What are high efficiency solar cells?
The development of photovoltaic technology has been centered on two main topics: one is to improve the photoelectric conversion efficiency; the other is to reduce production costs. If the efficiency of solar cells and photovoltaic modules is improved without increasing production costs, that is the best technical direction. Therefore, the development of high-efficiency photovoltaic modules is one of the most effective measures to further reduce the cost of photovoltaic power generation.
High-efficiency crystalline silicon photovoltaic modules mainly rely on the conversion efficiency and output power of the battery itself. Therefore, whether it is a single crystal silicon battery or a polycrystalline silicon battery, it is necessary to develop a special process to achieve a further increase in efficiency. At present, the mainstream high-efficiency battery types on the market mainly include SE, PERC, PERT, HIT, IBC, MWT, etc. Here is a brief introduction to these batteries, and a brief description of the corresponding packaging technology and component characteristics. Let us understand what is a high-efficiency solar cell.
SE (Selective Emitter) battery is a selective emitter battery, which is mainly manufactured by selective diffusion process. The main feature is high phosphorus doping in the metallized area and low phosphorus doping in the illuminated area to reduce contact resistance and improve short-wave response. Thereby improving battery efficiency. Such components need to consider the absorption of short-wave light by the packaging material, otherwise the short-wave response advantages of the SE battery will not be brought into play. Choosing suitable materials for matching, the power of SE battery modules is generally increased by about 2W.
PERC (Passivated Emitter and Rear Cell) is an AlOx/SiNx: H double-layer passivation film added to the back of a conventional all-aluminum back field solar cell, and the back electrode contacts the substrate through an opening penetrating the passivation film. Due to the use of back passivation, the recombination rate of photo-generated carriers on the back surface of the battery is reduced, and the long-wave response of the solar cell is improved, thereby improving the conversion efficiency of the solar cell. PERC can be used in p-type monocrystalline and polycrystalline cells. The efficiency can be increased by about 1%, and the corresponding module power is more than 10W higher than that of ordinary battery modules. It is suitable for use in low-irradiation areas where scattered light accounts for a large amount. This type of module has been mass-produced. The packaging process of PERC cells is exactly the same as that of conventional crystalline silicon cells.
PERT (Passivated Emitter, Rear Totally-diffused) solar cells are high-efficiency crystalline silicon solar cells that use n-type monocrystalline silicon wafers as the substrate. Since phosphorus-doped n-type silicon wafers are used as substrates, there is no light-induced attenuation caused by “B-O pairs” (boron-oxygen pairs). The front side of the n-type PERT solar cell is a boron-doped p+ emitter, and the back side is a whole phosphorus-doped n+ back surface field. Generally speaking, the power of the n-type PERT battery module can be more than 5W higher than the power of the p-type PERC.
The p-type PERC cell and the n-type PERT solar cell can use grid electrodes on both the front and back sides. Light can be injected into the battery from the front and back sides of the battery to generate electricity at the same time. Such a battery is called a double-sided battery. In the early days, a transparent backplane was used on the back of the double-sided battery. However, the temperature of the backplane will rise due to the power generation on the back, which is prone to serious yellowing. Therefore, the structure of double-sided glass is usually used for component packaging.
The HIT (Heterojunction with Intrinsic Thin-layer) solar cell uses a heterojunction structure of amorphous silicon and monocrystalline silicon, using n-type monocrystalline silicon wafers as the substrate, and a layer of intrinsic amorphous silicon is sequentially deposited on the front side The film and a layer of p-type amorphous silicon film are used as emitters, and a layer of intrinsic amorphous silicon film and a layer of n-type amorphous silicon film are sequentially deposited on the back surface as the back surface field. Because the band gap of amorphous silicon is larger than that of monocrystalline silicon, and the amorphous silicon film has an excellent passivation effect on the surface of monocrystalline silicon wafers, the open circuit voltage of HIT solar cells far exceeds that of monocrystalline silicon solar cells, the highest Up to 750mV. Therefore, HIT batteries have the advantages of high conversion efficiency and good high-temperature characteristics. The efficiency of HIT battery is 1%~1.5% higher than that of conventional battery. Correspondingly, the power is 15~20w higher. The module temperature coefficient is about -0.29%/℃ (generally crystalline silicon battery is -0.44%/℃), and the temperature is stable. Better, so it has a better power generation effect, especially in high-temperature areas. The HIT battery process is simple and the preparation temperature is low, but the equipment requirements are very high. At present, there is no very mature equipment, so the yield rate is relatively low and the cost is relatively high. From the perspective of technological development trends, HIT batteries will become a new mainstream technology direction. HIT solar cells can also be made into double-sided cells. Due to the particularity of HIT cell materials, it is recommended that photovoltaic modules adopt a double-layer glass packaging structure.
IBC (Interdigitated Back Contact) solar cells are interdigitated back contact solar cells, invented by R.J. Schwartz and M.D. Lammert in 1975. IBC solar cells use n-type monocrystalline silicon wafers with high minority carrier life as the substrate. The emitter and back field are located on the back of the battery and are arranged in a finger-shaped arrangement. The front of the battery does not have any grid lines, which is completely eliminated. The light is blocked. Therefore, IBC solar cells have very high short-circuit current and conversion efficiency. The efficiency of the IBC battery is more than 1.5% higher than that of the conventional battery, so the module power is more than 15W higher. At the same time, the temperature coefficient of the IBC battery is about -0.38%/℃, which is better than the ordinary crystalline silicon battery, and the actual outdoor power generation performance has a greater advantage. The IBC solar cell can combine the emitter and base electrodes at the two ends of the battery, and use specially designed tinned copper sheets to connect two adjacent cells in series during welding, and the lamination method is the same as that of conventional cells. In addition, there are no metal grid lines and interconnecting bars on the front of the IBC photovoltaic module, the color is the same, and it is very beautiful.
MWT (Metal Wrap Through) solar cells and EWT (Emitter Wrap Through) solar cells are the other two types of back-contact solar cells. Among them, the MWT solar cell retains the thin grid on the front of the battery, and the main grid is led to the back of the battery through the small hole penetrating the silicon chip, which eliminates the sun’s shading by the main grid. The EMT battery goes a step further. The front emitter is led to the back of the battery through a small hole through the silicon chip, so that there is no grid electrode on the front of the battery. Because the emitter electrode of these two batteries is surrounded by the aluminum backing area on the back, the electrode welding method of conventional batteries cannot be used. Usually an interdigital conductive back plate is used, and conductive adhesive or low-temperature welding conductive paste is used. The electrodes of the battery and the conductive backing plate are glued together, and an effective connection is formed between the battery and the conductive backing plate in the subsequent lamination process, and finally the assembly is packaged.
As mentioned above, SE cells and PERC cells are the same as conventional solar cells, and their component production processes are exactly the same. The welding method of IBC solar cells is different from that of conventional solar cells, but the packaging process is basically the same. PERT cells and HIT cells are generally packaged into double-sided modules, that is, the opaque backplane used in conventional modules is replaced with glass or transparent backplane, so that light can enter the cell from the back of the module, which improves the actual power generation efficiency of the module. The packaging methods of MWT solar cells and EWT solar cells have changed greatly, and the cost of conductive backplanes is higher, which makes the promotion of these two technologies encounter greater difficulties.
The above-mentioned high-efficiency solar cells are highly valued by R&D personnel and manufacturers. At present, PERC solar cells have been successfully mass-produced and will develop rapidly in recent years and become mainstream products. The development of MWT and EWT solar cells in recent years has encountered some setbacks, but they are constantly improving and developing. The development of PERT and HIT solar cells has begun to accelerate, and their market share will also rise steadily in the next few years. IBC solar cells have always been Sunpower’s exclusive products, but in recent years, several domestic companies have begun to master the key technologies of IBC solar cells, and mass production is likely to occur. However, the current cost is relatively high, and they are mainly used in racing cars and high-end yachts. , Aircraft and BIPV (Building integrated PV) and other high-end markets.