Patent Application
20150255659
The present disclosure relates to solar modules and specifically to thin film solar modules.
Conventional thin film solar modules, such as CIS-based solar modules (e.g., copper indium gallium (di)selenide or copper indium aluminum (di)selenide), have only a single junction and their conversion efficiency is limited by large thermalization losses.
Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.
The following disclosure provides many different embodiments, or examples, for implementing different features of the subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.
Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.
Likewise, terms concerning electrical coupling and the like, such as “coupled,” “connected” and “interconnected,” refer to a relationship wherein structures communicate with one another either directly or indirectly through intervening structures unless expressly described otherwise.
The coupling layer is an translucent material and adheres the top circuit 110A to the bottom circuit 120B. In embodiments, the coupling material layer is an ethylene vinyl acetate (EVA) or poly vinyl acetate (PVA).
Each circuit 110A, 110B is composed of sub-cells by monolithic integration via P1/P2/P3 scribing connections. In embodiments, the top circuit 110A and bottom circuit 110B are separately fabricated with different cell widths Wa (for the top circuit 110A) and Wb (for the bottom circuit 110B). Each circuit has active (sub-cell) areas 120 arranged and separated from one another by dead areas 130. The cell width is the combined width of an active area 120 and a dead area 130.
The layer structure in top circuit 110A and bottom circuit 110B are “mirror” symmetrical with respect to one another, as described in more detail below and as illustrated in the figures. Briefly, the top circuit 110A and bottom circuit 110B are oriented with respect to one another such that the absorber layers are disposed between the substrates of the circuits. Also, in addition to being flipped relative to one another, the top circuit 110A should be arranged in parallel to the bottom circuit 110B such that the scribing lines of the circuits are arranged in parallel rather than perpendicular to one another. The difference in the cell widths also ensures that dead areas 130 in the top and bottom circuits do not align and overlap with one another.
Turning to
(1) A front substrate layer 162A, such as formed from glass or plastic.
(2) A p-type TCO (transparent conducting oxide (i.e., optically transparent and electrically conductive)) layer 164A, such as, in embodiments, a TCO layer selected from a group of In2O3:Sn (ITO), SnO2:F (FTO), ZnO:Al (AZO), and ZnO:B (BZO). In embodiments, this p-type TCO layer 164A has an optical transmittance ≧90%, a sheet resistance ≦15 ohmn/square centimeters, and a temperature tolerance ≧600° C.
(3) A “top” absorber layer 166A that is, in embodiments, selected from a group of Cu(In,Al)(Se,S)2 and Cu(In,Ga)(Se,S)2 compounds. The thickness of the top absorber layer 166A is ≧100 nm.
(4) A “top” buffer layer 168A, such as one formed of cadmium sulfide (CdS). In embodiments, the thickness of “top buffer layer” is ≧30 nm.
As illustrated on the right side of
The graded band gap toward the top buffer layer 168A is controlled by the gradient of S/(Se+S) ratio in Cu-III-(Se,S)2 compounds. The graded band gap toward the p-type TCO layer 164A is controlled by the gradient of III atoms in the Cu-III-(Se,S)2 compounds.
(5) A n-type TCO layer 170A, which in embodiments is a n-type transparent conductive layer selected from a group of ZnO:Al(AZO), ZnO:B(BZO), In2O3:Sn(ITO), and SnO2:F(FTO). In embodiments, this n-type TCO layer 170A has an optical transmittance ≧90% and a sheet resistance ≦15 ohmn/square centimeters.
Turning to
(1) A back substrate layer 162B, which may be formed from, for example, glass, metal, foil, or plastic.
(2) A “back” electrode layer 164B, which may be formed from molybdenum.
(3) A “bottom” absorber layer 166B, which in embodiments is composed of Cu(In,Ga)(Se,S)2 compounds.
(4) A “bottom” buffer layer 168B, which in embodiments is a cadmium sulfide(CdS) layer. The thickness of “bottom buffer layer” may be ≧30 nm.
As shown on the right side of
(5) A n-type TCO layer 170B, which in embodiments is a n-type transparent conductive layer selected from a group of ZnO:Al(AZO), ZnO:B(BZO), In2O3:Sn(ITO), and SnO2:F(FTO). In embodiments, this n-type TCO layer 170B has an optical transmittance ≧90% and a sheet resistance ≦15 ohmn/square centimeters.
It can be seen from
As noted above, the top and bottom thin film solar module circuits 110A, 110B are separately fabricated and these two circuits are assembled by a coupling material layer. The electrical interconnect between the top and bottom circuits 110A, 11B could be alternatively series or parallel. This allows for the tailoring the design to either high voltage or high current applications. If a series connection is desired, P-side of the bottom circuit 110B is connected to the N-side of top circuit 110A, and the N-side of the bottom circuit 110B is connected to the P-side of the top circuit 110A. If a parallel connection is desired, the P-side of the bottom circuit 110B is connected to the P-side of the top circuit 110A, and the N-side of the bottom circuit 110B is connected to the N-side of the top circuit 110A. As will be understood by those of ordinary skill, the solar module circuit will have interconnects forming electrical busses that can be connected via external connections (e.g., by leads) to provide the desired series or parallel connections (described above) between the top and bottom circuits 110A and 110B.
The output current difference between the top and bottom circuits causes power loss when the circuits are in series, and the output voltage differences between the top and bottom circuits cause power losses when the circuits are in parallel. In embodiments, this current/voltage mismatch between the top and bottom circuits 110A, 110B will cause power loss when the circuits are series/parallel. Any such mismatch can be reduced by appropriate design of the cell width in the top and bottom circuits. In embodiments, the output current/voltage could be modified by different cell width between the top and bottom circuits, i.e. wider cell width results in higher output current and lower output voltage. Therefore, the mismatch problem could be addressed by modifying cell width in each circuit.
At steps 224 to 244 shown at the left side of
At step 246, the coupling material (layer 150) is used to mechanically couple the bottom and top circuits together to provide a tandem thin film solar module 100.
In embodiments, the tandem thin film solar module described herein provides a tandem graded band gap profile that provides several benefits, including, for example, reduced thermalization loss, reduced minority carrier collection loss, and reduced recombination loss, which improve conversion efficiency. These benefits are illustrated in part in connection with
Turning first to the benefit of reduced thermalization loss, in embodiments, the tandem graded band gap profile is composed of a high band gap profile (i.e., Eg=1.5-2 eV) in the top absorber layer 166A provided of the top circuit 110A, and a low band gap profile (i.e., Eg=1.04-1.2 eV) in the bottom absorber layer 166B of the bottom circuit 110B. The high/low band gap structure improves utilization of incident light spectrum by reduced thermalization loss (shown in
Turning to the benefit of reduced minority carrier collection loss, the tandem graded band gap profile has back surface fields (BSF) toward p-n junctions by gradient of III atoms. This is illustrated in
Turning now to the benefit of reduced recombination loss, the tandem graded band gap profile has enlarged band gaps near p-n junction. This can be provided by the sulfur incorporated into the absorber layers (steps 212, 234). The enlarged band gaps attributable to the sulfur-incorporation reduces recombination (
The tandem thin film solar cell module can also prevent or substantially reduce CdS absorption loss to maximum benefits from CdS buffers. In conventional “CIS-based” thin film solar cells, CdS is the best buffer to reduce interface recombination loss. However, the conventional layer structure suffers CdS absorption loss, which means high energy photons (>2.4 eV) are absorbed by CdS and lose energy via recombination in the CdS layer. In the tandem thin film solar cell module described herein, high energy photons (>2.4 eV) will be absorbed by top absorber layer 166A before arriving at the CdS buffer layers 168A, 168B. This layer structure management approach prevents CdS absorption loss and also has benefits of reduced interface recombination loss from the CdS buffers.
Unlike conventional CIS-based thin film solar modules, there is no dead area in the tandem thin film solar modules. That is, the tandem module converts incident light of all optical paths to electrical power under illumination. conventional “CIS-based” thin film solar module have 5-10% total dead area that contribute no electrical power under illumination. In contrast, and with reference to
As described herein, in embodiments of the tandem “CIS-based” thin film solar module, the solar module has a tandem graded band gap profile that provides for reductions in thermalization loss, minority carrier collection loss, and recombination loss. In embodiments, the layered structure also helps prevent CdS-buffer absorption loss. In embodiments, the structure of each circuit can also be arranged to provide reduce or eliminate dead areas of no-absorption in the design, allowing for conversion of incident light from all optical paths to electrical power under illumination. In embodiments, the interconnect between top and bottom circuits in this invention could be alternatively series or parallel to provide high voltage or current applications and reduce current/voltage mismatch through appropriate design of cell width in each circuit.
In one embodiment of a tandem solar module, the solar module includes a first thin film solar cell circuit comprising one or more thin film solar cells; a second thin film solar cell circuit disposed underneath the first thin film solar cell circuit and comprising one or more thin film solar cells; and a transparent coupling layer disposed between the first and second thin film circuits and securing the first and second thin film circuits together in a stack. Each solar cell circuit comprises a multi-layer structure, the multi-layer structure including a substrate, a first conductive layer formed over the substrate, a buffer layer, an absorber layer formed between the first conductive layer and the buffer layer, and a second conductive layer formed over the buffer layer, wherein the first thin film solar cell circuit and second thin film solar cell circuit are oriented with respect to one another such that the absorber layers are disposed between the substrates of the circuits, and wherein the first and second solar cell circuits have different bandgap profiles.
In another embodiment, the tandem solar module include a first thin film solar cell circuit comprising one or more thin film solar cells; a second thin film solar cell circuit disposed underneath the first thin film solar cell circuit and comprising one or more thin film solar cells; and a transparent coupling layer disposed between the first and second thin film circuits and securing the first and second thin film circuits together in a stack. Each solar cell circuit comprises a multi-layer structure, the multi-layer structure including a substrate, a first conductive layer formed over the substrate, a buffer layer, an absorber layer formed between the first conductive layer and the buffer layer, and a second conductive layer formed over the buffer layer. The first thin film solar cell circuit and second thin film solar cell circuit are oriented with respect to one another such that the absorber layers are disposed between the substrates of the circuits. The first and second solar cell circuits each have a double-graded bandgap profile, with bandgaps increasing approaching an interface between the absorber layer and the first conductive layer and approaching an interface between the absorber layer and the buffer layer. The first thin film solar cell circuit is configured for high energy photon absorption and the second thin film solar cell circuit is configured for low energy photon absorption.
In one embodiment, a method of forming a tandem solar module is provided. The method includes the steps of forming a first thin film solar cell circuit comprising one or more thin film solar cells; forming a second thin film solar cell circuit comprising one or more thin film solar cell, wherein each solar cell circuit comprises a multi-layer structure, the multi-layer structure including a substrate, a first conductive layer formed over the substrate, a buffer layer, an absorber layer formed between the first conductive layer and the buffer layer, and a second conductive layer formed over the buffer layer; disposing the second thin film solar cell circuit below the first thin film solar cell circuit; and coupling the first and second thin film solar cell circuits together with a transparent coupling layer with the first thin film solar cell circuit and second thin film solar cell circuit are oriented with respect to one another such that the absorber layers are disposed between the substrates of the circuits, wherein the first and second solar cell circuits have different bandgap profiles.
The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure.
