Monocrystalline silicon modules are currently the fastest-developed photovoltaic modules and have been widely used in space and on the ground. The solar cells in the monocrystalline silicon module use high-purity monocrystalline silicon rods as raw materials, with a purity requirement of 99.999%. At present, the monocrystalline silicon modules used on the ground use solar-grade monocrystalline silicon rods, and the head and tail materials and waste monocrystalline silicon materials processed by semiconductor devices can also be used to make monocrystalline silicon rods for solar cells after multiple drawing. The single crystal silicon rod is cut into silicon wafers with a thickness of about 0.3 mm, and the raw silicon wafers to be processed are made through processes such as forming, polishing, and cleaning. To process raw silicon wafers into solar cells, the silicon wafers must be doped and diffused first. Generally, the dopants are trace amounts of boron, phosphorus, and zinc. Diffusion is carried out in a high-temperature diffusion furnace made of quartz tubes, so that a PN junction is formed on the silicon wafer. Then use the screen printing method to print the prepared silver paste on the silicon wafer to make grid lines. After sintering, the back electrode is made at the same time, and the surface with the grid lines is coated with an anti-reflection source to prevent a large number of photons. It is reflected off by the smooth surface of the silicon wafer to form a single piece of monocrystalline silicon module(Figure 1).
Monocrystalline silicon solar cells have the highest conversion efficiency and the most mature technology. At present, the conversion efficiency of mass-produced ordinary monocrystalline silicon modules can reach more than 18%. However, with the continuous improvement of the conversion efficiency of monocrystalline silicon modules, it has become more and more difficult to continue to optimize the technology and process of crystalline silicon cells to further improve the efficiency of the modules. Revolutionary high-efficiency technologies are needed to further improve module efficiency and reduce costs. Based on this, high-efficiency photovoltaic power generation technology has been continuously developed.
High-efficiency photovoltaic power generation technology is the photoelectric conversion effect obtained by optimizing the cell structure, improving the production process of mainstream modules or using new materials, so that the performance of the module is greatly improved. High-efficiency modules mainly introduce thermal oxidation surface passivation technology, light trapping effect and increased back field theory to achieve higher conversion efficiency. High-efficiency modules mainly include IBC (interdigitated back contact) modules, HIT (hetero-junction with intrinsic thin-layer) modules, PERC (passivated emitter and rear cell) series modules, MWT (metal wrap through) modules, EWT (emitter wrap through) modules, OECO (obliquely evaporated contact) modules, etc.
(1) IBC modules
The IBC modules are made of N-type substrate material, and the front and rear surfaces are covered with a layer of thermal oxide film. The front side uses silicon dioxide or silicon oxide/silicon nitride composite film combined with the N layer as the front surface electric field, and is made into a suede structure for anti-reflection. The back surface is made into a cross-type junction with P and N staggered intervals by a diffusion method, and metal contact holes are opened on the silicon oxide to realize the contact between the electrode and the emitter region or the base region. The positive and negative electrodes of the IBC battery are on the back of the battery sheet. The surface of the battery is black and there is no metal grid at all, which can greatly increase the conversion efficiency of the battery. At present, the conversion efficiency of IBC modules has exceeded 25%.
Removing the front metal gate has many advantages: reducing the front shading loss, which is equivalent to increasing the effective semiconductor area; reducing the assembly cost of modules; the cross-arranged emitter and base electrodes almost cover most of the back surface, which is good for drawing current ; Beautiful appearance (Figure 2).
(2) HIT module
HIT modules are intrinsic thin film heterojunctions. The PN junction of a conventional battery is composed of crystalline silicon of the same material with the opposite conductivity type, which is a homojunction battery; while the PN junction of a HIT battery is composed of two different semiconductor materials-amorphous silicon/crystalline silicon. The HIT battery uses N-type monocrystalline silicon wafer as the substrate. The intrinsic layer, doped layer, TCO and printed electrodes are deposited on both sides of the cleaned and textured N-type C-Si. This symmetrical structure facilitates the reduction of process equipment and steps. . The conversion efficiency of HIT modules has exceeded 24.7% (Figure 3).
HIT modules have the following advantages: ① Symmetrical structure, capable of generating electricity on both sides: Because the structure of the HIT battery is symmetrical, both sides can generate electricity after being exposed to light, and can be made into double-sided battery modules. Under the same light conditions, the current that can be generated on the back of the module is about 80% of that of the front, and the power generation is higher than that of a single-sided module. ②The low-temperature manufacturing process protects the carrier life: Since the silicon-based film is used to form the PN junction, the highest process temperature is the amorphous silicon film formation temperature (about 200°C). This avoids the high temperature (about 900°C) of the PN junction formed by the traditional thermal diffusion type crystalline silicon solar cell, and reduces the thermal damage and deformation of the silicon wafer. ③High open circuit voltage characteristics: Because the intrinsic thin film ia-Si; H is inserted between the crystalline silicon and the doped thin film silicon, and the atomic hydrogen generated during the PECVD deposition of the amorphous silicon thin film can passivate the interface, it is effective The surface defects of crystalline silicon are reduced, so the open circuit voltage of the HIT cell is high, which can improve the conversion efficiency. ④Good temperature characteristics: When the working temperature of the photovoltaic module increases, its output power and conversion efficiency decrease with the increase of temperature. Due to the amorphous silicon thin film/crystalline silicon heterojunction in the HIT battery structure, the temperature characteristics are more excellent, with a temperature coefficient of -0.25%/℃, which is close to half of the temperature coefficient of a crystalline silicon battery -0.45%/℃. Compared with conventional crystalline silicon modules, HIT modules with the same power under STC can generate 8% to 10% more electricity in a day.
(3) PERC series modules
The common feature of the three types of back passivation (PERC/PERT/PERL) structures is that a passivation layer is added to the traditional aluminum back field to better prevent electrons from recombining on the back surface.
PERC technology uses SiNx or Al2O3 to form a passivation layer on the back of the battery as a back reflection layer to increase the absorption of long-wave light, while maximizing the potential difference between the P-N poles and reducing electronic recombination, thereby improving the conversion efficiency of the battery. The conversion efficiency of PERC modules has exceeded 23.6% (Figure 4).
PERT (passivated emitter, rear totally-diffused) technology uses KOH to etch the surface of the monocrystalline silicon cell to form an inverted pyramid structure. The shallow phosphor diffuses to form the front surface field (FSF), and the shallow boron on the back diffuses to form the emitter junction, and then grows high on both sides. The high-quality SiO₂ passivation layer realizes heavy phosphorus diffusion under the front electrode through a photolithography process, and heavy boron diffusion at the point contact of the back electrode. Although the battery with this structure has high efficiency, the production process is complicated and it is not suitable for industrial production.
The principle of PERL (passivated emitter, rear locally-diffused) battery technology is to passivate the emitter and diffuse locally on the back. In 1990, the University of New South Wales used BBr3 localized diffusion at the contact hole on the back of the battery to prepare a PERL battery based on the structure and technology of the PERC battery.
PERL and PERT are heavily diffused (partially or completely) materials with the same polarity as the silicon wafer after adding a passivation layer to form another back electric field. Among the three types of modules, PERC has the most obvious price advantage and has higher market competitiveness.
(4) MWT modules
MWT battery, namely metal wound battery, or metal perforated wound battery. Compared with conventional batteries, MWT battery technology uses laser drilling and back wiring technology to eliminate the main grid lines of the front electrode. The current collected by the fine grid lines of the front electrode is led to the back through the silver paste in the hole. The positive and negative electrode points are distributed on the back of the cell, which effectively reduces the shading of the front grid line, improves the conversion efficiency, and reduces the consumption of silver paste and the minority carrier recombination loss at the metal electrode-emitter interface.
The metal electrode winding can also realize a double PN junction structure, that is, the front junction and the back junction are connected through a metallization channel to collect carriers together, which improves the efficiency of separating and collecting carriers. For silicon linings with low minority carrier lifetimes With this structure, a higher short-circuit current can still be obtained, which reduces the requirements for Si materials.
(5) EWT modules
The difference between EWT and MWT batteries is that in EWT batteries, the grid lines that transfer power are also transferred to the back. Similar to the MWT battery, the EWT battery also connects the upper and lower surfaces by drilling micro holes in the battery. Compared with the approximately 200 through holes per silicon wafer of the MWT battery, the EWT battery has approximately 20,000 through holes per silicon wafer, so laser drilling becomes the only process that can meet the speed of commercial scale. The EWT battery that produces industrialized large-area silicon wafers has high technical requirements, such as the realization of non-destructive laser cutting, the limitation of electrode shape by screen printing, the filling depth of the metal in the hole, and the optimization of the emitter series resistance. Because there are no grid lines and electrodes on the front of the EWT battery, the module assembly is easier, and at the same time, because the shading loss is avoided and the double-sided carrier collection is realized, the photo-generated current is greatly improved.
(6) OECO modules
OECO is an obliquely evaporated metal contact silicon solar cell. Compared with other high-efficiency batteries, it has the advantages of novel structure design, simple production, no loss of electrode materials, low cost and suitable for mass production. The OECO battery structure is based on metal-insulator-semiconductor (MIS) contact, using surface trench morphology to obscure the extremely thin oxide tunnel layer and obliquely vaporize low-cost aluminum as the electrode, without photolithography, electrode burn-through, and under-electrode Heavy doping and high-temperature processes can form high-quality contacts, and a large number of battery electrodes can be evaporated at one time. More importantly, when the production area of this battery is expanded from 4c㎡ to 100c㎡, the efficiency is only slightly reduced from 21.1% to 20%, which is still in the high-efficiency range, so this battery is more suitable for industrialization. The surface of the battery is composed of many neatly arranged square grooves. The shallow emitter N is located on the upper surface of the silicon wafer. There is a very thin oxide tunnel layer on it. The aluminum electrode is deposited obliquely on the side of the groove, and then PECVD is used. Evaporated silicon nitride as a passivation layer and anti-reflection film.