A solar cell is a device that responds to light and converts light energy into electricity. Many materials can produce the photovoltaic effect, such as monocrystalline silicon, polycrystalline silicon, amorphous silicon, gallium arsenide, and copper indium selenide. Their power generation principles are basically the same; the photovoltaic process will now be described using crystalline silicon as an example. P-type crystalline silicon can be doped with phosphorus to obtain N-type silicon, forming a PN junction.
When light shines on the surface of a solar cell, some photons are absorbed by the silicon material. The energy of the photons is transferred to the silicon atoms, causing electrons to transition and become free electrons. These free electrons accumulate on both sides of the PN junction, creating a potential difference. When an external circuit is connected, a current flows through the external circuit under the influence of this voltage, generating a certain output power. Essentially, this process converts photon energy into electrical energy.
Basic characteristics of solar cells
The basic characteristics of solar cells include three fundamental properties: polarity, performance parameters, and current-voltage characteristics. These are explained in detail below.
1. Polarity of solar cells
Silicon solar cells are generally fabricated with a P+/N type structure or an N+/P type structure. P+ and N+ represent the conductivity type of the semiconductor material in the front-side light-illuminating layer of the solar cell; N and P represent the conductivity type of the semiconductor material in the back-side substrate of the solar cell. The electrical performance of a solar cell is related to the characteristics of the semiconductor materials used in its manufacture.
2. Performance parameters of solar cells
The performance parameters of a solar cell consist of open-circuit voltage, short-circuit current, maximum output power, fill factor, and conversion efficiency. These parameters are indicators of the performance of a solar cell.
3. Current-voltage characteristics of solar cells
A PN junction solar cell comprises a shallow PN junction formed on the surface, a strip-shaped and finger-shaped front ohmic contact, a back ohmic contact covering the entire back surface, and an anti-reflective layer on the front. When the cell is exposed to the solar spectrum, photons with energy less than the bandgap Eg do not contribute to the cell's output. Only photons with energy greater than the bandgap Eg contribute energy Eg to the cell's output; energy greater than Eg is dissipated as heat. Therefore, the impact of this heat on the cell's stability and lifespan must be considered during the design and manufacturing of solar cells.
Performance parameters of solar cells
1. Open circuit voltage
Open-circuit voltage UOC: The output voltage of a solar cell when it is exposed to a light source of 100 mW/cm2 with its terminals open.
2. Short-circuit current
Short-circuit current (ISC): This is the current flowing through the two ends of a solar cell when it is placed under the illumination of a standard light source and short-circuited at the output terminal.
3. High output power
The operating voltage and current of a solar cell vary with the load resistance. Plotting the operating voltage and current values corresponding to different resistance values yields the current-voltage characteristic curve of the solar cell. The maximum output power, denoted by Pm, is achieved when the selected load resistance value maximizes the product of the output voltage and current. The operating voltage and current at this point are called the optimal operating voltage and optimal operating current, denoted by Um and Im, respectively.
4. Fill factor
Another important parameter of solar cells is the fill factor FF, which is the ratio of maximum output power to the product of open-circuit voltage and short-circuit current.
FF (Fill Factor) is an important indicator of the output characteristics of a solar cell. It represents the maximum power output of a solar cell under optimal load; a higher FF value indicates a higher output power. The value of FF is always less than 1. In reality, due to the influence of series and parallel resistances, the actual fill factor of a solar cell is lower than the ideal value given by the above formula. Series and parallel resistances have a significant impact on the fill factor. The larger the series resistance, the greater the decrease in short-circuit current, and the greater the reduction in fill factor; the smaller the parallel resistance, the greater the current in this part, the greater the decrease in open-circuit voltage, and the greater the reduction in fill factor.
5. Conversion efficiency
The photoelectric conversion efficiency of a solar cell refers to the maximum energy conversion efficiency when the optimal load resistance is connected in the external circuit. It is equal to the ratio of the solar cell's output power to the energy incident on the solar cell surface. The photoelectric conversion efficiency of a solar cell is an important parameter for measuring cell quality and technological level. It is related to the cell's structure, junction characteristics, material properties, operating temperature, radiation damage from radioactive particles, and environmental changes.
