BACKGROUND OF THE INVENTION

1. FIELD OF THE INVENTION

The present invention generally relates to a solar cell, and more particularly to a method of making a concentrated photovoltaic (CPV) cell.

2. DESCRIPTION OF RELATED ART

Electric power may be generated in several forms such as nuclear power, wind power, hydraulic power, thermal power, or solar power. And the form of electric power is generally selected based on factors such as safety, cost, environmentally friendliness, life-time, or resource availability. Green power, such as solar power, geothermal power, or hydraulic power, has recently been getting more attention. Solar power, in particular, is taken more seriously since it minimizes pollution to the environment.

The generation of solar power may be classified into two types : solar thermal and photovoltaic . The solar thermal type of generation uses heat collected from the sun or derives electric power from the heat; and the photovoltaic type of generation transforms light radiation to electric power via a solar cell according to photovoltaic effect.

The solar cell commonly used is either a bulky panel photovoltaic cell or a concentrated photovoltaic (CPV) cell. The bulky panel photovoltaic cell, also named silicon-based solar cell, has an active layer made from a silicon wafer or thin film, where the silicon wafer may be monocrystalline silicon, multicrystalline silicon or ribbon silicon, and the thin film may be cadmium telluride (CdTe) , copper indium gallium selenide (CIGS) , or amorphous silicon (A-Si) . The bulky panel photovoltaic cell has a simple structure, it does not require additional optical elements, but it has low conversion efficiency (8- 1 0%) . Accordingly, the bulky panel photovoltaic cell has an intermediate overall system cost, for example, at US $ 6/ W _p , where W _p stands for peak watt.

The concentrated photovoltaic cell, also named chemical compound-based solar cell, has an active layer made from group III-V compound such as gallium arsenic (GaAs) , indium gallium arsenic (In _x Gai - _x As) , aluminum phosphorus (A1P) , or gallium phosphorus (GaP) . FIG . 1 shows a cross section of a conventional concentrated photovoltaic, which includes, from bottom to top, a substrate 10 , a gallium arsenic (GaAs) layer 1 1 , an indium gallium arsenic (InGaAs or In _x Gai - _x As) layer 12 , a gallium phosphorus (GaP) layer 13 , and an aluminum phosphorus (A1P) layer 14.

The compounds mentioned above have a narrow band gap range, for example, 0.36-2.45 electron volts (eV) . Some band gap values of the aforementioned compounds are listed in Table 1 as follows :

Table 1

FIG . 2 shows an energy band diagram. When a solar cell absorbs light energy higher than the maximum band gap Eg (i . e . , 2.45 eV) , an electron 20 at a valence band Ev may thus be over-excited above a conduction band Ec. As the electron 20 returns to the conduction band Ec, heat will be generated. As a result, light energy is wasted, and a cooling system or a heat dissipation device may be further needed to dissipate the generated heat.

Most of the compounds mentioned above are toxicant and fragile such that manufacturing cost is high and yield is low. Moreover, as gallium arsenic (GaAs) in the conventional solar cell shown in FIG . 1 has a tolerance of merely 10 ³ / cm ² to defect density, yield is problematic. The compounds mentioned above are normally made by expensive metalorganic chemical vapor deposition (MOCVD) , which cannot deposit a thick layer to increase conversion efficiency. Further, the conventional concentrated photovoltaic need be made in a high temperature (> 5O0 °C ) , which therefore raises manufacturing cost. Compared to the bulky panel photovoltaic, the concentrated photovoltaic has a smaller dimension and higher conversion efficiency (25-45%) , but requires additional optical elements to focus light (e . g. , by using a lens with an area ratio of 1 : 1 000) and demands for a cooling system or a heat dissipation device . Accordingly, the concentrated photovoltaic has a high overall system cost, for example, at US$ 1 0/ W _P .

Since conversion efficiency of the conventional concentrated photovoltaic cell cannot be effectively improved to lower overall system cost, a need has thus arisen to propose a novel solar cell to improve the efficiency of the concentrated photovoltaic cell.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the embodiment of the present invention to provide a method of making a solar cell that uses group Ill-nitride compounds to replace the conventional group III-V compounds, uses low temperature chemical vapor deposition or physical vapor deposition to replace the conventional metalorganic chemical vapor deposition (MO CVD) , and uses laser as excitation energy. Accordingly, the solar cell of the embodiment is capable of improving conversion efficiency, decreasing dimension and lowering overall system cost by using low temperature process

According to one embodiment, a base is first provided, and a substrate is formed on a surface of the base. In one embodiment, the base/ substrate includes silicon/ silicon carbide; while in another embodiment, the base/ substrate includes aluminum/ aluminum oxide. Subsequently, a buffer layer is selectively formed on the substrate. A plurality of group Ill-nitride layers are deposited on or above the substrate or the buffer layer, wherein the plurality of group Ill-nitride layers are deposited using laser as excitation energy, and band gap values of the group Ill-nitride layers progressively increase in a direction away from the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross section of a conventional concentrated photovoltaic cell;

FIG. 2 shows an energy band diagram;

FIG.3A to FIG.3C show a method of making a solar cell and an associated structure according to one embodiment of the present invention; and

FIG. 4 illustrates relationship between a solar cell structure and corresponding absorbable sunlight wavelength.

DETAILED DESCRIPTION OF THE INVENTION

FIGs. 3A to FIG. 3C show a method of making a solar cell (or photovoltaic cell or photoelectric cell) and an associated structure according to one embodiment of the present invention. A solar cell may convert light energy (e.g., sunlight energy) to electric energy. Only composing elements pertinent to the embodiment are shown in the figures. In other words, one or more layers may be inserted between two shown adjacent layers when necessary. The dimensions of the shown elements are not depicted to a proportionate degree.

According to a first embodiment of the invention, as shown in FIG. 3A, a silicon (Si) base 30 is first provided. Subsequently, a silicon carbide (SiC) substrate 31 is formed on a surface of the silicon base 30. In a preferred embodiment, the silicon carbide substrate 31 has a thickness of, but is not limited to, about 2-10 micrometers (/ m). Relevant details about forming silicon carbide substrate 31 may be referred, for example, to US Patent Publication No. 2009/0045412 to Udagawa et al., entitled "Method for Production of Silicon Carbide Layer, Gallium Nitride Semiconductor device and Silicon Substrate," the disclosure of which is incorporated herein by reference.

According to a second embodiment of the invention, the structure shown in FIG. 3A may be made by, first providing an aluminum (Al) base 30, which is subjected to hard-anodizing to form an aluminum oxide (AI2O3) substrate 31 on a surface of the aluminum base 30. Aluminum oxide, commonly called sapphire, is harder than unprocessed aluminum. In a preferred embodiment, the aluminum oxide substrate 31 has a thickness of, but is not limited to, about 2-10 micrometer. Anodizing is an electrolytic process that places metal to be treated in acids (e . g. , sulfuric acid) and passes a current through the metal. Relevant details about anodizing may be referred, for example, to US Patent Publication No . 20 1 1 / 0 146795 to Chang et al. , entitled "Structure and Preparation of CIGS-Based Solar Cells Using an Anodized Substrate with an Alkali Metal Precursor," the disclosure of which is incorporated herein by reference . The hard-anodizing used in the embodiment is different from the ordinary anodizing. For example, the hard-anodizing operates at a temperature lower than the anodizing, and the current passing in the hard-anodizing is higher than the anodizing. After hard-anodizing, a surface of the aluminum oxide substrate 3 1 includes a grain boundary having crystals with single orientation, for example, C-plane or A-plane .

Afterward, as shown in FIG . 3B, a buffer layer 32 is formed on or above the substrate 3 1 . In this specification, "up" or "top" is a direction that is away from the base 30, that is, facing a light source; and "down" or "bottom" is a direction that faces towards the base 30. The buffer layer 32 may be used to alleviate stress between subsequent layers and the substrate 3 1 . The buffer layer 32 of the embodiment may include a material of gallium nitride (GaN) , which may be deposited on the substrate 3 1 by chemical vapor deposition. The gallium nitride in the embodiment has a tolerance to defect density up to 1 0 ⁹ / cm ² , while gallium arsenic (GaAs) used in the conventional solar cell has a tolerance to defect density of merely 10 ³ /cm ² . Although a buffer layer 32 is illustrated in the embodiment, it is appreciated that the buffer layer 32 may be omitted. As shown in FIG. 3C, a number of group Ill-nitride layers

33A-33D are formed on or above the buffer layer 32. Each group Ill-nitride layer 33A-33D includes nitride atoms and at least one of group III atoms (e.g., aluminum, gallium, indium, etc). Taking FIG. 3C as an example, the group Ill-nitride layers 33A-33D include, from bottom (i.e., near the substrate 31, which is equivalent to the silicon carbide substrate of the first embodiment or the aluminum oxide substrate of the second embodiment) to top (i.e., away from the substrate 31), an indium nitride (InN) layer 33A, an indium gallium nitride (InGaN or In _x Gai- _x N) layer 33B, an indium aluminum gallium nitride (InAlGaN) layer 33C, and an aluminum nitride (AIN) layer 33D.

According to one aspect of the embodiment, the band gap values of the group Ill-nitride layers 33A-33D progressively increase in a direction away from the substrate. Some band gap values of the group Ill-nitride layers 33A-33D are listed in Table 2 as follows:

Table 2

FIG . 4 illustrates relationship between a solar cell structure and corresponding absorbable sunlight wavelength, what is illustrated is only a portion (the visible portion) of the sunlight spectrum where the horizontal axis represents the light intensity and the vertical axis represents wavelength in micrometer. According to the equation E=h* (c/ λ ) , wavelength λ is in inverse proportion to light energy E, where h is Planck's constant and c is the speed of light. According to the structure disclosed in the embodiment, a layer with higher band gap value is nearer a light source (e . g. , sunlight) such that over-excited electrons and associated generated heat may be substantially prevented or reduced when the electrons are excited by light energy to a conduction band from a valence band. With respect to the conventional solar cell with a maximum band gap value of 2.45 eV, light energy higher than the maximum band gap value will result in electron over-excitation and associated generated heat.

According to FIG . 4 and Table 2 , as the solar cell structure disclosed in the embodiment possesses wide band gap range, i. e . , 0.7-6.3 eV, which may correspond to wide light energy (or wavelength) range . On the other hand, the conventional solar cell has narrow band gap range , e . g. , 0.36-2 .45 eV, which corresponds to narrow light energy (or wavelength) range . As discussed above, large light energy (or small wavelength) causes electron over-excitation and associated heat. Accordingly, the solar cell of the embodiment has conversion efficiency higher than the conventional solar cell. Moreover, the buffer layer (GaN) 32 of the embodiment has a band gap value of 3.4 eV, which contributes to improvement in conversion efficiency. For example, when partial light energy is not absorbed by the group Ill-nitride layers 33A-33D , the passing light energy may probably be absorbed by the buffer layer 32 and the silicon carbide substrate 3 1 . According to a rough estimation, conversion efficiency of the embodiment may be higher than 60%, while conversion efficiency of the conventional solar cell may be 45% at best. In addition to wide band gap range, there is greater probability for material combinations available in the embodiment than the conventional solar cell, such that more layers may be formed to further improve conversion efficiency. According to a third embodiment of the invention, the group

Ill-nitride layers 33A-33D may be deposited by adopting chemical vapor deposition (CVD) and using laser as excitation energy, which is used to perform pyrolytic to carry out thermochemical decomposition such that reaction gas (or reactant) is decomposed and then recombined to form a deposited film. Alternatively, the embodiment may use laser as excitation energy to perform photolytic to carry out photochemical reaction such that reaction gas is decomposed and then recombined to form a deposited film. Relevant details about pyrolytic and photolytic may be referred, for example, to US Patent No . 5 ,4 1 7 , 823 to Narula et al. , entitled "Metal-Nitrides Prepared by Photolytic/ Pyrolytic decomposition of Metal-Amides," the disclosure of which is incorporated herein by reference .

Some reactants adoptable in CVD of the embodiment are

In addition to CVD used in the first through the third embodiments, the group Ill-nitride layers 33A-33D may be deposited by adopting physical vapor deposition (PVD) . Compared to CVD , PVD uses lower temperature and pressure (e . g. , vacuum) . In PVD , elements or compounds to be deposited are vaporized and then condensed to form a film within line of sight. Relevant details about forming the group Ill-nitride layers using PVD may be referred, for example, to US Patent No . 6, 7 1 6 , 655 to Nagai et al. , entitled "Group III Nitride Compound Semiconductor Element and Method for Producing the Same ," the disclosure of which is incorporated herein by reference . According to a fourth embodiment of the invention, the group

Ill-nitride layers 33A-33D may be deposited by adopting pulsed laser deposition, which is one method of PVD . In the embodiment, source substance in a target is impinged and vaporized by using high power laser beam as excitation energy, and is then condensed to form a film. Relevant details about depositing nitride compounds using laser as excitation energy may be referred, for example, to a literature by A. Perrone et al. , entitled "An Overview on Nitride Film Deposited by Reactive Pulsed Laser Ablatio," Lasers and Electro-Optics Europe, 2000, Conference Digest, the disclosure of which is incorporated herein by reference . For the foregoing, the embodiment uses laser as excitation energy such that a lower temperature (< 5O0°C ) may be adopted to reduce thermal budget or cost as compared to a conventional method (e . g. , MOCVD) . Moreover, thicker group Ill-nitride layers 33A-33D may be formed to improve conversion efficiency as compared to MOCVD .

As wider band gap range, greater probability for available material combinations, and thicker layers may be obtained according to the embodiments, conversion efficiency higher than that of the conventional solar cell may thus be attained . Accordingly, focusing optical elements with smaller dimensions may be used or even omitted, and a cooling system or a heat dissipation device may be simplified or reduced in size or even omitted, such that overall dimensions and cost of the solar cell may be substantially reduced. Further, overall converted electric power may be improved owing to material characteristics (e . g. , tolerance to defect density and toughness) of group Ill-nitride compounds used in the solar cell.

Although specific embodiments have been illustrated and described, it will be appreciated by those skilled in the art that various modifications may be made without departing from the scope of the present invention, which is intended to be limited solely by the appended claims .