positioned on a thin, flexible, stainless steel substrates (Fig. 5 (a)) and also protected by high end polyvinyl fluoride (PVF), ethylene-tetrafluoroethylene (ETFE) layers. For multi-junction solar cell as shown in Fig. 5 (b), each silicon layer is tuned in order to convert different wavelength bands in the solar spectrum into electrical energy. The different layers of this innovative triple junction cell designed to absorb blue, green and red light from the sun. As a result, conversion efficiency is superior even in low and diffused light conditions. With a total thickness of < 1 µm, the typical triple junction cells are very flexible. Optimal kilowatt-hour input can be provided by the TF membrane made of a-Si cells. Solar cells based on the triple junction technology can perform excellent even under diffuse light conditions thus the annual energy yield and performance ratio are significantly higher for suboptimal roof orientations (e.g. flat roofs) than all present crystalline silicon technologies. TF triple junction technology demonstrated exceptionally high performance and temperature resistance, weightless, mechanically resistant and flexible (SolarPV.co.UK; IPS Rubber Fuse, 2014).

Fig. 5: (a) Thin film triple junction technology, (b) Thin film triple junction amorphous solar cell (Courtesy: IPS Rubber Fuse; Global Solar Mall).

2.2.3 Amorphous Thin Film Tandem: Amorphous silicon films serving as photoelectric conversion thin films and microcrystal silicon films are deposited on a glass substrate to form tandem-junction thin-film silicon solar panels. With the aid of two kinds of chemical vapor deposition (CVD) systems, tandem type cell is formed by depositing photoelectric conversion thin films of amorphous on the top of the cell and a microcrystalline silicon on the bottom of the cell as indicated in Fig. 6. To effectively confine the light in the photoelectric conversion films (a-Si and µc-Si), tandem silicon solar cell has the structure of an i-layer sandwiched by p- and n-layers both in top and bottom of the cells (Yasuo Shimizu, 2010).

Fig. 6: Structure of tandem type thin film silicon solar cell (Courtesy: Yasuo Shimizu).

460 Banupriya Balasubramanian and A. Mohd Ariffin, 2014 Australian Journal of Basic and Applied Sciences, 8(6) April 2014, Pages: 455-468

2.2.4 Cadmium Telluride (CdTe) Thin Film: Cadmium and tellurium are the parent compounds of cadmium telluride. It features a simple semiconductor film stack sequentially deposited onto a transparent conducting oxide (TCO) coated sheet of glass substrate (Fig. 7 (a)). The manufacturing process is continuous and automated using large area substrates. Fig. 7 (b) depicts the CdTe solar module (Utility Scale Solar Power Plants, 2012). CdTe based solar cell is a highly efficient material for thin-film cells and able to absorb more light. CdTe is more suited for large scale production and also easier to deposit when compared to other thin-film materials. CdTe technology has experienced significant growth in the recent years. The technology is more affordable since it is possible to fabricate large quantities of solar cell within a short time period. Properties of CdTe allows better distribution of photons in the solar spectrum so CdTe modules can even absorb weak and diffuse light such as dawn and dusk or cloudy weather and convert it to electricity. Thus CdTe thin film modules can produce more electricity with similar output power ratings than the conventional cells (Jayakumar, 2009).

Fig. 7: (a) CdTe solar cell structure, (b) Cadmium telluride solar module (Courtesy: Leading Companies; PHYS ORG).

2.2.5 Copper Indium Gallium (di) Selenide – Cu (ln, Ga) Se2 (CIGS): CIGS is a group of compound consisting of copper, indium, gallium and selenium. Compared to the crystalline silicon, CIGS absorption coefficient is higher, yet much greater thickness of film is required than a-Si cells. However its raw material (indium) is scarce and therefore costly but they are needed in small quantities only. CIGS is still in the first stage of its commercial production. Among all the thin film PV module technologies, CIGS has the highest probability in module conversion efficiency (Utility Scale Solar Power Plants, 2012). The fabrication process of CIGS cells is more difficult than CdTe cells resulting in high cost to attain similar high efficiency (Technology Roadmap, 2010). Fig. 8 (a) and (b) illustrates the cross- sectional schematic and a typical CIGS thin film photovoltaic solar cell respectively. Even though there is a smaller market for CIGS technology than cadmium telluride, it has the potential to enter in to a new market so- called Building Integrated Photo Voltaic (BIPV) market. Compared to other thin film technologies, conversion efficiency of CIGS solar cells is higher (about 2%). Each 1% of efficiency is more or less equal to 6% of reduction in overall expenses because of decrease in land, modules, inverters and labor. Therefore if the efficiency is increased by 2% then it leads to a significant savings of expenditure of about 12%. CIGS are deposited by low cost evaporation techniques rather relying on expensive vacuum deposition processes. The energy conversion efficiency of CIGS solar cells is estimated to outweigh CdTe solar cells. Hypothetically, maximum efficiency of about 30% can be reached by both CIGS and CdTe solar cells but realistically CIGS is likely to be better than CdTe solar cells with an estimated industry efficiency of about 25% (Solar Cell Central, 2014).

Fig. 8: (a) CIGS structure, (b) CIGS solar cell (Courtesy: Solar Cell Central; Solar Novus Today). 461 Banupriya Balasubramanian and A. Mohd Ariffin, 2014 Australian Journal of Basic and Applied Sciences, 8(6) April 2014, Pages: 455-468

2.2.6 Copper Indium (di) Selenide (CIS): CIGS cells are the descendant of Copper Indium Selenide (CIS) solar cells. Sometimes the abbreviations CIS and CIGS are used interchangeably. The construction of CIS and CIGS cells are alike. CIGS cells have high performance ratio and received a lot of responsiveness by solar manufacturers although more than 14% of solar cell conversion efficiency is attained by CIS cells (Fig. 9 (a)). Top and bottom layers are the two types of semiconductor layers in CIS cell. The top layer or which is a window layer allows almost all incident light to reach the bottom layer which is made of CIS. Once the light has been absorbed by the bottom layer electrons are then generated which effectively drive electrical currents. The above detailed explanation can also be applied for CIGS cells however gallium will be added in the equation. Wuerth Solar, a German based manufacturer is presently producing CIS solar cells (non-gallium) (CalFinder, 2014).

2.2.7 Micro Crystalline Thin Film: Remarkable breakthrough has been achieved in the thin-silicon technology by obtaining both p- and n- type microcrystalline silicon (μc-Si). This emerging technology was first introduced by Matsuda in 1983. This new form of μc-Si material has been used to make the first thin-film silicon solar cells by Hattori et al. (1987). The demand for absorbing material with high quality and long term stability in the 1990‟s proliferate the attention towards intrinsic microcrystalline silicon. Accordingly Faraji et al. (1992) have reported the development of very-high-frequency, VHF-PECVD and Rath et al. (1997) utilized the hot-wire CVD (HW-CVD) process. c- Si:H films can be considered as a mixture of different structural components of crystallites, amorphous tissue, grain boundaries and voids. Since the application of this material as an absorber layer in thin film solar cells had become successful, c-Si:H seems to be the most popular choice of material for SPV. Notably it has attracted many attentions due to its stability when subjected to notorious light-induced degradation than amorphous silicon. Compared to a-Si, microcrystalline silicon exhibits significant advantage of more electron mobility and thus higher electrical conductivity due to the existence of the silicon crystallites. It becomes an important material for the use in a-Si solar cells since it shows sufficient absorption in the red and infrared part of the solar spectrum (Finger et al., 2003). Fig. 9 (b) depicts a microcrystalline silicon solar cell (5 cm in diameter) formed on a honeycomb-textured substrate.

Fig. 9: (a) CIS solar cell, (b) Microcrystalline silicon solar cell (Courtesy: Brown Dog Gadgets; Advanced Industrial Science and Technology).

2.2.8 Dye Sensitized Solar Cell (DSSC) (TiO2): A dye-sensitized solar cell (DSSC, DSC or DYSC) is a kind of thin film solar cells, promising for inexpensive, large-scale solar power applications. DSSC is a sort of photo-electro-chemical solar device which has becoming the subject of active research in the context of cheaper photovoltaic devices as a root of sustainable energy. DSSCs are based upon the porous nano-crystalline titania (TiO2) film combined with an efficient light-absorbing dye (EprintsUnife, 2014). Firstly dye-sensitized solar cell technology was presented by a chemist, Michael Gratzel in 1991 and therefore known as Gratzel cells. In this type of solar cell, titanium dioxide (TiO2) is coated with light sensitive dye which converts photons into electrons and so the electrons travel through the surrounded electrolyte to create an electric current. The mechanism of this solar cell is more or less similar to the photosynthesis in plants. The production of conventional silicon solar cells requires pure silicon which is fairly expensive material and also costly semiconductor equipment which uses large amount of energy. However the production of these DSSCs normally involves abundantly available titania, an oxide of titanium which is cheaper as well. It is composed of a photosensitive layer made of extremely small, nano-sized semiconductor crystals over a thin layer of TiO2. DSSC drive an electrical current once the photons have been absorbed by the photosensitive layer where it can excite electrons that then flow into the titanium dioxide. Previously a liquid electrolyte had been used for transferring the electrons from one layer to another. Now in the 462 Banupriya Balasubramanian and A. Mohd Ariffin, 2014 Australian Journal of Basic and Applied Sciences, 8(6) April 2014, Pages: 455-468 third generation of solar cells particularly in Gratzel cells, a dye made of amorphous organic material is used to coat the titanium dioxide. Light excites the dye and the excited electrons are then transferred to the TiO2 which acts like a roadway for the electrons (electricity) travelling through the cell. A dye transfers the excited electrons to the TiO2 which separates the charge. Fig. 10 (a) and (b) depicts the working principle and the DSSC panel (Osaka Gas, 2014) Dye-sensitized solar panels show power production efficiencies of around 11 to 12%. Although the efficiency is lesser than many of the best TF cells, dye-sensitized panels has many benefits as follows: superior price to performance ratio, made of low cost materials and therefore less expensive, simple and cheaper to manufacture, rely on easy to produce and abundant raw materials. Compared to standard silicon-based solar panels, dye-sensitized panels are robust with the ability to engross and convert photons into useful electrical energy. DSSCs have multiple platforms of applications than more rigid silicon panels since it is possible to manufacture pliable cells using flexible substrates. Nevertheless it is not commonly used as compared to conventional silicon solar cell due to its lower energy conversion rate (Energy Harvesting Journal, 2014).

Fig. 10: (a) Working principle of DSSC, (b) Dye sensitized solar panel (Courtesy: Osaka Gas; World of ).

2.2.9 Plastic / Polymer Solar Cell: Plastic solar cell technology is based on organic semiconductor materials of conjugated polymers and molecules as well called as “plastic solar cells” as displayed in Fig. 11 (a). This cell can be manufactured at a low cost, and its usage of lightweight malleable substrates, non-toxic, good power conversion efficiencies are its noteworthy advantages (Jeroen Van Duren, 2014). The architecture of polymer solar cells consists of photo active layer for absorbing light which is sandwiched between two electrodes. Active layer comprises of a polymer electron donor (p-type) and fullerene-derivative electron acceptor (n-type) mixture. Glass or clear plastic foil is used as a substrate. To let the light pass through into the active layer, one of the electrodes is transparent. Typically the transparent conducting electrode is made of indium oxide (ITO) and the other electrode may contain metallic material such as aluminum, calcium or magnesium as in Fig. 11 (b). Once the photons are absorbed, they will excite electrons in the active layer. As a result, electrons will be stimulated inside the polymer from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) (Fig. 11 (c)). This excitation of an electron leaves a hole in the HOMO level. The two parts of hole and the excited electron are not totally independent as they may form a bound state called as exciton. Excited electron and hole can recombine in most organic molecules and the absorbed energy may be released as either radiative or non-radiative energy which is not useful to generate electricity. Still recombination occurs in polymer solar cells but not instantaneous. Alternatively, due to the ionization potential of the donor and electron affinity of the acceptor, exciton is detached into a free hole and a free electron which are then transported to different electrodes. After dissociation, in order to re-combine with the hole, the electrons are then forced to travel from one electrode through an external circuit to the other electrode and so driving the current (Lasse Bo Lumholdt Riisager, 2009).

2.2.10 Nano Crystal / Quantum Dot (QD) Solar Cell: New routes for inexpensive solar cells have been developed by the assimilation of nanoparticle materials which comprise of quantum dots, rods or multiple rods of various sizes in PV devices. One of the promising materials for the next generation solar cells is the very recently emerged organic semiconductor based nano crystals (also known as Quantum Dots). QDs have been extensively explored due to its properties of unique size 463 Banupriya Balasubramanian and A. Mohd Ariffin, 2014 Australian Journal of Basic and Applied Sciences, 8(6) April 2014, Pages: 455-468 and composition dependent absorption (Saim Emin et al., 2011). Quantum dots are so tiny which may be orders of magnitude of few nanometers than that of general ordinary bulk semiconductors made of macroscopic materials and are regarded as zero-dimensional (Paul Hemphill et al., 2006). QD based solar cell concepts accompanied with energy transfer is a sintered nanocrystal solar cell with sequential layer-by-layer assembly. Blue photons will be absorbed by the smallest nano crystals in the top layer which would convert the highest energy photons in to exciton. Upon photo excitation of the cell represented by arrows in Fig. 12 excitons would pass through to the second or third layer and then reach the p-n junction. Electric field will cause the exciton to split into electrons and holes to move towards the opposite terminals. Charge hopping will be circumvented in this method (Los Alamos National Laboratory, 2014).

Fig. 11: (a) Polymer solar cell, (b) Schematic presentation of a polymer solar cell, (c) Charge separation within the active layer (Courtesy: Lasse Bo Lumholdt Riisager; Daniele Di Nuzzo et al).

Fig. 12: Schematic of Quantum Dot solar cell (Courtesy: Los Alamos National Laboratory).

2.3 Silicon Thin Film Technology: Silicon thin film cell combines both crystalline and amorphous silicon cell design features in one structure. Typically PE-CVD from silane and hydrogen gas is the common deposition method used by the silicon TF cells. Silicon thin film technology can give rise to one among the solar cells of nano crystalline, apex silicon, Silicon on Glass (SiOG) and Heterojunction with Intrinsic Thin layer (HIT) based on the deposition parameters.

2.3.1 APex Silicon: Apex silicon cells are the first application of the production of thin film process with crystalline silicon cells on a cost efficient substrate. The thick silicon wafer is replaced by the electrically conductive ceramic substrate containing silicon. The photovoltaic layer of the apex material is formed as a continuous sheet of thin polycrystalline silicon to the required thickness may be of between 0.03 mm and 0.1 mm. The properties of large format solar cells are similar to the conventional polycrystalline cells. Apex solar cells offer the advantage of lower cost even though it requires the processing temperature of 900oC to 1000oC. Since the consumption of high-grade semiconductor material is reduced and this type of solar cell can also be produced in a matter of minutes (Deutsche Gesellschaft, 2008). At the Solar Energy Industries Association (SEIA) conference held in Orlando, Florida, apex solar cells and modules were introduced. Apex material is designed as a continuous sheet to the required thickness in a matter of minutes which differs from conventional solar cells made using batch- style processes derived from the computer chip industry. In many ways this production method is similar to modern low-cost methods for manufacturing plate glass or sheet steel. Lower cost and greater size flexibility are the significant benefits that have been offered by apex solar cells (PR Newswire, 2014). 464 Banupriya Balasubramanian and A. Mohd Ariffin, 2014 Australian Journal of Basic and Applied Sciences, 8(6) April 2014, Pages: 455-468

2.3.2 Silicon on Glass (SiOG): A new SiOG process starts with substrate consists of high quality crystalline silicon layer on industry- standard Corning EAGLE XG™ glass. Aluminoborosilicate glass used as substrate is thin and has more than 90% optical transmission. Also, this material is currently commercially available in large sizes. It is processed at temperatures below its strain point of 665°C for maintaining its dimensional stability (Corning, 2014). The silicon wafer and glass substrates are cleaned and the silicon wafer is implanted with hydrogen ions that can penetrate the glass substrate to a desired depth. The two substrates of silicon and glass are heated to a temperature below the glass strain point and are applied with a high voltage. By simultaneously applying the heat and voltage, silicon and glass are bonded together (Couillard et al; Kishor et al., 2007).

Fig. 13: (a) Schematic drawing of Corning‟s SiOG process, (b) 370 x 470 mm SiOG substrate (Courtesy: Cites et al; Dawson-Elli et al).

If the heating and therefore temperature increases, then the mobility of positive ions in glass will be better. High electric field across the silicon and glass induces the diffusion of these elements near the glass surface. As described in Fig. 13 (a), the resultant is a strong bond with a unique structure. Silicon tile will be separated along the implanted layer during the anodic bonding process which will result in the formation of thin film silicon on the glass. Finished SiOG substrate sample is depicted in Fig. 13 (b). The final step is the thinning process to remove residual damage from the ion implant and to obtain the desired silicon surface roughness and the film thickness uniformity better than 1%. This is only possible because of anodic bonding process, resulting in bond strength of greater than 15 J/m2 which exceeds the thermally bonded or deposited films. Other material systems can also utilize this SiOG technology. as the semiconductor and high temperature glasses (up to 900°C) as the insulator is the few examples of successful application of other materials in this process (Cites et al., 2009).

2.3.3 Heterojunction with Intrinsic Thin layer (HIT): HIT cell of Fig. 14 (b) is attracting more and more attention since the efficiency is much higher. HIT solar cell is a hybrid solar cell that combines both high efficiency mono-crystalline and ultra-thin amorphous silicon cell design features in one structure as shown in Fig. 14 (a). The band gap of amorphous hydrogenated silicon developed by PECVD is wider than that of crystalline material. By the insertion of wide band gap amorphous silicon, hetero-interface will be formed with the underlying mono crystalline silicon wafer and so highly recombination-active (ohmic) contacts from the crystalline surface will be displaced. These films can be doped relatively easy to form a junction. The junction will be formed with the underlying n-type crystalline wafer with the uppermost heavily doped p-type thin amorphous silicon layer. The high performance of the cell is secured by the intervening, very thin intrinsic amorphous silicon layer. The subsequent rear side of the wafer or the back surface contact of the cell is of the reversed polarity. On both front and rear surfaces of the silicon layers, an antireflective transparent conductive oxide (TCO) is deposited by physical vapor deposition (PVD) because of poor carrier mobility of the charge carriers, even if the conductivity of heavily doped amorphous silicon is quite low. Lateral carrier transport is allowed and made by a screen-printed metallic contacting grid. HIT cell can respond to light from both directions due to the patterned rear contact. Also rear of the module is wide-open and exposed to the light scattered from the surroundings which will optimize the output power in large scale installations. Presently the total processing sequence of HIT succeeded in increasing the average production efficiency by almost 17.2% which is identical to the range claimed by the buried contact solar cell production sequence. The efficiency of modules by the HIT cell developers, Sanyo is higher than those manufactured by BP Solar due to the tighter packaging of these cells into modules. BP Solar is manufacturing HIT cell with the nominal efficiencies of 15.2% (Martin A. Green, 2003).

465 Banupriya Balasubramanian and A. Mohd Ariffin, 2014 Australian Journal of Basic and Applied Sciences, 8(6) April 2014, Pages: 455-468

2.4 Special Solar Cell Technology: Currently many research teams are developing new technologies based on new solar cell architectural designs. Hybrid and Buried Contact Solar Cell (BCSC) are the few types of such special solar cell technologies.

Fig. 14: (a) HIT solar cell structure, (b) HIT solar cell (Courtesy: Mikolasek Miroslav; Sun Store Solar Power Solutions).

2.4.1 Hybrid Solar Cell: One of the PV solar module manufacturer and as well a solar cell innovator, Silevo is scaling up with the new manufacturing technique and novel type of solar cell compared to standard silicon solar cells. This innovative solar cell makes use of silicon in part, at the same time relies on diverse materials for other cell components. So it is possible to be more productive and the performance ratio and cell‟s conversion efficiency can be improved. As illustrated in Fig. 15 (a) the architecture of the Silevo hybrid solar cell consists of N-type crystalline substrate that couples tunneling oxide layers with the base layer and some more doped with thin film passivation layers to alter the voltage and current of the cells on which it adds a TCO layer. Fig. 15 (b) portrays the hybrid solar cell. This hybrid technology is the first to combine all of the three key materials in a single solar cell. By far, the most prevalent main material for solar cells is amorphous silicon and the PV industry has spent plenty of venture capital money for firmly manufacturing amorphous-silicon based solar panels. Nevertheless compared to the cells that use only silicon or even other alternative materials, the electricity produced by the amorphous-silicon alone is too small (GIGAOM, 2014). One type of hybrid solar cell is a combination of both organic and inorganic materials which take advantage of the extremely high carrier mobility of inorganic semiconductor photovoltaic materials (Nano Tech Web, 2014).

Fig. 15: (a) Hybrid solar cell structure, (b) Hybrid solar cell (Courtesy: GIGAOM; Solar Feeds Network).

2.4.2 Buried Contact Solar Cell (BCSC): In the early 1980s, buried-contact solar cell design of Fig. 16 was developed at University of New South Wales (UNSW). This has been considered as a low cost approach and benefits of this concept while testing in the lab has been developed for commercial availability of this cell. The key feature of this approach is the laser grooving operation in which laser cuts deep narrow grooves into the top silicon surface through the previously lightly diffused oxide layer and the underlying dielectric coating. A subsequent second diffusion to these grooved areas will heavily dope the non-grooved areas which expose fresh silicon and a heavier phosphorous 466 Banupriya Balasubramanian and A. Mohd Ariffin, 2014 Australian Journal of Basic and Applied Sciences, 8(6) April 2014, Pages: 455-468

(dielectric) is confined to the grooves by the masking oxide. Similarly, the dielectric confines a subsequent coating with a nickel/copper/silver trilayer formed by electroless (autocatalytic) deposition techniques. In addition, dielectric will act as an antireflection coating for the final cells. The core advantage of this BCSC method is that the quality of the silicon on the surface region of the cell need not be sacrificed, which allows responding to the blue wavelengths. There are also advantages of lower cost and performance benefits. BP Solar, one of the world‟s largest solar cell manufacturers commercialized this technology as BP solar new high efficiency buried contact Saturn solar cells and has also expanded its production capacity. Over the coming years, growth and the market share of this technology is expected to increase as a whole (Martin A. Green, 2003).

Fig. 16: Buried Contact Solar Cell (BCSC) structure (Courtesy: Martin A. Green).

Conclusion: The deployment of various solar PV technologies has been continuing to advance at a rapid pace in order to produce power efficiently at low cost and to reduce the dependency on other means such as coal and fossil fuel to generate electricity. The implementation of SPV technologies can reduce excessive emission of carbon dioxide; which contributes significantly to the global warming problem. A wide range of solar PV technologies has been thoroughly reviewed in this paper. The main objective of this review paper is to offer a brief summary on the current state of the art solar photovoltaic technologies with its deposition process, benefits and drawbacks in terms of efficiency and cost. REFERENCES

Advanced Industrial Science and Technology [Online]. Available: https://www.aist.go.jp, [accessed 20.01.2014] Basic Photovoltaic Principles and Methods, 1982. Technical Information Office, Solar Energy Research Institute (SERI) Brown Dog Gadgets [Online]. Available: http://www.browndoggadgets.com/collections/, [accessed 15.01.2014] CalFinder [Online]. Available: http://solar.calfinder.com/, [accessed 15.01.2014] Cites, J. S., J. G. Couillard and K. P. Gadkaree, 2009. Silicon-on-Glass (SiOG) Substrate Technology: Process and Materials Properties. IEEE International SOI Conference, pp: 1-2 Corning [Online]. Available: http://www.corning.com/displaytechnologies/, [accessed 23.12.2014] Couillard, J. G., K. Gadkaree and J. F. Mach, 2007. Glass-Based SOI Structures. US Patent 7176528 Daniele Di Nuzzo., Stefan Meskers and Rene Janssen. Polymer solar cells: „see‟ what is going on inside [Online]. Available: http://www.tue.nl/en/, [accessed 29.01.2014] Dawson-Elli, D. F., C. A. Kosik Williams, J. G. Couillard, J. Cites, R. G. Manley, G. Fenger and K. D. Hirschman, 2007. Demonstration of High Performance TFTs on Silicon-on-Glass (SiOG) Substrate. ECS Transactions, 8(1): 223-228 Deutsche Gesellschaft, 2008. Planning and Installing Photovoltaic Systems: A Guide for Installers, Architects and Engineers. Earthscan Emerging Tech from Japan [Online]. Available: https://www.semiconportal.com/en/, [accessed 26.02.2014] Energy Harvesting Journal [Online]. Available: http://www.energyharvestingjournal.com/, [accessed 25.01.2014] EprintsUnife. Dye Sensitized Solar Cells: principles and mechanisms [Online]. Available: http://eprints.unife.it/, [accessed 23.01.2014] EXPO Solar [Online]. Available: http://exposolar.org/2010/, [accessed 22.02.2014] 467 Banupriya Balasubramanian and A. Mohd Ariffin, 2014 Australian Journal of Basic and Applied Sciences, 8(6) April 2014, Pages: 455-468

Faraji, M., S. Gokhale, S. M. Choudari, M. G. Takwale and S. V. Ghaisas, 1992. High mobility hydrogenated and oxygenated silicon as a photo-sensitive material in photovoltaic application. Appl. Phys. Lett, vol. 60, pp: 3289-329 Finger, F., R. Carius, T. Dylla, S. Klein, S. Okur and M. Gunes, 2003. Stability of microcrystalline silicon for thin film solar cell applications. IEE Proceedings - Circuits, Devices and Systems, vol. 150, pp: 300-308 Friedrich Sick and Thomas Erge, 1996. Photovoltaics in Buildings: A Design Handbook for Architects and Engineers. London, United Kingdom: James & James (Science Publishers) Ltd GIGAOM [Online]. Available: http://gigaom.com/cleantech/silevo-unveils-hybrid-solar-cell-tech-chinese- factory/, [accessed 01.12.2014] Global Solar Mall [Online]. Available: www.globalsolarmall.com, [accessed 10.02.2014] Green Rhino Energy [Online]. Available: http://www.greenrhinoenergy.com/solar/, [accessed 12.02.2014] Green Technologies Research [Online]. Available: http://greentechresearch.wordpress.com/, [accessed 23.02.2014] Handbook for Solar Photovoltaic (PV) Systems, 2011. Energy Market Authority and Building and Construction Authority (BCA) publications Hattori, Y., D. Kruangam, K. Katoh, Y. Nitta, H. Okomoto and Y. Hamakawa, 1987. Tech. Dig, PVSEC-3, pp: 171 IEA‟s Energy Technology Perspectives (ETP) publication [Online]. Available: https://www.iea.org/publications, [accessed 14.05.2013] Indium Corporation Blogs [Online]. Available: http://blogs.indium.com/blog/thin-film-technology, [accessed 22.02.2014] IPS Rubber Fuse [Online]. Available: http://www.ipsukltd.co.uk/photovoltaic-systems.php, [accessed 10.02.2014] Jayakumar, P, 2009. Resource Assessment Handbook. Asia and Pacific Center for Transfer of Technology (APCTT) Jeroen Van Duren. The Basic Theory of Plastic Solar Cells [Online]. Available: http://www.interpv.net/, [accessed 28.01.2014] Kishor, P., Gadkaree, Kamal Soni, Shang-Cong Cheng and Carlo Kosik-Williams, 2007. Single-crystal silicon films on glass. Journal of Materials Research, 22(9): 2363-2367 Lasse Bo Lumholdt Riisager, 2009. Polymer Solar Cells: Steps Towards Improving the Power Conversion Efficiency. Master‟s Thesis, Aalborg University, Aalborg, Denmark Leading Companies [Online]. Available: http://solarcellcentral.com/companies_page.html, [accessed 08.02.2014] Los Alamos National Laboratory [Online]. Available: http://quantumdot.lanl.gov/devices.shtml, [accessed 26.01.2014] Martin A. Green, 2003. Crystalline and thin-film silicon solar cells: state of the art and future potential. Solar Energy, 74 (3): 181-192 Mikolasek Miroslav, 2009. Current status and progress in the new generation‟s silicon based solar cells. Posterus Portál pre odborné publikovanie, Elektrotechnika, Študentské práce Nano Tech Web [Online]. Available: http://nanotechweb.org/cws/article/tech/50536, [accessed 03.12.2014] Osaka Gas [Online]. Available: http://www.osakagas.co.jp/csr_e/charter02/energy.html, [accessed 24.01.2014] Paul Hemphill, Christian Lawler and Ryan Mansergh, 2006. Physics 4D Dr. Ataiiyan, Quantum Dots PHYS ORG [Online]. Available: http://phys.org/news/2011-06-efficiency-flexible-cdte-solar-cell.html, [accessed 08.02.2014] PR Newswire [Online]. Available: http://www.prnewswire.com/, [accessed 26.12.2014] Pure Point Energy [Online]. Available: http://purepointenergy.blogspot.com/, [accessed 26.02.2014] PVResources [Online]. Available: http://www.pvresources.com/Introduction/Technologies.aspx, [accessed 28.02.2014] Rath, J. K, 1996. Hot Wire CVD: A One-Step Process to Obtain Thin Film Polycrystalline Silicon at a Low Temperature on Cheap Substrates. Technical Digest, 9th Int. PVSEC, pp: 227 Renesis [Online]. Available: http://www.renesis.com.tr/eng/, [accessed 12.02.2014] Saim Emin, Surya P. Singh, Liyuan Han, Norifusa Satoh and Ashraful Islam, 2011. Colloidal quantum dot solar cells. Solar Energy, 85(6): 1264-1282 Samsung SDI [Online]. Available: http://samsungsdi.com/nextenergy/dssc-solar-cell-structure-principle.jsp, [accessed 22.02.2014] SNe Research [Online]. Available: http://www.sneresearch.com/eng/, [accessed 28.02.2014] Solar Cell Central [Online]. Available: http://solarcellcentral.com/solar_page.html, [accessed 05.01.2014] Solar Feeds Network [Online]. Available: http://www.solarfeeds.com/hybrid-thin-film-solar-cells-by- silevo/, [accessed 02.12.2014] 468 Banupriya Balasubramanian and A. Mohd Ariffin, 2014 Australian Journal of Basic and Applied Sciences, 8(6) April 2014, Pages: 455-468

Solar Novus Today [Online]. Available: http://www.solarnovus.com/index.html, [accessed 05.01.2014] SolarPV.co.UK [Online]. Available: http://www.solarpv.co.uk/solar-pv-laminates.html, [accessed 10.02.2014] SolarVis Energy Limited [Online]. Available: http://www.solarvisenergy.co.uk/pVSolarPanelTypes/, [accessed 25.02.2014] Spheral Technology in the Photovoltaic Industry, 2005. Note #PV1-0303.01, The Prometheus Institute research notes. Sun Store Solar Power Solutions [Online]. Available: http://www.sunstore.co.uk/Sanyo-DIRECT-HIT- H250E01-black-MC3.html, [accessed 25.12.2014] Technology Roadmap: Solar photovoltaic energy, 2010. International Energy Agency publications Utility Scale Solar Power Plants: A Guide For Developers and Investors, 2012. International Finance Corporation (IFC) World of Photovoltaics [Online]. Available: http://www.worldofphotovoltaics.com/, [accessed 24.01.2014] Yasuo Shimizu, 2010. Tandem Type Thin-film Silicon Photovoltaic Module Production Turnkey Line. Ulvac Solar Technical Journal (English) No. 72E