With the increasingly tense global energy, solar energy has become a new type of energy has been vigorously developed, in which we use the most in life is the solar cell. Solar panels are based on semiconductor materials, the use of photoelectric materials to absorb light energy after the photoelectric conversion, so that it generates electric current, so how does the working principle of solar panels? Solar cell is a device that directly converts light energy into electrical energy through the photoelectric effect or photochemical effect. When sunlight hits a semiconductor, some of it is reflected off the surface and the rest is absorbed or transmitted by the semiconductor. Some of the absorbed light, of course, becomes heat, and some photons collide with the valence electrons of the atoms that make up the semiconductor, resulting in electron-hole pairs. In this way, light energy in the form of generating electron-hole pairs into electrical energy.
Solar cell working principle
1, photovoltaic effect.
Solar cell energy conversion is based on the semiconductor PN junction photovoltaic effect. As mentioned earlier, when light irradiation to the semiconductor photovoltaic device, energy greater than the width of the silicon forbidden band of photons across the reflective film into the silicon, in the N region, depletion zone and P region in the excitation of photogenerated electrons - hole pairs.
Depletion region: Immediately after the photogenerated electron-hole pairs are generated in the depletion region, they are separated by the built-in electric field, the photogenerated electrons are sent into the N-region, and the photogenerated holes are pushed into the P-region. According to the depletion approximation condition, the carrier concentration at the boundary of the depletion region is approximated to be 0, i.e., p=n=0.
In the N region: after the creation of the photogenerated electron-hole pair, the photogenerated hole diffuses toward the P-N junction boundary, and once it reaches the P-N junction boundary, it is immediately subjected to the effect of the built-in electric field, and is pulled by the electric field force to make a drifting motion, crossing the depletion region into the P region, and the photogenerated electrons (multiplets) are left in the N region.
In the P-region, the photogenerated electrons (oligons) also enter the N-region first by diffusion and then by drift, and the photogenerated holes (multiplets) remain in the P-region. This creates an accumulation of positive and negative charges on both sides of the P-N junction, so that the N-region stores an excess of electrons and the P-region has an excess of holes. This results in the formation of a photogenerated electric field in the opposite direction to the built-in electric field.
1. In addition to partially offsetting the role of the potential electric field, the photogenerated electric field also makes the P region positively charged, the N region negatively charged, in the thin layer between the N region and the P region to generate electric potential, which is the photogenerated voltammetric effect. When the battery connected to a load, photocurrent from the P area through the load flow to the N area, the load that is the power output.
2. If the P-N junction open circuit, you can measure the electric potential, called the open-circuit voltage Uoc. For crystalline silicon batteries, the typical value of the open-circuit voltage of 0.5 ~ 0.6 V. 3.
3. If the external circuit is short-circuited, there is a photocurrent in the external circuit that is proportional to the energy of the incident light, and this current is called the short-circuit current Isc.
Factors affecting the photocurrent:
1. The more electron-hole pairs generated in the interface layer by light, the greater the current.
2. The more light energy absorbed by the interfacial layer, the larger the interfacial layer, i.e., the larger the cell area, the larger the current formed in the solar cell.
3. The N-region, depletion region and P-region of the solar cell can produce photogenerated carriers;
4. The photogenerated carriers in each region must cross the depletion region before compounding in order to contribute to the photocurrent, so the solution to the actual photogenerated current must take into account the various factors such as generation and compounding, diffusion and drift in each region.
Solar cell equivalent circuit, output power and fill factor
(1) Equivalent circuit
In order to describe the operating state of the battery, the battery and load system is often modeled with an equivalent circuit.
1. constant current source: in constant light, a solar cell in working condition, its photocurrent does not change with the operating state, in the equivalent circuit can be regarded as a constant current source.
2. Dark current Ibk: Part of the photocurrent flows through the load RL, which establishes a terminal voltage U at both ends of the load, which in turn is positively biased at the PN junction, giving rise to a dark current Ibk in the opposite direction of the photocurrent.
3. In this way, the equivalent circuit of an ideal PN homojunction solar cell is drawn as shown in Fig.
4. Series resistance RS:Due to the contact of the electrodes on the front and the back, as well as the fact that the material itself has a certain resistivity, additional resistance is inevitably introduced in both the base region and the top layer. The current flowing through the load through them will inevitably cause losses. In an equivalent circuit, their total effect can be expressed in terms of a series resistance RS.
5. Shunt resistance RSh:Due to leakage at the edges of the battery and leakage from metal bridges formed at microcracks, scratches, etc., when making metallized electrodes, etc., a portion of the current that should have passed through the load is short-circuited, and the magnitude of this effect can be equated with a shunt resistance RSh.
When the current flowing into the load RL is I and the terminal voltage of the load RL is U, it is obtained:
P in the equation is the output power obtained at the load RL when the solar cell is irradiated.
(2) Output power When the current flowing into the load RL is I and the terminal voltage of the load RL is U, it is obtained:
P in the equation is the output power obtained at the load RL when the solar cell is irradiated. When the load RL is changed from 0 to infinity, the output voltage U is changed from 0 to U0C, and at the same time the output current is changed from ISC to 0. The load characteristic curve of the solar cell can thus be drawn. Any point on the curve is called the operating point, the line between the operating point and the origin is called the load line, the inverse of the slope of the load line is equal to RL, and the operating point corresponding to the horizontal and vertical coordinates of the operating voltage and current.
Adjust the load resistance RL to a certain value Rm, get a point M on the curve, corresponding to the working current Im and the working voltage Um product is the largest, that is: Pm = ImUm
The point M is generally called the optimum working point (or maximum power point) of the solar cell, Im is the optimum working current, Um is the optimum working voltage, Rm is the optimum load resistance, and Pm is the maximum output power.
(3) Filling factor
1. Maximum output power and (Uoc × Isc) ratio is called the fill factor (FF), which is used to measure the output characteristics of solar cells is one of the important indicators.
2. Filling factor characterizes the strengths and weaknesses of solar cells, in a certain spectral irradiance, the larger the FF, the more "square" curve, the higher the output power.
4, the efficiency of solar cells, factors affecting efficiency
(1) solar cell efficiency.
Solar cell irradiated, the output electric power and incident light power ratio η is called the efficiency of the solar cell, also known as photoelectric conversion efficiency. Generally refers to the maximum energy conversion efficiency when the external circuit is connected to the optimal load resistance RL.
In the above equation, if At is replaced by the effective area Aa (also known as the active area), that is, from the total area deducted from the area of the grid line graphic area, so as to calculate the efficiency to be higher, which should be noted when reading domestic and foreign literature.
Prince of the United States first calculated the theoretical efficiency of silicon solar cells for 21.7%. 1970s, Wolf (M. Wolf) has done an exhaustive discussion, but also get the theoretical efficiency of silicon solar cells in the AM0 spectral conditions for 20% to 22%, and later modified it to 25% (AM1.0 spectral conditions).
To estimate the theoretical efficiency of a solar cell, all possible losses that may occur between the incident light energy and the output electrical energy must be accounted for. Some of these losses are material and process related, while others are dictated by fundamental physical principles.
(2) Factors affecting efficiency
To summarize, to improve the efficiency of solar cells, the three basic parameters, open circuit voltage Uoc, short circuit current ISC and fill factor FF, must be improved. And these three parameters are often interlocked, if a unilateral increase in one of them, may therefore reduce the other, so that the total efficiency not only did not improve but also decreased. Therefore, in the selection of materials, design process must be considered holistically, and strive to maximize the product of the three parameters.
1. Material bandwidth.
The open circuit voltage UOC increases with the increase of the energy band width Eg, but on the other hand, the short circuit current density decreases with the increase of the energy band width Eg. As a result, a peak in solar cell efficiency can be expected to occur at a defined Eg. The highest efficiency can be expected from solar cells made of materials with Eg values between 1.2 and 1.6 eV. A direct band gap semiconductor is preferred for thin film cells because it absorbs photons near the surface.
2. Temperature.
The diffusion length of an oligon increases slightly with temperature, so the photovoltaic current also increases with temperature, but the UOC decreases sharply with temperature. The filling factor decreases, so the conversion efficiency decreases with increasing temperature.
3.Irradiance.
The short circuit current increases linearly with increasing irradiance and the maximum power keeps increasing. Focusing sunlight on a solar cell can make a small solar cell produce a large amount of electricity.
4. Doping concentration.
Another factor that has a significant effect on UOC is the semiconductor doping concentration. The higher the doping concentration, the higher the UOC. However, when the impurity concentration in silicon is higher than 1018/cm3 is called highly doped, due to high doping caused by the forbidden band contraction, impurities can not be fully ionized and the decline in the lifetime of the oligon and other phenomena collectively referred to as the high doping effect, should also be avoided.
5. Photogenerated carrier composite lifetime.
For the semiconductor of solar cell, the longer the photogenerated carrier complex life, the larger the short circuit current will be. The key to achieve a long life is to avoid the formation of composite centers in the process of material preparation and cell production. In the process, appropriate and often related processes, can make the composite center removed, and extend the life.
6. Surface compounding rate.
A low surface composite rate helps to improve Isc. The composite rate on the front surface is difficult to measure and is often assumed to be infinite. A type of cell called a backside field (BSF) is designed to diffuse an additional layer of P+ on the backside of the cell before depositing the metal contact.
7. series resistance and metal grid lines.
Series resistance comes from the lead, metal contact grid or cell body resistance, while the metal grid line can not pass through the sunlight, in order to maximize Isc, the metal grid line should occupy the smallest area. Generally make the metal grid line into a dense and thin shape, you can reduce the series resistance, while increasing the light transmission area of the battery.
8. Adoption of velvet cell design and selection of high-quality reflection-reducing film.
Relying on the surface pyramid-shaped square cone structure, multiple reflections of light, not only reduces the reflection loss, but also changes the direction of light in the silicon forward and extends the optical range, increasing the photogenerated carrier yield; zigzagging velvet surface and increase the area of the PN junction, thereby increasing the collection rate of photogenerated carriers, so that the short-circuit current increased by 5% to 10%, and improve the red light response of the battery.
9. The effect of shadows on solar cells.
Solar cells can suffer from uneven irradiation due to shadowing, etc., and the output power is greatly reduced.
At present, the application of solar cells from the military field, aerospace field into industry, commerce, agriculture, communications, household appliances and utilities and other sectors, especially can be decentralized in remote areas, mountains, deserts, islands and rural areas in order to save the cost of very expensive transmission lines. However, at the present stage, its cost is still very high, send out 1kW of electricity need to invest tens of thousands of dollars, so the large-scale use is still subject to economic constraints.
However, in the long run, with the improvement of solar panel manufacturing technology and the invention of new light-electricity conversion devices, the protection of the environment and the huge demand for renewable clean energy, solar cells will still be the use of solar radiation energy is a more practical and feasible way to use the sun's energy for the future of mankind's large-scale use of solar energy to open up a wide range of prospects.
