SOLAR BATTERY

SOLAR BATTERY

A solar battery is simply a battery charged with energy from solar panels.

keywords : SOLAR BATTERY


Profile

Battery ProfileSunbright Power, leading manufacturer focused in design and produce maintenance free sealed lead acid battery in China. The company registered capital of 8 million USD, with a total investment 70 million USD. It covers an area of 220 acres, 70,000 square meter production plant, and annual production capacity of one million KVAh. The batteries made by Sunbright include backup batteries applied in telecommunications, Power Plant, UPS battery, fire alarm system, emergency lighting and efficient energy storage batteries used in solar energy, wind energy and, as well as motive power batteries for electric vehicles, golf carts, electric forklift, electric traction trucks and other fields. All products are CE certificated, UL certificated, and TLC, ROSH certificated. SBB has won good reputation from market. In the year 2008, SBB is the only power supplier for Mount Everest section of Olympic torch route.

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Assembling Buildings in-Production Test-Research-Center

Photovoltaic industry
On July 31, 1993, the German government issued the newly revised Renewable Energy Law ( RenewableNergylaw or Emeuerbacre - Energyen - Gesetz, EEG ) in Bonn. In this year, the electricity price of solar photovoltaic grid in Germany was three times that of retail electricity, that is, eight times that of industrial electricity.Then, in 2004, global solar's PV output exceeded 1GW, and the total installed capacity in the world over the years exceeded 3GW.The photovoltaic industry has been growing at a rate of more than 30 % since 2000.However, due to Germany's policy influence, the growth rate in 2004 exceeded 60 %.
At present, the SOLAR BATTERY on the market are still crystalline silicon SOLAR BATTERY?In 2004, 36 % were monocrystalline silicon SOLAR BATTERY, 58 % were polycrystalline silicon SOLAR BATTERY and SOLAR BATTERY with silicon.Of the 6 % thin film SOLAR BATTERY, amorphous silicon thin film SOLAR BATTERY account for 4 %, while tin oxide thin film SOLAR BATTERY and copper steel gallium selenium thin film SOLAR BATTERY add up to about 2 %.
The production technology of crystalline silicon SOLAR BATTERY is continuously improving, and its component cost [ 2 ] has dropped from $ 5 / WP in the early 1990s to $ 2.5 / WP in 2004.However, compared with conventional power generation costs, solar photovoltaic power generation is still too expensive.Under the climate of northwest Europe, the cost of photovoltaic system is $ 0.7 / kw, h, which is obviously too high.The main reason for the high price is the high cost of 250300 thick silicon wafers, which account for more than 50 % of the component cost.At the beginning of the photovoltaic industry, people realized that if the cost of using electronic grade silicon as the raw material for SOLAR BATTERY was too high, they would seek to develop polysilicon materials for SOLAR BATTERY.Compared with electronic grade silicon, solar grade silicon requires less parameters such as purity.In addition, the thinner silicon wafer, the more efficient use of Si and the increase in component conversion efficiency from 13 % 15 % to 18 % 20 % will reduce the cost of crystalline silicon SOLAR BATTERY by 23 times by 2020 [ 3' 4 ].In 20052008, there was a shortage of polycrystalline silicon raw materials in the photovoltaic industry, so the price of polycrystalline silicon soared.The shortage of polycrystalline silicon is temporary, but the large-scale industrialization and commercialization of thin-film SOLAR BATTERY is an inevitable trend, which will make the cost of thin-film solar cell modules less than $ 1 / WP.Developing thin film SOLAR BATTERY.Reducing the cost of photovoltaic power generation requires not only large-scale production and increasing market share, but also more scientific research and efforts to solve various existing technical defects.
Another important indicator of solar cell technology is the energy payback period ( PBT ), that is, how many years of electricity generated by SOLAR BATTERY can recover the energy consumed in the manufacturing process.The energy recovery period of crystalline silicon solar cell module is 23 years, while that of thin film solar cell module is less than 1 year.Since the conversion efficiency of thin film SOLAR BATTERY is lower than that of crystalline silicon SOLAR BATTERY, the comparison of energy recovery period EPBT also needs to consider the application occasions.Thin - film SOLAR BATTERY are more suitable for roofs and square arrays of grid-connected power generation, while crystalline silicon SOLAR BATTERY are more suitable for self-supporting applications with supports.The so-called system balance ( BOS' ) is the cost part of the solar photovoltaic system except for SOLAR BATTERY components.When self-supporting is applied to crystalline silicon SOLAR BATTERY with higher conversion efficiency, the system balance is effectively saved, especially the cost of the support [ 5 ].
Industrialization of Thin Film Solar Cells
Due to the great commercial success of first solar's cadmium telluride thin-film solar cell technology in the United States, it is the leading manufacturer of SOLAR BATTERY components in global solar. In recent years, various thin-film solar cell technologies have attracted a large amount of venture capital and private equity investment.The 2010 U.S. film " Wall Street: Money Never Sleeps" was released by Michael?Douglas starred in a major commercial negotiation about thin film solar cell technology.
In 20072009, the western developed countries, mainly the United States, invested more than $ 3 billion in the venture capital and private equity of thin film SOLAR BATTERY, and also fostered a large number of start-up enterprises, resulting in the " gold rush" of thin film SOLAR BATTERY, as shown in the actual situation.However, U.S. First Solar's market value in January 2010 was $ 13.54 billion, and even the $ 3 billion venture capital was only 22 % of First Solar's market value, so the success of First Solar's cadmium telluride thin-film solar cell technology can be seen.
Definition of film
How to define " thin film" and " thick film" exactly?Film definition [ 7 ] is: random nucleation occurs through solidification or chemical reaction to accumulate atoms, ions or molecules on the substrate.Since the thickness of various films varies from several nm to several tens of FXM, it is difficult to define them from the thickness but from the preparation process.The chemical, metallurgical and physical properties of the film also depend largely on the film thickness and other deposition parameters.In addition, there are also so-called thick film technologies, such as chipping and grinding bulk materials, screen printing, electrophoresis, slurry spraying, plasma spray gun and ablation, etc.The techniques for preparing thin films include plasma, sputtering, evaporation, liquid phase deposition, etc.According to the structural characteristics and electrical quality, the films are divided into crystalline, amorphous and intermediate forms.This book will introduce to readers the concept structure, fabrication process and characteristic performance of various thin film SOLAR BATTERY.
film absorbing layer material
In the traditional sense, the material of the solar cell absorption layer is an inorganic semiconductor, and the semiconductor junction separates electron and hole carriers.The semiconductor junction can be either a homogeneous junction or a heterojunction. Although there are many kinds of semiconductor materials, there are very few potential SOLAR BATTERY.The conditions for becoming an ideal solar cell absorbing layer material are: band gap of 11.5 eV;The absorption coefficient of the solar spectrum in the wavelength range of 3501000 nm reaches 104105 cm - 1.The quantum efficiency of excited carriers is higher.The diffusion length is longer.The composite speed is low.If the material meets the above-mentioned conditions and is low in cost and easy to obtain, it can be used as a material for preparing thin film SOLAR BATTERY.
From the point of view of process, production and reproducibility, it is a simpler and more direct method to produce thin film SOLAR BATTERY with elemental semiconductors.This is why today's SOLAR BATTERY all use Si as the basic material.The band gap of crystalline silicon is 1.1 eV, because Si exhibits indirect band gap characteristics for photons with energy below 3.4 eV, Fen is not an ideal material for preparing thin film SOLAR BATTERY.However, basic research on crystalline silicon thin film SOLAR BATTERY is still being carried out.If a low temperature deposition process is used, hydrogenated microcrystalline silicon ( MC - Si: H, hereinafter referred to as microcrystalline silicon ) can be grown.The crystalline silicon thin film grown by high temperature deposition is similar to bulk crystalline silicon.Because of the low absorption coefficient, a crystalline silicon film without light trapping ( optical enhancement or optical confinement ) should be at least 30pm thick to fully absorb incident light.Since the refractive index of Si is as high as 4, the optical path length of light inside crystalline silicon can be increased by 50 times.Therefore, light trapping is an important basic research field of crystalline silicon thin film SOLAR BATTERY.
Si can also be deposited in amorphous form.However, pure A - Si is not suitable for preparing SOLAR BATTERY because there are too many dangling bonds and the inter-band state density of 〉 1019 CRRT 3, resulting in the recombination of excess carriers and Fermi level pinning O. After A - Si is added to H to form hydrogenated amorphous silicon ( A - Si: H, hereinafter referred to as amorphous silicon ), the film properties are greatly improved. H can passivate dangling bonds and reduce the inter-band state density to below 1016 CNT 3.The main raw materials for deposition of A - Si: H films are SiH4 and H2. In non-equilibrium plasma, the covalent bond of precursor SiH is broken and decomposed into SiH (?The active group of < 4 ) is grown into a - si: h on the substrate.Compared with crystalline silicon, A - Si: H has several superior properties as an absorption layer of thin film SOLAR BATTERY.First, the band gap of A - Si: H can be adjusted by changing deposition conditions and deposition methods.Moreover, the doping of A - Si: H is relatively simple and it is easy to prepare heterojunctions.Finally, the absorption coefficient of A - Si: H to the solar spectrum is higher.PTC - Si: H and nanocrystalline silicon can also be prepared by H2 dilution under suitable deposition conditions.If the grain size or crystal lite and the volume fraction of the grains are very small, these grains ( grain or crystal lite ) will catalyze the crystallization of the remaining a - si: h under annealing = MC - si: h material deposited in this way has a smaller defect density, which is more stable than a - si: h photoluminescence attenuation effect.The recently developed material for improving conversion efficiency is a non-uniform mixture of a - si: h and medium grain size - si: h.
In the academic circles in the 1980s, the research of amorphous silicon thin film SOLAR BATTERY based on A - Si: H was very popular and could even challenge the dominant position of crystalline silicon SOLAR BATTERY.The market share of amorphous silicon thin film SOLAR BATTERY is very limited because of stability problems and low conversion efficiency compared with crystalline silicon.However, there is a shortage of polysilicon materials, and the multi-junction technology of Si: H and nanocrystalline silicon provides a new opportunity for the industrialization of amorphous silicon thin film SOLAR BATTERY.
Recently, some scientific research teams have introduced the method of using C as a thin film solar cell material, such as diamond - like carbon ( DLC ) [ 8: fullerene [ 4 ] in spherical shell carbon molecules.These new technologies have not been developed for a long time and are only academic topics, so this book will not elaborate on them in detail.
Hi - V semiconductors represented by GaAs and InP have direct band gaps and are ideal solar cell materials, but the cost of precursors and deposition equipment is too high to realize large-scale industrial applications.The process for preparing DI - V semiconductor thin films is molecular beam epitaxy ( MBE ) or metal organic chemical vapor deposition MOCVD.
Compared with M - V semiconductors, I - DI - VI and II - VI semiconductors represented by CUI NSE2 and CdTe are simpler in production process, lower in material and equipment costs, and more suitable for preparing large-area thin film SOLAR BATTERY.In CUI NSE2 and CdTe, inner and outer surfaces such as grain boundaries and interfaces are inherently purified, resulting in a slower recombination rate for excess carriers.Even if the grain size of polycrystalline material is several mm, the low recombination speed of grain boundaries makes the conversion efficiency of copper indium gallium selenium thin film solar cell and cadmium telluride thin film solar cell higher.In contrast, the grain boundary of polysilicon SOLAR BATTERY will form a higher recombination speed.In addition, I - M - VI and N - VI polycrystalline semiconductors can be deposited on various substrates such as glass and stainless steel sheets, and can also be applied to low-temperature deposition processes to avoid obtaining large grain sizes by high-temperature deposition processes such as epitaxial growth.Moreover, I - IH - VI semiconductors can adjust the band gap.For example, replacing SE in Cu in SEZ with S can improve the band gap.This characteristic can realize the gradient band gap, which is beneficial to the collection of excess carrier _ current, and can eventually develop multi-junction batteries.As the number of elements in ternary and quaternary compounds increases, more combinations of elements can better adjust the properties of semiconductor materials.
In addition to the inorganic semiconductor absorption layer, organic semiconductor materials can also be used to prepare SOLAR BATTERY. Polymer or plastic materials have unique photoelectric properties, mechanical properties and I properties.The organic semiconductor material does not generate only charges after absorbing photons, but bound electron-hole pairs, i.e., excitons' have a binding energy of about eV.The exciton with energy has no net charge and needs to diffuse to the separation position so that the charge can be separated and transported to the contact electrode.Since the band gap of most semiconductor polymers is greater than 2.0 eV, only 30 % of human light can be absorbed.Typical low carrier ( exciton ) mobility requires an absorption layer thickness < 100nm.The light trapping structure makes such a thickness sufficient to absorb most human-emitted photons, while the low refractive index is not appropriate for such a thin film thickness.More importantly, organic semiconductor materials can be prepared from solution at room temperature, suitable for flexible substrates, and use simple, low-cost and low-energy deposition techniques such as spin coating or printing to produce low-cost devices.Although the conversion efficiency of such devices is still low, they have unique applications in low-power SOLAR BATTERY.In addition to improving the stability of conjugated polymers, the problems faced by the large-scale marketing of organic SOLAR BATTERY also require the use of mixed polymers, composite polymers or dyes to match the band gap of organic materials to the solar spectrum in order to achieve higher conversion efficiency.
Classification of Thin Film Solar Cells
Based on the above discussion of solar cell semiconductor absorber materials, we can systematically introduce different types of thin film solar cell technologies.
Crystal silicon thin film solar cell
There are many ways to prepare crystalline silicon thin film SOLAR BATTERY.Among them, the preparation method of epitaxial crystalline silicon thin film solar cell and crystalline silicon solar cell is very close.This method is to epitaxially grow a crystalline silicon film of good electrical quality on a low-cost Si carrier substrate.This solar cell structure is very similar to crystalline silicon SOLAR BATTERY, so its preparation process is very similar to the current process in photovoltaic industry.The special case is the epitaxial layer stripping ( WT - OH ) technique, which is to strip the porous crystalline silicon template before or after the cell process to obtain a crystalline silicon thin film solar cell.
In addition to this epitaxial growth technique for Si substrates, there are other methods that do not require Si substrates.Because of the film deposition temperature ( t ) > 600 c, the substrate cannot use easily melted glass, but needs to use low - cost, high-melting ceramic or graphite.Since the two substrates are not conductive, a novel interdigital contact structure is required.The Si layer deposited on these substrates is microcrystalline silicon # - Si: H or polysilicon, and the grain size is determined by the growth temperature and supersaturation state during deposition.Because the grain size of 110 is not easy to achieve higher solar cell conversion efficiency, liquid phase recrystallization is often used to improve the final grain size on high temperature resistant ceramic substrates, while laser recrystallization or rapid thermal annealing RTA can be applied to substrates that cannot withstand > 650 C high temperatures.
amorphous silicon thin film solar cell
Amorphous silicon A - the absorption layer of amorphous silicon thin-film SOLAR BATTERY can be easily doped into P - type and N - type to form homogeneous junctions.Due to the short life of carriers and the low mobility of carriers, the simple diffusion of excess carriers cannot achieve effective collection, so amorphous silicon thin film SOLAR BATTERY need to contain a drift - dominated region to improve the collection of carriers.In this way, the structure of the solar cell is a P - I - N structure with an intrinsic layer between the N + type layer and the type layer.Because the doping concentration of the intrinsic layer is very low, the electric field will expand in the intrinsic layer.As mentioned earlier, the properties of materials and semiconductor junction devices are severely restricted by the photo-induced attenuation effect caused by metastable defects, i.e. the S - W effect.The photoinduced attenuation effect of amorphous silicon thin film SOLAR BATTERY and the reduction of the intrinsic layer electric field caused by it can be solved by reducing the thickness of H layer, so that photogenerated carriers can reach the contact electrode only by moving a small distance.However, thinning of the A * * Si: H layer will reduce the light absorption and require front and back contact to form a diffuse light trapping structure to enable the absorption layer to absorb photons better.After a period of time, high-intensity research and development of deposition technology and device structure have resulted in single-junction and multi-junction devices with high conversion efficiency and certain stability.A typical A - Si: H based multi-junction SOLAR BATTERY.
Due to the high cost of Ge precursors, the electrical properties of hydrogenated amorphous silicon germanium A - SiGe: H are lower than A - Si: H. Much research has focused on replacing A - SiGe: H with microcrystalline silicon - Si: H.This formed the concept of non - microcrystalline laminated SOLAR BATTERY, with A - Si: H as the top SOLAR BATTERY and MC - Si: H as the bottom SOLAR BATTERY.
The effect on device performance depends on the device structure.Because CDS diffusion will occur during the growth of high temperature CIGS thin film, the upper layer configuration performance of Cu - In - Ga - Se thin film solar cell is not as good as that of the substrate configuration.The laboratory conversion efficiency of the upper layer configuration was recorded as 10.2 %, while that of the substrate configuration was 19.2 %.
Cadmium telluride thin film SOLAR BATTERY are more suitable for fabrication into an upper layer configuration because CdTe surfaces are exposed to contact electrodes.Moreover, the good characteristics of CdS diffusion in the process can reduce lattice mismatch between CdS and CdTe O cadmium telluride thin film SOLAR BATTERY using borosilicate glass in high temperature deposition > 600 C and soda-lime glass in low temperature deposition of 60500' C ..CdTe can also be deposited on thin metal sheets such as stainless steel, Mo, Ni, Cu, etc.Mo is more suitable for CdTe deposition due to its superior thermal properties.
Organic solar cell
The so-called " organic" SOLAR BATTERY need to be properly defined.The category of this term is SOLAR BATTERY with organic layers as essential components.The basic steps of solar photovoltaic conversion are photon absorption, generation of charged carriers, carrier transport and carrier collection through contact electrodes.Specifically, " organic" SOLAR BATTERY mean that at least the first two steps are implemented by an organic layer.According to this definition, organic SOLAR BATTERY include both full organic SOLAR BATTERY and hybrid organic SOLAR BATTERY.
Because organic SOLAR BATTERY have great potential to reduce the cost of active layer materials, they have always been one of the hot spots in solar cell research.Earlier, the structure of organic SOLAR BATTERY was similar to the P - N junction or P - I - N junction of other thin film SOLAR BATTERY, with a conversion efficiency of only 1 %. The main limitation was that the diffusion length of excitons was too short, thus the exciton separation was insufficient.After that, the breakthrough came from the concept of planar heterojunction, and then evolved into an interface distributed in vivo to capture excited carriers and increase the separation rate of excitons.Dye - sensitized SOLAR BATTERY are such hybrid organic SOLAR BATTERY.In the pores of the porous tiq layer, a single layer of sensitizer is absorbed by the pore wall.After the photons are absorbed, excited electrons in the dye molecules are quickly transported to the conduction band of tiq, and then the electrons diffuse to the contact electrode through the porous network.The oxidized sensitizer molecules absorb the electrons of the liquid electrolyte in the pores and reduce to the initial state.The small-area dye-sensitized solar cell can achieve a conversion efficiency of 11 %, while the large-area dye-sensitized solar cell can achieve a conversion efficiency of 5 % and 7 %.At present, the research directions of dye-sensitized SOLAR BATTERY include increasing the absorption of sensitizers in red and infrared light bands, replacing liquid electrolyte with solid conductors and improving the stability of the cells.
Dye - sensitized SOLAR BATTERY are mixed organic SOLAR BATTERY, while bulk heterojunction SOLAR BATTERY are completely organic SOLAR BATTERY.Among them, two organic components are mixed, one with donor characteristics and the other with acceptor characteristics.Excitons are effectively separated at the interface between the donor and the acceptor, then permeate through the donor and acceptor networks and are collected by selective exposure.The complete organic solar cell with P3HT as donor material and PCBM as acceptor material achieves a conversion efficiency of 5 %.PCBM is very suitable as a acceptor material, and its exciton separation rate is high, and its transport time is on the order of Loofs.It has also been reported that replacing PCBM with other polymers with acceptor properties is a widely concerned donor material, which can not only improve the response of red light and infrared light, but also improve the stability of the device.
For the sake of completeness, it is also necessary to mention the inorganic bulk heterojunction, i.e. the extremely thin absorption layer ( eta ) structure.
Thin film solar cell module
The structure of the thin film solar cell module is divided into two types.A module structure is similar to a crystalline silicon solar cell and is suitable for any thin film solar cell based on silicon wafer substitutes.Both epitaxial growth and crystalline silicon thin film SOLAR BATTERY fabricated on graphite substrates form this type of assembly.The concept is to connect the front contact of the SOLAR BATTERY and the back electrode of the adjacent SOLAR BATTERY with a metal strip.
Another kind of thin film solar cell module structure is monolithic integration, and there is no clear distinction between cell and module technology.The production of monolithic integrated thin film solar cell modules involves the sequential deposition of different thin films on large area substrates.A typical polycrystalline thin film solar cell adopts an upper layer configuration, and after cleaning the glass substrate, transparent conductive oxide TCO deposition, window layer preparation and absorption layer preparation are carried out respectively.Generally, three laser scribing or mechanical scribing are required to define, interconnect and isolate the SOLAR BATTERY.Metallization after the second scoring makes the batteries defined after the first scoring interconnected in several ways.The assembly is prepared after lamination.The cross section of a typical monolithic integrated thin film solar cell module.The actual process, device structure and materials vary from manufacturer to manufacturer.The advantage of monolithic integration in the thin film solar cell module process is that the area lost by the module can be minimized.Several key cost factors for the production of large-area thin-film solar cell modules are: yield, raw material cost and capital expenditure for purchasing equipment.The yield of production depends on many factors, especially the uniformity of the deposition process and the laser scribing process to isolate and interconnect individual cells.Because defective devices can seriously affect the performance of the entire assembly, the uniformity of large area devices is very critical, which is both an advantage and a disadvantage of monolithic integration.In principle, the monolithic integration process has greatly saved costs, and several steps required for the production of silicon wafer-based components have been greatly simplified.However, the potential to reduce costs can only be realized through the high yield of large-area component production.
An introduction to
In order to greatly reduce the cost of existing crystalline silicon SOLAR BATTERY, it is necessary to reduce the material usage of high purity silicon in typical solar cell structures.In crystalline silicon SOLAR BATTERY, most of the crystalline silicon materials only serve as mechanical carriers for SOLAR BATTERY, and most of the light absorption occurs only in the area up to 30pm.When a certain method is used to maximize the light trapping structure, the minimum 0.5 active layer thickness is sufficient to achieve a conversion efficiency w of 15 %.It is a development direction to reduce Si usage by thinner silicon wafers. However, if a cell is produced from a silicon wafer with a thickness of less than 200 mm, the yield of the process will have a large crack propagation problem.In order to avoid such problems, special substrate types have been developed, such as tri - crystalline silicon material [ 2 ] or thin edge - limited thin film growth ( edge definitedflm - fed growth, EFG ) with silicon [ 3 ].
A more promising way to reduce the cost of SOLAR BATTERY is to grow a very thin active layer of crystalline silicon on a cheap carrier.Such carriers may be ceramic substrates or even glass substrates, and the S layer that needs to be deposited at low temperature and deposited on top of these substrates using other solar cell processes 0 will be microcrystalline silicon or polysilicon, and the grain size will be determined by the growth temperature and supersaturation state of S layer deposition.For microcrystalline silicon thin film SOLAR BATTERY on glass, the grain size is in the range of 1100 nm, and the conversion efficiency of up to 10 % is reported [ 4 ].On the other hand, materials with a grain size of 110 ftm have proved difficult to achieve high conversion efficiency of SOLAR BATTERY [ 5' 63 ], but fundamental progress has recently been made in this field [ 7' 8 ].On ceramic substrates that can withstand high temperatures, liquid phase recrystallization ( liquidphaserecrystallizationp 1q ) is often used to increase the final grain size, while laser recrystallization or rapid thermal annealing ( RTA ) is developed for substrates that can only withstand > 65o c high temperatures for a limited time [ 11' 12 ].
The core idea of forming the thin film solar cell technology in this chapter is to epitaxially grow crystalline silicon thin films with higher electrical properties on low-cost Si carriers [ 13, 14 ].When people discuss thin-film solar cell technology, they do not pay special attention to crystalline silicon thin-film SOLAR BATTERY based on epitaxial growth of active layers on highly doped low-cost Si carrier substrates.The typical structure of this thin film solar cell is very similar to that of a classical crystalline silicon solar cell.In this way, the added investment and financial risks will be minimized, while most other thin film solar cell technologies require huge investment in production lines, which has become a major obstacle to their industrialization.Finally, using the on-line product quality monitoring tool in the crystalline silicon solar cell production line, the process yield of the silicon wafer replacement method can reach a higher level.For other thin film solar cell technologies, deposition of an active layer on a large area substrate larger than 1m2 will cause serious problems in uniformity and reproducibility, and it will not be easy to achieve similar yield of crystalline silicon thin film SOLAR BATTERY.
The similarity between the basic structure of epitaxial crystalline silicon thin film SOLAR BATTERY and that of classical crystalline silicon SOLAR BATTERY seems to give the impression that the potential of epitaxial growth technology to reduce costs is limited.However, a careful analysis reveals that this is not the case.After analyzing the cost structure of the polysilicon solar cell module, we can see that more than 50 % of the module cost is related to the crystalline silicon substrate [ 15' 16 ].Although there was a shortage of polycrystalline silicon raw materials in the photovoltaic industry in 20052008 and the cost of crystalline silicon substrate soared, a large amount of investment and capacity expansion in the production of polycrystalline silicon raw materials will meet the demand of the crystalline silicon photovoltaic industry [ 17 ].However, the cost of solar grade silicon raw materials is predicted to be in the range of 1520 / kg.Based on such a historical trend of 5 % annual reduction in raw material cost and unit WP crystalline silicon usage [ 18 ], the cost of crystalline silicon solar cell module will reach the range of $ 1.2 / WP and the conversion efficiency will approach 20 % [ 16 ].Si is not a rare element on the earth. Metallurgical grade silicon ( Mg - Si ) reduced with quartz sand costs only 12 / kg.Therefore, the technical route of the Siblin crystalline silicon thin film solar cell is based on Mg - Si or high purity metallurgical grade silicon ( UMG - Si ) and can achieve a cost of less than 5 / kg.If the cost of epitaxial Si deposition process with high production rate is less than 10 / m2, even if the conversion efficiency is only 15 %, the final component cost will also be in post.91/W。Scope: In addition to such cost potential, epitaxial silicon thin film SOLAR BATTERY will no longer be affected by polysilicon raw material supply in the photovoltaic industry' epitaxial silicon thin film SOLAR BATTERY use low-cost Fen substrates because of their doping and impurity containment levels, and cannot achieve sufficient solar cell conversion efficiency in the substrates.The Si substrate may be water-tight silicon, such as by chemical vapor deposition ( CVD ) ⑽ or liquid phase epitaxy ( LPE ) [ 2 ].21 ] Epitaxial growth of Mg - Si or UMG - Si ingots on the substrate growth belt silicon ( RGS ) can also be used as SL substrates for epitaxial crystalline silicon thin film SOLAR BATTERY.
Growth of epitaxial layers with appropriate doping levels and lower defect density on top of these substrates can result in crystalline silicon thin film SOLAR BATTERY with better performance [ 22' 23 ].Through the secondary ion mass spectrometry ( SIMS ) distribution, it was observed that the epitaxial layer on the top of the contaminated substrate did contain lower impurity concentration than the substrate.SIMS used in materials science and surface science can analyze the composition of solid surfaces and thin films, and is the most sensitive surface analysis technology.In SIMS, a first focused ion beam is sputtered onto the sample surface, and an outgoing second ion beam is collected and analyzed.According to the second ion beam, the mass spectrometer can determine the elements, isotopes and molecular components of the surface.SIL SO is a polycrystalline silicon material first prepared by casting method in Germany Wacker as early as 1975.
This chapter will introduce the technical details of epitaxial crystalline silicon thin film SOLAR BATTERY from the perspective of deposition technology and epitaxial layer structure.We will discuss the development of solar cell technology and its potential to achieve higher conversion efficiency in laboratory conditions or industrial environments, and will pay special attention to the application of epitaxial crystalline silicon thin film solar cell technology in commercial production.This includes not only the concept and development of high production rate deposition technology and solar cell process modification, but also the method of minimizing epitaxial layer thickness.This requires increased light absorption and trapping within the effective volume of the SOLAR BATTERY.Readers may think that light trapping is a difficult problem because both the substrate and the active layer are made of crystalline silicon, which will greatly reduce the reflection of light at the interface between the Si substrate and the active layer.In order to solve this inherent problem, the new method is based on the buried mirror, and we will discuss different methods to realize the buried mirror.Ge alloy is another way to increase solar cell absorption.
Deposition technique
We will discuss different deposition techniques for growing epitaxial layers in order of deposition temperature, from the technique with the highest deposition temperature to the technique with the lowest deposition temperature.This classification method also reflects the number of experimental results and the maturity of related technologies.This is an easy-to-understand fact, because the epitaxial layer thickness required for crystalline silicon thin film SOLAR BATTERY is much larger than in typical microelectronic device applications, with the exception of power devices.The epitaxial layer thickness required for crystalline silicon thin film SOLAR BATTERY is in the 530 range, and a high growth rate' rate' is required to avoid excessive deposition time.At low deposition temperature, the surface mobility of adsorbed atoms is small, and the adsorbed atoms do not have enough time to relax to the lattice position, which will increase the number of crystal defects.As a result, at lower deposition temperatures, additional energy in addition to thermal energy needs to be supplied to increase surface mobility and achieve high-quality epitaxial growth.These additional energies can be supplied by accelerated ion or plasma technology.
thermally assisted chemical vapor deposition
For epitaxial crystalline silicon thin-film SOLAR BATTERY, the most widely studied deposition technique is thermal assisted chemical vapor deposition ( TA - CVD ), the principle of which is that the S precursor and doping gas ( Doping Gas or Dopant Gas ) undergo thermally assisted non-uniform decomposition on the heated S surface.Since the 1970s, TA - CVD has been used in solar cell preparation, and now it is widely used in Europe % and Japan [ 27 ] to prepare crystalline silicon thin film SOLAR BATTERY.TA - CVD has several types of reaction chambers, including batch and single wafer systems?A single wafer system is a lateral flow reaction chamber, with gas drawn from one end of the chamber and discharged from the other end.The wafer is laid flat on a graphite pedestal coated with SiC and heated by radiation under thermal isolation.Because of the need to achieve extremely fast heating and cooling rates, such a technique is also called rapid thermochemical vapor deposition ( rt - CVD ) m.RT - CVD was introduced in the 1980s. The organization that specializes in this technology and applies it to crystalline silicon thin film SOLAR BATTERY is the Institute of Solid State Electronics and Systems ( INESS ) in Strasbourg, France [ 3D ].RT - CVD prevents S from depositing on the furnace wall at low temperature, reduces the time required to heat and cool the substrate, and uses all the energy to heat the substrate instead of the furnace wall.In addition to a single wafer system, the batch multi-wafer reaction chamber includes a flat reaction chamber and a cylindrical reaction chamber. A planar or cylindrical substrate holder can guarantee the required uniformity and aerodynamic conditions and realize low-pressure chemical vapor deposition ( LPC VD ).
The advantage of CVD deposition technology is that there are a large number of senior process experts in microelectronics.The existing epitaxial deposition system and the corresponding process level can make the thickness and doping distribution of the active layer highly reproducible and uniform' for large area substrates with 200300 mm side length, the typical doping uniformity and thickness uniformity are in the range of several percentage points.As a matter of fact, the parameters of microelectronic applications are much more stringent than those of photovoltaic applications, and about 10 % of the thickness uniformity and doping concentration uniformity of photovoltaic applications is sufficient.However, the technical route of crystalline silicon thin film SOLAR BATTERY using Ta _ CVD also has several inherent disadvantages.First, the Si precursor used in Ta - CVD is toxic, corrosive and has a high explosion risk.Moreover, obtaining a high growth rate on the order of several / im / min requires a deposition temperature in the range of up to 1000 - 20001.
The electrical characteristics of CVD growth epitaxial layer through life measurement research, the typical life of single crystal silicon substrate is on the order of several ms, and the life of polysilicon substrate is on the order of 1.2.2 liquid phase epitaxy and electroplating
Solution growth ( SG ) is fundamentally different from chemical vapor deposition CVD, using a liquid medium instead of a gas environment as a precursor source. When SG is applied to grow epitaxial layers on a crystal substrate, also known as liquid phase epitaxy LPE [ 31 \ In SG, Si is grown from a metal melt, typically Sn or In, sometimes Cu or A1E32' 33 ] metal melt contains saturated Si and is then slowly cooled.When cooled to a certain extent, the metal reaches supersaturation and the crystalline silicon layer will be deposited from the metal melt onto the substrate through non-uniform nucleation.The typical deposition temperature is in the range of 700900 C, lower than that of thermally assisted chemical vapor deposition TA - CVD, and the growth rate is on the order of 1.
In addition to the simple concept, the main advantage of LPE technology is that the growth system is close to the thermal equilibrium state and Si atoms in the melt show a larger diffusion coefficient, both of which contribute to improving the crystallization quality of the grown Si film.At the same time, the properties close to the thermal equilibrium state also have serious negative effects, because nucleation on non-silicon substrates or along Si surface defects is very difficult, and non-uniform Si layers or even non-continuous Si layers containing quite crystalline defects are often formed on the substrates.In the case of non-silicon substrates such as graphite, the solution to this problem is to deposit a Si seed layer using another technique.When an epitaxial layer is grown on the substrate growth zone silicon RGS T2 or the powder growth zone silicon ( SSP ) [ 2i ] using LPE technology, the epitaxial layer thickness in the grain boundary region is often much smaller than that in the grain because the higher energy associated with the defect suppresses the epitaxial layer growth near the defect.In the region where the epitaxial layer is particularly thin, the N + emitter diffuses, has direct contact with the P + substrate, forms a leakage junction, and has a lower fill factor ( FF ).The cooling rate is high, but the uniformity of large area deposition is still a problem.
Because of the low supersaturation degree in the LPE growth process, the epitaxial layer grown by LPE has lower defect density and less excess carrier recombination than the epitaxial layer grown by CVD [ 34' 35 ].The image of electron beam induced current with respect to the partially masked structure proves that the recombination rate m of LPE epitaxial layer is small because LPE growth tends to reach the minimum energy distribution and reduces lattice dislocation.Because of the distribution coefficient between the liquid phase and the solid phase, impurities are also contained in the molten metal solution.Epitaxial crystalline silicon thin film SOLAR BATTERY grown by LPE can generally reach minority carrier lifetime of number / NS10 / IS [ 36 ].
EBIC is a semiconductor analysis technology based on scanning electron microscope SEM. Electron - hole pairs are generated in the semiconductor by microscopic electron beams for analyzing buried junction regions, defects or minority carrier characteristics of the semiconductor.
The other SG is Si electroplating from salt solution, and an epitaxial layer [ 37 ] can also be grown.Electroplating refers to the technique of forming a uniform, dense and well-bonded semiconductor, metal or alloy deposit on the surface of a workpiece by electrolysis.
LPE can easily realize in-situ doping gradient in the base of the active layer.If the dopant is reduced during the growth process, the doping gradient will form a forward electric field.The positive electric field helps to collect minority carriers, and as a result, the effective diffusion length ( leff ) can be increased.
Where h is the minority diffusion length in the absence of an electric field;NR is the impurity concentration at the substrate - epitaxial layer interface ( DopingLevelEtubStrate - EpiLayerinterFace );N ( is the impurity concentration o on the epitaxial layer surface although we expect the doping gradient effect to fundamentally improve the performance, the performance improvement has been proved to be very limited in most cases and only slightly obvious without limited light trapping structure and small minority carrier diffusion length.It has even been reported that the method of adding doping elements to form negative electric field during the growth of epitaxial layers [ deleted ].Although this view seems to contradict the previous conclusion, it can be understood that the minority carrier concentration gradient under illumination is large and relatively unaffected by the doping distribution, while the high doping in the junction region will cause a high open circuit voltage ( v ).
Near - space gas phase transport
As another epitaxial layer preparation technique, near-space vapor transport ( csvt ) has high chemical efficiency, which refers to the ratio of si growth to si supply [ 4'' < ] \ although csvt technology was known in the 1960s [ 42 ], csvt has recently been re-studied by the U.S. national renewable energy laboratory ( nrel ).In CSVT technology, Si is transported from a solid source to a substrate.The driving force of Si transport comes from the temperature difference between the solid source and the substrate?The smaller spacing between the solid source and the substrate results in greater transport efficiency and minimizes the & loss on the cavity wall.CSVT technology can deposit epitaxial layers on highly doped monocrystalline silicon or polycrystalline silicon substrates.
The epitaxial layer obtained by CSVT in the laboratory has good electrical properties, and uses atmospheric iodine vapor transport ( API VT ) to achieve a high deposition rate under the condition of relatively low substrate temperature.The substrate temperature is 650850 C, the solid source temperature is 1300 C, the growth rate reaches the order of 13 mm / min, and it is relatively insensitive to the substrate temperature.A model has been established to explain the atypical dependence of growth rate on temperature.The model considers the arrival rate of Si L2, the exit rate of SIL 4 and surface migration, and obtains the expression of growth rate scale:
JOC ( T — A T EXP ( 1.4 )
where t is the substrate temperature;T Source is the solid source temperature;Q is the surface migration activation energy.Two opposite trends make the growth rate ruler insensitive to substrate temperature t.For the classical CVD, the surface migration depending on the substrate temperature t is more frequent, which will increase the growth rate and the deposition rate.However, the temperature TW URA of the solid source in API VT is unchanged, and the temperature difference between the solid source and the substrate is small. Less frequent surface migration will reduce the arrival rate and growth rate.
Ion assisted deposition
Ion assisted deposition ( IAD ) is based on electron gun evaporation ( EGE ) and partial ionization of Si [ 43 ].The external voltage or applied voltage accelerates the movement of Si ions toward the substrate.A typical acceleration voltage is 20V, which creates the least etch pits on the monocrystalline silicon epitaxial layer [ 44 ].
The energy provided by accelerating ions increases the mobility of adsorbed atoms on the surface.Therefore, the epitaxial growth of IAD technology can achieve a high deposition rate of 0.5 mm / min at a low temperature of 435 C ..The hall mobility of single crystal epitaxial layer increases with the deposition temperature and reaches a value equal to the crystal silicon with the temperature > 540 c.The majority carrier mobility of the IAD grown B doped thin film almost reaches the theoretical value of crystal silicon with doping concentration in the range of 1016 - 102 cm - 3.The electrical properties of the film strongly depend on the crystal orientation of the substrate.The diffusion length of Si thin film on crystal plane is at most one order of magnitude lower than Si thin film on crystal plane. The beam-induced current ( OBIC ) measures non-uniform current collection in the epitaxial layer grown on polysilicon substrate, which is consistent with other low-temperature epitaxial growth [ 45 ].Photoluminescence ( pl ) and deep level transient spectra dlts show that a low deposition temperature of < 500 c will result in a large defect distribution.The point defect density of Si epitaxial layer on crystal plane is at most 4 orders of magnitude lower than Si thin film on crystal plane.The temperature-dependent quantum efficiency ( TQE ) measurement was also used to study the composite properties of IAD - grown crystalline silicon thin film SOLAR BATTERY [ 46 ].The diffusion length depends on Shockle - Reed - Hall recombination caused by shallow energy level defects with activation energies of 70110 eV and 160 - 210 eV, respectively.Under the conditions of preheating > 810 c and deposition temperature > 650 c, IAD can realize si epitaxial layer with minority carrier diffusion length of 40 [ 47 ].
OBIC is a semiconductor analysis technology that uses a scanning laser beam to induce current in a semiconductor sample. Collecting and analyzing the induced current can produce images representing the characteristics of the sample, so as to be able to detect and locate defects or abnormal structures in the semiconductor sample.PL is the process by which matter absorbs photons and then re - radiates them.DLTS is a semiconductor testing tool for studying electrically activated defects. It can measure defect density and other basic defect parameters in materials, and some parameters are considered as characteristic parameters for determining and analyzing related defects.
Plasma enhanced chemical vapor deposition and electron cyclotron resonance chemical vapor deposition can not only accelerate ions, but also increase energy and surface mobility' by means of low-temperature deposition to achieve high-quality epitaxial growth.By low energy plasma enhanced chemical vapor position ( PECVD ), high current plasma discharge containing low energy ions can produce a relatively high deposition rate and ensure no damage to the wafer surface.The main application of this technology is to grow SiGe relaxation buffer layer with composition gradient for advanced metal oxide semiconductor ( MOS ) devices. Such PECVD technology can also prepare epitaxial crystalline silicon thin film SOLAR BATTERY in principle.
Electron cyclotron resonance chemical vapor deposition ( ECR - CVD ) can also provide high-concentration plasma to greatly increase surface mobility and make the deposition temperature of epitaxial layers < 400 c [ 49 ].
increasing the light absorption of the epitaxial layer
Because crystal silicon has an indirect band gap and the absorption coefficient of wavelength > 900 nm is very small, the epitaxial layer with a thickness of 20 FIM cannot effectively absorb human light with a larger wavelength.In order to increase the light absorption, the epitaxial layer needs to be modified, and a special structure can be grown to increase the optical path length or to form an alloy with Ge to reduce the band gap.
Epitaxial growth on textured substrates
The first technique to increase the light absorption of epitaxial layers is to chemically texturize the substrate before epitaxial growth.However, this technology still has shortcomings.The facet formed by epitaxial growth will flatten the textured structure, thus reducing the effectiveness of the textured process.Moreover, since the surface before epitaxial growth is already quite rough, the defect density of the epitaxial layer will be very high, and as a result, it is difficult to achieve a higher open-circuit voltage.
The textured surface can also be prepared mechanically.From the figure, we can clearly see that on the textured surface modified after epitaxial growth, the crystal grains near the crystal planes are well preserved, while the original textured surface in other crystal directions is modified by different facets.Scanning Electron Microscope ( SEM ) is a common electron microscope that scans the surface of a sample with a grating pattern and forms an image through a high-energy electron beam.Electrons interact with the atoms that make up the sample, and the resulting signals contain information such as the surface morphology, composition and conductivity of the sample.
Through this technique.Chemical vapor deposition ( CVD ) realized the growth of 100cm2 large area epitaxial crystalline silicon thin film SOLAR BATTERY on a high grade metallurgical grade silicon UMG - Si substrate.Using the contact electrode screen printing in the industrialized solar cell process, a conversion efficiency of 12 % and 13 % was achieved.Moreover, the thickness of the epitaxial layer is only 1520 mm [ 22' 51 ].
silicon germanium alloy
One technique to directly increase the short circuit current density ( 9 ) is to increase the absorption coefficient of crystalline silicon thin film SOLAR BATTERY with an alloy of S and Ge.The small band gap of SiGe alloy can increase the infrared absorption of the SOLAR BATTERY.However, the open circuit voltage V of the solar cell will be reduced at the same time.In crystalline silicon SOLAR BATTERY, such losses exceed the increase in short-circuit current density.In the epitaxial crystalline silicon thin film solar cell [ 52 ] prepared with: r < 10 % si ge and substrate, this problem has been confirmed.However, at least in theory, the reduction of open circuit voltage VOC of crystalline silicon thin film SOLAR BATTERY is limited.Because surface recombination exceeds in vivo recombination, most of the loss of open-circuit voltage can be theoretically eliminated by Si / SiJ - Ge, the back surface field of heterojunction ( BSF ).
The experimental study shows that the relaxed si ge epitaxial layer of epitaxial crystalline silicon thin film SOLAR BATTERY limits ge content to < 20 %, while theoretical calculation shows that higher ge content will benefit from collision ionization?A better crystalline silicon thin film solar cell can be prepared on a highly doped monocrystalline silicon substrate by chemical vapor deposition CVD or liquid phase epitaxy LPE with a thicker relaxation system -, % layer grown epitaxially ( within the range of 0 % to 20 % ).In an environment of 40 m Torr low pressure and 700 - 800 C temperature, CVD can epitaxially grow SiGe alloys on graphite substrates using a lamp - heated system ( e.g.Si H2 Cl 2 and Ge H4 are precursors of Si and Ge, respectively.Compared with the smart changes required for electronic applications, the thickness of SiGe alloy layer required for epitaxial crystalline silicon thin film SOLAR BATTERY is relatively large, so it is desirable to achieve as high a growth rate as possible.Increasing the growth rate requires a higher deposition temperature, but the 06 content at high temperature will be small' as shown in actual .10'.Therefore, epitaxial crystalline silicon thin film SOLAR BATTERY generally require a moderate growth rate of 0.150.2 mm / min' because there is some lattice mismatch between the Si substrate and the layer, it is necessary to introduce a buffer layer between the Si substrate and the active layer.When Si. Ge, the thickness of the layer reaches 1015, far exceeding the critical thickness of growth [ S4 ], mismatch dislocations caused by lattice mismatch relax the strain of the lattice.The lattice constants of Si and Ge differ by about 4 %.However, the lattice constant of SiGe alloy varies linearly between & and Ge's lattice constant, which conforms to Vegard's' Slaw' 0 fen substrate with a certain dislocation density in the inclined SIT -. Ge. epitaxial layer.TEM is a common microscope technology. A beam of electrons transmits through a very thin sample and interacts with the thin sample during transmission. The image formed by this interaction is magnified and focused on imaging devices such as a screen, photographic film or CCD camera.
The electron beam induced current ebic was used to measure the relaxed si -. ge. as - deposited layer as a schottky diode, and the diffusion length [ 55 ] was obtained.For epitaxial layers with buffer layers grown by CVD and LPE, although the defect density is on the order of 10s cm - 2, the effective diffusion length reaches 80100 mm.The proper term " effective diffusion length" is used because the epitaxial layer is only 20 thick.It has also been proved that at least at room temperature, the dislocation of CVD grown epitaxial layer is not more than that of LPE grown epitaxial layer.
EBIC is a semiconductor analysis technology based on scanning electron microscope SEM, which relies on electron-hole pairs generated in the semiconductor by microscopic electron beams and can be used to analyze buried junction regions, defects or minority carrier characteristics of the semiconductor.
The laboratory process for preparing relaxed Six -. Ge layer on P + Si substrate and forming epitaxial crystalline silicon thin film SOLAR BATTERY includes solid source diffusion, surface passivation and evaporation of contact electrodes [ 56 ].In order to prepare high performance epitaxial sh -. ge, passivation of the surface of thin film SOLAR BATTERY is particularly important.Although plasma enhanced chemical vapor deposition PECVD can passivate the nitrogen surface on the front surface, the blue light response of the cell is lower than that of the reference crystalline silicon thin film solar cell.The blue light response can be improved if an S - coating is added on top of the cell structure.Si / sii -. ge. transformation is very important to the position of the junction region. the defect density of the si cap layer is on the order of 107 CNT2, which can reduce the interface recombination near the interface between the diffused emitter si cap layer and the sii - ge base.Internal quantum efficiency ( internal quantumefaciency, / q ¬The curves show that the red light response of the base can be enhanced by alloying with Ge, while the blue light response can be improved by Si coating.
The short-circuit current density of epitaxial sit -. ge. thin-film SOLAR BATTERY is higher than that of ordinary crystalline silicon thin-film SOLAR BATTERY with similar active layer thickness and similar impurity distribution, while the short-circuit current density is?However, the open-circuit voltage and fill factor ff of epitaxial si. ge thin-film SOLAR BATTERY are lower than those of the reference ordinary crystalline silicon thin-film SOLAR BATTERY, as shown in actual .1 ..However, the conversion efficiency V of epitaxial Si. Ge and thin film SOLAR BATTERY is lower because the reduction degree of open circuit voltage is greater than the increase degree of short circuit current density.
Quantum dot solar cell
In order to avoid the crystal defect caused by the relaxed SiGe layer thickness exceeding the critical thickness, other methods have recently been proposed and tested.One method is to use sk growth ( str anski - krastanovgrowth ) to embed the grown ge layer into a si crystal matrix to form a three-dimensional island.The embedded Ge layer will increase the absorption of infrared light at the base of the cell to achieve higher photo-generated current and overcome the loss of open-circuit voltage of the heterostructure.Stacked self-assembled germanium quantum dots ( stacked self - assembled quantum dots ) can be prepared in the intrinsic region of Si - based P - I - N junction diodes to form quantum dot SOLAR BATTERY [ 57 ].By gas source molecular beam epitaxy ( GS - MBE ), Ge quantum dots are epitaxially grown on the P - type 3 crystal plane substrate by SK growth.Moreover, it was observed that EQE increased with the number of stacking layers [ 58 ].By ultra-high vacuum molecular beam epitaxy ( UHV - MBE ), up to 75 layers of Ge can be grown at intervals, each layer of Ge being about 8 atomic monolayers thick, separated by 916 nm Si spacer layers, using a standard 10 ¢ 1cm P - type Si substrate.Using sb as a surfactant, the island density can be increased to > 1011 cm - 2.The island is covered from above by a 200 nm thick N - type Si layer as the emitter of the solar cell.The measurement of photogenerated current confirms that quantum dot SOLAR BATTERY have higher infrared region response than standard crystalline silicon thin film SOLAR BATTERY.
In order to introduce more completely, GaAs layer grown on Si carrier has received extensive attention for SOLAR BATTERY in space applications.Ge template layers are often grown between the & substrate and GaAs active layer to eliminate lattice mismatch [ % 6 ] between si and GaAs \ however, this technique is beyond the scope of this chapter.
Buried back mirror
Although it can be clearly proved that SiJ -. Ge, alloy or Ge quantum dots increase the photogenerated current of epitaxial crystalline silicon thin film SOLAR BATTERY, the experiment still cannot prove that the increase in current is enough to offset the decrease in voltage, so that the conversion efficiency decreases instead.Therefore, it is necessary to develop a light trapping structure between the substrate and the S epitaxial layer, which requires adding a medium with different refractive indices between the Si substrate and the epitaxial layer to form a buried back mirror and allow epitaxial growth.In general, there are two basic fabrication techniques for buried back mirrors, namely porous silicon interlayer and epitaxial lateral overgrowth.The porous structure formed by anodic plating in HF - containing solution can control the refractive index, the porous silicon can be used as a template for epitaxial growth, and the crystallization information of the substrate is isolated from 0 while the epitaxial lateral overgrowth on the dielectric or metal layer leaves some " windows" that retain the crystallization information of the Si substrate.
Porous silicon interlayer
Electrochemical etching is a good technique for preparing porous silicon, which can form multiple Bragg mirrors and be applied to optical resonant cavities.The refractive index of the multiple Bragg mirror layer can be controlled by porosity, which is determined by the current density of electrochemical etching, HF concentration in solution and other anode plating conditions.Since porous silicon retains the crystal information of the original crystal substrate being etched and the Si deposition process is an ordered deposition based on the original crystal substrate structure, electrochemical etching is an ideal technique for preparing buried back mirrors [ 61' % ].
In order to simulate and optimize the buried back mirror of porous silicon interlayer, a model of light propagation in porous silicon can be established based on the theory of electromagnetic wave propagation in irregular media [ 63 ].In the model, Si is used as matrix material, irregular spherical pores are used as scattering particles, and the reflection part and diffuse scattering part of light scattering are treated separately.
In the experiment of preparing the porous silicon interlayer buried back mirror, the Si deposition process requires higher temperature, while the F - hole silicon is unstable at high temperature, making it difficult to maintain the mirror characteristics.The problem is that porous silicon tends to enlarge the spherical pores' and reconstruct to a low energy form.In addition, after preparing the porous silicon intermediate layer, Si deposition will lead to the phenomenon of filling pores, and some of the deposited SL will enter the pore structure and reduce the porosity.There are two techniques to solve these problems.
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