Patent Application
20140261689
The present disclosure relates to a photovoltaic device, and more particularly to a photovoltaic device having an improved efficiency and methods of forming the same.
Photovoltaic modules, devices, or cells, can include multiple layers (or coatings) created on a substrate (or superstrate). For example, a photovoltaic device can include a barrier layer, a transparent conductive oxide layer, a buffer layer, and a semiconductor layer formed in a stack on a substrate. Each layer may in turn include more than one layer or film. For example, a semiconductor window layer and a semiconductor absorber layer together can be considered a semiconductor layer. Additionally, each layer can cover all or a portion of the device and/or all or a portion of a layer or a substrate underlying the layer. For example, a “layer” can include any amount of any material that contacts all or a portion of a surface. Cadmium telluride has been used for the semiconductor layer because of its optimal band structure and a low cost of manufacturing.
The window layer is typically the top layer in a single-junction photovoltaic cell of a photovoltaic device formed from a high band gap material to allow transmittance of sunlight to underlying layers of the device. Annealing processes involving CdCl2 known in the art may affect a thickness and/or performance of window layers formed from CdS. Thus, the annealing process must be tailored to minimize prolonged exposure of the CdS window layer to CdCl2 and/or the elevated temperatures of the annealing process. Furthermore, a photovoltaic device having a window layer formed from a material with a band gap greater than that of CdS would result in a more efficient photovoltaic device. Accordingly, it would be desirable to develop a more efficient photovoltaic device.
Concordant and congruous with the instant disclosure, a more efficient photovoltaic device has surprisingly been discovered.
In an embodiment of the invention, A photovoltaic device comprises a substrate; a zinc-containing layer disposed on the substrate; and a CdTe semiconductor layer disposed adjacent to the zinc-containing layer.
In another embodiment of the invention, a photovoltaic device comprises a substrate; a ZnO1-xSx layer disposed on the substrate; and a CdTe layer disposed adjacent to the ZnO1-xSx layer.
In another embodiment of the invention, a method of forming a photovoltaic device comprises the steps of applying a ZnO1-xSx layer on a substrate; and applying a semiconductor layer on the ZnO1-xSx layer, wherein the ZnO1-xSx layer is one of a window layer and a buffer layer.
The above, as well as other advantages of the present invention, will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment when considered in the light of the accompanying drawings in which:
The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should also be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features. In respect of the methods disclosed, the order of the steps presented is exemplary in nature, and thus, is not necessary or critical unless recited otherwise.
Referring now to the drawings, there is illustrated in
The photovoltaic device 10 of
As shown in
For purposes of simplicity in illustrating the invention, only the substrate 12, the TCO layer 14, the ZnO1-xSx window layer 16, the absorber layer 18, and the back contact layer 24 are shown. However, one of ordinary skill in the art should understand that one or more additional layers including, but not limited to, a barrier layer, additional buffer layers, a dielectric layer, and the like, formed from the same or different materials, may also be used in the photovoltaic device 10 of the present disclosure.
By using the ZnO1-xSx window layer 16 instead of a traditional window layer, such as CdS which has a lower quantum efficiency and absorbs at least some of the light in the blue light spectrum (around about 475 nm), a quantum efficiency of the photovoltaic device 10 in the blue light spectrum is improved. The ZnO1-xSx window layer 16 has improved optical transmission over that of CdS, thereby resulting in the improved photovoltaic device 10. A graph of quantum efficiency versus light wavelength is shown in
Furthermore, compared to a band gap of approximately 2.43 eV for CdS, the band gap of the ZnO1-xSx window layer 16 may be selectively varied between about 2.55 eV and about 3.60 eV by altering the stoichiometry of the ZnO1-xSx window layer 16, as best shown in
By selectively varying and increasing the band gap of the ZnO1-xSx window layer 16 by altering the stoichiometry thereof, the optical response of the photovoltaic device 10 may be tuned and the conduction band offset optimized. Another limitation of using a CdS window layer is the lack of control over the rate flux of the material once a CdTe/CdS structure is exposed to CdCl2 and annealed, as known in the art. A more aggressive CdCl2 annealing step at elevated temperatures and repeated heat treatments has been shown to improve the quality of the CdTe semiconductor layer, but prolonged exposure to the CdCl2 and prolonged exposure to elevated temperatures may lead to a minimized CdS window layer, thus affecting the performance of the photovoltaic device. However, due to a relatively higher melting point, the ZnO1-xSx window layer 16 provides a material more resistant to dissolution or interaction with adjacent layers of the photovoltaic device 10 during the CdCl2 annealing process. Consequently, a more aggressive CdCl2 annealing process may be used that prolongs the absorber layer 18 exposure to CdCl2 and elevated temperatures may be used to result in an improved absorber layer 18 without adversely affecting the ZnO1-xSx window layer 16.
The band gap of the ZnO1-xSx window layer 16 may be selectively varied between about 2.55 eV and about 3.60 eV by manufacturing processes for controlling the stoichiometry thereof (see
Using an RF sputtering process, the ZnO1-xSx window layer 16 is formed by using a ceramic ZnS target with Ar and O2 as working and reactive gases, respectively. With the substrate 12 between about 300° C. and about 400° C. and an RF power at about 300 W, the ZnO1-xSx window layer 16 may have a thickness of between about 100 nm and about 700 nm. Using an ALD process, the TCO layer 14 is exposed to a zinc precursor (e.g., diethyl zinc) and subsequently converted to ZnO using a water pulse (or other oxygen containing fluid) and an H2S pulse (or other sulfur containing fluid) for further inclusion of sulfur ions in the ZnO1-xSx window layer 16. By controlling the duration of the pulses, the stoichiometry of the ZnO1-xSx window layer 16 may be controlled. Due to the nature of the ALD process, typical growth of the ZnO1-xSx crystals may occur at relatively low temperatures from about 100° C. to about 200° C., thus minimizing costs associated with processing at higher temperatures.
Using an APCVD process, a temperature of the substrate 12 is typically higher than temperatures used for RF sputtering processes and ALD processes. For the APCVD process, an upper limit of the temperatures use during the APCVD process are around a softening temperature of the material forming the substrate 12 (e.g., soda lime glass). Thus, the tolerance of the ZnO1-xSx window layer 16 to excessive temperatures such as temperatures approaching the softening temperature of the substrate 12 facilitates the use of the APCVD process to result in a photovoltaic device 10 as described herein.
The photovoltaic device 210 of
As shown in
For purposes of simplicity in illustrating the invention, only the substrate 212, the TCO layer 214, the ZnO1-xSx buffer layer 216, the absorber layer 218, and the back contact layer 224 are shown. However, one of ordinary skill in the art should understand that one or more additional layers including, but not limited to, a barrier layer, additional buffer layers, a dielectric layer, and the like, formed from the same or different materials, may also be used in the photovoltaic device 210 of the present disclosure.
By using the ZnO1-xSx buffer layer 216, a quantum efficiency of the photovoltaic device 210 in the blue light spectrum may be improved. A graph of quantum efficiency versus light wavelength is shown in
The ZnO1-xSx buffer layer 216 also may provide a more resistive material to fluxing or interaction with adjacent layers of the photovoltaic device 210 during the CdCl2 annealing process. Consequently, a more aggressive CdCl2 annealing process may be used that prolongs the exposure of the absorber layer 218 to CdCl2 and elevated temperatures may be used to result in an improved absorber layer 218 without adversely affecting the ZnO1-xSx buffer layer 216 and underlying n-type semiconductor layer 220 and/or p-type semiconductor layer 222.
In one embodiment of the invention, the ZnO1-xSx buffer layer 216 may be grown in crystalline form of the Wurtzite variety on the photovoltaic device using one of the following: an RF sputtering process, a chemical vapor deposition from gas phases process (e.g., APCVD, LPCVD, MOCVD), through vaporization (e.g., close-spaced vaporization, VTD), pyrolysis process, or atomic layer deposition (ALD) process.
Using an RF sputtering process, the ZnO1-xSx buffer layer 216 is formed by using a ceramic ZnS target with Ar and O2 as working and reactive gases, respectively. With the substrate 212 between about 300° C. and about 400° C. and an RF power at about 300 W, the ZnO1-xSx buffer layer 216 may have a thickness of between about 100 nm and about 700 nm. Using an ALD process, the TCO layer 214 is exposed to a zinc precursor (e.g., diethyl zinc) and subsequently converted to ZnO using a water pulse (or other oxygen containing fluid) and an H2S pulse (or other sulfur containing fluid) for further inclusion of sulfur ions in the ZnO1-xSx buffer layer 216. By controlling the duration of the pulses, the stoichiometry of the ZnO1-xSx buffer layer 216 may be controlled. Due to the nature of the ALD process, typical growth of the ZnO1-xSx crystals may occur at relatively low temperatures from about 100° C. to about 200° C., thus minimizing costs associated with processing at higher temperatures.
Using an APCVD process, a temperature of the substrate 212 is typically higher than temperatures used for RF sputtering processes and ALD processes. For the APCVD process, an upper limit of the temperatures use during the APCVD process are around a softening temperature of the material forming the substrate 212 (e.g., soda lime glass). Thus, the tolerance of the ZnO1-xSx buffer layer 216 to excessive temperatures such as temperatures approaching the softening temperature of the substrate 212 facilitates the use of the APCVD process to result in a photovoltaic device 210 as described herein.
The photovoltaic device 310 of
As shown, the zinc-containing layer is a ZnO1-xSx layer 316 disposed between an n-type semiconductor layer 320 and a p-type semiconductor layer 322. The ZnO1-xSx layer 316 forms a composite window layer with the n-type semiconductor layer 320. The band gap of the ZnO1-xSx layer 316 may be tuned by affecting a ratio of S/O. The ZnO1-xSx layer 316 may be deposited using a number of deposition techniques, including ALD, APCVD, sputtering, and CBD. Sulfur content may be controlled as a function of deposition technique and deposition conditions, as desired. The ZnO1-xSx layer 316 may be grown in crystalline form of the Wurtzite type using RF sputter techniques, APCVD, LPCVD, MOCVD, through vaporization (close spaced vaporization, vapor transport deposition), pyrolysis, or atomic layer deposition (ALD).
As shown in
For purposes of simplicity in illustrating the invention, only the substrate 312, the TCO layer 314, the ZnO1-xSx layer 316, the absorber layer 318, and the back contact layer 324 are shown, However, one of ordinary skill in the art should understand that one or more additional layers including, but not limited to, a barrier layer, additional buffer layers, a dielectric layer, and the like, formed from the same or different materials, may also be used in the photovoltaic device 310 of the present disclosure.
By using the ZnO1-xSx layer 316, a quantum efficiency of the photovoltaic device 310 in the blue light spectrum may be improved. A graph of quantum efficiency versus light wavelength is shown in
The ZnO1-xSx layer 316 also may provide a more resistive material to fluxing or interaction with adjacent layers of the photovoltaic device 310 during the CdCl2 annealing process. Consequently, a more aggressive CdCl2 annealing process may be used that prolongs the exposure of the absorber layer 318 to CdCl2 and elevated temperatures may be used to result in an improved absorber layer 318 without adversely affecting the ZnO1-xSx layer 316 and underlying n-type semiconductor layer 320 and/or p-type semiconductor layer 322.
In one embodiment of the invention, the ZnO1-xSx layer 316 may be grown in crystalline form of the Wurtzite variety on the photovoltaic device using one of the following: an RF sputtering process, a chemical vapor deposition from gas phases process (e.g., APCVD, LPCVD, MOCVD), through vaporization (e.g., close-spaced vaporization, VTD), pyrolysis process, or atomic layer deposition (ALD) process.
Using an RF sputtering process, the ZnO1-xSx layer 316 is formed by using a ceramic ZnS target with Ar and O2 as working and reactive gases, respectively. With the substrate 312 between about 300° C. and about 400° C. and an RF power at about 300 W, the ZnO1-xSx layer 316 may have a thickness of between about 100 nm and about 700 nm. Using an ALD process, the TCO layer 314 is exposed to a zinc precursor (e.g., diethyl zinc) and subsequently converted to ZnO using a water pulse (or other oxygen containing fluid) and an H2S pulse (or other sulfur containing fluid) for further inclusion of sulfur ions in the ZnO1-xSx layer 316. By controlling the duration of the pulses, the stoichiometry of the ZnO1-xSx layer 316 may be controlled. Due to the nature of the ALD process, typical growth of the ZnO1-xSx crystals may occur at relatively low temperatures from about 100° C. to about 200° C., thus minimizing costs associated with processing at higher temperatures.
Using an APCVD process, a temperature of the substrate 312 is typically higher than temperatures used for RF sputtering processes and ALD processes. For the APCVD process, an upper limit of the temperatures use during the APCVD process are around a softening temperature of the material forming the substrate 312 (e.g., soda lime glass). Thus, the tolerance of the ZnO1-xSx layer 316 to excessive temperatures such as temperatures approaching the softening temperature of the substrate 312 facilitates the use of the APCVD process to result in a photovoltaic device 310 as described herein.
While certain representative embodiments and details have been shown for purposes of illustrating the invention, it will be apparent to those skilled in the art that various changes may be made without departing from the scope of the disclosure, which is further described in the following appended claims.
This application claims the benefit of priority of U.S. Provisional Patent Application Ser. No. 61/789,281 filed on Mar. 15, 2013 hereby incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61789281 | Mar 2013 | US |
