2V batteries are often used on large ships as backup for the emergency power system. These 2V batteries are often used in series connection. 12V (= 6 cells) or 24V (= 12 cells) battery systems can be assembled with 2V cells.
where ace is the absorption coefficient;n is the refractive index;and 9 are constants describing the state density distribution shapes of the valence band and conduction band extended states, respectively;and b is the optical bandgap pre - factor.In the case of crystals, the distribution of state density near the band edge depends on the square root of photon energy e, equation ( 5.3 ) describes the so-called taoko curve [ 26 ], and the corresponding optical band gap is called taoko optical band gap.When the state density distribution near the band edge is linear 4 = 9 = 1, the optical band gap is called cubic optical band gap.The ceramic optical band gap of the device quality intrinsic A - Si: H is in the range of 1. 71. 8 eV, which is 0. 10. 2 eV larger than the cubic optical band gap of the same material.In A - Si: H, the optical band gap increases with the increase of H concentration of the thin film [ 39 \ A - Si: H refractive index reaches a maximum of about 5.0 in the band of about 425 nm, and decreases with respect to wavelength in the band of 350 900 nm in the solar spectrum.The refractive index decreases slightly in the band larger than 900 nm, and the refractive index of the device mass a - h at 900 nm > 3.6.
The electrical properties of amorphous silicon a - si: h often use dark conductivity?, optical conductivity txph and mobility lifetime.Measuring these characteristics is a standard method for obtaining quality information of A - Si: H materials for photovoltaic applications.
The dark conductivity ( CRJ ) of device quality intrinsic A - Si: H is generally less than 1XL0 _ 1 FI _ 1cm - 1.In order to measure the activation energy of m and dark conductivity, the current to be measured is very low, on the order of pa.A single layer of a - si: h film measured in this way was deposited on high resistivity glass ( e.g. corning 1737 ) and a coplanar metal electrode separated by < 1 mm and 12 cm was deposited on the sample.It is necessary to pay special attention to humidity and ensure that impurity diffusion does not affect current measurement.Therefore, the measurement is usually carried out in vacuum or in an inert gas atmosphere, and the sample is annealed at 150 C for half an hour before the test so as to reduce the humidity on the surface of the film.Generally, a voltage of 100 V is applied to an A - Si: H film with a thickness of L m to obtain a current of tens of Pa, so the dark conductivity is:
Where u is the applied voltage d is the measured current;/ is the length of the electrode;W is the distance between the electrodes 5.3.6 to measure the state density
We will extensively discuss various methods for determining state density in amorphous silicon A - Si: H or microcrystalline silicon MC - Si: H here.
There are many ways to relate the light absorption of amorphous silicon A - A: H films or devices to the state density distribution.Sub - band gap absorption is of particular concern because it reflects the transition of local states in the band gap.However, the sub-band gap absorption is weak, so some indirect methods based on measuring the secondary effect are used to determine the absorption coefficient.In the photothermal deflection spectrum PDS, the deflection of the detection laser beam can reflect the refractive index change of the medium in contact with the A - condition: H film, which depends on the heat generated after the A - Si: H film absorbs monochromatic light and dissipates from the film into the medium.Other techniques are based on the measurement of spectrum-dependent optical conductivity, such as the steady-state optical current generation method CPM [ 43 ], the two-beam optical conductivity DBP [ M ] and the recently adopted Fourier transform optical current generation spectrum FTPSE 45.
Photothermal deflection spectrum
In the photothermal deflection spectrum PDS, part of the photon energy absorbed by the thin film is converted into heat energy, and the heat of the thin film is dissipated in the adjacent medium, and the refractive index of the medium changes.By detecting the refractive index change of the medium with the laser beam, one can calculate the light absorption of the film by the deflection of the detection beam.Amorphous silicon A - S: H film samples are immersed in optically transparent and thermally conductive liquid, and monochromatic pump beams are irradiated on the samples.The position-sensitive detector connected to the phase-locked amplifier can measure the deflection signal.
The advantage of PDS technology is high sensitivity, which can measure the film?The absorption value of 10 _ 5.For a typical A - Si: H film with a thickness of 1, the absorption coefficient of the lowest LO CNC1 can be measured.PDS technology is sensitive to the surface state, so PDS results tend to overestimate the state density of defect states.
Steady - state photoinduced current method
The steady photogenerated current method CPM based on measuring optical conductivity is widely used to determine the sub-band gap absorption spectrum of thin film semiconductors, especially A _ Si: H ..During CPM measurement, the steady photogenerated current h in the sub-band gap region is measured as a function of photon energy e.The light absorption in this area is weak, and the exponential function can be expanded into power series, exp ( 1?1. Applying this approximation to Equation ( 5.9 ), the optical conductivity can be expressed as:
Fourier Transform Photogenerated Current Spectrum
Recently, Fourier transform photo-generated current spectroscopy FTPS technology has been applied to determine sub-band gap absorption.Fourier Transform Infrared Spectrometer ( FTIR ) is used as an interferometer.The amorphous silicon A - Si: H sample is used as an external detector.The spectrometer is equipped with a SiC hot rod and a white light source so that the spectrum of 0.4 - 1.8 eV can be measured.The sample may be a thin film deposited on a substrate or a complete solar cell.The circuit connecting the sample has a low noise voltage source and a current amplifier.After the photo-generated current of the sample is amplified by the preamplifier, the digital-to-analog converter ( A / D converter ) digitizes the output signal of the preamplifier.Then, the computer Fourier transforms the signal in the time domain into the signal in the frequency domain.FTIR signals from the sample are normalized to FTIR signals independent of the spectral detector.The advantage of FTPS technology is that it is sensitive to low-energy photons, and the measurement of sub-band gap absorption has increased from about 0.8 eV to 0.4 eV in the steady-state photogenerated current method CPM and the two-beam optical conductivity DBP technology.Moreover, the measurement time of FTPS is reduced to only a few minutes.
The absorption coefficient determines the defect density
Through the sub-band gap absorption region of the absorption coefficient spectrum, the amorphous 18.104.22.168 space charge method can be obtained by a simple method
Other methods use the space charge region characteristics of amorphous silicon A - Si: H interface to obtain defect density in the mobility gap, such as field effect technology [ 49 ], deep level transient spectrum DLTS [ 50 ] and space charge limiting current SCLC [ 51' 52 ].
Deep - level transient spectrum
Deep level transient spectroscopy ( DLTS ) is a particularly sensitive technique, which is different from electron spin resonance ESR and can be used to measure non-paramagnetic defects.DLTS is a conventional technique for determining the energy level and defect density in semiconductors, and it is almost the most commonly used technique for measuring the deep energy level of crystalline semiconductors.DLTS has been proved to be a valuable state density measurement method for amorphous silicon A - Si: H [ 5 ( )' 53 ].The earlier capacitive DLTS can only measure lightly doped A - Si: H thin films, while the later charged DLTS ( Q - DLTS ) can measure undoped A - Si: H thin films 1.
Q - DLTS samples are usually metal oxide semiconductor MOS structures. A 1 - thick A - Si: H layer is deposited on a highly doped N + type crystalline silicon substrate as a back contact.In order to successfully realize the Q - DLTS experiment of undoped A _ Si: H, a very thin insulator layer needs to be prepared on the surface of A - Si: H ..The insulator layer can obviously reduce the leakage current of the sample so that the relative and charge transients can be ignored, and the Fermi level of the A - Si: H thin film moves with the applied bias voltage.The translucent A1 layer usually serves as the top electrode of the MOS structure.By applying a bias voltage pulse to the MC ) s sample, the fermi level moves along with the band mobility, and the energy state in the a - si: h band gap is filled with carriers.After each fill pulse, the transient current of the external circuit needs to be measured as a function of temperature.The charge emitted by the occupied trap state is determined by integrating the measured current.The charge released at a specific temperature is proportional to the state density of a specific energy in the mobility gap of a - si: h material.Q - DLTS technology is suitable for studying the change of state density distribution in mobility gap caused by photo-induced attenuation or particle-induced attenuation, so that the complex characteristics of A - Si: H exposed to light or particles can be better understood by 1 5 ].
In this paper, the specific requirements for device quality intrinsic A - Si: H and A - SiGe: H in the preparation of amorphous silicon thin-film solar 2V battery are summarized, as shown in actual. 1.
Photoinduced attenuation effect
Amorphous silicon A - Si: H will change greatly in electrical properties under illumination due to inherent photo-induced attenuation effect or S - W effect [ 24 ].Since the photoinduced attenuation effect was observed, people have spent a lot of energy trying to understand the mechanism of the change of a - si: h structure and photoelectric properties caused by incident light [ 56 - 59 ].Photoinduced attenuation of a - si: h thin film and amorphous silicon thin film solar 2V battery is a meta - stable or meta - stable state, i.e. the attenuation effect is reversible and can be eliminated by annealing at a temperature > 150 c.
It is believed that photoaging treatment can increase more dangling bond defects, and dangling bond is considered to be the main cause of photoinduced attenuation effect.According to the change of absorption coefficient measured by the double-beam optical conductivity DBP, the defect density in A - Si: H will be obviously increased by photo - aging.The sample was irradiated by a red He - Ne laser ( A = 633 nm ) with a human light intensity of about 40MW - cm - 2O in the photon energy range of 0. 81. 4 eV, and the sub-band gap absorption increased with the illumination time.This shows that the defect density also increases with the illumination time.Saturation of A - S: H photo-induced defects is an important characteristic.The saturation value of defect density at room temperature is about 2x1017 cm - 3, and the saturation value is almost independent of different human light intensity and temperature change below 70 C [ then.
The change of Q - DLTS signal of charge type deep level transient spectrum after photoaging can give information of defect state energy distribution Q - DLTS measurement shows that with the time of photoaging treatment, the number of positively charged defects decreases, the density of negatively charged defects almost remains unchanged, while the dangling bond of electrically neutral defects increases significantly by m.
- annealed state I
photoaging treatment state A
Temperature / ruler
In fact. 5 The Q - DLTS signal of the charge deep level transient spectrum of amorphous silicon A - H after different light aging treatment irradiation time has been known more about the mechanism and characteristics of different types of defects of A - Si: H [ 55 ] by using Q - DLTS technology, and it has been found that besides dangling bonds, other types of defects also have an important influence on the photo-induced attenuation effect of A - Si: H ..The positively charged defect state above the center of the band gap is related to the complex combination of S - dangling bonds and H molecules.Floating bonds covalently bonded to adjacent 5 Si atoms form negatively charged defect states below the center of the band gap.However, the photoinduced attenuation effect in A - Si: H is still a complicated phenomenon, and many problems need to be solved.The exact role of H atom, weak Si - Si bond and Si - H bond, and the formation of metastable defects are still the focus of research.The computer simulation of A - Si: H network is helpful to these studies.Up to now, there is no universally accepted theoretical model to explain the formation of metastable defects in A - Si: H and all related experimental phenomena [ 58 ].
Primary crystalline silicon and polymorphous silicon
Up to now, the research on the preparation of amorphous silicon A - Si: H by PEC VED technology has shown that' H2 dilutes SiH and the source gas is beneficial to obtaining a more stable high quality A - Si: H ..Diluted SiH: The prepared amorphous silicon thin film solar cell showed better performance than the traditional undiluted sample under illumination [ 64.65 ].Due to several good properties' H2 dilution A - Si: H has received a lot of attention, this material is now also called the hydrogen - Si dilution ratio ( R = H2 / SiH4 ) of the primary crystalline Si 1 - 66 - 1, which can roughly define the deposition conditions under which the growth of the primary crystalline Si occurs.As the film evolves from amorphous to amorphous - microcrystalline mixed phase and to the final microcrystalline phase, the growth of primary crystalline silicon will undergo a special characteristic change.TEM is a common microscope technology. A beam of electrons transmits through a very thin sample and interacts with the thin sample during transmission. The image formed by this interaction is magnified and focused on imaging devices such as a screen, photographic film or CCD camera.TEM image of transmission electron microscope observed about 1pm thick primary crystalline silicon deposited on glass substrate under the condition of 2 = 30.It should be noted that the primary crystalline silicon is defined as the complete amorphous phase, and if the amorphous phase changes to the mixed phase, the film cannot be called the primary crystalline silicon.Based on the in-situ real-time ellipsometry study, the phase diagram of amorphous to microcrystalline phase transition can be determined, and the film thickness at which the transition occurs under certain conditions can be predicted [ 67 ].In the case of low dilution ratio ( feet < 10 ), the film must be in an amorphous phase without the characteristics of Cr > 10 grown from primary crystalline silicon, and the critical thickness at r > 10 will be determined. the a - si: h surface beyond this critical thickness will become very rough.The stability and quality improvement of the primary crystalline silicon layer compared with the traditional pure SiH., or the low H2 dilution deposition A - Si: H is attributed to the enhancement of the intermediate-range order in the improved intermediate-range ordered O medium range, which can be determined by measuring the half-value full width ( Fullwidthalfmaximum, A20 ) of the X - ray diffraction peak ( scattering angle 20 about 28.5 ), the typical value of A - Si: H is A20 about 6, while the primary crystalline silicon decreases to A20 about 5 [ 68 \ An important feature of the growth of the primary crystalline silicon is that the film grows as an amorphous or microcrystalline phaseIf the non - microcrystalline laminated cell does not contain the traditional A - S: H interlayer, it is difficult to directly grow intrinsic native crystalline silicon on the doped microcrystalline silicon MC - Si: H layer.On A - Si: H, the nucleation of grains will be suppressed.Phase diagram is an important tool for optimizing microcrystalline silicon thin film solar energy and amorphous silicon thin film solar energy, and can control the film growth in the desired crystal phase.
In addition to the 3 - condition: SiH, which is highly diluted than H2, polymorphosicon ( PM - PM - Si: H ) can also be prepared. The features of the structure include many plasma - formed dispersed nano - grains, which are embedded in a more relaxed polymorphous network than the standard A - Si: H structure.In contrast to the native crystalline silicon, a thicker PM - Si: H film can be prepared because its structure does not depend on the thickness or characteristics of the substrate, while the concentration of smaller PM - Si: H grains is about 2 %.Although the H concentration of PM - Si: H is high in the range of 15 % to 20 %, elliptical polarization spectrum measurements show that PM - Si: H films have higher density than standard A - Si: H films.The special structure of PM - Si: H forms a lower defect density ( about 5x1011 cm - 3 ) and is much less affected by photoaging than A - Si: H.In particular, the improved hole transport is very helpful for photovoltaic applications of PM - Si: H.
doping of amorphous silicon
The purpose of doping semiconductor materials is to artificially control the type and size of material conductivity, and the doping method is to add a certain amount of special impurity atoms in intrinsic semiconductors.Amorphous silicon A - Si: H uses the same main doping elements as crystalline silicon: B is doped as P type and P is doped as N type.In 1975, Speai and Leco MBER in university of dundee, England, first reported that A - Si: H can be doped by adding B and P [ 2 ].It is an important breakthrough to realize effective doping in A - Si: H, because doping of A - Si: H is considered impossible for a long time.By depositing by glow discharge after mixing the source gases of PH3 or SiH, the dependence of room temperature conductivity ( CTUT ) of A - S: H on the ratio of doping gas to SiH and gas can be changed by changing the ratio of doping gas, and the variation range of coffee can reach 108 times.The dark conductivity activation energy of intrinsic materials can be reduced from 0.70.8 eV to 0.15 eV of N type or 0.3 eV of P type.
The continuous irregular network is considered to easily contain impurity atoms such as P or B, and the coordination arrangement corresponds to the bonding mode with the lowest energy of impurity atoms.This characteristic of the continuous irregular network is contrary to the crystal structure, the crystal is # pass ordered, and the impurity atoms are forced to adapt to the coordination arrangement of the matrix atoms.In a continuous irregular network, if the impurity atom has n valence electrons, the optimal number of covalent bonds ( OptimumUmberoFCoValentBonds, Z ) for coordination arrangement is:
This is the " 8 _ n rule" ( 8 - n rule ) proposed by Mott in 1969, which describes the atomic coordination arrangement of continuous irregular networks [ 71 ].
According to the 8 - N rule, a P atom with five valence electrons will form three covalent bonds with adjacent atoms, thus binding to a continuous irregular network.The sum si mode has lower and more stable energy than the neutral donor PS and is also called defect compensation donor
The defect compensation donor formed by p atoms and the defect compensation acceptor formed by b atoms are the main doping mechanism of a - si: h.This mechanism was first proposed by Street. The most important conclusion of the auto-compensation model [ 72 ] O is that the doping of A - Si: H will inevitably result in a large number of dangling bonds.The defect density of doped a - si: h is 23 orders of magnitude higher than that of intrinsic a - si: h.The diffusion of carriers in doped A _ Si: H is very small compared with monocrystalline silicon.Therefore, the amorphous silicon thin film solar cell cannot be designed as a P - N junction, and the intrinsic layer with relatively few defects needs to be inserted between the P - type layer and the N - type layer to form a P - I - N junction.
Another important difference between A - Si: H and monocrystalline silicon is that Fermi level cannot move close to mobility edge.When the concentration of B atoms in P - type A - Si: H increases, the Fermi level cannot move within 0.30 eV of the valence band mobility edge.However, when the concentration of P atoms in N - type A - Si: H increases, the Fermi level cannot move within 0.15 eV of the valence band mobility side.The reason for this phenomenon is that the existence of the tail state and the defect state in the band gap does not allow the Fermi level to move too close to the mobility edge.The formation of more space charges in the tail state in the form of exponential function will be accompanied by the movement of Fermi level to the mobility side to compensate for the charges formed by ionized doped atoms.In this way, the movement of Fermi level will also reduce the probability of doping atoms forming doped state modes.The limitation of Fermi level shift in the doped layer will reduce the built-in voltage ( VBI ) of the p _ I _ n junction of amorphous silicon thin film solar 2V battery, thus reducing the open-circuit voltage of 5.3.10 amorphous silicon alloy, which was originally mainly determined by the band gap
The purpose of preparing amorphous silicon A - Si: H alloy in photovoltaic applications is to shift the light absorption spectrum to a higher or lower photon energy range, thereby adjusting the optical sensitivity of A - Si: H to different parts of the solar spectrum.The absorption spectrum of A - Si: H can be finely tuned by changing the H content in the film [ 39 ], which requires changing deposition conditions such as substrate temperature or H2 dilution of SiH4.The larger absorption spectrum shift comes from the alloy of A - Si: H and C, O, N or Ge.The alloy is similar to the doping process, and only the appropriate gas needs to be added to SiH4 source gas before chemical vapor deposition CVD process.SiH4 can be mixed with CH4 or 06 redundantly to obtain hydrogenated amorphous silicon carbon A - SiC: H or hydrogenated amorphous silicon germanium A - SiGe: H, respectively.Compared with A - Si: H, the absorption spectrum edge of A - SiC: H moves toward high photon energy, while the absorption spectrum edge of A - SiGe: H moves toward low photon energy.Absorption coefficient spectra of p - type a - sic: h and a - sige: h.A simple A - condition: H alloy process realizes the design of solar cell with alloy absorption layer structure.
At present, only A - SGE: H alloy has been proved to be an ideal absorber for multi-junction amorphous silicon thin film solar 2V battery.Earlier studies of A - SiGe: H show that?, by changing the proportion of Ge H4 in the mixed gas, the prepared A - SII Ge H layer has an adjustable optical band gap in the range of 1.7 evcr = 0 ) to 1.0 evcc = l ).If the Ge content in the thin film increases, the photoelectric characteristics related to photovoltaic application of A - SiGe: H alloy will obviously deteriorate compared with the device quality A - Si: H ..The traditional deposition parameters to achieve device quality a - si: h cannot produce high quality a - si -. ge h materials with z > 0.3.After that, a lot of deposition process research was carried out, hoping to improve the quality of A - SiGe: H material.The main method used is to dilute the mixed gas of SiH4 and Ge H4 with a large amount of H2?, the deposition temperature of a _ sige: h decreases from 250 c to 180 c while maintaining a high material quality [ 75 \ because the thermal damage of the bottom layer is reduced, the application of a - sige: h absorption layer deposited at low temperature can improve the performance of amorphous silicon thin film solar 2V battery.
The key parameter to determine the quality of A - SL: H material devices is the state density distribution in the bandgap.The measurement of the sub-band gap absorption of A - SiGe: H alloy shows that even if the band gap is reduced to 1.25 eV, the Ulbach energy is almost unchanged, and its value is close to about 50 RN EVC 3 of the device mass A - Si: H.This also proves that the defect density of A - SiGe: H alloy generally increases with the increase of Ge content in the material.At present, the A - SiGe: H layer is used for the bottom cell of a stacked cell or the middle cell and the bottom cell of a triple junction cell, thereby absorbing long wavelength photons more effectively.After carefully optimizing the process of diluting mixed source gas with H2, the growth of A - SiGe: H belongs to the category of primary crystalline silicon close to the transition state [ 77 ].The human intensity of open circuit voltage can indicate the transition from amorphous phase to microcrystalline phase.The device quality A - SiGe: H alloy for laminated batteries requires a ceramic optical band gap of about 1.45 eV, as shown in actual .1.
B - doped A - SiC: H is a standard P - type layer material for microcrystalline silicon thin film solar 2V battery [ 8 ].Since bonding B reduces the band gap of A - SiC: H, the best P - type layer requires a trade-off between conductivity and light absorption.In addition, a - sic: h with low defect density can be applied to the thin buffer layer of the p / I interface of the solar cell with high conversion efficiency, thereby preventing the diffusion of b atoms from the p - type layer to the I - type layer and the diffusion of photogenerated electrons from the I - type layer to the p - type layer.
Deposition of amorphous silicon
The first technology to successfully deposit amorphous silicon A - Si: H layer is the RF - driven glow discharge process W of SiH4.After that, various deposition techniques were studied to improve the quality of A - Si: H and increase the deposition rate.Methods for preparing A - Si: H films can be divided into two categories:
* chemical vapor deposition CVD: after the precursor gas with Si is decomposed, the vapor phase is deposited on the substrate;Physical vapor deposition ( PVD ): sputtering si target to obtain si atoms' to grow a - si: h thin film.
Sputtering deposition of A - Si: H is not commonly used, so it will not be discussed in detail here.RF glow discharge method is now called RF plasma enhanced chemical vapor deposition ( RFPECVD ) and is still the most commonly used A - Si: H deposition technology in laboratory and industrial mass production.The bottleneck of RFPECVD technology is the relatively low deposition rate (?12A/s）。If the amorphous silicon thin film solar cell technology is to be upgraded to mass production, a high deposition rate is very important, requiring 1020a / so 5.4.1 RF plasma enhanced chemical vapor deposition
The most common method for depositing device quality A - Si: H is radio frequency plasma enhanced chemical vapor deposition ( RFPECVD ) with a plasma excitation frequency of 13.56 MHz.The role of plasma is to provide an energy source for decomposing SiH4 gas. Secondary electrons formed by plasma accelerate in the electric field, and electrons after accumulating energy collide with SiH4 molecules to decompose SiH4 molecules into free radicals.Free radicals are radicals containing one unpaired electron.As atoms form molecules, electrons in chemical bonds must appear in pairs, so free radicals with high reactivity can seize an electron from other substances everywhere to form stable substances.Free radicals adhere to the surface of the growth film, thereby growing an amorphous silicon A - H layer.Since the device uses a - Si: H thin film with a thickness of about 0.5 mm, a - Si: H must be deposited on a suitable substrate carrier.Some of the energy transferred to molecules by collisions with electrons is visible radiation, so plasma enhanced chemical vapor deposition ( PECVD ) technology is also called glow discharge.An important advantage of PECVD deposition is that the deposition temperature of device quality A - Si: H is usually 200250 C, while the low-temperature process allows the use of various low-cost materials as substrates, such as glass, stainless steel sheets and flexible plastic sheets.
The structure of PECVD deposition system is relatively simple, including five main parts:
Stainless steel high vacuum reaction chamber: with capacitively coupled parallel electrodes, RF power feedthrough, substrate support and substrate heating device;
The gas system comprises a large flow controller and a plurality of gas valves, adjusts the gas flow required for depositing the intrinsic layer and the doping layer, and controls the gas pressure in the reaction chamber;
Supu system: Usually, the reaction gas is treated with full-wheel molecular pump and mechanical rotary pump, because under repeated impact of moving solid surface, momentum in a specific direction can be transferred to gas molecules, and the turbine rotating blades of the turbine molecular pump rapidly impact gas molecules from inside the pump to leave the reaction chamber, thus forming a high vacuum.Exhaustion system: the residual gas after the treatment process of the scrubber or combustion chamber;Electronic and control systems: DC or RF power generators with matching devices and vacuum meters, barometers and thermometers.
SiH4 plasma deposition of a - Si: H layer can be described as four steps [ 18 ]:
In a reaction in the gas phase, SiH4 molecules are excited and decomposed by electron collisions, producing various free radicals, neutral molecules, positive ions, negative ions and electrons.In plasma, molecules, ions and free radicals undergo secondary reactions to generate various reaction substances.Finally, larger Si - H groups are formed, such particles are almost dust or powder, and neutral substances will diffuse to the substrate.Positive ions bombard the growing film.The negative ions will be captured by plasma.The free radicals interact with the surface of the growth film, such as free radical diffusion, chemical bonding, H adhesion on the surface or H separation on the surface.The relaxation of H, Si atomic network is released inside the film.
SiH3 free radicals are generally considered as the main free radicals in A - Si: H growth because the outermost layer of most growth surfaces is HM, SiH3 free radicals cannot directly form bonds with growth films, but diffuse on the surfaces until they encounter dangling bonds.Si H3 radicals contribute to the growth of thin films by bonding with dangling bonds.SiH3 needs enough dangling bonds to bond with the growth film, and H atoms often form dangling bonds when they leave the surface.H atoms can leave the surface by thermal excitation or can be extracted from the surface by SiH3 radicals, thus forming a dangling bond and a SiH4 molecule [ 78 ].Although other free radicals do not contribute much to the growth of the film, they play an important role in the properties of the film.SiH2 and higher order free radicals have higher viscosity coefficient than SiH3 and can be directly bonded to the surface of outer layer H [ 26 ].However, the effect of these free radicals on the film growth will lead to poor film quality, so it is necessary to avoid the existence of these free radicals in the plasma.In addition, ions can also contribute 10 % of A - Si: H film growth [ 81 \ Overall, the deposition process is a complex phenomenon of chemical reaction between gas and surface, and the deposition parameters controlling the reaction are gas composition, gas flow rate, gas pressure, RF power density, ion excitation frequency, substrate temperature, electrode structure, etc.Luft and TSUO described the deposition conditions of A - Si: H alloy in detail [ 82 ], which was also discussed in other references [ 79 ].Through RFPECVD system, the typical deposition parameters of a - Si: H uniform thin film with device quality prepared in the laboratory are:
SiH4 flow: 2050 Seem;
Process pressure: 0.50.7 mbar;
Substrate temperature: 200250 C;
RF power density: 2050 MW / cm2;
Electrode distance: 13m;
Typical deposition rate: 12a / s.
According to these conventional conditions, it can be simply calculated that it takes 2550 min to deposit a 300 nm thick A - Si: H absorbing layer for solar 2V battery.Such a production rate is too slow for large-scale production, and only a deposition rate of 10 - 20a / s can meet the requirements of large-scale production.People are doing a lot of research to increase the deposition rate of A - Si: H, but the requirements will not affect the material quality.
The core parameter that determines the deposition rate is the power absorbed by the plasma. Higher power will produce higher electron concentration and electron temperature in the plasma [ 79 ] and promote the decomposition of SiH4 molecules to form higher concentration and other free radical concentration, thus increasing the deposition rate.However, an increase in power will lead to higher order SiH, free radical deposition conditions, and eventually powder formation.Under such deposition conditions, the concentration of H and Si H2 bonds in the film will increase' and the film quality will decrease [ 83 ].In order to form a tight si atomic network under the condition of high deposition rate, it is necessary to suppress the formation of sihs >.For this reason, the electron temperature of the plasma should be reduced and the deposition temperature should be increased.Increasing the excitation frequency of plasma can reduce the electron temperature of plasma.A high deposition temperature of < 350 c can result in a high deposition rate, thus promoting sih diffusion to the growth surface and limiting h on the growth surface.However, such high absorption layer deposition temperature will cause thermal damage U5 to the solar cell bottom layer \ inhibiting high-order SiH4 radicals and short-life radicals in the gas phase has become the main method for depositing A - Si: H with high deposition rate and high material quality.In order to achieve a higher deposition rate, PECVD process for depositing A - Si: H has been developed in two directions:
The operating conditions of PECVD technology were further studied, and high pressure and high power RFPECVTF 853, VHF - PECVD 3 and MW - PEC VTF 87 were developed.Development of novel deposition techniques, such as hot wire chemical vapor deposition HWCVDFM and expanded thermal plasma chemical vapor deposition ETP CVD 1: 89 ] 05.4.2 direct plasma enhanced chemical vapor deposition
High pressure and high power RF plasma enhanced chemical vapor deposition high pressure and high power RF plasma enhanced chemical vapor deposition RFPECVD technology improves the deposition pressure and RF power density of RFPECVD, while VHF - PECVD improves the plasma excitation frequency of RFPECVD, so they are all called direct plasma enhanced chemical vapor deposition D PECVD technology.The high-pressure and high-power RFPECVD can prepare device quality A - Si: H thin films at a deposition rate of 12 people / s, with a higher deposition pressure of 510 mbar, a higher RF power density of 270530 MW / cm2, and a mixed gas of SiH4 and H2 as the source gas [ 85 ].If the flow ratio of SiH4 and H2 is adjusted or the deposition pressure is changed at a fixed flow ratio, a transition from amorphous to microcrystalline phase will occur.The A - Fen: H absorption layer prepared at 190 C deposition temperature and 12 person / s deposition rate achieves a stable conversion efficiency of 6.5 % compared with the standard A - Si: H intrinsic layer prepared at 190 C deposition temperature and low H dilution, and the band gap of the high-rate amorphous silicon thin film solar cell is increased by 50 mV, thus the typical high open circuit voltage reaches 0.880.9 V, which makes the high-rate cell suitable as A - Si: H / A - Si: H or A - Si: H / A - A.
Similar band gap broadening phenomenon was also observed in high H content polymorphous silicon PM - Si: H [ 69 ].The deposition conditions of PM - Si: H are close to the state where powder is formed.The conditions for forming the secondary reaction plasma of the powder are to increase the deposition pressure, RF power density and electrode distance, and to reduce the substrate temperature.The key factor for deposition of PM - Si: H is to bring the plasma close to the transition condition so that tiny crystalline particles of 35 nm size are formed in the plasma and embedded in the grown A - Si: H thin film.The conversion conditions of plasma require higher h dilution, hydrogen - silane dilution scale > 30.The higher band gap of PM - Si: H compared to A - Si: H and the improved stability of PM - Si: H make it an ideal material for high V ( ) C laminated batteries and triple junction top batteries.
VHF plasma enhanced chemical vapor deposition
Very high frequency plasma enhanced chemical vapor deposition ( VHF - PECVD ) is another direct plasma enhanced chemical vapor deposition D PECVD.Researchers at the University of New Chastel in Switzerland raised the plasma excitation frequency of PECVD from 13.56 MHz to 150 MHz [ 86 ] and now call 30 - 300 MHz the VHF range.They confirmed that raising the plasma excitation frequency from 13.56 MHz to 70 MHz can increase the deposition rate from 3a / s to 10a / s monotonically, and maintaining a good amorphous silicon a - si: h film quality to a higher plasma excitation frequency changes the distribution function of electron energy in the plasma, thus making the decomposition rate of the source gas faster and achieving a higher deposition rate.The higher plasma excitation frequency allows a higher plasma power density to be applied.And still in the powder-free operating range [ 91 ].
Using VHF - PECVD technology, researchers from New Chastel University recently prepared a P - I - N amorphous silicon thin film solar cell [ 92 ] with a stable conversion efficiency of 9.5 %, which is the laboratory conversion efficiency record of the same type of cell.The deposition rate of the a - si: h intrinsic layer is about 5 persons / s, the thickness of the absorbing layer is only 0.25 mm, the short-circuit current density is very high, the initial value is > 18ma / CNT 2, and the stable value is > 17.5 ma / crit 2.The glass substrate on which the A - Si: H thin film is deposited is plated with Zn layer prepared by LPC VD by low-pressure chemical vapor deposition, and the light trapping effect is improved by using the antireflection film.
Recently, the Kais commercial PECVD deposition system in Oricon, Switzerland, has been modified to deposit A - S1: H and Si: H with a plasma excitation frequency of 40 MHz [ 21 ].The KAI - M reaction chamber uses a glass plate substrate with a size of 1.4m2.The initial conversion efficiency of a single junction amorphous silicon thin film solar cell reaches 10 %.
The VHF - PECVD equipment developed by Mitsubishi Heavy Industries of Japan can deposit a large area substrate of 1. LMX 1. 4 m, using a stepped electrode and a phase modulation method [ 93 ].The basic plasma excitation frequency is 60 MHz, and other frequencies can be selected according to the required deposition uniformity.The average deposition rate reached 11a / s and the thickness uniformity remained within 18 %.The stable aperture conversion efficiency of the P - I - N amorphous silicon thin film solar cell module reached 8 %, and commercial production began in October 2002.
The main problems of commercial large-scale application of VHF - PECVD are:
The uniformity of film thickness deposited on a large area substrate is affected by high frequency ( > 60 MHz ) electromagnetic standing waves.power is effectively coupled into the plasma.A 120-mV input, fully integrated dual-mode charge pump in 65-nm CMOS for thermoelectric energy harvester
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