PERC (Passivated Emitter and Rear Cell), or passivated emitter and rear cell technology, was first proposed by Australian scientist Martin Green in 1983 and is now becoming a new generation of conventional technology for solar cells. PERC has continuously broken efficiency records in recent years and will become the most cost-effective technology in the next three years.
(Single-sided perc battery structure)
perc technology The conversion efficiency is improved by adding a dielectric passivation layer on the back side of the cell. The better efficiency levels in standard cell structures are limited by the tendency of photogenerated electrons to recombine. PERC cells maximize the potential gradient across the PN junction, which allows for a more stable flow of electrons, reduced electron recombination, and higher efficiency levels.
The advantages of PERC technology also lie in its compatibility with other high-efficiency cell and component technologies, and its potential to continuously improve efficiency and power generation capacity. By combining technologies such as multi-busbar, selective emitter and TOPCon, PERC cell efficiency can be further improved; combined with diamond wire cutting and black silicon technology, the cost performance of multicrystalline cells can be improved. Bifacial PERC cells achieve bifacial power generation with almost no additional cost, and achieve a 10%-25% power generation gain on the system side, greatly enhancing the competitiveness and future development potential of PERC technology.
Overview of the process
The production process of PERC cells includes: depositing a back passivation layer and then opening it to form a back contact. These are two important steps in addition to the conventional photovoltaic cell production process. In addition, the edge isolation step based on a chemical wet bench needs to be slightly adjusted for back polishing. In other words, the velvet pyramid structure on the back of the silicon wafer needs to be dissolved away. The degree of polishing varies depending on the selected technology. Therefore, both the passivation film deposition equipment and the film opening equipment (which can use either laser or chemical etching) require additional processing equipment on the traditional cell production line. For the less used laser edge isolation process production line, a chemical wet bench needs to be added for back polishing.
Passivation film
Impurities and defects inside and on the surface of silicon wafers can have a negative impact on the performance of photovoltaic cells. The passivation process is to reduce the impact of defects by reducing the recombination of surface carriers, thereby ensuring the efficiency of the cell.
Surface passivation of crystalline silicon solar cells has always been a top priority in design and optimization. From the early days of only back electric field passivation, to front silicon nitride passivation, to the PERC design of passivation of local opening contacts on the back with dielectric layers such as silicon oxide, aluminum oxide, and silicon nitride. The core of the PERC concept is to add a full-coverage back passivation film to conventional photovoltaic cells.
Passivation mainly reduces the recombination rate and increases the minority carrier lifetime through the following two methods: one is chemical passivation, which saturates various defect states on the interface and reduces the interface defect concentration, thereby reducing the recombination centers in the bandgap; the other is field effect passivation, which accumulates charges to form an electrostatic field at the interface, thereby reducing the minority carrier concentration.
Back passivation material
In the selection of passivation film materials. Aluminum oxide (Al2O3) has a high charge density and can provide good passivation for P-type surfaces. It is currently widely used as a back passivation material for mass production of PERC cells. In addition to aluminum oxide, silicon oxide (SiO2), silicon oxynitride, etc. can also be used as back passivation materials.
In addition, in order to fully meet the back passivation conditions, a layer of silicon nitride (SiNx) needs to be coated on the surface of aluminum oxide to protect the back passivation film and ensure the optical performance of the back of the battery. Therefore, the back passivation of PERC batteries mostly adopts Al2O3/SiNx double-layer structure.
Figure: Crystalline silicon photovoltaic cell passivation
At present, the industry's PERC battery technology route has basically gone through three stages. The first stage is to directly upgrade on the conventional production line, which can increase the efficiency by 1%; the second stage is to add thermal oxidation process and optimize etching and diffusion matching to increase the efficiency to 21.7%; in the third stage, the SE technology that is about to be promoted on a large scale will increase its efficiency to 22% in mass production.
Regardless of the process stage, the selection of growth equipment for the core back passivation film layer is very important, involving the key points of plant layout, automation matching, and overall process optimization.
Table: Development of PERC Cell Process Route
Back side passivation process
◎ Plasma enhanced chemical vapor deposition
Plasma enhanced chemical vapor deposition is a chemical vapor deposition reaction that uses the physical effect of glow discharge to activate particles. It is a thin film deposition technology that integrates plasma glow discharge and chemical vapor deposition. In the plasma field formed by glow discharge, due to the huge difference in the mass of electrons and ions, the process of exchanging energy through collision between the two is relatively slow. Therefore, there is no uniform temperature inside the plasma, only the so-called electron gas temperature and ion temperature. From a macroscopic point of view, the temperature of this plasma is not high, but it is in an excited state inside. Its electron energy is enough to break the molecular bonds and cause the generation of chemically active substances (activated molecules, atoms, ions, atomic groups, etc.), so that chemical reactions that originally need to be carried out at high temperatures, when in the plasma field, reduce the reaction temperature due to the electrical activation of the reaction gas, so that a solid film can be formed on the substrate at a lower temperature or even at room temperature.
◎Thermal oxidation method
In the process of manufacturing solar cells, the silicon wafers with pn junctions are placed in a high-temperature furnace and reacted with oxidants at high temperatures to grow a layer of SiO2 film, which plays a passivation role on the surface of the solar cell. The SiO2 film prepared by thermal oxidation has a large number of fixed positive charges in the thermally oxidized silicon dioxide, which will produce a field effect passivation effect, reduce the defect density on the surface of the silicon wafer, and obtain a low surface recombination rate.
◎Atomic layer deposition
Atomic layer deposition is a process in which different gas-phase precursor reactants are alternately introduced into the reactor, chemically adsorbed and reacted on the deposition substrate to form a thin film. The deposition process is controlled at the atomic level by limiting the surface reactants. With the precursor trimethylaluminum and water as reactants, a series of reactions constitute an ALD cycle, and Al2O3 thin film is deposited on the surface of n-type crystalline silicon. The required film thickness can be obtained by controlling the number of cycles. The biggest advantage of atomic layer deposition is its self-limiting property, so the thickness and quality of the film can be accurately controlled, so that it has good step coverage and large-area thickness uniformity. Based on the above advantages of atomic layer deposition, J. Schmidt et al. used atomic layer deposition to prepare Al2O3 as a back surface passivation film to prepare a PERC solar cell with an efficiency of 20.6%. Its disadvantage is also obvious, that is, the lower growth rate, because the two gas extraction processes in each cycle reaction take several seconds, while the reaction time of the precursor is only a few milliseconds, which limits the speed of atomic layer deposition to about 2nm/min.
