METHOD AND APPARATUS FOR FORMING A TRANSPARENT CONDUCTIVE OXIDE USING HYDROGEN
A method and apparatus for forming a crystalline cadmium stannate layer of a photovoltaic device by heating an amorphous layer in the presence of hydrogen gas.
This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 61/606,512 filed on Mar. 5, 2012, which is hereby incorporated by reference in its entirety.
TECHNICAL FIELDDisclosed embodiments relate to the field of photovoltaic devices, which include photovoltaic cells and photovoltaic modules containing a plurality of cells, and more particularly, to a method and apparatus for forming a transparent conductive oxide using hydrogen gas.
BACKGROUNDPhotovoltaic devices can include semiconductor material deposited over a substrate such as glass, for example, with a first layer of the semiconductor material serving as a window layer and a second layer of the semiconductor material serving as an absorber layer. The semiconductor window layer forms a junction with the semiconductor absorber layer where incident light is converted to electricity.
Photovoltaic devices can also include a transparent conductive oxide (“TCO”) layer to conduct electrical charge. One TCO material which is often used is crystalline cadmium stannate. This is because of crystalline cadmium stannate's low sheet resistance and high light transmissivity.
One conventional method of forming crystalline TCO layer is to deposit an amorphous layer of cadmium and tin oxide onto a substrate and to then transform the deposited amorphous layer to a crystalline form. This is done by annealing the amorphous layer at a high temperature (e.g., typically a temperature greater than 550° C.), in a low oxygen partial pressure environment (i.e., an oxygen-deficient or reduced atmosphere) for a sufficient amount of annealing time (e.g., at least 10 minutes).
To provide the low oxygen partial pressure environment, current photovoltaic device manufacturing processes advocate forming the semiconductor window layer, which may be made of cadmium sulfide, on the amorphous TCO layer before it is annealed. Doing so deprives the amorphous layer of oxygen that may be available in an ambient processing atmosphere. In addition, the cadmium sulfide layer over the amorphous TCO layer encourages any oxygen that may be present in the amorphous TCO layer to diffuse out of it. Specifically, oxygen that diffuses out of the amorphous TCO layer may react with the cadmium sulfide to form cadmium oxide which may evaporate at temperatures of about 600° C. and above and sulfur dioxide which will diffuse into the deposition ambient. This reaction then produces oxygen vacancies in the amorphous layer. Each oxygen vacancy acts as an electron donor which, once the amorphous TCO layer is transformed to a crystalline form, helps with electrical conductivity. Thus, the window layer is used as a reducing agent because it creates the needed oxygen-deficient atmosphere that promotes the oxygen vacancies in the TCO layer.
However, aiming the cadmium sulfide window layer on the TCO layer before the amorphous TCO layer is annealed requires a longer annealing time, or a higher annealing temperature or both than would have been needed otherwise to transform the amorphous layer to the crystalline form. Using high temperatures for long periods of time can damage glass substrates. For example, glass substrates will often begin to soften at a temperature of about 550° C. and above. Thus, subjecting the glass substrates to such a high annealing temperature (i.e., greater than 550° C.) for such a relatively long time (i.e., 10 minutes or more), increases the risk of damaging the substrates. Specifically, the glass substrates may begin to soften and warp at the high annealing temperatures applied for such long annealing periods of time. Further, the high annealing temperature has a tendency to ionize sodium atoms or molecules present in the glass substrates, which over time may diffuse to other layers of the devices. Diffusion of sodium ions in certain layers of the devices may adversely affect device performance. In addition, a long annealing time decreases productivity and also subjects the annealing chamber to conditions favorable to chamber degradation, which can require remediation. Finally, the high annealing temperature used to transform the amorphous layer into crystalline is one of many high temperatures to which the devices may be subjected while being processed. For example, other layers have to be annealed at high temperatures. Thus, the devices may be subjected to a plurality of high thermal cycles. These thermal cycles may weaken the glass and subject it to a high degree of breakage.
Accordingly, a method of transforming an amorphous TCO layer to a crystalline form which mitigates against these potential problems is desired.
In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments that may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to make and use them, and it is to be understood that structural, logical, or procedural changes may be made to the specific embodiments disclosed without departing from the spirit and scope of the invention.
Embodiments described herein provide for a method of forming a TCO layer by heat-treating or annealing an amorphous TCO layer of cadmium and tin oxide, for example, in the presence of hydrogen to transform the amorphous TCO layer to at least a partially, if not completely, crystalline form. According to a disclosed embodiment, the amorphous TCO layer may be annealed before formation of the window layer. This allows for much lower annealing temperatures. Amorphous TCO layer annealing in the presence of hydrogen may occur in the same environment as, for example, a multiple-zone oven, but prior to, a semiconductor deposition process, for example, vapor transport deposition, close space sublimation, evaporation, sputtering or other semiconductor deposition process.
Further, just as in the case of using cadmium sulfide as the reducing agent, the hydrogen gas is also used as a reducing agent. For example, the hydrogen gas, similarly to the cadmium sulfide, shields the amorphous TCO layer from any oxygen present in the deposition environment and thus creates an oxygen-deficient environment. In addition, the hydrogen gas diffuses into the amorphous TCO layer where it reacts with oxygen within the amorphous TCO layer to form water on or within the amorphous TCO layer which evaporates during the annealing process. Again, just as in the case of using cadmium sulfide as the reducing agent, oxygen molecules that have reacted with the hydrogen gas will produce vacancies in the amorphous TCO layer. These oxygen molecules will act as electron donors which, once the amorphous TCO layer is transformed to a crystalline form, will help with electrical conductivity.
Referring now to
The TCO stack 170 may be formed adjacent to the substrate layer 110 and may include a plurality of layers. For example, the TCO stack 170 may include a barrier layer 120 adjacent to the substrate layer 110, an amorphous TCO layer 130 adjacent to the barrier layer 120, and a buffer layer 140 adjacent to the amorphous TCO layer 130, though the buffer layer 140 may be omitted. The barrier layer 120 is used to lessen diffusion of sodium or other contaminants from the substrate layer 110 to other layers of the device 100. These other layers may include layers of semiconductor material 180 (see
Layers 120, 130 and 140 of the TCO stack 170 can be formed using any suitable technique, such as sputtering, for example, as described in U.S. patent application Ser. No. 12/860,115, entitled “Doped Transparent Conductive Oxide,” filed on Aug. 20, 2010, which is hereby incorporated by reference in its entirety. The layers of the TCO stack 170 may also be formed using other deposition techniques, such as, for example, low pressure chemical vapor deposition, atmospheric pressure chemical vapor deposition, plasma-enhanced chemical vapor deposition, thermal chemical vapor deposition, spin-on deposition, and spray-pyrolysis.
The amorphous TCO layer 130 can be of any suitable thickness. For example, amorphous TCO layer 130 can have a thickness of about 10 nm to about 1000 nm. The amorphous TCO layer 130 can include any ratio of cadmium to tin suitable for the resulting cadmium stannate. For example, the cadmium-to-tin atomic ratio can be about 2:1. The amorphous TCO layer 130 can also have any surface roughness (Ra) as well as any suitable average optical absorption. The amorphous TCO layer 130 may have an average optical absorption of less than about 20% in the wavelength range of about 400-850 nm, and a surface roughness of less than about 1 nm.
Following formation of the TCO stack 170 (
Referring to
The transformation zone illustrated generally as 403 in
The diffuser 260 may be omitted and the hydrogen gas/mixture may be introduced via one or both of input lines 240, 250 and diffuse within the transformation zone 403 under ambient conditions. The rate of travel of coated substrate 401 through transformation zone 403 allows the coated substrate 401 to be in the transformation zone 403 for a long enough time for the conversion of the amorphous material to crystalline to occur.
The transformation zone 403 may include one or more heaters 230 to bring the temperature up to as well as to maintain a desired processing temperature (i.e., a temperature between 500° C. to 650° C.). The heating can be conducted anywhere from 3 minutes to 25 minutes, depending on the temperature used. As an example, the coated substrate 401 can be heated for about 25 minutes at about 500° C., or for about 3 minutes at about 650° C. The heating provided by the one or more heaters 230 can provide radiated heating, convective heating, and/or resistive heating.
The multiple-zone oven 400, 500 may be a controlled ambient oven, in which load/exit locks, i.e., a chamber or zone that includes one or more doors, or gas separation curtains, i.e., fast-moving streams of inert gas, provided in entry and exit zones 402 and 406, for example, may be used to keep hydrogen gas/mixture inside the ovens 400, 500.
Referring again to
Referring again to
Now referring again to
Referring to
For that TCO conversion using hydrogen gas, 2,000 sccm (standard cubic centimeter) flow of 2.9% hydrogen gas diluted with helium and 4,000 sccm pure helium were sourced into transformation zone 403 to reach a 0.97% hydrogen gas concentration in the transformation zone 403. The use of hydrogen gas therefore permits a significant reduction in TCO annealing temperature. Note that the magnitude of the reduction can be changed by adjusting the hydrogen concentration in the transformation zone.
The embodiments described above are offered by way of illustration and example. It should be understood that the examples provided above may be altered in certain respects and still remain within the scope of the claims. It should be appreciated that, while the invention has been described with reference to the above example embodiments, other embodiments are within the scope of the claims. It should also be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various features and basic principles of the invention.
Claims
1-18. (canceled)
19. An apparatus for forming a crystalized cadmium stannate layer of a photovoltaic device, comprising:
- a multiple-zone oven comprising separate transformation and semiconductor zones;
- at least one heater for heating the transformation zone;
- a transport mechanism for transporting a substrate containing an amorphous layer containing cadmium and tin through the multi-zone oven;
- a first gas source for supplying hydrogen gas into the transformation zone for contact with the transported substrate.
20. The apparatus of claim 19, wherein the semiconductor zone is configured for a semiconductor deposition technique selected from the group consisting of vapor transport deposition, close space sublimation, evaporation and sputtering.
21. The apparatus of claim 20, wherein the semiconductor zone is configured for vapor transport deposition.
22. The apparatus of claim 19, further comprising a diffuser arranged inside the transformation zone for diffusing the hydrogen gas.
23. The apparatus of claim 19, wherein the first gas source is configured to supply a gas mixture comprising hydrogen gas and an inert gas into the tranformation zone for contact with the transported substrate.
24. The apparatus of claim 23, wherein the inert gas comprises one of argon, nitrogen and helium.
25. The apparatus of claim 24, wherein the inert gas comprises helium.
26. The apparatus of claim 19, further comprising a second gas source for supplying an inert gas into the transformation zone.
27. The apparatus of claim 26, wherein the inert gas comprises one of argon, nitrogen and helium.
28. The apparatus of claim 27, wherein the inert gas comprises helium.
29. The apparatus of claim 19, wherein the at least one heater is configured to heat the transformation zone to a temperature of about 500° C. to about 650° C.
30. The apparatus of claim 19, wherein the hydrogen gas converts the amorphous layer into a crystalized cadmium stannate layer.
31. The apparatus of claim 30 further comprising a sputtering apparatus in the transformation zone for depositing a buffer layer over the crystalized cadmium stannate layer.
32. The apparatus of claim 19, wherein the transformation zone contains hydrogen gas at a concentration of about 0.01% to about 10%.
33. The apparatus of claim 31, wherein the hydrogen gas concentration is about 1%.
Type: Application
Filed: Dec 28, 2015
Publication Date: May 11, 2017
Inventors: Benyamin Buller (Sylvania, OH), Markus Gloeckler (Perrysburg, OH), David Hwang (Perrysburg, OH), Rui Shao (Sylvania, OH), Zhibo Zhao (Novi, MI)
Application Number: 14/981,258