Process for making multi-crystalline silicon thin-film solar cells

Dichlorosilane and diborane are deposited on the titanium-based alloy film to grow a p+ type back surface field film. The temperature is raised to grow a p− type light-soaking film on the p+ type back surface field film. Phosphine is deposited on the p− type light-soaking film to form an n+ type emitter. Thus, an n+-p−-p+ laminate is provided on the titanium-based alloy film. SiCNO:Ar plasma is used to passivate the n+-p−-p+ laminate, thus forming an anti-reflection film of SiCN/SiO2 on the n+ type emitter. The n+-p−-p+ laminate is etched in a patterned mask process. A p− type ohmic contact is formed on the titanium-based alloy film. The anti-reflection film is etched in a patterned mask process. The n+ type emitter is coated with a titanium/palladium/silver alloy film that is annealed in hydrogen. An n− type ohmic contact is formed on the n+ type emitter.

BACKGROUND OF INVENTION

1. Field of Invention

The present invention relates to a process for making multi-crystalline silicon thin-film solar cells and, more particularly, to a high-temperature process for making multi-crystalline silicon thin-film solar cells based on plasma-enhanced chemical vapor deposition.

2. Related Prior Art

Silicon-based solar cells are generally made in low-temperature processes based on plasma-enhanced chemical vapor deposition (“PECVD”). An amorphous or microcrystalline silicon film is coated on a substrate of glass, aluminum, silicon, stainless steel or plastics. A back contact is made of aluminum, gold, silver or transparent conductive oxide such as indium-tin oxide (“ITO”) and zinc oxide.

The primary advantage of the low-temperature processes is the wide variety of materials that can be used to make the substrates. However, they suffer drawbacks such as defective silicon films, low photoelectrical conversion efficiencies and low light-soaking stability. In the PECVD, while coating the microcrystalline silicon film, a silicon material is highly diluted in hydrogen according to the following notion:
[H2]/[SiH4]>15

That is, the concentration or flow rate of H2is more than 15 times as high as that of SiH4. The problems with the PECVD include a low growth rate of the film, a long process and a high cost.

Regarding the making of the multi-crystalline silicon solar cells, there are various techniques such as solid phase crystallization (“SPC”) and aluminum-induced crystallization (“AIC”).

The SPC is based on the PECVD. In the SPC, an amorphous silicon film is deposited, intensively heated and annealed at a high temperature. Thus, a multi-crystalline silicon film with a grain size of 1 to 2 micrometers is made.

There are however problems with the low-temperature processes for making multi-crystalline silicon solar cells based on the PECVD. Firstly, many defects occur in the silicon films. Secondly, the photoelectrical conversion efficiencies are low. Thirdly, the light soaking stabilities are low. Fourthly, the growth rates of the films are low. Sixthly, the processes are long. Seventhly, the costs are high.

Referring toFIGS. 11 through 15, in the AIC, a substrate71is coated with an aluminum film72. An amorphous silicon film73is coated on the aluminum film72based on the PECVD and annealed at a temperature of 575 degrees Celsius for a long time to form a seed film74. Then, it is subjected to an epitaxial process such as the PECVD or an electron cyclotron resonance chemical deposition (“ECR-CVD”) to make a multi-crystalline silicon film75. The AIC however involves many steps and takes a long time. The resultant grain size is 0.1 to 10 micrometers.

A conventional silicon-based tandem solar cell includes an upper laminate and a lower laminate. The upper laminate is an amorphous silicon p-i-n laminate. The lower laminate is a microcrystalline silicon p-i-n laminate. Thus, the infrared and visible light of the sunlit can be converted into electricity. However, the photoelectrical conversion efficiency of the conventional silicon-based tandem solar cell deteriorates quickly.

Concerning the process for making multi-crystalline silicon solar cells based on the AIC, the processes are long for including many steps and therefore expensive. As for the conventional silicon-based tandem solar cell, the photoelectrical conversion efficiency deteriorates quickly.

SUMMARY OF INVENTION

It is the primary objective of the present invention is to provide a process for making a tandem solar cell.

To achieve the primary objective, a titanium-based alloy film is provided on a ceramic substrate. Dichlorosilane and diborane are deposited on the titanium-based alloy film to grow a p+type back surface field film. The temperature is raised to grow a p−type light-soaking film on the p+type back surface field film. Phosphine is deposited on the p−type light-soaking film to form an n+type emitter. Thus, an n+-p−-p+laminate is provided on the titanium-based alloy film. SiCNO:Ar plasma is used to passivate the n+-p−-p+laminate, thus forming an anti-reflection film of SiCN/SiO2 on the n+type emitter. The n+-p−-p+laminate is etched in a patterned mask process. A p−type ohmic contact is formed on the titanium-based alloy film. The anti-reflection film is etched in a patterned mask process. The n+type emitter is coated with a titanium/palladium/silver alloy film that is annealed in hydrogen. An n−type ohmic contact is formed on the n+type emitter.

Other objectives, advantages and features of the present invention will become apparent from the following description referring to the attached drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Referring toFIG. 1, there is shown a process for making multi-crystalline silicon thin-film solar cells according to the preferred embodiment of the present invention.

Referring toFIGS. 1 and 2, at11, a ceramic substrate21is provided. The ceramic substrate21is made of aluminum oxide. The thickness of the substrate21is 0.1 to 1.0 mm.

The ceramic substrate21is coated with a titanium-based alloy film24(FIG. 6). The titanium/silicon alloy film24may be made of TiSi2, TiN, TiC, TiB2or TiCxNy. The titanium-based alloy film24can be provided in three subroutines.

In the first subroutine, at12(FIGS. 1 and 3), a titanium film22is coated on the ceramic substrate21in an e-gun evaporation system at 250 degrees Celsius. The thickness of the titanium film22is 1000 to 5000 angstroms.

At13a(FIGS. 1 and 4), dichlorosilane is deposited on the titanium film22in an atmospheric pressure chemical vapor deposition (“APCVD”) apparatus4, at 800 to 1100 degrees Celsius. The dichlorosilane and the titanium film22exchange silicon atoms and titanium atoms to form the titanium/silicon alloy film24. The grain size of the titanium/silicon alloy film24is larger than 1 micrometer. The sheet resistance of the titanium/silicon ally film24is lower than ohm/cm2.

In the second subroutine, at12(FIGS. 1 and 3), a titanium film22is coated on the ceramic substrate21in an e-gun evaporation system at 250 degrees Celsius. The thickness of the titanium film22is 1000 to 5000 angstroms. At13b(FIGS. 1 and 5), an amorphous silicon film23is coated on the titanium film22in a plasma-enhanced chemical vapor deposition (“PECVD”) apparatus. Alternatively, the amorphous silicon film23may be coated on the ceramic substrate21before the titanium film22is coated on the amorphous silicon film23. In either case, the ratio of the thickness of the amorphous silicon film23to the thickness of the titanium film22is 2:1.

The titanium film22and the amorphous silicon film23are heated in a high-temperature annealing apparatus5at 700 to 900 degrees Celsius so that they exchange titanium atoms and silicon atoms, thus forming the titanium/silicon alloy film24. Then, the temperature in the APCVD apparatus5is raised to a value-higher than 1000 degrees Celsius for the epitaxial growth of the grains. The size of the grains of the titanium/silicon alloy film24is larger than 1 micrometer. The sheet resistance of the titanium/silicon alloy film24is lower than ohm/cm2.

In the third subroutine, dichlorosilane and titanium tetrachloride are made to react with each other to form the titanium/silicon alloy film24in the APCVD apparatus4.

Referring toFIGS. 1 and 7, at15, dichlorosilane and diborane are made to exchange silicon atoms and boron atoms in the APCVD apparatus4at 900 to 1000 degrees Celsius, thus forming a type multi-crystalline silicon back surface field film25.

The temperature in the APCVD apparatus4is raised to a value higher than 1000 degrees Celsius. More dichlorosilane and diborane are made to exchange silicon atoms and boron atoms, thus forming a p−type multi-crystalline silicon light-soaking film26on the p+type multi-crystalline silicon back surface field film25, which is used as a seed layer. The epitaxial growth of the p−type multi-crystalline silicon light-soaking film26is 0.5 micrometer/minute and lasts for 30 minutes. The thickness of the p−type multi-crystalline silicon light-soaking film26is 1 to 15 micrometers. The size of the grains261of the p−type multi-crystalline silicon light-soaking film26is larger than 10 micrometers. The concentration of the boron atoms in the p−type multi-crystalline silicon light-soaking film26is 1016to 1017#/cm3.

At 800 to 1000 degrees Celsius, phosphine is deposited on the p−type multi-crystalline silicon light-soaking film26, thus executing the n+type deposition of the phosphor atoms of the phosphine on the p−type multi-crystalline silicon light-soaking film26. That is, an n+type multi-crystalline silicon emitter27is form on the p−type multi-crystalline silicon light-soaking film26. The thickness of the n+type multi-crystalline silicon emitter27is smaller than 1000 angstroms. The concentration of the boron atoms in the n+type multi-crystalline silicon emitter27is 1018to 1019#/cm3. The n+type multi-crystalline silicon emitter27, the p−type multi-crystalline silicon light-soaking film26and the p+type multi-crystalline silicon back surface field film25together form a n+-p−-p+laminate1.

Referring toFIGS. 1 and 8, at16, SiCNO:Ar plasma is provided in a PECVD apparatus6. Silane, nitrous oxide and methane are used as the raw materials of the SiCNO:Ar plasma while argon is used as a carrier. The SiCNO:Ar plasma passivates the n+-p−-p+laminate1. Hence, the dangling bonds of the silicon atoms on the surface271of the n+type multi-crystalline silicon emitter27are filled. The dangling bonds of the silicon atoms at the grain boundaries262between the grains261of the p−type multi-crystalline silicon light-soaking film26are also filled. The dangling bonds of the silicon atoms in the p+type multi-crystalline silicon back surface field film25are also filled. Moreover, an anti-reflection film28of SiCN/SiO2is coated on the n+type multi-crystalline silicon emitter27.

Referring toFIGS. 1 and 9, at17, potassium hydroxide solution is used to etch the multi-crystalline silicon laminate in a patterned mask process. The substrate21and the titanium/silicon alloy film24are not etched at all. A p−type ohmic contact29is made on the titanium/silicon alloy film24.

Referring toFIGS. 1 and 10, at18, the anti-reflection film28are etched in a patterned mask process so that portions of the n+type multi-crystalline silicon emitter27are exposed from the anti-reflection film28. A titanium/palladium/silver alloy film30is provided in the exposed portions of the n+type multi-crystalline silicon emitter27and annealed in the high-temperature annealing apparatus5. Finally, an n−type ohmic contact31is provided on the titanium/palladium/silver alloy film30.

As discussed above, the multi-crystalline silicon laminate1includes the ceramic substrate21and the titanium/silicon alloy film24used as the seed layer. The APCVD apparatus6is used in the high-temperature process for the exchange of the silicon atoms and the boron atoms, thus forming the p+type multi-crystalline silicon back surface field film25and the p−type multi-crystalline silicon light-soaking film26. Then, the phosphor atoms and the silicon atoms are exchanged so that the n+type multi-crystalline silicon emitter28is made. The SiCNO:Ar plasma is used to passivate the laminate1. The patterned mask process is used to make the p−type ohmic contact29on the titanium/silicon alloy film24. The patterned mask process is used to coat the titanium/palladium/silver alloy film30on the n+type multi-crystalline silicon emitter27and provide the n−type ohmic contact31on the n+type multi-crystalline silicon emitter27.

Solar cells made in the process according to the present invention exhibits several advantages. The ceramic substrate21is inexpensive, refractory and chemically stable, and can be integrated with materials for construction.

The titanium/silicon alloy film24is environmentally friendly, abundant and inexpensive. The titanium/silicon alloy film24ensures the integrity of the multi-crystalline silicon laminate1since its thermal expansion coefficient is matched with that of the ceramic substrate21and the p+type multi-crystalline silicon back surface field film25.

The solar cells provide a high photoelectrical conversion efficiency and excellent light-soaking stability because the PEVCD apparatus6is used in the high-temperature process to passivate the multi-crystalline silicon films that would otherwise involve high mobility and a large diffusion length, and take long for recombination.

Moreover, the process of the present invention provides a high epitaxial growth rate and a high crystal quality.