(1) The I-V characteristic curve connects different loads to the solar cell, and the obtained power output will be different. ∞). The volt-ampere characteristic test circuit is shown in Figure 1. Connect the solar cell, change the load resistance value from 0 to ∞, and measure the current and voltage of the load, the volt-ampere characteristic curve of the solar cell can be drawn as shown in Figure 2.
The volt-ampere characteristic curve can reflect many important properties of solar cells, such as open circuit voltage, short circuit current, maximum output, power, fill factor, etc. At the same time, the parallel resistance of the solar cell can be qualitatively judged by the rate of change of the curve when V→0. The parallel resistance includes the leakage resistance of the P-N junction and the leakage resistance of the cell edge, etc., which are mainly caused by the unclean edge of the silicon wafer or defects in the body. When → 0, the parallel resistance is larger, and the performance of the solar cell is better; otherwise, the performance of the solar cell is poor. The series resistance of the solar cell can be qualitatively judged by the rate of change of the curve when I→0. The series resistance mainly includes the sheet resistance of the diffusion layer, the bulk resistance of the base material itself, the contact resistance between the electrode and the semiconductor, the resistance of the electrode, etc. . When →∞, the series resistance is small, and the performance of the solar cell is better; otherwise, the performance of the solar cell is poor. To learn more about batteries, visit Tycorun Battery.
(2) Carrier collection rate Determine the carrier collection coordinates as shown in Figure 3. When photons enter the interior of the solar cell, the photons will excite the carriers to generate photogenerated carriers (positive and negative electron pairs). At the same time, the light energy entering the solar cell will attenuate negatively exponentially with the depth of entering the solar cell.
But not all photogenerated carriers can be effectively collected by the P-N junction. Therefore, we care about the ratio of the contribution of photogenerated carriers to the short-circuit current to the total number of photogenerated carriers, which is defined as the carrier Collection rate, denoted by fc. Deduced from the relevant theories of semiconductor physics, (4) can be obtained:
In the formula, x is any value on the coordinate axis of the solar cell in Fig. 3; La is the diffusion length of electrons.
It can be seen from equation (4) that the collection rate of photogenerated carriers in semiconductor solar cells is related to the distance from the space charge region, and the farther away from the space charge region, the stronger the exponential decay is. The carrier drift velocity caused by the electric field is much greater than its diffusion velocity, so the collection rate of photogenerated carriers can be considered to be 1 in the space charge region. Because the diffusion length Ln has the following relationship with the carrier lifetime τn (5):
where Dn is the electron diffusion coefficient.
It can be seen that, except for the space charge region, the diffusion length and minority carrier lifetime are the keys to determine the carrier collection rate. The relationship between the carrier collection rate and the cell space is shown in Figure 6.
(3) Junction depth and surface recombination rate In general, the junction depth is measured in microns or nanometers, and is defined as the distance from the solar cell surface (illuminated surface) to the point where the doping concentration of the diffusion layer is equal to the doping concentration of the substrate. From the surface of the solar cell deep inside the semiconductor material, the spectral excitation rate decays exponentially. However, when the carrier collection rate is the highest, it appears in the space charge region. If you want to collect as many photogenerated carriers as possible, you need to make the space charge region as close to the surface of the solar cell as possible, that is, to make the junction depth as small as possible. , in order to make the high carrier excitation rate region and the high collection rate region closer, so as to obtain more residual carriers to improve the short-circuit current.
The annihilation of minority carriers at the semiconductor surface is called surface recombination. The semiconductor surface is the termination surface of the crystal lattice, and there are many defects, and the influence of pollution or external factors will increase the number of defects, and these large numbers of defects will become the generation-recombination center of carriers. Therefore, for the recombination of minority carriers, there is a strong recombination effect on the semiconductor surface. Since the semiconductor surface is in contact with the outside world, the semiconductor surface is very sensitive to external environmental factors, and the large semiconductor surface will greatly affect the performance of the semiconductor device. In semiconductor physics, we use the surface recombination velocity to characterize the strength of the surface recombination. The surface recombination velocity can also be understood as the speed at which the carriers flow out of the surface. The unit of the surface recombination velocity is cm/s. For semiconductor solar cells made by inversion diffusion, the surface N-type semiconductor concentration is 4 orders of magnitude higher than the base region P-type semiconductor doping concentration, and the high surface doping makes the surface recombination rate about 10 times higher than the recombination rate in the solar cell. 8 orders of magnitude. The surface of the solar cell also needs to be made into a grid electrode to collect charges, so we hope to reduce the contact resistance between the metal grid line and the semiconductor material through high surface doping, and the junction depth will be deepened by high doping through diffusion. Therefore, junction depth and surface recombination have become the opposing factors of high-efficiency solar cells. Through optimized calculation, the junction depth of common crystalline silicon semiconductor solar cells is generally 200~400nm.
(4) Lateral resistance of the top region The lateral resistance of the top region is also called the surface lateral resistance. The lateral resistance of the top region has a very important influence on the electrical performance output parameters of the semiconductor solar cell. The surface of the top region is generally the ohmic contact between the metal grid line and the N-type semiconductor. When the solar cell is connected to a load, the current flows perpendicular to the surface inside the solar cell, and the surface of the top region needs to flow laterally along the surface to achieve The purpose of being collected by the shed line. The flow of current during operation of the solar cell is shown in Figure 7. The surface lateral resistance, the contact resistance between the grid line and the semiconductor and the resistance of the grid line itself constitute the main part of the series resistance of the solar cell.
If the distance between two adjacent gate lines is x, the junction depth is d, and the gate line length is l, since the top region is very thin (only 1/10 of the thickness of the solar cell), it is approximately considered that the top region thin layer is uniformly doped, and set If the resistivity is ρa, the resistance R between two adjacent gates can be expressed as (8):
For a very thin top layer, assuming that the electron mobility is p and the acceptor impurity concentration is NA, then (9):
In semiconductor physics, the electrical conductivity of a thin layer is described by the resistivity per unit thickness, which is called sheet resistivity or sheet resistance, and is represented by Ω/□, then the sheet resistance R of the top layer of the solar cell can be calculated by the following formula (10):
The lateral resistance is part of the series resistance of the solar cell, and we want its value to be smaller. However, it can be seen from equation (10) that the mobility μn, the acceptor impurity concentration NA and the junction depth d are mutually restricted, so it is necessary to comprehensively consider the optimization process. At the same time, the larger the lateral resistance of the top region, the larger the power loss caused. We can use wider grid lines and narrow grid line spacing to reduce series resistance, but it will bring greater optical loss. Combining these factors, for silicon solar cells, the sheet resistance is generally 30~100Ω/□.