A sealed lead acid battery or gel cell is a lead acid battery that has the sulfuric acid electrolyte coagulated (thickened) so it cannot spill out. They are partially sealed, but have vents in case gases are accidentally released for example by overcharging.
adding H to the plasma;
increasing plasma energy;
Increase the plasma excitation frequency.
The first two methods can be observed through the optical emission spectrum of plasma, so deposition of PTC - Si: H thin films usually requires a series of different silicon concentration ( Silane concentration, SC = CSIH. I3 / [ SiH _ T + H2 ] ).Because the / IC - Si: H layer has a large internal stress, the adhesion of thick films on glass substrates is another problem [ 2.
In the following, we will discuss the main physical properties of undoped PTC - Si: H layer and doped MC - Si: H layer, as well as their effects on microcrystalline silicon thin film solar SLA BATTERY.
undoped microcrystalline silicon layer
The intrinsic layer of undoped microcrystalline silicon MC - Si: H was deposited on different substrates to analyze the basic physical and chemical properties of the - Si: H material as the active layer of microcrystalline silicon thin film solar SLA BATTERY.The Bian C - Si: H layer can be prepared by plasma enhanced chemical vapor deposition PECVD using SiH4 and H2 precursor gases as described earlier.X - ray diffraction and Raman spectroscopy can show the change of silicon baking concentration SC, and make the deposited film in A - Si: H / MC - Si: H transition state ( A - Si: H / GC - Si;Htransition）［29］。The three main diffraction peaks of X - ray diffraction correspond to the,, and crystal planes.In some spectra, a small peak of about 38.5 appears, which comes from the A1 contact electrode.When SC is 7.5 %, the X - ray diffraction spectrum of the active layer shows the characteristic of A - Si: I4.When SC reaches 7 %, MC - Si appears in X - ray diffraction spectrum:
H - specific diffraction peaks.Therefore, we believe that
The SC corresponding to Si: H transition is between 7 % and 7.5 % [ 3 ].
In X - ray diffraction crystal testing technology, X - rays are exposed to atoms
Diffraction occurs due to scattering of electrons outside the nucleus.Since regular arrangement of atoms in a crystal will produce regular diffraction images, the distances and spatial arrangements among various atoms in a molecule can be calculated accordingly.X - ray diffraction is a useful method to analyze the spatial structure of macromolecules.Raman spectroscopy is a spectral technique for studying vibration, rotation and other low frequency modes of microscopic systems.In the process of measuring Raman spectrum, monochromatic laser light in visible light, near infrared light or near ultraviolet light interacts with molecular vibration, phonons or other excited forms in the system, causing inelastic Raman scattering, thus causing the laser spectrum to move to high frequency or low frequency, and the frequency shift gives information about phonon modes in the system.
In the following, we will discuss in detail the changes of the active layer microstructure corresponding to different SC's, and focus on the active layer microstructure close to the A _ Si = transition point.Then, we will discuss the nucleation and growth of active layers on different substrates, a simple growth model, and the optical properties and electron transport properties of Si: H materials.
Microcrystalline silicon MC - Si: H is a relatively complex material. Its microstructure is amorphous silicon A - Si: H wrapped - Si: H group, while PTC - Si: H group is composed of nanocrystalline silicon with smaller grain size.TEM plan of transmission electron microscopy shows that the # - Si: H groups composed of nanocrystalline silicon with a diameter of 10 - 20nm are embedded in A - Si: H, and A - Si: H, grain boundaries and cracks separate different groups.Because the Si grain of the group material is on the nm scale, this is why / ZC - Si: H is sometimes called " nanocrystalline silicon" T31, while microcrystalline silicon comes from the group size on the order of magnitude, which limits coplanar electron transport [ 32 ].TEM is a common microscope technique, in which a beam of electrons is transmitted through a very thin sample.During transmission, it interacts with a thin sample, and the image formed by this interaction is magnified and focused on an imaging device such as a fluorescent screen, photographic film, or CCD camera.
In general, the MC - Si: H microstructure depends on the preparation conditions and the substrate on which nucleation and growth occur.Schematic diagram of MC - Si: H microstructure evolution with silane concentration SC.If the transition state of 3 - & # / # - 31 is carefully analyzed, it can be found that the top of the substrate is a continuous complete a - si: h layer, the top of which is a starting layer with crystal nucleus, and then a mixed phase layer containing amorphous and microcrystalline phases extends outward from the crystal nucleus point to the condensation threshold, so that the conical groups grow into MC - si: h columnar crystals.A typical cone aperture angle is 15?20°［34'35］。
Typical nanocrystalline silicon has a diameter of several nm and is less sensitive to deposition conditions.The overall characteristics of these MC - Si: H microstructures and the change of the type I layer as a function of deposition conditions will provide a good help for the research and development of type I layers of similar thickness in P - I - N junction or N - I - P junction microcrystalline silicon thin film solar SLA BATTERY [ 34 ].
The working principle of AFM is to mount the probe at one end of an elastic microcantilever and fix the other end of the microcantilever. When the probe scans the surface of the sample, the repulsive force between the probe and the atoms on the surface of the sample will cause the microcantilever to deform slightly, so that the slight deformation of the microcantilever can be used as a direct measure of the repulsive force between the probe and the sample.A laser beam reflected from the back of the microcantilever to the photodetector can accurately measure the micro deformation of the microcantilever, thus reflecting the surface morphology and other surface structures of the sample by detecting the atomic repulsion force between the sample and the probe.
The crystallinity of the active layer is one of the main material parameters of microcrystalline silicon thin film solar SLA BATTERY.As mentioned earlier, the growth of MC - Si: H results in a degree of crystallinity that depends on depth, and the material generally near the substrate has a smaller degree of crystallinity.Therefore, the experimental method to evaluate the crystallinity of type I layer is very important to determine the characteristics of microcrystalline silicon thin film solar SLA BATTERY.TEM is a very useful technique, which can be used to directly observe the distribution of amorphous and microcrystalline phases, and the numerical analysis of TEM microscopic images can determine the crystal volume fraction [ 4 ].The shortcomings of TEM technology are long sample preparation time and damage to the sample layer.X - ray diffraction can also measure the average crystallinity of type I layer, but the measured crystal volume fraction has a relatively large experimental error.
Raman spectroscopy is the fastest and simplest experimental technique for measuring the crystallinity of MC - Si: H ..The Raman spectrum of a - si: h is a wide peak at 480 cm - 1, and the integral intensity ia of the peak is proportional to the volume fraction in the stimulated volume of a - si: h.MC - Si: H Raman spectrum is characterized by an asymmetric peak at 520 cm - 1 and a Raman spectrum position of monocrystalline silicon at 520cn 1.However, the asymmetry of MC - Si: H peak is due to the lower wave number tail state, i.e. the smaller peak at about 510 cT1, which represents the wurtzite Si structure caused by small size grains [ 41 ] or twins less than 10 nm.510 cn t1 and 520 cm - 1, the two peak values related to MC - si: h, have an integral strength l, which can be considered to be proportional to the volume fraction of MC - si: h.Therefore, the IJD, + FA ) ratio obtained by deconvolution and integration of Raman curves can be defined as Raman crystallinity to quantitatively describe each MC - Si: H layer and microcrystalline silicon thin film solar cell.In order to be more accurate, the quantitative measurement of crystal volume fraction, i.e. Raman crystallinity, needs to be corrected by a correction factor to obtain the true value of crystal volume fraction 126.96.36.199 nucleation and growth
Crystallinity is a key microstructure parameter. In order to obtain a higher open-circuit voltage for microcrystalline silicon thin film solar SLA BATTERY, it is necessary to focus on controlling the crystallinity of the intrinsic layer.As mentioned earlier, the crystallinity of the microcrystalline silicon / - 3 layer depends on the thickness # - # groups can nucleate on the substrate, while the MC - Si: H layer close to the transition state needs to be deposited on the continuous amorphous silicon A - Si: H starting layer.Therefore, the crystal nucleus density of microcrystalline phase is an important parameter to control the growth of MC - Si: H ..Measuring the crystal nucleus density of MC _ Si: H deposited on different rough or flat substrates near the transition state can confirm that the crystal nucleus density mainly depends on the chemical properties of the substrate.Another important aspect of MC - Si: H nucleation is local epitaxy.In P - I - N or N - I - P type microcrystalline silicon thin film solar SLA BATTERY, doped SPED layer ) is used as the city view, while I wake up and grow M locally on the nucleation layer.If the crystallinity of W is high, the type I layer grown later will have a higher crystal nucleus density.
The microcrystal phase has a growth tendency perpendicular to the surface of the local substrate, especially in the region where it has just begun to grow tens of nm above the substrate.This phenomenon has nothing to do with growth on a flat substrate, but it is important for growth on a rough substrate.The transparent conductive oxide TC can be used as the light trapping structure and contact layer of microcrystalline silicon thin film solar SLA BATTERY and become a rough substrate grown by - Si: H, with typical root mean square surface roughness on the order of tens of nm.When TCO is used as a rough substrate, amorphous phase will accumulate at the bottom of the substrate groove [' grain boundaries extend from the bottom of the groove to the top of the device, so the substrate morphology has an important influence on the average crystallinity and grain boundary density of the active layer [ 46 ].
The unique advantage of growing - Si: H thin films is that grains can grow at temperatures much lower than the melting point of Si.The formation of microcrystalline phase at such a low temperature is far from the equilibrium state, and therefore kinetics or dynamics needs to be considered.At present, the physical and chemical model of MC - Si: H growth describes the possible gas phase reaction between SiH4 and H2.For example, H + SiH4 - SiH3 + H2, whose equilibrium determines the concentration of H atoms.Through experiments under various deposition conditions, the microscopic effects of H atoms with high H atom concentration and high crystallinity were observed as follows:
Preferred etching model: H atom preferred etching removes amorphous phase but does not help to grow microcrystalline phase @;Surface diffusion model: H atoms combine exothermically on the surface, providing enough local energy % 493 for crystallization;Chemical annealing model: the infiltration of human H atoms causes chemical annealing to crystallize M in the amorphous phase.
Although these models can predict the necessary physical and chemical conditions for crystallization, they cannot explain the conical microstructure of microcrystalline phase growth ( see 188.8.131.52 Section ).From the point of view of statistical mechanics, it is possible to give a model to simulate grain growth, but at present there is no statistical growth model to explain the following phenomena:
Conical crystalline region;
The growth of microcrystal phase is perpendicular to the plane of local substrate.
After crystallization, the surface roughness increases steadily with the change of cumulative thickness.
In order to simulate the microstructure characteristics of F - Si: H grown in the unbalanced growth process at these low temperatures, our research team developed a simple discrete dynamic model of crystal growth based on Porter's model.In this numerical model [ 51' 523 ] which can simulate the roughness change and microstructure during # - s: h growth, square particles are used to represent the free radicals or atomic species of the growth film, and each particle can drop a randomly selected columnar lattice against the growth surface.When the particle reaches the growth surface, it relaxes to the lowest lattice position among the eight adjacent lattice positions.The selection principle of this newly arrived particle orientation is:
If the largest of the eight adjacent lattice orientations has a larger number, exceeding the " crystallization threshold", this orientation is the orientation of the newly grown particles.If the maximum number of orientations in the adjacent 8 lattices is below the crystallization threshold, the newly grown particles randomly select the possible lattice orientations.
According to such a particle orientation selection principle, the orientation assigned to growing particles may represent a crystalline orientation, with certain crystalline regions being filled with particles of the same orientation, while amorphous regions being filled with particles of different orientations.In such a model, the parameters are reduced to 3:
Total number of possible crystallographic orientations;
Possibility of etching.
The microstructure simulation results obtained can be directly compared with TEM micrographs.In the figure, the black part is the substrate, the mottled part is an amorphous phase starting layer composed of particles with different orientations, and various gray parts are conical crystalline regions.
This discrete dynamic model can simulate the microstructure growth process with time. It is the first model to simulate the main growth characteristics ( microstructure and surface roughness changes ) of Si: H with a simple selection principle.The model interprets growth as local self-assembly of particles under the influence of neighboring lattices.Now our research team is improving this simpler kinetic model and will further consider the crack and nanocrystalline silicon structure in the group.
Optical Bandgap of Microcrystalline Silicon PC - Si: H?5 =: 1.1 eV, which is good' with the band gap of monocrystalline silicon and both are indirect band gap semiconductors.In order to play the role of photo generation yer ( BP < source layer or absorption layer ) in microcrystalline silicon thin film solar SLA BATTERY, the type I layer needs to have a higher absorption coefficient in the solar radiation spectrum range' for am 1.5 spectrum, the 14ev range contains 85 % of the total human light energy.For monocrystalline silicon, amorphous silicon Si: H and MC _ Si: H, the absorption coefficient A is a function of photon energy.Penetration is the reciprocal of the absorption coefficient, which is 1 / a.The data of Si: H have been corrected to eliminate the influence of light scattering in the real measurement process [ 53 ].If there is no such correction, in areas where light absorption is weak ( photon energy is less than 1.1 eV ), the as - measured absorption coefficient is 510 times higher than the value in the curve, which is significantly higher than that of single crystal silicon, because the nm - level roughness formed by growth will cause a certain light trapping effect.
If a thin film is deposited on a glass substrate, two methods are required to measure the absorption spectrum in the above seven orders of magnitude:
High absorption coefficient region: reflection-transmission spectrum measurement;
Low absorption coefficient region: technologies with higher sensitivity, such as photothermal deflection spectroscopy ( PDS ), constant photocurrentspectroscopy ( CPM ) [ 54' 55 ], Fourier transform photocurrentspectroscopy ( ftps ) 1 - 56 ( see section 4.3 ).
PDS needs to measure the refractive index change caused by human light heating medium, and is especially suitable for applications where transmission spectrum is not suitable.In the PDS, the degree of refraction of the probe laser beam is proportional to the temperature gradient of the transparent medium near the surface. From the degree of deflection of the beam, the excitation radiation absorbed by the medium can be determined.CPM is widely used to determine the sub-band gap absorption spectrum of thin film semiconductors, especially A - Si: H.FTPS technology is based on the spectral dependence of measuring optical conductivity, using Fourier transform infrared spectrometer FTIR as interferometer and light source, and A - Si: H sample as detector.
The absorption coefficient spectra of A - Si: H and - Si: H consist of three parts:
The high absorption coefficient part: > 1.1 ev, caused by excitation of carriers between energy bands;The Ulbach tail state in the form of exponential function: 0.91.1 eV, caused by the disorder of static structure;Low photon energy fraction: about 0.8 eV, caused by defect density in the center of the band gap.
For monocrystalline silicon, A - Si: H or MC _ Si: H, the high absorption coefficient part is the main part of the absorption coefficient spectrum.The band-to-band absorption region is followed by a tail state that decreases exponentially in the low photon energy range.In monocrystalline silicon, the absorption tail state varying with temperature can also be observed, but such tail state is attributed to the thermal disorder state ( e.g.On the other hand, in disordered semiconductors such as A - Si: H, due to the disorder of the static structure, a more obvious exponential absorption tail state, called the Ulbach tail state, can be observed.In Si: H, Ulbach tail states caused by structural disorder can also be observed.With the decreasing exponential function, the data with Si: H below 1.1 eV can be fitted to obtain the slope of the Ulbach tail state. The exponential function is called the Ulbach parameter.The typical Ulbach parameter of device level / IC - Si: H is 3540 MeV, while the Ulbach parameter of device level A - Si: H is higher.Because there is a certain charge in the tail state, this reduces the electric field intensity ( F ) of the intrinsic layer, and a smaller Ulbach tail state is beneficial to material application in solar SLA BATTERY, which is also one of the reasons why microcrystalline silicon thin film solar SLA BATTERY use a relatively thick ( > 2mm ) absorption layer.
In the low photon energy range ( about 0.8 eV ), the absorption coefficient depends on the defect density in the center of the band gap, which acts as a recombination center.Therefore, the absorption coefficient is an important characteristic of the material quality of microcrystalline silicon thin film solar SLA BATTERY.The recombination of T - layer is one of the main factors that limit the performance of the device.The typical device level MC - Si: H has a lower absorption coefficient of 0.10.3 cm -' ( after scattering light correction ) at = 0.8 eV, while the absorption coefficient immediately after measurement is 13 cm " 10O, which is lower than the absorption coefficient of device level A - Si: H such as = 1.2 eV after photo attenuation.P - SHH intrinsic layer has a lower defect density, which is another reason why microcrystalline silicon thin film solar SLA BATTERY use a thicker absorption layer.
An important step in obtaining FTPS absorption spectrum is calibration.Although the calibration process of A - Si: H is well known, the calibration process of MC - Si: H is more critical.Because of the natural surface roughness just after growth ( as - grown ), light scattering improves the absorption coefficient just after measurement.The Ulbach slope does not depend on the calibration process, while the defect - related absorption coefficient ( DEFECT - RELATED DABSORPTION OFFICIENT ) is particularly dependent on the calibration process.Since the absorption coefficients of monocrystalline silicon and / IC - Si: H at 1.35 eV are the same, a simple way to calibrate the spectrum is to adjust the measured MC - Si: H absorption coefficient according to the value at 1.35 eV.However, even if such calibration has been used, it is not easy to measure the absolute value of the absorption coefficient.In a specific case, various light scattering characteristics will affect the measured value of absorption coefficient, such as different thickness of each thin film layer and different roughness of each thin film deposited on transparent conductive oxide TCO.
Electronic transport characteristics
For electrons and holes, the mobility lifetime product ( r ) can be conveniently used to describe the electron transport characteristics, because the two physical characteristics of drift length ( LD ) and diffusion length can be described at the same time, fully representing the material characteristics required by solar cell devices to collect charges.In microcrystalline silicon, PR of majority carriers can be obtained by measuring optical conductivity, R1 - 33,59 of minority carriers can be obtained by steady-state photogenerated carrier grating ( SSPG ) experiments, and experiments need to be carried out immediately after deposition to prevent the influence of post - oxidation.The electron transport in both measurement techniques occurs in a plane parallel to the substrate, also called coplanar electron transport, while the carrier transport direction of P - BuN type or N - I - P type microcrystalline silicon thin film solar SLA BATTERY is perpendicular to the substrate plane.As mentioned earlier, the MC - Si: H microstructure is columnar, and it can be observed that the highly microcrystal phase with columnar growth and cracks exhibits anisotropic electron transport characteristics.The surface photovoltage ( SPV ) measurement technique can determine the PR of the growth direction, and can prove the anisotropic electron transport characteristics ⑽ compared with the PX obtained from the above coplanar measurement.For the device-level MC - Si: H approaching the A - Si: H / MC _ Si: H transition state, we can assume that the degree of anisotropy is very low, as described below.the Zr value are measured in a direction parallel to that substrate.
Surface photogenerated voltage SPV technology is widely used to measure the diffusion length and surface potential of semiconductors, which is helpful to solve the minority carrier transport in determining the p _ n junction characteristics.As a non-contact method, SPV is an ideal method to describe compound semiconductor ohmic contacts and special device structures.
The measured values of photogenerated conductivity and bipolar diffusion length can be calculated to obtain the product of mobility lifetime and drift length, as shown in actual .1.The households of - Si: H and the households of amorphous silicon A - Si: H before photo-induced attenuation after annealing are basically on the order of: an order of magnitude, while the " R" of A - Si: H after photo-induced attenuation is one order of magnitude smaller than that of A - Si: H before photo - induced attenuation.These F's are far lower than the Z / R measurements of solar grade polysilicon wafers.Therefore, the diffusion-limited P _ N junction configuration cannot effectively collect charges from A - Si: H MC - Si: H, while microcrystalline silicon thin film solar SLA BATTERY and amorphous silicon thin film solar SLA BATTERY are often designed to rely on drift P - I - N junctions or N - I - P junction devices, so that the electric field in the type I layer can realize effective collection of photogenerated carriers' drift length LD given by the field strength F = 1V / mm, which can be inferred that microcrystalline silicon thin film solar SLA BATTERY can realize effective collection within the number thickness range of the type I layer. Such collection is also observed through experiments.In addition, it was observed that the electric field strength was uniform in the type I layer with the number of FXM thick P - I - N microcrystalline silicon thin film solar SLA BATTERY [ S5 ], due to the relatively low defect density ( see 184.108.40.206 Section ).
The study of the effect of the microstructure of MC - Si: H on the coplanar electron transport characteristics shows that [ 32 \ / Zr ] is not affected by the size of nano - grains, but will decrease with the decrease of group size.In these samples, the bipolar diffusion length LMB is almost 1 / 2 of the group size, which can be understood as the bipolar diffusion length is limited by the group grain boundary.According to document [ 66 ], if Si: H does not contain conical groups, but is in a state of A - Si: H / MC - Si: H transition and consists of nanocrystalline silicon embedded in a - Si: H matrix, the activation energy of dark conductivity is greater than 0.5 eV, and the electron transport characteristics will be limited by the size of nanocrystalline grains and no longer depend on the size of / xc _ Si: H groups.
doped microcrystalline silicon layer
Historically, microcrystalline silicon MC - Si: H was first used as a high quality doping layer in amorphous silicon thin film solar SLA BATTERY.In fact, - Si: H doping is easier to achieve than amorphous silicon A - Si: H doping.Compared with a _ si: h, - si: h doping can make the fermi level closer to the conduction band bottom more easily.E ) or valence band top ( ev ) o dark conductivity activation energy eau can be used as a rough estimate of the energy level distance between fermi level and band edge.In doping A - Si: H, it is not easy to realize ￠ lower than 2001 ;（：1。In the doped MC - Si: H, it is convenient to make it lower than 50meV, so that the doped layer has higher conductivity.
For doped layers, crystallinity is also an important parameter.The doped Si: H layer requires higher crystallinity to achieve higher conductivity.The crystallinity of P - type doped layer is a key issue to consider. B doping and low deposition temperature are very sensitive and will seriously affect the growth of microcrystalline phase.In particular, in order to reach the maximum crystallinity of the doping layer with a value of > = 16, the ideal range of deposition temperature is about 180 c the thickness of the p - type layer is another key factor for forming high crystallinity.In fact, it is difficult to obtain a thin P - type layer with high crystallinity.In general, the thickness of the P - type layer with maximum conductivity is at least greater than 20. The growth of the doped layer is affected by the surface roughness and chemical properties of the substrate.On the ZnO layer with high surface roughness prepared by LPC VD by low pressure chemical vapor deposition, the growth of P - Si: H has the characteristics of village bottom [ 67 ].If!The type 1 layer is deposited on the transparent conductive oxide TCO, and it can be observed that the crystallinity depends on the substrate type 1, which means that when the substrate type changes, the deposition conditions of the doped Si: H layer need to be re - optimized.
Microcrystalline silicon thin film solar cell
About 10 years ago, microcrystalline silicon MC - Si: H was identified as a promising thin film absorbing layer material, thus making C - Si: H a supplement to amorphous silicon A - Si: H to achieve a silicon-based thin film solar cell with higher conversion efficiency.The differences between C - Si: H and A - Si: H in photovoltaic applications are mainly as follows:
MC - Si: H has a band gap close to that of monocrystalline silicon, at room temperature?1.1 eV, while the band gap of A - Si: H is much higher, close to 1.75 eV;Due to the crystal characteristics of Si: H, the absorption coefficient of MC - Si: H with indirect band gap in visible light band is lower than A - Si: H, so the microcrystalline silicon thin film solar cell needs a thicker type I layer ( at least 1 mm ), while the typical thickness of amorphous silicon thin film solar cell is 0.3 PIM;Photoinduced attenuation effect or s - w effect will reduce the initial efficiency of amorphous silicon thin film solar SLA BATTERY ( initial efficiency, 683 ), but MC _ si: h basically has no photoinduced attenuation effect.
Up to now, the laboratory conversion efficiency record of single junction microcrystalline silicon thin film solar cell AM 1.5 is slightly higher than 10 % ( the cell size is smaller ) [ 18 ].However, the stable conversion efficiency of single junction amorphous silicon thin film solar SLA BATTERY after photo-induced attenuation can also reach a similar level [ 7 ].Therefore, the commercial mass production of single-junction microcrystalline silicon thin film solar cell module is not mature' in fact, because the type I layer has a larger thickness, - si: h deposition is much slower than a _ si: h, and the cost is higher' if the single-junction microcrystalline silicon thin film solar cell can achieve higher conversion efficiency through research and development, this situation can be changed.
At present, the best way to use microcrystalline silicon thin film solar SLA BATTERY is to have a non - microcrystalline laminated structure with a bottom cell and a top cell with a - si: h ( see section 4.3.3 ).Assuming that all photons with energy greater than the band gap are absorbed by the active layer and that the fill factor ff and the open-circuit voltage are close to the theoretical ideal value, it can be simply proved that the combination of the band gaps of 1.1 ev and 1.75 ev is very close to the theoretical ideal band gap, thus achieving the maximum conversion efficiency of am 1.5 spectrum [ 71' 72 ].
The condition that ATM thickness Si: H absorbs all energy larger than band gap photons is far from being realized, so the necessary light trapping structure and optical optimization are critical for microcrystalline silicon thin film solar SLA BATTERY ( see Section 4.3.1 ).Subsequently, the structure, typical electrical characteristics and methods to improve the performance of the single junction microcrystalline silicon thin film solar cell will be introduced ( see section 4.3.2 ).Finally, the photoelectric characteristics of non - microcrystalline laminated SLA BATTERY are discussed ( see section 4.3.3 ).
For all types of thin film solar SLA BATTERY, it is necessary to minimize the thickness of the thin film and improve the effective optical path length through proper optical optimization and light trapping.Optics optimization is very important for amorphous silicon thin film solar SLA BATTERY and more important for microcrystalline silicon thin film solar SLA BATTERY.As mentioned earlier, as an indirect band gap semiconductor, microcrystalline silicon P - SHH has a much lower absorption coefficient than amorphous silicon A - Si: H, and the deposition rate of PTC - Si: H is limited, so the thickness and deposition time of type I layer are the key cost factors for large-scale production of microcrystalline silicon thin film solar cell modules.Since optical optimization and light trapping structure depend on the choice of substrate material type, we will first discuss alternative substrate materials.
Choosing suitable substrate materials is a necessary prerequisite for mass production of low-cost solar cell modules.Up to now, most thin film solar SLA BATTERY use glass substrates plated with transparent conductive oxide TCO.Commercial TCO manufacturers generally deposit TCO on cheap soda-lime glass to achieve cost - effectiveness.However, microcrystalline silicon thin film solar SLA BATTERY prepared on more expensive glass with low Fe content can achieve higher conversion efficiency, and lower light absorption in red and near infrared bands can increase the full utilization of microcrystalline silicon FTC - Si: H response to long wavelength bands.Through the completely transparent substrate and contact layer, the microcrystalline silicon thin film solar cell can be prepared into a P - I - N type stacking structure, and human light enters the active layer from the P type layer.Such an upper layer configuration has a TCO layer with high surface roughness, and a rough TCO / P type # - Si: H interface can realize the required optical optimization and light trapping structure ( see 220.127.116.11 Section ).On the other hand.The P - I - N type upper layer glass substrate also serves as a cover glass for packaging design.Although the glass is fragile and relatively heavy, the glass substrate can resist the influence of severe weather on the solar cell module to ensure the long-term stability of the module for outdoor use.
Another choice of substrate material is stainless steel sheet and is suitable for roll - to - roll process. However, microcrystalline silicon thin film solar SLA BATTERY on stainless steel sheet substrates are not yet able to fabricate monolithic components using integrated interconnection.
Using non-conductive flexible substrates such as plastic sheets to develop lightweight and non-fragile components is also an important research direction at present [ 72 - 74 ].The single junction microcrystalline silicon thin film solar cell with plastic sheet substrate prepared in the laboratory can already approach the performance of microcrystalline silicon thin film solar cell with TC glass as substrate.
Microcrystalline silicon thin-film solar SLA BATTERY made of opaque and UV - attenuating substrates such as stainless steel sheets or plastic sheets often adopt an N - I - P type stacking structure, and human light still enters the active layer from the P type layer, but the P type layer is the last semiconductor layer deposited in the continuous deposition process, and then a TCO layer and a protective layer with a laminated packaging function are prepared. Indiumtin oxide ( ITO ) or ZnO are used as TCO.In general, the N - I - P type substrate configuration may allow almost any material to be used as the substrate because plasma enhanced chemical vapor deposition PECVD with low deposition temperature is used.On the other hand, one of the main cost factors for substrate configuration is the top protective layer material, which requires high transparency, UV protection and long-term stability.The cost-effectiveness of substrate configuration of microcrystalline silicon thin film solar SLA BATTERY is also the focus of research and development.
In the N - I - P type substrate configuration, the effective light trapping structure can be formed by texturing the substrate or directly preparing a textured surface layer on the substrate by trimming the surface roughness of the back mirror.N - I - P type microcrystalline silicon thin film solar SLA BATTERY often use hot silver to prepare irregular metal mirrors [ 76 ], or use structured substrates such as periodic gratings specially adjusted to broadband reflectivity as back mirrors [ 75 ].After TCO is deposited on a metal mirror or periodic grating, the surface still has the shape of a back mirror, so light scattering can occur at the MC - Si: H / TCO interface.One of the main advantages of the N - I - P type substrate configuration is that the fluff with light trapping effect is formed at the bottom of the cell before the Si active layer is deposited, giving more options to the process method and substrate material.
Transparent conductive oxide
Transparent conductive oxide ( TCO ) is required for both the upper layer configuration and the substrate configuration of microcrystalline silicon thin film solar SLA BATTERY.The TCO layer becomes a transparent contact electrode on the front surface of the human light entering cell.On the back surface in front of the back mirror, the TCO layer can improve the optical characteristics through four matching of refractive index and also can act as a diffusion barrier.Microcrystalline silicon thin-film solar SLA BATTERY have some strict requirements on the front TCO layer and the back TCO layer, as shown in actual .2 ..The P - I - N type upper layer configuration has particularly strict requirements on the front surface TCO layer, which is worth discussing.
The front surface TCO layer of the upper layer configuration and the substrate configuration needs to meet two requirements:
High conductivity keeps the series resistance of microcrystalline silicon thin film solar SLA BATTERY at a low level, and the typical square resistance cannot be greater than 10FL / O;The high transmittance or transmission throughout the solar spectral pivot minimizes the loss of absorbed and reflected light, with a typical average absorbance in the 400 - 1100 nm range of less than 6 % 7 %.
The front TCO layer of the P - I - N upper layer configuration also requires the following two additional requirements:
Higher surface roughness results in better light scattering ability.
It has high chemical stability in the process of preparing Si active layer by plasma enhanced chemical vapor deposition PECVD.The three most commonly used TCO materials for thin film solar SLA BATTERY are:
indium tin oxide ITO;
ITO can meet the two requirements of the upper layer configuration and the substrate configuration for the front surface TCO layer, and ITO is indeed used as the front contact for the N - I - P type substrate configuration of microcrystalline silicon thin film solar cell M ..However, ITO is unstable in SiH plasma and cannot be used as a front surface TCO layer of a P - I - N type upper layer configuration microcrystalline silicon thin film solar cell.If Si thin film is deposited on ITO, it will surely cause serious problems such as in diffusion and reduction of minority carrier lifetime in Si active layer.
S NO2 is another common TCO.Glass plated with S NO2: F is already a commercial raw material, usually used as TCO for amorphous silicon thin film solar SLA BATTERY and components.The surface roughness of S NO2: F contributes to better light scattering and trapping.However, S NO2: F can not be directly used as the front surface TCO layer of the microcrystalline silicon thin film solar cell on the upper layer, because there will be a high concentration of H plasma during the next deposition of microcrystalline silicon MC _ Si: H, which will cause a certain reduction reaction of the oxide layer and affect the transmittance.Some laboratories cover the S NO2: F layer with a very thin layer of 21!0 layer [ 77 ] or TiO 2 layer [ 78 ] before S NO2: F can be used as the front surface TCO layer of microcrystalline silicon thin film solar SLA BATTERY.
Our lab focuses on preparing amorphous silicon thin film solar SLA BATTERY and microcrystalline silicon thin film solar SLA BATTERY using ZnO as TCO layer.In fact, ZnO has considerable advantages as TCO:
The supply of abundant, non-toxic and harmless materials;
can be easily deposited at a low temperature;
No reduction reaction occurs when exposed to subsequent high concentration H plasma.
ZnO can be conveniently prepared by sputtering or low-pressure chemical vapor deposition LPC VD.The aluminum-doped zinc oxide ( ZnO: Al ) prepared by sputtering is basically flat, so the deposited ZnO: A1 does not have the light trapping effect required by the TCO layer on the front surface.However, the necessary light trapping structure T79 can be obtained after the NM - level texturing by wet chemical etching.
On the other hand, ZnO deposited by LPC VD has several obvious advantages [ 8' 81 ].LPC VD is a very simple process and can be easily upgraded to a maximum area of 1m2, with deposition rate exceeding 2 nm / s.Due to the columnar crystal growth with obvious crystal plane preferred orientation, the deposited ZnO layer has a better texture.TEM cross-section shows that the typical thickness of ZnO columnar microstructure is 1 mm, and each pyramid structure has a ZnO grain protruding from its surface.Scanning electron microscope ( SEM ) is a common electron microscope, which scans the surface of a sample with a grating pattern through a high-energy electron beam and forms an image.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.
Typical transmittance spectra of ZnO prepared by LPC VD under low pressure chemical vapor deposition.In the short wavelength region, the transmittance is limited by ZnO optical band gap ( 3.2 eV ).In the region near the river, the transmittance is limited by free carrier absorption.Free carrier absorption means that after the semiconductor material absorbs human light, it does not excite valence band electrons to jump II to the conduction band, but excites valence band electrons to obtain kinetic energy and jump to a higher energy level in the same valence band, so free carrier absorption is dominated by infrared sheath.In order to apply ZnO as front contact, it is often necessary to balance the square resistance and transmittance of ZnO.Square resistivity is defined as the resistivity of the film divided by the thickness of the film.If the film thickness is increased, the square resistance decreases, but the light absorption increases and the transmittance decreases.In order to minimize the square resistance, it is necessary to minimize the resistivity of the film.If the carrier concentration is increased to decrease the resistivity, it will also increase the light absorption of ZnO thin films in the near infrared band.Listen, the carrier mobility of TCO needs to be increased to reduce resistivity and square resistance.High mobility TCO is a key area of recent research and development, and it is of special significance for microcrystalline silicon thin film solar energy with available spectral range extending to near infrared band.In order to achieve a high mobility TCO, people are not only trying to improve the ZnO layer, but also trying new TCO room materials [ 82 ].
The optical optimization of the P - I - N type upper layer configuration depends on the TCO / P type layer interface in two aspects.First, low refractive index materials and high refractive index materials ( Kiss?The rough interface between 3.7 ) will reduce the reflection, because the index matching by Jin depends on the effective index given by the volume fraction of the interface.This effect of surface roughness on reflection reduction can be applied to the corresponding band of the whole solar cell spectrum.In order to minimize reflectivity, it is necessary to improve the root mean square surface roughness of TCO / P layer interface as much as possible.Another rough TCO / P layer interface is the light trapping structure of Si layer.The angle of some human light passing through diffuse transmission is larger than the critical angle, and then total reflection occurs in the propagation process in the Si layer, greatly increasing the optical path length.Trapping light is important to the band where light absorption is weak because most of the light at these wavelengths is reflected by back contact.In this case, the scattering of the front interface and the back interface can increase the optical path length in the Si layer by 45 times.
In order to carry out more effective optical optimization of microcrystalline silicon thin film solar SLA BATTERY, it is necessary to further study the TCO mean square surface roughness and geometric structure and optimize the feature shape and feature size of the TCO surface.However, numerical simulation has been able to calculate the effect LM of specific TCO on the spectral response curve and short-circuit current density of microcrystalline silicon thin film solar SLA BATTERY.On the whole, higher surface roughness will lead to higher surface roughness. On the other hand, TC 0 mmwm with higher surface roughness will reduce the open circuit voltage and filling factor of microcrystalline silicon thin film solar SLA BATTERY. Therefore, a lot of research work is still needed to optimize TCO of single junction microcrystalline silicon thin film solar SLA BATTERY.
Single junction microcrystalline silicon thin film solar cell
The single-junction microcrystalline silicon wafer is very sensitive to the electric dragon. We made Falhenggong at Swiss Petel University. Several laboratories subsequently developed the P - I - N type upper layer configuration and N - I - P type substrate configuration with conversion efficiency in the range of 8 % to 10 % UUO.A typical P - I - N type upper layer is configured with a SLA BATTERY structure.The transparent conductive oxide TCO here is prepared from LPC VD by low pressure chemical vapor deposition.Determining the state-of-health of maintenance-free aircraft batteries
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