Photovoltaic materials abundance limitations literature review/Thin film solar cells with improved performance

ZnO/CdS/Cu(In,Ga)Se, THIN FILM SOLAR CELLS WITH IMPROVED PERFORMANCE^[1][edit | edit source]

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^[2][edit | edit source]

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^[3][edit | edit source]

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^[4][edit | edit source]

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^[5][edit | edit source]

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^[6][edit | edit source]

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^[7][edit | edit source]

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^[8][edit | edit source]

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]^[9][edit | edit source]

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^[10][edit | edit source]

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^[11][edit | edit source]

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^[12][edit | edit source]

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^[13][edit | edit source]

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^[14][edit | edit source]

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^[15][edit | edit source]

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^[16][edit | edit source]

"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^[17][edit | edit source]

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.