Solar Panel Principle_How Solar Cells Work

As global energy becomes increasingly scarce, solar energy has been vigorously developed as a new energy source. Among them, solar cells are the most used in our lives. Solar cells are mainly based on semiconductor materials. Photoelectric materials absorb light energy and then undergo photoelectric conversion to generate current. So what is the working principle of solar cells? Solar cells are devices that directly convert light energy into electrical energy through the photoelectric effect or photochemical effect. When sunlight shines on a semiconductor, part of it is reflected by the surface, and the rest is absorbed or transmitted by the semiconductor. Of course, some of the absorbed light turns into heat, and other photons collide with the atomic valence electrons that make up the semiconductor, thus generating electron-hole pairs. In this way, light energy is converted into electrical energy in the form of electron-hole pairs.

1. Physical basis of solar cells


When sunlight irradiates the p-n junction, the electrons in the semiconductor release electrons due to the light energy they obtain, and accordingly, electron-hole pairs are generated. Under the action of the barrier electric field, the electrons are driven to the N-type region and the holes are driven to the P-type region, so that the N-type region has excess electrons and the P-type region has excess holes. Thus, a photogenerated electric field in the opposite direction to the barrier electric field is formed near the p-n junction.
If there is a P-N junction in the semiconductor, a barrier electric field is formed on both sides of the P-type and N-type interface, which can drive electrons to the N-type region and holes to the P-type region, so that the N-type region has excess electrons and the P-type region has excess holes, and a photogenerated electric field in the opposite direction to the barrier electric field is formed near the P-N junction.

There are more than a dozen known semiconductor materials for making solar cells, so there are many types of solar cells. At present, the most mature and commercially valuable solar cells are silicon solar cells. Let's take silicon solar cells as an example to explain in detail the working principle of solar cells.

1. Intrinsic semiconductors


The conductivity of a substance is determined by its atomic structure. Conductors are generally low-valent elements, and their outermost electrons can easily break free from the nucleus and become free electrons. Under the action of an external electric field, they move in a directional manner to form an electric current. For high-valent elements (such as inert gases) or polymer substances (such as rubber), their outermost electrons are strongly bound by the nucleus and are difficult to become free electrons, so their conductivity is extremely poor and they become insulators. Commonly used semiconductor materials silicon (Si) and germanium (Ge) are both tetravalent elements. Their outermost electrons are neither as easy to break free from the nucleus as conductors, nor as tightly bound by the nucleus as insulators, so their conductivity is between the two.


Pure semiconductors are made into single crystals through a certain process, which is an intrinsic semiconductor. The atoms in the crystal form a neatly arranged lattice in space, and adjacent atoms form covalent bonds.

The covalent bonds in the crystal have extremely strong binding force. Therefore, at room temperature, only a very small number of valence electrons obtain enough energy due to thermal motion (thermal excitation) to break free from the constraints of the covalent bonds and become free electrons. At the same time, a hole is left in the covalent bond. The atom becomes positively charged due to the loss of a valence electron, or the hole is positively charged. In intrinsic semiconductors, free electrons and holes appear in pairs, that is, the number of free electrons and holes is equal.

If a free electron meets a hole during its movement, it will fill the hole and make both disappear at the same time. This phenomenon is called recombination. At a certain temperature, the number of free electrons and hole pairs generated by intrinsic excitation is equal to the number of free electrons and hole pairs in recombination, so a dynamic equilibrium is achieved.


Band theory:
1. When electrons in a single atom move around the nucleus, the electrons in each orbit have specific energy;
2. The closer the orbit is to the nucleus, the lower the electron energy;
3. According to the principle of minimum energy, electrons always occupy the lowest energy level first;
4. The energy band occupied by valence electrons is called the valence band;
5. There is a forbidden band above the valence band, and there is no energy level occupied by electrons in the forbidden band;
6. Above the forbidden band is the conduction band, and the energy level in the conduction band is the energy level that the valence electrons can occupy when they break free from the covalent bond and become free electrons;
7. The width of the band gap is represented by Eg, and its value is related to factors such as the material of the semiconductor and the temperature it is in. At T=300K, Eg=1.1eV for silicon; Eg=0.72eV for germanium.

2. Impurity semiconductors

Impurity semiconductors: Impurity semiconductors can be obtained by adding a small amount of impurity elements to intrinsic semiconductors through diffusion processes.


Depending on the impurity elements added, N-type semiconductors and P-type semiconductors can be formed; by controlling the concentration of the impurity elements, the conductivity of the impurity semiconductor can be controlled.


N-type semiconductors: When pentavalent elements (such as phosphorus) are added to pure silicon crystals to replace the position of silicon atoms in the crystal lattice, N-type semiconductors are formed.

Since the outermost layer of the impurity atom has five valence electrons, in addition to forming covalent bonds with the surrounding silicon atoms, there is one more electron. The extra electron is not bound by the covalent bond and becomes a free electron. In N-type semiconductors, the concentration of free electrons is greater than the concentration of holes, so free electrons are called majority carriers and holes are minority carriers. Since impurity atoms can provide electrons, they are called donor atoms.
P-type semiconductor: P-type semiconductors are formed by doping trivalent elements (such as boron) into pure silicon crystals to replace the position of silicon atoms in the crystal lattice.

Since the outermost layer of impurity atoms has three valence electrons, when they form covalent bonds with the surrounding silicon atoms, a "vacancy" is generated. When the outermost electrons of the silicon atom fill this vacancy, a hole is generated in its covalent bond. Therefore, in P-type semiconductors, holes are majority carriers and free electrons are minority carriers. Because the vacancies in impurity atoms absorb electrons, they are called acceptor atoms.

3. PN junction


: Using different doping processes, P-type semiconductors and N-type semiconductors are made on the same silicon wafer, and a PN junction is formed at their interface.

Diffusion movement: Matter always moves from a place with high concentration to a place with low concentration. This movement caused by concentration difference is called diffusion movement.
When a P-type semiconductor and an N-type semiconductor are made together, at their interface, the concentration difference of the two carriers is very large, so the holes in the P region must diffuse to the N region. At the same time, the free electrons in the N region must diffuse to the P region, as shown in the figure.


Since the free electrons diffused to the P region recombine with the holes, and the holes diffused to the N region recombine with the free electrons, the concentration of majority carriers near the interface decreases, and a negative ion region appears in the P region and a positive ion region appears in the N region. They cannot move, which is called the space charge region, thus forming a built-in electric field ε.
As the diffusion movement proceeds, the space charge region widens, the built-in electric field increases, and its direction points from the N region to the P region, just preventing the diffusion movement from proceeding.
Drift movement: The movement of carriers under the action of the electric field force is called drift movement.


When the space charge region is formed, under the action of the built-in electric field, the minority carriers produce drift movement, the holes move from the N region to the P region, and the free electrons move from the P region to the N region. In the absence of external electric field and other excitation, the number of majority carriers participating in diffusion motion is equal to the number of minority carriers participating in drift motion, thus achieving dynamic equilibrium and forming a PN junction, as shown in the figure. At this time, the space charge region has a certain width, the potential difference is ε =Uho, and the current is zero.

2. Working Principle of Solar Cells


1. Photovoltaic Effect:


The basis of solar cell energy conversion is the photovoltaic effect of semiconductor PN junction. As mentioned above, when light shines on semiconductor photovoltaic devices, photons with energy greater than the silicon bandgap pass through the anti-reflection film into the silicon, and stimulate photogenerated electron-hole pairs in the N region, depletion region and P region.
Depletion region: After the photogenerated electron-hole pairs are generated in the depletion region, they are immediately separated by the built-in electric field, and the photogenerated electrons are sent to 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 approximately 0, that is, p=n=0.

In the N region: after the photogenerated electron-hole pairs are generated, the photogenerated holes diffuse to the PN junction boundary. Once they reach the PN junction boundary, they are immediately affected by the built-in electric field and are pulled by the electric field force to drift, crossing the depletion region into the P region, while the photogenerated electrons (majority carriers) are left in the N region.
In the P region: the photogenerated electrons (minority carriers) also enter the N region due to diffusion and then drift, while the photogenerated holes (majority carriers) remain in the P region. In this way, the accumulation of positive and negative charges is formed on both sides of the PN junction, so that the N region stores excess electrons and the P region has excess holes. Thus, a photogenerated electric field in the opposite direction to the built-in electric field is formed.

1. In addition to partially offsetting the effect of the potential barrier electric field, the photoelectric field also makes the P region positively charged and the N region negatively charged, and an electromotive force is generated in the thin layer between the N region and the P region. This is the photovoltaic effect. When the battery is connected to a load, the photocurrent flows from the P region through the load to the N region, and the load obtains power output.


2. If the two ends of the PN junction are open, this electromotive force can be measured, which is called the open circuit voltage Uoc. For crystalline silicon cells, the typical value of the open circuit voltage is 0.5~0.6V.


3. If the external circuit is short-circuited, a photocurrent proportional to the incident light energy will flow through the external circuit. This current is called the short-circuit current Isc.
Factors affecting the photocurrent:


1. The more electron-hole pairs generated by light in the interface layer, the greater the current.
2. The more light energy the interface layer absorbs, the larger the interface layer, i.e., the battery area, and the greater the current formed in the solar cell.
3. The N region, depletion region and P region of the solar cell can all generate photogenerated carriers;
4. The photogenerated carriers in each region must cross the depletion region before recombination to contribute to the photocurrent, so the actual photogenerated current must take into account various factors such as generation and recombination, diffusion and drift in each region. Solar cell


equivalent circuit, output power and fill factor


(1) Equivalent circuit

In order to describe the working state of the battery, the battery and load system are often simulated by an equivalent circuit.

1. Constant current source: Under constant illumination, the photocurrent of a working solar cell does not change with the working state. In the equivalent circuit, it can be regarded as a constant current source.
2. Dark current Ibk: A part of the photocurrent flows through the load RL, establishing a terminal voltage U at both ends of the load. In turn, it is forward biased to the PN junction, causing a dark current Ibk in the opposite direction to the photocurrent.
3. In this way, the equivalent circuit of an ideal PN homojunction solar cell is drawn as shown in the figure.
4. Series resistance RS: Due to the contact between the front and back electrodes and the certain resistivity of the material itself, additional resistance is inevitably introduced in the base region and the top layer. When the current flowing through the load passes through them, it will inevitably cause loss. In the equivalent circuit, their total effect can be represented by a series resistance RS.
5. Parallel resistance RSh: Due to leakage at the edge of the battery and leakage of metal bridges formed at microcracks and scratches when making metallized electrodes, part of the current that should pass through the load is short-circuited. The magnitude of this effect can be equivalent to a parallel resistance RSh.
When the current flowing into the load RL is I and the terminal voltage of the load RL is U, it can be obtained:

The P in the formula is the output power obtained on the load RL when the solar cell is illuminated.

⑵ Output power
When the current flowing into the load RL is I and the terminal voltage of the load RL is U, we can get:

P in the formula is the output power obtained on the load RL when the solar cell is irradiated.
When the load RL changes from 0 to infinity, the output voltage U changes from 0 to U0C, and the output current changes from ISC to 0, thus drawing the load characteristic curve of the solar cell. Any point on the curve is called the working point, and the line connecting the working point and the origin is called the load line. The inverse of the slope of the load line is equal to RL, and the horizontal and vertical coordinates corresponding to the working point are the working voltage and working current.

When the load resistance RL is adjusted to a certain value Rm, a point M is obtained on the curve, and the product of the corresponding working current Im and working voltage Um is the largest, that is: Pm=ImUm.
Point M is generally called the optimal working point (or maximum power point) of the solar cell, Im is the optimal working current, Um is the optimal working voltage, Rm is the optimal load resistance, and Pm is the maximum output power.

⑶ Fill factor
1. The ratio of maximum output power to (Uoc×Isc) is called fill factor (FF), which is one of the important indicators used to measure the output characteristics of solar cells.

2. The fill factor characterizes the quality of solar cells. Under a certain spectral irradiance, the larger the FF, the more "square" the curve is, and the higher the output power.

4. Efficiency of solar cells and factors affecting efficiency
⑴ Efficiency of solar cells:
When a solar cell is irradiated, the ratio of the output power to the incident light power is called the efficiency of the solar cell, also known as the photoelectric conversion efficiency. It generally refers to the maximum energy conversion efficiency when the external circuit is connected to the optimal load resistance RL.

In the above formula, if At is replaced by the effective area Aa (also called active area), that is, the area of ​​the grid pattern is deducted from the total area, the calculated efficiency will be higher. This should be noted when reading domestic and foreign literature.


Prince of the United States first calculated that the theoretical efficiency of silicon solar cells was 21.7%. In the 1970s, M. Wolf made a detailed discussion and also obtained that the theoretical efficiency of silicon solar cells was 20%~22% under AM0 spectral conditions, and later revised it to 25% (AM1.0 spectral conditions).


To estimate the theoretical efficiency of solar cells, all possible losses from incident light energy to output electrical energy must be calculated. Some of them are losses related to materials and processes, while others are determined by basic physical principles.


⑵ Factors affecting efficiency


In summary, to improve the efficiency of solar cells, it is necessary to improve the three basic parameters of open circuit voltage Uoc, short circuit current ISC and fill factor FF. These three parameters are often mutually restrained. If one of them is increased unilaterally, the other may be reduced, so that the total efficiency is not only not increased but decreased. Therefore, when selecting materials and designing processes, it is necessary to consider all aspects and strive to maximize the product of the three parameters.


1. Material band width:


The open circuit voltage UOC increases with the increase of the band width Eg, but on the other hand, the short circuit current density decreases with the increase of the band width Eg. As a result, it is expected that the peak of the solar cell efficiency will appear at a certain Eg. Solar cells made of materials with Eg values ​​between 1.2 and 1.6 eV are expected to achieve the highest efficiency. Direct bandgap semiconductors are more desirable for thin-film batteries because they can absorb photons near the surface.


2. Temperature:


The diffusion length of minority carriers increases slightly with the increase of temperature, so the photocurrent also increases with the increase of temperature, but UOC decreases sharply with the increase of temperature. The filling factor decreases, so the conversion efficiency decreases with the increase of temperature.

3. Irradiance:


As the irradiance increases, the short-circuit current increases linearly, and the maximum power increases continuously. Focusing sunlight on a solar cell can make a small solar cell generate a large amount of electricity.

4. Doping concentration:


Another factor that has a significant impact 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, it is called high doping. The bandgap shrinkage, impurities cannot be completely ionized, and minority carrier lifetime reduction caused by high doping are collectively called high doping effects, which should also be avoided.

5. Photogenerated carrier recombination life:


For the semiconductor of the solar cell, the longer the recombination life of the photogenerated carriers, the greater the short-circuit current. The key to achieving long life is to avoid the formation of recombination centers during material preparation and battery production. During the processing, appropriate and frequent related process treatments can remove the recombination centers and extend the life.


6. Surface recombination rate:


Low surface recombination rate helps to increase Isc. The recombination rate on the front surface is difficult to measure and is often assumed to be infinite. A cell design called back field (BSF) is that a layer of P+ additional layer is diffused on the back of the cell before depositing the metal contact.


7. Series resistance and metal grid:


Series resistance comes from the lead, metal contact grid or battery body resistance, and the metal grid cannot transmit sunlight. In order to maximize Isc, the area occupied by the metal grid should be minimized. Generally, making the metal grid dense and thin can reduce the series resistance and increase the light-transmitting area of ​​the cell.


8. Use velvet cell design and select high-quality anti-reflection film:


Relying on the pyramid-shaped square cone structure on the surface, the light is reflected multiple times, which not only reduces the reflection loss, but also changes the direction of light in silicon and extends the optical path, increasing the production of photogenerated carriers; the tortuous velvet surface increases the area of ​​the PN junction, thereby increasing the collection rate of photogenerated carriers, increasing the short-circuit current by 5% to 10%, and improving the red light response of the cell.


9. The impact of shadows on solar cells:


Solar cells will be unevenly illuminated due to shadows, etc., and the output power will be greatly reduced.

At present, the application of solar cells has entered the industrial, commercial, agricultural, communication, household appliances and public utilities sectors from the military and aerospace fields. In particular, they can be used in remote areas, mountains, deserts, islands and rural areas to save expensive transmission lines. However, at the current stage, its cost is still very high. It takes tens of thousands of dollars to generate 1kW of electricity, so large-scale use is still subject to economic restrictions.


However, in the long run, with the improvement of solar cell manufacturing technology and the invention of new light-to-electricity conversion devices, the protection of the environment and the huge demand for renewable clean energy in various countries, solar cells will still be a more practical way to utilize solar radiation energy, which can open up broad prospects for large-scale use of solar energy in the future.