12 battery may refer to: Car battery (most batteries for vehicles and UPS are 12 volts) Lantern battery. A23 battery, a small battery (roughly 2/3 of an AAA battery in length) made for RF transmitters.
In this section, we will discuss in depth the important characteristics of polysilicon thin films:
Average grain size;
Grain boundary defect density;
Grain boundary recombination velocity SGB;
Different interpretations of polycrystalline silicon grain boundary effect:
Effect of grain size on open circuit voltage.
Grain size is a key parameter of polysilicon film.The grain size of polysilicon layer is not uniform and needs to be described by distribution.For simplicity, we often use " average grain size".The average grain size is the average of all grain diameters, which is different from the area weighted average grain size obtained by dividing the total area by the number of grains. The experimental method for determining the grain size by a difference of 2 times between the average grain size and the area weighted average grain size is to use grain boundary crosscutting technique in the image obtained by optical microscope or electron microscope [ 68 ].In TEM images of transmission electron microscopy, the grain size is the average of the distances between the two grain boundaries of all visible grains, and the more grains selected, the better.In the SEM top view of the scanning electron microscope, a straight line is used to cross the image, and the grain size is equal to the length of the straight line divided by the number of grain boundaries intersecting the straight line, so that the average grain size can be obtained by repeating it several times.It is found that the average grain size obtained by grain boundary crosscutting technique is slightly higher than the area weighted average grain size.
Composite because two adjacent grains with < 110 > grain orientation can in principle only form lower energy grain boundaries [ 12' 1163 ].However, the experiment does not prove that such materials have better performance. devices with stronger < 110 > preferred orientation films give very low performance and the best performance polysilicon thin film solar 12V battery are based on solid-phase crystallization SPC.However, SPC layer generally does not have " 110" preferred orientation.Reports [ 12,116 ] say that the SPC layer of Japan's sanyo motor has " 110" suede, but the document [ 103,117 ] of sanyo motor does not confirm this, which may be a confusion of the concepts of nucleation layer and active layer, and an inadequate explanation of the superior performance of the " 110" preferred orientation is unnecessary.
Some people think that even if grain boundaries have medium SGH, very small grains ( at the end of grain size distribution ) will have a great negative effect on I.Very small grains can be completely depleted so that Fermi pinning occurs in the center of the band gap [ 118 ].Since these regions extend deep into the base, and SH Ockley - Read - Hall recombination is most effective when the electron concentration and hole concentration are the same order of magnitude, we can think that the performance of the device will be greatly affected.Although this effect may have some importance, it cannot directly explain the difference between polycrystalline silicon and polycrystalline silicon on grain boundary recombination.In fact, MC - Si: H devices include a larger depletion region, and according to this mechanism, they will be affected by a larger recombination, but this is not the case.
The difference in crystal boundary effect between / xc - si: h and polycrystalline silicon materials can be explained by impurity segregation at grain boundaries.Segregation refers to the phenomenon of uneven distribution of each constituent element in the alloy during crystallization.Impurity segregation at grain boundaries can greatly change the electrical properties of polycrystalline silicon semiconductor materials, and it can be considered that the electrical activity at grain boundaries is entirely the result of impurity segregation [ 119 \ If the temperature for preparing polycrystalline silicon thin films is higher, impurity segregation is more obvious.During the annealing process exceeding 6001, O will diffuse to the grain boundary E12 U21 ].If the annealing time or temperature increases, the O concentration on the grain boundary will increase and the grain boundary barrier will increase.
IMEC, IME C98 [ M ] of Belgian Inter - School Microelectronics Research Center;University of New South Wales, Australia ( Unsw ) [ 124 ];IMEC05Cl25]；
ETL [ 3 ] of Japan Electronic Technology Research Institute;
German Institute of Optoelectronics Technology ( IPHT ) [ 126 ].
The effective diffusion length [ 12 ] can be further deduced by using tar etto's method.The theoretical calculation can also give the diffusion length corresponding to SGB, so that the degree of grain boundary recombination can be easily determined.The SBB of Sanyo Motor, Zhong Yuan and CSG Solar data points is less than 10000 cm / s, and the SGB of Sanyo Motor data points is even close to 1000 cm / so, which is different from the report [ 12 ]. We cannot distinguish between the two types of data, and the data points cover the entire SGB range of 1000 - 107 cm / s.The material quality of polysilicon film depends on the degree of hydrogen passivation.The grain boundary of Si: H is passivated by in-situ hydrogen, while the hydrogen passivation of polysilicon film is the latter process.Although this increases the difficulty of preparing polycrystalline silicon thin film solar 12V battery, the passivation of polycrystalline silicon grain boundaries can achieve the same effect as MC _ Si: H ..
Process for batteries and components
Polysilicon thin film solar 12V battery are similar in structure to standard crystalline silicon solar 12V battery, except that the active layer is particularly thin.The structure of the semiconductor device for collecting carriers is also a P - N junction, the emitter is highly doped, and the base is moderately or lightly doped.The base doping concentration of several polysilicon thin film solar cell technologies is very low, on the order of 1015 cm - 3.In general, the base does not need to be intentionally doped, and preferential diffusion of O donor impurities or dopants will result in sufficient base doping concentration.However, the inter-charged region does not extend to the entire base, unlike amorphous silicon thin film solar 12V battery, the main mechanism for collecting photo-generated carriers is not drift but diffusion.The back surface of the polysilicon thin film solar cell has a highly doped back surface field BSF, which facilitates the formation of contact electrodes.If the diffusion length approaches or exceeds the base thickness, the field effect formed by BSF keeps minority carriers away from the back surface.
Like other thin film solar 12V battery, the design of polysilicon thin film solar 12V battery is also divided into an upper layer configuration and a substrate configuration.CSG Solar in Germany uses the upper layer configuration, while Sanyo Motor in Japan uses the substrate configuration.In the upper layer configuration, human light enters the cell through the upper layer glass with supporting function, while the active layer is deposited on the upper layer glass.In the substrate configuration, an active layer is deposited on an opaque substrate having a supporting effect.Although the upper layer configuration here uses the P - type base, the N - type base is also applicable in this configuration.On the contrary, most of the polysilicon thin film solar 12V battery configured on the substrate use P - type bases.The upper layer configuration has some inherent advantages:
If the area is large enough, the upper layer glass with better mechanical strength can be used as the front surface of the assembly without additional cover glass, which can effectively reduce the cost of assembly production.Since neither the front contact nor the back contact covers the active layer, the shielding loss can be minimized or even ignored.The designed metallization pattern can have a lower series resistance and can be used to prepare a wider metal gate line without forming shadows, thus ensuring a sufficient short-circuit current density.
For most thin film solar 12V battery, it is difficult to collect current from the active layer contacting the substrate.If the substrate is electrically conductive ( e.g., stainless steel sheet ), the substrate can form ohmic contact with the active layer.Become back contact.However, most polysilicon thin film solar 12V battery use an insulating substrate.In principle, a conductive layer such as transparent conductive oxide TCO of amorphous silicon thin film solar cell or Mo back electrode of copper indium gallium selenium thin film solar cell can be covered on the substrate before the active layer is deposited on the substrate.Because the preparation of polysilicon thin films often requires higher temperatures, this severely limits the choice of intermediate conductive layers.One possible conductive material is that the layer has the required thermal stability and conductivity [ 127 ].However, a better way is to take advantage of the natural conductivity of polysilicon.Compared with amorphous silicon A - Si: H and microcrystalline silicon Si: H, polysilicon has lower resistivity and depends on grain structure and doping concentration.If the lateral resistance is high, a certain resistance loss will be formed and the voltage at the contact electrode will be lower than the voltage at the center of the area between the gate lines.The highly doped polysilicon thin layer can provide enough conductivity to substantially avoid such resistance loss.This forms a local contact mode, using P + BSF as the positive contact by etching to collect the current in a large area around it.The optimal contact density depends on the trade-off between series resistance loss, local contact area loss and local contact process limitations.
preparation of P - N junction
The most mature method for preparing P - N junction on polysilicon film is the standard technology applied to crystalline silicon solar 12V battery, i.e. P diffusion of P - type polysilicon film at 700 900 C high temperature.Although the doping distribution is not as easy to control as the standard crystalline silicon material, the diffusion of polysilicon film is faster, and crystal defects such as grain boundaries and dislocations increase the diffusion speed.The preferential diffusion of dopants along grain boundaries forms doping peaks, and the N - type vertical junction extends into the P - type base.
Scanning Capacitance Microscope ( SCM ) places the probe electrode on the sample surface, scans the sample, and determines the characteristics of the sample surface from the change of electrostatic capacitance between the probe and the surface.SCM gives an image of doping spikes [ 128 ].Doping spikes have a great impact on carrier collection, especially at the highly electrically active polysilicon grain boundary D29, and can be effectively utilized to improve the performance of polysilicon thin film solar 12V battery |: 13' 115' 13 ( see Section 3.2.3 ).However, the vertical junction increases the surface of the p - n junction, which increases the recombination current in the space charge region and decreases the open circuit voltage w.The degree of diffusion increase in grain and grain boundaries depends on the preferred orientation and growth rate of polysilicon [ 131 ].
In addition to the high temperature dopant diffusion, another technique for preparing the P - N junction is to grow the emitter in situ while depositing the base.This process has been successfully applied to high temperature plasma enhanced chemical vapor deposition PECVD [ 92 ] and solid phase crystallization SPCU 5,1m.Although in-situ growth emission does not require an additional step of preparing P - N junction, it can greatly simplify the process, but it also reduces the control of doping distribution, especially when high temperature deposition process is used.
Another technique for preparing P - N junctions on polysilicon films is to deposit amorphous silicon A - Si: H layers to form heterojunctions.Japan's sanyo motor is in the leading position in this process and developed the intrinsic thin layer heterojunction hit 12V battery [ 132 ] in the early 1990s ( see section 3.6.1 ).A thin A - Si: H intrinsic layer about 5 nm thick is deposited on the polysilicon film first, and then a highly doped P + type A - Si: H emitter is deposited.Since the lateral conductivity of the A - Si: H layer is low, the P + emitter needs to be combined with the transparent conductive oxide TCO.Although Sanyo Motor only reported the results of N - type base devices, heterojunction emitters can also be applied to P - type emitter devices to form N + type A - Si: H emitters.Recently, P - type polycrystalline silicon thin films prepared by CVD on ceramic substrates have been reported, realizing a polycrystalline silicon thin film solar cell of up to 520 mV.
Compared with standard solar grade silicon materials, polycrystalline silicon thin films contain a large number of defects ( 1016 CNR 3 orders of magnitude ) at grain boundaries and inside grains.In order to achieve higher device performance, hydrogen passivation of these defects is very important.Some defects can be eliminated by short-term high-temperature annealing.For solid-phase crystallized SPC polysilicon grown on glass substrates, RTA can increase the open-circuit voltage by more than 30mV [ 1M' 13.However, in the process of high temperature annealing, the phenomenon of " blurring" of doping distribution will occur, which will increase the doping concentration in the lower doping region.If the thermal budget is very high, two regions with high doping concentration and opposite doping types will contact each other, resulting in a lower shunt resistance.
Hydrogen passivation has attracted H atoms in polycrystalline silicon thin film materials, and its principle is quite complicated, which is still the focus of many studies.A simpler explanation is that H atoms combine with dangling bonds to remove defect energy levels from the band gap.There are many research results on hydrogen passivation of polysilicon solar 12V battery [ 134 - 136 ].In the preparation process of the anti-reflection film of the polysilicon solar cell.Plasma enhanced chemical vapor deposition PECVD can effectively passivate hydrogen while forming Si3N4, which is now the standard process #' 138' for crystalline silicon solar 12V battery.Hydrogen passivation can increase polysilicon solar 12V battery by up to tens of mV, while hydrogen passivation has a far-reaching significance for polysilicon thin film solar 12V battery, usually increasing their VOE by more than 200 mV [ 97,106,133,139 ]
The typical process temperature for plasma hydrogenation is 400 C, but studies show that higher temperatures can achieve more effective hydrogen passivation [ 97 ].The plasma hydrogenation of CSG Solar in Germany uses the high temperature of SiO' C ..A certain plasma [ 1 6 ] is still maintained during the cooling process.
The typical hydrogen passivation time is 30 min. Longer hydrogen passivation will create new defects in the grain and counteract the passivation effect [ 14.Recently, it has been reported that hydrogen passivation in RTA process of standard crystalline silicon solar 12V battery can also be effectively applied to polycrystalline silicon thin film solar 12V battery [ 141 ].
Polysilicon thin film solar cell technology requires that after depositing a polysilicon thin film with certain structure and characteristics on a large area substrate, the large area thin film is isolated into many interconnected areas to form independent 12V battery.In order to connect the 12V battery in series, it is necessary to isolate the entire active layer above the substrate by wet etching, dry etching or laser scribing.If laser scribing is used, special measures need to be taken to ensure that local melting does not form p + - n + junctions with low shunt resistance.
When separate batteries are isolated, they need to be connected in series to realize integrated interconnection?It is the most convenient method to integrate interconnection and metallization at the same time.The positive contact of one 12V battery is connected to the negative contact of the other 12V battery, and the connected metal strip can fill the isolation groove. CSG Solar of Germany uses this metallization integration interconnection method [ 15 ].The integrated interconnection does not require grid lines, and current flows from one cell to another cell, passing through each interconnection area in a distributed manner, and requires contact electrodes at both ends of the assembly to collect larger current.In order to avoid short circuit of 12V battery caused by interconnection metal, CSG Solar fills the isolation groove with insulating resin.This integrated interconnection process greatly simplifies the module production and reduces the loss of series resistance, which is an important advantage of polysilicon thin film solar cell industrialization over traditional crystalline silicon solar cell modules.
Research Results of Polysilicon Thin Film Solar Cells
In this section, we will review the main research results of polysilicon thin film solar 12V battery.Different from the development of crystalline silicon solar 12V battery, the research and development of polycrystalline silicon thin film solar 12V battery is not continuous and gradual, but intermittent and jumping. The research results come from the contributions of several research institutions and companies.
As early as the 1970s, polysilicon thin film solar 12V battery were first studied in the United States.The preparation method is high-temperature vapor deposition, and the conversion efficiency is only within the range of 1 % 3 %.Although this stage accumulated rich knowledge and laid a solid foundation for subsequent research and development, the research project stopped [ 52 ] in the late 1970s.Until the 1990s, research on polysilicon thin film solar 12V battery continued in Japan.
HIT 12V battery
Japan sanyo motor has done a lot of research work on polysilicon thin film solar 12V battery, and began to report the research results in 1990 [ 132 ].First, plasma enhanced chemical vapor deposition PECVD was used to deposit a thicker amorphous silicon A - Si: H thin film on Shi Ying or metal substrate. Polysilicon thin film formed after solid-phase crystallization SPC has obvious non-artificial N - type doping characteristics [ 117 ], and then two layers of A - Si: H were deposited successively, i.e. undoped intrinsic layer and heavily doped P + type emitter, to obtain an intrinsic thin layer heterojunction ( HIT ) cell structure.In 1995, 1cm2 HIT 12V battery achieved a conversion efficiency of 9.2 % [ 1M ], which was already superior to microcrystalline silicon thin film solar 12V battery.However, for some reasons, Sanyo motor stopped the research and development of HIT 12V battery based on polysilicon film in 1996, and replaced polysilicon film with crystalline silicon wafer with high minority carrier life as its base, using A - Si: H as its intrinsic and emitter, the HIT 12V battery manufactured in the laboratory achieved a conversion efficiency of up to 21.5 % in 2005 [ m2 ], and successfully realized the large-scale production of commercial HIT 12V battery.
Star 12V battery
Japanese Zhong Yuan's famous hybrid photovoltaic module is called amorphous silicon condition: the combination of H and microcrystalline silicon, and commercial products have long been on the market.In the hybrid photovoltaic module, the A - Si: H top cell was deposited on the cover glass plated with transparent conductive oxide TCO, and then the bottom cell was prepared by plasma enhanced chemical vapor deposition PECVD.Because A - Si: H top 12V battery are sensitive to temperature, the deposition temperature of bottom 12V battery is required to be low enough.
However, Zhong Yuan did a lot of research work on polysilicon films in the 1990s.During most of the 1990s, Zhong Yuan reported that the grain size of polysilicon film was at least 0.1 pm and the effective doping concentration was 1016 CNT 3 [ 143' 14.Although no details of the deposition technique were published, the reported PECVD deposition temperature was < 550 c.These parameters are not consistent with MC - Si: H, which has a grain size of several tens nm, is an undoped intrinsic semiconductor and has a deposition temperature of about 200 C ..This shows that the textured back mirror enhanced absorption ( STAR ) cell reported by Zhong Yuan in the literature is composed of a real polysilicon thin film bottom cell and a - Si: H top cell, with a polysilicon grain size of 0.1 mm, crystallinity close to 100 %, and is prepared by PEC VED at 550 C high temperature.
STAR 12V battery achieves a high conversion efficiency of 10.1 %. It can be said that STAR 12V battery is by far the best polysilicon thin film solar 12V battery.The superior characteristics of STAR 12V battery come from the effective light trapping structure. The suede surface on the front surface and the rough mirror on the back surface increase the absorption of human light by the active layer [ 146 ].
CSG 12V battery
In addition to Japan's Sanyo Motor and Zhong Yuan, Germany's CSG Solar also has major research and development achievements in polycrystalline silicon thin film solar 12V battery, and its crystal silicon ( CSG ) 12V battery have entered the commercial stage.The predecessor of CSG Solar was Pacific Solar, which was founded in 1995, but only published its first paper in 2002, reporting a 7.3 % component conversion efficiency [ 147 ]. Subsequent papers published more technical details [ 15' W6' with the highest conversion efficiency of the CSG 12V battery being 8.8 % of the 96 cm2 sub-component and the open circuit voltage VOC of a single 12V battery approaching 500 mV.In 2004, new investors changed the name of Pacific Solar to CSG Solar, and set up production lines in East Germany, with production planned for 2006.
CSG 12V battery have P - junctions, similar to the standard structure of crystalline silicon solar 12V battery.A layer of amorphous silicon A - Si: H was prepared on the surface of borosilicate glass by sol-gel containing Shi Ying beads, then plasma enhanced chemical vapor deposition PECVD was used to prepare amorphous silicon A - Si: H layer on the surface of borosilicate glass, followed by solid phase crystallization SPC to obtain a 1.5 thick polysilicon film.Although the thickness of the active layer is very thin, the better light trapping structure achieves a current density of up to 28mA / cm3.The treatment of polysilicon film by defect annealing and hydrogen passivation reduces the defect density of the active layer and enables the component to achieve better performance.The defects of polycrystalline silicon films were annealed by rapid thermal annealing RTA at 900 C for a short time, which was much higher than the strain point and softening point of borosilicate glass substrate.CSG Solar's plasma hydrogenation process is particularly effective for SPC polysilicon thin films, increasing 260 mV.At present, the conversion efficiency of CSG 12V battery is close to that of microcrystalline silicon thin film solar 12V battery, and the production line with high production rate has been built and its upgrade is relatively simple [ 149 ].3.6.4 Other research results Several research teams around the world are developing polycrystalline silicon thin film solar 12V battery and are committed to polycrystalline silicon thin film preparation technology with higher crystallization quality and higher growth rate than solid phase crystallization SPC process.If the crystal quality and grain size are continuously increased, a polysilicon thin film solar cell with a conversion efficiency of 14 % and 15 % will be finally developed.Here, we will introduce the research results of several teams, but the results of these open-circuit voltages and conversion efficiency will certainly be improved in the next few years.
UNSW of the University of New South Wales in Australia has studied several technical routes of polysilicon thin film solar 12V battery, all based on medium temperature substrates and non-ultra-high vacuum evaporation technology, so as to achieve the low cost target ⑽ of polysilicon thin film solar 12V battery.Evaporated silicon solid-phase crystallization ( EVA ) 12V battery is similar to that of CSG Solar in Germany, except that the preparation process of amorphous silicon A - Si: H replaces plasma enhanced chemical vapor deposition PECVD with evaporation.The other two technical routes are epitaxial deposition of AIC seed layer induced by aluminum on glass substrate.Aluminum - induced crystallization ion-assisted deposition ( Alicia ) 12V battery uses ion-assisted deposition IAD to prepare epitaxial absorption layer, while aluminum - induced crystallization solid-phase epitaxy ( Alice ) 12V battery uses a - Si: H layer deposited on AIC seed layer to carry out solid-phase epitaxy SPE.Unsw's three kinds of polysilicon thin film solar cell technology have all realized more than 400 mV, of which 454 mV is EVA.However, the conversion efficiency is still very low, only 1 % to 2 %, because the solar cell process is still not optimized.
The Hahn Maitenaz Institute in Berlin, Germany, used a similar process, but focused more on AIC and ECR - CVD low-temperature epitaxial deposition on glass substrates [ 152 ].Its polycrystalline silicon thin film solar cell uses heterojunction emitters and reports a conversion efficiency of up to 378 mV and 1 % 2 % [ 153 \ The polycrystalline silicon thin film solar cell technology developed by IPHT of German Institute of Optoelectronics is called LayeredlaseRCrystallization ( LLC ) [ 126 ].The P + type layer forms a seed layer by laser crystallization, and an epitaxial layer is prepared on the porch layer by PECVD combined with KrF excimer laser.The laser pulse can crystallize the A - Si: H layer deposited by PECVD in situ to form an epitaxial layer with high crystal quality.The small area polysilicon thin film solar cell achieves a conversion efficiency of up to 510 mV and 3.6 %.
All the above research teams focus on the process of medium temperature substrates, which can withstand temperatures exceeding 600 C in a short period of time, while others are studying the preparation of polycrystalline silicon thin film solar 12V battery on high temperature substrates such as ceramics.
In the mid - 1990s, IMEC, Belgium's inter-school microelectronics research center, began to study polycrystalline silicon thin-film solar 12V battery using high-temperature chemical vapor deposition CVD.Previous studies focused on controlling the nucleation at the early stage of deposition in order to obtain a better grain size and achieve a conversion efficiency of " 115" of 5.5 % on the oxidized silicon wafer model substrate.Now IMEC is studying the high-temperature CVD epitaxial thickening of AIC seed layer on ceramic substrate [ 154 ]. Polycrystalline silicon thin-film solar 12V battery with heterojunction emitters and interdigital contacts have achieved a conversion efficiency of 5.9 %. The French Semiconductor Physics and Application Laboratory ( PHYSICAL ) has used several methods to control the grain size of polycrystalline silicon. The substrates are ceramic materials such as A12O3, mullite, Si3N4, and chemical vapor deposition ( CVD - OGL ) on glass layer
NarainstituteofScience and Technology ( Naist ) in Japan focuses on depositing polysilicon thin films directly on heterogeneous substrates by high-temperature CVD, and controls the grain size by intermittent supply method [ 57' 65 ], achieving an average grain size of 10, a maximum of 410 mV and a conversion efficiency of 2 % 3 % [ 156 ].
National Renewable Energy Laboratory NREL uses iodine vapor transport deposition to deposit polycrystalline silicon thin films on heterogeneous substrates' polycrystalline silicon thin film solar 12V battery with heterojunction emitters have a conversion efficiency of 460 mV and 3.5 % L87'.
Polysilicon thin film solar cell is a relatively new thin film solar cell technology' with higher cost reduction potential and higher brewing and fresh change.Objective To investigate the effect of crystal crystallization SPC on the I5 - day contact and extinction of the female's aged crystal. The conversion efficiency of the sub-assembly reached about 9 %.In order to achieve higher performance, a lot of research work has been carried out to improve the crystallization quality, and finally the single junction 12V battery achieves 12 % component conversion efficiency and corresponding large-scale production process.
In many ways, silicon-based thin-film solar 12V battery are the ideal technology choice for large-scale production of solar cell modules at low cost in the future:
Will not use scarce raw materials or toxic raw materials in large quantities;
The energy recovery period EPBT is shorter than the existing crystalline silicon solar cell.Production lines with capacity exceeding 10 mwp / a have demonstrated components upgraded to large areas ( > 1m2 );Monolithic integration connects the batteries in series into a complete assembly;
Typical low temperature processes allow the use of a wide variety of substrate materials, including low cost flexible plastic sheet substrates.
In the first 20 years of the development of silicon-based thin-film solar 12V battery ( about 1970 - 1990 ), the research focus was on amorphous silicon thin-film solar 12V battery prepared from amorphous silicon a - si: h and its alloy hydrogenated amorphous silicon germanium ( a - sige: h ) and hydrogenated amorphous silicon carbon ( a - sic: h ) ( see chapter 5 ).Although amorphous silicon thin-film solar 12V battery are mature technologies in industry, they still do not occupy a large market share of photovoltaic power generation and are rarely used in large-scale power generation systems, but are only limited to small-scale power generation systems such as hand-held calculators.The main reason for this commercial situation is the relatively low conversion efficiency of amorphous silicon thin film solar 12V battery ( 5 % 7 % ), while the S - W effect or light - induced attenuation effect is an important reason for limiting the conversion efficiency.Nevertheless, amorphous silicon thin film solar 12V battery are gradually entering the photovoltaic power generation market, especially in the field of photovoltaic building integration ( BIPV ), because amorphous silicon thin film solar 12V battery have unique structural and aesthetic advantages.
In the early 1990s, our research team at the University of New Chastel in Switzerland first proposed microcrystalline silicon Si: H as a new thin film material for photovoltaic applications.MC - Si: H is more specifically hydrogenated microcrystalline silicon, but we will simply call it microcrystalline silicon.Microcrystalline silicon thin-film solar 12V battery can use the same equipment and similar processes as amorphous silicon thin-film solar 12V battery, but - Si: H has very different material properties than A - Si: H:
Complex material microstructure, and the microstructure varies greatly depending on deposition conditions.High sensitivity to pollution;
The lower band gap is 1.1 eV for C - Si: H and 1.71.8 eV for A - Si: H, so MC - Si: H can be used to absorb the solar spectrum in the near infrared band.Indirect band gap semiconductors require a thicker absorption layer and a more effective light trapping structure than A _ Si: H for lower absorption coefficient in the visible band of the solar spectrum.More moderate photoinduced attenuation effect.
Microcrystalline silicon thin film solar cell is still a research topic of great concern, especially many aspects of material properties and device physics.This chapter will introduce the structure, process and characteristics of microcrystalline silicon thin film solar 12V battery.
The laboratory conversion efficiency of single-junction microcrystalline silicon thin-film solar 12V battery can reach 9 % and 10 %, and that of individual record-keeping 12V battery can reach slightly higher than 10 %.Because the microcrystalline silicon thin film solar cell is still in the early stage of research and development, people hope that the conversion efficiency record will be greatly improved in the next few years, and especially hope for a more effective light trapping structure.
However, the best way to use microcrystalline silicon thin film solar 12V battery is to use non - microcrystalline laminated 12V battery, i.e. the combination of bottom 12V battery and a - si: h top 12V battery. at present, its small-area laboratory 12V battery have achieved a stable conversion efficiency in the range of 11 % to 12 %.Japan's Zhong Yuan keeps an initial laboratory conversion efficiency record of 14.7 % for non-microcrystalline laminated 12V battery, and the introduced non-microcrystalline laminated thin film solar cell module achieves a stable conversion efficiency of more than 8 %.This chapter will fully discuss the advantages and disadvantages of non-microcrystalline laminated batteries.
Microcrystalline silicon deposition technology
Development of microcrystalline silicon
Microcrystalline silicon or nanocrystalline silicon usually refers to si films prepared by plasma enhanced chemical vapor deposition PECVD or similar techniques at low temperature ( < 400 c ) with pure sih or sih4 / H2 mixed gas, whereas polysilicon films refer to si films prepared by chemical vapor deposition CVD at high temperature ( > 600 c ) ( see chapter 3 ).The low temperature process of MC - Si: H has three advantages:
There are more kinds of substrates to choose from;
the problem of substrate pollution can be reduced;
Hydrogen passivation is carried out simultaneously with film preparation to more effectively passivate defects and crystal boundaries.
In 1968, VE PREK and MARE CEK discovered that - Si: H [ 1 ] can be synthesized at low temperature by chemical transport technology.Such a chemical transport technique is similar to PECVD in two respects:
deposition of SiH4 - related free radical precursor;
H atoms play an important role in the growth of thin films.
In 1980, MC - Si: H [ 2' 3 ] was deposited by PECVD with SiH4.The earliest application of MC - Si: H in solar 12V battery was the preparation of P - type window layer in P - I - N amorphous silicon thin film solar 12V battery.The high conductivity of Si: H and the low absorption coefficient in visible light band are particularly useful [ 5 ].However, the microcrystalline silicon thin-film solar cell with hydrogenated microcrystalline silicon Si: H photosensitive layer is not promising because of the poor quality and N - type characteristics of the undoped layer at first.
In 1994, a research team from the University of New Chastel reported that the conversion efficiency of the single-junction microcrystalline silicon thin film solar cell was > 7 %, and the process used was very high frequency glow discharge. At present, the growth rate of MC - Si: H is the bottleneck limiting the commercialization of microcrystalline silicon thin film solar 12V battery, and the development of MC - Si: H layer deposition technology with high growth rate ( > 5a / s ) is the current research focus.We will discuss in detail:
High pressure depleted chemical vapor deposition HPD - CVT F8 -;
Microwave Plasma Enhanced Chemical Vapor Deposition MW - PECVD [ 10 - N ];Hot wire chemical vapor deposition HW CVD [ 12U.
Among them, the deposition of MC - Si: H by VHF - GD is the most advantageous, and the latter three deposition techniques can also replace VHF - GD to prepare MC - Si: H thin films.These technologies are still under study. Although the deposition rate in the laboratory has reached 10 people / s, there have been no reports of large-scale production of such a high deposition rate other than Zhong Yuan, Japan.
VHF glow discharge
In 1987.Vhf glow discharge ( VHF - GD ) deposition technology has been applied to increase the deposition rate of amorphous silicon A - Si: H film ( and later microcrystalline silicon MC - Si: H film ) at the University of New Chastel in Switzerland.
Standard plasma enhanced chemical vapor deposition PECVD for preparing A - Si: H or S: H has a diode-type reaction chamber.The conventional PECVD power source with plasma excitation frequency of 13.56 MHz is replaced by the VHF power source with plasma excitation frequency of 30300 MHz, which is VHF - GD deposition technology.Because the electron thermal velocity in the plasma is much higher than that of the positive ions, the plasma needs to have a positive voltage to maintain electrical neutrality.The plasma sheath near the substrate accelerates positive ions and confines electrons to the plasma.A higher plasma excitation frequency can reduce the plasma sheath thickness, thus reducing the plasma impedance, enhancing the decomposition of SiH4, and reducing the energy of ions impacting the growth surface.Because the ion bombardment growth surface is softer ( but stronger ), VHF - GD is easier to grow MC - S: H than PECVD with the traditional 13.56 MHz plasma excitation frequency.
The research shows that VHF - GD has the potential to become a high deposition rate process.Several research teams have confirmed that if the plasma excitation frequency is increased from the standard 13.56 MHz to VHF range, the deposition rate of A - Si: H or FIC - Si: H can be increased 410 times.
The relationship between deposition rate and plasma excitation frequency can give the maximum deposition rate for a specific deposition reaction chamber.
If the plasma excitation frequency increases, the deposition rate increases, and the plasma sheath thickness and ionic impedance decrease, which can be attributed to the basic physical characteristics of the capacitor glow discharge plasma.On the other hand.Due to the limitation of the design of the reaction chamber, a higher plasma excitation frequency forms a certain parasitic shunt capacitance in the reaction chamber, so in a higher plasma excitation frequency range, the deposition rate decreases with the plasma excitation frequency.Hein Tze et al. measured the increasing relationship between ion flux and plasma excitation frequency on the growth plane, and suggested that higher ion flux and lower energy ion bombardment would cause softer but stronger bombardment.From these physical modifications, it can be seen that a higher plasma excitation frequency can couple radio frequency energy into the plasma instead of the plasma sheath, so that the plasma has a higher electron concentration and a better decomposition of SiH4 % 16, and a higher free radical flux and ion flux are received on the growth surface, so the deposition rate increases and the growth of PIC - Si: H improves.
Other deposition techniques
If microcrystalline silicon Si: H is prepared by high pressure depletion chemical vapor deposition ( HPD - CVD ), the gas pressure in the plasma reaction chamber replaces the conventional deposition gas pressure O of 0.3 Torr with a high pressure of 710 Torr, and ions bombarding the growth surface lose most of the energy in the process of colliding with the plasma due to the high pressure of 710 Torr, the energy supplied to the plasma is greatly increased, so HPD - CVD effectively increases the deposition rate without affecting the material quality.In order to obtain MC - Si: H instead of amorphous silicon A - Si: H, plasma conditions require SiH4 to be depleted and the plasma to have a relatively high H concentration.Optimizing HPD - CVD needs to solve the problems of high H consumption, powder formation and uneven active layer.The team from the German Juelich Research Center has obtained a microcrystalline silicon thin film solar cell with good performance through HPD - CVD. HPD - CVD with a plasma excitation frequency of 13.56 MHz can reach a deposition rate of 5 people / s, while the combination of HPD - CVD and VHF - GD ( 95 MHz plasma excitation frequency ) can reach a deposition rate of 11a / s [ 18 ], realizing a single junction microcrystalline silicon thin film solar cell with a conversion efficiency of 9.8 %.Some Japanese research teams reported that microcrystalline silicon thin film solar 12V battery have slightly lower conversion efficiency, but the deposition rate has reached 2040a / s range [ 19' 2 ]. The current research focus is to design a large-area reaction chamber suitable for large-scale production based on the combination of HPD - CVD and VHF - GD [ 21 ].
Compared with plasma enhanced chemical vapor deposition ( PECVD ) with plasma excitation frequency of 13.56 MHz, microwave plasma enhanced chemical vapor deposition ( MW - PECVD ) also has lower ion bombardment intensity and higher MC - Si: H deposition rate, but there is no report on the performance of microcrystalline silicon thin film solar 12V battery [ 22 ].
Hot wire chemical vapor deposition ( HW CVD ) is another technique for preparing MC - Si: H. A metal filament ( such as tungsten filament ) heated to 16002000 C can thermally decompose SiH4.Since there is no plasma in HW CVD, ion bombardment harmful to the quality of the active layer can be completely avoided.Up to now, HW CVD has achieved better quality single junction microcrystalline silicon thin film solar 12V battery at lower deposition rates.Although HW CVD also achieved MC - Si: H growth at higher deposition rates, satisfactory results of microcrystalline silicon thin film solar 12V battery have not been obtained at higher deposition rates [ 13, 23 ].Because a certain amount of low energy ion bombardment is beneficial to MC - Si: H growth, the lack of plasma in HW CVD is indeed a major disadvantage.In fact, the combination of HW CVD and VHF - GD has given better results [ 2.At present, specific equipment manufacturers are trying to solve some important technical problems faced by HW CVD deposition JNC - Si: H:
Uniformity of large area deposition;
Replace the filament regularly;
Overheating of growth film caused by heat radiation from filament.
Since the discovery of microcrystalline silicon Si: H in 1968, it has been known that the necessary condition for depositing MC - Si: H is that the growth surface should have a high H concentration of 2.The methods of increasing H atom concentration on the growth surface are as follows.Battery state of health estimation through coup de fouet
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