This series designed for small UPS/EPS, Emergency lighting (medical, insurance, bank, tax administration and recreation), Spot light, energy saving lamp, Railway and airline signals, Alarming and security, Instruments and meters, electrical D/C power sources.
Voltage: 6V, 12V; Capacity 1.2Ah to 28Ah
One technique is to use low-energy plasma enhanced chemical vapor deposition w - energy plasma _ enhancedcalcvavodship, lepecvdf as a representative low-temperature deposition technique to maximize the retention of porous silicon structure.The deposition of Si thin film by this technique can form better epitaxial layer quality on top of porous silicon without damaging the multi-layer porous silicon structure.The high resolution X - My Diffraction and TEM cross section of the transmission electron microscope verify that this technique does not damage the porous silicon structure by Si deposition.At 590 C, the growth of the 10 PAN thick epitaxial layer has a high deposition rate of about 3 nm / s.TEM analysis shows that the interface of porous silicon / epitaxial layer forms a high defect density and the defect diffuses to the whole epitaxial layer.If the deposition temperature rises to 645 C, the defect density will decrease.
In X - ray diffraction crystal testing technology, X - rays are scattered by electrons outside the nucleus and diffracted.A regular arrangement of atoms in a crystal produces a regular diffraction image.The distance and spatial arrangement between various atoms in a molecule can be calculated accordingly.X - ray diffraction is a useful method to analyze the spatial structure of macromolecules.
Another technique for preparing a porous silicon interlayer buried back mirror is epitaxial growth at high temperature, and depending on the inevitable reconstruction, a good experimental result of 1 3 has been achieved.Electrochemical etching is carried out on a highly p - doped monocrystalline silicon substrate to form a low porosity / high porosity stack, and the obtained porous silicon has a multilayer structure.Subsequently, the samples were thermally assisted chemical vapor deposition ( TA - CVD ) at high temperature to prepare epitaxial layers.SEM images of scanning electron microscope can show such a structure. The original pore size of porous silicon is only on the order of several nm. Large - scale reconstruction realizes larger pores and wider pore walls, resulting in a structure of low porosity / high porosity.
In the method for preparing the high porosity / low porosity multilayer porous silicon buried back mirror at high temperature, the reconstructed structure has better performance of multiple Bragg mirrors.The internal reflectivity of the porous silicon / epitaxial layer interface can be expressed as a function of wavelength.The internal reflectivity increases with the number of layers of porous silicon, and the maximum internal reflectivity of 80 % can be reached when 15 layers are stacked in a wide wavelength range.The measurement of resistivity shows that the technique of preparing porous silicon interlayer buried back mirror will not affect the majority carriers transported vertically, and it is an ideal way to develop high current density epitaxial silicon thin film solar 6V battery.
Liquid phase epitaxy LPE can prepare Si epitaxial layer M on porous silicon.First, the porous silicon is formed on the or crystal plane by HF anode plating, then annealed in H2 atmosphere, and finally, the epitaxial layer is grown on the porous silicon by LPE at different temperature distributions.Thin films grown on porous silicon with crystal planes have pyramid structures, but it is difficult to obtain condensation.On the other hand, an epitaxial layer with a particularly uniform structure can be obtained on the porous silicon on the crystal plane.
In addition to acting as an epitaxial growth template, the porous silicon buried layer can also be used as a gettering layer to prevent impurities from diffusing from the Si carrier substrate into the effective epitaxial layer.The preparation of crystalline silicon thin film solar 6V battery on metallurgical grade silicon Mg - Si low cost substrates with porous silicon as gettering layer has been explained in principle [ 67 ].
Epitaxial lateral overgrowth
Through the opening of insulator SiO _ 2, epitaxial lateral overgrowth technology can realize preferential epitaxial growth of Si.As shown in actual. 14.The insulator SiO _ 2 obtained by thermal oxidation has the function of a masking layer.The preferred characteristics of chemical vapor deposition CVD come from controlling the balance between Si etching and growth in Cl atmosphere.In order to grow excessively on the insulator SiO 2, the lateral growth rate needs to be faster than the longitudinal growth rate, and the opening size needs to be controlled at twice the thickness of the epitaxial layer.
Due to the close thermal equilibrium state, LPE growth in liquid phase epitaxy is easier to obtain a higher aspect ratio.In most cases, a smooth epitaxial layer surface is obtained by growing on a Si substrate with crystal planes.As shown in actual. 17, the in-situ texturing on the crystal plane A substrate is also an advantage.Another technique for epitaxial lateral overgrowth of buried back mirrors is electroplating.On the Si substrate with metal masking pattern, the proof of concept was realized by the electro - epi - epitaxial lateral overgrowth process ⑽ of the liquid phase, as was not the case in the actual .17.The Si thin film was grown on a strip-shaped substrate masked by W by current-induced crystallization through a liquid-phase metal solution ( Si saturated Bi melt ).W is selected as the masking layer because W is resistant to high temperature during electroplating and has stable chemical properties in the metal melt.The growth temperature is in the range of 8001150 C and the current density of 220 mA / cm2 is added to the Si / metal melt interface to increase the lateral growth.W masking substrate with strip opening pattern of 10 width and 100 spacing realized the growth of continuous Si epitaxial layer with an area of 1cm2.
Laboratory and Industrial Achievements
Epitaxial crystalline silicon thin-film solar 6V battery have the advantage of low-cost S - substrates, and have many laboratory results based on monocrystalline silicon and polycrystalline silicon substrates.In the 1990s, several laboratories reported the research results of epitaxial silicon thin film solar 6V battery grown on highly doped monocrystalline silicon substrates, proving the high conversion efficiency potential.The growth technology of epitaxial layer is mainly chemical vapor deposition CVD m and liquid phase epitaxy LPE m the laboratory results of highly doped monocrystalline silicon substrate confirm the potential of high conversion efficiency of epitaxial crystalline silicon thin film solar 6V battery, as shown in actual. 2.An oxide intermediate layer can be prepared by oxygen injection isolation ( SIMOX ) or back etching, and a light trapping structure for burying the back mirror can be formed by silicon on insulator ( SOI ).Some reports believe that the lattice mismatch between the highly doped Si substrate and the epitaxial layer will lead to crystal defects in the epitaxial layer.Although such lattice mismatch is small, from the perspective of the larger thickness of the epitaxial layer, strain relaxation will occur by introducing mismatched dislocations [ 73 \ If epitaxial crystalline silicon thin film solar 6V battery are grown by CVD on highly doped polysilicon substrates ( with silicon, metallurgical grade silicon Mg - Si, P + type polysilicon ), the laboratory conversion efficiency is obviously lower than that of epitaxially grown 6V battery on monocrystalline silicon substrates.The process step of introducing H into the epitaxial layer proved to play a key role in passivating epitaxial layer defects [ 19' 22' % 51 ].H - inducing technologies include remote plasma hydrogenation ( RPH ) or nitriding sintering, which can make the conversion efficiency of small-area epitaxial silicon thin film solar 6V battery with a thickness of 20 close to 14 %.
The epitaxial crystalline silicon thin film solar cell grown by LPE has good electrical characteristics, and the open circuit voltage is greater than 660 mV ( AMI. 5, 25 C ) [ 72 ].Epitaxial crystalline silicon thin film solar 6V battery prepared by LPE in the laboratory can achieve a conversion efficiency of 17 % and 18 % [ 74 ].One of the important process steps to realize these high conversion efficiency 6V battery is to remove most of the heavily doped substrate from which the epitaxial layer is grown, thereby increasing the reflection on the back surface.With this method, the conversion efficiency of the epitaxial crystalline silicon thin film solar cell can be increased by nearly 25 %, while the thickness of the cell can be reduced to 30pm.As mentioned earlier, LPE can directly dope the epitaxial layer according to the set doping distribution.If Ga doping gradient is introduced into the thin film, a drift field is formed in the base of the solar cell, which can increase the effective minority carrier diffusion length and increase the long wavelength response [ 75 ].When the epitaxial crystalline silicon thin film solar cell grown by LPE has a drift field, the independent side confirmed the conversion efficiency of 16.4 %.
The small area epitaxial silicon thin film solar cell grown on the polysilicon substrate LPE has achieved a conversion efficiency of 15.4 %, which further confirms the potential of conversion efficiency [ 76 ].
Achievements of industrialization
The so-called industrialized solar cell refers to a large area ( > 20 cm2 ) solar cell, and the production process is similar to the existing process in the photovoltaic industry.Although the basic process flow for producing epitaxial crystalline silicon thin film solar 6V battery is very similar to that for producing classical crystalline silicon solar 6V battery, it needs to be described independently in this chapter for two reasons.First of all, the preparation of large-area solar 6V battery is a sign of the maturity of specific epitaxial deposition technology, and it is especially necessary that large-area 6V battery can still meet the requirements of thickness uniformity and doping uniformity.Secondly, although there is a great similarity with the production process of crystalline silicon solar 6V battery, process steps such as the preparation of front surface textured surfaces need to be readjusted in the production of epitaxial crystalline silicon thin film solar 6V battery.If the conventional texturing process of crystalline silicon solar 6V battery is used, the epitaxial layer of 1020 households will be destroyed.
The results of the industrial epitaxial silicon thin film solar cell can be summarized, as shown in actual .3.The monocrystalline silicon substrate grown by the Krousky method achieves a conversion efficiency of 14 %, while the highly doped polycrystalline silicon substrate achieves a conversion efficiency of 13 %.The technological process used is the sintering of tubular or in-line P doping, screen printing of front and back electrodes, and anti - reflective coating ( ARC ) [ 8' 81 ].
The measurement of beam induced current OBIC shows that the diffusion length distribution is uneven in the whole epitaxial silicon thin film solar cell, the maximum effective diffusion length LDF reaches 25, and the average value is about 15 PTM ⑽.Through the detection of infrared lock-in thermography, the local shunt path is related to a certain extension [ 83 ].In the infrared phase-locked thermal imaging technology, the study sample is subjected to periodic thermal excitation, a high degree of attenuation is generated inside the sample and scattered " thermal waves" are recorded after reaching the near surface area.In order to reduce the reflection on the front surface and increase the optical path length, the front surface of the epitaxial crystalline silicon thin film solar cell needs to be textured on the p - type si epitaxial layer before forming the n + type emitter so that the n + type emitter grows on the textured surface.However, the epitaxial crystal silicon solar cell needs to modify the classic suede process, and the most important problem is to reduce the loss of the A epitaxial layer due to suede preparation and the initial epitaxial layer thickness required before suede preparation as much as possible.The process of texturing industrial polycrystalline silicon solar 6V battery is a chemical method, using HF and HNO 3 to perform wet chemical etching on the damaged surface to form textured surfaces with etching pits of 110 in diameter.However, this wet chemical etching process produces uneven textured surfaces on the epitaxial layer, forming deeper pits because the surface is free from defects of sawing damage.For this reason, a new plasma texturing technology using SF6 has been developed. The plasma is formed by a microwave antenna.Plasma textured surface can minimize the loss of epitaxial layer, while reducing reflection, only 2 PTM thick epitaxial layer is lost.The epitaxial layer was prepared by chemical vapor deposition CVD on a high purity metallurgical grade silicon UMG - Si substrate, achieving a conversion efficiency of nearly 13 %.
Recently, epitaxial crystalline silicon thin film solar cell modules grown on highly doped monocrystalline silicon or polycrystalline silicon substrates have been reported [ 8.The component based on the monocrystalline silicon substrate has a pore size of 368 cm2 and a conversion efficiency of 12.2 %, while the component based on the polycrystalline silicon substrate has a pore size of 576 cm2 and a conversion efficiency of 10.2 %.Such industrialization results prove the rapid maturity of epitaxial crystalline silicon thin film solar cell technology.
The film growth process based on liquid phase epitaxy LPE can also be used to prepare industrialized epitaxial crystalline silicon thin film solar 6V battery.For CVD - grown epitaxial crystalline silicon thin film solar cell technology, the laboratory conversion efficiency record is only 1 % different from the industrial conversion efficiency ( taking Silos polysilicon as an example ), but for LPE growth technology, the conversion efficiency is much different because LPE is difficult to achieve the thickness uniformity of large-area deposited thin film.Through phosphorus slurry diffusion and LPE film growth, only epitaxial crystalline silicon thin film solar 6V battery with conversion efficiency of 10.0 % can be realized [ 85 ].1.4.3 Epitaxial crystalline silicon thin film solar 6V battery with a novel structure using front gate lines as contact electrodes can be prepared in a substrate configuration or an upper layer configuration.However, silicon wafer-based crystalline silicon solar 6V battery have novel contact electrode forms.If these two concepts are integrated into a single device, the two main loss mechanisms of crystalline silicon solar 6V battery can be reduced simultaneously.The concept of epitaxial crystalline silicon thin film solar cell is realized by epitaxial lateral overgrowth.The epitaxial crystalline silicon thin film solar cell obtained by replacing the traditional metal front gate line with a semiconductor embedded gate line has a conversion efficiency of 7.8 % on the aperture area, a better isolated contact electrode, a negligible series resistance power loss and a shadow of < 1 % on the aperture area.
high production rate deposition
Although the production process of epitaxial crystalline silicon thin film solar 6V battery is very similar to that of classical crystalline silicon solar 6V battery, the development of this technology needs to solve two problems.
The first problem is the commercialization of low-cost Si substrates.In a short period of time, epitaxial crystalline silicon thin film solar 6V battery based on metallurgical grade silicon Mg - Si substrates can achieve a conversion efficiency of 14 % 15 %.There is a need to greatly increase the production capacity of Mg - Si raw materials to produce Mg - Si ingots and silicon wafers on a large scale.
The second problem is to develop deposition equipment with high production rate and realize the growth of epitaxial layers with a thickness of 520 p ( m ) at a rate of several m7h.The commercial epitaxial reaction chamber developed in microelectronics needs to achieve thickness uniformity and doping uniformity on the order of 1 %.In contrast, epitaxial crystalline silicon thin film solar 6V battery require much higher production rates.Similar upgrade requirements also exist in the technical field of thin-film solar 6V battery based on amorphous silicon A - Si: H or microcrystalline silicon Si: H, but it is difficult to increase the production rate for epitaxial crystalline silicon thin-film solar 6V battery.In order to achieve high growth rate and high crystallization quality on the order of FXM / min, the ideal temperature range is 8001300 C, and the materials to be used can withstand high temperatures.In addition to the high temperature requirements for deposition, there is also a need to prevent contamination during the growth of the epitaxial layer of crystalline silicon and to have strict requirements on the purity of the raw materials used in order to improve the chemical efficiency, that is, the ratio of Si in the active layer to Si in the gas or liquid.However, chemical vapor deposition CVD and liquid phase epitaxy LPE use corrosive gases and liquids, respectively.In the condition deposition process, high chemical efficiency needs to deplete gas, while high thickness uniformity cannot deplete gas, so the balance between chemical efficiency and thickness uniformity is required.Even if the thickness uniformity and doping uniformity are required to be on the order of 10 %, the contradiction between chemical efficiency and thickness uniformity still needs comprehensive consideration.
Even if these problems are satisfactorily solved, the reliability and safety of the long-term use of the deposition equipment still need to be demonstrated.The production cost of epitaxial crystalline silicon thin film solar 6V battery also needs to be optimized.Acceptable costs require a comprehensive consideration of investment costs, gas, pedestal and other consumables costs and maintenance costs, which obviously depend on the achievable conversion efficiency.Assuming that the component conversion efficiency is 14 %, the price of 1 / WP means an epitaxial layer growth cost of up to 20 / m2.
Obviously, it is still difficult to realize such costs.However, several new concepts of high production rate deposition reaction chamber are being developed to solve the main problems faced.These concepts can realize large-scale production of CVD and LPE processes, which will be discussed further below.Because CVD and LPE are more mature than other deposition techniques such as ion-assisted deposition IAD, people have focused on large-scale production research of CVD and LPE.It will still take several years to finally integrate these reaction chamber concepts into the existing solar cell production line.
Chemical vapor deposition suitable for mass production
The concept of continuous chemical vapor deposition ( CON CVD ) is close to the on-line process used in most crystalline silicon solar cell production lines, which is very suitable for large-scale production.CON CVD concept is based on the continuous wafer transport process, through which epitaxial deposition takes place in the thermal reaction region.In order to improve the deposition efficiency from vapor Si to solid Si and reduce parasitic deposition, CON CVD relies on tube - in - tube WM reading [ 87.88 ].The inner tube is a reaction chamber, the rectangular substrate forms the side wall of the reaction chamber, and the reaction chamber is sleeved in the outer tube.The outer tube is filled with a large amount of H2 and inert gas, and the anti - F gas Si HCl: H' is introduced into the reaction chamber for deposition.Two rows of substrates continuously slide through the reaction chamber of CON CVD, which solves the contradiction between F lateral uniformity and gas depletion, and all substrates pass through the same reaction environment.In order to reduce the diffusion of reaction gas from the reaction chamber to the outer tube, it is necessary to slightly increase the gas pressure in the outer tube.
If CON CVD uses a gas curtain system, the isolation between the atmospheric pressure inside the reaction zone and the atmospheric pressure outside the reaction zone can be ensured.The gas curtain system sends the substrate away and out of the reaction chamber in a continuous manner without gas exchange between the reaction chamber and the laboratory.According to the diffusion mode of one gas relative to another flowing gas, a small gas flow rate on the order of 111 / 5 can reduce the diffusion gas concentration to a negligible value at several cm in the opposite direction of the flowing gas.CON CVD gas curtain system can be installed at both ends of the reaction tube.
The first CON CVD equipment using the concept of gas curtain system was designed and built by Fraunhoferistitusolar Solar Systems ( FHG - ISE ) in Flawn, Germany [ 87, 88 ].Growth of epitaxial layers on highly doped polysilicon substrates requires an ambient temperature of 1150 C and can achieve a deposition rate of 1.5 mm / min, a production rate of 1.2m7h and a conversion efficiency of 12.5 %.
The main characteristics of CON CVD are:
An open system with a gas curtain and two rows of continuously moving substrates;Through resistance heating, the substrate temperature is up to 1300 C;SiC reaction tubes with a diameter of 30cm;
cm long graphite reaction chamber;
graphite carriers for transporting substrates in columns;
The width of the substrate is 10 cm or 20 cm, and the maximum length is 40 cm.
The 10 cm wide substrate was placed in 2 columns on the same carrier.Reaction gases H2, Si HCl 3FFL B2 H 6 pass through the human reaction chamber, H2 and inert gases pass through the human outer tube;The deposition rate is 5 mm / min, the production rate is 1.4 mz / h, and the thickness of si epitaxial layer is 30 mm ..
FHG - ISE researchers made a preliminary cost prediction for CON CVD.If the length of the reaction chamber is increased from 40 cm to 200 cm and the width of the reaction chamber is 20 cm, the production rate can reach 150000m2 / a, and the Si epitaxial layer growth cost is finally 10 / m2, meeting the previously proposed target of 20 / m2 epitaxial layer growth cost.The growth cost of epitaxial layer mainly comes from consumed H2, which can be further reduced by advanced H2 recovery technology [ 87 ].
Convection - assisted chemical vapor deposition
As a recently developed large-scale production process, convection-assisted chemical vapor deposition ( CO CVD ) can effectively use thermal convection to control gas flow in the reaction chamber and recover underutilized gas precursors by convection.The angle between the reaction chamber and the horizontal direction is < >, and convection can be controlled through the inclination angle a, while thermal convection will increase the flow of precursor gas and the stability of the flow.Cold gas is sent to the reaction chamber and flows down the side wall of the cold chamber.The side wall opposite to the side wall of the cold chamber is a heated substrate, and gas flows upward along the heated substrate to deposit on the substrate to form an epitaxial layer, and the heated substrate drives the gas to convect.Part of the gas that has undergone chemical reaction leaves the reaction chamber through the outlet, while part of the gas that is far away from the substrate remains in the convection gas for downward circulation.Such an internal reflow mechanism can reduce Si HCl 3 required for the growth of an epitaxial layer with a certain thickness.The tilt angle of the substrate and the bottom wall of the reaction chamber are critical to reflow.
Batch epitaxial reaction chamber
Although con CVD and co CVD with convection-assisted chemical vapor deposition have made great progress in the feasibility of large-scale production, the reliability and safety of these two concepts still need to be demonstrated.In the field of microelectronics, batch epitaxial reaction chambers have been developed. With SiH as Si precursor, wafers of 200 mm and 300 mm size have been developed. The production rate is on the order of 100 wafers / h, which is equivalent to the production rate of 5 m2 / h.Like the preparation of power electronic devices, batch-type epitaxial reaction chambers can also be used for large-scale production of epitaxial crystalline silicon thin film solar 6V battery, but the polysilicon deposition process needs to be improved to form a thicker polysilicon layer, the temperature is slightly increased by 100200' c, and the uniformity requirement is reduced to 10 %. such batch-type epitaxial reaction chambers can achieve the required target cost.
For the first time, LPC VL 913 was used in the batch epitaxial reaction chamber with better epitaxial layer quality.One end of the batch-type epitaxial reaction chamber is an air pump system connected with a resistance heating quartz tube, while the other end is a Shi Ying window, which realizes thermal isolation through a vacuum quartz clock container.Batch epitaxial reaction chambers can simultaneously grow epitaxial layers on 20 wafers, and Shi Ying inserts that can be easily removed are placed on the side walls of the reaction chambers so that Si layers will not be deposited on the side walls of the reaction chambers.The placement direction of the wafer may be longitudinal or transverse.
Another concept of batch epitaxy reaction chamber is the so-called stacking epitaxy reaction chamber ( SER ) [ 93 ].SER technology has been developed since the 1980s based on densely arranged resistance heated graphite pedestal [ 94 ].SER can achieve efficient and uniform heating.At present, a sample of SER has been built and includes a reflux system for H2.A high H2 flow rate in SER is required to make the temperature of the gas in the chamber low enough to prevent the formation of micro - dust.If there is no H2 reflux, the economic benefits of SER are not obvious.
Liquid phase epitaxy suitable for mass production
Liquid phase epitaxy LPE technology suitable for mass production can also be divided into on-line type and batch type.However, the large M - mode production of LPE has not yet reached the maturity level of chemical vapor deposition CVD.
The temperature difference method and the temperature difference method are quasi-continuous technologies with good prospects, which can grow epitaxial layers on the large-area polysilicon substrate of locmxloc, and are important directions for the large-scale production research of liquid phase epitaxy LPE.TDM was first used to deposit DI - V semiconductor layers for light-emitting devices. A temperature gradient of M perpendicular to the substrate surface will generate thermal sub - driving forces for TDM growth epitaxial layers.The temperature gradient forms a concentration gradient of the melt, which is the driving force for the growth of epitaxial layers on the substrate.Since the temperature of the substrate is constant during the growth process, the temperature-dependent solubility does not affect the composition of the epitaxial layer.
TDM can grow Si epitaxial layer from IR VGA melt, and can achieve a growth rate of up to 0.3 mm / min.In the 30 - thick epitaxial layer grown on monocrystalline silicon and polycrystalline silicon substrates, the minority carrier lifetime reaches 510 ms.Since hydrogen passivation is not performed, such performance is quite excellent.
Batch liquid phase epitaxy
If the liquid phase epitaxy LPE produced on a large scale adopts a continuous on-line process, it is not comparable to a batch process because the dissolved Si will continuously decrease and the solution needs to be replaced after each epitaxial growth.In batch-type liquid phase epitaxy, 16 substrates are simultaneously immersed in " infinite source", making batch-type liquid phase epitaxy very suitable for mass production [ 98' m ].One of the unique advantages of batch-type liquid phase epitaxy is that it can be remelted by melting high purity metallurgical grade silicon UMG - Si in the crucible for O' before each growth process, instead of using electronic grade silicon as the silicon source material, the upgrading of Si material by batch-type liquid phase epitaxy uses less energy than the purification of metallurgical grade silicon.The conversion efficiency of 3cm2 small-area epitaxial silicon thin film solar 6V battery grown by batch liquid phase epitaxy reached 10 %, and the process can be compatible with commercial production.
Epitaxial crystalline silicon thin-film solar 6V battery are different from other thin-film solar 6V battery and closer to mature crystalline silicon solar 6V battery, which is a transition from silicon-based solar technology to monolithic integration technology.Although not necessarily as successful and effective as other thin film solar 6V battery, epitaxial crystalline silicon thin film solar 6V battery still have considerable cost reduction potential.However, epitaxial crystalline silicon thin film solar 6V battery can reduce the dependence of photovoltaic industry on polysilicon raw materials.The industrialized crystalline silicon thin film solar cell with mature technology has achieved a conversion efficiency of 13 %, while the light trapping structure can further improve its performance.The buried back mirror formed by the porous silicon interlayer can effectively increase the optical path length and make the conversion efficiency close to that of a common crystalline silicon solar cell.Among several deposition techniques studied, chemical vapor deposition ( CVD ) is the most mature and obtains the best 6V battery performance.The precondition for commercialization of epitaxial silicon thin film solar 6V battery in the future is to develop high production rate epitaxial growth equipment for large-scale production, which has become the most important topic for research and development of epitaxial silicon thin film solar 6V battery at present.
For decades, scientists studying solar energy technology have been seeking to fabricate thin-film solar 6V battery using heterogeneous substrates made of non-silicon materials and Si coating processes.The advantage of this silicon - based thin film solar cell concept is that it can greatly reduce the consumption of expensive solar grade silicon and can completely eliminate the silicon wafer cutting process.So.Compared with the traditional crystalline silicon solar cell module based on silicon chips, silicon-based thin film solar 6V battery will have great cost reduction potential.
In order to realize the technical concept of silicon-based thin-film solar 6V battery, researchers began to prepare amorphous silicon A - Si: H thin films on glass sheets or glass plates as substrates 30 years ago.The advantages of glass substrate are low cost, abundant sources, high transparency and so on.However, with the progress of research and development, the conversion efficiency of amorphous silicon thin film solar 6V battery has stagnated and the typical component conversion efficiency is limited to 6 % 8 %.This conversion efficiency is too low, and the larger area of amorphous silicon thin film solar 6V battery will bring additional costs, so the expected cost reduction potential of silicon thin film solar 6V battery cannot be realized.Subsequently, in order to improve the conversion efficiency of amorphous silicon thin-film solar 6V battery, people introduced a new cell concept by stacking two or three P - N junctions and then monolithically integrating them to generate photo-generated carriers [ 1 ].It has also been found that microcrystalline silicon - Si: H is suitable for preparing such laminated batteries.The grain size of Si: H is on the order of submicron, and a large number of Si dangling bonds are passivated by hydrogen.The electrical and optical properties of / IC - Si: H are similar to those of polycrystalline silicon with large grain size.In a double-junction stacked cell, C - Si: H is the top cell that receives human light, while MC - Si: H is the bottom cell, the conversion efficiency before light aging treatment of large-area laboratory components can be increased to more than 13 % [ 2 ], and the conversion efficiency after stabilization can reach 10 % m ..Even with such performance improvement, amorphous silicon thin-film solar 6V battery still do not enter the market on a large scale.The traditional silicon wafer-based crystalline silicon solar cell is still the mainstream of the photovoltaic market, now accounting for 93 % of the global market share, and this trend will continue to W for quite some time.The reason for this market pattern is that the silicon wafer production process is mature and reliable, and the crystalline silicon material enables the module to achieve stable performance and higher conversion efficiency.Therefore, people will think of combining the concepts of " crystalline silicon" and " thin film" to research and develop a heterogeneous substrate crystalline silicon thin film solar cell with the potential to reduce costs.The heterogeneous substrate crystalline silicon thin film solar cell can combine the advantages of the thin film solar cell described above with the high conversion efficiency of the crystalline silicon solar cell.Since 1015 years ago, heterogeneous substrate crystalline silicon thin film solar 6V battery have been studied and developed.Soon, the study shows that the minority carrier diffusion length needs to reach 23 times the thickness of Si active layer to make the conversion efficiency unaffected by the quality of the active layer [ 5 ].If the harmful effects of grain boundaries are considered, the conversion efficiency of solar 6V battery is quantitatively related to the grain size of polysilicon layer by % 7 ].According to these calculations, in order to achieve a conversion efficiency of more than 15 %, the grain size of S is required to reach at least 100 orders of magnitude.
Directly depositing si on a heterogeneous substrate can only achieve a grain size of < 5, and does not depend on the deposition method and the deposition temperature.Therefore, the focus of research and development of heterogeneous substrate crystalline silicon thin film solar 6V battery is to improve the grain size of the layer just after deposition through recrystallization.All the preparation methods of crystalline silicon thin film solar 6V battery with heterogeneous substrates for recrystallization adopt certain heat treatment.The limitation of process temperature depends on the choice of substrate type.Another kind of polycrystalline silicon thin film solar cell passes the " low temperature method" and uses a glass substrate, and the process temperature cannot exceed 55o c.The maximum grain size achieved is in the range of 10.Related solar cell conversion efficiency is limited to 8 % [ 8 ].For a more detailed introduction to polysilicon thin film solar 6V battery, see Chapter 3.
In the " high temperature method" described in this chapter, the substrate does not limit the process temperature by definition.The recrystallization process temperature of the liquid phase can even exceed the melting point of si, i.e 1420 c, which is the most effective method to increase the grain size, and the grain size of the microcrystalline layer can exceed 100 mm ..
This chapter will discuss in detail the high-temperature preparation process of crystalline silicon thin-film solar 6V battery with heterogeneous substrates. The key technologies are:
Selection of suitable substrate materials;
Developing on-line Si deposition methods;
Perfect high-speed recrystallization.
This section will outline the requirements of heterogeneous substrate crystalline silicon thin film solar cell performance on material selection, active layer characteristics and process details, current research focus, problems and possible solutions.Each important topic will be discussed in detail in different sections.We will use many examples to highlight the typical treatment methods and improvement effects in the experimental work, hoping to give a vivid description of the research results.
The heterogeneous substrate crystalline silicon thin film solar cell can be based on any high temperature stable substrate such as ceramic or low cost silicon tape.The substrate is plated with an intermediate layer, which mainly serves to prevent impurities from diffusing from the substrate to the Si layer.The requirements to be met by the substrate and the intermediate layer, as well as the current research progress, will be discussed in detail in Section 2.2.
The process of forming Si layer is often divided into three main steps.
In the first step, the S layer with high B doping ( about 1019 CNC 3 ) is deposited on the surface of the intermediate layer. The commonly used S deposition technique is atmospheric chemical vapor deposition ( APCVD ) with high process temperature. Section 2.4 will discuss its basic principle and test results.
Since that lay just after deposition is si: h
Recrystallization was carried out in three steps.Large - area melting and zone melting, zone - melting crystallization ( ZMR ) are all feasible methods.ZMR is a more commonly used technology and will be discussed in this chapter.Section 2.3 will discuss the principle of ZMR, the characteristics of the Si film obtained and the development of ZMR equipment?
The recrystallized Si layer is called a seed layer, and a good seed layer helps epitaxially deposit normal B doping (?Io 17c m " 3" si layer, forming an absorption layer of a heterogeneous substrate crystalline silicon thin film solar cell.The research progress of this process step and the development of CVD equipment will be described in Section 2.4.
After three process steps, P + / P high-low junction will be formed on the interface between the quintessence layer and the epitaxial absorption layer, acting as the back surface field BSF and effectively reducing the surface recombination on the back surface of the epitaxial layer.
In principle, there are two technical routes for preparing solar 6V battery from the above-mentioned stacked layers.If the substrate or intermediate layer is not conductive, the metal contacts of the base and emitter are placed on the front surface.Using laser scribing or mechanical scribing to isolate the thin film regions and connecting them to each other, a monolithic integrated assembly with a single large area substrate is realized.If both the substrate and the intermediate layer are electrically conductive, the metallized structure of the mature crystalline silicon solar cell can realize the contact electrodes of the base and emitter.The base contact is prepared on the back side of the substrate and the emitter contact is formed on the front side of the stack layer.If this process is feasible, the stacked layer including the substrate, the intermediate layer and the Si layer can be called a recrystallized " silicon wafer substitute"!, because in principle such silicon wafer substitutes can be processed into 6V battery and components by conventional processes and equipment of crystalline silicon solar 6V battery.Section 2.5 will describe in detail the experimental results of crystalline silicon thin-film solar 6V battery with heterogeneous substrates and the production process of recrystallized silicon wafer substitutes.
Heterogeneous substrates and intermediate layers
Almost all thin film solar cell concepts are based on depositing an absorbing layer on a substrate.In most cases, an intermediate layer needs to be prepared between the substrate and the thin film to reduce the influence of impurity diffusion on the performance of the solar cell.High temperature deposited heterogeneous substrate crystalline silicon thin film solar 6V battery have higher requirements for both heterogeneous substrates and intermediate layers.In the following, we will discuss the characteristics and latest research and development results that heterogeneous substrates and intermediate layers need to achieve.
The high temperature process for preparing crystalline silicon thin film solar 6V battery with heterogeneous substrates has higher Si deposition rate, larger grain size after recrystallization and lower defect density of epitaxial layers.However, the necessary temperature stability limits the choice of heterogeneous substrate materials because the following requirements need to be met:
Mechanical stability and chemical stability: A basic requirement for heterogeneous substrates is mechanical stability in high temperature processes.Screen printing and laminating require bending strength of about 150m PAM.As for chemical stability, heterogeneous substrate materials need not react chemically with liquid phase Si during zone melting and recrystallization ZMR.There is also a need to prevent harmful impurities from diffusing from the heterogeneous substrate to the Si active layer.For these two chemical stability problems, attracting the middle layer to block can reduce the harsh requirements on heterogeneous substrates.Finally, heterogeneous substrates need to be able to withstand various chemicals in solar cell processes, and replacing standard wet processes with dry processes can make this issue less important [ 12 ].
Smoothness and thickness uniformity: In order to smoothly carry out ZMR, a certain substrate flatness is required so that the focus line is on a fixed plane.The maximum acceptable curvature of 15 OMMX 15 OMM silicon wafers is in the range of 0.51.0 mm.Heterogeneous substrates with curvature exceeding this value will not only cause problems in ZMR, but also will not be compatible with standard crystalline silicon solar cell devices.Thickness uniformity is important for each process step.However, the thickness uniformity is the key factor of ZMR. Only when the thickness uniformity is guaranteed can the temperature distribution be uniform and stable recrystallization be realized.
Smaller surface roughness: It is required to achieve smaller surface roughness and lower surface porosity, so that Si deposition and recrystallization can form high-quality films.In general, the surface roughness needs matching of L / 3B \ thermal expansion coefficient ( TEC ) smaller than the thickness of the seed layer: during Si deposition, the heterogeneous substrate is heated to about 1200 C, while during recrystallization, the local substrate temperature may even exceed 1400 C ..The thermal expansion of the heterogeneous substrate needs to be matched with Si [ 14 ] to prevent cracks in the heterogeneous substrate at the cooling stage after the high temperature process.
Thermal shock resistance: Thermal shock resistance is another kind of thermal performance that heterogeneous substrates need to meet. It is found that heterogeneous substrate materials can withstand rapid heating and cooling processes in high-temperature processes.
Low cost: The cost of heterogeneous substrate and Si active layer needs to be far lower than the standard polysilicon sheet to realize the cost-effective O - assumption of 12 % conversion efficiency for heterogeneous substrate crystalline silicon thin film solar 6V battery, and the maximum cost limit given by cost calculation is 5070 / m2 [ 15 ].For large-scale production, the estimated cost of heterogeneous substrates is 2050 / m2 [ 15' 16 ].Astro power Inc., acquired by general electric ( ge ), calculated the cost of a heterogeneous substrate of $ 9 / m2 [ 17 ].If ceramic is selected as the heterogeneous substrate material, casting is recommended because other processes such as hot pressing are too expensive.
low cost heterogeneous substrate
The low-cost heterogeneous substrates that can be selected for the heterogeneous substrate crystalline silicon thin film solar cell include:
Si3N4 ceramics [ 19' 2 ];
Silicon - impregnated silicon carbide ( SISISIC ) ceramics [ 22 ];Reaction bonded silicon carbide ( Rb SiC ) ceramics [ 23 ];Ceramic based on AI2O3
Andalusite ceramics [ 25' 26 ];
Silicon AlON ceramics [ 27.2 illusions;Graphite [ 12.29 ];
Powder growth zone silicon SSP [ 28,3 ].
Not all of these heterogeneous substrate materials meet the requirements set forth in the previous section.For example, the thermal expansion coefficient 7tc of ai2o3 is very different from si, so a12o3 substrate can only directly deposit si and cannot use zone melting recrystallization zmr.In order for the reader to better understand the following chapters, it is necessary to introduce these low-cost heterogeneous substrate materials in more detail.
There are two methods to prepare Si3N4 ceramics: sintering Si3N4 powder to form sintered silicon nitride ( SSN );Or by nitriding to form reactive bonded silicon nitride ( RBS in ) ossn has the advantage that the material density is higher than RBS in [ l8 ].However, as a heterogeneous substrate, RBS in has a cost advantage.The T ￠ C of the two Si3N4 materials are slightly smaller than Si and have better thermal shock resistance.Germany h c starck ceramics produces SSN and hot-pressed RBS in, while the Netherlands energy research center ( ERCN ) prepares a certain amount of cast SSN ceramics.
Zr SiO 4 comes from the natural mineral zirconium sand and has great cost advantage.ZrSi 4 substrate is non-conductive while white ZrSiO 4 is easy to trap light.It has been reported that ZrSi 4 substrate is prepared by uniaxial hot pressing, but casting is also a feasible process.
SISISIC is an important non-oxide ceramic among various heterogeneous substrate materials.After the liquid phase Si fills the pores of SiC ceramics, a high density material without pores is formed.SISISIC has high stability and good thermal conductivity and conductivity [ 18 ] O The usual preparation process of SISIC ceramics is equal pressure hot pressing molding, and the production cost is high.
A more economical heterogeneous substrate is RB SiC ceramics prepared by tape casting [ 23 ].German h c starck ceramics uses tape casting to produce Rb sic ceramics. the process is as follows: the slurry obtained by mixing si, c and sic particles with a binder is coated on a 40 cm wide ribbon substrate, trimmed and shaped, and sintered at a temperature of over 1500 c.During sintering, & and C react to form SiC and combine with SC particles that already exist.The resulting ribbon Rb SiC has the potential to reduce the cost of heterogeneous substrates.Compared with SISISIC, RB SiC contains open pores, which increases the difficulty of wet chemical process.
FHG - ISE and Siemens developed SSP silicon growth process [ 31' 323 ].At first, the SSP tape silicon growth process includes surface melting and region melting.After that, the process applied to the heterogeneous substrate crystalline silicon thin film solar cell was modified to have only surface melting d.The thickness of the heterogenous substrate formed by SSP with silicon is about 600 mm, and there is no need to level the surface mechanically or chemically.
Both the ceramic and the silicon-containing heterogeneous substrate have been used to study crystalline silicon thin film solar 6V battery.People are still continuously optimizing substrate composition and preparation methods, but the corresponding 6V battery still lack the process reproducibility of conventional crystalline silicon solar 6V battery.At present, there are no heterogeneous substrates in the market that can meet the requirements of high-temperature process very well.However, the research and development of heterogeneous substrate crystalline silicon thin film solar 6V battery is making positive progress.
In order to study the heterogeneous substrate crystalline silicon thin film solar 6V battery, in addition to the low-cost heterogeneous substrates with silicon and ceramic materials mentioned above, standard monocrystalline silicon or polycrystalline silicon wafers will also be used as model substrates for Si thin film optimization.Single crystal silicon is often grown by the Krousky method, while polycrystalline silicon is often grown by the DirectionAlDiofficialSystem ( DSS ). The resulting silicon wafer is oxidized or SiC coated, and is often used as a model substrate to study ZMR, Si deposition and optimization of the performance of heterogeneous substrate crystalline silicon thin film solar 6V battery.The semiconductor industry can be used for these experiments by reusing test silicon wafers discarded after more than a dozen times and polysilicon wafers damaged by wire cutting and etching.
The low cost level and chemical purity of heterogeneous substrates often cannot be satisfied at the same time.The Si active layer can hardly reach a certain purity without preparing an intermediate layer that plays a blocking role.The chemical properties of metal elements in elemental form are unstable, while the metal oxide needs to meet the following requirements to prepare the intermediate layer more appropriately:
Mechanical stability and chemical stability: The most important requirement is that the intermediate layer achieves certain mechanical stability and chemical stability in the zone melting recrystallization ZMR process at temperatures up to 1450 C ..Otherwise, problems such as non-ideal deposition of Si seed layer, non-uniform ZMR, complete damage of seed layer structure, contamination of Si layer by substrate impurities, etc. may occur.A low-voltage CMOS DC-DC converter for a portable battery-operated system
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