RU2487438C1 - Photocell of space laser radiation detector-converter - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: invention relates to wireless transmission of electrical energy between spacecraft based on directed electromagnetic radiation from one spacecraft to a photoelectric converter-based detector-receiver of a second spacecraft. The photocell of a space laser radiation detector-converter has p-type and n-type semiconductor layers, alternating contact strips on the working side of the photocell and a continuous ohmic contact on the rear side of the photocell, wherein a diffraction grating is placed on the working side of the photocell with given thickness δ of its photoactive region, said diffraction grating being made from opaque parallel contact strips with width b, which alternate with a constant spacing Δ and make up an ohmic contact with the semiconductor layer of the photocell, on which electromagnetic radiation of the laser with wavelength λ falls normally. The diffraction grating is made such that it satisfies given relationships that a protected by the present invention.

EFFECT: invention enables to increase efficiency and specific values of photocurrent of the photoelectric converter in space.

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Description

The invention relates to the field of wireless transmission of electrical energy between spacecraft (SC) based on directional electromagnetic radiation from one SC to a receiver-converter, based on a photoelectric converter (PEC), a second SC.

Currently, a number of new areas based on the use of laser radiation have been determined in space technology. Among them, a very promising direction should be considered the transfer of energy between the spacecraft through the optical channel in wireless power transmission systems. Currently, each spacecraft is equipped with its own electric energy generation system. However, there is an alternative way of energy supply, involving the use of centralized power plants and the transfer of energy to spacecraft-consumers using electromagnetic radiation. In this case, it is possible to implement a centralized power supply scheme for both individual spacecraft and their groupings, which expands their functionality and increases their resource / 1 /. The energy transfer of electromagnetic radiation by a laser in space between spacecraft has advantages over terrestrial systems, where the laser beam, passing through the atmosphere, undergoes significant absorption and scattering. In addition, the divergence of radiation as a result of atmospheric distortion, when using the ground laser, requires special correction systems.

In this case, the transmission of energy by a laser beam is considered, where a photoelectric converter acts as a receiver-converter of electromagnetic energy.

The modern technical level of the photomultiplier is quite high - multi-junction photomultipliers on GaAs have an efficiency of ~ 40%; in the case of conversion of concentrated radiation, an efficiency of up to 70% is predicted / 1 /. Given the high monochromaticity of laser radiation, the efficiency of even conventional solar cells using silicon can reach 20-30%. Specialized photomultipliers designed to operate in a narrow infrared range have high efficiency (up to 40%) even in a single junction design, and high efficiency also for thin-film photomultipliers based on semiconductors with a diamond-like structure. The specific gravity of space solar cells in the modern version is 4-6 kg / m ² . The operating temperature range for the elements of the system for wireless energy transfer in the infrared range is 10-20 ° C for laser diodes and up to 60 ° C for the photomultiplier / 1 /.

The use of a laser to transmit energy of monochromatic radiation will increase the efficiency of energy-receiving receivers in comparison with conventional solar cells, where spectral energy losses are characteristic. In this case, the heating of the solar cells due to spectral losses is significantly reduced. In addition, the use of laser radiation with a high energy flux density, without fear of overheating of the solar cells, can reduce the cost of the panels several times and improve their overall dimensions.

There are known designs of p-n junction photoelectric converters, where the electron-hole junction in the photocell is created by doping a plate of a single-crystal semiconductor material of a certain type of conductivity with an impurity, which provides the creation of a surface layer with conductivity of the opposite type 12, p. 88 /.

So in / 3 / is a semiconductor photoelectric generator that contains a substrate, semiconductor layers, an antireflection coating, metal contacts. Moreover, according to the invention, on the front side of the generator there are many deposited layers forming planar diode n ⁺ -pp ⁺ or p ⁺ -nn ⁺ , or n-p structures connected in series in the direction of radiation propagation. One or two linear sizes of each diode structure does not exceed the diffusion length of minority charge carriers in the base region. The thickness of the diode structure in the direction of radiation propagation is inversely proportional to the maximum radiation absorption coefficient in the semiconductor material. B / 4, p. 65 / shows the design of silicon PECs made in the form of a diode structure with a pn junction on the front side, current-collecting metal contacts to the doped layer in the form of a comb, a solid back contact and an antireflection coating on the front (working) side . The process of manufacturing a photomultiplier is based on diffusion doping of the front side with phosphorus / 4, p.127 /, chemical deposition of the nickel contact / 4, p.135 /, selective etching of the contact pattern and the application of antireflection coating. In / 5 /, the design of the photomultiplier is given, where the contact grid is made on a thin silicon semiconductor wafer and includes narrow conductors passing along one axis of the photoelectric transducer at a step distance from each other, and two wide conductive conductors crossing narrow conductors at an angle of 90 ° , for connecting down conductors. The metal coating of narrow conductive conductors is made at a distance of not more than three millimeters from the edges of the wide conductive conductors, and its total area is 95-98% of the total surface area of all narrow conductive conductors.

The above constructions relate to photovoltaic converters for direct conversion of solar energy into electrical energy using solar cells and have a common disadvantage associated with the incomplete use of the solar radiation incident on the photomultiplier to create a photocurrent through the p-n junction. This is due to the fact that sunlight is not monochromatic, but contains electromagnetic waves of various frequencies. And since the properties of PEC semiconductor materials depend on the wavelength of the incident radiation (for example, the absolute refractive index, reflection coefficient, absorption coefficient, etc.), it is difficult to create a PEC design operating in the optimal mode.

As a prototype, the design of a highly efficient concentrator solar cell / 6 / was adopted, including p-type and n-type semiconductor layers grown on an n-GaAs substrate, front ohmic contact in the form of contact strips on the front side of the photocell and continuous ohmic contact on the back side photocell. To increase the efficiency of the photocell, pulling fields were created in the base layer and the front doped layer due to inhomogeneous doping. Contact strip shading was about 15%. Ohmic contacts on both the front and back are made on the basis of Au.

The disadvantage of this photocell is the insufficient conversion efficiency of the concentrated radiation and ohmic losses associated with the spreading resistance in the front doped layer when current flows along the surface of the photocell to the contacts.

The task of the invention is to increase the efficiency and specific values of the photocurrent photomultiplier.

The above result is achieved by the fact that the photocell of the receiver of laser radiation in space, containing semiconductor alloyed and base layers of p-type and n-type, alternating contact strips on the working side of the photocell and continuous ohmic contact on the back of the photocell, on the working side of the photocell, with a given thickness δ of its photoactive region, a diffraction grating is installed, made of opaque parallel to each other contact strips of width b, alternating with a constant w Δ and constituting an ohmic contact with the semiconductor layer of the photocell, onto which the electromagnetic radiation of the laser with a wavelength λ is incident normally, while the diffraction grating is made so that the distance between the contact strips (Δ-b) and the length of the contact strips (L) correspond to the relation

$$\left(\Delta -b\right)<<L,\left(\mathrm{one}\right)$$

and the width b and the step Δ of the contact strips of the diffraction grating satisfy the relations

$$b<2\cdot k\cdot {\left(\delta \cdot \lambda \right)}^{\mathrm{one}/2}/\left(\mathrm{one}-k\right),\left(2\right)$$

$$\Delta =b/k,\left(3\right)$$

where k is the specified coefficient of shadowing by the contact strips of the radiation-sensing surface of the solar cell;

δ is the specified thickness of the photoactive region, commensurate with the diffusion length of minority charge carriers.

The total thickness of the doped and base layers forming the photoactive region is comparable with the diffusion length of minority charge carriers / 4, p. 87, p. 122 /, since the diffusion length determines the photoactive region in which the absorbed photon is likely to turn into an electric current through pn- transition. Typical values of the diffusion length are in the range of 10 ^-6 -10 ^-4 m, depending on the concentration of charge carriers and the method of formation of the semiconductor structure / 7, p.207, 208 /.

Improving the conversion efficiency of intense laser radiation fluxes in the proposed design of the photocell with a diffraction grating on the working side is based on the phenomenon of deviation of the propagation of electromagnetic waves from the laws of geometric optics, or diffraction / 8, p.664 /. When a plane monochromatic wave is incident on a diffraction grating from a laser source with a wavelength of λ, electromagnetic waves bend around the opaque contact strips that form the diffraction grating and the wave penetrates into the geometric shadow region behind the contact strips. This leads to an increase in the path length traveled by the radiation in the photocell and allows one to reduce the losses on the passage of radiation in the main absorption band. An increase in the fraction of absorbed radiant flux leads to an increase in the total number of photoelectrons and photoholes created by radiation per unit time on both sides of the pn junction. In addition, the series resistance of the photocell is reduced by reducing the spreading resistance in the front doped layer as current flows along the surface of the photocell to the contacts. For a photocell with a diffraction grating, the possibility of generating charge carriers in almost the entire volume of the photoactive region is characteristic due to the formation of additional sources of carrier generation, which are sources of secondary waves. Thus, an increase in the generation function in the base region, an increase in the efficiency of conversion of electromagnetic radiation leads to an increase in the efficiency in the proposed design of the solar cell in real operating conditions in comparison with the known structures.

The essence of the invention is illustrated in figure 1, which schematically shows the design of a photocell with a pn junction. The photocell consists of: a doped layer 1, a base layer 2, a pn junction 3, a semiconductor substrate 4, a back metal contact 5, a diffraction grating 6, contact strips 7, an antireflection coating 8, a working surface 9, a photoactive region 10 incident on photocell of electromagnetic radiation 11.

The photocell works as follows. Normally plane monochromatic electromagnetic radiation 11 of a laser with a wavelength of X is incident on a working surface 9 with a diffraction grating 6. Electromagnetic radiation 11 passes between the opaque contact strips 7 of the diffraction grating 6 through a transparent antireflection coating 8 into the photoactive region 10 with a given total thickness of 5 doped 1 and base 2 layers. The diffraction grating 6 is a combination of a large number of narrow opaque contact strips 7 of width b, alternating with a constant pitch Δ and making ohmic contact with the semiconductor doped layer 1 of the photocell. Since the design of the diffraction grating 6 satisfies the relations (Δ-b) << L, b <2 · k · (δ · λ) ^1/2 / (1-k) and Δ = b / k, i.e. if the diffraction conditions are met, then the phenomenon of enveloping by electromagnetic waves of the contact strips 7 and the penetration of electromagnetic radiation 11 into the geometric shadow region will be observed. Electromagnetic radiation 11, along the edges of each contact strip 7, enters the photocell, into its photoactive region 10, at some angle to the plane of the pn junction 3, which leads to an increase in its path length in the photoactive region 10 and a more uniform distribution of electromagnetic radiation energy over the volume of the photoactive region 10 of the semiconductor. Active absorption of photons occurs with a maximum absorption coefficient of electromagnetic radiation, due to the appropriate selection of semiconductor material for a given wavelength λ of monochromatic laser radiation. Photoactive absorption is accompanied by the formation of electron-hole pairs and the appearance of excess charge carriers. Nonequilibrium charge carriers assembled to the pn junction 3, due to the presence of the contact potential difference, are separated on it: minority carriers freely pass through the pn junction 3, while the main ones are delayed. For example, when a p-type doped layer 1 and an n-type base layer 2 are made, the electrons pass from the doped layer 1 to the base layer 2 and then through the semiconductor substrate 4 to the rear metal contact 5, and the holes in the opposite direction. Thus, under the influence of laser electromagnetic radiation with a single wavelength λ, a current of minority non-equilibrium charge carriers — photoelectrons and photo holes — will flow through the pn junction 3 in both directions.

Expressions (1) - (3) determine the conditions of wavefront diffraction during normal incidence of coherent laser waves on the working surface of a photocell with a diffraction grating and reflect the possibility of increasing the efficiency of conversion of laser radiation energy into electricity. The diffraction grating is a combination of a large number of narrow opaque contact strips separated by gaps with a constant pitch, making ohmic contact with the semiconductor layer. Under condition (1), each gap between the contact strips of the diffraction grating, with a sufficient degree of accuracy, can be mistaken for a long slit and diffraction from the slit can be considered. The positive effect is based on the phenomenon of deviation of the propagation of electromagnetic waves from the laws of geometric optics, or diffraction / 8, p. 644 /, and manifests itself in a violation of the straightness of the propagation of rays, enveloping waves of obstacles, in the penetration of electromagnetic radiation into the geometric shadow. When energy is transmitted by a laser beam from one spacecraft to another in space, the divergence angle is very small, and the distance between the spacecraft is large enough, which allows us to assume with sufficient accuracy that a plane monochromatic wave is incident on the photocell of the receiving-converting device. When a plane monochromatic wave is incident on the photocell from a laser source with a wavelength of λ, each surface gap between the opaque contact strips will be a source of coherent secondary waves, starting from a distance h

$$h\sim {[\left(\Delta -b\right)/2]}^2/\lambda ,\left(\mathrm{four}\right)$$

when diffraction phenomena are clearly observed / 8, p.665 /. In a first approximation, wave diffraction is the effect of transverse diffusion of the beam amplitude along the front of the propagating waves / 8, p.666 /. For the design of the photocell with a diffraction grating made of alternating metal contact strips on the working side of the photocell, it is necessary to consider the transverse diffusion of the beam amplitude along the front of the cylindrical wave. The phenomenon of transverse diffusion of amplitude along the wave front has a local character and is relatively pronounced in zones of effective diffusion. In the approximation of transverse diffusion of amplitude along the front of a plane wave, in the design with a diffraction grating under consideration, the effective diffusion zone will be a parabolic cylinder with a vertex of a parabola along each edge of each contact strip. Moreover, two parabolic zones of effective diffusion from two adjacent contact strips will merge at a distance h ~ [(Δ-b) / 2] ² / λ from the diffraction grating of the photocell / 8, p.666 /. Considering that the deviation of electromagnetic waves from the rectilinear direction becomes significant when δ> h and, taking the shading coefficient to the contact strips of the beam-sensing surface of the solar cell known and equal to the ratio k = b / Δ, from (4) we obtain the right-hand side of inequality (2) for the width strips b ohmic contact. Obviously, the minimum size of the strip width b of the ohmic contact, which is made of metals with low resistivity, will be limited by the characteristic parameter of the crystal lattice - lattice constant / 9, p. 322 /. From where, choosing the width of the contact strip b from (2) and setting the permissible value of the shading coefficient k, we obtain relation (3) to determine the required step of the contact strips Δ.

We give an example of a specific implementation of the photocell of the receiver-converter of laser radiation in space. Suppose that the photocell is made in the form of a square 3.5 × 3.5 mm in size, on the radiation-receiving surface of which a diffraction grating 6 with contact strips 7 based on Au is installed along one of the sides of the square, i.e. with strip lengths L = 3.5 mm = 3500 μm.

Assume that normally monochromatic electromagnetic radiation of a laser with a wavelength of λ = 0.8 μm is incident on the diffraction grating 6 of the photocell. As a semiconductor material, we select GaAs as the material having the highest absorption coefficient for a given laser wavelength, in comparison with other semiconductors / 2, p. 93 /. The photocell of the receiver-converter of laser radiation in space contains p-type epitaxial semiconductor layers (doped layer 1) and n-type epitaxial layers (base layer 2), suppose, with a thickness of 1.0 μm and 3.0 μm, respectively, on the semiconductor substrate 4, for example from n-GaAs. Solid ohmic contact 5, on the back of the photocell, we assume, is made on the basis of Au. The antireflection coating 8 is assumed to be made of ZnS. Thus, on the working side of the photocell, with a thickness δ = 4.0 μm of its photoactive region 10, the electromagnetic radiation of the laser normally falls. Using relations (1), (2) and (3), we determine the parameters of the diffraction grating 6 by setting the shading coefficient k = 0.2. We define the restriction on the choice of the width of the contact strips b, calculating the right side of inequality (2)

b <2 · k · (δ · λ) ^1/2 / (1-k) = 2 · 0.2 · (4.0 · 0.8) ^1/2 / (1-0.2) = 0, 89 microns.

Where do we choose the width of the contact strips satisfying relation (2), for example, we take b = 0.5 μm.

From relation (3) we find the required step of the contact strips in the diffraction grating Δ = b / k = 0.5 / 0.2 = 2.5 μm.

Obviously, the relation (1) (Δ-b) << L holds, namely (2.5-0.5) = 2.0 μm << 3500 μm.

Thus, for the proposed photocell design, conditions (1), (2) and (3) are satisfied, i.e. the conditions of the envelope by electromagnetic waves, with a length of λ = 0.8 μm, of the contact strips of the diffraction grating and the penetration of electromagnetic waves into the region of the geometric shadow behind the diffraction grating are satisfied. Around the obstacles (contact strips) of the diffraction grating, electromagnetic waves will enter the photocell at an angle to the plane of the pn junction, which will lead to an increase in its path length in the semiconductor and a more uniform distribution of radiation energy over the volume of the photoactive region with a thickness of δ = 0.4 microns. Active absorption of photons occurs with the maximum absorption coefficient of electromagnetic radiation, due to the appropriate selection of semiconductor material for a given wavelength λ of the laser, in almost the entire volume of the photoactive region. The absorption is accompanied by the formation of electron-hole pairs and the appearance of excess charge carriers. Nonequilibrium charge carriers assembled to the pn junction, due to the presence of the contact potential difference, are separated on it: minority carriers freely pass through the pn junction, while the main carriers are delayed. Thus, under the influence of laser electromagnetic radiation with a single wavelength λ, a current of minority non-equilibrium charge carriers — photoelectrons and photoholes — will flow through the pn junction in both directions.

Thus, the application of the proposed design of the photocell of the receiver-converter of laser radiation in space allows you to increase the efficiency and specific values of the photomultiplier of the photomultiplier due to:

1) increasing the length of the path traveled by radiation in the photoactive region of the photocell, which allows to reduce its thickness and thus increase the collection coefficient of current carriers and improve the energy-mass characteristics of the receiver-converter;

2) reduction of losses due to radiation in the main absorption band;

3) increasing the proportion of absorbed radiant flux, which increases the total number of photoelectrons and photoholes created by radiation, per unit time on both sides of the p-n junction;

4) reducing the series resistance of the solar cell by reducing the spreading resistance in the front doped layer with the passage of current along the surface of the solar cell to the contacts;

5) generation of charge carriers in almost the entire volume of the photoactive region.

LITERATURE

1. Prospects for the use of wireless transmission of electrical energy in space transport systems // Gribkov AS, Evdokimov RA et al. // Izv. RAS. Energy 2009. No2. S.118-123.

2. V.A. Griliches, P.P. Orlov, L. B. Popov. Solar energy and space travel. M .: Nauka, 1984.

3. RF patent No. 2357325, cl. H01L 31/04, H01L 1/18, publ. 05/27/2009.

4. Vasiliev A.M., Landsman A.P. Semiconductor photoconverters, M .: Soviet Radio, 1971

5. RF patent No. 2303830, cl. H01L 31/0224, H01L 1/18, publ. 07/27/2007.

6. Highly efficient concentrator (2500 suns) AlGaAs / GaAs - solar cells // Andreev VM, Khvostikov VP, Larionov VR et al. // Physics and Engineering of Semiconductors, 1999, Volume 33, Issue 9, pp. 1070-1072.

7. Sh. Chang. Energy conversion. M .: Atomizdat, 1965.

8. Physical encyclopedia. M .: Soviet Encyclopedia, Volume 1, 1988.

9. Physical encyclopedic dictionary. Moscow, "Soviet Encyclopedia", 1983.

Claims (1)

A photocell of the receiver of laser radiation in space, containing semiconductor doped and base layers of p-type and n-type, alternating contact strips on the working side of the photocell and continuous ohmic contact on the back of the photocell, characterized in that on the working side of the photocell, with a given thickness δ of its photoactive region, a diffraction grating is made made of opaque parallel to each other contact strips of width b alternating with a constant pitch Δ and comprising ohms contact with the semiconductor layer of the photocell, onto which the electromagnetic radiation of the laser with a wavelength λ is incident normally, while the diffraction grating is made so that the distance between the contact strips (Δ-b) and the length of the contact strips (L) correspond to the relation (Δ-b) << L, and the width b and the step Δ of the contact strips of the diffraction grating satisfy the relations
b <2 · k · (δ · λ) ^{l / 2} / (1-k), Δ = b / k,
where k is the specified coefficient of shadowing by the contact strips of the radiation-sensing surface of the solar cell;
δ is the specified thickness of the photoactive region, commensurate with the diffusion length of minority charge carriers.

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