Rechargeable Battery

diffusion length
The transport of minority carriers is often described by diffusion length.In polycrystalline silicon, the diffusion length cannot be directly obtained from the material properties, but is the result of different diffusion and recombination mechanisms in different regions of the device.It is necessary to define a comprehensive effective diffusion length LEO among the existing different models [ 4 ], and the model based on the Shackley' SFILAMENTTHEORY' is accurate, concise and most practical.According to Schotley's filament theory model, polysilicon material consists of an array of grains with the same square cross section.
Where ln is the electron diffusion length between grains;6。Is the first solution of the following equation:
where A is the grain size;SSI is the speed of grain boundary recombination.Dn is the electron diffusion coefficient between grains.The equation for smaller A and medium SGB ( SGBA / D, 4 ) and relatively larger LN, LK can be simplified to:
In order to achieve high conversion efficiency, polysilicon thin film solar Rechargeable Battery need to be much higher than the thickness of the active layer, and at the same time, the recombination speed of the back surface needs to reach medium.Since the active layer is generally only a few thick layers, the requirements are not as high as those of crystalline silicon solar Rechargeable Battery, and 10 is sufficient to achieve high conversion efficiency.
numerical simulation
Several theoretical papers reported the numerical simulation of polycrystalline silicon thin film solar Rechargeable Battery, with emphasis on the effects of grain size, grain boundary recombination velocity and dislocation density [ 8 - 12 ].Recently, some literatures discussed the effect of doping peaks formed by preferential diffusion of dopants along grain boundaries on the performance of polycrystalline silicon thin film solar Rechargeable Battery during emitter preparation, and analyzed the cell performance by numerical simulation ( see section 3.5.2 ).The doping peak of the columnar structure penetrates into some or all of the active layer along the grain boundary, and the formed structure is called a three-dimensional emitter ( Treble ) cell based on local enhanced diffusion [ 13 ].Under the condition of short minority carrier diffusion length, the vertical junction formed by preferential diffusion of dopants can enhance the collection of carriers in the material, thus increasing the conversion efficiency of polysilicon thin film solar Rechargeable Battery.Assuming that the thickness of P - type base with acceptor doping concentration 7VA = 1017 cm _ 3 is 8 mm, the thickness of P + type back surface field BSF with team = IO 19 cm _ 3 is 2, the average grain size is 3 mm, and minority carrier lifetime is 0.5, the dependence of conversion efficiency on grain boundary recombination velocity SGB and grain boundary dopant diffusion depth ( XGB ) is given by numerical simulation.The commercial two-dimensional semiconductor device simulation software ISE - DESSIS realizes such analysis.When SGB = 104 cm / s and XKB = 2.5 mm, the conversion efficiency is about 12 %.When the passivation of grain boundaries is good enough ( SGB = 103 cm / s ), the effect of preferential diffusion of dopants will not be obvious, the conversion efficiency can reach more than 13 %, and it has nothing to do with XGB.
Selection of substrate
Unlike crystalline silicon solar Rechargeable Battery, polycrystalline silicon thin film solar Rechargeable Battery are fabricated on heterogeneous substrates with good mechanical strength.If the substrate material is an insulator and can be produced in a large area, the cost of Rechargeable Battery technology and component production can be reduced by using a single piece of $ link.
The selection of the substrate depends on several important material parameters, as shown in actual .1.Since the main purpose of research and development of thin film solar Rechargeable Battery is to reduce costs, the first parameter to be considered is the substrate price.Secondly, the softening point ( Soften in PO1 )' indicates the maximum temperature the substrate can withstand, and the designed solar cell process temperature needs to be lower than the softening point'. Another key substrate parameter is the thermal expansion coefficient TEC, which needs to be as close as possible to the 4x106 of crystalline silicon. If the TEC of the substrate and the active layer mismatches, the difference in expansion or contraction of the material will cause excessive stress and even cracks or even cracks during the temperature rise or fall.Whether the 7TC requirements are strict or not also depends on the maximum process temperature.If the substrate material is transparent, you can also choose to prepare an upper layer configuration so that human light can enter the device through the transparent substrate.
Sodium - calcium glass is an ideal substrate for polycrystalline silicon thin-film solar Rechargeable Battery. Float glass technology produced on a large scale can achieve very low unit area cost.As the cover plate and back plate in glass / glass laminate assembly, soda lime glass is the standard raw material for the production of crystalline silicon solar cell assembly.The front cover glass allows only a low concentration of metal impurities, preferably tempered ultra-white glass.The softening point of the standard sodium Korean glass is only about 580 C, so it cannot withstand the temperature for too long.Moreover, the soda-lime glass is much larger than Si, which brings problems to the preparation of large-area polycrystalline silicon thin film solar Rechargeable Battery.
In principle, metal substrates are more cost - effective.Stainless steel sheet substrates are also often used in amorphous silicon thin film solar Rechargeable Battery.The TEC of metal is usually much larger than Si, which is prone to cracking and breakage.Conductive substrates seem to be an advantage, making the preparation of electrodes simpler, but not able to be monolithically interconnected, and there seems to be no potential to reduce the cost of component production in this respect.One way to solve this problem is to plate an insulating layer on the conductive metal substrate.Amorphous silicon thin-film solar Rechargeable Battery and microcrystalline silicon thin-film solar Rechargeable Battery use this technology [ 14 ], but there are no reports of its application in polycrystalline silicon thin-film solar Rechargeable Battery.
Borosilicate glass is more suitable to be used as the substrate of polycrystalline silicon thin film solar Rechargeable Battery in many aspects, its tec is closer to si and can withstand process temperatures greater than 800 c.Borosilicate glass can even withstand high temperatures greater than 900 C in a relatively short period of time, such as the defect annealing process step [ 15 ] of CSG Solar in Germany, where borosilicate glass is used.
If the process temperature is required to be > 1000 c, tec of some special glass called " high temperature glass" needs to be well matched with si, and prediction shows that high temperature glass will achieve very low cost in large-scale production.
If the required process temperature is much higher than 1000 C, only the ceramic substrate can be considered.Several kinds of ceramic substrates have been studied.Al2O3 with microcrystalline structure is widely used in electronic packaging industry.Some properties of A12O3 are very suitable for polycrystalline silicon thin film solar Rechargeable Battery. A12O3 not only has high mechanical strength, but also has good diffuse reflection properties.However, the TEC of A12O3 is twice as large as that of Si, which is not conducive to the preparation of large-area batteries.Andalusite ceramic is a solid mixture of a12o3 and SiO 2 in a certain concentration ratio.The optical properties of mullite ceramics are similar to a12o3, but its 7tc matches & very well [ 18 ].Large - scale production can reduce the cost of mullite ceramics to an ideal range.Other N - based ceramic substrates, such as Salem Si AlON ceramics [ 19 ], were also studied and prepared by mixing powders of Si3N4, A1N and A12O3.The TEC of Si AlON ceramics is close to Si, with good mechanical strength and slightly poor optical properties.
Before concluding the discussion of substrate requirements, the author wishes to emphasize a fact clearly expressed in the actual .1.Many literatures mistakenly believe that 600 C is the upper limit of glass substrate processing temperature.However, the broad sense of glass is not limited to low temperature. In fact, many types of special glass can be used in high temperature process.For high temperature process, borosilicate glass, aluminosilicate glass, glass - ceramics and other high temperature glasses can be selected as substrates.
As the indirect band gap semiconductor Si has a smaller absorption coefficient and the active layer of the polysilicon thin film solar cell is much thinner than that of the crystalline silicon solar cell, it is particularly necessary to design a better light trapping structure ( see Section 3.2.1 ).In light trapping structures, light scattering from rough surfaces can maximize internal reflectivity.In Japan Zhong Yuan's suede back mirror enhanced absorption STAR Rechargeable Battery, both the front surface and the back mirror are made of suede ( see section 3.6.2 ), while the CSG solar's glass-on-glass crystalline silicon CSG Rechargeable Battery flat glass is plated with bead-like suede [ 15 ] ( see section 3.6.3 ).Other texture options include sand blasting or shot blasting %, [ E2 ] of plasma texture and chemical reaction kiss involving A1.
Growth of Polysilicon Thin Films
Polysilicon films can be grown using a variety of different technologies to meet the needs of specific semiconductor devices.Technically and economically, photovoltaic applications require that the optimum thickness of the polysilicon film be LOF ZM or less.There are different deposition techniques for growing such polysilicon films on heterogeneous substrates:
chemical vapor deposition CVD;
Plasma enhanced chemical vapor deposition PECVD:
Ion - assisted deposition of IAD;
Liquid phase epitaxy LPE;
Solid Phase Crystallization SPC.
In this section, we will review the different process steps for growing polysilicon thin films and the grain size and preferred orientation of the polysilicon layer.
Grain enlargement technique
In order to give full play to the potential of high conversion efficiency of polysilicon thin film solar Rechargeable Battery, researchers are working hard to obtain polysilicon thin films with ideal performance and hope to obtain the largest grain size possible.If Si is directly deposited on a heterogeneous substrate, the crystal quality is generally poor and the grain size is also small.In order to achieve a larger grain size of 10pm, some grain enlargement techniques are often used.
Nucleation control method
The growth of any form of polycrystalline silicon film on a heterogeneous substrate starts from the nucleation period, and then the crystal nucleus grows until a complete film is formed.In general, the higher the crystal nucleus density during the nucleation period, the smaller the grain size of the polysilicon film.On the other hand, if the density of crystal nuclei is too low and the distance between crystal nuclei is too large, it will take too long to complete the film growth and even no condensation will occur at all.Therefore, in order to obtain an ideal polycrystalline silicon thin film, nucleation control methods need to be adopted early in the crystallization or deposition process, so as to achieve the crystal nucleus density corresponding to the maximum crystal grain size.
Different film growth techniques have different nucleation control methods, which will be discussed separately below.
Seed layer method
Different from the nucleation control method, the seed layer method is another seed layer method to increase the grain size to separate the initial nucleation process from the growth of the active layer.it is first necessary to form a film have a larger grain size, i. e., a see layer.The seed and silks layer catches and locks silks, but the crane's thick and hungry seeds are willing to be used as the active layer of polysilicon thin film solar Rechargeable Battery.After the seed layer is formed, the crystal structure of the seed layer is reproduced by an epitaxial deposition process with faster film growth to form a thicker absorption layer.The seed layer method solves the following two problems separately:
high crystallization quality of the film is realized;
The absorption layer of a specific doping distribution is deposited at a sufficient growth rate.
There are two better ways to grow the seed layer for polysilicon thin film solar Rechargeable Battery:
Laser crystallization;
Aluminum induced crystallization ( AIC ).
Laser crystallization uses laser pulses to locally melt the Si layer and induce crystallization.Since only a small volume of Si melts into a liquid state in a short time, the whole substrate is still at a low temperature, so laser crystallization can be compatible with low-cost low-temperature substrates.Even on ordinary glass substrates, laser crystallization can achieve high electrical quality and large area uniformity, so the electronic industry uses laser crystallization to prepare thin film transistors ( TFT ) for flat panel displays.There are several different techniques for laser crystallization over a large area.At present, the standard technology applied to TFT is excimer laser annealing ( ELA ), and amorphous silicon A - Si: H thin films are crystallized by repeated pulses of a wider excimer laser beam.Because the excimer is not a stable molecule, the excimer laser is a pulsed laser, which is produced by filling the cavity with a mixture of different rare gases and halogen gases with different wavelengths.The grain size obtained by ELA is relatively small ( 0.1 - 0.5 mm ).Some newer laser crystallization techniques are also in the research stage and can produce polycrystalline silicon thin films with better crystal quality and larger grain size, of which the most promising is the sequential lateral solidification ( SLS ) process 25 ].SLS process forms crystal grains in the seed layer for subsequent further lateral crystallization, and local melting and crystallization are spatially and temporally separated from the previous crystallization process.The smaller the thickness, laser power or pulse frequency of the A - Si: H thin film is, the larger the strain rate is, the faster the crystallization is, and the smaller the grain size is [ 26 ].The SLS process has successfully prepared polycrystalline silicon material [ 27 ] with a typical grain size of 24 mm, even reaching a grain size of 7 mm under certain conditions [ 28 \ In principle, if A - Si: H is moderately doped, laser crystallization can be directly used.The research team of Japan's Electronic Technology Research Institute ( ETL ) reported that the conversion efficiency of small-area ceramic substrate polycrystalline silicon thin-film solar Rechargeable Battery is up to 6.5 % [ 29,3 ] through laser crystallization.However, due to the lack of doping distribution control, laser crystallization is difficult to realize the fabrication of large area devices.Since the entire Si volume is in the liquid phase and the resulting doping distribution is uniform throughout the thickness range, it is not possible to achieve the highly doped back surface field BSF required for the lateral transport of majority carriers. Laser crystallization is mainly applied to the preparation of highly doped seed layers for subsequent epitaxial deposition [ 31 \ The main advantage of the laser crystallization process is that a low-cost glass substrate or flexible substrate can be used.Because the speed of scanning the whole device surface with a narrow laser beam is very slow, the key problem in the industrial application of laser crystallization is not only the maintenance cost but also the production rate.However, with the continuous development of laser technology, such problems will be solved.
In addition to laser crystallization, another seed layer method is metal-induced crystallization, which can achieve a larger grain size and ensure the substrate is in a low temperature state.Metals such as Al, Ni or Pd can crystallize A - Si: H at low crystallization temperature to obtain larger grain size [ 32 - 34 ].In the process of Ni metal-induced crystallization, silicide precipitates will form as crystallized crystallizing point in the initial stage of high temperature annealing.Each Ni particle can form a grain, and before the random nucleation is completed, the a - si: h layer is fully crystallized with a grain size of > 100.But.These grains contain many low-angle sub-crystal boundaries and also have serious metal contamination.In order to prevent contamination, the metal-induced crystallization of A1 is more appropriate. Although AIC technology makes the polysilicon film have a higher concentration of A1, A1 can be used as the acceptor dopant of Si and diffuses relatively slowly in the thin film. The back surface field BSF formed by the P + type layer conforms to the design of the polysilicon thin film solar cell.In the AIC process, adjacent Si layers and A1 layers are exchanged with each other, and A - Si: H is converted into polysilicon during high temperature annealing. Therefore, AIC is also called aluminum - induced layer exchange ( ALI LE ) [ 35, 36 ] O Si layers and A1 layers each hundreds of nm thick are generally prepared by evaporation, magnetron sputtering or plasma enhanced chemical vapor deposition PECVD.It is also necessary to form an important A12O3 separator with several nm thickness between the Si layer and the A1 layer. Usually, the A12O3 separator can be formed directly by oxidizing A1 before Si deposition.Focusing ion beam ( FIB ) microscope gives a cross-sectional micrograph of AIC process on SiO 2 substrate.If the correct parameters are selected, after annealing at 500 C for 2h, a continuous polysilicon film can be formed below the mixed layer of A1 and Si, and the layer exchange can be successfully completed.
After the AIC process is completed, the A1 layer can be removed by preferential etching ( e.g., using H3PO4 treatment ).The grain size in the range of pn > 50pm depends on the substrate and process.The aspect ratio of the grains is very small because the grain thickness is only 100500 RUN and depends on the deposited A1 layer.The exact mechanism of AIC is still unclear, but nucleation theory and diffusion finite dynamics can explain AIC to some extent [ 37 ].The following parameters have an effect on AIC process:
Annealing temperature and time;
A1 grain size [ 38' 39 ];
Deposition techniques used [ 4' 41 ];
The surface roughness of the substrate.
If the parameters are selected for rapid crystallization, more crystal nuclei and smaller grain size will appear.Therefore, the final grain structure comes from the trade-off between acceptable process time and optimal crystallization quality.The majority of AIC research work is carried out on glass substrates and glass needs to be as close as possible to the ideal flatness.The AIC process can also be applied to other types of substrates such as ceramics [ 12' 13 ]'. However, excessive surface roughness of the substrate may cause excessive nucleation, which needs to be solved by depositing a flat layer of flowable oxide on the substrate before AIC.The AIC seed layer shows a preferred orientation of < 100 > [ 36, 46, 47 ], which is very favorable for subsequent epitaxial growth.Through several process parameters, the angle of preferred orientation can be adjusted within a certain limit.The secondary crystal grains or islands on the seed layer will form undesirable epitaxial growth and more grain boundaries. The methods for removing the secondary crystal grains include:
removing M by chemical mechanical polishing ( CMP );High temperature ( > 130 C ) ultrasonic H3PO is processed to remove the secondary grain %' If it is confirmed that the secondary grain is completely embedded in A1 layer after AIC process, it can be easily removed by A1 etching [ 51 \ 3.4.2 3.4.2.1 chemical vapor deposition technology for polysilicon film growth
Chemical vapor deposition ( CVD ) is the deposition of gaseous reactants into solid thin films by chemical reaction, which is widely used to grow epitaxial layers in microelectronics and can achieve better dopant concentration and thin film thickness control.The precursors for depositing Si active layer are SiH4, SiH2 Cl 2 or Si HCl 3.A series of Si deposition processes in CVD technology include:
The precursor gas is heated and decomposed into smaller molecules for gas phase reaction;
Gas diffusion;
Surface reaction.
The deposition temperature of CVD is in the 8001200 C range and depends on the precursor.CVD is one of the main technologies to study polycrystalline silicon thin film solar Rechargeable Battery. For a detailed review, please refer to [ 52 ].
CVD can be applied to the above two different grain growth techniques:
In the seed layer method, CVD grows an epitaxial layer on the seed layer;In the nucleation control method, CVD forms the entire absorption layer and performs nucleation control at the beginning of deposition.
As part of the seed layer method, CVD is used relatively directly.Different from other epitaxial layer technologies, CVD can obtain a better epitaxial layer without being affected by the crystal orientation and surface H content of the substrate before epitaxial growth.If the substrate is smooth and continuous, in principle CVD can make the active layer have a lower defect density.This has been confirmed by epitaxial crystalline silicon thin film solar Rechargeable Battery and heterogeneous substrate crystalline silicon thin film solar Rechargeable Battery prepared at high temperatures.For polycrystalline silicon thin film solar Rechargeable Battery prepared by the seed layer method, some recent reports show that if the seed layer has better quality, the seed layer with inconsistent crystal orientation can still grow a better epitaxial layer [ 44 ].
Up to now, most of the work of preparing polysilicon thin film solar Rechargeable Battery by high temperature CVD is direct deposition on heterogeneous substrates.The substrate material capable of obtaining a uniform active layer with a large area is as follows:
The oxidized silicon wafer has a SiOP 5 - 57 surface;
Graphite [ 58 ];
High temperature glass [ 59 ];
Eight, 120.——65］；
Celeron Si AlON ceramics and siliconized silicon oxynitride aluminum ( Si - Si AlON ) ceramics mullite ceramics [ 67' 64 ].
When Si is directly deposited on a heterogeneous substrate, the grain size will be determined by nucleation at the early stage of deposition.During the nucleation period, the formed crystal nucleus begins to capture free Si atoms on the surface of the substrate.In the existing grain growth process, new grains will be formed among them.After coagulation, the grains are further epitaxially grown until the entire active layer is formed.Because the grains have different crystal orientations, the grain boundaries of polycrystalline silicon thin films are formed by the mutual contact of the grains.Scanning electron microscope ( SEM ) is a common electron microscope, which scans the surface of a sample with a grating pattern and forms an image through a high energy electron beam.Electrons interact with the atoms that make up the sample, and the resulting signals contain information such as the surface morphology, composition and conductivity of the sample.
Obviously, the number of crystal nuclei before condensation determines the grain size of the active layer.The lower the crystal nucleus density, the larger the crystal grain size.If the crystal nucleus density is, in CNT 2, and the average grain size of the active layer is LG, in cm, then there is an approximate relationship??Lg_2。When the required average grain size of the active layer is 10 mm = l ( t3cm, the crystal nucleus density should be on the order of 106 cm - 2.
Another factor affecting grain size is competitive growth.After the initial deposition, some grains will grow faster than others.The size of these fast growing grains on the surface is much larger than the size at the substrate interface, forming V - shaped grains.Competitive growth is related to deposition rate depending on crystal orientation.
Finally, grain boundary migration is another potentially important factor affecting grain boundary size.It can be considered that grain growth is not the mutual condensation of small grains, but the result of grain boundary migration.If the deposition process has obvious grain boundary migration, the final grain size can be much larger than the grain size predicted by the crystal nucleus density at the beginning of deposition.Grain boundary migration mainly depends on temperature.Under normal conditions of conventional CVD, grain boundary migration is so slow that it does not play a significant role in the whole deposition process [ SS ].However, some conditions can significantly increase grain boundary migration:
High doping concentration
Point defect injection mechanism, such as oxidation M;A1 alloy
Some studies on nucleation and thin film growth are based on silicon oxide wafers as ideal heterogeneous substrate models, and show that grain size does depend on grain density [ 57.72.73 ].The average grain size of the complete active layer can be predicted fairly accurately by the crystal nucleus density after deposition for several min.In order to achieve a grain size of 10, a crystal nucleus density of about 106 cm - 2 is required.Many experiments show that nucleation on heterogeneous substrates also determines the structure of the final active layer.Because the surface has many preferential nucleation sites, the crystal nucleus density is usually large, so the grain size of the active layer is only 1 or less.Possible nucleation control methods include:
adding HC1 [ 55 ] to the precursor gas;
The intermittent supply method injects precursor gas instead of stable flyer _'
The intermediate layer medium is deposited on the substrate to change nucleation [ 74 ].
Many reports of direct CVD on heterogeneous substrates mention a strong < 110 > preferred orientation, but the observed preferred orientation crystal plane is not limited to high temperature CVD process because it is common for many deposition techniques with high growth rates [ 79 - 82 ].In the active layer with fine grains, the " 110" grains account for the majority, and their characteristics determine the advantages in competitive growth. The { 110 } crystal plane has a higher growth rate, while the U11 } facet grows slowly, and the probability of occurrence is lower. " 83' 84 \ Thin films with a stronger" 110 " preferred orientation usually have too many twins, so the preferred orientation of CVD - prepared active layers means poor material quality.In the active layer containing larger grains, competitive growth is not important, does not have a stronger < 110 > preferred orientation, and tends to contain random orientation e85.
Iodine vapor transport deposition is an alternative to CVD.I steam reacts with Si source to form transport agent SIL 4, SII 4, which decomposes on the substrate to obtain polysilicon thin films [ 86, 87 ].Iodine vapor transport deposition at a temperature of about 9 8 C has a faster growth rate and a larger grain size of about 20pm.
The earlier method of increasing grain size was to deposit Si layer on the surface with lower viscosity.The low viscosity surface increases the mobility of Si adsorbed atoms and enables Si crystal nuclei to rotate on the surface, resulting in large and flat grains with low energy at grain boundaries.Graef, Gilling and BL OEM have introduced liquid-phase chemical vapor deposition ( Cv Doll ) to obtain an active layer with a grain size of up to 100 mm using Sn layer?.CV Doll first proposed M by Rasman IS in the 1960s in the name of liquid phase epitaxy LPE.At the temperature of CVD, large crystal grains can be formed on A1 substrate by using glass melted into liquid phase, and a bipolar device with better performance can be obtained.The CV Doll technology under study has great potential, but there are still problems in controlling viscosity and impurity pollution at high temperatures.
Plasma enhanced chemical vapor deposition
Plasma enhanced chemical vapor deposition PECVD also uses precursor gases, but the energy that breaks chemical bonds comes from plasma.Therefore, the temperature required for PECVD to achieve a certain growth rate is lower than that of high temperature chemical vapor deposition CVD.The high-energy electrons in the plasma collide with the precursor gas molecules and decompose the gas molecules, causing related chemical reactions.Moreover, ion bombardment of the surface of the substrate by positively charged ions in the plasma can change the chemical composition of the surface.The samples of direct plasma enhanced chemical vapor deposition ( D PECVD ) are directly exposed to the plasma, while the samples of remote plasma enhanced chemical vapor deposition ( R PECVD ) are placed in the afterglow position of the plasma. The reaction chamber structures of D PECVD and R PECVD are quite different.The different ranges of plasma excitation frequencies from several kHz to several GHz will give different types of excitation.The most commonly used PECVD reaction chambers are based on radio frequency parallel plates, in which hot wire chemical vapor deposition ( HW CVD ) WL and electron cyclotron resonance chemical vapor deposition ECR - have a high deposition rate and a certain number of bombardment ions.In HW CVD, the filament catalyst, which is several cm away from the substrate surface and heated to 13002000 C, decomposes Sihi and H2 source gases at high temperature.In ECR - CVD, when the frequency of the alternating electric field matches the frequency of the electrons moving around the magnetic lines, the cyclotron resonance will form a high plasma density.
The PECVD process for depositing amorphous silicon A - Si: H or microcrystalline silicon has a lower deposition temperature ( 100300 C ), while the preparation of polysilicon active layer for polysilicon thin film solar Rechargeable Battery requires a higher process temperature.Here, we still need to discuss the seed layer method and the direct deposition method respectively.
PECVD, as an epitaxial technique to thicken the crystal layer, focuses on the epitaxial layer with better quality and thickness.Epitaxial growth at low temperatures is difficult, especially on non-ideal seed layers.On the whole, it is difficult to obtain high-quality epitaxial growth for grains with larger deviation from the < 100 > crystal direction, and epitaxial growth cannot even occur at a distance of several hundred nm.However, at 585' C, Cr - CVD has grown a 70 % area 2 thick epitaxial layer on the seed layer after aluminum-induced crystallization of AIC [ 48 ].
Although PECVD directly deposits polysilicon layer with simple process, high growth rate and low substrate temperature, the research results are less than those of CVD.Fu Yuki et al. studied the effect of deposition conditions on preferential orientation and growth rate on silicon oxide wafer ⑽.REE Hal et al. achieved good results in fabricating polycrystalline silicon thin film solar Rechargeable Battery on 800 C graphite substrate by PECVD. Japanese Zhong Yuan realized a high performance suede back mirror enhanced absorption STAR cell using high temperature PECVD process ( see Section 3.6.2 ).
Ion assisted deposition
Ion - assisted deposition IAD is based on electron gun evaporation of EGE and subsequent partial ionization of Si. Si ions bombard the substrate through a typical acceleration voltage of 20V.Accelerating the energy provided by ions can increase the mobility of adsorbed atoms on the surface.At a low temperature of 435 C, IAD technology can obtain epitaxial deposition with a deposition rate of up to 0.5 mm / min, and achieve a better quality epitaxial layer on the polysilicon substrate at a low temperature.At a deposition rate of 0.8 mm / min and a deposition temperature of 650 C, the epitaxial thin film grown on the < 100 > crystal - oriented silicon wafer IAD has a minority carrier diffusion length of 40 and a dislocation density E95 lower than 103 CNC2.However, for most low-temperature epitaxial deposition techniques, the quality of the epitaxial layer is highly dependent on the substrate orientation [ 96 ], and the epitaxial layer grown on the < 111 > wafer orientation IAD has a dislocation density of at least two orders of magnitude higher.
At a substrate temperature of about 600 C, on borosilicate glass plated with a thicker aluminum-induced crystallization AIC seed layer, IAD prepared a polysilicon thin film solar cell with better performance [ 97 \ Special attention should be paid to preventing surface contamination and passivating the surface with hydrogen, which is critical for low-temperature epitaxial deposition.In the initial stage, a lower deposition rate is needed to realize the epitaxial growth of non-ideal crystal grains.After rapid thermal annealing RTA and hydrogenation, the open-circuit voltage of polysilicon thin-film solar cell samples is up to 420 mV.
rheotaxial
Liquid phase epitaxy LPE, also known as solution growth SG, exposes the substrate to Si saturated metal melt solute and then lowers the temperature. Si precipitates from the metal melt solution due to supersaturation and is deposited on the substrate to form an epitaxially grown Si thin film.The metal solutes used are in, ga, a1, etc. the process temperature range of liquid phase epitaxy LPE is 700950' c.Because the thermodynamic driving force of LPE is relatively small, there will be no unnecessary deposition on the sidewall of the deposition apparatus.
If LPE is carried out on a silicon wafer, the defect density of the epitaxial layer is low and the minority carrier lifetime is long. If LPE is carried out on an epitaxial substrate, it is difficult to achieve the required quality of the device.Because of the small driving force, it is not easy to get enough crystal nuclei on the non-silicon substrate, thus making Si crystal grains relatively isolated and the space between the crystal grains larger, so that a continuous layer cannot be formed, which will affect the preparation of the device.Since a continuous layer can be formed before the LPE, the seed layer method will make the LPE more feasible.Different combinations of seed layer deposition and LPE deposition have been tried, but the research results of active layer have not yet met the requirements of the device.In many cases, Si cannot completely cover the entire substrate.Even if the substrate is completely covered, the Si active layer does not look tight, much like a lot of separated large grains that are close to each other, but are not connected to each other.Kuhn le et al. gave an explanation for this phenomenon [ 1 2 ]. a lower supersaturation of lpe will affect the critical grain size, and a lower supersaturation means a larger critical grain size.Since all the small grains in the seed layer are smaller than the critical grain size, they are dissolved in the metal melt solute during the LPE process, which is favorable for the average grain size, but will form a discontinuous layer.
Solid phase crystallization
Solid Phase Crystallization ( SPC ) of Amorphous Silicon A - Si: H is the most studied and the best technology for preparing polysilicon thin film 1 " 3' 1.A - Si: H can be deposited by plasma enhanced chemical vapor deposition PECVD, sputtering or evaporation.
A typical A - Si: H film is L - 3 mm thick and can be undoped, moderately doped, highly doped or multilayer films with different doping levels.After deposition, the A - Si: H thin film needs to be annealed at 550700 C for tens of hours.The higher the temperature, the faster the full crystallization is achieved, but the smaller the grain size.However, if the temperature is too low, complete crystallization cannot be achieved within the allowed time range.Therefore, the operable temperature compromise is 600 C ..TEM shows the cross-sectional image of SPC polycrystalline silicon film after evaporation of A - Si: H annealing.TEM is a common microscope technology. A beam of electrons transmits through a very thin sample and interacts with the thin sample during transmission. The image formed by this interaction is magnified and focused into imaging devices such as a screen, photographic film or CCD camera.The typical grain size of the solar-grade SPC film is 12 pairs of specific SPC annealing temperatures, and the grain size presents log - normal distribution [ 1971 < ) 9 ], most of which are small grains, and the average grain size is 35 times smaller than the maximum grain size.Moreover, the quality between grains is far from perfect, with many defects such as twins and dislocations, and the electrical quality after growth is poor.If the device is fabricated with such an active layer, the open-circuit voltage will be 200 mV 0 + - 105 \ However, the subsequent annealing and hydrogen passivation processes can greatly improve the electrical quality of such polysilicon and increase V … to more than 500 mV.It is noted that the diffusion of typical dopants such as P, B and A1 is slow at 600 C, while the thermal budget after SPC is still low, and the doping distribution formed during A - Si: H deposition can be maintained after crystallization.
Compared with other types of polycrystalline silicon films, SPC polycrystalline silicon materials show a preferred orientation of < 111 >, which is due to the anisotropic crystallization rate, especially the ability to grow < 111 > crystal grains perpendicular to the substrate.
There are still different ways to improve the material quality of SPC polysilicon layer:
Nucleation control method: during the deposition of the first layer of a - s: h thin film, nucleation is controlled so as to first nucleate and then affect the crystallization of the entire stack.High doping levels can reduce the crystallization threshold, especially in N - type Si.Another important aspect is the appearance of small grains in a - si: h.Japan sanyo motor realized high quality SPC polycrystalline silicon thin film with highly doped n - type initial layer, and the nano-crystal grain has < 110 > preferred orientation [ 1 3 ].
Seed layer method: First prepare a seed layer with larger grain size, then deposit A - Si: H on the seed layer, and finally carry out SPC.This process applied to polysilicon thin films can also be called solid phase epitaxy ( SPE ).SPE has been tried on different types of seed layers, especially aluminum induced crystallization of AIC seed layer [ 111 ], but there is still no standard SPC available.
Compared with other active layer growth technologies, SPC has many advantages:
The process is simple and the cost benefit is realized.
Low process temperature requirements;
the quality of that active lay is relatively high;
easy to carry out large-scale production;
realize in-situ junction growth.
SPC has the disadvantage of long production cycle and is difficult to achieve the structural material quality required by polysilicon thin film solar Rechargeable Battery with a conversion efficiency of 12 %.
Characteristics of Polysilicon Films
The Li-ion rechargeable battery: a perspective【JB Goodenough, KS Park - Journal of the American Chemical …, 2013 - ACS Publications】
Recent advances in rechargeable battery materials: a chemist's perspective【MR Palacin - Chemical Society Reviews, 2009 - pubs.rsc.org】
New Nanostructured Li2S/Silicon Rechargeable Battery with High Specific Energy【Y Yang, MT McDowell, A Jackson, JJ Cha, SS Hong… - Nano …, 2010 - ACS Publications】
Li− air rechargeable battery based on metal-free graphene nanosheet catalysts【E Yoo, H Zhou - ACS nano, 2011 - ACS Publications】
The rechargeable aluminum-ion battery【N Jayaprakash, SK Das, LA Archer - Chemical Communications, 2011 - pubs.rsc.org】
High-Rate LiFePO4 Lithium Rechargeable Battery Promoted by Electrochemically Active Polymers【YH Huang, JB Goodenough - Chemistry of Materials, 2008 - ACS Publications】
Electrolyte roadblocks to a magnesium rechargeable battery【J Muldoon, CB Bucur, AG Oliver, T Sugimoto… - Energy & …, 2012 - pubs.rsc.org】
Rechargeable battery with memory that contains charging sequence data【D Malackowski - US Patent 6,522,101, 2003 - Google Patents】
Emerging N‐Type Redox‐Active Radical Polymer for a Totally Organic Polymer‐Based Rechargeable Battery【T Suga, H Ohshiro, S Sugita, K Oyaizu… - Advanced …, 2009 - Wiley Online Library】
Rechargeable battery pack and operating system【AE Seman Jr, DC Brotto, DT Trinh, F Qin - US Patent 7,941,865, 2011 - Google Patents】
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