The P-N junction is the "heart" of photovoltaic cells. According to the type of P-N junction, photovoltaic cells can be divided into homojunction cells and heterojunction cells. Among them, the homojunction cell mainly achieves doping on the same type of silicon wafer (P-type or N-type) by means of diffusion, thereby obtaining a P-N junction. The P-type region and N-type region of the heterojunction cell are composed of different types of semiconductor materials, which can be divided into doped type and non-doped type.

When a P-type semiconductor and an N-type semiconductor are combined, due to the high concentration of holes in the P-type semiconductor and the high concentration of electrons in the N-type semiconductor, thermal diffusion will be formed. That is, holes in the P-type semiconductor diffuse to the N-type region, and electrons in the N-type semiconductor diffuse to the P-type region. Then negative charges are formed in the P-type region, while positive charges are formed in the N-type region, forming a built-in electric field between the two. Under light conditions, photons with energy greater than the forbidden band width are absorbed, and electron-hole pairs are generated on both sides of the PN junction, and they are separated from each other under the action of the built-in electric field, thereby generating a photoinduced current.

"Collection probability" describes the probability that the carriers generated by light irradiation on a certain area of the battery are collected by the P-N junction and participate in the current flow. Its size is related to the distance that photogenerated carriers need to move and the surface characteristics of the battery. The farther away from the dissipative region, the lower the probability of being collected, and surface passivation can increase the probability of collecting carriers at the same location.

What is Diffusion? Diffusion describes the movement of one substance within another. The essence lies in the Brownian motion of atoms, molecules and ions, causing diffusion from places with high concentration to places with low concentration. The manufacture of crystalline silicon solar cells adopts the method of high temperature chemical thermal diffusion to achieve doping junction. Thermal diffusion uses high temperature to drive impurities through the silicon lattice structure, this method is affected by time and temperature, and requires 3 steps: pre-deposition, push-in and activation.

Three indicators of diffusion: square resistance, junction depth, and surface concentration

The square resistance value is mainly a comprehensive characterization of the surface concentration and junction depth, and its influence on the cell parameters mainly includes the following three points: 1) The depth of the diffusion P-N junction directly affects its absorption of short-wave light, so the diffusion within a certain range The shallower the P-N junction (the higher the square resistance value), the higher the current value; 2) The doping concentration of the diffused phosphorus element affects the conductivity of its N-type silicon part to a certain extent, so the higher the doping concentration (the square resistance value The smaller the value is, the higher the fill factor is; 3) Generally speaking, within a certain range, the open circuit voltage increases as the diffusion concentration increases.

2.1. Homojunction: phosphorus expansion and boron expansion

In a homojunction battery, the P-type region and the N-type region are the same type of semiconductor material, and a P-N junction is generally formed by doping. Common doping methods include: 1) tubular diffusion (low pressure, normal pressure); 2) ion implantation + annealing; 3) coating source diffusion (screen printing, spin coating, spray coating, roller printing). At present, most of them use low-pressure tubular diffusion.

Phosphorus diffusion: P2O5 produced by the decomposition of POCl3 is deposited on the surface of the silicon wafer, P2O5 reacts with silicon to form SiO2 and phosphorus atoms, and forms a layer of phosphosilicate glass on the surface of the silicon wafer, and then the phosphorus atoms diffuse into the silicon. Boron diffusion: B2O3 produced by the decomposition of BBr3/BCl3 is deposited on the surface of the silicon wafer, B2O3 reacts with silicon to form SiO2 and boron atoms, and forms a layer of borosilicate glass on the surface of the silicon wafer, and then the boron atoms diffuse into the silicon. It can be seen from the above that whether it is boron diffusion or phosphorus diffusion, it is necessary to form boron atoms or phosphorus atoms to diffuse into the silicon substrate. Boron diffusion is more difficult than phosphorus diffusion. The reason is that the solid solubility of boron atoms in the silicon matrix is low, so the temperature of boron expansion must reach above 1000 °C. And when the surface doping amount is high, it is easy to form boron accumulation on the surface, that is, boron-rich layer (BRL), which poses challenges for subsequent cleaning.

For boron expansion, there are currently two routes of BBr3/BCl3. BBr3 is a liquid at room temperature, and its safety is relatively good, but the generated B2O3 is viscous and requires DCE cleaning, resulting in high maintenance costs. BCl3 is a gas at room temperature, and its safety is relatively poor, but the generated B2O3 is granular and easy to clean. The disadvantage is that the B-Cl bond energy is greater and it is not easy to decompose, resulting in low utilization at the diffusion temperature.

According to ITRPV forecast, the BBr3 route will still occupy the majority of the market in the future, but the proportion of the BCl3 route will gradually increase, reaching about 40% of the market share by 2032.

2.2. Homojunction: SE

Why is SE (Selective Emitter) needed? The reason is that conventional crystalline silicon solar cells use uniformly high-concentration doped emitters. A higher concentration of doping can improve the ohmic contact between the silicon wafer and the electrode and reduce the series resistance, but it is also easy to cause higher surface recombination. To this end, it is necessary to use selective emitter (SE) technology to perform high-concentration doping deep diffusion at and near the contact between the metal gate line (electrode) and the silicon wafer, and low-concentration doping shallow diffusion in the area other than the electrode. .

The advantages of the SE structure: 1) The heavy doping under the electrodes makes the contact resistance lower than that of conventional batteries, thereby improving the fill factor; 2) The light doping between the electrodes can effectively reduce the recombination of carriers when they flow laterally in the diffusion layer 3) The short-wave band of sunlight is basically absorbed on the front surface of the silicon wafer, and shallow diffusion can improve the excitation efficiency of short-wave band sunlight, thereby increasing the short-circuit current; 4) Form an n++-n+/ p++ The high-low junction of -p+ can reduce the recombination of minority carriers under the electrodes and increase the open circuit voltage. In summary, SE better balances the contradiction between the contact resistance between metal and semiconductor and photon collection than conventional batteries. Based on the technical difficulty of boron expansion, it is more difficult to make SE based on boron expansion than phosphorous expansion SE. At present, two technical routes of primary boron expansion and secondary boron expansion are mainly developed.

According to Tongwei's "Preparation of Ultra-thin Tunneling Oxide Layer Based on PECVD Technology and Application of Poly-Si in TOPCon Batteries", there are currently five common boron-expanded SE solutions in the industry, among which the laser film opening route is currently the most mature. plan. From the perspective of mass production prospects, the Etch-back route and the laser direct doping route are the most likely routes to achieve mass production. Among the five schemes, the etching slurry route, boron slurry route, and Etch-back route all require external development of different slurries.

2.3. Heterojunction: doped and undoped

Essentially, thermal diffusion is a method of achieving doping to form a PN junction on the same semiconductor. Other methods include ion implantation, vapor deposition, etc. For example, by depositing intrinsic amorphous silicon and doped amorphous silicon on the surface of crystalline silicon by vapor deposition, since crystalline silicon and amorphous silicon do not belong to the same semiconductor material, the formed P-N junction is called a heterojunction.

Heterojunction cells are doped using vapor deposition rather than diffusion. One problem brought about by this is that the bandgap of amorphous silicon on the front surface is small, resulting in serious optical parasitic absorption, which limits the increase of saturation current; and the low doping efficiency of the amorphous silicon layer leads to low cell yield. This has triggered the application and exploration of doping-free heterojunction cells. N-type transition metal oxide (TMO) materials have been attempted as hole transport layers. According to relevant data from Sun Yat-Sen University, MoOx is used instead of p-type doped amorphous silicon in HJT cells, and the highest conversion efficiency has reached 23.5%.

2.4. Non-junction doping

In addition to forming P-N junctions, the doping process is also used to form high-low junctions. The so-called high-low junction refers to the establishment of a concentration gradient of the same impurity between the battery substrate and the bottom electrode to prepare a P-P+ or N-N+ high-low junction to form a back electric field, which can improve the effective collection of carriers and improve solar energy efficiency. The long-wave response of the battery increases the short-circuit current and open-circuit voltage, and this battery is called a "back field battery". A typical case is that in TOPCon cells, boron doping is used on the front to form a P-N junction on the N-type silicon wafer, and N-type polysilicon made of phosphorus doping is used on the back to play the role of high and low junctions. In the HJT battery, the i-layer amorphous silicon on the front surface forms a P-N junction with the n-type silicon substrate, and the n-type amorphous silicon on the back surface forms a high-low junction with the n-type silicon substrate.

In a broad sense, as long as the electric field is established through the concentration gradient of the same impurity, thereby affecting the structure of carrier collection, it can be called a high-low junction. Such as the selective emitter in boron expansion/phosphorus expansion, the aluminum back field in BSF cells, the local aluminum back field in PERC cells, and the silver-aluminum paste fine grid on the front surface of TOPCon cells.

2.5. Diffusion furnace

Domestic phosphorus diffusion equipment for PERC electric field has been fully localized, and equipment suitable for large silicon wafers and large production capacity has been developed. Considering the uniformity of thermal field and gas field, the placement modes of silicon wafers include horizontal, vertical, and similar. PE type vertical and many other modes. Boron diffusion equipment has higher requirements than phosphorus diffusion, mainly reflected in: uniformity, long diffusion time, high diffusion temperature, and parasitic OSF dislocation of silicon wafer.

Uniformity problem: the core is the uniformity of gas field and thermal field. Vertical placement and horizontal placement have their own advantages and disadvantages. Vertical placement is conducive to heat radiation transfer, but not conducive to airflow transmission; horizontal placement is conducive to airflow transmission, but shields heat radiation. As the size of silicon wafers becomes larger and thinner, the uniformity of vertical placement is challenged. On the one hand, large silicon wafers lead to a longer gas movement distance between two silicon wafers, and the resistance increases; on the other hand, the curvature of thin silicon wafers when placed vertically get bigger. The Laplace is placed horizontally back to back, and the airflow enters from the port and the side, which not only increases the uniformity of the airflow, but also places the silicon wafers back to back, which are naturally compressed under the action of gravity, which can reduce winding plating.

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