Similarly, when phosphorus atoms are doped into the silicon substrate, the phosphorus atom, with its five electrons in its outermost shell compared to the silicon atom's four-electron structure, becomes highly reactive, resulting in an N-type semiconductor. Crystalline silicon solar cells primarily use silicon semiconductor materials as the substrate to create large-area planar PN junctions. This involves diffusing phosphorus atoms in a diffusion furnace onto a P-type silicon wafer approximately 15 cm × 15 cm in size, creating a thin, heavily doped N-type layer. Then, through etching and PECVD, an anti-reflection coating is deposited on the entire N-type layer surface to reduce sunlight reflection loss. Finally, metal grid lines are screen-printed on the diffusion surface as the front contact electrodes of the solar cell. A metal film is printed on the etched surface as the back ohmic contact electrodes of the solar cell, and the cells are then sintered and encapsulated.
When photons of a certain energy strike a solar cell, many new electron-hole pairs are generated. As the cell material continuously absorbs light, the intensity of the incident light decreases, causing the density of electron-hole pairs inside the cell to gradually decrease along the incident direction. Under the influence of this concentration difference, the electron-hole pairs diffuse inwards. When the electron-hole pairs reach the PN junction boundary, they are broken up by the built-in electric field. Holes and electrons are pushed towards the P-region and N-region respectively. If the circuit is open at this point, these photogenerated electrons and holes will accumulate around the P-region and N-region respectively. The P-region will gain an additional positive charge, and the N-region will gain an additional negative charge. The accumulated positive and negative charges in the P-region and N-region will generate a photogenerated electromotive force in the PN junction. If the positive and negative terminals of the solar cell are connected at this time, a current will be generated. Thus, a photogenerated current will be generated inside the PN junction, flowing from the N-region to the P-region.
Schematic diagram of photocurrent
I. Formation of P-type semiconductors
As shown in the figure, the positive charge represents a silicon atom, and the negative charge represents the four electrons orbiting the silicon atom.
When boron is doped into silicon crystals (as shown in the diagram below), the negative charge represents the four electrons surrounding the silicon atom. The yellow area represents the doped boron atom. Because boron atoms only have three electrons around them, blue holes are created, as shown in the diagram. These holes are unstable due to the lack of electrons and easily absorb electrons to neutralize them, forming a P-type semiconductor.
II. Formation of N-type semiconductors
As shown in the diagram above, the positive charge represents a silicon atom, and the negative charge represents the four electrons orbiting the silicon atom.
After phosphorus atoms are incorporated (as shown in the diagram above), because a phosphorus atom has five electrons, one of these electrons becomes highly active, forming an N-type semiconductor. Yellow represents the incorporated phosphorus atom, and red represents the extra electron.
III. Formation of PN Junction
A P-type semiconductor and an N-type semiconductor are tightly joined together—a connection at the atomic radius scale, with no gaps. This results in the following physical process at the interface between the N-type and P-type semiconductors. It's important to note that in reality, solar cells cannot achieve a PN junction by connecting P-type and N-type cells together, because molecular-level splicing is impossible. In actual production, N-type cells are typically fabricated by single-sided diffusion on a P-type silicon substrate.
In the diagram, the blue circles represent majority electrons, and the red circles represent majority holes. The concentration of majority electrons in an N-type semiconductor is much greater than the concentration of minority electrons in a P-type semiconductor; the concentration of majority holes in a P-type semiconductor is much greater than the concentration of minority holes in an N-type semiconductor. Therefore, diffusion occurs at the interface between the two semiconductors due to the concentration difference of charge carriers, as shown in the diagram above.
As diffusion proceeds, on the N-region side of the interface, impurities become positive ions as electrons diffuse into the P-region; on the P-region side, impurities become negative ions as holes diffuse into the N-region. Since impurities are immobile within the crystal lattice, a thin layer of positive ions forms on the N-region side of the N-type semiconductor interface, and a thin layer of negative ions forms on the P-type side. This ion layer creates an electric field, directed from the N-region to the P-region, called the internal electric field, as shown in the figure below.
The presence and direction of the internal electric field impede diffusion, limiting its further development. Minority carriers also exist in semiconductors; the electric force of the internal electric field acts on these carriers, causing them to drift.
We call the internal electric field pointing from the N-region to the P-region a PN junction. Simply put, an N-type semiconductor contains more holes, while a P-type semiconductor contains more electrons. When P-type and N-type semiconductors are combined, a potential difference is formed at the interface, which is the PN junction.
When a battery module is irradiated, the ratio of the output electrical power to the incident light power is called the efficiency of the battery module, also known as the photoelectric conversion efficiency.
The theoretical limit of efficiency for traditional crystalline silicon solar cells is 28.8% (excluding silicon-based composite solar cells).
Solar photovoltaic (PV) power generation is a renewable energy technology that converts solar energy into electrical energy. It uses photovoltaic panels to convert sunlight into direct current (DC), which is then converted into alternating current (AC) by an inverter, supplying power to homes, businesses, and public utilities. In this article, we will detail the working principles, advantages, and applications of solar PV power generation.
First, let's understand how solar photovoltaic (PV) power generation works. A solar photovoltaic panel consists of multiple photovoltaic cells, most of which are made of silicon and are also known as crystalline silicon solar cells. When sunlight shines on the photovoltaic panel, the light energy reacts with the semiconductor material of the photovoltaic cell through the photoelectric effect, generating electron-hole pairs. These electron-hole pairs form an electric current within the photovoltaic cell, which is then drawn out through metal electrodes to form direct current (DC). Next, an inverter converts the DC power into alternating current (AC) to meet the electricity needs of homes and businesses.
Solar photovoltaic power generation has many advantages. First, it is a clean energy source that does not produce greenhouse gases and pollutants such as carbon dioxide, making it environmentally friendly. Second, solar energy is a widely available resource, accessible almost globally. Compared to fossil fuels, solar energy is an infinitely renewable energy source, capable of continuously supplying electricity. Furthermore, the operating and maintenance costs of solar power systems are relatively low, which can reduce energy costs in the long run.
Solar photovoltaic (PV) power generation is being widely applied across various sectors. In homes, people can install solar PV systems to convert sunlight into electricity for lighting, appliances, and other electrical equipment. In remote areas or regions without electricity, solar PV power generation has become a reliable energy supply method. For businesses and industrial sectors, solar PV power generation systems can reduce energy expenditures and contribute to sustainable development. Furthermore, solar PV power generation can also be used in public utilities, such as solar PV power plants and solar streetlights.
However, solar photovoltaic (PV) power generation also faces some challenges. First, its efficiency is relatively low and needs further improvement. Second, solar power generation is affected by weather and seasonal changes; for example, power generation decreases on cloudy days, at night, and during winter when there is less sunlight. Furthermore, the construction and operation and maintenance costs of solar PV systems are high, requiring substantial investment. Nevertheless, with technological advancements and cost reductions, solar PV power generation will gradually become a mainstream energy source.
In summary, solar photovoltaic (PV) power generation is a sustainable and clean energy technology with broad application prospects. It converts sunlight into electricity through photovoltaic panels, providing power to homes, businesses, and public utilities. The advantages of solar PV power generation include its environmental friendliness, renewable nature, and low cost, but it also faces some challenges. With technological advancements and cost reductions, solar PV power generation will play an even more important role in the future, driving sustainable development and energy transition.
