"Photovoltaic effect", referred to as "photovoltaic effect", English name: Photovoltaic effect, refers to the generation of uneven semiconductors or between different parts of semiconductors and metals combined by lightPotential differencePhenomenon.When the PN junction is irradiated by light of appropriate frequency, due to the effect of the built-in electric field, an electromotive force or photogenerated voltage is generated in the semiconductor. If the PN junction is shorted, current will appear.The photoelectric effect caused by this built-in field is called photovoltaic effect.[5]
As early as 1839, Becqurel, a French scientist, discovered that light can make potential differences between different parts of semiconductor materials.This phenomenon was later referred to as "photovoltaic effect".American scientist in 1954Chapin andKarl-Pearson A practical monocrystalline silicon solar cell was first made in Bell Laboratories in the United States, and a practical photovoltaic power generation technology that converts solar energy into electrical energy was born.The working principle of solar cells is based on the photovoltaic effect of semiconductor PN junction, which is an effect of electromotive force and current generated by the change of charge distribution state in the object when the object is illuminated.That is, when sunlight or other light irradiates the PN junction of the semiconductor, there will be a voltage on both sides of the PN junction, called photogenerated voltage, which will short-circuit the PN junction and generate current.
Photovoltaic power generation is a technology that uses the photovoltaic effect of semiconductor interface to directly convert light energy into electrical energy.The key element of this technology is the solar cell.After the solar cells are connected in series, they can be packaged and protected to form a large area of solar cell modules, which together with power controller and other components form a photovoltaic power generation device.The advantage of photovoltaic power generation is that it is less restricted by the region, because the sun shines on the earth;The photovoltaic system also has the advantages of safety, reliability, no noise, low pollution, no fuel consumption and transmission lines can be set up to generate electricity and supply power locally, and the construction period is short.Photovoltaic effect is referred to as photovoltaic effect, which refers to the phenomenon that the light causes the potential difference between different parts of heterogeneous semiconductor or semiconductor and metal combination.
Photovoltaic effect
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The sun shines on the semiconductorP-n junctionA new hole electron pair is formed. Under the effect of the electric field of the p-n junction, the holes flow from the n-area to the p-area, and the electrons flow from the p-area to the n-area. After the circuit is connected, a current is formed.This is the working principle of photoelectric effect solar cells.
Formation of P-N junction
Homogeneous junction can be formed by doping a piece of semiconductor into P and N regions.Because the activation energy of impurities is very small, impurities are almost all ionized into acceptor ion NA - and donor ion ND+at room temperature.Due to the concentration difference of carriers at the interface of PN region, they will diffuse to each other.It is assumed that at the moment of formation of the junction, the electrons in the N area are many, and the electrons in the P area are few, so that the electrons flow from the N area into the P area, and the electrons meet the holes and recombine again, so that there are few electrons near the junction surface in the original N area, and the remaining un neutralized ions ND+form positive ionsspace charge 。Similarly, after the hole diffuses from the P region to the N region, a negative space charge is formed by the immobile acceptor ion NA -.An immovable ion zone (also called depletion zone, space charge zone and barrier layer) is generated on both sides of the interface between P zone and N zone, so a space electric dipole layer appears, forming an internal electric field (called built-in electric field). This electric field resists the diffusion of multi carriers in the two zones, and helps the drift of minority carriers, until the diffusion current is equal to the drift current to achieve balance,Stable built-in electric fields are established on both sides of the interface.[1]
photoelectric effect
Photovoltaic effect refers to the phenomenon that the light causes a potential difference between heterogeneous semiconductors or between different parts of semiconductors and metals.First of all, it is a process of transforming photons (light waves) into electrons and light energy into electrical energy;Secondly, it is the process of voltage formation.With voltage, it is like building a high dam. If the two are connected, a current loop will be formed.
When the P-N junction is illuminated, both the intrinsic and extrinsic absorption of photons by the sample will produce photogenerated carriers (electron hole pairs).However, only a few carriers excited by intrinsic absorption can cause photovoltaic effect.The photogenerated holes generated in the P region and the photogenerated electrons generated in the N region are all multi carriers, which are blocked by the potential barrier and cannot cross junction.Only the photogenerated electrons in the P region and the photogenerated holes in the N region and the electron hole pairs (minority carriers) in the junction region can drift over the junction under the built-in electric field when they diffuse near the junction electric field.Photogenerated electrons are pulled to the N region, and photogenerated holes are pulled to the P region, that is, the electron hole pairs are separated by the built-in electric field.This leads to the accumulation of photogenerated electrons near the boundary of the N region and photogenerated 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 thermal balance P-N junction, and its direction is from the P region to the N region.This electric field lowers the potential barrier, which is the photogenerated potential difference. The P end is positive and the N end is negative. At this time, Fermi energy levels are separated, resulting in voltage drop. Add electrodes on both sides of the silicon chip and connect the voltmeter.yesCrystalline siliconFor solar cells, the typical value of open circuit voltage is 0.5~0.6V.The more electron hole pairs generated by light in the interface layer, the greater the current.The more light energy absorbed by the interface layer, the larger the area of the interface layer, that is, the larger the current formed in the solar cell.
In fact, not all photogenerated carriers generated contribute to photogenerated current.Let the diffusion distance of hollow holes in N region within the lifetime τ p be Lp, and the diffusion distance of electrons in P region within the lifetime τ n be Ln.Ln+Lp=L is much larger than the width of the P-N junction itself.Therefore, it can be considered that the photogenerated carriers generated within the average diffusion distance L near the junction contribute to the photocurrent.The generated electron hole pairs whose positions are more than L away from the junction area will be all recombined in the diffusion processphotoelectric effectNo contribution.
Power generation mode
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Solar power generation There are two ways of solar power generation, one is light heat electricity conversion, the other is light electricity direct conversion.
(1) The light heat electricity conversion mode uses the heat energy generated by solar radiation to generate electricity. Generally, the solar collector converts the absorbed heat energy into steam of working medium, and then drives the steam turbine to generate electricity.The former process is light heat conversion process;The latter process is the thermal electric conversion process, which is the same as the 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 plants.
(2) Photoelectric direct conversion mode This mode uses the photoelectric effect to directly convert the solar radiation energy into electric energy. The basic device of photoelectric conversion is the solar cell.Solar cell is a device that directly converts solar energy into electrical energy due to photovoltaic effect. It is a semiconductor photodiode. When the sun shines on the photodiode, the photodiode will convert the solar energy into electrical energy and generate current.When many cells are connected in series or parallel, they can become a solar cell array with relatively large output power.Solar cells are a new type of power supply with great prospects. They have three advantages: permanence, cleanness and flexibility. Solar cells have a long life. As long as the sun exists, they can be invested once and used for a long time;Compared with thermal power generation and nuclear power generation, solar cells will not cause environmental pollution.[2]
Current equation
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Compared with the thermal balance, when there is light, an additional current (photocurrent) Ip will be generated in the P-N junction, which is in the same direction as the P-N junctionReverse saturation currentI0 is the same, generally Ip ≥ I0.here
I=I0eqU/KT - (I0+Ip)
Let Ip=SE, then
I=I0eqU/KT - (I0+SE)
Open circuit voltage Uoc
P-N junction under lightExternal circuitThe voltage from end P to end N in open circuit, that is, the U value when I=0 in the current equation above:
0=I0eqU/KT - (I0+SE)
Uoc=(KT/q)ln(SE+I0)/I0≈(KT/q)ln(SE/I0)
Short circuit current Isc
When the P-N junction is exposed to light, the external circuit is short circuited, flows out from the P end, passes through the external circuit, and the current flowing in from the N end is called the short circuit current Isc.That is, I value when U=0 in the above current equation, then Isc=SE.
Parameter relationship
Uoc and Isc are two important parameters of P-N junction under light. At a certain temperature, Uoc is logarithmic to illuminance E, but the maximum value does not exceed the contact potential difference UD.Under weak light, Isc has a linear relationship with E.
a) In the thermal equilibrium state without light, the NP type semiconductor has a unified Fermi level, and the barrier height is qUD=EFN-EFP.
b) Under stable illumination, the external circuit of the P-N junction is open. Due to the accumulation of photogenerated carriers, the photogenerated voltage Uoc no longer has a unified Fermi level, and the barrier height is q (UD Uoc).
c) Under stable light, the circuit outside the P-N junction is short circuited, there is no photogenerated voltage at both ends of the P-N junction, and the barrier height is qUD. The photogenerated electron hole pair is separated by the internal electric field and flows into the external circuit to form a short circuit current.
d) With light and load, part of photocurrentloadThe voltage Uf is established on the P-N junction, and the other part of the photocurrent is offset by the forward current caused by the forward bias voltage of the P-N junction. The barrier height is q (UD Uf).[1]
energy band
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Under the condition of heat balance, there is a uniform EF in the junction region;The relationship between EC, EF and E ν is the same as that before the formation of the knot at the position far away from the knot region.
From the energy band diagram, when N-type and P-type semiconductors exist alone, there is a certain difference between EFN and EFP.When N type and P type are in close contact, electrons flow from the side with high Fermi level to the side with low Fermi level, while holes flow in the opposite direction.At the same time, the built-in electric field is generated, and the direction of the built-in electric field is from area N to area P.Under the action of the internal electric field, the EFN will move down together with the entire N band, and the EFP will move up together with the entire P band, until the Fermi level is flattened to EFN=EFP, and the carrier stops flowing.In the junction area, the conduction band and the valence band bend correspondingly, forming a potential barrier.The height of potential barrier is equal to the difference of Fermi energy levels when N-type and P-type semiconductors exist alone:
qUD=EFN-EFP
have to
UD=(EFN-EFP)/q
q: Electronic power
UD: contact potential difference or built-in potential
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 P-N junction, the greater the UD.
For bandwidth forbidden materials, Ni is small, so UD is also large.[3]
Photovoltaic materials
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Ferroelectric condition
Among many photovoltaic materials, ferroelectric materials have attracted much attention due to their abnormal photovoltaic effect (photovoltaic voltage is not limited by the crystal band gap (Eg), and can even be 2~4 orders of magnitude higher than Eg, reaching 103~105V/cm) [3].
Half a century ago, people found ferroelectric photovoltaic materials in various ferroelectric materials with non central symmetry, which can produce stable photovoltaic effect along the direction of polarization.It is generally believed that the photovoltaic effect of ferroelectric materials originates from their spontaneous polarization [5]. One of the prominent features of ferroelectric photovoltaic 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 materials is always opposite to the polarization direction.The difference between the ferroelectric photovoltaic effect and the traditional p-n junction is that in the traditional p-n junction, the photoexcited electron hole pairs are rapidly separated by the built-in field in the p-n junction, drift in the opposite direction, finally reach the electrode, and then are collected by the electrode.
Therefore, theoretically, the photogenerated voltage generated by p-n junction solar cells is limited by the semiconductor band gap width, which is generally less than 1V.For the ferroelectric photovoltaic effect, the photogenerated voltage obtained in the experiment is proportional to the polarization intensity and the distance between the electrodes, and is not limited by the band gap width. It can reach 104V.The higher the photogenerated voltage of the solar cell, the more electric energy generated and the higher the efficiency.
Mechanism of ferroelectric photovoltaic effect
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Although the research on ferroelectric photovoltaic effect has been carried out for decades, no one has been able to pinpoint the principle of this material's photovoltaic process until now, and the origin of the abnormal photovoltaic effect of ferroelectric materials has also been controversial.Generally speaking, there are many factors affecting the photogenerated voltage of ferroelectric materials, such as the distance between two electrodes, the intensity of light, the conductivity of materials, the intensity of residual polarization, crystal orientation, grain size, oxygen vacancies, domain walls and interfaces.But in essence, ferroelectric photovoltaic effect[4]There are mainly the following mechanisms:
(1) Bulk photovoltaic effect
This mechanism believes 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 stable current (photogenerated current: Js) generated under light is related to the properties of non centrosymmetric ferroelectric materials.In a crystal with non central symmetry, the probability of electron transition from the state with momentum k to the state with momentum k is different from the probability of electron transition from the state with momentum k to the state with momentum k, resulting in asymmetric momentum distribution of photogenerated carriers, which can form a stable current under light.
The total current density (J) through ferroelectric materials can be expressed as J=JS+(σ d+σ ph) E
Where, σ d and σ ph respectively represent the conductance of ferroelectric materials in dark field and bright field, namely 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).Since the distance between electrodes is usually large, and the dark conductivity and photoconductivity of most ferroelectric materials are very low, solar photovoltaic devices made 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=+σ σ From the above formula, it can be seen that if the total conductivity (σ d+σ ph) does not obviously depend on light intensity, the open circuit voltage Voc increases linearly with Ioc (or Js).
(2) Domain wall theory
When Yang et al. studied the photovoltaic effect of bismuth ferrite (BFO) films, they found that the photogenerated voltage in BFO increased linearly with the increase of the number of domain walls in the polarization direction, and no obvious photovoltaic effect was observed perpendicular to the polarization direction (Fig. 2b and 2d).According to the domain wall theory, because the polarization intensity will produce a component perpendicular to the domain wall, the voltage generated at the domain wall is~10mV, and the width of the domain wall is about 2nm, so the electric field generated at the domain wall by polarization is as high as 5 × 106V/m, which is far greater than the internal electric field in the p-n junction, and is considered as the origin of the abnormal photovoltaic effect of ferroelectric materials,It is also the main driving force for separating photogenerated carriers.Because there are many electric domains in ferroelectric materials, after being polarized, the domains are connected end to end, and domain walls are like connected nano photovoltaic generators, and the photogenerated voltage gradually accumulates along the polarization direction.This mechanism is similar to the concept of series solar cells, whose output voltage is the sum of each unit.
If the distance between the two electrodes is larger, there will be more electric domains, and the photovoltage generated between the two electrodes under light will be higher. This model can well explain the anomalous photovoltaic effect.In addition, due to the continuous photocurrent generated under light, domain walls are used as current sources in some literatures, and the total photogenerated voltage Voc is determined by the current density, conductivity of ferroelectric materials under light and the distance Jsc between electrodes.Different from 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 recombination speed of carriers excited by light outside the domain wall is very fast, and the bulk photovoltaic effect can be ignored.
Alexe et al. believed that the recombination of carriers within the domain in BFO was not as fast as expected.The photovoltaic effect in BFO single crystal was studied by photoelectric atomic force microscope and piezoelectric atomic force microscope.Further research shows that the lifetime of photogenerated carriers in BFO is~75 μ s, which is equivalent to the results obtained at the domain wall.Although the anomalous photovoltaic effect can be well explained by the domain wall theory, that is, the photogenerated voltage can be far greater than the band gap width, there are some experimental phenomena that can not be explained only by the magnetic domain wall theory, and the volume photovoltaic effect theory must be taken into account.For example, according to the domain wall model, since the fall of the potential at the domain wall is caused by the polarization charge, 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 the incident light in ferroelectric materials such as BFO, which indicates that the origin of the anomalous photovoltaic effect of ferroelectric materials is more complex than expected.
In the ferroelectric photovoltaic effect, because both the electric domain and the volume effect contribute to the photogenerated current, if they are long, the photogenerated current is larger, on the contrary, the photogenerated current is smaller, which can explain why no photocurrent is observed parallel to the domain wall in the experiment of Yang et al.
(3) Schottky junction effect
When the ferroelectric material contacts with the electrode to form a Schottky barrier, 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, and the generated photocurrent is largely determined by the depth of the Schottky barrier and depletion layer.According to this model, the magnitude of the photogenerated voltage in the Schottky barrier is still limited to the band gap of ferroelectric materials. The voltage caused by the Schottky effect is often ignored in the early stage of the study of ferroelectric photovoltaic effect, because it is far lower than the abnormal photogenerated voltage in most ferroelectric crystals.But Schottky effect is becoming more and more important in ferroelectric thin film photovoltaic devices, because the photovoltaic voltage output of these devices is usually small.
Generally speaking, in the ferroelectric photovoltaic device with sandwich structure composed of the same electrode and ferroelectric material, the contribution of photocurrent generated by the Schottky barrier does not exist, because the two Schottky junctions composed of the same electrode and ferroelectric material are back-to-back and contain each other, so the generated photogenerated voltage and current cancel each other.However, if different types of electrodes are used, the enhancement of photovoltaic effect in ferroelectric photovoltaic devices with vertical structure can be achieved.Because the Schottky junction effect is independent of the polarization direction of ferroelectric materials, the contribution of Schottky junction and bulk photovoltaic effect to photocurrent can be distinguished according to this feature.However, some researchers believe that the height of Schottky barrier can be adjusted by applying an electric field to ferroelectric materials to change their polarization direction.Moreover, when the polarization direction of Schottky barrier and ferroelectric material changes, the sign of photogenerated voltage also changes.
For example, in the ferroelectric diode with vertical structure composed of Au/BFO/Au, the photogenerated current and photogenerated voltage change with the change of polarization direction.At first, the photovoltaic effect of BFO thin films was considered as the main reason for this phenomenon, but subsequent studies showed that the Schottky barrier change of BFO thin films in the polarization process was mainly caused by the migration of oxygen vacancies. When the oxygen vacancy migration was frozen at low temperature, the photovoltaic effect no longer changed with the change of polarization direction.
(4) Depolarization field effect
For the ferroelectric thin film in the 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 BFO film as an example, its residual polarization intensity is about 100 μ C · cm-2, and the electric field generated by unshielded polarized charge can reach 3 × 1010V/m.
When the ferroelectric film contacts with metal or semiconductor, the surface charge caused by residual polarization will be partially shielded by the free charge in the metal or semiconductor.In general, the reason why the surface charge is not completely shielded is that the center of gravity of the polarized charge and the free compensation charge do not coincide, and an electric field, namely depolarization field, is generated in the entire ferroelectric film.
The depolarization field may be very large. For example, for BTO films with a thickness of 10~30nm, the depolarization field in the sandwich structure composed of BTO and SrRuO3 electrodes is about 45 × 106V/m.Such a high depolarization field is considered to be the main driving force for separating photogenerated carriers, which also indicates that the anomalous photovoltaic effect is closely related to the shielding degree of polarized charges.
The distribution of shielding charge depends on the properties of ferroelectric materials and metals (or semiconductors), such as residual polarization, free charge density and dielectric constant.On the other hand, the influence of unshielded polarized charges on the depolarization field mainly depends on the thickness of the ferroelectric layer: a thin ferroelectric layer results in a large depolarization field.
In general, the depolarization field generated by the contact between semiconductor and ferroelectric material is larger than that generated by the contact between metal and ferroelectric material. This is because semiconductor materials have smaller free charge density and larger dielectric constant, resulting in weaker shielding effect.[4]
Scope of application
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1. Solar power supply for users: (1) small power supply of 10-100W is used for military and civilian life in remote areas without power, such as plateau, island, pastoral area, border post, etc., such as lighting, television, radio recorder, etc;(2) 3-5 KW family roof grid connected power generation system;(3) Photovoltaic water pump: solve drinking and irrigation of deep water wells in areas without electricity.
2. Traffic field: such as beacon lights, traffic/railway signal lights, traffic warning/marker lights, Yuxiang street lights, high-altitude obstacle lights, highway/railway radio phone booths, unattended road crew power supply, etc.
3. Communication/communication field: solar unattended microwave relay station, optical cable maintenance station, broadcast/communication/paging power system;Rural carrier telephone photovoltaic system, small communicator, soldier GPS power supply, etc.
4. Petroleum, marine and meteorological fields: solar power supply system for cathodic protection of oil pipelines and reservoir gates, domestic and emergency power supply of oil drilling platforms, marine detection equipment, meteorological/hydrological observation equipment, etc.
5. Power supply of household lamps: such asCourtyard lamp, street lamp, portable lampCamping lampMountaineering lamp, fishing lamp, black light lamp, rubber tapping lamp, energy-saving lamp, etc.
6. Photovoltaic power stations: 10KW-50MW independent photovoltaic power stations, wind solar (diesel) complementary power stations, charging stations of various large parking plants, etc.
7. Solar buildings: The combination of solar power generation and building materials will enable future large-scale buildings to be self-sufficient in electricity, which is a major development direction in the future.
8. Other fields include: (1) matching with automobiles: solar cars/electric vehicles, battery charging equipment, automobile air conditioners, ventilators, cold drink boxes, etc;(2) Regenerative power generation system of solar hydrogen production and fuel cell;(3) Power supply for seawater desalination equipment;(4) Satellite, spacecraft, space solar power station, etc.[2]
Solar powerIts basic principle is "photovoltaic effect".The task of solar energy experts is to complete the work of making voltage.The photoelectric conversion is completed because the voltage is to be producedSolar cellIt is the key to solar power generation.
Solar energy is the most important basic energy among all kinds of renewable energy. Biomass energy, wind energy, ocean energy, water energy, etc. all come from solar energy. In a broad sense, solar energy includes all kinds of renewable energy above.As a kind of renewable energy, solar energy refers to the direct conversion and utilization of solar energy.It belongs to solar thermal utilization technology to convert solar radiation energy into heat energy through conversion device, and it is calledSolar thermal power generation, also belongs to this technical field;The conversion of solar radiation energy into electric energy through conversion devices is a solar photovoltaic power generation technology. Photoelectric conversion devices usually use the photovoltaic effect principle of semiconductor devices for photoelectric conversion, so they are also calledSolar PVTechnology.The study of solar photovoltaic technology can effectively increase the level of energy utilization, improve the utilization rate of clean energy, thus reducing environmental pollution, improving energy carrying capacity, and is conducive to the realization of an environmentally friendly society.In the field of photovoltaic power generation, mankind has carried out a lot of exploration and gained a lot of valuable experience.
In the 1950s, there were two major technological breakthroughs in the field of solar energy utilization: one was that Bell Laboratories in the United States developed 6% practical monocrystalline silicon cells in 1954; the other was that Tabor in Israel put forward the concept and theory of selective absorption surface and successfully developed selective solar absorption coatings in 1955.These two technological breakthroughs have laid a technological foundation for the entry of solar energy utilization into the modern development period.
Since the 1970s, in view ofConventional energyWith the limited supply and the increasing pressure on environmental protection, many countries in the world have set off a boom in the development and utilization of solar energy and renewable energy.In 1973, the United States formulated a government level solar power generation plan, and in 1980 officiallyPhotovoltaic power generationIncluded in the public power planning, the cumulative investment reached more than 800 million dollars.In 1992, the US government issued a new photovoltaic power generation plan and set a grand development goal.Japan formulated the "Sunshine Plan" in the 1970s, and merged the "Moonlight Plan" (energy saving plan), "Environmental Plan" and "Sunshine Plan" into the "New Sunshine Plan" in 1993.Germany and other EC countries and some developing countries have also formulated corresponding development plans.Since the 1990s, the United Nations has held a series of summit meetings attended by leaders of all countries to discuss and formulate the world's strategic plan for solar energy, the international solar energy convention, the establishment of the International Solar Energy Fund, etc., and promote the development and utilization of global solar energy 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 an important part of the sustainable development strategy formulated by countries.
Since the "Sixth Five Year Plan", our government has always included research and development of solar energy and renewable energy technologiesNational Science and Technology Research Program, which has greatly promoted the development of solar energy and renewable energy technology and industry in China.[2]
