The crystalline silicon solar cell is actually a large planar diode. For n-type cells, the base of the cell is n-Si, and the front surface of the base is heavily doped by diffusion to form a p+ emitter, and the p+ emitter is connected to the n-Si substrate. The contact forms a p+-n junction, the back surface of the substrate is heavily doped by diffusion or ion implantation to form an n+ back field, and the n+ back field contacts the n-Si substrate to form an n+-n high-low junction. There is a built-in electric field inside the p+-n junction and the n+-n high and low junction, which can separate the electron-hole pairs generated by illumination. The separated electrons are output through the back electrode above the back field, and holes are output through the front electrode above the emitter. to the external circuit to drive the load to run.
As shown in Fig. 1(a), the structure of the n-PERT bifacial cell is: metal electrode, front surface AR coating, boron doped emitter, n-type silicon, phosphorus doped back field (BSF), backside AR membrane and back electrode. Compared with single-sided battery, n-PERT double-sided battery mainly lies in the difference of back structure. The back of double-sided battery uses high-transmittance SiNx as passivation/anti-reflection film. The back metal electrode is the same as the front metal electrode, which occupies the battery The area is ~3%; while the back electrode of the single-sided cell is covered with full metal, as shown in Fig. 1(b).
FIG. 2 is a schematic diagram of the power generation principle of double-sided cells and single-sided cells. As shown in the figure, when sunlight hits the n-PERT double-sided cell, some light will be reflected by the surrounding environment and irradiate to the back of the n-PERT double-sided cell. This part of the light can pass through the SiNx material and be absorbed by silicon , the excited electron-hole pairs are separated by the n+-n high-low junction, thereby contributing to the photocurrent and efficiency of the cell (Fig. a). However, the backside of the single-sided cell is completely covered by the metal electrode, and the thickness of the metal electrode is ~10 μm. The light cannot penetrate the backside metal electrode and be absorbed by the silicon. Therefore, the single-sided cell can hardly utilize the light entering the cell from the backside. When the reflectivity is not zero, bifacial cells have higher power generation efficiency than monofacial cells.
Figure 3(a) shows the IV characteristic curves of the front and back sides of the bifacial cell. It can be seen that under STC conditions, the front power of the n-PERT bifacial cell can reach 5.2W and the back power can reach 4.7W. The rate is 90%. The efficiency gain of n-PERT double-sided cells relative to single-sided cells is difficult to measure by a single cell. Generally, multiple solar cells are connected in series or parallel to the electrodes, and EVA, glass, backplane and other materials are used for packaging. , becomes the component to measure the efficiency gain. The front of the double-sided battery module is encapsulated with glass + EVA, and the back can be encapsulated with EVA + glass or EVA + transparent backplane, so as to ensure that sunlight can penetrate the encapsulation material to the back of the battery, as shown in Figure 3(b) .
Figure 4(a) The theoretically calculated maximum power and maximum current of the double-glass module under different back-reflection conditions, (b) the power output gain of the actual double-glass module relative to the cement floor of the polycrystalline module under different reflection conditions ( All components are south facing, 30 degree inclined installation)
When the installation orientation, inclination and height of the double-glass module are fixed, the power generation gain of the double-glass module is mainly related to the reflectivity of the ground on the back of the module. Take a double-glass module whose electrical performance parameters are shown in Table 1 as an example. The theoretically calculated values of the maximum power and maximum current in the case of backside reflection are shown in Fig. 4(a). It can be seen from 4(a) that the maximum output power and maximum current value of the module are positively correlated with the reflectivity of the back surface, and increase with the increase of the back surface reflectivity; moreover, the maximum output power curve and the maximum current curve have similar growth trends , indicating that the increase in component power is mainly due to the increase in component current, and the increase in component current is due to the increase in the reflectivity of the backside, which allows more light to be absorbed and utilized.
In actual operation of the double-glass module, compared with the polycrystalline module installed on the cement floor, the efficiency gains on different floors are shown in Fig. 4(b). From 4(b), it can be seen that no matter on cloudy or sunny days, the power generation gain of double-glass modules is the largest on the ground painted with white paint, followed by aluminum foil, and the lowest on lawn, and both are higher than single-sided polycrystalline modules. Therefore, it is proved from both theoretical and practical application that the back of the bifacial cell can generate electricity, which has the effect of improving the output power of the module.
The double-glass modules manufactured by Jolywood bifacial cells can generate electricity by utilizing backscattered/reflected light and have the advantages of greater power output, as well as the following advantages:
1) The working temperature is lower than that of conventional components; the back of the battery is made of high-transmittance SiNx material, and the light of the infrared part can penetrate the battery and is not absorbed by the battery, while the back of the conventional battery is an all-metal electrode, which will absorb infrared light. It is shown that the temperature of the double-glass module power generation system under normal operation is 5~9°C lower than that of the conventional single-glass module.
2) The temperature coefficient is lower than that of conventional modules; when the operating temperature of the module is 1°C higher than the standard temperature, its output power will be reduced by 0.42%. Under the same temperature rise, the power loss of the N-type module is smaller than that of the conventional P-type module, and the power loss is 0.4% or less.
3) Excellent weak light response; due to the high minority carrier lifetime of the N-type base material, the N-type crystalline silicon module exhibits better power generation characteristics than the conventional P-type crystalline silicon module under weak light.
4) Flexible installation methods and wide application range; especially suitable for distributed power generation systems such as roofs, fences, complementary fishing and light, complementary agricultural light, sound insulation walls, and areas with a lot of snow.
