Photocells for industrial use
At solar power plants (SPP), different types of solar cells can be used, but not all of them meet the set of requirements for these systems:
- high reliability with a long (up to 25-30 years) service life;
- high availability of raw materials and the possibility of organizing mass production;
- acceptable from the point of view of the payback period for the costs of creating a transformation system;
- minimum energy and mass costs associated with managing the energy conversion and transmission system (space), including the orientation and stabilization of the station as a whole;
- ease of maintenance.
Some promising materials are difficult to obtain in the quantities required to create solar power plants due to limited natural reserves of raw materials or the complexity of their processing. Certain methods for improving the energy and operational characteristics of solar cells, for example by creating complex structures, are poorly compatible with the possibility of organizing their mass production at low cost, etc. K: Wikipedia: Articles without sources (type: not specified)[ source not specified 4257 days
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High productivity can be achieved only by organizing fully automated production of solar cells, for example, based on tape technology, and creating a developed network of specialized enterprises of the appropriate profile, that is, in fact, an entire industry comparable in scale to the modern radio-electronic industry K: Wikipedia: Articles without sources (type : not specified) [ source not specified 4257 days
]. The production of photovoltaic cells and the assembly of solar panels on automated lines will ensure a manifold reduction in the cost of the battery.
The most likely materials for solar cells are considered to be silicon, Cu(In,Ga)Se2 and gallium arsenide (GaAs), and in the latter case we are talking about heterophotoconverters (HPCs) with the AlGaAs-GaAs structure. K: Wikipedia: Articles without sources (type: not specified) [ source not specified 4257 days
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Operating principle of a photocell
A photocell is a device that produces a photocurrent when exposed to the sun. The practical possibility of obtaining electrical energy from light was developed by MU physics professor A.G. Stoletov. He explained the origin of the photoelectric effect and was able to convert solar energy into photoEMF. Based on this, today electric vacuum and semiconductor photocells are manufactured, which are used in measuring, control and automatic equipment.
The device is a quartz or glass flask. A photosensitive layer of alkali metal (cathode) is applied to its inner side. It contacts the wire connected to the negative pole of the power source. In the middle of the device is an electrode called the anode. It is connected to the positive pole. When exposed to light, electrons are released from the cathode. In an electromagnetic field, they rush towards the anode, creating a current in the circuit.
Semiconductor photocell
The semiconductor has a stable structure. The atoms are tightly linked by a covalent bond. If the quantum energy is enough to break the bond between the electron and the atom, the electron becomes free. In its place, a hole is born - a positive charge equal to the charge of the electron. If a potential difference is applied, an electric current will appear.
The weak conductivity of pure semiconductors is increased with the help of impurities of other substances in order to obtain more free positively or negatively charged particles:
- impurities that create an excess of electrons form an n-type semiconductor.
- impurities that create an excess of holes are a p-type semiconductor.
If you combine an n-type and a p-type material, a redistribution of charges will occur at the boundary: holes will move into the n-region, and electrons will move into the p-region, until a double layer of charges, called a pn junction, and an electric field arise at the boundary. preventing their further redistribution. If the circuit is closed, an electric current will appear proportional to:
- luminous flux intensity;
- semiconductor area;
- duration of light exposure.
Photocell device
Application
Photocells have become a part of our lives. A photo relay allows us to enter the subway, controls the processes of modern production, ensures the safety of people and mechanisms, and controls the quality of products according to the classical scheme. A photocell that reacts to the light flux is installed at its input. The signal is amplified and supplied to a relay in the executive circuit, controlling the operation of engines, machine tools and entire systems used in everyday life and in production.
The study of light continues today. Scientists are confident that the potential of the photon is colossal, and helium energy will soon change light in all respects.
Electrovacuum photocells
An electrovacuum (electronic or ion) photocell is a diode in which a photocathode in the form of a thin layer of a substance emitting photoelectrons is applied to the inner surface of a glass container. The anode is usually a metal ring that does not interfere with light reaching the photocathode. Electronic photocells create a high vacuum, while ionic cells contain an inert gas, such as argon, under a pressure of several hundred pascals (several millimeters of mercury). Cathodes are usually antimony-cesium or silver-oxygen-cesium.
Photocells with external photoelectric effect are manufactured in two types: electronic photocells and ion photocells. In electronic solar cells, current is generated only by electrons leaving the cathode under the influence of light. In ionic photocells, the photoemission current increases due to the occurrence of a non-self-sustaining discharge. The structure of a vacuum photocell is shown in Figure 2. Two electrodes are placed in a glass container from which air has been pumped out: cathode K and anode A.
Rice. 2. Vacuum photocell design
The cathode in the form of a thin photosensitive layer is deposited on the inner
surface of the cylinder. The anode is made in the form of a ring located in the center of the cylinder. The leads from the cathode and anode are made through the leg to the lower base.
The properties and features of photocells are reflected by their characteristics. Anode (volt-ampere) characteristics of an electronic photocell Iph = f(ua) at Ф = const, shown in Fig. 3, a, show a pronounced saturation mode. For ion photocells (Fig. 22.2,b), such characteristics at first are almost the same as for electronic photocells, but with a further increase in the anode voltage due to gas ionization, the current increases significantly, which is estimated by the gas gain coefficient, which can be equal to from 5 to 12 The energy characteristics of the electronic and ion photocell, which give the dependence Iph = f(Ф) at Ua = const, are shown in Fig. 4. Frequency characteristics of sensitivity give the dependence of sensitivity on the frequency of modulation of the light flux. From Fig. Figure 5 shows that electronic photocells (line 1) have low inertia. They can operate at frequencies of hundreds of megahertz, but ion photocells (curve 2) exhibit significant inertia, and their sensitivity decreases already at frequencies of several kilohertz.
Rice. 3. Anode characteristics of an electronic (a) and ionic (b) photocell
Rice. 4. Energy characteristics of electronic (1) and ionic (2) photocells
Rice. 5. Frequency characteristics of electronic (1) and ion (2) photocell
The photocell is usually connected in series with the load resistor RH (Fig. 6). Since photocurrents are very small, the resistance of the photocell to direct current is very high and amounts to units or even tens of megaohms. The resistance of the load resistor is also preferably high. The voltage received from the light signal is removed from it. This voltage is supplied to the input of the amplifier, the input capacitance of which is shunted by resistor RH. The greater the resistance RH and the higher the frequency, the stronger this shunting effect and the lower the signal voltage across the resistor RH.
Rice. 6. Photocell connection diagram
The main electrical parameters of photocells are sensitivity, maximum permissible anode voltage and dark current. For electronic photocells, the sensitivity reaches tens, and for ion photocells - hundreds of microamperes per lumen. Dark current is the current in the absence of irradiation. It is explained by thermionic emission of the cathode and leakage currents between the electrodes. At room temperature, thermal emission current can reach 10-10 A, and leakage currents - 10-7 A. In special designs of photocells, it is possible to significantly reduce leakage currents, and thermal emission current can be reduced only by cooling the cathode to very low temperatures. The presence of dark current limits the use of photocells for very weak light signals.
Electrovacuum photocells have found application in various automation devices, in sound cinema equipment, and in instruments for physical research. But their disadvantages - the impossibility of microminiaturization and rather high anode voltages (tens and hundreds of volts) - have led to the fact that these photocells are currently replaced by semiconductor radiation receivers in many types of equipment.
An electrovacuum (electronic or ion) photocell is a diode in which a photocathode in the form of a thin layer of a substance emitting photoelectrons is applied to the inner surface of a glass container. The anode is usually a metal ring that does not interfere with light reaching the photocathode. Electronic photocells create a high vacuum, while ionic cells contain an inert gas, such as argon, under a pressure of several hundred pascals (several millimeters of mercury). Cathodes are usually antimony-cesium or silver-oxygen-cesium.
Photocells with external photoelectric effect are manufactured in two types: electronic photocells and ion photocells. In electronic solar cells, current is generated only by electrons leaving the cathode under the influence of light. In ionic photocells, the photoemission current increases due to the occurrence of a non-self-sustaining discharge. The structure of a vacuum photocell is shown in Figure 2. Two electrodes are placed in a glass container from which air has been pumped out: cathode K and anode A.
Rice. 2. Vacuum photocell design
The cathode in the form of a thin photosensitive layer is deposited on the inner
surface of the cylinder. The anode is made in the form of a ring located in the center of the cylinder. The leads from the cathode and anode are made through the leg to the lower base.
The properties and features of photocells are reflected by their characteristics. Anode (volt-ampere) characteristics of an electronic photocell Iph = f(ua) at Ф = const, shown in Fig. 3, a, show a pronounced saturation mode. For ion photocells (Fig. 22.2,b), such characteristics at first are almost the same as for electronic photocells, but with a further increase in the anode voltage due to gas ionization, the current increases significantly, which is estimated by the gas gain coefficient, which can be equal to from 5 to 12 The energy characteristics of the electronic and ion photocell, which give the dependence Iph = f(Ф) at Ua = const, are shown in Fig. 4. Frequency characteristics of sensitivity give the dependence of sensitivity on the frequency of modulation of the light flux. From Fig. Figure 5 shows that electronic photocells (line 1) have low inertia. They can operate at frequencies of hundreds of megahertz, but ion photocells (curve 2) exhibit significant inertia, and their sensitivity decreases already at frequencies of several kilohertz.
Rice. 3. Anode characteristics of an electronic (a) and ionic (b) photocell
Rice. 4. Energy characteristics of electronic (1) and ionic (2) photocells
Rice. 5. Frequency characteristics of electronic (1) and ion (2) photocell
The photocell is usually connected in series with the load resistor RH (Fig. 6). Since photocurrents are very small, the resistance of the photocell to direct current is very high and amounts to units or even tens of megaohms. The resistance of the load resistor is also preferably high. The voltage received from the light signal is removed from it. This voltage is supplied to the input of the amplifier, the input capacitance of which is shunted by resistor RH. The greater the resistance RH and the higher the frequency, the stronger this shunting effect and the lower the signal voltage across the resistor RH.
Rice. 6. Photocell connection diagram
The main electrical parameters of photocells are sensitivity, maximum permissible anode voltage and dark current. For electronic photocells, the sensitivity reaches tens, and for ion photocells - hundreds of microamperes per lumen. Dark current is the current in the absence of irradiation. It is explained by thermionic emission of the cathode and leakage currents between the electrodes. At room temperature, thermal emission current can reach 10-10 A, and leakage currents - 10-7 A. In special designs of photocells, it is possible to significantly reduce leakage currents, and thermal emission current can be reduced only by cooling the cathode to very low temperatures. The presence of dark current limits the use of photocells for very weak light signals.
Electrovacuum photocells have found application in various automation devices, in sound cinema equipment, and in instruments for physical research. But their disadvantages - the impossibility of microminiaturization and rather high anode voltages (tens and hundreds of volts) - have led to the fact that these photocells are currently replaced by semiconductor radiation receivers in many types of equipment.
