The most popular material for creating solar cells right now on the photovoltaic market is silicon, which comes in three primary varieties: monocrystalline silicon solar cells, polycrystalline silicon solar cells, and amorphous silicon solar cells. The first two reasons a specific thickness is needed when absorbing solar energy since the materials are indirect bandgap semiconductors. Because the PN junction is relatively thick (often greater than 200 microns), more silicon raw material must be used, which raises the cost. The cost of a solar panel is still considerable, and the silicon waste it produces is also quite substantial. Silicon is a crucial and highly adaptable semiconductor.

Amorphous silicon has a wide spectrum of light radiation absorption, a small needed thickness, and is a direct bandgap semiconductor. As a result, thin film solar cells constructed of amorphous silicon may be produced incredibly thin. The market prefers amorphous silicon due to its lower cost and superior performance, and the entire thickness of the light absorption sheet is roughly 1 micron.

Ⅰ. Characteristics of amorphous silicon solar cells

Low cost

1. Light can be completely absorbed by the silicon material while using less material. The amorphous silicon should be one micron thick, and the single crystal should be 200 microns thick (amorphous silicon has a large light absorption coefficient).

2. Silane is the primary raw material utilized in the manufacture of high-purity polysilicon. The chemical industry can provide this gas in big volumes at a low cost. 6%).

3. Crystalline silicon solar cells range in thickness from 240 to 270 um, a difference of more than 200 times. There are a staggeringly vast number of semiconductor grades needed for wide-scale manufacture. 65-70% of the price of the total solar cell is made up of the cost of silicon wafers alone. One watt in China. The cost of silicon material for crystalline silicon solar cells has risen to more than RMB22.

Humans use sunlight to produce electricity on a big scale, and there is no other option from the standpoint of raw material supply other than amorphous silicon solar cells and other thin-film solar cells.

Easy to form large scale

The pn junction and the accompanying laminated structure can only be produced by modifying the gas phase composition or gas flow; the production can be fully automated because the core method is suited for the fabrication of a-Si alloy thin films with very large areas without structural faults.

Variety and versatility

Thin-film a-Si solar cells are simple to integrate, and the device power, output voltage, and output current may be arbitrarily developed and made. This makes it simple to build a variety of products that are suitable for various applications. It is useful for creating low-power power sources for indoor use, such as watch batteries, calculator batteries, etc. because of the high light absorption coefficient and low dark conductance. The silicon mesh structure of the a-Si film has high mechanical qualities that make it ideal for use in light solar cells on flexible substrates. Building-integrated batteries can be created using a variety of adaptable manufacturing techniques, making them ideal for the installation of rooftop solar power systems for homes.

Amorphous silicon can avoid the issue of lattice mismatch between the material and the substrate that must be taken into account in the formation of crystals since it lacks the periodic atomic arrangement needed for crystals. As a result, it may be easily applied across a vast area and can be placed on practically any substrate, even cheap glass substrates.

Good performance

Amorphous silicon thin-film cells generate 15% more power annually than single-crystalline silicon cells do under the same lighting circumstances. The efficiency-to-mass ratio of amorphous silicon cells is also the highest (i.e., the material is light and efficient), and it is six times higher than that of monocrystalline cells, making it appropriate for the future construction of space solar power plants.

Ⅱ.  Development history of amorphous silicon solar cells

Since 1974, researchers have been studying amorphous silicon solar cells after realizing the potential use of doped amorphous silicon thin films in solar cells.

The conversion efficiency at the time was less than 1%, according to Carlson of RCA, who created amorphous silicon solar cells using metal-semiconductor and p-i-n device architectures.

1977: Carlson raises amorphous silicon solar cells' conversion efficiency to 5.5%.

Japanese authorities first use integrated amorphous silicon solar cells in 1978.

1980: Using a metal-insulator-semiconductor (MIS) structure, ECD created an amorphous silicon solar cell with a conversion efficiency of 6.3%; pocket calculator for silicon solar cells.

1982 saw the introduction of watches, chargers, radios, and other goods incorporating amorphous silicon solar cells.

Beginning in 1984, composite solar cells made on amorphous silicon were used as standalone power sources.

The most promising solar cells are those made of amorphous silicon. As a result, its standing within the semiconductor solar cell industry as a whole is improving. Amorphous silicon solar cells account for practically all of the portion used for civilian purposes and make up around one-third of the total solar cell production in the world today in terms of electric power.

Ⅲ.  Amorphous silicon solar cell structure

Figure. 1

Figure. 2

In contrast to monocrystalline silicon solar cells, which typically have a p-n structure, amorphous silicon solar cells typically have a p-i-n structure. This is due to the fact that lightly doped amorphous silicon has a smaller Fermi level shift, and the band bending will also be smaller if a material is lightly doped on one side and substantially doped on the other side is employed. If the p+-n+ junction is directly made using severely doped p+ and n+ materials, then the minority carrier lifetime is low due to the larger density of defect states in the heavily doped amorphous silicon material, and the performance of the battery is decreased. a horrible idea. As a result, the active collector area is often formed by depositing an undoped amorphous silicon layer between the two substantially doped layers.

In contrast to crystalline silicon solar cells, where photogenerated carriers primarily travel due to diffusion, photogenerated carriers in amorphous silicon solar cells are primarily generated in the undoped i-layer. Photogenerated carriers in amorphous silicon solar cells primarily rely on the solar cell's electric field to cause their drift motion.

The top, heavily doped layer in amorphous silicon solar cells is thin and nearly transparent, allowing incident light to pass through to the undoped layer and produce free photogenerated electrons and holes. The higher built-in electric field likewise spreads essentially from this point, and the photogenerated carriers are promptly swept to the n+ and p+ sides after being produced.

Since undoped amorphous silicon is essentially a weak n-type material, it can be made into an i-type with the Fermi level centered by adding a trace amount of boron when depositing the active collector region. This helps to increase solar energy. battery efficiency In order to ensure that the active collector region is naturally doped with boron when the p-layer is formed, the deposition sequence is frequently set up as p-i-n during the actual preparation process. The transparent conductive substrate cell will always face the p+ layer according to this deposition sequence, while the opaque substrate cell will always face the n+ layer.

Tandem amorphous silicon cells The theoretical maximum conversion efficiency for single-junction solar cells, even when made of crystalline materials, is often just around 25% under AM1.5 light. This is true because the solar spectrum's energy distribution is quite broad, and any semiconductor can only absorb photons whose energy is greater than the value of its own band gap. The remaining photons either transfer energy to the atoms of the battery material itself, causing the material to heat up, or they travel through the battery and are absorbed by the metal on the backside and converted to heat. By producing photogenerated carriers, none of this energy can be transformed into electricity. Not only that, the thermal effect of these photons can also increase the battery operating temperature and degrade the battery performance.

in order to make the most of solar energy's efficiency throughout a larger range of wavelengths. In order to create cells with a spectral response that is near to the spectrum of sunlight, people divide the solar spectrum into many parts and choose materials whose energy gaps are best matched to these regions. A solar cell with this kind of construction is known as, as illustrated in the figure: This battery is laminated.

Figure. 3

Ⅳ.  Amorphous silicon thin film solar cell production and manufacturing process

The following equipment is primarily used in the production of amorphous silicon thin film solar cells: conductive glass cleaning and edging equipment, large-scale amorphous silicon thin film PECVD production equipment, infrared and green laser engraving equipment, large magnetron sputtering production equipment, and component testing equipment.

1. Grinding: Make careful to chamfer the eight edges of the clear conductive glass and remove any sharp edges from all of the glass's corners.

2. Once-off cleaning: thoroughly washing and drying the transparent conductive glass surface (double-sided).

3. To prevent the sub-battery from being short-circuited, laser-engrave lines on the clear conductive glass, reserve a specific number of substrates, and use a multimeter to measure > 1M (megaohm); in production, we use 20K. The conductive film was used for the measurement (double junction product: there are 39 cells, and the sub-cell spacing is 15.5mm).

4. Secondary cleaning: washing, drying, and making sure the transparent conductive glass surface is spotless.

5. Loading: Insert the conductive glass into the frame of the workpiece so that it can be heated up and coated. You must watch out that the conductive glass film surface doesn't reverse during the loading operation.

6. Preheating: Warm the conductive glass to the temperature needed for PECVD deposition (215°C in the preheating furnace), making sure that the glass is heated evenly.

7. PECVD: Under vacuum, a large-area uniform PIN layer is applied to the conductive glass. The battery chip shows no glaring chromatic aberration or stripes after deposition. When the battery chip is exposed to light from a flashlight, there are no evident pinhole phenomena.

8. Cooling (unloading): After cooling in the workpiece rack, the glass is unloaded. Glass should not be loaded into the workpiece rack at high temperatures since doing so could cause it to flex or break.

9. Use a 532 green laser to scribe lines on the partially finished chip where the silicon layer has been placed, leaving a conductive channel free for attaching the sub-battery. The laser spot is uniform, smooth, and clean; it can't be round or crooked, and it can't have burrs.

10. PVD (magnetron sputtering): The back electrode (AL or AZO+AL) is plated, the battery chip does not peel after aluminum plating, and the resistance of the back electrode is less than 10 ohms.

11. Three laser scribing: To complete the series connection of the sub-cells, scribe lines on the partially finished chip with an aluminized back electrode.

12. Sweep the edge: To reach the insulation state, clear the 10mm area along the chip's four edges.

13. Annealing: Recombining the thin film material's microstructure through heat treatment to increase its stability can increase the battery chip's conversion efficiency.

14: Check: Check the battery chip's electrical specifications.

15. Back-pressure: Back-pressure fixes a chip's flaws to increase the chip's conversion efficiency.

Among all photovoltaic cells, the production procedure for amorphous silicon thin film solar cells is the quickest. But from film design to process control, the requirements for vacuum PECVD fabrication of amorphous silicon semiconductor films are highly stringent.

While the large-area modules created by the technique have not yet attained the authorized efficiency of 6% on most manufacturing lines, the current laboratory preparation of small-area cells has approximately 15% photoelectric conversion efficiency. The limit of large-area components is, theoretically, between 85% and 90% of that of small-area components. A production line's technical sophistication can be inferred from the variation in photoelectric conversion efficiency between small-area components and other components.

Due to the performance of almost all other types of photovoltaic modules (including the currently hotly contested microcrystalline silicon thin-film cells) in actual operating conditions of high temperature, cloudy days, and low illumination, only a few amorphous silicon production lines in the world have produced modules with a certified efficiency of more than 8%. However, under these actual operating conditions, amorphous silicon modules alone function superbly, leading to various types of large-area thin-film cells producing modules with certified photoelectric conversion efficiencies of over 12% in the past, even at scale. With their lowest manufacturing costs, moderate, and not as bad as misreported photoelectric conversion efficiency, as well as their beautiful translucent modules, bendable, high-quality Light and unbreakable flexible components, and other applications that only have the superior performance of amorphous silicon, amorphous silicon photovoltaic modules cannot be eliminated for power generation applications.

Ⅴ.  Defects in amorphous silicon materials - photodegradation effect

The light-induced deterioration effect is a severe drawback of hydrogen-doped amorphous silicon thin films used in solar cells. Due to the weak Si-H bond (bond energy 323), H is quickly lost and a significant number of Si dangling bonds are produced when the hydrogenated amorphous silicon film is exposed to strong light or current for an extended period of time, decreasing the electrical characteristics of the film. This loss of H behavior also results in a chain reaction, as the dangling bond of the lost H attracts the H atom on the neighboring bond, loosening the Si-H bond surrounding it, leading the nearby H atom to combine into H 2, which (H-H bond energy 436) makes it easier to produce H2 bubbles. The performance of solar cells will be significantly impacted by the photoelectric conversion efficiency, which will degrade as illumination time increases. Amorphous silicon solar cells have a limited conversion efficiency since the material itself is not responsive to the long-wavelength part of the solar radiation spectrum due to its optical band gap of 1.7 eV.

In order to avoid the weakening of the absorption of incident light caused by the thinning of the thickness, a multi-level solar cell group can be produced by connecting numerous batteries in series. As a result, the thickness of the i-layer in the battery can be lowered. absorb. Tandem solar cells replicate one or more p-i-n single junction solar cells on their constructed counterparts.

Figure. 4

One PIN junction is layered on top of another, as seen in the above diagram. The idea behind tandem amorphous silicon solar cells is that any existing semiconductor material can only absorb photons whose energy is higher than its energy gap value because of the large energy distribution in the solar spectrum. Less energetic photons from sunlight will pass through the battery, be absorbed by the metal of the back electrode, and be transformed into heat energy. In contrast, high-energy photons with energies above the energy gap width will transfer their energy to the battery material itself through the pyrolysis of photogenerated carriers. The material itself heats up due to the atoms in its lattice structure. None of these energies can be converted into useful electrical energy by photogenerated carriers and delivered to the load. Therefore, the theoretical maximum conversion efficiency for single-junction solar cells is often only around 25%, even if they are comprised of crystalline materials. If the sunlight spectrum can be broken down into several separate parts, the battery should be made of a material that has the best energy band width match for each of these parts, and it should be superimposed from the outside to the inside in the order of the energy gap from the largest to the smallest. In order to maximize the conversion of light energy into electrical energy, the light is used by the outermost wide-gap material cell and the longer wavelength light can be transmitted into and used by the narrower energy-gap material cell. Laminated cells are one type of such cell structure.

The efficiency of amorphous silicon cells with reduced conversion efficiency brought on by photo-induced degradation, in which the H-H bond is broken and the Si-H bond is re-formed, can also be restored to 80%-97% of the original value after annealing at 130-175 degrees Celsius, a capability that other batteries lack.

Ⅵ.  Amorphous silicon cell performance influencing factors and development prospects

The long-range disordered random network structure of the amorphous silicon structure strongly scatters the carriers, making it unable to gather the carriers efficiently. In general, the p-n structure of monocrystalline silicon solar cells is not used to increase the conversion efficiency and stability of amorphous silicon solar cells. This is so because amorphous silicon with light doping has a tiny Fermi level shift. The energy band bending will be lessened and the battery's open circuit voltage will change if both sides are mildly doped or if one side is weakly doped and the other side is strongly doped. Limitation; Due to the large density of defect states and short minority carrier lifetime in the severely doped amorphous silicon material, the cell performance will be poor if the p+-n+ junction is created directly using heavily doped p+ and n+ materials. In order to create a p-i-n structure, an undoped amorphous silicon layer (i-layer) is typically placed between two strongly doped layers.

In contrast to crystalline silicon solar cells, where carriers migrate primarily due to diffusion, photogenerated carriers are primarily generated in the undoped i-layer in amorphous silicon solar cells. The battery's electric field is mostly what drives the small's drift movement. When an amorphous silicon battery uses a pin structure, it can operate in the presence of light, but because of the effect of light-induced deterioration, the battery's performance is unstable and its conversion efficiency gradually decreases over time. As a result, the battery's structure and manufacturing process need to be further optimized.

The transparent conductive film, window layer characteristics (such as optical band gap width, conductivity, and doping concentration), window layer activation energy, and window layer light transmittance are the main variables affecting the conversion efficiency and stability of amorphous silicon cells. Overrate), the energy gap matching and interface state (density of the interface defect state), the thickness of each layer (particularly the thickness of the I layer), and the design of the solar cell, among other factors. Amorphous silicon thin-film cells typically consist of heterojunctions that have been integrated, stacked, or otherwise built.

Amorphous silicon cells are produced using a straightforward technique that uses little energy and little heat, and its market share is growing every year. Amorphous silicon thin-film technology is currently used by more than half of thin-film solar cell businesses, and it is anticipated that within a few years, amorphous silicon thin-film will occupy a significant share in future thin-film solar cells. The two main issues with existing amorphous silicon thin-film cells, however, are low photoelectric conversion efficiency and light-induced deterioration. People must intensify their exploration of novel device structures, novel materials, novel processes, and novel technologies in order to increase efficiency and stability.

For instance, stacked and integrated battery structures are used; in the area of transparent conductive films, a transparent conductive film that not only has low resistivity but also has the ability to block ion pollution, increase incident light absorption, and have anti-radiation properties is used to replace the current ITO, ZnO, ZnO#Al, and other conductive films; in the area of window layer materials, investigate new window layer materials with wide optical band gap and low resistance materials, such as amorphous silicon carbon, amorphous silicon oxygen, microcrystalline silicon, microcrystalline silicon carbon, etc.; The amorphous silicon film preparation techniques of RF-PECVD, ultra-high vacuum PECVD, very high frequency (VHF) PECVD, and microwave PECVD can be enhanced to increase the carrier transport capability, electron density, and photon life of the film. Performance, stability, etc.; with regard to interface treatment, buffer layer insertion and hydrogen passivation technology can be utilized to lower interface recombination loss and enhance battery short-circuit current and open-circuit voltage. Although the main barriers to the large-scale industrial production of amorphous silicon thin film solar cells are their poor efficiency and unstable performance, numerous solutions to improve amorphous silicon thin film cells are yet conceivable. There will be widespread use of thin-film solar cells.