A method for improving the photoelectric conversion efficiency of GaAs thin-film solar cells

Abstract

The present invention discloses a method for improving the photoelectric conversion efficiency of GaAs thin-film solar cells. Taking the window layer as the active layer of the solar cell, fixed charges are introduced into the antireflection structure, induced charges appear on the outer surface of the window layer, energy band bending occurs near the surface, and minority carriers are hindered from flowing to the surface for recombination. On the one hand, this structure increases the absorption efficiency of short-wave photons, and on the other hand, it reduces the surface recombination of minority carriers, thereby improving the collection efficiency of minority carriers, short-circuit current and conversion efficiency.

Description

Technical Field

[0001] The present invention relates to the technical field of III-V group compound semiconductor solar cells, and particularly to a method for improving the photoelectric conversion efficiency of GaAs thin film solar cells. Background Art

[0002] III-V group compound semiconductor materials are widely used in solar cells due to their high carrier mobility, direct bandgap and other advantages. However, due to the high surface recombination rate of III-V group compounds, in order to reduce surface recombination, a thin Al 0.8 Ga 0.2 As window layer (generally less than 100 nm) is often fabricated on the cell surface. Taking GaAs material as an example, the purposes are: on the one hand, since Al 0.8 Ga 0.2 As has very good lattice matching with GaAs, the surface recombination rate of GaAs material can be significantly reduced. On the other hand, Al 0.8 Ga 0.2 As has a relatively wide bandgap width, and only short-wave photons with energy greater than the bandgap width are absorbed, and most photons will pass through the window layer and enter the GaAs active layer. Therefore, only a very small part of the photo-generated carriers are generated in the window layer.

[0003] Although the window layer can reduce the GaAs surface recombination rate, due to the low absorption efficiency of GaAs material for short waves, most short-wave photons cannot be absorbed and are lost; on the other hand, although Al 0.8 Ga 0.2 As has a relatively wide bandgap, it is still inevitable that photons are absorbed in the window layer and carriers are generated. However, this part of carriers is recombined due to the high surface recombination rate of AlGaAs, generating recombination current. To solve the existing problems, it can be addressed from two aspects: on the one hand, the Al component can be adjusted to obtain a suitable bandgap width, the thickness of the window layer can be increased, the absorption efficiency of short-wave photons in the window layer can be enhanced, and the utilization rate of sunlight can be improved; on the other hand, in order to improve the carrier collection efficiency in the window layer, the surface carrier recombination rate must be reduced. Summary of the Invention

[0004] In view of the problems existing in the prior art, the present invention provides a method for improving the photoelectric conversion efficiency of GaAs thin film solar cells.

[0005] The technical solution of the present invention is as follows:

[0006] A method for improving the photoelectric conversion efficiency of GaAs thin film solar cells, comprising the following steps:

[0007] 1) Using the window layer as a photon absorption layer to enhance the absorption efficiency of short-wave photons;

[0008] 2) Adjust the Al component of the window layer to obtain an appropriate bandgap width;

[0009] 3) Increase the thickness of the window layer to enhance the absorption efficiency of short - wave photons in the window layer;

[0010] 4) Construct a MIS solar cell structure, that is, separate the antireflection film and the window layer with an insulating layer,

[0011] 5) Introduce fixed charges into the antireflection film to generate induced charges on the surface of the window layer, thereby causing band bending.

[0012] Further, the window layer uses Al x Ga 1-x As material, where x = 0.3 - 0.4; the window layer can also use other materials for the window layer of solar cells;

[0013] Further, the antireflection film uses a single - layer dielectric material thin film or a double - layer dielectric material thin film or a photonic crystal array structure; such as single - layer SiO2 / TiO2 / Si3N4, etc. or double - layer antireflection films MF2, ZnS, etc., or a dielectric photonic crystal array structure, etc.;

[0014] Further, introducing fixed charges into the antireflection film includes negative charges or positive charges.

[0015] Further, the MIS solar cell structure means that the metal and the semiconductor / antireflection film are separated by an insulator;

[0016] Further, the window layer is a semiconductor window layer.

[0017] Further, the insulating layer uses Al2O3.

[0018] Further, by changing the concentration of fixed charges (N fix ) in the antireflection film, the concentration of induced charges on the surface of the window layer is changed, causing the surface barrier height to change.

[0019] The beneficial effects of the present invention are as follows:

[0020] 1) The present invention uses Al 0.4 Ga 0.6 As as part of the active layer of the solar cell, which has a strong absorption effect on short - wave photons of the sun. Due to the existence of the potential barrier, the photo - generated minority carriers in this area can be collected by the PN junction, improving the photo - generated current of the solar cell.

[0021] 2) By increasing the concentration of fixed charges, the surface recombination rate of the window layer can be made close to zero, and there is no need to perform surface passivation processes anymore.

[0022] 3) Compared with traditional solar cells, the short-circuit current is significantly higher, resulting in a higher conversion efficiency.

[0023] 4) This method is suitable not only for GaAs solar cells but also for III-V compound semiconductor solar cell structures with a window layer as an active absorption layer passivation layer. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a schematic structural diagram of an embodiment of the present invention;

[0025] Figure 2 It is a schematic diagram of the barrier height corresponding to different fixed charges of the present invention;

[0026] Figure 3 It is a comparison diagram of J-V curves of the MIS structure of the present invention corresponding to different fixed charges when the AM1.5 terrestrial solar spectrum is vertically incident;

[0027] In the figure: 1. GaAs active layer; 2. Al 0.4 Ga 0.6 As window layer; 3. Al 0.3 Ga 0.7 As back surface layer; 4. Al2O3 insulating layer; 5. ZnS thin film layer; 6. MgF2 thin film layer; 7. GaAs substrate; 8. Positive electrode; 9. Back electrode. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0028] The present invention will be further described below in conjunction with the drawings and embodiments.

[0029] A method for improving the photoelectric conversion efficiency of GaAs thin-film solar cells adopts a new MIS solar cell structure (i.e., metal / insulating layer / semiconductor). By adjusting the Al component, the bandgap width of the appropriate window layer is obtained to enhance the absorption of short-wave photons. At the same time, fixed charges are introduced into the antireflection film to generate induced charges on the surface of the window layer, causing band bending and hindering the flow of minority carriers to the surface, thereby reducing the surface recombination rate and enhancing the collection efficiency of minority carriers. This concept is not only applicable to GaAs solar cells but also has very good effects on III-V compound semiconductor solar cells with a high surface recombination rate.

[0030] Embodiment:

[0031] A new type of MI-Al 0.4 Ga 0.6 As / GaAs / Al 0.3 Ga 0.7 As solar cell structure, as Figure 1 shown, this solar cell is composed of a GaAs absorption layer (GaAs active layer 1) and Al0.4 Ga 0.6 As absorption layer (Al 0.4 Ga 0.6 As window layer 2) together constitute the active layer of the solar cell, Al 0.3 Ga 0.7 As back surface layer 3, Al2O3 insulating layer 4, double-layer antireflection layer ZnS thin film layer 5 and MgF2 thin film layer 6, GaAs substrate 7, positive electrode 8 and back electrode 9 are composed.

[0032] In this embodiment, the induced barrier height of the window layer is changed by changing the fixed charge concentration in the antireflection layer;

[0033] As Figure 1 shown, the thickness of the n-Al 0.4 Ga 0.6 As absorption layer is 50 nm (doping concentration is N D = 1×10 18 cm -3 ), the thickness of the n-GaAs emitter region is 50 nm (N D = 2×10 18 cm -3 ), the thickness of the p-GaAs base region is 150 nm (N A = 1×10 17 cm -3 ), N A represents the doping concentration of the P region; the back surface layer is an n-Al 0.3 Ga 0.7 As layer with a thickness of 100 nm (NA = 1×10 18 cm -3 ), the ZnS thin film layer 5 and the MgF2 thin film layer 6 (double-layer antireflection film) are 100 nm and 50 nm respectively, the thickness of the Al2O3 insulating layer 4 is 10 nm, the materials of the positive electrode 8 and the back electrode 9 are silver (Ag) with a thickness of 200 nm, and the GaAs substrate 7 is 3000 nm.

[0034] In this embodiment, since the window layer is an n-type region, the fixed charge in the antireflection layer is a positive charge, and the surface barrier of the window layer bends downward, acting as a hindrance to the movement of minority carrier electrons towards the surface, enhancing the collection efficiency of minority carriers; as the fixed charge concentration increases, the barrier height increases, and the reflection of carriers becomes stronger; as Figure 2 shown, the corresponding barrier heights under different fixed charges, the higher the fixed charge concentration, the more severe the downward bending of the energy band, and the stronger the hindrance to minority carrier holes.

[0035] The above embodiments are only used to illustrate the present invention, rather than limiting the scope of the present invention. All structural transformations made without creative labor starting from the concept of the present invention fall within the protection scope of the present invention.

Claims

1. A method for improving the photoelectric conversion efficiency of GaAs thin-film solar cells, characterized in that, It includes the following steps: 1) Using the window layer as the photon absorption layer to enhance the absorption efficiency of short-wave photons; The window layer uses Al x Ga 1-x As material, where x = 0.3 - 0.4; 2) Adjusting the Al component of the window layer to obtain an appropriate bandgap width; 3) Increasing the thickness of the window layer to enhance the absorption efficiency of short-wave photons in the window layer; 4) Constructing a MIS solar cell structure, that is, separating the antireflection film and the window layer with an insulating layer; 5) Introducing fixed charges into the antireflection film to generate induced charges on the surface of the window layer, thereby causing band bending; the introduction of fixed charges into the antireflection film includes negative charges or positive charges.

2. A method for improving the photoelectric conversion efficiency of a GaAs thin film solar cell according to claim 1, characterized in that, The antireflection film is a single-layer dielectric material thin film or a double-layer dielectric material thin film or a photonic crystal array structure.

3. A method for improving the photoelectric conversion efficiency of a GaAs thin-film solar cell according to claim 1, characterized in that, The window layer is a semiconductor window layer.

4. A method for improving the photoelectric conversion efficiency of a GaAs thin film solar cell according to claim 1, characterized in that, The insulating layer uses Al2O3.

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