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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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Blocking impurity diffusion: The intermediate layer needs to be an effective diffusion barrier layer to block the diffusion of electrically active impurities from the heterogeneous substrate to the Si layer.Of course, the intermediate layer needs to be quite pure and cannot become an impurity source by itself.Considering the ZMR high temperature process, this requirement is very high, which greatly limits the selection of intermediate layer materials.
Better wettability: if the substrate, the intermediate layer and the si layer all have better wettability, the zmr process will be more reliable.However, many reports think that SiO _ 2 is not as wet as liquid Si as the intermediate layer.
In addition to the barrier property, the middle layer has two functions:
Realize integrated series connection and become part of the high-level contact electrode structure;As a back mirror, it plays a role of trapping light.
On the model substrate formed by monocrystalline silicon or polycrystalline silicon wafers, the preparation of the model substrate crystalline silicon thin film solar cell does not require the intermediate layer to form a diffusion barrier layer, but the intermediate layer has other functions.Here, the intermediate layer ensures that the growth of the Si seed layer is similar to the growth on the coated ceramic, and the Si active layer can be electrically isolated from the monocrystalline silicon or polycrystalline silicon wafer substrate as necessary.
In principle, any carbide, nitride or oxide can be used as the interlayer material.However, compounds such as silicates and even elemental layers such as high - hum carbon can also be used to prepare intermediate layers.Of course, it is necessary to avoid transition metals and other harmful elements, as compound cations or elemental metals in the intermediate layer, because the transition metals will dissolve in the liquid phase Si and become contaminated.
Up to now, only SIQ, SIK and SiC layers have been studied as intermediate layers.SIQ can be formed by plasma enhanced chemical vapor deposition PECVD or commercial spin on glass ( SOG ) using thermal oxidation.However, only PECVD is used to deposit the SiNi layer.As the accumulation of siq and sik, Ono ( siq / sik / siq ) has also been studied m' 26, 34.The SiQ layer and SIR layer are not conductive, while the conductive SiC layer is prepared by PECVD at low temperature or APCVD at high temperature.The thickness of the intermediate layer ranges from 0.54 for low temperature deposition to 10 households for high temperature deposition of SiC.
Light trapping structure
Because crystalline silicon is an indirect band gap semiconductor, it needs a certain light trapping structure to improve the conversion efficiency of heterogeneous substrate crystalline silicon thin film solar POWER BATTERY.If the optical path length is not increased, the advantages of crystalline silicon film, i.e. the potential of high open-circuit voltage VW under the condition of low material quality requirements, cannot be reflected.As a matter of fact, companies that use low temperature technology to realize commercialization have all focused their research and development on light trapping structures, such as Japanese Zhong Yuan [ 37 ] and German CSG Solar [ 8' 38 ].
By using the analysis method, the light trapping structure combination between the plane front surface and the ideal scattering back reflector is obtained [ 39 ].The core of this light trapping structure is Lambert reflector.The lambertian mirror will reflect uniformly in all directions regardless of the angle of incidence of the light.In the light trapping structure with lambertian mirrors, only a small fraction of scattered light will leave the front surface, while all reflected light with an angle of 0 greater than the critical angle will experience total reflection o in the wavelength range of 8001200 nm, with the critical angle of the si - air interface being about 16 degrees and the critical angle ft of the si - SiO 2 interface being about 24 degrees [ 4 degrees ].
After calculation, the irregular structure such as Lambert mirror can increase the optical path length by 4N2 times, while n is the refractive index of si relative to adjacent materials [ 41' 42 ], and the optical path length of si relative to air increases by about 50 times.Numerical simulation shows that even more complex structures are difficult to exceed the performance of ideal reflective structures such as Lambert mirrors, but in fact, ideal Lambert mirrors are difficult to achieve [ 44 ], and the more realistic scattering back mirror simulation uses von model [ 5' 45 ] o moreover, the middle layer on the back surface of the si layer is not a mirror but a dielectric mirror.The dielectric mirror will have transmission loss, and the critical angle depends on the refractive index n.The transmission loss is a big disadvantage for the intermediate layer ( n city 5 of sic ) made of high refractive index materials.
The numerical calculation shows that the most effective light trapping structure is that both the front surface and the middle layer have a gold tower structure?
The combination of the front surface of the pyramid structure and the middle layer of the planar mirror is often used as a light trapping structure' based on the model substrate as shown in actual .2 ..The etched polycrystalline silicon wafer and ceramic substrate have enough surface roughness, and the corresponding light trapping junction is between actual .2 and actual .2.Because the grain growth direction obtained by zone melting recrystallization ZMR tends to U00 ) crystal plane, the textured surface of Si surface can be easily obtained.Through KOH and isopropanol alkaline solution, the required front surface pyramid texture' impurity diffusion' can be prepared by standard anisotropic wet chemical etching method
The intermediate layer not only has the function of trapping light, but also is a diffusion barrier layer for substrate impurities.In principle, in a heterogeneous substrate crystalline silicon thin film solar cell, any chemical element is impurity pollution to the Si active layer, and the impurity element we focus on is defined by the following three principles:
Elements with high concentration in raw materials: transition metals Fe, Ni, Ti and other elements A1 exist in metallurgical grade silicon Mg - Si and Si - based ceramics [ 46 ].The concentration of these impurity elements can reach up to 102 CNT 3, and most of them are easy to migrate at high temperatures.Even if a small amount of impurities are transported to the Si active layer, it will have a great negative impact on the performance of the heterogeneous substrate crystalline silicon thin film solar cell.
It is harmful to the performance of solar POWER BATTERY: among all the polluting elements, the transition metal element is particularly harmful and will seriously reduce the conversion efficiency of solar POWER BATTERY [ 47 ].
The diffusion speed of impurity elements is faster: in the heterogeneous substrate crystalline silicon thin film solar cell, the diffusion speed of transition metal in Si increases with the temperature rising, especially faster than that of other elements at high temperature.Among them, Fe, Cr and V have fairly high diffusion coefficients. If the impurity elements Fe, CI or V pass through the intermediate layer, they will be almost uniformly distributed in the Si active layer in a very short time.Ti also has a large diffusion coefficient, but tends to separate on the Si surface, so it is not as easy to migrate as Fe, Cr and V [ 5.
According to these three standards, transition metals Fe, Cr and V are the most important impurities in heterogeneous substrate crystalline silicon thin film solar POWER BATTERY.Therefore, the intermediate layer needs to act as an effective diffusion barrier to the diffusion of these impurity elements.SiO _ 2, sin, and sic have not done much systematic research on quantitative parameters of Fe, Cr, and v diffusion barrier properties, but have done less research at the high temperatures required for heterogeneous substrate crystalline silicon thin film solar POWER BATTERY.However, some non-solar research reports the data of diffusion barrier layer [ 51 ]. Detailed experimental information shows the temperature-dependent diffusion coefficients of Fe, Cr, V, SiO 2 and SiN. The intermediate layer is prepared by plasma enhanced chemical vapor deposition PECVD.The report also gives simulation results, proving SiO 2 and Qiao!It can effectively stop the diffusion of impurities.Moreover, the basic process of heterogeneous substrate crystalline silicon thin film solar cell described in Section 2.1 can be used.If the intermediate layer is prepared by PECVD, the SiO 2 diffusion barrier layer is more effective than SiN. A more practical method can measure the effectiveness of the diffusion barrier layer.That is, after all processes are completed, the Si layer deposited on the heterogeneous substrate is subjected to composition analysis.Glowdischaragemassspectroscopy ( GDMS ) measured the 2 - thick SiO 2 interlayer L2 deposited by PECVD on silicon-infiltrated silicon carbide SIIC ceramic substrate to obtain ZMR and impurity concentration distribution of the deposited layer after epitaxial growth.In the GDMS method, sputtering will form a crater that enters the sample from the depth of the surface, thus realizing the continuous analysis of the chemical composition of the material in the crater. Therefore, the horizontal sputtering time has a certain relationship with the depth of the sample, but cannot be directly equivalent.GDMS measurements show that the ceramic substrate is contaminated with a large amount of impurities, including B and transition metals Fe, Ti, V, Cr, Mn and Ni.In the diffusion barrier layer and the back surface field BSF ( p + - si region in the figure ), the concentration of all impurity elements is greatly reduced, and only Fe and Ni concentrations exceeding 0.01 ppma are detected in the base ( partsperemillationatoms, impurity concentration ppma equals 5x1016cm _ 3 ).This value is equivalent to a reduction of 23 orders of magnitude compared to the concentration in the substrate.This study also reported that the application of ONC intermediate layer can also reduce the transition metal concentration in the p + region, while the impurity concentration in the epitaxial layer is below the detection limit.Composition analysis of heterogeneous substrate crystalline silicon thin film solar cell structure is a good method to measure the effectiveness of diffusion barrier layer.Research and analysis show that SiO _ 2 and sin layers are effective diffusion barriers and can even be applied to ceramic substrates with high impurity concentrations and extremely high process temperatures greater than 1450 c.However, the study of SiC interlayer is not enough.
Zone melting recrystallization
For the heterogeneous substrate crystalline silicon thin film solar cell discussed in this chapter, zone melting recrystallization ZMR is a core technology.This process largely determines the crystalline and electrical quality of Si films.This section begins with the development of ZMR, and then introduces the growth process and characteristics of ZMR film.Then, the effect of scanning speed on film quality is discussed, which is the main determinant of process cost.Then it demonstrates the concrete implementation of ZMR system. In - situ process control is an important feature of automatic high-speed process.The ZMR results on ceramic substrates will be presented at the end of this section.
Development of Zone Melting Recrystallization
If Si thin films are deposited on heterogeneous substrates by chemical vapor deposition CVD at high temperatures, typical grain sizes are on the order of magnitude.Up to now, using the traditional crystalline silicon thin film solar cell process, this Si thin film has been prepared as a P - N junction, and the highest conversion efficiency is less than 6 % [ 26' 53' 54 ].However, by solid-phase recrystallization or liquid-phase recrystallization, the grain size can be increased by several orders of magnitude, thus realizing a heterogeneous substrate crystalline silicon thin film solar cell with higher conversion efficiency.
Recrystallization technology applied to heterogeneous substrate crystalline silicon thin film solar POWER BATTERY needs to meet certain requirements, and the cost of the recrystallization process steps is less than 20 / m2.
The production rate of relevant equipment is at least 5m2 / h;
Recrystallization equipment cannot be too complicated and has high reliability.The process needs to be controlled automatically.
A grain size greater than 100 pm is achieved.
ZMR is one of the few technologies that meet the above requirements.The heat source in this design is a linear halogen lamp.The common principle of all ZMR methods is to focus the heat source on a narrow sample area and scan to melt the film in this area.However, after passing through the focus line, the melt cools to below the melting point and uneven nucleation occurs, resulting in stable grains that can be used as seed layers for subsequent epitaxial growth.Because of the anisotropy of crystal growth rate [ 55 ], grains with a specific crystal orientation quickly become the majority.This geometric selection method has been known in the Bridgman process, which is a better method for polycrystalline silicon ingot growth of crystalline silicon solar POWER BATTERY [ 57 ].The grain size of ZMR growth can reach several mm wide and several cm long.
The pioneer of ZMR technology was Leitz, who applied for a patent for the growth of single crystal thin films of luminescent materials as early as 1950 [ 58 ].In the 1960s, ZMR was used to grow low melting point semiconductors such as Ge or InSb [ 59 ], and ZMR experiments for Si growth were not successful.However, ZMR technology once again became a research focus in the 1980s and was used to develop silicon-on-insulator ( SOI ) thin films, making the complementary metal oxide semiconductor ( CMOS ) structure of high-speed integrated circuits very small, with lower capacitance and avoiding parasitic transistors. SOI thin films are also used in various high-power devices prepared by insulator #.Givargizov has a comprehensive comment on the results of this stage of research ⑽.Miaou lis et al. also established a theoretical model for ZMR SOI growth [ 61 ].
In the application of SOI technology, ZMR wafers cannot be used with oxygen injection isolation SIMOX or wafer bonding.However, several research teams have tested four different ZMR technologies in heterogeneous substrate crystalline silicon thin film solar POWER BATTERY, as shown in actual .1.
Among the four ZMR technologies developed, the main difference is the difference in heat sources:
Electron beam: The electron beam is generated in a vacuum environment and can melt Si film very quickly [ 62 ].The scanning speed is as high as several cm / s, and the thermal load of the substrate can be lower.The defect density of the crystal grain is relatively high, but due to the lower substrate temperature.The electron beam ZMR is particularly suitable for glass substrates.
Laser: The laser beam scans the Si layer in the two-dimensional direction to form a melting region of Si [ 63 ].Because the laser focus points need to overlap, the scanning speed formed is relatively low and the production rate is limited.Similar to the electron beam ZMR, heating and cooling are very fast, and the thermal load of the substrate is also very low, forming crystals with high defect density.Laser ZMR is also suitable for glass substrates.
Graphite Strip Heater: Under vacuum or inert gas atmosphere, heating graphite strips close to Si surface with electric current as ZMR heater substrate has higher thermal load, but has smaller temperature gradient and slower cooling.The ZMR realized by graphite strip heater can make the crystal quality very high.
Linear halogen lamps: Incoherent light sources such as linear halogen lamps are often focused through an optical system as a heat source for regional melting.The characteristics of the Si layer formed are similar to those of graphite strip heaters, but higher reliability and better process control can be achieved, and the linear halogen tungsten lamp can completely avoid C pollution of the Si layer.The following sections will focus on ZMR process of linear halogen lamps.
With regard to the development of ZMR technology, it is worth mentioning Japanese mitsubishi electric Research [ 65 ].The research team used graphite strip heaters and non-coherent light sources to heat at the same time to realize a thin-film solar cell with a high-temperature route ZMR, TF FIM 1cmx LCM conversion efficiency of 16.45 % m and an area of 10cmxlOCM conversion efficiency of 16 % [ w ] - ZMR technology has made progress mainly in three aspects in the past 10 years:
Equipment development;
Process control;
Numerical simulation o
Numerical simulation benefits from advances in computer technology.A more detailed description of progress in these areas' see Section 2.3.2' Section 2.3.41 and Section.
Mechanism of Zone Melting Recrystallization
In order to understand the mechanism of zone melting recrystallization ZMR, it is necessary to analyze the morphologies of ZMR grown films through experiments.The ZMR - grown thin film will show typical defect structure by shot etching.The etching of Si thin film is a preferential etching or selective etching.According to the ratio of 2: 1, 48 % HF solution and 0.15 mol solution of k2cr 2o 7 ( 1 mol solute and 1l water solution ) can be mixed to obtain a shot etching solution [ the black line defect appearing in the shot etching fen film is a small angle grain boundary.Small angle grain boundary refers to grain boundary formed by dislocation angle less than 11.Because some grain boundaries are present in the same grain, they are also called subgrain boundaries.Sub - crystal boundaries are parallel to the scanning direction and are regularly spaced apart from each other in most cases.Different subgrain boundaries will merge into Y - type condensation, and new grain boundaries will also appear.The origin of the subgrain boundary has been studied experimentally and theoretically [ 6975 ].Givargizov's works also summarize different theoretical models ⑽.Optical microscope ( OpticalMicrost Knoy ) is a type of microscope that uses visible light and lens system to display large and small sample images and observe them directly with the naked eye.It is also one of the oldest microscopes, and its design can be traced back to about 1600 years.
The formation of subgrain boundary can be further understood by analyzing the morphology of crystal surface.The appearance of subcrystal boundary structure was first explained by quenching experiments [ 7' 73 ]. The structure after rapid cooling is considered to be an image of the crystal plane, and the morphology is not a complete plane but a facet growth.Subsequently, the in-situ observation of the CCD camera ( ChargeCoupledVicecamera ) gave further information.The CCD camera was first applied to the laser beam ZMR [ W ], then to the graphite strip heater [ 74 ] and the incoherent light source [ 8 ].Different from the results of quenching, in-situ observation gives information of transient development and stability of crystal surface.In - situ observation is also the basis of advanced process control ( see section ).
At present, most people think that the formation of sub-crystal boundary and the specific crystal surface morphology are caused by the instability of solid-liquid interface supercooling.Over - cooling means that even if a liquid is cooled at a temperature below freezing point, it still remains in its original liquid state.
Leamy et al. M and Lemons et al [ 8Y attributed the formation of sub-crystal boundary to the supercooling of the component due to impurity separation in front of the crystal, which is well known in the Czochralski growth technique, and will form micropore morphology 1183' 8430. However, non-planar morphology can also be observed in static heating, while there is no supercooling of the component in static heating.IM et al. confirmed by careful microscopic in-situ observation that the static graphite strip heater ZMR will form a non-planar solidification surface morphology M.While im et al. attributed the observed morphology to " radiation" supercooling o in such a model, supercooling was interpreted as being caused by different reflectivities of the solid phase and the liquid phase.Reflectivity I < ( 0.6 ) of liquid phase si is calculated from refractive index and extinction coefficient of wavelength a = 0.6 households [ 85 ].
For wavelengths a > 0.6, the extinction coefficient of solid phase si can be ignored [ 1 ] and k ( 0.6fzm ) = 30 %.More detailed analysis requires further consideration of the corresponding radiance because the temperatures of the S film and the filament are different M ..Then, the reflectivity of liquid phase S is much higher than that of solid phase Si by 70 % and 30 %.Therefore, the radiation absorbed by the liquid phase Si is smaller than that of the solid phase Si, and supercooling of the liquid phase Si and overheating of the solid phase S occur simultaneously at the solid-liquid phase interface.
Although the mechanisms of component supercooling and radiation supercooling are different, both theoretical models will lead to the same results, and both have the basic relationship between the temperature gradient of the solid-liquid phase interface and the depth of supercooling region ( WD ).For an explanation of this relationship, see Practical 5.Assuming that the initial plane of the solid-liquid phase interface is at the A - line position, this structure is very unstable and perturbations will form prominent, so that the supercooling area will expand to the B - line position, and the depth of the supercooling area will be such that the solid-liquid phase interface can self-adjust and become a more stable form.For the cell structure shown in actual. 5, most Si near solidification plane A line is solid phase and most Si at relative position B line is liquid phase within the supercooling region.The maximum distance between the growing tip and the inner corner of the cell was measured as the depth of the supercooled region.In both cases, the depth of the supercooling zone depends on the temperature gradient of the solid-liquid interface.The smaller the temperature gradient, the greater the depth of the supercooling area.When the temperature rises, the change of interface morphology can be observed, from dendrite to cell crystal and then to flat crystal % 873.The terminology of crystal growth morphology is somewhat inconsistent.Some authors will distinguish between cell growth and facet growth [ 8 ].Here, according to a certain naming method [ 6 ], we believe that cell growth includes circular growth and facet growth.For the actual small-plane cell-crystal interface shown in .5, the sub-crystal boundary spacing;and that depth WSC of the supercool area.Such a small-plane cell structure has the lowest defect density and is confirmed by experimental results [ 61' 87' 88 ].However, the film growth at the dendrite or flat interface has a much larger defect density.
The choice of heat source has an important influence on the temperature gradient and film quality, as shown in actual .1.As a heat source, graphite strip heater or linear crane lamp can form a lower temperature gradient than laser or electron beam heat source, thus obtaining better crystallization quality.
Another practical aspect discussed in. 1 is common impurity pollution.In the ZMR process, a typical Si film is in contact with at least one SiC ) 2 layer:
SiO _ 2 intermediate layer;
SiO 2 or SiN coating to prevent aggregation of molten Si and balling - upeffect.
Therefore, it can be found in the recrystallization layer that the O concentration even reaches the dissolution limit [ 71,9? \ For the graphite strip heater ZMR, the graphite strip is another source of pollution, resulting in a higher C concentration.
As mentioned earlier, the most stable ZMR growth and the lowest defect density' can be observed in the interface morphology of the cell crystal. In the case of a small-plane cell crystal, the interface consists of { 111 } crystal planes and is the crystal plane with the lowest Si growth rate.Both the scanning direction and the surface normal are in < 100 > crystal direction.The preferred orientation of grains in polycrystalline solids will affect the orientation of textured surfaces.
The experimental study shows that the percentage of crystal grains on { 100 } crystal plane depends on the thickness of A film and scanning speed [ 9 ].If the thickness of the thin film is less than the critical thickness of 25 households, the < 100 > crystal orientation will have an absolute advantage based on the anisotropic free energy of the interface between crystalline silicon and SiO 2, and Biegelsen et al explained the preferred orientation ⑽.By anisotropic etching, the < 100 > textured surface can be simply prepared into a light trapping structure.
Zone melting recrystallization seed layer
Grain size increases
The most important role of zone melting recrystallization ZMR is to increase the grain size of the seed layer. See Section 2.3.2 for an explanation of its mechanism.After the seed layer before and after ZMR is etched by shot etching, the cross section of its crystal structure can be observed:
Just - after - deposition layer before ZMR: In a typical just - after - deposition layer, the grain size is on the order of magnitude, and the first 5 PAN part contains smaller submicron-sized grains.
Differential Interference Phase Difference Microscopy ( DIC ) is a microscope technique for observing uncontaminated transparent samples.DIC decomposes two mutually orthogonal polarized beams from a polarized light source. The two coherent beams irradiate the plane of the sample at a certain distance, pass through the sample and then merge to interfere. The observed interference is sensitive to the optical path difference.Because the optical path difference is the product of the refractive index and the geometric optical path length, the phase difference determined by the optical path difference will change the pattern of the interference fringes and thus give an accurate image of the topography.
ZMR is particularly effective for the 15 - thick seed layer grown on SiO _ 2 - plated monocrystalline silicon substrate.Under the optimized condition of ZMR, the seed layer obtained is even close to monocrystalline silicon.The optimum conditions for ZMR are:
The melting region has a very uniform power distribution;
The thickness of the substrate and each layer of film is very uniform.
The covering layer is very stable, and some reports even use SiO _ 2 / sin _ 3 ¢ lamination.
For the case close to the optimized conditions, the grain width of the obtained seed layer is about 1cm, the grain length is close to the substrate length, the grain size slightly deviates from the ideal < 100 >, and the grain area increases 109 times from about 1mm2 to about 10cm2.If the radiation and temperature have a certain distribution change or the film thickness is not uniform, the more deviation from ZMR optimization conditions, the less ideal results will be achieved and the grain size will be further reduced.For heterogeneous substrate crystalline silicon thin film solar POWER BATTERY, very strict cost requirements will use non-ideal substrates, ZMR devices that will not be overly complicated, and non-uniform seed layers, which are typical results of these conditions.The grain width is generally in the range of 0.55 mm and the grain length is on the order of several cm, still increasing the grain area by 107108 times.The grains are no longer perfectly parallel, but will tilt relative to the scanning direction, which causes new grains to be formed in the intersecting areas of the grains tilted to each other, so that the grown grains are generally shorter than the length of the substrate.At the edge of the actual. 7 sample, the S layer did not undergo recrystallization, and the surface showed uneven light gray, clearly indicating the increasing effect of ZMR on grain size.
Dislocation density of seed layer
The formation mechanism of grain boundary defects in ZMR process is discussed.After the ZMR process step, the highly doped Si thin film needs to be epitaxially grown to form a normally doped thicker Si thin film in order to prepare a heterogeneous substrate crystalline silicon thin film solar cell.Therefore, the relationship between defects of ZMR seed layer and epitaxially grown thin films is very important.
The ZMR seed layer after polishing and etching is shown in actual .8 ..Because dislocation density is macroscopically measured as etch pit density ( EPD ), EPD is much higher than background and can be observed as black stripes.Two typical grains can be distinguished, and their EPD is very different:
Grain A: There is an obvious included angle between the sub-crystal boundary direction and the scanning direction. Jetd background value is about 1x106 cm - 2, while EPD in high dislocation density area is 5x107 cm - 2, which is 50 times larger than EPD background value.Grain B: The sub-crystal boundary direction is almost parallel to the scanning direction, and the background value is similar to that of Grain A, while the EPD in the high dislocation density region is 7x106 cm - 2, which is 7 times larger than the background value of EPD, the area of high dislocation density is much smaller than that of Grain A, and the EPD value is several times smaller than that of Grain A ..
In fact, the black stripe region with high dislocation density in .8 is the sub-crystal boundary of ZMR seed layer and will extend to the epitaxial layer grown on the crystal layer.According to the image of the inclined cross section, the high dislocation density region of the epitaxial layer does come from the sub-crystal boundary of the ZMR seed layer.
It is found that the dislocation density in the seed layer and the epitaxial layer has a clear correlation, and the dislocation density in the epitaxial layer is higher than that in the seed layer.According to the dislocation density, it can be divided into three cases:
There is no clear subgrain boundary in the seed layer and no obvious dislocation density distribution in the epitaxial layer.
In the process of preparing heterogeneous substrate crystalline silicon thin film solar cell, Si thin film needs to be epitaxially grown on ZMR seed layer.Through the study of ZMR seed layer, we realized that the optimized ZMR process is the key technology to ensure the high quality epitaxial layer, so as to realize the uniform film growth on the uniform substrate.Therefore, it is necessary to strengthen research, develop and implement these requirements, and meet the cost constraints.
Melting Recrystallization in High Scanning Speed Zone
Within the allowable range of zone melting recrystallization ZMR process adjustment, higher costs can be directly translated into an increase in scanning speed.In principle, ZMR is not a high investment process, so the main investment cost comes from the process cost per unit area.Further calculation shows that in order to make ZMR cost less than 10 / m2, the scanning speed of the process must be higher than 100t nm / min and the production rate of each equipment must be higher than 10000 m2 / a [ 9s ].Although the scanning speed reported in most literatures is in the 1020 mm / min range, in the early 1990s, Japanese mitsubishi electric studied the effect of higher scanning speed on crystal structure and defect density 91'.They focus on the relatively thin seed layer with a maximum thickness of 1 mm and a maximum scanning speed of 360 mm / min.The main results of the research work in mitsubishi electric are as follows:
For films with a thickness greater than lmm, an increase in scanning speed is harmful because even a moderate scanning speed will greatly increase the defect density.
The thinner the seed layer, the smaller the defect density increases.When the film thickness is about 0.3 mm and the defect density is less than 106 cm - 2, a high scanning speed can be achieved.
This result is suitable for ideal substrates, and higher crystal quality requires thin films.However, the low-cost substrate used in the heterogeneous substrate crystalline silicon thin film solar cell does not conform to this research situation.Due to the high surface roughness, the very thin ZMR seed layer is not compatible with low-cost substrates.The typical average surface roughness of the low-cost substrate is in the range of 25pm, which is nearly 10 times higher than the optimized seed layer of the mitsubishi electric experiment.
FHG - ISE of the Hoff Solar System Research Institute in Flawn, Germany, conducted a similar study to mitsubishi electric, but the seed layer thickness was 2 and 8 mm and the scanning speed was in the range of 10 - 100 mm / min m.Although the relationship between scanning speed and etch pit density EPD is consistent with mitsubishi electric's research results, there are some contradictions in the conclusion about thickness: the seed layer with 8 thickness has better crystal quality than the seed layer with 2 @ thickness.The latest research focus of FHG - ISE research team shifted from seed layer thickness ( 2PM and 8F IM thick seed layer ) to scanning speed ( 20400 mm / min ) and EPD difference before and after epitaxial layer preparation.
The influence of scanning speed * degree on the morphology of melting region: the morphology of melting region depends on scanning speed in three aspects: width, facet growth size and geometric stability.First of all, the higher the scanning speed, the wider the melting area, thus realizing complete melting and facet growth.When the scanning speed team = 20 mm / min, the width of the melting zone is 1.1 mm, while when the scanning speed vs = 200 mm / min, the width of the melting zone is 3.3 mm, an increase of 3 times.Secondly, the average cell size of facet growth also increases with scanning speed.According to Section 2.3.2, the size of cell crystal growth is directly related to the distance between sub-crystal boundaries, so higher scanning speed will result in lower defect density and slower temperature gradient change at the liquid-solid phase interface.According to the mechanism of component supercooling and radiation supercooling in Section 2.3.2, a lower temperature gradient on the solidification surface will result in a larger cell size.Finally, a larger scanning speed will reduce the geometric stability of facet growth.As the scanning speed increases, the facets will move laterally, disappear or combine with each other more frequently.For the sub-crystal boundary, this means that at higher scanning speeds, the sub-crystal boundary is no longer relatively parallel, but is more frequently combined or split.As a result, when the scanning speed increases, the increase of cell size and the decrease of defect density will be compensated or overcompensated by the influence of geometric instability.
Effect of scanning speed on grain shape: After ZMR with scanning speed of 20 mm / min and 300 mm / min, a 12 - thick seed layer was obtained.Obviously, a more regular structure can be obtained at a low scanning speed, which illustrates the geometric stability of the above-mentioned facet growth.In the sample with low scanning speed, the typical grain width is in the range of 13 mm, the maximum width reached by a single grain is 13 mm, the typical grain length is several cm, and there are almost no small grains smaller than 1 mm in size.However, in the sample with high scanning speed, the maximum grain width is several mm, small grains with dark spots or dark lines can be observed in the large-sized grains, and grains with different colors have different crystal orientations.The enlarged observation shows that the grain size is still in the hundreds of households, so such grain size will not affect the performance of the heterogeneous substrate crystalline silicon thin film solar cell too much.
The effect of scanning speed on defect density: As mentioned earlier, grain-to-grain defects are the most important factor affecting the heterogeneous substrate crystalline silicon thin film solar cell.The number distribution of etch pit density epd is a very useful method to describe the defect density of the layer.The limit of EPD only affects the actual epitaxial layer of. L3, making the portion of the number distribution curve exceeding IO 7 CNT2 and the EPD average smaller.In practice. 13, the average EPD value of ZMR seed layer does not obviously depend on the scanning speed of 50350 mm / min, and the average EPD value of 3 - 5 Xi O6 cm " 2 still meets the requirements of heterogeneous substrate crystalline silicon thin film solar POWER BATTERY.In fact. 13 shows that the EPD average value of the epitaxial layer is about 3xio 7 cm - 2, which is almost an order of magnitude larger than that of the zmr seed layer.This result is consistent with the data given by mitsubishi electric [ 99 ].Moreover, this also confirms the discussion that the defect density increases in the epitaxial layer ( see section ).The EPD of ZMR seed layer is obviously better than that of epitaxial layer. This discovery expands the conclusion in the Festival: considering defect density, a ZMR process with high scanning speed and high crystal quality is to ignore the epitaxial growth process steps and directly use the thickened ZMR seed layer as the active layer of a heterogeneous substrate crystalline silicon thin film solar cell.Of course, such an approach will cause further problems, such as the solar cell performance of ZMR seed layer and the reduction of surface recombination speed on the back surface are very important to improve the conversion efficiency of heterogeneous substrate crystalline silicon thin film solar POWER BATTERY.
Zone melting recrystallization equipment
As mentioned earlier, ZMR with non-coherent light source such as linear halogen lamp as heat source can produce the best quality seed crystal layer with ideal grain structure, defect density and process-related pollution.Therefore, ZMR process of linear halogen lamp heat source has realized the best performance of heterogeneous substrate crystalline silicon thin film solar cell, which is being studied by several research and development teams.The main problem in the development of ZMR equipment is to increase the production rate. Therefore, several aspects should be studied:
Enlarge the film width;
Reduce sample loading and unloading time;Increase ZMR scanning speed;
Improve automatic process control.
At present, ZMR is probably the most advanced ZMR equipment in these aspects and is running FHG - ISE at Hoff Solar System Research Institute in Flawn, Germany [ 98.1 Gas Equipment Configuration
In the early 1980s, it was reported that S thin films were prepared by zone melting recrystallization ZMR using non-coherent light sources as heat sources for silicon-on-insulator SOI applications in microelectronics [ 1 1 ].Ten years later, ZMR was applied to the research of heterogeneous substrate crystalline silicon thin film solar POWER BATTERY, first reported by Japanese mitsubishi electric in 1996 [ 1 2 ], and then reported by German Flawn Hoff Solar System Research Institute FHG - ISE in 1998 [ 94 ].FHG - ISE reported an improved equipment process [ 1 3 ] in 2001.Although there are several versions of ZMR equipment, the core wells of the configuration have not changed.The elliptical reflector O linear tooth tungsten lamp with cylindrical focusing mirror is placed on the first focusing line of the elliptical reflector, while the Si film sample is placed on the second focusing line.The bottom heater is a heater array composed of linear tungsten halogen lamps to uniformly preheat the sample.The sample moves on a second focus line perpendicular to the elliptical mirror to complete the ZMR.By observing the melting area formed by the focusing heater with a CCD camera, the width of the melting area can be controlled by image analysis.The mitsubishi electric research team used hardware analysis equipment to control the width of the melting area, while FHG - ISE used visual software to achieve similar functions ( see Section ).mitsubishi electric can achieve a uniform ZMR sample size of 6 " ( about 150 mm in diameter ), while FHG - ISE is 100 mm ..
The upgraded equipment can handle a substrate with a maximum width of 400 mm [ 1d ], this equipment called " zmr 400" has a larger furnace body and a corresponding larger linear halogen crane lamp, the loading part has a load lock, and the sample transport has a feed-through of migwe t, but other principles applicable to small-sized equipment have not changed.The feedthrough is an electronic, physical or mechanical connection between the outside and the vacuum chamber.ZMR 400 achieves uniform recrystallization of samples larger than 300 mm in width, but the production rate is low.
Therefore, ZM R4 ⑺ is extended to a completely on-line type. This continuous ZMR process is called " ZMR 400 CON", where CON means continuous [ 98 ].At the front end and rear end of the furnace body, the load lock is replaced by a gas curtain, and the sample is transported from the loading part to the unloading part through a stepping mechanism.The bottom heater can control the power density in the transport direction and the vertical transport direction, thus improving the lateral uniformity of the melting area.mitsubishi electric proved that such bottom heater configuration is effective for ZMR equipment.The improved sample loading and transportation system raised the production rate of ZMR 400 CON to a maximum of 10m2 / h.
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