Photovoltaic materials abundance limitations literature review

Background

Photovoltaics (PV) is a method of generating electrical power by converting solar radiation into direct current electricity using semiconductors that exhibit the photovoltaic effect. Photovoltaic power generation employs solar panels composed of a number of solar cells containing a photovoltaic material. Materials presently used for photovoltaics include monocrystalline silicon, polycrystalline silicon, amorphous silicon, cadmium telluride, and copper indium gallium selenide/sulfide. The study of these material in terms of their availability, abundance, efficiency, lifetime usage, doping density, energy produced per gram, cost and several other parameters will help us in future to determine which materials would be the best to use in solar cells. Photovoltaics

Overview articles

Abundance of materials in the lithosphere^[1]

This ref shows the percentage of materials available to us in the form of raw material in the earth's crust.

1. Silicon, 270,000 ppm
2. Gallium, 19 ppm
3. Indium, 0.160 ppm
4. Tellurium, 0.001 ppm

Mineral availability^[2]

1. Tellurium Primary and intermediate producers further refined domestic and imported commercial-grade metal and tellurium dioxide, producing tellurium and tellurium compounds in high-purity form for specialty applications. Tellurium was increasingly used in the production of cadmium-tellurium-based solar cells and was the major end use for tellurium. It states that the world refinery production and reserves for Telluirium was 24,000 metric tons in 2012. But The figures shown for reserves include only tellurium contained in copper reserves. These estimates assume that more than one-half of the tellurium contained in unrefined copper anodes was actually recovered.

2. Indium Indium’s abundance in the continental crust is estimated to be approximately 0.05 part per million. Trace amounts of indium occur in base metal sulfides—particularly chalcopyrite, sphalerite, and stannite—by ionic substitution. Indium is most commonly recovered from the zinc-sulfide ore mineral sphalerite. The average indium content of zinc deposits from which it is recovered ranges from less than 1 part per million to 100 parts per million. Although the geochemical properties of indium are such that it occurs with other base metals—copper, lead, and tin— and to a lesser extent with bismuth, cadmium, and silver, most deposits of these metals are subeconomic for indium. The world refinery production for Indium was 670 metric tons in 2012.

3. Gallium The average content of gallium in bauxite is 50 parts per million (ppm). U.S. bauxite deposits consist mainly of subeconomic resources that are not generally suitable for alumina production owing to their high silica content. Recovery of gallium from these deposits is therefore unlikely. Some domestic zinc ores contain as much as 50 ppm gallium and, as such, could be a significant resource. World resources of gallium in bauxite are estimated to exceed 1 billion kilograms, and a considerable quantity could be present in world zinc reserves. The foregoing estimate applies to total gallium content; only a small percentage of this metal in bauxite and zinc ores is economically recoverable. World primary gallium production capacity in 2012 was estimated to be 474 tons; refinery capacity, 270 tons; and recycling capacity, 198 tons.

Materials Availability Expands the Opportunity for Large-Scale Photovoltaics Deployment^[3]

Solar photovoltaics have great promise for a low-carbon future but remain expensive relative to other technologies. This paper has examined 23 promising semiconducting materials. Twelve composite materials systems were found to have the capacity to meet or exceed the annual worldwide electricity consumption of 17 000 TWh, of which nine have the potential for a significant cost reduction over crystalline silicon. They found that devices performing below 10%power conversion efficiencies deliver the same life time energy output as those above 20% when a 3/4 material reduction is achieved. By one recent estimate, the U.S. could achieve 69% of electricity and 35% of total energy consumption by 2050 entirely with existing PV technologies but may require almost half a trillion dollars in subsidies.

Today, modules represent ∼54% of the total installed cost for existing PV technologies, notably mono- and polycrystalline silicon, where silicon processing is estimated to be ∼85% of the energy input of the finished module (10–13). Aggressive development of nonsilicon-based PV materials has changed the PV landscape, offering exciting near-term cost reductions for material systems like copper indium gallium selenide (CIGS) and cadmium telluride (CdTe). the average quality of copper ore has gone from 2.4% to 1% in the last 100 years. The fraction of recoverable zinc that has already been placed in use is 19%, and indium, a secondary metal byproduct of zinc mining, has seen its price increase 400% in the past five years due to an increase in demand from the digital display market. One such hypothetical analysis demonstrated that a CdTe device with a cell thickness of 2 µm and operating at 10% power conversion efficiency was found to be capable of producing 0.3 TW per year before material scarcity was to become a limiting factor. Total annual electricity potential P in terawatt hours (TWh) is calculated by the following equation:

P=I·η·A·C·H/beta·10^12 where I is the solar spectrum intensity taken as a global air mass index (AM1.5G) of 1000 W/m2, A is the annual production per mineral in metric tons, C is the capacity factor for operation taken at 20%, and H is the number of hours per year.

Solar cell efficiency tables (version 42)^[4]

This paper consolidates the information in tables showing the highest independently confirmed efficiencies for solar cells and modules. The first new result in Table I is a very recent result, the demonstration of 19.6% efficiency for a 1-cm2 CdTe cell fabricated by GE Global Research and measured by the Newport Technology and Application Center. This is a significant improvement upon the previous best efficiency of 18.3% for a CdTe cell of this size. The second new cell result in Table I is an improvement in the performance of a 1-cm2 nanocrystalline silicon solar cell to 10.7%. A third new result in Table I is an improvement in efficiency to 8.2% for a 25-cm2 organic cell submodule fabricated by Toshiba and measured by the Japanese National Institute of Advanced Industrial Science and Technology (AIST). A third new result in Table I is an improvement in efficiency to 8.2% for a 25-cm2 organic cell submodule fabricated by Toshiba and measured by the Japanese National Institute of Advanced Industrial Science and Technology (AIST).

The first new result in Table II is a new record for a large area silicon module. An efficiency of 22.4% is reported for a 1.6-m2 silicon module fabricated by SunPower and measured by NREL. The second new result reported in Table II is a record for any polycrystalline thin-film module, with 16.1% total area efficiency reported for a 0.72-m2 CdTe module fabricated by First Solar [10] and again measured at NREL.

The first new result in table III is a new efficiency record for a large-area silicon cell with 24.7% efficiency reported for a 102-cm2 HIT cell (Heterojunction with Intrinsic Thin-layer cell) fabricated on an n-type silicon wafer substrate by Panasonic and measured by AIST. The second new result in Table III is 20.8% efficiency for a small area (0.25 cm2) crystalline GaInP cell fabricated and measured by NREL. The third new result is 20.4% efficiency for a small area (0.52 cm2) flexible CIGS cell developed by EMPA, the Swiss Federal Laboratory for Materials Science and Technology, and measured at FhG-ISE.

Table IV reports one new result for concentrator cells. A new record of 44.4% for the conversion of sunlight by any means is reported for a 0.165-cm2 multijunction cell operating at a concentration of 302 suns (direct irradiance of 302 kW/m2). The cell is an InGaP/GaAs/InGaAs triple junction device fabricated by Sharp and measured at FhG-ISE.

Silicon^[5]

Silicon is the eighth most common element in the universe by mass, but very rarely occurs as the pure free element in nature. It is most widely distributed in dusts, sands, planetoids, and planets as various forms of silicon dioxide (silica) or silicates. Over 90% of the Earth's crust is composed of silicate minerals, making silicon the second most abundant element in the Earth's crust (about 28% by mass) after oxygen. Elemental silicon also has a large impact on the modern world economy. Although most free silicon is used in the steel refining, aluminum-casting, and fine chemical industries, the relatively small portion of very highly purified silicon that is used in semiconductor electronics (< 10%) is perhaps even more critical. Because of wide use of silicon in integrated circuits, the basis of most computers, a great deal of modern technology depends on it. Metallurgical grade silicon is commercially prepared by the reaction of high-purity silica with wood, charcoal, and coal in an electric arc furnace using carbon electrodes. At temperatures over 1,900 °C (3,450 °F), the carbon in the aforementioned materials and the silicon undergo the chemical reaction SiO2 + 2 C → Si + 2 CO. Liquid silicon collects in the bottom of the furnace, which is then drained and cooled. The silicon produced in this manner is called metallurgical grade silicon and is at least 98% pure. As of September 2008, metallurgical grade silicon costs about US$1.45 per pound ($3.20/kg), up from $0.77 per pound ($1.70/kg) in 2005. Solar grade silicon cannot be used for semiconductors, where purity must be extreme to properly control the process. Bulk silicon wafers used at the beginning of the integrated circuit making process must first be refined to "nine nines" purity (99.9999999%), a process which requires repeated applications of refining technology. The majority of electronic grade silicon crystals grown for device production are produced by the Czochralski process, (CZ-Si) since it is the cheapest method available and it is capable of producing large size crystals.

Twenty‐four percent efficient silicon solar cells with double layer antireflection coatings and reduced resistance loss^[6]

This paper report the increase in the cell efficiency by a combination of several mechanisms. One is the reduction of recombination at cell surfaces using atomic hydrogen passivation of silicon/silicon dioxide interfaces. Joule resistive losses in the cell have been reduced by a process which allows different thickness for fine and coarse features in the top cell metallization. Finally, reflection losses have been reduced by the use of a double layer antireflection coating. It gives the fabrication details that was used to generate the cells. They have stressed on the SiO2 layer to be as thin as possible so as to keep the recombination losses in oxide covered area to be low. Metal contact to the cell rear is made through small holes in this oxide (10micron X 10micron) to keep the contact area as small as possible due to the inferior recombination properties of this region. Immediately beneath these rear contact areas, the silicon is heavily doped (with boron) to further suppress contact recombination by sup-pressing minority carrier concentrations. For the present de-vices, the top surface contact was made via a contact stripe through the oxide of 2micron width, slightly narrower than in earlier devices to further reduce the recombination loss at this contact.

19.8% efficient “honeycomb” textured multicrystalline and 24.4% monocrystalline silicon solar cells^[7]

in this paper a substantially improved efficiency for a multicrystalline silicon solar cell of 19.8% is reported together with an incremental improvement in monocrystalline cell efficiency to 24.4%. The improved multicrystalline cell performance results from enshrouding cell surfaces in thermally grown oxide to reduce their detrimental electronic activity and from isotropic etching to form an hexagonally symmetric ‘‘honeycomb’’ surface texture. This texture reduces reflection loss as well as substantially increasing the cell’s effective optical thickness by causing light to be trapped within the cell by total internal reflection.

The 1.5 V cm multicrystalline material used in this work was prepared using a modified directional solidification approach,4 sawn into wafers and processed into cells of 260 micron thickness with the structure shown in Fig. 1. An important feature is the use of thin, thermally grown silicon dioxide layers to almost completely enshroud the cell. This oxide reduces recombination along cell surfaces, improving both the collection probability for carriers photogenerated near the surface and the cell open-circuit voltage. The ‘‘inverted-pyramid’’ surface texturing used in monocrystalline PERL cells is not suited to multicrystalline substrates since it relies on anisotropic etching to expose intersecting {111} crystal planes forming pyramid sides.Such texturing not only reduces front surface reflection but increases the cell’s effective optical thickness. Weakly ab-sorbed light is reflected from the cell’s rear and strikes the top surface texture internally. Experimentally, the inverted-pyramid structure has been the most effective in trapping this light by total internal reflection, increasing the cell’s effec-tive optical thickness by factors as high as 40. It has shown that the monocrystalline silicon cells using inverted pyramids have incresed the efficiency from 24% to 24.4%. The lower reflection from the tex-tured samples results from increased absorption in this rear reflector due to multiple light passes across the cell. The results suggest light trapping in the honeycomb textured cell is comparably effective to that in inverted-pyramid cells.

24.5% Eciency Silicon PERT Cells on MCZ Substrates and 24.7% Eciency PERL Cells on FZ Substrates^[8]

This model showed the highest efficiency achieved with crystalline silicon solar cells. They were able to achieve 24.7% using passivated emitter. rear locally diffused (PERL) structure. The major advantage of the PERL cells is passivation of most of the cell surface areas with high quality TCA(trichloroethane) grown SiO2. Both front and rear metal contact areas are also passivated by heavily phosphorus and boron diffused regions. The inverted pyramid front surface and the rear surface mirror form an excellent light trapping scheme. Hence, the PERL cells give very high open-circuit voltage and short-circuit current density on FZ substrates with high post-processing minority carrier lifetimes. The PERL cell metallisation pattern has also been recently re-optimised to further reduce the metallisation shading and resistance losses.

Polycrystalline silicon^[9]

Polycrystalline and paracrystalline phases are composed of a number of smaller crystals or crystallites. Polycrystalline silicon is a material consisting of multiple small silicon crystals. Polycrystalline cells can be recognized by a visible grain, a "metal flake effect". Semiconductor grade (also solar grade) polycrystalline silicon is converted to "single crystal" silicon ie the randomly associated crystallites of silicon in "polycrystalline silicon" are converted to a large "single" crystal. Single crystal silicon is used to manufacture most Si-based microelectronic devices. Polycrystalline silicon can be as much as 99.9999% pure. Ultra-pure poly is used in the semiconductor industry, starting from poly rods that are two to three meters in length. One major difference between polysilicon and a-Si is that the mobility of the charge carriers of the polysilicon can be orders of magnitude larger and the material also shows greater stability under electric field and light-induced stress. This allows more complex, high-speed circuity to be created on the glass substrate along with the a-Si devices, which are still needed for their low-leakage characteristics. When polysilicon and a-Si devices are used in the same process this is called hybrid processing. Upgraded metallurgical-grade (UMG) silicon (also known as UMG-Si) solar cell is being produced as a low cost alternative to polysilicon created by the Siemens process. UMG greatly reduces impurities in a variety of ways that require less equipment and energy than the Siemens process. UMG is about 99% pure which is three or more orders of magnitude less pure and about 10 times less expensive than polysilicon ($1.70 to $3.20 per kg from 2005 to 2008 compared to $40 to $400 per kg for polysilicon). It has the potential to provide nearly-as-good solar cell efficiency at 1/5 the capital expenditure, half the energy requirements, and less than $15/kg. At $50/kg and 7.5 g/W, module manufacturers spend $0.37/W for the polysilicon.

Cadmium telluride photovoltaics^[10]

Cadmium telluride has a bandgap of 1.5 eV and CdTe PV is the only thin film photovoltaic technology to surpass crystalline silicon PV in cheapness for a significant portion of the PV market, namely in multi-kilowatt systems. CdTe PV is considered the ecologically leading technology as it provides a solution to key ecological issues including climate change, energy security, and water scarcity. On a life cycle basis, CdTe PV has the smallest carbon footprint, lowest water use, and fastest energy payback time of all solar technologies. CdTe cell efficiency is approaching 20% in the laboratory with a world record of 19.6% as of 2013. There has been a lot of research on fabricating CdTe cells on flexible substrate since 1999. In 2009, EMPA, the Swiss Federal Laboratories for Materials Testing and Research, demonstrated a 12.4% efficient solar cell on flexible polyimide substrate. Recently, First Solar, Inc. and GE/Primestar have made a series of advances in research cell efficiencies with the most recent record being made in 2013 at 19.6%. In 2013, the record module efficiency was 16.1%. First Solar’s record-breaking module efficiency marks a substantial increase from the 14.4% record of the previous year.[36] These achievements demonstrate that CdTe’s efficiency potential is far from saturated.

Cadmium flows and emissions from CdTe PV: future expectations^[11]

From the abstract of this paper we get to know of the future prospects of CdTe thin film technology by stating that it already represents the largest contributor to non-silicon based photovoltaics worldwide. On the basis of the finding done, they found out that while CdTe PV may account for a large percentage of future global Cd demand, its role in terms of Cd sequestration may be beneficial. They also calculated that its potential contribution to yearly global Cd emissions to air and water may well be orders-of-magnitude lower than the respective current Cd emissions rates in Europe. It states that the global demand for primary cadmium was approximately 0.6% in 2008 while its market share is expanding very rapidly, already representing 5% of the total market for photovoltaics (EPIA and Greenpeace, 2008), with a single producer now supporting a production capacity of 1.1 GW/year (First Solar, 2009a). It talks about the certain assumptions it made to conclude with a take which gave them the required information for a future of cadmium use till 2050.

Cadmium is an extremely toxic material and it occurs in small amounts in zinc ores, so that Zn producers do not have the option of not mining Cd. Zn extraction and processing have grown for the last three decades, from approximately 5.5 million tonnes per year in the early 1970s to about 11 million tonnes per year today (USGS, 2009a and USGS, 2009b). In contrast, total (primary+secondary) Cd demand expanded much more slowly in the 1970s and ’80s, and since remained virtually stable at roughly 20,000 tonnes/year (USGS, 2009a and USGS, 2009c). In fact, since the global production of Zn has increased much faster than the corresponding demand for Cd, the annual amounts of raw Cd generated are entirely determined by Zn production rates. In their analysis thay calculated that the global potential production of primary Cd from processing Zn ores was approximately 33,000 tonnes per year, based on an average Cd/Zn ratio in the ores of 0.003 (UNEP, 2006). Based on the information taken from the research groups in Europe they have also stated the cadmium emission that occur due to combustion of fossil fuels in coal- and oil-fired power plants and boilers, accounting for over 60% of the total. The average Cd content in coal reportedly ranges from 0.1 g/tonne (Pacyna and Pacyna, 2001) to 3 g/tonne (Swaine, 1995); petroleum oil has a comparatively lower Cd content, ranging 0.002–0.2 g/tonne (Karlsson et al., 2004). Other important sources of atmospheric Cd emissions are from producing and recycling galvanized iron and steel, as well as the life cycle of non-ferrous metal industrial products containing Zn, together adding up to approximately 15% of the total emissions. A third relevant source is the cement sector, contributing over 10% of the total. The metal industry account for 71 % od Cd emission into the water and in soil, The sedimentary phosphate rocks from which virtually all the commercial phosphate is produced naturally contain cadmium in concentrations from about 15 to over 200 mg (Cd)/kg (P2O5) (EC, 2001 and Oosterhuis et al., 2000).

The Impact of Tellurium Supply on Cadmium Telluride Photovoltaics^[12]

CdTe sales are growing rapidly, but there is concern about projecting hundredfold increases in power production relative to current production with CdTe PV modules. One reason is that Te, a humble nonmetal that is actually abundant in the universe, is as rare as many of the precious metals recovered from Earth's crust. CdTe module production costs have dropped from over $2/W in 2004 to $0.84/W in 2010, the lowest in photovoltaics (2). A key advantage of CdTe for thin-film devices is that it can be deposited rapidly.

Any favorable projections for CdTe PV will be moot if its contribution to power production is severely limited by Te supply. For the present technology, generating 1 gigawatt (GW) of power requires 91,000 kg (91 metric tons, MT) of Te (a cost of about $20 million). If all of the PV delivered in 2009—7 GW—had been produced with CdTe, about 640 MT of Te would have been required, which is comparable to its present annual production.

If PV is to supply 10% of the projected demand of electricity worldwide in 2030, the per annum growth rate must be 18.5%; for 25% of world electricity, it must be 25%. In 2030, the annual production would require 200 GW/year (at 10% electricity supply) or 670 GW/year (at 25% supply). For the current CdTe modules, 19,000 or 61250 MT of Te per year, respectively, would be needed, equivalent to an increase in supply by a factor of about 40 to more than 100, respectively.

The crustal abundance of Te is similar to that of platinum, but the actual recovery of Te is more than twice that of platinum. This paper gives the analysis to reduce the CdTe thickness which will ultimately reduce the use of tellurium use and the cost associated with it.

Future recycling flows of tellurium from cadmium telluride photovoltaic waste^[13]

According to the European Photovoltaic Industry Association, photovoltaic energy has the potential to contribute up to 13% to the global electricity supply by 2040. A part of this electricity production will come from thin-film photovoltaic technologies. From various thin-film technologies available on the market today, low-cost cadmium telluride photovoltaics (CdTe-PV) can be considered the market leader with a market share of 5% at annual production. There are however two major concerns about this technology: first, the potential negative environmental impacts of cadmium contamination from CdTe-PV; and second, the possible shortage of the metal tellurium in the future. To overcome the shortage of tellurium, studies have been made to recycle the tellurium from CdTe modules. It has been said in the paper that with this recycling even with the growing market of CdTe, the total tellurium demand will start to decline after 2020. Thus, the CdTe-PV industry has the potential to fully rely on tellurium from recycled end-of-life modules by 2038.

The highlights of the study estimating future recycling flows of tellurium from CdTe-PV waste are:

1. At present, overspray from CdTe deposition is the largest waste stream.
2. The Te demand, after peaking around 2020, is expected to decline.
3. Even at peak times a supply shortage of Te is implausible.
4. The CdTe-PV industry could rely on Te from recycled end-of-life modules by 2038.

Life cycle materials and water management for CdTe photovoltaics^[14]

This paper summarizes the material management aspect related to CdTe technology. It gives information on actively managing raw materials throughout the product life cycle can help to manage cost and conserve resources for large-scale PV deployment. The recent developmental research made on this technology includes solar cell and module efficiency being increased to 19.6% and 16.1%, respectively. Future projections include improving CdTe solar cell efficiency to 22% as well as achieving an average CdTe production line module efficiency of 17% by 2017. It introduces the factor of life cycle materials management is water management, where specific strategies include minimizing electricity use in PV module manufacturing, improving PV module efficiency, deploying tracking systems, developing a water balance for PV manufacturing facilities, minimizing grading during construction, using dry brush cleaning methods during operation, and recycling end-of-life systems.

The concern is to actively managing raw materials throughout the product life cycle can help to manage cost and conserve resources for large-scale PV deployment. In the case of cadmium telluride (CdTe) PV, semiconductor materials can undergo nearly closed loop materials management. Cadmium is obtained from the waste by-products of zinc refining, and tellurium from the waste by-products of copper refining. After being compounded and deployed in CdTe PV modules, CdTe can be recycled with approximately 95% yield from end-of-life modules. After further refining to semiconductor-grade purity, the CdTe is reused in the production of new PV modules. It estimates that from 2013 to 2017, module cost per Watt is to decline from $0.63–0.66 to $0.38–0.41 through a combination of increased line utilization and throughput, improved module conversion efficiency, and variable cost reduction. Over the same period, utility-scale system cost is projected to decline from $1.59 to $0.99 per Watt due to the module as well as balance of system (electrical, structural, design, and project-specific) cost reductions.

It also give information in tables which compares factors like efficiencies, open circuit voltage, fill factor and material inventory required for CdTe technology.

FIRST SOLAR SETS CdTe MODULE EFFICIENCY WORLD RECORD^[15]

First Solar, Inc. today announced it set a new world record for cadmium-telluride (CdTe) photovoltaic (PV) module conversion efficiency, achieving a record 16.1 percent total area module efficiency in tests confirmed by the U.S. Department of Energy's National Renewable Energy Laboratory (NREL). The new record is a substantial increase over the prior record of 14.4 percent efficiency, which the Company set in January 2012. Separately, First Solar also set a record for CdTe open circuit voltage (VOC), a critical parameter for PV performance, reaching 903.2 millivolts (mV) in NREL-certified testing. This new record marks the first substantial increase in CdTe VOC in over a decade of international R&D. The new records come just six weeks after First Solar announced a new world record for CdTe solar cell efficiency of 18.7 percent.

Copper indium gallium selenide solar cells^[16]

Copper indium gallium selenide (CuInxGa1-xSe2 or CIGS) is a direct bandgap semiconductor useful for the manufacture of solar cells. Because the material has a high absorption coefficient and strongly absorbs sunlight, a much thinner film is required than of other semiconductor materials. Devices made with CIGS belong to the thin-film category of photovoltaics (PV).CIGS is one of three mainstream thin-film PV technologies, the other two being cadmium telluride and amorphous silicon. CIGS is a I-III-VI2 compound semiconductor material composed of copper, indium, gallium, and selenium. The material is a solid solution of copper indium selenide (often abbreviated "CIS") and copper gallium selenide. The bandgap varies continuously with x from about 1.0 eV (for copper indium selenide) to about 1.7 eV (for copper gallium selenide).

CIGS is mainly used in the form of polycrystalline thin films. The best efficiency achieved as of October 2013 was 20.8%. A team at the National Renewable Energy Laboratory achieved 19.9% new world record efficiency. These efficiencies are different from module conversion efficiencies. Two of the leading manufacturers of CIGS thin-film PV have hit new record highs in module conversion efficiencies. The U.S. National Renewable Energy Laboratory has confirmed 13.8% efficiency of a large-area (meter-square) production panel, and 13% total-area (and 14.2% aperture-area) efficiency with some production modules. In September 2012 the German Manz AG presented a CIGS solar module with an efficiency of 14.6 % on total module surface and 15.9 % on aperture, which was produced on a mass production facility. MiaSolé obtained a certified 15.7% aperture-area efficiency on a 1m2 production module, and Solar Frontier claimed a 17.8% efficiency on a 900 cm2 module. The use of gallium increases the optical band gap of the CIGS layer as compared to pure CIS, thus increasing the open-circuit voltage. In another point of view, gallium is added to replace as much indium as possible due to gallium’s relative availability to indium. Advantage of CIGS compared to CdTe is smaller amount of toxic material cadmium are present in CIGS cells.

ZnO/CdS/Cu(In,Ga)Se, THIN FILM SOLAR CELLS WITH IMPROVED PERFORMANCE^[17]

This paper reviews the increase in the efficiency in the CIGS technology using deposition conditions for the ZnO window layer. The highly doped part of the window, ZnO:Al, has been replaced with IT0 on some devices and a comparison is made. Also, ZnO/CdSICuInSe, and ZnO/CdS/Cu(In,Ga)Se, thin film devices exhibiting active area conversion efficiencies of 15.4% and 16.9%, respectively, are demonstrated. They have shown that t CuInSe, films grows with a <112> prefered orientation, especially when stoichiometric or slightly Cu-rich. Investigating large numbers of layers deposited at various conditions and on different substrates it can be concluded that many factors, not always under control, influence the texture of the films. The device structure is explained with a soda lime float glass of 0.8 micron thick as the substrate. It gives the process flow of depositing the material to get efficient solar cells. The paper showed that the textured sample on the soda lime glass substrate had better performance.they also said that the oriented film on soda-lime is substantially better than the randomly oriented one.

Thin film PV manufacturing: Materials costs and their optimization^[18]

This paper does the analysis of the solar materials to lower the cost of production. The paper shows that the cost of production could be $0.33/Wp which corresponds to module efficiencies of about 15% and module manufacturing costs of about $50/m2. It gives a graph of Record laboratory thin-film solar cell efficiencies from 1975 to 2000. The writer has used The First Solar data is the basis for a baseline thin film manufacturing process that is used throughout the paper. Other technologies are compared using this baseline and data from the public domain. There is a listing of the similarities between all thin film technologies.

1. A transparent encapsulating top sheet (usually glass) that protects from the environment.
2. A transparent and conductive top thin film (transparent, conductive oxide) or metal grid that carries away current.
3. A thin (1–4 μ), central sandwich of semiconductors that form one or more junctions to separate charge.
4. A back contact that is often a metal film.
5. A back sheet that protects from the environment and that could be supportive (rigid or flexible).

It further give the material analysis and categorizes them as active and inactive materials. From the First Solar inventory, it is seen that the total cost of the PV panel is $44/m2 for 10% efficiency. Of that, only about $5/m2 is for the active materials defined previously. Thus about $39/m2 of thin film module costs will be for encapsulants, pottants, buss bars, wires, connectors, and the like. There is a table that list the raw material and its cost. Thus relavent conclusions are made for the reduction in cost in future.

Toward GW/year of CIGS production within the next decade^[19]

This paper gives the statistical study in the improvement in the CIGS thin film industry and that annual production volume of CIGS thin-film PV modules will exceed 1 GW/year within the next 10 years. The projected total energy world consumption in the years 2050 and 2100 has been estimated to be 28 and 46 TW, respectively. Currently PV industry is growing at a rate of 35–40% each year. Continued growth of the photovoltaic sector at rates ∼25% would increase the production level from 1.7 GW in 2005 to 380 GW in 2030 and thus would satisfy a significant fraction of the world energy demand in future. The author give the analysis of the cost of electricity generation in USA to be approximately US¢ 30 kWh−1. He compares this amount with different countries like Japan, Germany and implementaions are also being done in countries like Spain, Italy, Canada and France. Detailed calculations of existing and projected costs of capital, material, utilities, labor, maintenance, etc for CdTe, a-Si:H and CIGS thin-film PV technologies have also shown that production cost may be lowered to $1 Wp−1 when such production volumes are achieved. During 1976–2003, the PV module price followed a 80% learning curve dropping from $65 Wp−1 to $3.50 Wp−1 corresponding to an increase in the cumulative PV production. The writer gives the price analysis of silicon from 2003 to 2006. At present PV industry uses more Si wafers than the IC industry. In 2008, the tonnage of polysilicon used in the PV industry is expected to exceed that used in IC industry.

Cadmium telluride and copper–indium (In)–gallium (Ga)–selenide (Se)-sulfide (CIGS) solar cells face the problems of the supply of tellurium and indium, respectively. The total production of indium is 950 t/year of which 450 t is new indium while 500 t is recycled from monitor industry [10]. At present, the flat panel industry uses 800 t/year. Indium price was $60–70 kg−1 toward the end of 2002. Average price of indium rose to ∼$ 166 kg−1 in 2003 and $ 970 kg−1 in 2005 because of the demand of flat panel displays. The energy demand to produce 1 kg of indium is 3,611 kWh that is ∼50 times more than what is required to produce 1 kg of aluminum. Indium (1 kg) can produce up to 50 kW of CIGS PV cells that can amortize the energy in a few weeks. In fact, more energy is required to produce the sodalime glass used in the module.

Of the various technologies for the fabrication of CIGS absorber, co-evaporation of Cu, In, Ga and Se is used by Würth Solar and Global Solar. Würth Solar is fabricating monolithically interconnected CIGS cells on 60 cm×120 cm size sodalime glass substrates and has efficiency of 13%an average aperture area efficiency of 11% for the production substrate size of 7200 cm2. Similarly Global Solar have reported best efficiency of 10.4% and spec-sheet efficiency of 11% for an aperture area 8709 cm2.Global Solar has a current annual production of 3 MW/year. Therefore worldwide PV cell and module annual production which is estimated to be 1.75 GW/year during 2006 is expected to grow to 5, 34 and 380 GW/year, respectively, in 2010, 2020 and 2030 while annual production of thin-film PV production which is estimated to be 150 MW/year during 2006 is expected to grow to 1, 7.5 and 133 GW/year, respectively, in 2010, 2020 and 2030. Thus the share of thin-film PV production is expected to grow from 8.6% of the total production in 2006 to 20, 22 and 35%, respectively, in 2010, 2020 and 2030.

19.9%-efficient ZnO/CdS/CuInGaSe2 Solar Cell with 81.2% Fill Factor^[20]

This paper describes the method used by NREL in CIGS technology to obtain a record 19.9% efficiency. The have provided the fabrication of the device as per the deposited materials and graph with the lamp and substrate temperatures. They have described the 3 major difference in processing of the device with respect to previous model.

1. Ga and In were deposited in the normal ratios, however inthe last 10 s, 25 A˚ of indium was delivered in absence of Ga.
2. The sample was then subjected to a 2.5 min anneal in Se while the sample temperature was maintained at ~600 C.
3. A 2-min, 200 C air anneal was performed after the CdS deposition.

They believe that these three empirical processing changes create a near-surface region in the CIGS with reduced recombination. They also compared the last six devices in terms of current density - voltage (J-V) parameters. Minimum bandgap, which is the main determinant of Jsc, occurs about 0.5 micron into the film and is unchanged by near-surface variations. They also mentioned that the CIGS is 2.2 micron thick via SEM cross-section or by mechanical profilometer. This thickness is about 0.5 micron thinner than previous record devices.

Amorphous silicon^[21]

Amorphous silicon (a-Si) is the non-crystalline allotropic form of silicon. Silicon is a fourfold coordinated atom that is normally tetrahedrally bonded to four neighboring silicon atoms. In crystalline silicon (c-Si) this tetrahedral structure continues over a large range, thus forming a well-ordered crystal lattice. In amorphous silicon this long range order is not present. Rather, the atoms form a continuous random network. Moreover, not all the atoms within amorphous silicon are fourfold coordinated. Due to the disordered nature of the material some atoms have a dangling bond. Physically, these dangling bonds represent defects in the continuous random network and may cause anomalous electrical behavior. Likewise, the material can be passivated by hydrogen, which bonds to the dangling bonds and can reduce the dangling bond density by several orders of magnitude. Hydrogenated amorphous silicon (a-Si:H) has a sufficiently low amount of defects to be used within devices such as solar photovoltaic cells, particularly in the protocrystalline growth regime.

While a-Si suffers from lower electronic performance compared to c-Si, it is much more flexible in its applications. For example, a-Si layers can be made thinner than c-Si, which may produce savings on silicon material cost. One further advantage is that a-Si can be deposited at very low temperatures, e.g., as low as 75 degrees Celsius. This allows for deposition on not only glass, but plastic as well, making it a candidate for a roll-to-roll processing technique. Once deposited, a-Si can be doped in a fashion similar to c-Si, to form p-type or n-type layers and ultimately to form electronic devices. The main advantage of a-Si in large scale production is not efficiency, but cost. a-Si cells use approximately 1% of the silicon needed for typical c-Si cells, and the cost of the silicon is by far the largest factor in cell cost.

Amorphous silicon based photovoltaics—from earth to the ‘‘final frontier’’^[22]

a-Si PV technology has positioned itself as the low-cost solution to the challenge of energy, environmental, and ecology—the e-tripos. It says that PV in general, and a-Si technology in particular, will undoubtedly play a significant role in providing clean, quiet, and renewable energy harvested from the inexhaustible sun. The volume of worldwide sales of a-Si PV modules was 34 MW in 2001, a phenomenal growth of 26% over the previous year. The advantages of a-Si mentioned are.

1. low cost.
2. Good conversion efficiency is another advantage for a-Si PV.
3. low cost manufacturing
4. use of a-Si in multi junction cells. Multi junction tendem cell help in reducing the SWE.

The p layer through which the light enters the solar cell is particularly important and should be as transparent and conductive as possible. It has a table that compares characteristics of a-Si:H/a-Si:H double-junction cells with two different tunnel-junction configurations. It is observed that a superior tunnel-junction leads to a higher cell efficiency. They have also discussed the process to make the roll to roll model of solar cells. Key factors leading to the attainment of high efficiency cells were also discussed.

A review of solar photovoltaic technologies^[23]

This paper gives a review of the current state of the art technologies that have develop[ed over the years in photovoltaics field. It has use a large set of reference to put forword the analysis that was done. For Amorphous (uncrystallized) silicon is described as the most popular thin-film technology with cell efficiencies of 5–7% and double- and triple-junction designs raising it to 8–10%. Yang et al. discussed the advances made in amorphous-Si PV technology that led to the achievement of an AM 1.5, 13% stable cell efficiency and set the foundation for the spectrum splitting triple-junction structure being manufactured by the roll-to-roll continuous deposition process as reviewed in the above paper.

For the thin film PV solar cells, Barnett et al. investigated that solar cells utilizing thin-film polycrystalline silicon can achieve photovoltaic power conversion efficiencies greater than 19% as a result of light trapping and back surface passivation with optimum silicon thickness. Powalla et al. assessed that all existing thin-film PV technologies, especially the Cu(In,Ga)Se2 (CIGS)-based technology, have a high cost reduction potential at high production volumes projecting futuristic challenge to combine high production volumes with high throughput, sufficient yield and superior quality to achieve efficiencies of above 11% and a maximum of 12.7%. The major purpose of this paper was to help solar PV system manufactures, academicians, researchers, generating members and decision makers.

Towards very low-cost mass production of thin-film silicon photovoltaic (PV) solar modules on glass^[24]

Production volume of PV modules increases at > 35% per year, but one is yet far from making a global impact on energy supply. One of the obstacles is given by the present high production costs of PV modules. The paper says that the use of modified fabrication equipment from the AM-LCD Display Industry is therefore a promising way to implement low-cost mass production. It has been mention that the PV modules have $3-4$ per Wp (peak-Watt) and for purchase of large quantities of crystalline silicon PV modules, prices as low as $2.60 per Wp are quoted in certain countries. Compound semiconductors, like CdTe and Cu (In, Ga) Se2 (CIGS), have attracted much attention in the past, and have led to relatively high laboratory efficiencies, but so far their large-scale industrialization has not reached volumes comparable to thin-film silicon based technologies. Beside the highly toxic materials Cd, an ultimate materials shortage, particularly of In, followed by Te could appear.

Low cost concepts for thin film silicon solar cell manufacturing are being tried to develop. By choosing silicon, we can benefit from the efforts of the Display Industry, where amorphous silicon is widely used for AM-LCD (Active Matrix Liquid Crystal Display) production. Thin silicon films, with amorphous or polycrystalline structure, can be deposited on glass, on stainless steel and on plastic substrates, by PECVD (Plasma-Enhanced Chemical Vapor Deposition), from a mixture of silane (SiH4) and hydrogen (H2), and, partly depending on the substrate material, at temperatures from 150 to 500 °C. There are figures to represent the different absorption coefficient for different materials. One has, to use p–i–n diodes, where the photo-generation takes place in the i-layer and transport and collection are drift-assisted. The Transparent Conductive Oxide (TCO, here ZnO) used as transparent electrode, on the side where the light enters the solar cell. This feature is common to all thin film solar cells. It also gives issues related to the fabrication process of the solar cells. This paper conclusives says that it is necessary to reach to module manufacturing costs of 1 U.S. $ per peak-Watt and the ways in which we can do it.

[HIGH-EFFICIENCY AMORPHOUS SILICON DEVICES ON LPCVD-ZNO TCO PREPARED IN INDUSTRIAL KAITM-M R&D REACTOR]^[25]

This paper presents a study on the optimal i-layer thickness for high efficiency amorphous silicon p-i-n solar cells deposited on doped LPCVD-ZnO. It rightly states that both, the manufacturing costs and the light-soaking stability of the modules can be beneficially influenced by appropriately reducing the a-Si:H absorber layer thickness. For this purpose, rough TCOs were used for enhancing the light-trapping within the device. TCO film properties, such as high transmission, high conductivity, excellent light-scattering capabilities (visible and near-infrared range), and a surface morphology suited for the growth of high quality thin films are mandatory for high efficient silicon solar devices. Previous studies demonstrated that an excellent quality amorphous silicon i-layer (at 3.35 Å/s) could be deposited by using an excitation frequency of 40.68 MHz. In this new study, the i-layer deposition rate was reduced to 1.75 Å/s. The polycrystalline films were growing as pyramids and this rough surface difusses the light. This effect yields to an efficient light-scattering into the silicon device and, hence, high Jsc-values. For the amorphous silicon the deposition rate of the new i-layer was reduced from 3.35 Å/s to 1.75 Å/s. Both considered i-layers have an optical bandgap (Tauc’s) of 1.73 eV.

The fundamental learning from this study is that front and back LPCVD-ZnO contacts for a-Si:H singlejunction solar cells, can provide very high stable Jsc-value (16 mA/cm2) in combination with good Voc- and FF-values even with a thin i-layer (180nm). When test were conducted on this new type of a-Si solar cell a remarkably high stabilized efficiency of 10.09 ± 0.3 % was confirmed.

Effect of structural variations in amorphous silicon based single and multi-junction solar cells from numerical analysis^[26]

In this paper single and multi junction amorphous silicon have been analysed. Effects of thickness and doping concentration of different layers as well as the operating temperature on cell efficiency have been investigated with a view to find a more efficient and stable cell. They used Analysis of Microelectronic and Photonic Structures (AMPS-1D) simulator for the numerical analysis of the solar cell wrt various parameters.

1. For the single junction cell, the maximum efficiency of 19.62% has been achieved for a thickness of 500 nm of i-layer with temperature gradient of 0.23%/C.
2. In case of double junction cell, the highest efficiency of 20.19% was found for top i-layer thickness of 700 nm with temperature gradient of 0.17%/C.
3. For the triple junction cell, the maximum efficiency of 21.89% was found with the top i-layer thickness of 600 nm with temperature gradient of 0.18%/C.

Hydrogenated amorphous silicon (a-Si:H) and its alloys have become important semiconductor materials for solar cells due to lower cost and the ease of fabrication merits. Multi junction a-Si have better efficiency and the stacked solar cell also shows better stability as the problem of photodegradation associated with amorphous silicon solar cells is less manifested in it than its single junction counterpart.

The Prospects of Amorphous Silicon PV: Down, But Hardly Out^[27]

This article give the statistical analysis of the amaorphous silicon in market from 2009 to 2012. It has information on the prices and 2 graph that represents the $/W. The market cost for single junction a-Si would be $1.20 per watt with a gross margin profile of 20%. In the analysis it is said that in 2012 the market price for a-Si would be about 95 cents to a dollar a watt, implying a fully loaded 2012 cost of about 80 cents a watt.

Polycrystalline Thin-Film Solar Cell Technologies^[28]

This paper present the significant progress in the thin film technology especially in CdTe and CIGS. It states that the worldwide estimated projection for thin-film PV technology production capacity announcements (including a-Si/thin Si) which was estimated at 3700 MW in 2006 is now estimated at more than 8000 MW by 2010. Thin-film CdTe solar cells are high absorption coefficient and therefore very thin absorber layer are needed to absorb the photons. . Laboratory efficiencies of 16.5% for thin-film CdTe solar cell has been demonstrated by NREL scientists. For CIGS technology fabricating high-efficiency modules, low-cost, and reliable thin-film power modules can be produced. This material has demonstrated the highest total-area, conversion efficiency for any thin-film solar cells in the range of 19.3% to 20.0%, fabricated by NREL scientists. In conclusion it stated that in the United States, market share for thin-film (includind a-Si) was about 65% in 2007 compared to less than 10% in 2003, and has surpassed Si shipments in the year 2007.

Life cycle assessment of solar PV based electricity generation systems: A review^[29]

This paper presents a review of the life cycle assessments (LCA) of solar PV based electricity generation systems. The complete production process from extraction of silica to final panel assembly has been considered. Life cycle assessment of amorphous, mono-crystalline, poly-crystalline and most advanced and consolidate technologies for the solar panel production has been studied. Life cycle assessment (LCA) is a technique for assessing various aspects associated with development of a product and its potential impact throughout a product's life. LCA can help in improvement, reduce impact on human health and materials reduction. This paper give a good analysis of the amorphous silicon using a set of references in terms of CO2 emission. Some highlights of the analysis are.

1. The primary energy consumption for the construction of the PV power plants ranges from 13,000 to 21,000 kWh/kWp & the life cycle CO2 emission is 3.360 kg-CO2/kWp.
2. For the production of PV modules and BOS components of grid-connected PV systems, the energy pay-back period was found to be 2.5–3 years for rooftop installation and 3–4 years for multi-megawatt ground mounted system. Similarly, the CO2 emissions of the rooftop system were calculated as 50–60 g/kWhe now and probably 20–30 g/kWhe in the future.
3. For PVL136 thin film (amorphous) modules including BOS, the manufacturing of one PVL136 module consumes 371 MJ of primary energy in materials and 1490 MJ of primary energy as process energy. The life CO2 emissions were 34.3 g-CO2eq/kWhe.
4. The study done by Ito et al on life cycle analysis for 100 MW very large scale PV (VLS-PV) systems at Gobi desert using amorphous silicon (a-si) solar cell modules, the life cycle CO2 emissions were found out to be 15.6 and 16.5 g-CO2eq/kWhe considering temperature of the desert 5.8 and 30.2 °C, respectively.

Similar kind of analysis has been given for mono crystalline and polycrystalline silicon PV systems and in conclusion the LCA was good for amorphous silicon by comparison.

Health and Safety Concerns of Photovoltaic Solar Panels^[30]

The life cycle of a c-Si panel starts with mining of crystalline silica in the form of quartz or sand. The raw material is then refined in industrial furnaces to remove impurities to produce metallurgical grade silicon (~98% pure silicon). The metallurgical grade silicon is then further refined to produce high purity polysilicon for use in the solar and semiconductor industry. It should be noted that e only 2% of global silica sand production is utilized in the production of metallurgical grade silicon. In order to transform industrial grade silica sand into metallurgical grade silicon, the silica is combined with carbon in the form of charcoal, coal, or coke in an electric arc furnace in a process called carbothermic reduction. In order to reach a purity level acceptable for use in manufacture of semiconductor devices, metallurgical grade silicon must go through two additional purification steps. The primary output from this purification process is polysilicon, the precursor to the silicon wafers used to manufacture the integrated circuits at the heart of most electronics as well as monocrystalline photovoltaic solar cells. In the first step, pulverized metallurgical grade silicon is combined with hydrogen chloride gas and a copper catalyst in a fluid bed reactor to produce trichlorosilane. Trichlorosilane is the primary chemical feedstock for the production of polysilicon. To produce polysilicon, the trichlorosilane is subjected to a distillation process until the desired purity level is achieved. The purified trichlorosilane is then used to deposit very pure polysilicon in a chemical vapor deposition reactor. This process, commonly referred to as the Siemens process, accounts for as much as 98% of the world's polysilicon production.

Life Cycle Analysis of Silane Recycling in Amorphous Silicon-Based Solar Photovoltaic Manufacturing^[31]

The current manufacturing process for Si:H-based PV cells varies between different substrates, methods, and manufacturers,but the actual deposition of a-Si:H and µc-Si:H layers are formed by running the substrate through a set of chambers andexposing it to a mixture of hydrogen and silane using plasma-enhanced chemical vapor deposition (PECVD) (Street, 2000;Wronski and Carlson, 2001). The layers of a-Si:H PV are constructed in a p-i-n or n-i-p device structure. The p-layer isusually a boron-doped a-Si:H or a-SiC:H , followed by an i-layer of either undoped a-Si:H or amorphous silicon germaniuma-SiGe:H and an n-layer of phosphorous doped a-Si:H (Izu and Ellison, 2003; Street, 2000; Wronski and Carlson, 2001;Wronski et al., 2002).For the tandem cell structures, µc-Si:H has been shown to be able to act as an active layer in p-i-n solar cells (Meier, et al.,1998; Keppner, et al., 1999; Meier, et al., 2006;). Compared to the bandgap of a-Si:H (1.8eV), μc-Si:H was found to have asignificantly lower energy bandgap of around 1 eV and thus the combination of both materials (two absorbers with differentgap energies) leads to a tandem cell structure, referred to as the “micromorph” cell, with superior performance to bothsingle and double junction a-Si:H-based cells because of increased use of the solar spectrum (Meier, et al., 2006)

Defects in photovoltaic materials and the origin of failure to dope them^[32]

"The doping limit rule for n type doping" A material cannot be doped successfully n-type if its Conduction Band Minimum (CBM) is too close to vacuum (i.e., its electron affinity is too small). This is the case for diamond, AIN, CuGaSe, etc. In these cases, electron-killer defects such as cation-vacancy will form and compensate the electron-producing agent. Conversely. a material can be doped successfully n-type if its CBM is as far away from vacuum as possible (large electron affinity). This is the case in ZnO, SnO₂. InP. etc.

"The doping limit rule for p type doping" A material cannot be doped p-type if its VBM is too far from the vacuum level (intrinsic work-function is too big). This is the case for common oxides such as ZnO. MgO. CaO. etc. In this case, hole-killers such as cation recently been attributed to H incorporation into n-type ZnO as a donor.

Crystalline Silicon Photovoltaic Cell Technology: Meeting The Challenge For Utility Power^[33]

This paper tells us about the early evolution of photovoltaic materials where the present research was focused on improving the efficiency of solar cells using crystalline silicon. It gives statistical analysis of the reduction in the cost as there was evolution in the technology used from 1970's to 1980's. It presented an economical analysis of flat plat and concentrator modules with respect to cell efficiency. It also showed results pertaining to a wide variety of crystalline silicon technology paths including both single crystal and polycrystalline silicon in flat plates with all tracking approaches and line-focus and point-focus concentrator systems. Through its finding it showed that highly efficient crystalline silicon could be used to be make solar cell at lower cost as compared to other materials present at that time.

Silicon: Evolution and future of technology^[34]

Monocrystalline silicon is also used in the manufacturing of high performance solar cells. Since, solar cells are less demanding than microelectronics for as concerns structural imperfections, monocrystaline solar grade (Sog-Si) is often used, single crystal is also often replaced by the cheaper polycrystalline silicon. Monocrystalline solar cells can achieve 21% efficiency. [Monocrystalline siicon]

The writers of one of the chapters W.Heywang, K.H.Zaininger were the pioneer men who had seen the evolution of the silicon technology over the years. They summarized certain properties of silicon during their study.

1. It is abundant easy to obtain and low cost.
2. It is a single crystal with substrate diameter as large as 12 inches in which defects could be eliminated or selectively utilized.
3. Not brittle and can easily be handled with an excellent mechanical substrate.
4. Adequate thermal conduction to take away electrically generated heat.
5. Thin crystalline films can be grown over substrate having different electrical properties via epitaxy.
6. Thin crystalline films can be grown over insulators to provide improved isolation, speed and lower capacitance.
7. Novel films of III - V compounds containing quantum dots can be grown onto substrate by CVD or MBE.
8. Doping (n type or p type)can improve the conductive properties of silicon using diffusion or ion implantation.\
9. It is not light sensitive and is stable under various light conditions.

Solar cell surface characterization^[35]

Majority of solar cells are produced using monocrystalline or large grained polycrystalline silicon. To reduce manufacturing cost industries use lower quality siliocon refered as metallurgical grade silicon which has about 98% purity. For this material the light absorbtion efficiency is very low. Increasing the effective optical thickness of the silicon surface is the most reliable way to increase cell efficiency. It is called as surface texturing and depends on the nature of silicon. On the other hand in multicrystalline silicon, texturing is not so effective because most of the grains have dislocated orientation. Surace texturing on this has the disadvantage that different grain etch at different rates giving steps at grain boundaries which proves to be a problem for the subsequent processess.

Producer responsibility and recycling solar photovoltaic modules^[36]

In this paper Pearce et al have given vital information on the recycling of the photovoltaic materials in 5 major technologies vis polycrystalline silicon, crystalline silicon, amorphous silicon, cadmium telluride and CIGS. From this analysis the postulated formulas for mass of recovered semiconductor and glass, and profit from resale of recovered semiconductor and glass.

Thermodynamic limitations to nuclear energy deployment as a greenhouse gasmitigation technology^[37]

To both replace fossil-fuel-energy use and meet the future energy demands, nuclear energy production would have to increase by 10.5% per yearfrom 2010 to 2050. Global warming is already occurring, and if combustion of fossil fuels continues, temperatures are projected to rise by between 1.8°C and 4°C in the next 100 years. the US National Energy Policy Development Group stated, “Nuclear power today accounts for 20% of our country’s electricity. This paper shows how they could reduce the GHG emission by using efficient methods.

Global electricity production using different sources^[38]

World electricity generation rose at an average annual rate of 3.6% from 1971 to 2009, greater than the 2.1% growth in total primary energy supply. This increase was largely due to more electrical appliances, the development of electrical heating in several developed countries and of rural electrification programmes in developing countries. The share of electricity production from fossil fuels has gradually fallen, from just under 75% in 1971 to 67% in 2009. This decrease was due to a progressive move away from oil, which fell from 20.9% to 5.1%. Oil for world electricity generation has been displaced in particular by dramatic growth in nuclear electricity generation, which rose from 2.1% in 1971 to 17.7% in 1996. However, the share of nuclear has been falling steadily since then and represented 13.4% in 2009. The share of coal remained stable, at 40-41% while that of natural gas increased from 13.3% to 21.4%. The share of hydro-electricity decreased from 22.9% to 16.2%. Due to large development programmes in several OECD countries, the share of new and renewable energies, such as solar, wind, geothermal, biofuels and waste increased. However, these energy forms remain of limited importance: in 2009, they accounted for only 3.3% of total electricity production for the world as a whole.

Towards sustainable photovoltaics: the search for new materials^[39]

This paper talks about the material scarcity that are used in PV systems and to find alternative to them and says if PVs is to make a significant contribution to satisfy global energy requirements, issues of sustainability and cost will need to be addressed with increased urgency. In terms of primary (thermal) power rather than energy, the corresponding 2008 figure is 13.2 TW, of which less than 0.8 TW was generated by non-nuclear renewable resources (primarily hydroelectric). The total power of the Sun’s radiation that is incident on the Earth can be calculated from the solar constant (1.366 kW m−2) and the cross-sectional area of the Earth. The result is 1.4×1017W, i.e. around 5000 times the estimated primary power requirement for 2050. If we assume that we could cover 1 per cent of the Earth’s land surface with solar arrays operating at a power efficiency of 10 per cent, a rough calculation based on the land area illuminated by the Sun and losses owing to weather and seasons indicates that photovoltaics (PVs) could generate around 25 TW. The current contribution of PVs to the total world energy requirement is still very small, with total worldwide installed capacity just over 20 GWp.

The bandgaps of different materials are compared and the bandgap of the CIGS system can be tuned by controlling the In/Ga ratio. CdTe thin film solar cells CdTe-based solar panels are currently the most rapidly expanding thin-film PV technology, with module sales from First Solar reportedly passing the 1 GWp mark in 2009. This paper therefore attempted to show that thin-film PVs are having an increasing impact on renewable energy strategies. Existing technologies based on well-known semiconducting materials are driving down cost and carbon footprint, and novel technologies are moving towards the market.

Materials availability for thin film (TF) PV technologies development: A real concern?^[40]

The authors have compared 6 different papers (Andersson et al, 1998; B A Anderson, 2000; Keshner and Arya, (report), 2004; Feltrin and Freundlich, 2008; V Fthenakis, 2009; Wadia et al., 2009) which have all talked about material availability for thin film solar cells. It shows the different assumptions on which all these papers have postulated their theory. Assumptions with respect to cell efficiencies, material thickness, material utilization for solar industry, recycled material useage etc. which all results in different numbers related to power production. It states that PV generation will play a major role in the future global energy mix up to 11% of global electricity by 2050. In 2009, c-Si had a 82% market domination and thin film (TF) technologies had 17%. CdTe TF modules are currently the least expensive to manufacture with a module cost production of $0.76/W. They suggest that the implications of rising material costs as a result of relative scarcity may be more significant to the future development of CdTe and CIGS technologies than any fundamental limit on material supply.

This paper compares certain different papers on material availability of indium and tellurium and says that the estimates of future availability of indium and tellurium used in the literature present several uncertainties, both in the data and in the methodologies adopted. Regarding the availability of material data shown by USGS every year, to collect this data would require a significant international co-operation which they assume to be unlikely. It states that In and Te could be extracted from Iron and Lead respectively which have not been estimated by USGS. Therefore they argue that there is more In and Te than that is estimated. These three factors (marginal reserves, improving geological knowledge and technological advance) are collectively referred to as ‘reserve growth’. Reserves, reserve growth, and 'yet to find' (YTF) can be collectively referred to as Ultimately Recoverable Resources (URR). To increase the power, one or all the 3 points could be implemented. 1) Increasing cell and module efficiencies. 2) Reducing the active layer thickness. and/or 3) Higher material utilisation during production.

It should also be pointed out that that recycled indium and tellurium are not fully accounted for by the literature in estimating materials’ future availability. Indium is increasingly reclaimed from deposition process of indium tin oxide (ITO) in liquid crystal displays (LCDs) manufacturing, which accounts for more than 50% of primary indium demand. This number could be increased to 92% based on the research. Also Average recovery rates of around 90–95% are possible for Te from CdTe end-of-life cells. Materials constrained potential of CdTe and CIGS technologies has to be assessed while acknowledging that future market size and cumulative installed capacity are to be satisfied by a mix of PV technologies. Thus, it is possible to conclude that absolute availability of indium and tellurium is not a constraint to future development and deployment of CIGS and CdTe PV technologies per se. More than 50% of refine Indium is used by ITO industry and only 2% in the solar industry. Thus ITO has been held accountable for the price increase in indium. However, 11% of tellurium was used in the solar industry in 2009 which might have a significant role in driving the prices. However, as some contributions in the literature have suggested, it is the price of indium and tellurium that could have a negative impact on cost reduction ambitions and future developments of CdTe and CIGS technologies. Ultimately the author says that there is more amount of indium and tellurium available that can be discovered which can be used in the solar cells. In conclusion, they have said that there is plenty of reserves available for In and Te, but the factor that posses a threat to the development of these technologies is the price hike in the near future.

Can solar power deliver?^[41]

Until 2013, mono crystalline and multi crystalline silicon modules have contributed to 90 % of installed PV capacity. It clearly states with number that even if we harvest just over 1% of solar energy available in a year, we exceed the annual power and electricity requirement. (Useable land area and whether conditions have been considered). Assuming if 15% efficiency cell were modules deployed in London, only 5% of its land area would be required to power the whole city. This shows that there is plenty of land available for solar installation. It quotes solar power as intermittent energy source, and need to be cascaded with other renewable energy source like wind to provide the energy needs. In projections of future energy supply, PVs are expected to play a significant role, delivering up to 16 per cent of global electricity by 2050. It states that the cost could be reduced using the new technological milestones. BOS has an important contribution in the cost of the module and new technologies are ensuring that BOS is reduced. In the growth of the PV industry every year, carbon emission play an important role. Its states for short term that higher growth of low carbon intensive technology will reduce the carbon effects and for longer term, with R&D, high efficiency cell would help in utilizing higher device area which will ultimately mitigate the carbon issue drastically.

Technology Roadmap: Solar Photovoltaic Energy^[42]

This technology roadmap estimates that: 1. By 2050, PV could provide 11% of global electricity production and avoid 2.3 gigatonnes (Gt) of CO2 emissions per year. 2. By 2020, PV is expected to reduce system and generation costs by more than 50%. PV residential and commercial systems will achieve the first level of grid parity. 3. Towards 2030, typical large-scale utility PV system generation costs are expected to decrease to USD 7 to USD 13 cents/kWh.

Assessing the dynamic material criticality of infrastructure transitions: A case of low carbon electricity^[43]

Criticality is currently defined as the combination of the potential for supply disruption and the exposure of a system of interest to that disruption. The European Commission defines critical materials as those at risk of supply disruption and which are difficult to substitute. The criticality assessment has been done in particular for Neodymium. Fig 2 shows the criticality of two scenarios of transition to low carbon electricity generation in the UK from 2012–2050. The results show that criticality in the Core Pathway increases more than threefold over the period from 2012 to 2050, with a step-change occurring in 2030, as shown with reference to 2012 values. This trend is even more dramatic in the Renewables scenario with a ninefold increase. The results of the case study demonstrate the importance of considering the dynamic analysis of the risk of material criticality.

Photovoltaic material resources^[44]

This chapter refers to materials critical in the thin film photovoltaic industry like Gallium, Indium and Tellurium and makes an analysis regarding it. Referring to different sources it says that Te ranges from critical to not critical material. Projections for 2050 vary by nearly 2 orders of magnitude with the lower end stating 100GW and upper end 10TW. This difference is due to the different assumptions made by the sources. The lower end of the projection for 2050 represents less than 2% per annum compounded growth suggesting little difficulty in increasing material suppy rates. The higher end of the range represents a growth rate of 20% per annum.

Considerations of resource availability in technology development strategies: The case study of photovoltaics^[45]

World electricity demand is continuously growing and is expected to be 28 000 TWh by 2030 and range between 50 000 TWh and 60 000 TWh in 2050. The pay back times of different PV technologies have been mentioned in this paper. The theoretical limit for energy production by solar based technology is the global irradiation which accounts to 1 083 000 000 TWh and the (state-of-the-art) technical limit is 444 000 TWh which takes into account issues like available land, electricity net configuration, conversion efficiency, etc. As the technical limit (444 000 TWh) is about 9–7.5 times the expected demand in 2050 as mentioned above (50 000–60 000 TWh), irradiation and technology should not be a limiting factor. As an example of material thickness, 3 micon of a CdTe absorber layer are only around 9 g of Cd and 9 g of Te per square meter. For c-Si, losses due to sawing and losses at ingot growing are in the range of 45–51%. In the case of CdTe and CIGS the material utilization is currently around 40% and 90% foe a-Si. In this scenario 25% of worlds electricity demand is supposed to be produced through PV installations by 2040. In absolute numbers this accounts for 10000 TWh of the produced electricity by 2040. For material availability specifically for tellurium, Even in the optimistic scenario cumulative tellurium demand is around 2.5 times the known reserves for 2040.

Sustainability of photovoltaics: The case for thin-film solar cells^[46]

This paper talks about the availability of material for solar applications, in particular, tellurium and indium. To ensure photovoltaics become a major sustainable player in a competitive power-generation market, they must provide abundant, affordable electricity, with environmental impacts drastically lower than those from conventional power generation. The recent reduction in the cost of 2nd generation thin-film PV is remarkable, meeting the production milestone of $1 per watt in the fourth quarter of 2008. This achievement holds great promise for the future. However, the questions remaining are whether the expense of PV modules can be lowered further, and if there are resource- and environmental-impact constraints to growth. The potential of thin-films in a prospective life-cycle analysis has been examined, focusing on direct costs, resource availability, and environmental impacts. These three aspects are closely related; developing thinner solar cells and recycling spent modules will become increasingly important in resolving cost, resource, and environmental constraints to large scales of sustainable growth. In conclusion he say that there is huge potential in solar power generation, and future R&D should concentrated on reducing the thickness of the material used in the thin film solar cells.

Considerations of resource availability in technology development strategies: The case study of photovoltaics^[47]

This paper has done an analysis on the tellurium and indium availability. Photovoltaic (PV) technologies have experienced considerable growth rates of up to 70% in the last years. This has been possible because of low total CO2 emissions and a positive overall energy balance for PV. Several institutions have developed future scenarios which show an increase in global electricity demand from 17 000 TWh in 2005 to some 60 000 TWh by 2050. A significant part of this amount should be supplied by PV installations. Based on selected scenarios material demand is calculated for four different PV technologies: crystalline silicon (c-Si), amorphous silicon (a-Si) in tandem configuration, cadmium tellurium (CdTe) and copper indium gallium diselenide (CIGS). As these technologies use rare metals it is shown, that particular scenarios are unlikely to be realized because of supply constraints and scarcity phenomena. Critical materials are silver, tellurium and indium. We consider photovoltaics as an appropriate example for the implementation of resource availability considerations into technology development strategies.

They have conducted sensitivy analysis on different solar cell technologies. Only a-Si in tandem configuration with μc-Si has, from today's point of view, the potential to be installed on a several terawatt level. They say that recycling of material would become a huge aspect when larger capacity power installations are done in the future.

Towards real energy economics: Energy policy driven by life-cycle carbon emission^[48]

This paper focusses on the policy required to have a good future in solar sector. It has done a comparison chart of the CO2 emission occuring due different energy sources. In the table it is mentioned that 1 kWH of energy generated due to combustion of coal produces 881.6 grams of CO2 in the atmosphere. However for solar PV, its in the range of 21-59 g/kWh. Also the energy payback time has been compared to fossil fuel and solar has been proved to be far more superior than any other source of energy used. Thus in conclusion they say, the challenges involved in meeting increasing electricity demand while simultaneously reducing carbon dioxide emissions can be met by the large-scale deployment of alternative energy technologies. However, deployment must be dependent on the life-cycle carbon emissions of each viable technology. Currently employed static life-cycle assessments incorrectly trivialize subtleties associated with rapid growth; this disparity enables the development of a global carbon Ponzi Scheme wherein the carbon mitigation potential of technologies is hindered by large-scale deployment. Dynamic life-cycle analyses offer a superior tool to evaluate deployment strategies for energy technologies. While there remains a need for rigorous simulation of carbon-neutral growth rates on both global and local scales, carbon-neutral growth rates paired with dynamic life-cycle assessments arm policy makers with standardized information needed to optimize electricity generation technology deployment for effective climate change mitigation.

Effect of packing factor on the performance of a building integrated semitransparent photovoltaic thermal (BISPVT) system with air duct^[49]

In this paper, an attempt has been made to study the effect of packing factor of semitransparent photovoltaic (PV) module integrated to the roof of a building, on the module and room air temperature, and electrical efficiency of PV module. Energy and exergy analysis have been carried out by considering different packing factors (0.42, 0.62, and 0.83) of PV module namely mono crystalline silicon (m-Si), poly crystalline silicon (p-Si), amorphous silicon (a-Si), cadmium telluride (CdTe), copper indium gallium diselenide (CIGS), and a heterojunction with thin layer (HIT). It is observed that the decrease in the temperature of PV module due to decrease in packing factor, increases its electrical efficiency. It is also found that the decrease in packing factor increases the room temperature. Maximum annual electrical and thermal energy is found to be 813 kWh in HIT and 79 kWh in a-Si PV module respectively with packing factor of 0.62

Technology Roadmap: Solar photovoltaic energy^[50]

1. Solar PV power is a commercially available and reliable technology with a significant potential for long-term growth in nearly all world regions. This roadmap estimates that by 2050, PV will provide around 11% of global electricity production and avoid 2.3 gigatonnes (Gt) of CO2 emissions per year. 2. Achieving this roadmap’s vision will require an effective, long-term and balanced policy effort in the next decade to allow for optimal technology progress, cost reduction and ramp-up of industrial manufacturing for mass deployment. Governments will need to provide long-term targets and supporting policies to build confidence for investments in manufacturing capacity and deployment of PV systems. 3. PV will achieve grid parity – i.e. competitiveness with electricity grid retail prices – by 2020 in many regions. As grid parity is achieved, the policy framework should evolve towards fostering self-sustained markets, with the progressive phase-out of economic incentives, but maintaining grid access guarantees and sustained R&D support.

Energy Technology Perspective^[51]

This report focuses on the stats related to energy and considers the scenarios and strategies to 2050. ETP 2010 presents updated scenarios from the present to 2050 that show which new technologies will be most important in key sectors and in different regions of the world. It highlights the importance of finance to achieve change, examines the implications of the scenarios for energy security and looks at how to accelerate the deployment of low-carbon technologies in major developing countries. It presents roadmaps and transition pathways for spurring deployment of the most important clean technologies and for overcoming existing barriers. With extensive data, projections and analysis, Energy Technology Perspectives 2010 provides decision makers with the detailed information and insights needed to accelerate the switch to a more secure, low-carbon energy future.

Reference </ref>

References

See also

• Evolution of microstructure and phase in amorphous, protocrystalline, and microcrystalline silicon studied by real time spectroscopic ellipsometry
• Improvement of solar cells performance by boron doped amorphous silicon carbide/nanocrystalline silicon hybrid window layers
• General Temperature Dependence of Solar Cell Performance and Implications for Device Modelling

Potential of solar electricity generation in the European Union member states and candidate countries