Patent Application
20100089440
The present invention relates to photovoltaic solar cells. In particular, dual junction InGaP/GaAs semiconductor solar cells are disclosed.
Photovoltaic cells often comprise semiconductors that convert solar radiation into electrical energy. Such semiconductors are generally solid crystalline materials that have an energy band gap between the valence band and the conduction band. When light is absorbed by the semiconductor, electrons in the lower-energy valence band may be excited to the higher-energy conduction band. As an electron is excited into the higher-energy conduction band, it leaves behind an unoccupied position, or an electron hole. These free electrons and electron holes contribute to the conductivity of semiconductors.
In order for light to excite an electron into the conduction band, the light, or photon, must have sufficient energy to overcome the band gap. If the light does not have sufficient energy to overcome the band gap, then the energy is merely absorbed as heat. Likewise, if the light has an excess of energy needed to excite the electron into the conduction band, the excess energy is converted into heat. Thus, an individual semiconductor having only one band gap can only convert a portion of the solar spectrum into electricity. However, if more than one semiconductor is arranged in a multi-junction tandem arrangement where each semiconductor has a different band gap, a larger portion of the solar spectrum can be absorbed and converted into electricity.
In a multi-junction tandem arrangement, the first semiconductor layer typically has a higher-energy band gap while the second semiconductor layer typically has a lower-energy band gap. Thus, as light strikes the top of the first semiconductor layer, higher-energy photons are absorbed in the first semiconductor layer and provide sufficient energy to excite electrons into the conduction band. Lower-energy photons, which do not provide sufficient energy to excite electrons in the first semiconductor layer, pass through the first semiconductor layer and are absorbed into the second semiconductor layer. The lower-energy photons provide sufficient energy to electrons in the second layer to excite the electrons into the conduction band of the second semiconductor layer.
Typically, semiconductors comprise one or more p-n junctions, which create electron flow as light is absorbed within the cell. A p-n junction is formed when a negatively doped (n-type) semiconductor material, is placed in contact with a positively doped (p-type) semiconductor material. Electrons present in the conduction band of the n-type layer diffuse across the junction and recombine with electron holes in the p-type layer. The combining of electrons and holes at the junction creates a barrier that makes it increasingly difficult for additional electrons to diffuse into the p-layer. This results in an imbalance of charge on either side of the p-n junction and creates an electric field that promotes the flow of current.
Often multi-junction solar cells only have two terminals. In a two-terminal device, the generated current flows through each of the connected semiconductor layers and thus, the current in each layer is generally the same. However, using a three-terminal structure enables independent current collection from each layer in the tandem semiconductor stack without the need for current matching between each cell.
Multi-terminal InGaP/GaAs solar cells are disclosed. The solar cell may include a GaAs first layer operatively connected with a first terminal. The GaAs first layer may have a band gap of approximately 1.43 eV. An InGaP second layer may be operatively connected with a second terminal and may have a band gap of approximately 1.84 eV. A middle lateral conduction layer may be disposed between the GaAs first layer and the InGaP second layer and may have a band gap higher than the band gap of the GaAs first layer.
In another embodiment, the solar cell may include a GaAs heterojunction subcell operatively connected with a first terminal. The GaAs bottom heterojunction subcell may have a band gap of approximately 1.43 eV. An InGaP homojunction subcell may be operatively connected with a second terminal and may have a band gap of approximately 1.84 eV. An InGaP middle lateral conduction layer may be disposed between the GaAs subcell and the InGaP subcell and may have a band gap of 1.93 eV and a sheet resistance of less than 10 ohm/sq. The InGaP middle lateral conduction layer may be operatively connected to a third terminal.
Another embodiment includes a photovoltaic solar cell arrangement. The arrangement may include an InGaN solar cell for converting photons having an energy between approximately 2.4 eV and approximately 2.6 eV into electric energy. In addition the arrangement may include a three terminal solar cell for receiving photons passing through the InGaN solar cell and converting photons having an energy of between approximately 1.84 eV and approximately 1.43 eV into electric energy. The three-terminal solar cell may include an InGaP layer associated with a first terminal. The InGaP layer may have a band gap of approximately 1.84 eV. An InGaP lateral conduction layer may be disposed below the InGaP layer and may have a band gap of 1.93 eV and a sheet resistance of less than 10 ohm/sq. The InGaP lateral conduction layer may be associated with a second terminal. A GaAs layer may be disposed below the InGaP lateral conduction layer and may have a band gap of approximately 1.43 eV. The GaAs layer may be associated with a third terminal.
In another embodiment a photovoltaic solar cell arrangement may include a first solar cell for converting light energy into electrical energy, and a prism for receiving the light energy passing through the first solar cell. The prism may be positioned to direct some of the light energy in a first direction toward a second solar cell and to direct some of the light energy in a second direction, generally orthogonal to the first direction, toward a third solar cell. Each of the second and third solar cells may be configured to convert the light energy into electrical energy. The second solar cell may include a first InGaP layer, a second InGaP layer disposed below the first InGaP layer, and a GaAs layer disposed below the second InGaP layer.
These and other features of the present teachings are set forth herein. Other features and advantages will become apparent to the one skilled in the art from the following drawings and description of various embodiments.
The skilled person will understand that the drawings described below, are for illustration purposes only. The drawings are not intended to limit the scope of the present teachings in any way.
The present invention relates to multi-terminal semiconductor solar cells. The solar cells may be dual junction solar cells comprising single junctions independently interconnected by a middle lateral conduction layer (MLCL). The solar cells include a GaAs subcell 12, InGaP subcell 10, and a MLCL 14 disposed therebetween. In addition, the solar cells may include a plurality of terminals. A first terminal 18 may be operatively connected to the GaAs subcell 12, a second terminal 16 may be operatively connected to the InGaP subcell 10 and a third terminal 20 may be operatively connected to the MLCL 14.
As illustrated in
Each semiconductor subcell within the solar cell comprises several layers. For instance, both the InGaP subcell 10 and the GaAs subcell 12 include an n-type window layer, an n-type emitter layer, a non-intentionally doped set-back layer, and a p-type back surface field layer. Moreover, the InGaP subcell 10 includes an InGaP base layer and the GaAs subcell 12 includes a GaAs base layer. A tunnel junction comprising an n-type layer and a p-type layer is formed between the InGaP subcell 10 and MLCL 14 and provides ohmic contact therebetween.
The above described solar cell may be used in conjunction with one or more other solar cells. Typically the hybrid optical concentrating system, illustrated in
In one embodiment, the second solar cell 30 has an InGaP subcell 10 stacked on a GaAs subcell 12 and converts the light energy between 1.84 eV-1.43 eV. The third solar cell 32 based on silicon or GaInAsP cell stacked on a GaInAs cell converts the light energy below 1 eV.
In one embodiment of
Table 1 illustrates the power generation of a two-terminal solar cell having a top InGaP subcell and a bottom GaAs subcell compared to a three-terminal solar cell having a top InGaP subcell 10, an InGaP MLCL 14, and a bottom GaAs subcell 12. A filter was applied to the two-terminal and three-terminal solar cells to simulate the presence of an InGaN solar cell mechanically stacked above the second solar cells. Various spectral conditions and top subcell band gaps were tested at approximately 13-sun AM1.5G concentration.
As shown in Table 1, two and three-terminal solar cell output power is nearly equal if the InGaP top subcell band gap is 1.9 eV for an unfiltered spectrum and 1.84 eV for a 2.6 eV filtered spectrum. Thus, the preferred band gap of the InGaP top subcell 10 in a three-terminal solar cell having a MLCL 14 is 1.84 eV when the three-terminal solar cell is used in conjunction with an InGaN solar cell. Similar testing, as shown in
The performance of the three-terminal solar cell is increased if the InGaN solar cell has a band gap of 2.4 eV and its output power exceeds 94 mW/cm2 or if the InGaN solar cell has a band gap of 2.6 eV and its output power exceeds 58 mW/cm2. Moreover, the power requirement for the InGaN cell is slightly relaxed if the band gap in the InGaP top subcell 10 is reduced to 1.84 eV. In this case, InGaN cell power is reduced by approximately 10 mW/cm2 from the previous values. Thus, in a three-terminal device, an InGaP top subcell 10 band gap of 1.84 eV is preferred to reduce the power requirement for the InGaN solar cell and to relax the band gap in the top subcell 10.
As used herein, the terms “having”, “containing”, “including”, “comprising” and the like are open ended terms that indicate the presence of stated elements or features, but do not preclude additional elements or features. The articles “a”, “an” and “the” are intended to include the plural as well as the singular, unless the context clearly indicates otherwise.
The present invention may be carried out in other specific ways than those herein set forth without departing from the scope and essential characteristics of the invention. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.
This invention was made with government support under Contract No. LOX497530 awarded by the Defense Advanced Research Projects Agency. The government has certain rights in the invention.
