Patent Application
20120100663
The present invention relates to fabrication of a CZTSSe thin film solar cell and more particularly, to techniques for controlling an amount of sulfur (S) and selenium (Se) in the CZTSSe thin film, thereby permitting control over a bandgap of the solar cell.
There is an increased demand for chalcogenide materials containing copper (Cu), zinc (Zn), tin (Sn), sulfur (S) and/or selenium (Se), such as CuZnSn(S,Se) (CZTSSe), for use as absorber layers in solar cells. Current techniques for producing CZTSSe thin film solar cells are described, for example, in T. K. Todorov et al., “High-Efficiency Solar Cell with Earth-Abundant Liquid-Processed Absorber,” Advanced Materials, vol. 22, 2010, pp. E156-E159” (reported solution process by control amount of S and Se compounds), Guo et al., “Synthesis of Cu2ZnSnS4 nanocrystal ink and its use for solar cells,” Journal of the American Chemical Society, vol. 131, 2009, pp. 11672-3 (reported for CuZnSnS then annealed with Se to add Se into the film) and M. Altosaar et al., “Cu2Zn1−xCdx Sn(Se1−ySy)4 solid solutions as absorber materials for solar cells,” Physica Status Solidi (a), vol. 205, 2008, pp. 167-170 (Se powder mixture to introduce Se).
The bandgap of the absorber layer in a solar cell affects what spectrum of light the solar cell absorbs and also the voltage it can extract. Thus, the desired bandgap can vary depending on the particular intended use of the device. Solar cells produced using conventional processes typically produce devices having a fixed bandgap. For example, for currently developed CZTS systems, the bandgap for CuZnSnS4 (pure S) is about 1.5 electron volts (eV), and the bandgap for CuZnSnSe4 (pure Se) is about 1.0 eV. These parameters may or may not be suitable for a given application.
Thus, techniques that permit one to control the bandgap during production of a solar cell would be desirable.
The present invention provides techniques for fabricating thin film solar cells. In one aspect of the invention, a method of fabricating a solar cell includes the following steps. A molybdenum (Mo)-coated substrate is provided. Absorber layer constituent components, two of which are sulfur (S) and selenium (Se), are deposited on the Mo-coated substrate. The S and Se are deposited on the Mo-coated substrate using thermal evaporation in a vapor chamber. Controlled amounts of the S and Se are introduced into the vapor chamber to regulate a ratio of the S and Se provided for deposition. The constituent components are annealed to form an absorber layer on the Mo-coated substrate. A buffer layer is formed on the absorber layer. A transparent conductive electrode is formed on the buffer layer.
A more complete understanding of the present invention, as well as further features and advantages of the present invention, will be obtained by reference to the following detailed description and drawings.
An absorber layer is then formed on the Mo-coated substrate. In this example, the constituent components of the absorber layer are copper (Cu), zinc (Zn), tin (Sn) and sulfur (S) and/or selenium (Se), i.e., CZTSSe. As shown in
Namely, for single junction solar cells, the optimum bandgap energy for the absorber layer is from about 1.2 electron volts (eV) to about 1.4 eV. For currently developed CZTSSe systems, the bandgap for CuZnSnS4 (pure S) is about 1.5 eV, and the bandgap for CuZnSnSe4 (pure Se) is about 1.0 eV. In compound semiconductors, the bandgap energy changes linearly with the composition. Thus, if the bandgap for pure S is about 1.5 eV and the bandgap for pure Se is about 1.0 eV, then the bandgap for Cu2ZnSn(S,Se)4, if S/(S+Se)=4x, is 1+x*0.5 eV. For example, as will be described in detail below, when the amount of Se is increased (relative to S) the open circuit voltage (Voc) of the device decreases while the short circuit current (Jsc) of the device increases.
The deposition of the absorber layer constituent components can be carried out in a number of different ways as described below. In each case, however, the S and Se constituent components are provided each from separate cracking cells. As will be described in detail below, a cracking cell provides multiple ways to regulate the flux of the S and the flux of the Se thus providing a precise control over the S/(S+Se) or Se/(S+Se) ratio of these components in the absorber layer.
According to the present teachings, the deposition of the S and Se from the cracking cells occurs via a thermal evaporation process. Thus, in one exemplary embodiment, the deposition of the Cu, Zn and Sn is also conducted using thermal evaporation, i.e., the Cu, Zn, Sn, S and Se are co-evaporated at same time. In this example, a Cu source, a Zn source and a Sn source are placed in a vapor chamber along with the Mo-coated substrate. The Cu source, Zn source and Sn source can be three crucibles containing Cu, Zn and Sn, respectively, placed in the vapor chamber with the Mo-coated substrate. The Cu, Zn, Sn, S and Se can then be deposited on the Mo-coated substrate with the S and Se being introduced to the vapor chamber from each of two cracking cells (one containing the S and the other containing the Se, i.e., the S source and the Se source, respectively).
This particular embodiment with exemplary cracking cells is shown in
The S/(S+Se) or Se/(S+Se) ratio is determined by the S flux and the Se flux into the vapor chamber. The more S flux, the higher the S/(S+Se) ratio will be. The more Se flux, the higher the Se/(S+Se) ratio will be. The use of cracking cells allows for control of the S and Se fluxes into the vapor chamber in a couple of different ways. First, the needle valve can be used to regulate the flow of S and/or Se into the cracking zone and hence into the vapor chamber (see, for example,
These flux adjustment measures can be operated independently (i.e., controlling the fluxes via adjustments to the needle valve or to the bulk zone temperature) or in combination (i.e., controlling the fluxes by varying both the needle valve position and bulk zone temperature). By way of example only, if the bulk zone in the S cracking cell is kept at 170 degrees Celsius (° C.), the S pressure inside the bulk zone is about 1×10−5 ton (an estimation). If the needle valve is closed to ‘0’, the flux of S is 0. If the needle valve is then opened to 100 mili-inch, there will be some flux, about 3×10-6 ton. If the needle valve is fully opened, the S flux will be same as the pressure in the bulk zone. So with the needle valve adjustments the S flux can be precisely and quickly tuned to flux from 0 to 1×105 torr. However if a flux higher than 1×10−5 ton is needed, then the bulk temperature needs to be further increased. The same procedure applies to the Se. A benefit to using the needle valve adjustment is that the S and Se bulk zones are typically very large and the temperature change requires 1 to 3 hours to stabilize, which is not desirable. Needle valve control is immediate.
Further, the cracking cell can be used to crack the S and Se molecules into smaller more reactive elements which will assist the material growth and can improve the quality of the resulting absorber layer. By way of example only, S8 molecules can be cracked into S4, S2 or even S1 molecules, and Se4 molecules can be cracked into Se2 molecules in the cracking zone. The temperature for the cracking zone is regulated separately from the bulk zone. For example, the cracking zone temperature is at least 100° C. higher than the bulk zone temperature because a cold cracking zone will condense the S or Se, and the condensed material will block the cell. Typically the cracking zone temperature for S/Se is from about 800° C. to about 1,000° C.
Alternatively, the Cu, Zn and Sn can be deposited on the Mo-coated substrate by a method other than thermal evaporation. By way of example only, other suitable deposition processes include, but are not limited to, sputtering, electron-beam evaporation, vacuum deposition, physical deposition or chemical deposition (such as chemical vapor deposition (CVD)). Each of these deposition processes are known to those of skill in the art and thus are not described further herein. In this alternative example, the Cu, Zn and Sn constituent components are first deposited on the Mo-coated substrate using one (or more) of these other deposition processes. Then the substrate is placed in a vapor chamber for the S and Se deposition which occurs via thermal evaporation as described herein.
Regardless of whether thermal evaporation is used exclusively, or in combination with another deposition method(s) for the Cu, Zn and Sn, the result will be a controlled S/(S+Se) or Se/(S+Se) ratio. As described above, the S/(S+Se) or Se/(S+Se) ratio affects the bandgap energy of the absorber layer. The S/(S+Se) or Se/(S+Se) ratio can be varied, using the present techniques, to attain a desired bandgap. It is notable that the relative amounts of the Cu, Zn and Sn have little, if any, effect on the bandgap energy of the absorber layer, especially when compared to the effect the amount of S relative to Se and vice versa does. Thus, the bandgap ‘tuning’ being described herein is achieved by replacing S with Se, or vice versa, rather than S or Se for any of the Cu, Zn and Sn in the absorber layer.
Once the constituent components have been deposited, the components are annealed to form CZTSSe absorber layer 202a on the Mo-coated substrate. See
As shown in
A transparent conductive electrode is then formed on buffer layer 402. The transparent conductive electrode is formed by first depositing a thin layer (e.g., having a thickness of from about 40 nm to about 100 nm) of intrinsic zinc oxide (ZnO) 502 on buffer layer 402. See
As shown in
As described above, the absorber layer constituent components (i.e., Cu, Zn, Sn, S and Se) can be deposited on the Mo-coated substrate using thermal evaporation with the S and Se being provided in controlled amounts using a cracking cell. An exemplary apparatus for this deposition process is shown illustrated in
As shown in
The Mo-coated substrate can be placed in the vapor chamber and then, as described above, the Cu, Zn, Sn, S and Se can be deposited on the Mo-coated substrate using thermal evaporation. The rectangles labeled “Cu,” “Zn,” “Sn,” “S” and “Se” are thermal effusion cells for Cu, Zn, Sn, S and Se, respectively. In this example, a pressure of from about 1×10−6 torr to about 1×10−8 ton is employed in the vapor chamber during the deposition. The needle valves in the cracking cells and/or the bulk zone temperatures of the cracking cells are adjusted to allow a precise amount of S and Se from the respective bulk zones into the cracking zones. Via the cracking zones, the S and Se are introduced into the vapor chamber at precisely controlled amounts.
Alternatively, as described above, a deposition process other than thermal evaporation may be used to deposit the Cu, Zn and Sn. In that instance, the Mo-coated substrate with the Cu, Zn and Sn having already been deposited thereon (e.g., by sputtering, electron-beam evaporation, vacuum deposition, physical deposition or chemical deposition) is placed in the vapor chamber and the S and Se are deposited by thermal evaporation. As above, a pressure of from about 1×10−6 torr to about 1×10−8 torr is employed in the vapor chamber during the deposition of the S and Se. Again the S and Se are dispensed from the cracking cell in precisely controlled amounts from the cracking cell.
The present techniques are described further by way of reference to the following non-limiting examples.
Furthermore, with quantum efficiency measurement, the bandgap energy of CZTSSe can be extracted.
Although illustrative embodiments of the present invention have been described herein, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be made by one skilled in the art without departing from the scope of the invention.
