GHOSH MALATHI (US)
LIU TONGYU (US)
MACNEIL JOHN (US)
| US4903102A | 1990-02-20 |
ELECTRONIC MATERIALS, September 1979, UCHIDA et al., "The Amorphous Silicon Solar Cell and its Application", pp. 111-115.
PROC. OF THE 16TH IEEE PV SPECIALIST CONF., 1982, MOLLER et al., "Low-Level Boron Doping and Light-Induced Effects in Amorphous Silicon PIN Solar Cells", pp. 1376-1380.
| 1. | An amorphous silicon film doped with boron in an amount effective to achieve photoconductivity degradation of less than about fifty percent. |
| 2. | An amorphous silicon film according to claim 1 wherein the amount of boron present in the silicon is less than about 5xl018 atoms/cm3. |
| 3. | An amorphous silicon film according to claim 1 wherein the photoconductivity degradation is less than about fifty percent after exposure to one sun illumination for at least 100 hours. |
| 4. | An amorphous silicon film according to claim 3 wherein the photoconductivity degradation is less than about fifty percent after exposure to one sun illumination for at least 500 hours. |
| 5. | An amorphous silicon film according to claim 4 wherein the photoconductivity degradation is less than about fifty percent after exposure to one sun illumination for at least 1000 hours. |
| 6. | An amorphous silicon film according to claim 1 wherein the photoconductivity degradation is less than about twentyfive percent. |
| 7. | An amorphous silicon film according to claim 6 wherein the film is exposed to one sun illumination for at least 1000 hours. |
| 8. | An amorphous silicon film according to claim 1 wherein the photoconductivity degradation is less than about ten percent. |
| 9. | An amorphous silicon film according to claim 8 wherein the film is exposed to one sun illumination for at least 1000 hours. |
| 10. | An amorphous silicon film according to claim 1 wherein the photoconductivity of the film is increased after exposure to illumination. |
| 11. | An amorphous silicon film according to claim 1 wherein the dark conductivity is less than about 5xlO"10 (ohms cm)"1. |
| 12. | A photovoltaic device comprising: (a) a player doped with electron accepting material; (b) an ilayer in communication with the player, the ilayer being doped with boron in an amount effective to achieve photoconductivity degradation of less than about fifty percent; and (c) an nlayer in communication with the ilayer, the nlayer being doped with an electron donating material. |
| 13. | A photovoltaic device according to claim 12 further comprising a transparent electrode in communication with the player. |
| 14. | A photovoltaic device according to claim 13 further comprising an electrically conductive electrode in communication with the nlayer. |
| 15. | A photovoltaic device according to claim 12 wherein the ilayer is doped with less than about 5xl018 atoms/cm3 boron. |
| 16. | A photovoltaic device according to claim 12 wherein the photoconductivity degradation of the ilayer is less than about fifty percent after exposure to one sun illumination for at least 1000 hours. |
| 17. | A photovoltaic device according to claim 12 further comprising a plurality of pin layers. |
| 18. | A photovoltaic device according to claim 12 wherein the photoconductivity degradation of the ilayer is less than about ten percent after exposure to one sun illumination for at least 1000 hours. |
| 19. | A photovoltaic device according to claim 12 wherein the efficiency of the device decreases less than about 10 percent after exposure to one sun illumination for at least 500 hours. |
| 20. | A photovoltaic device according to claim 19 wherein the efficiency of the device increases after exposure to one sun illumination for a least 500 hours. |
| 21. | A photovoltaic device having an nip structure, the device having an ilayer according to claim 1. |
| 22. | A photovoltaic device according to claim 12 further comprising a buffer layer located between the p layer and the ilayer. |
| 23. | A photovoltaic device according to claim 12 further comprising a buffer layer located between the i layer and the nlayer. |
| 24. | A method for making an amorphous silicon film comprising depositing a film of amorphous silicon on a substrate and doping the film with boron in an amount effective to achieve photoconductivity degradation of less than about fifty percent. |
| 25. | A method according to claim 24 wherein the film is doped with less than about 5xl018 atoms/cm3 boron. |
| 26. | A method according to claim 24 wherein the film is doped with boron in an amount effective to achieve photoconductivity degradation of less than about fifty percent after exposure to one sun illumination for at least 100 hours. |
| 27. | A method according to claim 24 wherein the film is doped with boron in an amount effective to achieve photoconductivity degradation of less than about fifty percent after exposure to one sun for at least 500 hours. |
| 28. | A method according to claim 24 wherein the film is doped with boron in an amount effective to achieve photoconductivity degradation of less than about fifty percent after exposure to one sun for at least 1000 hours. |
| 29. | A method according to claim 24 wherein the film is doped with boron in an amount effective to achieve photoconductivity degradation of less than about twenty five percent after exposure to one sun illumination for at least 100 hours. |
| 30. | A method according to claim 24 wherein the film is doped with boron in an amount effective to achieve photoconductivity degradation of less than about ten percent after exposure to one sun illumination for at least 500 hours. |
| 31. | A method for making a photovoltaic device comprising: (a) depositing a player of amorphous silicon doped with electron accepting material on a substrate; (b) depositing an ilayer of amorphous silicon in communication with the player, the ilayer being doped with boron in an amount effective to achieve degradation of photoconductivity of less than about fifty percent; and (c) depositing an nlayer of amorphous silicon in communication with the ilayer, the nlayer being doped with an electron donating material. |
| 32. | A method according to claim 31 further comprising coupling the player with a transparent electrode and coupling the nlayer with an electrically conducting electrode. |
| 33. | A method according to claim 31 further comprising providing a buffer layer between the player and the ilayer. |
| 34. | A method according to claim 31 wherein the i layer is doped with less than about 5xl018 atoms/cm3 boron. |
| 35. | A method according to claim 31 wherein the i layer is doped with boron in an amount effective to achieve photoconductivity degradation of the ilayer less than about fifty percent after exposure to one sun illumination for at least 500 hours. |
| 36. | A method according to claim 31 wherein the i layer is doped with boron in an amount effective to achieve degradation of efficiency of the device less than about ten percent after exposure to one sun illumination for at least 500 hours. |
| 37. | A method according to claim 31 wherein the i layer is doped with boron in an amount effective to achieve degradation of efficiency of the device less than about ten percent after exposure to one sun illumination for at least 1000 hours. |
FIELD OF THE INVENTION
This invention relates generally to amorphous materials such as silicon and more particularly to high photosensitivity amorphous films.
BACKGROUND OF THE INVENTION
Amorphous materials have found applications in semiconductor devices, including photovoltaic devices, as well as other applications, for at least 15 years. These applications can utilize amorphous materials such as silicon, germanium and one or more of the chalcogenides sulfur, selenium and tellurium. Generally, devices using amorphous materials have been manufactured by deposition of the amorphous material as a thin film on a substrate through chemical vapor deposition and/or plasma enhanced chemical vapor deposition techniques. The amorphous material may be deposited in a p-i-n structure having one or more junctions, and at least one layer of p-doped material, one layer of intrinsic amorphous material, and one layer of n-doped material. These thin film devices have been shown to be useful as photovoltaic solar cells, which are capable of converting solar energy into electricity.
Semiconductor devices made from thin films of amorphous material may generally comprise a p-i-n
structure. The p-layer may be an amorphous material such as silicon doped with an electron accepting material, such as boron. The intrinsic layer (or i-layer) may be a substantially pure amorphous material. The n-layer may be an amorphous material doped with electron donating materials. The i-layer is particularly useful where the p-layer and the n-layer are heavily doped. In such applications, the i-layer is believed to act as a strong photon absorber with a relatively low density of states which is efficient in generating charge carriers between the p-layer and the n-layer.
A well-recognized drawback of such devices is that amorphous materials, in particular silicon, suffer light-induced degradation upon exposure to intense sources of illumination. The inclusion of an i-layer was thought to be a primary source of light-induced degradation of p-i-n devices made from thin films of amorphous material. The degradation reduces the photoconductivity of the film and the efficiency of photovoltaic devices manufactured with the film. This degradation can occur over relatively short periods of time, on the order of tens of hours. While annealing of the film at temperatures of about 150°C may temporarily restore the photoconductivity and efficiency to original levels, it does not prevent immediate re-degradation of the film upon exposure to illumination. Consequently, degradation of amorphous films appears to be a major limiting factor for large scale commercialization of devices made from amorphous materials such as silicon. Light-induced degradation has been thought to be inherent in some amorphous materials. Work has therefore focused on designing multi-junction devices, that is devices having multiple sets of p-i-n layers, having much thinner individual layers, particularly at the top of the
device. The use of thinner layers would minimize the light-induced degradation suffered by the amorphous material due to the increased field strength experienced by the carriers. However, this approach requires complex manufacturing equipment since increasing the number of layers increases the number of deposition chambers that must be used in a multi-chamber system or increases the required cleaning steps between deposition of the various layers to prevent cross-contamination of the layers. Furthermore, multi-junction devices do not address the fundamental problem of degradation.
Other workers have attempted to employ dopant in a p-i-n device to reduce the effects of the degradation. Wronski, et al., Optoelectronic Properties of Boron Compensated Amorphous Silicon Solar Cells. Proceedings of 15th IEEE P.V. Specialists Conference, pp. 341-346 (1984), discloses that introduction of less than 1 ppm of boron in the amorphous silicon network improves the response of the solar cell to long wavelength radiation and adds to their stability on light exposure. However, while Wronski et al. discloses small changes to the photovoltaic properties of the described devices, Wronski et al. also discloses that prolonged illumination on these films leads to significant changes in the dark conductivity and photoconductivity.
OBJECTS AND SUMMARY OF THE INVENTION
In light of the foregoing, it is an object of this invention to provide a light stable amorphous material that has reduced light-induced degradation. It is a further object of this invention to provide high photosensitivity amorphous films which are stable under high intensity light illumination. It is a further object of this invention to provide a light stable amorphous
material which has stable photoconductivity and maintains a high photoconductivity to dark conductivity ratio.
It is a further object of this invention to provide solar cells which have increased stability and reduced light-induced degradation.
It is a further object of this invention to provide a method for making an amorphous material having reduced light-induced degradation. It is a further object of this invention to provide a method for making a photovoltaic device having reduced light-induced degradation. These and other objects of the invention are achieved through amorphous material including a dopant, such as boron, in small amounts effective to reduce light-induced degradation. The dopant preferably is added in the vapor deposition of the amorphous material and results in a concentration less than about 5xl0 18 atoms/cm 3 in the film.
This invention further includes the thin films made with the inventive amorphous materials and devices such as solar cells containing such thin films.
The invention further includes methods for forming the inventive amorphous material and methods for forming thin films and devices containing the amorphous silicon material.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a cross-sectional schematic view of multi-junction solar cells.
Figure 2 is a diagram of an apparatus used in the method of the present invention.
Figures 3 and 4 are graphs of the photoconductivity of films of the present invention versus time of light soaking.
Figure 5 is a graph of the photoconductivity of a
prior art film versus time of light soaking.
Figure 6 is a graph of percentage change of properties of prior art solar cell devices.
Figures 7 and 8 are graphs of the percentage change of properties of solar cell devices using the present invention.
Figure 9 is a cross-sectional schematic view of an amorphous silicon single junction solar cell suitable for use with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In the manufacture of p-i-n amorphous semiconductor devices, each layer can be formed by chemical vapor deposition processes. A preferred method for forming thin film layers of amorphous material is by using plasma enhanced chemical vapor deposition. In this process, a substrate is placed in a vapor deposition chamber. In the instance of amorphous silicon devices, a gaseous source of silicon, such as silane (SiH or disilane (Si 2 H 6 ) , is introduced into the chamber and subjected to a chemical reaction through activation by the energy present in a plasma, and typically heat. Where silane is utilized as the silicon source, the silane is decomposed and silicon and/or silicon carbide species are deposited on the substrate. The hydrogen acts to terminate dangling bonds in the deposited silicon structure and reduce the density of defects inherently present in amorphous materials deposited by conventional techniques. This enhances carrier mobility lifetime product. Other compounds, such as SiHCl 3 and SiF and others, can also be used as a source of high purity silicon material.
The p-layer can be formed by carrying out a deposition process in the presence of electron accepting material, such as boron in the form of diborane, B 2 H 6 , or
trimethyl boron, B(CH 3 ) 3 . The p-layer may be formed from amorphous material, or other materials such as polycrystalline or microcrystalline materials. The p- layer may further comprise materials such as silicon carbide, alone or in combination with amorphous silicon. The n-layer can also be formed by chemical vapor deposition techniques wherein electron donating material such as PH 3 is present with the silane during silicon deposition. The n-layer may be formed from amorphous material, or other materials such as polycrystalline or microcrystalline materials.
Referring to Figure 1, a multi-junction solar cell 110 used in prior art devices is shown. The cell contains a first p-i-n junction, having a p-layer 112, an i-layer 114 and an n-layer 116. A second p-i-n junction contains a p-doped film 118, an intrinsic layer 120 and an n-doped layer 122. A third p-i-n junction contains p-layer 124, i-layer 126, and n-layer 128. The cell contains a transparent electrode 130, which is located above the first p-layer and adjacent a source of illumination.
Electrode 130 preferably is made from tin oxide or zinc oxide. Electrode 130 has a high transparency, preferably approximately 80 percent, and low sheet resistivity, preferably less than about 10 Q/square. Electrode 130 may also be doped with a dopant such as boron, fluorine or aluminum. Cell 110 has a substrate 132 located below the last n-layer, which is preferably made from aluminum, zinc oxide, silver, stainless steel or a combination thereof. The layers preferably each comprise a film of amorphous material such as silicon, a silicon alloy or germanium. The p-layer preferably is approximately 100 Angstroms thick, the n-layer preferably 300 Angstroms thick, and the i-layer preferably approximately 1000-8000
Angstroms thick, and more preferably 3000-5000 Angstroms. The layers are deposited in a p-i-n arrangement on a substrate 132 preferably made from aluminum, silver or stainless steel. Substrate 132 has a high degree of back reflection to effectively use incident light. As noted above, these devices are complex and may only minimize the effects of degradation in use. Prior art multi- junction cells suffered degradation of efficiency of approximately 10-15 percent after 600 hours of illumination.
As used in this application, cell efficiencies and degradation are measured according to SERI (Solar Energy Research Institute, Colorado) standards. That is, cell efficiencies are measured with respect to the following conditions: AM 1.5 global solar reference spectrum, total illumination intensity of 1000 watts/meter 2 , and device temperature of 25°C, using recommended ASTM measurement procedures. A stable efficiency is defined as the efficiency achieved after 600 hours of continuous exposure to light having a spectral composition corresponding approximately to the AMI.5 global spectrum and an intensity of 1000 W/m 2 at a module temperature of 50°C under maximum power load conditions.
In the present invention, the i-layer is doped with a small amount of dopant, preferably an electron accepting material, which preferably is boron. Other materials which may be useful as dopants with amorphous materials include phosphorous (such as in the form PH 3 ) , xenon, fluorine, aluminum, gallium, indium and thallium. An i-layer doped according to the present invention is more stable and resistant to the light-induced degradation suffered by pure amorphous silicon. The photoconductivity of layers of amorphous material doped in accordance with the present invention degrade less
than about 50 percent after 1000 hours of exposure to one sun illumination. One sun is defined as 100 mW/cm 2 (or 1000W/m 2 ) . The photoconductivity of layers produced in accordance with the present invention preferably degrade less than ten percent, and may increase after exposure to illumination. This lessened decrease, or increase, of photoconductivity in accordance with the present invention yield single or ulti-junction cells whose efficiencies degrade to a lesser degree than those known in the art.
Doping can be carried out in a deposition chamber by introduction of a dopant gas in the chemical vapor deposition of the i-layer on a substrate. Other methods of adding the dopant to the i-layer may include ion implantation, thermal diffusion and other methods known in the art. In a vapor deposition process, the concentration of the dopant should be effective to achieve the beneficial increased stability of the final product. Preferably, the dopant is present in the gaseous state such that films formed according to the invention contain less than about 5xl0 18 atoms/cm 3 boron, as measured using secondary ion mass spectroscopy (SIMS) . More preferably the films contain from about 6xl0 16 to about 5xl0 18 atoms/cm 3 boron, and more preferably from about lxl0 17 to about lxl0 18 atoms/cm 3 boron.
The stability of the i-layer formed by the present invention permits practical use of a single junction cell. While still useful with the present invention, multi-junction cells are not necessary to achieve increased stability. The i-layers formed according to the present invention exhibit stable photoconductivity and photoconductivity to dark conductivity ratio. The efficiency of the layer is therefore stabilized after exposure to illumination. The photoconductivity and
efficiency may actually increase over time after treatment according to the present invention.
The i-layers formed with the present invention may be useful not only in photovoltaic devices, but also xerography drums and facsimile elements, as well as other applications which require a gray scale response and/or stable photoconductivity. Other useful applications may include thin film transistors, particle detectors, random access memory devices and other devices where stable electronic properties are useful.
Referring to Figure 2, formation of a more stable i- layer according to the present invention can be carried out as follows. A substrate 10, preferably made from glass coated with tin oxide, zinc oxide or ITO (indiu - tin oxide) , is positioned in a vapor deposition system 12. System 12 comprises an outer chamber 14 and an inner chamber 16. Inner chamber 16 is coupled by line 18 to branch 19 to vacuum gauge 28 and branch 20, through throttle valve 26 to branches 29A, 29B and 29C. Branch 29A runs from argon bleed line 98, which contains valve 98A, enabling argon to be flushed through the system to remove any contaminants. Branch 29B runs through rotary pump 32 to gas scrubber 34 and exits line 36 to exhaust. Branch 29C runs to vacuum gauge 38. Pressure in the inner chamber 16 is measured and maintained by vacuum gauge 28. Heater 48 and thermocouple 50 control the temperature of inner chamber 16, preferably at a temperature of about 180-260°C. View window 52 permits an operator to observe the inner chamber during processing.
An ion gauge 40 communicates with outer chamber 14 through valve 42, which preferably is a butterfly valve. The temperature in outer chamber 14 is maintained, preferably at about 150°C, by thermocouple 44 through the
use of heater 46.
A power source, preferably an RF source, communicates with inner chamber 16 through line 54. The power source comprises an RF generator 56, a matching network 58 and a power meter 60.
The outer chamber 14 may be provided with argon, or other inert materials from source 62 through line 64, which may contain suitable valving such as valve 63. A nitrogen source 66 preferably communicates through line 68, which may contain suitable valving such as valve 68A, to inner chamber 14. Line 70, provided in outer chamber 14, leads to branches 72 and 74. Branch 72 leads through valve 75 to pump 76, which preferably is a rotary pump, and thereafter to exhaust through line 78. Branch 74 leads through valve 77 to pump 79, which preferably is a turbo olecular pump, and through valve 80 to pump 76.
The source of amorphous material, dopant and inert material can be provided to inner chamber 16 through line 82, which may contain suitable valving such as valve 82A. The temperature in the line is maintained and controlled by heater 83 and thermocouple 84. Line 82 preferably communicates with a source of amorphous material 85, such as silicon in the form of silane or disilane through line 86. Dopant sources 87 and 88 communicate with line 82 through lines 89 and 90, respectively. Preferably, one dopant source comprises an electron donating material (n- dopant) and one source comprises an electron accepting material (p-dopant) . Hydrogen source 91 communicates through line 92 with line 82. Line 82 may also communicate with other sources of material conventionally utilized in deposition techniques, such as methane etc. Each of the lines in communication with line 82 may be provided with suitable valving or other flow controlling means such as valves 62A, 92A, 86A, 89A and 90A..
In operation, to deposit a doped i-layer according to the present invention, first the system is "roughed down" using pump 76, followed by turbomolecular pump 79 at a high vacuum, preferably 10 "7 Torr to evacuate the system. Pump 79 is then bypassed by turning off valve 77 and opening valve 75 to rotary pump 76, which operates to achieve a pressure of approximately 1 Torr. Argon is then pumped into the system, through lines 64 and 82, for a time sufficient to remove any dopant or other potential contaminants in the inner and outer chambers 14 and 16. Heater 46 and turbomolecular pump 79 are then turned on for a period of time sufficient to remove any residual water vapor present in the system, preferably for about 2 hours. When the chamber is ready for deposition, pump 79 is bypassed by closing valve 77 and opening valve 75. A source of amorphous material such as silicon, preferably in the form of silane, is introduced into the chamber through line 82 to achieve a pressure of preferably about 150 millitorr. Preferably, the silicon source is silane. A source of dopant, preferably boron in the form of diborane or trimethyl boron, is also introduced into the inner chamber 16 through line 82. The boron present in the inner chamber 16 should be sufficient to dope the silicon film with boron in an amount less than about 5xl0 18 atoms/cm 3 . Generator 56 is then turned on to generate about 1 watt of power in inner chamber 16. The power is preferably from an RF source at a frequency of about 13.56 Mhz. Deposition preferably takes place at a temperature of about 180-260°C and a pressure of about 150 millitorr. Deposition preferably takes place at a rate of less than 5 Angstrom/second, and more preferably at a rate of approximately 1 Angstrom/second. For a typical i-layer
used in photovoltaic devices, deposition is accomplished in approximately 1-2 hours. Generator 56 is turned off and substrate 10 is removed from the chamber.
In the manufacture of p-i-n devices according to the present invention, the p-layer is first deposited on the substrate, preferably in a layer approximately 100 Angstroms wide. The inner chamber is cleaned of any dopant using argon source 62, and the i-layer is deposited on the substrate in the manner described above. The n-layer is then deposited on the substrate over the i-layer. The n-layer preferably is approximately 300 Angstroms thick. Devices having an n-i-p structure may also be made in accordance with the present invention. An alternate apparatus which may be useful in practicing the present invention is disclosed in U.S. Patent No. 4,576,830, whose disclosure is incorporated herein by reference. Likewise, other known apparatus for deposition of amorphous materials may be used in accordance with the present invention. The stability of films made in accordance with the present invention is improved over the prior art, as is shown in the following Table I. Table I illustrates the stability in photoconductivity and dark conductivity achieved through the present invention. In particular, samples 2 and 3 are almost completely stable over a period of 1000 hours. By contrast, the photoconductivity of Sample 4, which does not utilize the present invention, degraded over one order of magnitude after prolonged exposure to illumination. The samples were placed on a transparent sheet and subjected to light at the intensities described in the Table for a period of time. The temperature was kept constant. The samples were approximately 2 inches x 2 inches in size.
Figures 3 to 5 demonstrate the improved stability of amorphous i-layers of the present invention. These Figures graph the photoconductivity of an i-layer made in accordance with the present invention versus time of light soaking. Figures 3 and 4 demonstrate that over time, the photoconductivity of films according to the present invention remains stable. Figure 5 is a graph of the photoconductivity of a prior art film. The film shows immediate degradation of photoconductivity upon exposure
to illumination.
Semiconductor devices made with i-layers of the present invention likewise exhibit improved resistance to light-induced degradation. Figures 6 to 8 graph the percentage change in the properties of devices versus time of exposure to illumination. Figure 6 shows the degradation of two p-i-n devices made without the benefits of the present invention. Voc is the open circuit voltage of the device, Jsc is the short circuit current, FF is the fill factor, and Eff is the efficiency of the device. For the prior art device whose properties are graphed in Figure 6, degradation in efficiency is seen almost immediately. The efficiency of the prior art device degraded by 30 to 40 percent over time. By contrast, Figures 7 and 8 depict the effect of light soaking on devices made according to the present invention. Efficiency actually increased initially after light soaking of both devices, and, as seen in Figure 7, even after 500 hours of light soaking the efficiency of one device was higher than the baseline level. The device of Figure 8 had an efficiency decrease of less than 5 percent after 500 hours of light soaking.
A single junction cell 140, which can be manufactured using stable i-layers of the present invention, is shown in Figure 9. The cell 140 has a p- layer 142, a stable i-layer 144 and an n-layer 146. The p-layer has adjacent thereto a transparent electrode 143 preferably made from zinc oxide or tin oxide. The electrode 143 has a high transparency, preferably on the order of 80 percent, and low sheet resistivity. The n- layer is coupled to a metal electrode 145, preferably made from zinc oxide and/or aluminum. The p-i-n structure may further include any other layers or elements known in the art, such as a buffer layer 147 made from silicon
carbide or other suitable material between the p-layer and the i-layer and/or between the i-layer and the n- layer. It should also be noted that the present invention, while preferred to include single junction devices, may also include multi-junction devices such as those depicted in Figure 1. Photovoltaic devices produced in accordance with the present invention can be of virtually any initial efficiency as is presently available in the art, and the initial efficiency will remain stable after exposure to illumination.
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