I. How to improve the efficiency of solar cells
A solar cell is a thin film of photovoltaic semiconductors that directly generates electricity using sunlight. Also known as a "solar chip" or "photovoltaic cell," it can instantly output voltage and generate current when a circuit is established, provided it receives sufficient illumination. In physics, this is called solar photovoltaic (PV), or simply photovoltaic.
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 photovoltaic effect, are the mainstream, while thin-film solar cells, which operate on the photochemical effect, are still in their infancy.
What do we mean by solar cell efficiency? Every device we use has a certain efficiency. Consider a machine that can produce 10 balloons per hour. Of these 10 balloons, two have holes or other types of defects. This means the machine's efficiency is 80%, because the machine absorbs the raw materials needed to produce 10 balloons but only converts 80% of it into usable output. Therefore, the efficiency of a device represents the amount of usable output produced per unit of input.
The theoretical efficiency of crystalline silicon solar cells is 25% (under AMO 1.0 spectral conditions). The theoretical efficiency of a solar cell is related to various possible losses before incident light energy is converted into current. Some of these factors are determined by the fundamental physics of the solar cell, while others are related to materials and manufacturing processes. From the perspective of improving solar cell efficiency, the following aspects should be addressed:
1. Reduce light reflection loss in solar cell thin films.
2. Reduce the forward current of the PN junction (commonly known as the dark current of a solar cell).
3. The width of the space charge region of the PN junction is reduced, and the recombination centers of the space charge region are also reduced.
4. Improve the minority carrier lifetime in silicon crystals, i.e. reduce the content of heavy metal impurities and other impurities that can act as recombination centers, crystal structure defects, etc.
5. When considering the thickness of each region of the silicon crystal in a solar cell and other structural parameters.
The main measures to improve the efficiency of solar cells are as follows, and the adoption of each measure often leads to corresponding new process technologies.
(1) Select a high-performance substrate silicon crystal with a long carrier lifetime.
(2) A textured or inverted pyramidal pitted surface structure is fabricated on the surface of the solar cell chip. A back mirror is fabricated on the back of the cell chip to reduce surface reflection and create a good light-blocking mechanism.
(3) Design the emitter junction structure reasonably to collect as many photogenerated carriers as possible.
(4) Use a high-performance surface passivation film to reduce the surface recombination rate.
(5) Adopt a deep junction structure and enhance passivation at the metal contact points.
(6) Reasonable electrode contact design to achieve low series resistance, etc.
II. What are the electrode materials for solar cells?
Solar cells are devices that directly convert light energy into electrical energy through the photoelectric effect or photochemical effect. Thin-film solar cells, which operate on the photoelectric effect, are the mainstream, while photochemical solar cells are still in their infancy. Sunlight shines on a semiconductor pn junction, forming new electron-hole pairs. Under the influence of the pn junction's electric field, holes flow from the n-region to the p-region, and electrons flow from the p-region to the n-region, forming a current when the circuit is connected. This is the working principle of a photoelectric effect solar cell.
Solar cells can be classified into two main categories according to their crystallization state: crystalline thin-film and amorphous film (hereinafter referred to as a-). The former is further divided into monocrystalline and polycrystalline forms.
Based on materials, solar cells can be classified into silicon thin-film, compound semiconductor thin-film, and organic thin-film types. Compound semiconductor thin-films are further divided into amorphous types (a-Si:H, a-Si:H:F, a-SixGel-x:H, etc.), Group III-V (GaAs, InP), Group II-VI (CdS system), and zinc phosphide (Zn3P2), etc. A table lists the classifications and applications of various types of solar cells.
