What Is the Photovoltaic Effect?

"Photovoltaic effect", referred to as "photovoltaic effect", English name: Photovoltaic effect. Refers to the phenomenon in which a potential difference occurs between different parts of an uneven semiconductor or a combination of semiconductor and metal. First, it is the process of converting photons (light waves) into electrons, and light energy is converted into electric energy; second, it is the process of forming voltage. With voltage, it is like building a dam. If the two are connected, a loop of current is formed.

Chinese name: Photovoltaic effect
Foreign name: Photovoltaic effect

nickname: Photovoltaic effect
Method: Illumination makes uneven semiconductor

Basic overview of photovoltaic effect

As early as 1839, French scientist Becqurel discovered that light can cause potential differences between different parts of semiconductor materials. This phenomenon was later called "photovoltaic effect", or "photovoltaic effect" for short. In 1954, American scientists Chapin and Pierson made the first practical single-crystal silicon solar cell in Bell Labs in the United States, and the practical photovoltaic power generation technology that converted solar energy into electrical energy was born. The basis of the working principle of solar cells is the photovoltaic effect of the semiconductor PN junction, which is an effect that generates an electromotive force and current when the charge distribution state in the object changes when the object is illuminated. That is, when sunlight or other light irradiates the PN junction of a semiconductor, a voltage appears on both sides of the PN junction, called a photo-generated voltage. If the PN junction is short-circuited, a current is generated.

Photovoltaic power generation is a technology that uses the photovoltaic effect of a semiconductor interface to directly convert light energy into electrical energy. The key element of this technology is solar cells. The solar cells can be packaged and protected after being connected in series to form a large-area solar cell module. A photovoltaic power generation device can be formed by adding a power controller and other components. The advantage of photovoltaic power generation is that it is less restricted by the region, because the sun shines on the ground; photovoltaic systems also have the advantages of safety, reliability, noiselessness, low pollution, on-site power generation without the need to consume fuel and erection of transmission lines, and the short construction period. Photovoltaic effect is simply referred to as photovoltaic effect, which refers to the phenomenon that a potential difference occurs between different parts of an uneven semiconductor or a combination of a semiconductor and a metal.

Photovoltaic effect

The sun shines on the semiconductor pn junction to form a new hole-electron pair. Under the action of the pn junction electric field, the holes flow from the n region to the p region, and the electrons flow from the p region to the n region. After the circuit is connected, a current is formed. This is how the photovoltaic effect solar cell works.

PN Formation of photovoltaic effect P-N junction

The homojunction can be doped with a semiconductor to form P and N regions. Due to the small activation energy of the impurities, the impurities are almost ionized into the acceptor ion NA- and the donor ion ND + at room temperature. Due to the difference in carrier concentration at the interface of the PN region, each of them must diffuse to the other. Imagine that at the moment when the junction is formed, the electrons in the N region are polytrons, and the electrons in the P region are minority electrons, so that the electrons flow from the N region into the P region, and the recombination of the electrons and the holes will occur, so in the original N region The electrons near the junction surface become very few, leaving the unneutralized ion ND + to form a positive space charge. Similarly, after the holes diffuse from the P region to the N region, a negative space charge is formed by the non-movable acceptor ion NA-. Ion regions (also known as depletion regions, space charge regions, and barrier layers) are generated on both sides of the interface between the P and N regions, and a space galvanic layer appears, forming an internal electric field (called a built-in electric field). Diffusion of multiple sons is counterproductive, and it helps the drift of minority sons until the diffusion current reaches equilibrium when the drift current is reached, and a stable built-in electric field is established on both sides of the interface. ^[1]

Photovoltaic effect

Photovoltaic effect refers to the phenomenon that a potential difference occurs between different parts of an uneven semiconductor or a combination of semiconductor and metal. First, it is the process of converting photons (light waves) into electrons, and light energy is converted into electric energy; second, it is the process of forming voltage. With voltage, it is like building a dam. If the two are connected, a loop of current is formed.

When the PN junction is illuminated, both the intrinsic absorption and the extrinsic absorption of the photon by the sample will generate photo-generated carriers (electron-hole pairs). However, only the minority carriers excited by intrinsic absorption can cause the photovoltaic effect. The photo-generated holes generated in the P region and the photo-generated electrons generated in the N region are multi-children, which are blocked by the potential barrier and cannot cross the junction. Only the photo-generated electrons in the P region and the photo-generated holes in the N region and the electron-hole pairs (minorons) in the junction region can drift through the junction under the action of the built-in electric field when diffused near the junction electric field. The photo-generated electrons are pulled toward the N region, and the photo-generated holes are pulled toward the P region, that is, the electron-hole pair is separated by a built-in electric field. This results in the accumulation of photo-generated electrons near the boundary of the N region and the accumulation of photo-generated holes near the boundary of the P region. They generate a photogenerated electric field opposite to the direction of the built-in electric field of the thermally balanced PN junction, with the direction from the P region to the N region. This electric field reduces the potential barrier, which is the difference between the photogenerated potential, the P terminal is positive, and the N terminal is negative. At this time, the Fermi level is separated, which causes a voltage drop. Add electrodes to both sides of the silicon wafer and connect to a voltmeter. For crystalline silicon solar cells, the typical value of the open circuit voltage is 0.5 to 0.6V. The more electron-hole pairs generated in the interface layer by light, the greater the current. The more light energy absorbed by the interface layer, the larger the area of the interface layer, that is, the battery area, and the larger the current generated in the solar cell.

In fact, not all photo-generated carriers produced contribute to photo-generated current. Let the diffusion distance of holes in the N region within the lifetime p be Lp, and the diffusion distance of the electrons in the P region within the lifetime n be Ln. Ln + Lp = L is much larger than the width of the PN junction itself. Therefore, it can be considered that the photo-generated carriers generated within the average diffusion distance L near the junction all contribute to the photocurrent. The electron-hole pairs generated at a position that is more than L from the junction region are recombined during the diffusion process, and do not contribute to the photoelectric effect of the PN junction.

Photovoltaic effect power generation

There are two ways of solar power generation, one is the light-heat-electric conversion method, and the other is the direct light-electric conversion method.

(1) The light-heat-electric conversion method uses the thermal energy generated by solar radiation to generate electricity. Generally, the solar thermal collector converts the absorbed thermal energy into working medium vapor, and then drives the steam turbine to generate electricity. The former process is a light-heat conversion process; the latter process is a heat-electric conversion process, which is the same as ordinary thermal power generation. The disadvantage of solar thermal power generation is its low efficiency and high cost. It is estimated that its investment is at least 5-10 times more expensive than that of ordinary thermal power stations.

(2) Light-to-electric direct conversion method This method uses the photoelectric effect to directly convert solar radiation energy into electrical energy. The basic device for light-to-electric conversion is a solar cell. A solar cell is a device that directly converts solar energy into electrical energy due to the photovoltaic effect. It is a semiconductor photodiode. When the sun shines on the photodiode, the photodiode will convert the solar light energy into electrical energy. Current. When many cells are connected in series or in parallel, it can become a square array of solar cells with relatively large output power. Solar cell is a promising new power source with three advantages: permanent, clean and flexible. The solar cell has a long life. As long as the sun exists, the solar cell can be invested once and used for a long time. In contrast, solar cells do not cause environmental pollution. ^[2]

Photovoltaic effect current equation

Compared with the thermal equilibrium, when there is light, an additional current (photocurrent) Ip will be generated in the PN junction, and its direction is the same as the reverse saturation current I0 of the PN junction, generally IpI0. at this time

I = I0eqU / KT-(I0 + Ip)

Let Ip = SE, then

I = I0eqU / KT-(I0 + SE)

Uoc Photovoltaic effect open circuit voltage Uoc

The voltage of the P terminal to the N terminal when the PN junction external circuit is open under the light, that is, the U value when I = 0 in the above current equation:

0 = I0eqU / KT-(I0 + SE)

Uoc = (KT / q) ln (SE + I0) / I0 (KT / q) ln (SE / I0)

Isc Photovoltaic effect short-circuit current Isc

The PN junction under the light, the external circuit is short-circuited, flows out from the P terminal, passes through the external circuit, and the current flowing from the N-terminal is called short-circuit current Isc. That is to say, the value of I when U = 0 in the above current equation results in Isc = SE.

PV effect parameter relationship

Uoc and Isc are two important parameters of the PN junction under light. At a certain temperature, Uoc and the illumination E have a logarithmic relationship, but the maximum value does not exceed the contact potential difference UD. Under low light, Isc has a linear relationship with E.

a) In the state of thermal equilibrium without light, NP-type semiconductors have a uniform Fermi level, and the barrier height is qUD = EFN-EFP.

b) The circuit outside the PN junction is open under stable light. The photogenerated voltage Uoc no longer has a uniform Fermi level due to the accumulation of photogenerated carriers, and the barrier height is q (UD-Uoc).

c) The external circuit of the PN junction is short-circuited under stable light. There is no photo-generated voltage across the PN junction. The barrier height is qUD. The photo-generated electron-hole pairs are separated by the built-in electric field and flow into the external circuit to form a short-circuit current.

d) There is light and a load. Part of the photocurrent establishes a voltage Uf on the load. Another part of the photocurrent is cancelled by the forward current caused by the forward bias of the PN junction. The barrier height is q (UD-Uf). ^[1]

Photovoltaic effect band

Under the condition of thermal equilibrium, the junction area has a uniform EF; at a position far away from the junction area, the relationship between EC, EF, and E is the same as that before the formation of the junction.

From the energy band diagram, when N-type and P-type semiconductors exist alone, there is a certain difference between EFN and EFP. When the N-type and the P-type are in close contact, the electrons flow from the high Fermi level to the low Fermi level, and the holes flow in opposite directions. At the same time, a built-in electric field is generated, and the direction of the built-in electric field is from the N area to the P area. Under the action of the built-in electric field, EFN will move down along with the entire N-zone energy band, and EFP will move up along with the entire P-zone energy band until the Fermi level is flattened to EFN = EFP, and the carriers stop flowing. At the junction, the conduction band and the valence band are bent accordingly, forming a potential barrier. The barrier height is equal to the difference between the Fermi energy levels when N-type and P-type semiconductors exist alone:

qUD = EFN-EFP

Get

UD = (EFN-EFP) / q

q: electronic power

UD: contact potential difference or built-in potential

For states outside the depletion zone:

UD = (KT / q) ln (NAND / ni2)

NA, ND, ni: acceptor, donor, intrinsic carrier concentration.

It can be seen that UD is related to the doping concentration. At a certain temperature, the higher the doping concentration on both sides of the PN junction, the greater the UD.

Forbidden bandwidth materials, ni is small, so UD is also large. ^[3]

Photovoltaic effect photovoltaic material

Photovoltaic effect ferroelectric conditions

Among many photovoltaic materials, ferroelectric materials have an abnormal photovoltaic effect (the photovoltaic voltage is not limited by the crystal forbidden band width (Eg), and can even be 2 to 4 orders of magnitude higher than Eg, reaching 103 to 105V / cm). It has attracted much attention [3].

Half a century ago, ferroelectric photovoltaic materials have been found in various ferroelectric materials with non-central symmetry, which can produce a stable photovoltaic effect along the direction of polarization. It is generally believed that the photovoltaic effect of ferroelectric materials originates from its spontaneous polarization [5]. One of the distinguishing characteristics of ferroelectric photovoltaics is that when the polarization direction changes under the action of an electric field, the photogenerated current also changes, and The direction of the photogenerated current inside the ferroelectric material is always opposite to the direction of polarization. The ferroelectric photovoltaic effect is different from the traditional pn junction: in the traditional pn junction, the light-excited electron-hole pairs are rapidly separated by the built-in field in the pn junction, drifting in the opposite direction, and finally reaching the electrode And then collected by the electrode.

Therefore, in theory, the photo-generated voltage generated by a pn-junction solar cell is limited by the semiconductor band gap width, and is generally less than 1V. For the ferroelectric photovoltaic effect, the photogenerated voltage obtained experimentally is proportional to the polarization intensity and the distance between the electrodes, without being limited by the band gap width, it can reach 104V. The higher the photovoltaic voltage of a solar cell, the more power it generates, and the higher the efficiency.

Photovoltaic Effect Ferroelectric Photovoltaic Mechanism

Although the research on the ferroelectric photovoltaic effect has been for decades, until now, no one can pinpoint the principle of the photovoltaic process of this material, and the origin of the abnormal photovoltaic effect of ferroelectric materials has been controversial. In general, there are many factors that affect the photogenerated voltage of ferroelectric materials, such as the distance between two electrodes, the intensity of light, the conductivity of the material, the residual polarization strength, crystal orientation, grain size, oxygen vacancies, and domains. Walls and interfaces. But in essence, the mechanisms of the ferroelectric photovoltaic effect ^[4] are mainly the following:

1 Photovoltaic effect (1) Bulk photovoltaic effect

This mechanism considers that the photogenerated voltage is generated inside the ferroelectric material, so it is called the bulk photovoltaic effect, and the ferroelectric material is used as the current source. The steady current (photogenerated current: Js) generated by light is related to the properties of a non-center symmetrical ferroelectric material. In a non-centrally symmetric crystal, the probability of an electron transitioning from a state with a momentum of k to a state of k is different from the probability of transitioning from a state with a momentum of k to a state of k, resulting in a momentum distribution of photogenerated carriers Asymmetric, so that a stable current can be formed under light.

The total current density (J) through the ferroelectric material can be expressed as J = JS + (d + ph) E

In the formula, d and ph respectively represent the conductance of the ferroelectric material in dark and bright fields, that is, dark conductance and photoconductivity; E = V / d is the electric field inside the ferroelectric material under light, which depends on the applied voltage (V) and The distance between the two electrodes (d). Because the distance between the electrodes is usually large, and the dark conductance and photoconductance of most ferroelectric materials are very low, solar photovoltaic devices composed of ferroelectric materials can be regarded as current sources. In ferroelectric materials, the open-circuit voltage Voc under light can be expressed as: V EJdd phocs = d = + As can be seen from the above formula, if the total conductivity (d + ph) does not depend significantly on the light intensity, The open circuit voltage Voc increases linearly with Ioc (or Js).

2 Photovoltaic effect (2) domain wall theory

When Yang et al. Studied the photovoltaic effect of bismuth ferrite (BFO) thin films, they found that the photogenerated voltage in BFO increased linearly with the number of domain walls in the polarization direction, and no obvious photovoltaic effect was observed in the direction perpendicular to the polarization direction ( Figures 2b and 2d). The domain wall theory believes that because the polarization intensity will produce a component perpendicular to the domain wall, the voltage generated at the domain wall is ~ 10m V, and the domain wall width is about 2 nm, so the electric field generated by the polarization at the domain wall As high as 5 × 106V / m, this value is much larger than the internal electric field in the pn junction. It is believed that the origin of the abnormal photovoltaic effect of ferroelectric materials is the main driving force for separating photo-generated carriers. Because there are many electrical domains in ferroelectric materials, the domains are connected end to end after being polarized, and the domain walls are like nano-scale photovoltaic generators connected in series. The photogenerated voltage gradually accumulates along the polarization direction. This mechanism is similar to the concept of a solar cell in series, and its output voltage is the sum of each cell.

If the distance between the two electrodes is larger, the more the electric domains are, and the higher the photogenerated voltage between the two electrodes under light, this model can well explain the anomalous photovoltaic effect. In addition, since continuous photocurrent is generated under light, the domain wall is used as a current source in some literatures, and the total photogenerated voltage Voc is determined by the current density, conductivity, and distance Jsc of the ferroelectric material under light. Unlike the bulk photovoltaic effect, the domain wall theory attributes the anomalous photovoltaic effect to the excitation of carriers at the domain wall. It is believed that the light-excited carriers outside the domain wall recombine quickly and the bulk photovoltaic effect can be ignored.

And Alexe et al. Believe that the recombination of carriers within the domain in BFO is not as fast as expected. The authors studied the photovoltaic effect in BFO single crystals using optoelectronic-atomic force microscope and piezoelectric atomic microscope, and found that a relatively large photogenerated current can be observed inside and outside the domain wall, indicating that the carrier recombination inside the domain is comparative weak. Further research found that the lifetime of photo-generated carriers in the BFO is ~ 75 s, which is comparable to the results obtained at the domain walls. Although the domain wall theory can well explain the anomalous photovoltaic effect, that is, the photogenerated voltage can be much larger than the forbidden band width. However, there are some experimental phenomena that cannot be explained simply by the magnetic domain wall theory, and the bulk photovoltaic effect theory must be considered. For example, according to the domain wall model, since the drop in potential at the domain wall is caused by polarized charges, the photocurrent does not depend on the polarization direction of light. However, researchers have observed the phenomenon that the photocurrent changes with the polarization direction of incident light in ferroelectric materials such as BFO, indicating that the origin of the abnormal photovoltaic effect of ferroelectric materials is more complicated than everyone expected.

In the ferroelectric photovoltaic effect, since the electric domain and body effects contribute to the photogenerated current, if the two are similar, the photogenerated current is large, and conversely, the photogenerated current is small, which can explain why in yang et al. No photocurrent was observed in the direction parallel to the domain wall in the experiment.

3 Photovoltaic effect (3) Schottky junction effect

When a Schottky barrier is formed by the contact between the ferroelectric material and the electrode, the energy band at the interface will bend, and the electron-hole pairs generated under light will be driven by the local electric field near the electrode. The photocurrent generated is largely caused by Schott The depth of the base barrier and depletion layer is determined. According to this model, the magnitude of the photogenerated voltage inside the Schottky barrier is still limited to the band gap of the ferroelectric material. The voltage caused by the Schottky effect is often ignored in the early stages of studying the ferroelectric photovoltaic effect. Because it is much lower than the abnormal photogenerated voltage in most ferroelectric crystals. But the Schottky effect is becoming more and more important in ferroelectric thin film photovoltaic devices, because the photovoltaic voltage output in these devices is usually relatively small.

In general, in a ferroelectric photovoltaic device with a sandwich structure composed of the same electrode and ferroelectric material, the contribution of the photocurrent generated by the Schottky barrier does not exist, because the same electrode and ferroelectric material The two Schottky junctions formed are back-to-back and contain each other, so the photo-generated voltage and current cancel out. However, if different types of electrodes are used, enhancement of the photovoltaic effect in a ferroelectric photovoltaic device with a vertical structure can be achieved. Since the Schottky junction effect is independent of the polarization direction of the ferroelectric material, the contribution of the Schottky junction and the bulk photovoltaic effect to the photocurrent can be distinguished based on this feature. However, some researchers believe that the height of the Schottky barrier can be adjusted by applying an electric field to a ferroelectric material to change its polarization direction. Moreover, when the Schottky barrier and the polarization direction of the ferroelectric material change, the sign of the photo-generated voltage also changes.

For example, in a ferroelectric diode with a vertical structure composed of Au / BFO / Au, both the photo-generated current and photo-generated voltage change as the polarization direction changes. The photovoltaic effect of the BFO thin-film body was initially considered to be the main cause of this phenomenon, but subsequent studies have shown that the change in the Schottky barrier of the BFO thin-film during polarization is mainly due to the migration of oxygen vacancies, and when When the oxygen vacancy migration is frozen at low temperature, the photovoltaic effect no longer changes with the polarization direction.

4 Photovoltaic effect (4) Depolarized field effect

For a ferroelectric thin film in a polarized state, the surface of the film has a high concentration of polarized charges. If the shielding effect is not considered, these high-density polarized charges will generate a huge electric field in the ferroelectric layer. Taking a BFO film as an example, its residual polarization intensity is about 100 C · cm-2, and the electric field generated by unshielded polarized charges can reach 3 × 1010V / m.

When a ferroelectric thin film is in contact with a metal or semiconductor, the surface charge caused by remanent polarization will be shielded by the free charge portion in the metal or semiconductor. In general, the surface charges are not completely shielded because the center of gravity of the polarized charge and the freely compensated charge do not coincide, and an electric field is generated inside the entire ferroelectric film, that is, a depolarized field.

The depolarization field may be large. For example, for a BTO film with a thickness of 10 to 30 nm, the depolarization field in a sandwich structure composed of BTO and Sr Ru O3 electrodes is about 45 × 106V / m. Such a high depolarization field is considered to be the main driving force for separating photo-generated carriers, and it also shows that the abnormal photovoltaic effect is closely related to the shielding degree of polarized charges.

The distribution of shielding charge depends on the properties of the ferroelectric material and the metal (or semiconductor), such as residual polarization strength, free charge density, and dielectric constant. On the other hand, the effect of unshielded polarized charges on the depolarization field depends mainly on the thickness of the ferroelectric layer: a ferroelectric layer with a small thickness results in a large depolarization field.

Generally speaking, the depolarization field produced by the contact between semiconductor and ferroelectric material is larger than the depolarization field produced by the contact between metal and ferroelectric material. This is because the semiconductor material has a smaller free charge density and a larger dielectric. Constant, resulting in a weaker shielding effect. ^[4]

Application scope of photovoltaic effect

1. User solar power supply: (1) small power supply ranging from 10-100W, used for military and civilian life power in remote areas without electricity, such as plateaus, islands, pastoral areas, border posts, etc., such as lighting, television, radio, etc .; (2) 3 -5KW family roof grid-connected power generation system; (3) Photovoltaic water pumps: solve drinking and irrigation of deep water wells in areas without electricity.

2. Transportation: such as beacon lights, traffic / railway signal lights, traffic warning / signage lights, Yuxiang street lights, high-altitude obstacle lights, highway / railway telephone booths, unattended road shift power supply, etc.

3. Communication / communication field: solar unattended microwave relay station, fiber optic cable maintenance station, broadcasting / communication / paging power system; rural carrier telephone photovoltaic system, small communication machine, soldier GPS power supply, etc.

4. Petroleum, marine, and meteorological fields: cathodic protection solar power systems for oil pipelines and reservoir gates, domestic and emergency power sources for oil rigs, marine detection equipment, and meteorological / hydrological observation equipment.

5. Power supply for home lamps: such as garden lights, street lights, portable lights, camping lights, climbing lights, fishing lights, black light lights, tap lights, energy-saving lights, etc.

6. Photovoltaic power station: 10KW-50MW independent photovoltaic power station, wind (solar) complementary power station, various large parking plant charging stations, etc.

7. Solar building: Combining solar power with building materials to enable self-sufficiency of large buildings in the future is a major development direction in the future.

8. Other fields include: (1) Matching with automobiles: solar cars / electric cars, battery charging equipment, car air conditioners, ventilation fans, cold drink boxes, etc .; (2) solar hydrogen fueling fuel cell regenerative power generation systems; (3) seawater Desalination equipment power; (4) satellites, spacecraft, space solar power stations, etc. ^[2]

Photovoltaic effect

The basic principle of solar power is the "photovoltaic effect". The task of the solar expert is to complete the task of manufacturing voltage. Because it is necessary to produce voltage, solar cells that complete photoelectric conversion are the key to solar power generation.

Solar (7 photos)

Solar energy is the most important basic energy source of various renewable energy sources. Biomass energy, wind energy, ocean energy, and water energy all come from solar energy. Broadly speaking, solar energy includes the above-mentioned various renewable energy sources. As a kind of renewable energy, solar energy refers to the direct conversion and utilization of solar energy. The conversion of solar radiant energy into thermal energy by a conversion device belongs to solar thermal utilization technology, and the use of thermal energy for power generation is called solar thermal power generation, which also belongs to this technical field. Solar photovoltaic power generation technology, photoelectric conversion devices usually use the photovoltaic effect principle of semiconductor devices to perform photoelectric conversion, so it is also called solar photovoltaic technology. Studying solar photovoltaic technology can effectively increase the level of energy utilization and increase the use of clean energy, thereby reducing environmental pollution and increasing energy load, which is conducive to the realization of an environment-friendly society. In the field of photovoltaic power generation, mankind has conducted a lot of exploration and gained many valuable experiences.

In the 1950s, two major technological breakthroughs occurred in the field of solar energy utilization: first, the Bell Labs in the United States developed a 6% practical single crystal silicon battery in 1954; The theory and development of selective solar absorbing coatings. These two technological breakthroughs have laid the technical foundation for the use of solar energy in the modern development period.

Since the 1970s, in view of the limited supply of conventional energy and the increasing pressure on environmental protection, many countries in the world have set off a wave of development and utilization of solar energy and renewable energy. In 1973, the United States formulated a government-level solar power generation plan. In 1980, photovoltaic power generation was officially included in public power planning, with a cumulative investment of more than $ 800 million. In 1992, the US government issued a new photovoltaic power generation plan, setting ambitious development goals. Japan formulated the "Sunshine Plan" in the 1970s. In 1993, it merged the "Moonlight Plan" (energy-saving plan), "Environmental Plan", and "Sunshine Plan" into "New Sunshine Plan". European Union countries such as Germany and some developing countries have also formulated corresponding development plans. Since the 1990s, the United Nations has held a series of summit meetings with leaders from various countries to discuss and formulate the world's solar strategic plan, the international solar convention, and the establishment of an international solar fund to promote the development and utilization of global solar and renewable energy. The development and utilization of solar energy and renewable energy has become a major theme and common action of the international community, and has become an important part of countries in formulating sustainable development strategies.

Since the "Sixth Five-Year Plan" period, the Chinese government has included research and development of solar energy and renewable energy technologies in the national scientific and technological research plan, which has greatly promoted the development of solar energy and renewable energy technologies and industries in China. ^[2]