Solar cells, also known as "solar chips" or "photovoltaic cells," are thin-film photovoltaic semiconductors that directly generate electricity using sunlight. When exposed to light under certain illumination conditions, they can instantly output voltage and, in the presence of a circuit, produce current. In physics, this is called solar photovoltaic (PV). Solar cells are devices that directly convert light energy into electrical energy through the photoelectric effect or photochemical effect. Crystalline silicon solar cells, which operate on the photoelectric effect, are the mainstream, while thin-film solar cells, which operate on the photochemical effect, are still in their infancy.
The working principle of solar cells is based on the photovoltaic effect of semiconductor PN junctions. Simply put, the photovoltaic effect is an effect where, when an object is exposed to light, the charge distribution within the object changes, generating an electromotive force and current. This effect can occur in gases, liquids, and solids, but the efficiency of converting light energy into electrical energy is particularly high in solids, especially in semiconductors. Therefore, the photoelectric effect in semiconductors has attracted considerable attention, been extensively studied, and led to the invention and manufacture of semiconductor solar cells. Furthermore, when sunlight or other light shines on the PN junction of a semiconductor, a voltage appears across the PN junction, called the photovoltage.
When P-type and N-type semiconductors are brought together, a special thin film forms at the boundary region between the two semiconductors. A negative voltage appears on the P-type side of the interface, and a positive voltage appears on the N-type side. This is because the P-type semiconductor has many holes, while the N-type semiconductor has many free electrons with a low concentration. Electrons in the N-segment are scattered in the P-segment, and holes in the P-segment are scattered in the N-segment. After scattering in the N-segment, an "internal electric field" is generated in the P-segment to prevent scattering. Once equilibrium is reached, a special film is formed, creating a potential difference, i.e., the pn transition. To date, most solar module manufacturers create N-type stripes on P-type silicon substrates using a diffusion process, with the PN (or N+/P) transition forming at the intersection of the two segments. The main structure of a solar panel is a large planar P-type node.
When a beam of light is emitted from a solar panel, it is absorbed in the boundary layer. Photons with sufficient energy can excite electrons in the covalent bonds of P-silicon and N-silicon, forming electron-hole pairs. Near the boundary layer, the electrons and holes are separated by the electric field of the space charge and then recombine. Electrons enter the positive voltage region N, and holes enter the negative voltage region P. This separation of charge in the boundary layer creates a measurable voltage between the P and H regions. At this point, electrodes can be added, and a voltmeter can be opened on both sides of the silicon wafer. For quartz silicon solar modules, the open-circuit voltage is typically 0.5-0.6V. The electron apertures generated by light in the boundary layer are more important than the current. The more light energy absorbed by the boundary layer, the larger the boundary layer area, and thus the more current the solar cells can draw.
Types of solar cells
Monocrystalline silicon solar cells are currently the most technologically mature type of cell, and they also have relatively high efficiency, so they are widely used in various power plants. However, the photoelectric effect caused by silicon is prone to degradation, and its stability is not very good. Therefore, the key issue for future development is how to solve the stability problem.
Monocrystalline silicon solar cells undoubtedly boast the highest conversion efficiency and still dominate in large-scale applications and industrial production. However, despite their high efficiency, the production of monocrystalline silicon solar cells requires large quantities of high-purity silicon material, involves complex processes, consumes significant amounts of electricity, and suffers from low planar utilization of solar cell modules, resulting in persistently high costs for monocrystalline silicon. Significantly reducing these costs is extremely difficult.
Polycrystalline thin-film batteries
Polycrystalline silicon thin-film solar cells are produced by growing polycrystalline silicon thin films on low-cost substrate materials. A relatively thin crystalline silicon layer serves as the activation layer, maintaining the high performance and stability of crystalline silicon solar cells while significantly reducing material usage and lowering overall cell costs. The working principle of polycrystalline silicon thin-film solar cells is the same as other solar cells: the photovoltaic effect is formed by the interaction of sunlight and semiconductor materials.
Organic polymer batteries
Also known as amorphous silicon solar cells, they consist of a transparent oxide (TCO) thin film layer, an amorphous silicon thin film PIN layer (the I layer being the intrinsic absorber layer), and a back electrode metal thin film layer. The substrate can be aluminum alloy, stainless steel, special plastics, etc. Its manufacturing method is completely different from that of monocrystalline and polycrystalline silicon solar cells, consuming very little silicon material and resulting in lower power consumption. This type of cell has lower material costs, higher utilization rate, is energy-saving and environmentally friendly, and the materials are relatively easy to obtain. Therefore, this type of cell is an inevitable trend in the future development of solar cells.
Multi-component thin-film solar cells
Multi-component compound thin-film solar cell materials are inorganic salts, mainly including gallium arsenide III-V compounds, cadmium sulfide, and copper inert selenide thin-film cells. Cadmium sulfide and cadmium telluride polycrystalline thin-film cells have higher efficiency than amorphous silicon thin-film solar cells, lower cost than monocrystalline silicon cells, and are easier to mass-produce. However, because cadmium is highly toxic, it causes serious environmental pollution.
Therefore, it is not the most ideal replacement for crystalline silicon solar cells. Gallium arsenide (GaAs) III-V compound cells can achieve a conversion efficiency of up to 28%. GaAs compound materials have an ideal optical bandgap and high absorption efficiency, strong radiation resistance, and are insensitive to heat, making them suitable for manufacturing high-efficiency single-junction cells. However, GaAs materials are expensive, which greatly limits the widespread use of GaAs cells.
