Patent Application
20250176305
This application claims the benefit under 35 USC 119(a) of Korean Patent Application No. 10-2023-0164397 filed on Nov. 23, 2023, in the Korean Intellectual Property Office, the entire disclosure of which is incorporated herein by reference for all purposes.
The present invention relates to a bifacial thin-film solar cell and a method of manufacturing the same. More specifically, the present invention relates to a bifacial thin-film solar cell capable of enhancing photovoltaic performance on the rear side by reducing interfacial resistance and recombination characteristics at the interface between the rear transparent electrode and the CIGS in implementing a CIGS-based bifacial thin-film solar cell, and a manufacturing method thereof.
CIGS (Cu(In1-xGax)(Se,S)2) thin film solar cells are in the spotlight as high-efficiency thin film solar cells with high photovoltaic efficiency of up to 23.6% and excellent stability. In addition, the CIGS thin film solar cells have a band gap that can be changed in the range of 1.0-1.7 eV compared to silicon solar cells with a fixed band gap of 1.1 eV, making them easy to apply according to usage requirements. Moreover, they have excellent electro-optical characteristics and can be applied to various fields such as buildings, automobiles, and military use due to their flexibility and light weight.
Meanwhile, a bifacial solar cell capable of generating photovoltaic power not only on the front surface but also on the rear surface of the solar cell is a technology that can improve photovoltaic power generation performance of the solar cell. Even in the case of the CIGS thin film solar cell, a bifacial type can be implemented by replacing the rear electrode made of Mo with a transparent conductive oxide (TCO). For example, a bifacial solar cell in the form of sequentially stacking a rear TCO, a light-absorbing layer (CIGS), a buffer layer, and a front TCO on a glass substrate can be implemented.
However, the bifacial solar cell incorporating this configuration demonstrates poor carrier transport properties at the TCO/CIGS interface, attributable to either work function mismatch between TCO and CIGS or elevated interfacial resistance resulting from GaOx reaction layer at the interface. Concurrently, it is characterized by an increased recombination rate of electron-hole (e-h) pairs at the interface, which, in conjunction, impairs the photovoltaic performance on the rear side. Therefore, it is necessary to reduce the rear interfacial resistance and suppress rear recombination characteristics.
First, in order to improve the rear interfacial transport properties, a method of applying a thin Mo thin film layer between the rear TCO and the CIGS or lowering the process temperature of the CIGS photoactive layer to 400° C. or less to suppress the formation of GaOx has been proposed. Those methods were highly effective in lowering the rear interfacial resistance. However, there is no evidence that the above methods of lowering the interfacial resistance are effective in suppressing the rear recombination.
As a method for improving the rear recombination characteristics, there has been proposed a method of forming a CIGS light-absorbing layer to have a composition distribution that increases a Ga/(Ga+In) ratio toward the TCO/CIGS interface so that a bandgap gradient occurs in the rear TCO direction. This approach was found to enhance the photocurrent generated by rear-side light incidence, operating on a principle that diminishes electron-hole (e-h) recombination. It does so by propelling photoexcited electrons away from the TCO/CIGS rear interface, facilitated by a gradient in the bandgap. This strategic displacement reduces e-h recombination near the interface. However, it's important to note that this method does not directly passivate the interface. Consequently, while it mitigates interface recombination to a degree, it does not eliminate it entirely, highlighting a fundamental limitation in fully suppressing interfacial recombination through this approach alone. (Efficiency boost of bifacial Cu(In,Ga)Se2 thin-film solar cells for flexible and tandem applications with silver-assisted low-temperature process (see Nature Energy, v. 8, p. 40-51, 2023)).
As another method, there has been proposed a technology in which Ga2O3 formed at the interface between the rear TCO and CIGS serves as a passivation layer, and conductive metal nanoparticles are dispersed within the Ga2O3, whereby a passivation by the Ga2O3 and an interfacial resistance reduction by metal nanoparticles dispersed in the Ga2O3 can be expected (Korean Patent No. 2009308). However, the Ga2O3, which is formed as a by-product in the growth process of the CIGS light-absorbing layer, acts as a source of high interfacial resistance and defects, as previously discussed, so the passivation effect by Ga2O3 is bound to be limited.
Meanwhile, as a structure for improving rear passivation characteristics and electrical characteristics, although not related to the bifacial solar cell, US Patent Publication No. 2018-0138347 discloses a structure in which a light reflective metal layer 104, an Al2O3 passivation layer 106, a light-absorbing layer 102, a buffer layer 802, and a transparent electrode 804 are sequentially stacked on a substrate, and a plurality of openings penetrating the Al2O3 passivation layer 106 is provided with an electrical contact 108. It describes that through this structure, the rear passivation characteristics by the Al2O3 passivation layer and the electrical characteristics by the electrical contact 108 that induces ohmic contact between the light-absorbing layer 102 and the light reflective metal layer 104 can be expected to be improved. The technology disclosed in US 2018-0138347 shares some technical goals with the present invention in terms of improving the rear passivation characteristics and electrical characteristics, but there are clear differences in whether it is a bifacial type, constituent materials of the rear passivation layer, and the coupling structure of the rear passivation layer and the conductive thin film pattern, which will be described later in the description of the present invention.
The present invention has been devised to solve the above problems, and its object is to provide a bifacial thin-film solar cell capable of enhancing photovoltaic performance on the rear side by reducing interfacial resistance and recombination characteristics at the interface between the rear transparent electrode and the CIGS in implementing a CIGS-based bifacial thin-film solar cell, and a manufacturing method thereof.
A bifacial thin film solar cell according to the present invention for achieving the above object is characterized by including: a rear transparent electrode stacked on a transparent substrate; a rear passivation layer stacked on the rear transparent electrode; a conductive thin film pattern formed in some regions on the rear passivation layer; a light-absorbing layer stacked on the front surface of the rear passivation layer including the conductive thin film pattern; a buffer layer stacked on the light-absorbing layer; and a front transparent electrode stacked on the buffer layer.
The rear passivation layer may be formed of TiOx or TaOx.
The electrical resistivity of TiOx may be greater than 1 Ωcm.
The rear passivation layer may be formed of TiOx or TaOx doped with any one element among Nb, Sb, and S.
The electrical resistivity of TiOx doped with any one of Nb, Sb, and S may be greater than 1 Ωcm.
The conductive thin film pattern may have a dot shape or a linear shape, and a plurality of conductive thin film patterns may be arranged on the rear passivation layer to be spaced apart from each other.
The total area of the plurality of conductive thin film patterns may not exceed 20% of the area of the rear passivation layer.
When the conductive thin film pattern has a dot shape, the conductive thin film pattern may have a length or diameter of 0.1 to 2 μm, a thickness of 0.1 to 2 nm, and a distance between the conductive thin film patterns which is equal to or less than a carrier diffusion length (LD) in the light-absorbing layer.
When the conductive thin film pattern has a linear shape, the linear conductive thin film pattern may have a width of 0.1 to 2 μm, a thickness of 0.1 to 2 nm, and a distance between the conductive thin film patterns which is equal to or less than a carrier diffusion length (LD) in the light-absorbing layer.
The conductive thin film pattern may be formed of molybdenum (Mo).
The stacked thickness of the rear passivation layer may be 2 to 4 nm.
The light-absorbing layer may be formed of CIGS (Cu(In1-x, Gax)(Se,S)2).
The rear transparent electrode and the front transparent electrode may be formed of any one of indium-based oxide, zinc-based oxide, and tin-based oxide, wherein the indium-based oxide may be any one of InOx, ITO, (W, Ce, Mo)-doped InOx, and IZO, the zinc-based oxide may be (Al, Ga, B, Ti, F, H)-doped ZnOx, and the tin-based oxide may be (F, Sb)-doped SnOx.
The buffer layer may be provided between the light-absorbing layer and the front transparent electrode, and the buffer layer may be formed of any one of CdS, InS(O,OH), ZnS(O, OH), ZnMgO, ZnTiO, ZnSnO, or a combination thereof.
The front transparent electrode, light-absorbing layer, and rear passivation layer in a specific region may be removed to provide a light-transmitting portion region in which the rear transparent electrode is exposed.
A P1 region may be provided in which the rear passivation layer and the rear transparent electrode are removed in a certain area, a P2 region may be provided in which the buffer layer and the light-absorbing layer are removed in a certain area, and a P3 region may be provided in which the front transparent electrode, the buffer layer, and the light-absorbing layer are removed in a certain area, wherein the rear transparent electrodes of neighboring cells may be insulated by the P1 region, the rear transparent electrode and the front transparent electrode of neighboring cells may be connected by the P2 region, and the front transparent electrodes of neighboring cells may be insulated by the P3 region.
A method of manufacturing a bifacial thin film solar cell according to the present invention is characterized by including the steps of: forming a rear transparent electrode on a transparent substrate; sequentially stacking a rear passivation layer on the rear transparent electrode; forming a conductive thin film pattern on the rear passivation layer; forming a light-absorbing layer on the front surface of the rear passivation layer including the conductive thin film pattern; and forming a front transparent electrode on the light-absorbing layer.
The step of forming a conductive thin film pattern on the rear passivation layer may include the substeps of: forming a mask that exposes some regions of the rear passivation layer on the rear passivation layer, depositing a conductive metal on the front surface of the rear passivation layer including the mask, and removing the mask to form a conductive thin film pattern in some regions of the rear passivation layer.
The method may further include a step of removing the front transparent electrode and the light-absorbing layer in a specific region to form a light-transmitting portion region in which the rear transparent electrode is exposed.
The method may further include a step of scribing the rear transparent electrode and the rear passivation layer in a certain area along the P1 region to divide them into a plurality of cells and insulating the rear transparent electrode between neighboring cells in a state in which the conductive thin film pattern is formed on the rear passivation layer before forming the light-absorbing layer.
The method may further include a step of exposing the rear passivation layer by scribing the light-absorbing layer along the P2 region after forming the light-absorbing layer.
The method may further include a step of insulating the front transparent electrode between neighboring cells by scribing the front transparent electrode and the light-absorbing layer along the P3 region in a state in which the front transparent electrode is stacked.
The bifacial thin-film solar cell according to the present invention has the following effects:
Through the combined structure of the rear passivation layer and the conductive thin film pattern, the rear passivation characteristics and rear interfacial transport characteristics can be improved, thereby enhancing the photocurrent characteristics by light incident from the rear of the bifacial thin film solar cell.
In implementing a bifacial thin film solar cell, the present invention proposes a technology capable of maximizing the photovoltaic performance on the rear side of a bifacial thin-film solar cell by passivating defects present at the interface between a rear transparent electrode and a light-absorbing layer to minimize carrier recombination at the interface, while improving an ohmic contact characteristics between the rear transparent electrode and the light-absorbing layer to facilitate a movement of holes (+) generated in the light-absorbing layer to the rear transparent electrode.
To implement this, the present invention presents a structure in which a rear passivation layer formed of TiOx or TaOx, an n-type metal oxide, is provided between the rear transparent electrode and the light-absorbing layer, and a conductive thin film pattern is provided on the rear passivation layer.
The rear passivation layer formed of TiOx (or TaOx) is provided between the rear transparent electrode and the light-absorbing layer to passivate defects present in the rear of the light-absorbing layer, thereby preventing electrons (−) generated in the light-absorbing layer from recombining with the defects and disappearing.
The conductive thin film pattern is provided on some regions of the rear passivation layer and inserted into the light-absorbing layer, minimizing interfacial resistance between the rear passivation layer formed of TiOx (or TaOx) and CIGS and inducing ohmic contact between the light-absorbing layer and the rear transparent electrode.
That is, under the stacked structure of the rear passivation and the conductive thin film pattern, the rear passivation layer in an region where the conductive thin film pattern is not provided serves to passivate the defects present in the rear of the light-absorbing layer, and the conductive thin film pattern provided on some regions of the rear passivation layer serves to lower the interfacial resistance between the light-absorbing layer and the rear passivation layer to induce ohmic contact between the light-absorbing layer and the rear transparent electrode.
In general, a passivation layer made of SiO2 or SiNx has been applied to passivate a p+ region (p-type substrate or p-type emitter) in crystalline silicon solar cells, and recently, it has been proven that Al2O3 is highly effective in passivating a p-type emitter of a n-type substrate. Al2O3 stacked on a silicon substrate has a large amount of negative fixed charges by heat treatment. These charges are mainly located at the interface between the Al2O3 thin film and the crystalline silicon substrate and forms a very strong negative electric field in the p+ region of the silicon surface, thereby pushing photo-excited electrons from the interface to show low recombination properties.
As such, Al2O3 is a highly effective material as a passivation layer for the p+ region of a crystalline silicon solar cell. Therefore, it may be considered naturally to apply Al2O3 as a passivation layer of a CIGS-based thin film solar cell. However, when Al2O3 is applied as a passivation layer in a crystalline silicon solar cell, a structure in which the electrode penetrates Al2O3 is required for ohmic contact between the electrode and the p+ region because Al2O3 is a non-conductor. Accordingly, if Al2O3 is applied as a passivation layer to a CIGS-based thin film solar cell, the penetration of Al2O3 is inevitably required for ohmic contact between the electrode and the light-absorbing layer. For this reason, it is difficult to apply Al2O3 as a passivation layer in a CIGS-based thin film solar cell. For reference, US 2018-0138347 proposes a structure in which an Al2O3 passivation layer 106 is provided without a penetration portion between a light reflection metal layer 10 and a light-absorbing layer 102, but the light reflection metal layer 10 is not an electrode where carriers are collected.
In the present invention, TiOx(or TaOx) is applied as the rear passivation layer of the CIGS-based thin film solar cell as described above. As is known, since TiOx and TaOx are n-type materials, it is theoretically unsuitable to apply an n-type material such as TiO2 for passivation of CIGS, a p-type semiconductor. This is because when the n-type material is applied as the passivation layer for passivation of the CIGS interface, the transfer of holes (+) is blocked by the n-type passivation layer independently of its passivation capability.
Despite this inadequacy in principle, the combined structure of the rear passivation formed of TiOx (or TaOx) and the conductive thin film pattern can resolve the barrier at the TiOx/CIGS interface that causes carrier blocking. In addition, since TiOx is not an insulator unlike Al2O3 which is a nonconductor, ohmic contact between the rear transparent electrode and the light-absorbing layer can be induced without penetration of TiOx. That is, the rear passivation characteristic is achieved by TiOx provided between the rear transparent electrode and the light-absorbing layer; the interfacial resistance between TiOx and CIGS is resolved by the conductive thin film pattern; and ohmic contact between the rear transparent electrode and the light-absorbing layer can be induced without penetration of TiOx by taking advantage of the fact that TiOx is conductive and that the conductivity of TiOx can be adjusted by changing the oxygen content (and/or impurity content) in the sputtering gas.
Hereinafter, a bifacial thin film solar cell according to an embodiment of the present invention will be described in detail with reference to the drawings.
Referring to
The light-absorbing layer 150 absorbs light to generate electron (−) and hole (+) pairs through photoelectric conversion, wherein the electron (−) generated in the light-absorbing layer 150 is moved to the front transparent electrode 170, and the hole (+) is moved to the rear transparent electrode 120. Here, the light-absorbing layer 150 and the front transparent electrode 170 form a p-n junction. Additionally, the light-absorbing layer 150 is formed of CIGS (Cu(In1-x, Gax)(Se,S)2).
The rear passivation layer 130 is provided between the rear transparent electrode 120 and the light-absorbing layer 150 to serve to passivate defects existing on the rear surface of the light-absorbing layer 150, thereby minimizing the disappearance of the photoexcited electrons (−) formed in the region of the light-absorbing layer 150 in contact with the rear transparent electrode 120 from the light-absorbing layer 150 by recombining with the defects.
The rear passivation layer 130 is formed of TiOx or TaOx, which is an n-type metal oxide. Additionally, TiOx or TaOx doped with any one element of Nb, Sb, and S may also be applied as the rear passivation layer 130. Most preferably, TiOx or Nb-doped TiOx (TNO, titanium niobium oxide) may be applied as the rear passivation layer 130. Hereinafter, for convenience of description, the description will be based on TiOx.
Although the light-absorbing layer 150 formed of CIGS is a p-type semiconductor, TiOx, an n-type metal oxide, is applied as the passivation layer of the light-absorbing layer 150 because this material has excellent passivation characteristics and can control its conductivity.
Al2O3 was previously mentioned as a representative passivation material for crystalline silicon solar cells in the ‘Background Art’. The Al2O3 has excellent passivation characteristics, but due to its electrical nonconductivity, the electrode must penetrate Al2O3, which is a passivation layer, in order for the electrode (e.g., the front electrode) to make ohmic contact with the p-type semiconductor layer.
Al2O3 has an energy band gap of about 7.0 to 7.6 eV and thus is a nonconductor, while TiOx, an n-type metal oxide, has an energy band gap of about 3.2 eV and thus is conductive.
As such, since TiOx, an n-type metal oxide, is conductive unlike Al2O3, which is a non-conductor, the penetration of the electrode into the passivation layer is not required when applied to the passivation layer. In addition, with regard to the passivation characteristics of TiOx, referring to the experimental example described later, it can be seen that when TiOx is applied to the rear passivation layer 130 at a thickness of 2 to 4 nm, the rear photoelectric conversion efficiency increases by two times or more compared to the case where TiOx is not applied, and these results disprove that the rear passivation characteristics are improved by TiOx.
As a requirement for improving photoelectric conversion efficiency of CIGS-based bifacial thin-film solar cells, the most important thing is to improve the rear recombination characteristics and rear interface transport characteristics, which can be achieved by applying TiOx as the rear passivation layer 130 and providing a conductive thin-film pattern 140 on the rear passivation layer 130.
As described above, it is confirmed through the experimental examples that the rear passivation characteristics, that is, the rear recombination characteristics, are improved through the application of TiOx to the rear passivation layer 130. However, although TiOx has conductivity, the absolute electrical conductivity of TiOx is not excellent. In addition, since TiOx is an n-type material, it hinders the transfer of holes (+) moving from the light-absorbing layer 150 to the rear transparent electrode 120. That is, high electrical resistance exists at the interface between TiOx and CIGS. Therefore, although the rear recombination characteristics can be improved only by applying TiOx, there is a limit to improving the rear interfacial transport characteristics.
In consideration of the above-mentioned point, the conductive thin film pattern 140 is applied. That is, the conductive thin film pattern 140 is provided in some regions on the rear passivation layer 130 formed of TiOx, whereby ohmic contact between the light-absorbing layer 150 and the rear transparent electrode 120 may be induced via the conductive thin film pattern 140, thereby improving the rear interface transport characteristics.
In this case, even though the conductive thin film pattern 140 is not provided in the form of penetrating the rear passivation layer 130, but is provided on the rear passivation layer 130, ohmic contact between the light-absorbing layer 150 and the rear transparent electrode 120 is possible, because the rear passivation layer 130 is made of TiOx rather than Al2O3. When Al2O3 is applied as the passivation layer as in the crystalline silicon solar cell discussed above, the electrode is required to penetrate the passivation layer due to the non-conductive properties of Al2O3. However, since TiOx has conductivity unlike Al2O3, ohmic contact between the light-absorbing layer 150 and the rear transparent electrode 120 can be mediated by adjusting the thickness of TiOx. As another example, U.S. Patent Publication No. 2018-0138347 configures the passivation layer 106 with Al2O3, indicating that the electrical contact 108 penetrates the Al2O3 passivation layer 106 for the electrical connection between the light-absorbing layers 150 and 102 and the light reflection metal layer 104.
In addition, the electrical conductivity of TiOx can also be controlled through the contents of oxygen and impurity, thereby controlling the ohmic contact between the light-absorbing layer 150 and the rear transparent electrode 120. The electrical resistivity of TiOx must be controlled to be greater than 1 Ωcm, and to this end, the oxygen content in the process gas must be maintained at a certain ratio or more during sputtering-based deposition of TiOx. For example, it needs to be adjusted at 0.2 to 0.3%. Here, the impurities refer to any one of Nb, Sb, and S.
As described above, the rear recombination characteristics and the rear interfacial transport characteristics can be improved by combining the TiOx rear passivation layer 130 and the conductive thin film pattern 140, wherein the TiOx rear passivation layer 130 and the conductive thin film pattern 140 need to have an optimal structure. For example, when the area and volume of the conductive thin film pattern 140 provided in some regions on the rear passivation are too large, the rear interfacial transport characteristics are improved, while the rear recombination characteristics due to the rear passivation layer 130 are deteriorated. When the area of the conductive thin film pattern 140 is too small, the rear recombination characteristics are improved, but the effect of improving the rear interfacial transport characteristics by the conductive thin film pattern 140 is insignificant.
Considering these points, the conductive thin film pattern 140 is provided in a dot shape (see
Specifically, when the conductive thin film pattern 140 forms a dot shape, a plurality of conductive thin film patterns 140 are disposed to be spaced apart on the rear passivation layer 130, and the total area of the plurality of conductive thin film patterns 140 should not exceed 20% of the area of the rear passivation layer 130. If it exceeds 20%, the rear interfacial transport characteristics are improved, but the rear recombination characteristics by the rear passivation layer 130 are deteriorated.
In addition, preferably, the dot-shaped conductive thin film pattern 140 is designed to have a length or diameter of 0.1 to 2 μm and a thickness of 0.1 to 2 nm for sufficient light transmission. Furthermore, the distance between the conductive thin film patterns 140 should be equal to or less than 2 to 5 μm, which is comparable to the carrier diffusion length LD of the light-absorbing layer 150.
When the conductive thin film pattern 140 is linear, the conductive thin film pattern 140 may be arranged in a lattice shape, and the distance between the conductive thin film patterns 140 is preferably designed to be equal to or less than 2 to 5 μm, which is comparable to the carrier diffusion length LD of the light-absorbing layer 150. In addition, the linear conductive thin film pattern 140 is preferably designed to have a width of 0.1 to 2 μm and a thickness of 0.1 to 2 nm for sufficient light transmission. As an example, the conductive thin film pattern 140 may be formed of molybdenum (Mo).
Meanwhile, the rear transparent electrode 120 and the front transparent electrode 170 may be formed of any one of indium-based oxide, zinc-based oxide, and tin-based oxide. The indium-based oxide is any one of InOx, ITO, (W, Ce, Mo)-doped InOx, and IZO, the zinc-based oxide is (Al, Ga, B, Ti, F, H)-doped ZnOx, and the tin-based oxide is (F, Sb)-doped SnOx.
The buffer 160 layer may be formed of any one of CdS, InS(O,OH), ZnS(O, OH), ZnMgO, ZnTiO, ZnSnO, or a combination thereof.
Hereinbefore, the bifacial thin film solar cell according to an embodiment of the present invention has been described. Next, a method of manufacturing a bifacial thin film solar cell according to an embodiment of the present invention will be described.
First, as shown in
The transparent substrate 110 may be a glass substrate, and the rear transparent electrode 120 may be stacked using any one of sputtering, vacuum evaporation, and solution processes. Additionally, the rear transparent electrode 120 may be formed of any one of indium-based oxide, zinc-based oxide, and tin-based oxide. The indium-based oxide is any one of InOx, ITO, (W, Ce, Mo)-doped InOx, and IZO, the zinc-based oxide is (Al, Ga, B, Ti, F, H)-doped ZnOx, and the tin-based oxide is (F, Sb)-doped SnOx.
Subsequently, a rear passivation layer 130 is stacked on the rear transparent electrode 120. Like the rear transparent electrode 120, the rear passivation layer 130 may be stacked using any one of sputtering, vacuum evaporation, and solution processes.
The rear passivation layer 130 may be formed of TiOx or TaOx, which is an n-type metal oxide, or may be formed of TiOx or TaOx doped with any one element among Nb, Sb, and S. Most preferably, the rear passivation layer 130 may be formed of TiOx or Nb-doped TiOx (TNO). In order to control the conductivity, that is, the electrical resistivity of TiOx or Nb-doped TiOx (TNO), the oxygen content and/or impurity content of TiOx or Nb-doped TiOx(TNO) is adjusted, and preferably, the electrical resistivity of TiOx must be controlled to be greater than 1 Ωcm. To this end, the oxygen content in the process gas must be maintained at a certain ratio or more during sputter deposition of TiOx, and for example, it needs to be adjusted at 0.2 to 0.3%.
In a state in which the rear passivation layer 130 is stacked, a conductive thin film pattern 140 is formed on the rear passivation layer 130. Specifically, a mask exposing some regions of the rear passivation layer 130 is formed on the rear passivation layer 130 (see
In the above process, when patterning the mask that exposes some regions of the rear passivation layer 130, the region exposed by the mask may form a dot shape or a linear shape. In addition, the distance between the dots or the distance between the lines should be equal to or less than 2 to 5 μm, which is comparable to the carrier diffusion length LD of the light-absorbing layer 150, and it is desirable to pattern the mask so that the length of dot or the width of linear shape is 0.1 to 2 μm. Furthermore, the total area of the region exposed by the mask should not exceed 20% of the area of the rear passivation. The region exposed by the mask corresponds to a region in which the conductive thin film pattern 140 is formed, and when the total area of the conductive thin film pattern 140 exceeds 20% of the area of the rear passivation, the rear passivation characteristic by the rear passivation layer 130 is deteriorated. In addition, when the conductive metal 140a is deposited in the above process, the conductive metal 140a is preferably deposited to have a thickness of 0.1 to 2 nm to secure sufficient light transmittance.
In a state in which the conductive thin film pattern 140 is formed on the rear passivation layer 130, a material of the light-absorbing layer 150, that is, CIGS is stacked on the front surface of the rear passivation layer 130 including the conductive thin film pattern 140, and then heat treatment is performed to form the light-absorbing layer 150 (see
Subsequently, when the buffer layer 160 and the front transparent electrode 170 are sequentially stacked on the light-absorbing layer 150, the method of manufacturing a bifacial thin film solar cell according to an embodiment of the present invention is completed. The buffer layer 160 may be formed of any one of CdS, InS(O,OH), ZnS(O, OH), ZnMgO, ZnTiO, ZnSnO, or a combination thereof, and the front transparent electrode 170 may be formed of the same material as the rear transparent electrode 120.
Meanwhile, the bifacial thin film solar cell of the present invention may be implemented in the form of a single integrated module. That is, it may be manufactured in a form in which a plurality of solar cells is provided on one substrate. In this case, scribing P1 for cell-to-cell insulation of the rear transparent electrode 120, scribing P2 for cell-to-cell connection between the rear transparent electrode 120 and the front transparent electrode 170, and scribing P3 for cell-to-cell insulation of the front transparent electrode 170 are required.
This will be described in detail as follows (see
In a state where the rear transparent electrode 120 and the rear passivation layer 130 are sequentially stacked on the transparent substrate 110, and the conductive thin film pattern 140 is formed on the rear passivation layer 130, the rear transparent electrode 120 and the rear passivation layer 130 are scribed in a certain area along a scribing line to divide them into a plurality of cells and to insulate the rear transparent electrode 120 between neighboring cells. In this case, the scribing line is referred to as a P1 region, the rear transparent electrode 120 is divided into a plurality of cells by the P1 region, and the rear transparent electrode 120 between neighboring cells is electrically insulated by the P1 region. The scribing process for the P1 region and the scribing processes for the P2 region and the P3 region, which will be described later, may be performed using a laser.
Subsequently, the light-absorbing layer 150 and the buffer layer 160 are sequentially stacked on the front surface of the substrate including the conductive thin film pattern 140. Accordingly, the light-absorbing layer 150 is also filled in the P1 region. Then, the buffer layer 160 and the light-absorbing layer 150 are scribed along the P2 region to expose the rear passivation layer 130.
In this state, the front transparent electrode 170 is stacked on the front surface of the buffer layer 160. In this case, the front transparent electrode 170 is also filled in the P2 region. As the front transparent electrode 170 is filled in the P2 region, the front transparent electrode 170 is electrically connected to the conductive thin film pattern 140 on the rear passivation layer 130. Here, the front transparent electrode 170 and the conductive thin film pattern 140 on the rear passivation layer 130 are provided in adjacent cells, respectively.
In a state in which the front transparent electrode 170 is stacked, the front transparent electrode 170, the buffer layer 160, and the light-absorbing layer 150 are scribed along the P3 region to insulate the front transparent electrode 170 between adjacent cells.
Through the above process, a structure in which a plurality of bifacial thin film solar cells is integrated on one transparent substrate 110 may be completed.
In addition to the integration of the above-described bifacial thin film solar cell, light-transmitting properties may be additionally provided. In a state in which the integration process is completed, as shown in
Hereinabove, the bifacial thin film solar cell and the manufacturing method thereof according to an embodiment of the present invention have been described. Hereinafter, the present invention will be described in more detail through experimental examples.
Indium tin oxide (ITO) was deposited to a thickness of 600 nm on a soda lime glass substrate by a sputtering process, and then a TiO2 thin film was deposited. The TiO2 thin films were deposited by varying their thickness (t) to 0, 1, 2, and 4 nm. A CIGS thin film was deposited on the TiO2 thin film at a substrate temperature of 450° C. by a 3-stage co-evaporation process. Subsequently, a CdS thin film was deposited by chemical bath deposition, i-ZnO and indium zinc oxide (IZO) thin films were deposited by sputtering, and then an Ag metal electrode pattern was deposited to complete a CIGS thin film solar cell.
For the completed CIGS thin film solar cell, light having an intensity of 1-sun is irradiated on the front and rear surfaces, respectively, and current-voltage characteristics are measured.
Referring to
On the other hand, in the case of a thin film solar cell with a TiO2 thin film applied at 2 nm and 4 nm, it can be seen that the fill factor (FF) is significantly reduced under the front light incident condition because the interfacial resistance is very high, but the photocurrent is greatly increased under the rear light incident condition (photocurrent ratio of 0.354-0.361), thereby improving the ITO/CIGS interface passivation characteristics.
In order to resolve the high resistance of the ITO/CIGS interface, a Mo ultra-thin layer of 0.5 nm thickness was applied to the thin film solar cell of Experimental Example 1. The Mo layer (0.5 nm thick) has a light absorption rate of 2% or less, can transmit most of the light, and can be expected to improve interfacial resistance characteristics.
According to the jV results measured under the front light incident condition (see
On the other hand, the jV results measured under the rear light incident conditions (see
These results can be said to show that the introduction of Mo in some regions can be expected to achieve both the effect of improving the rear passivation characteristics by TiO2 and the interfacial transport characteristics by Mo.
TEM and EDS analysis were performed on ITO/TiO2(4 nm)/CIGS and ITO/TiO2(4 nm)/Mo(0.5 nm)/CIGS respectively manufactured in Experimental Example 1 and Experimental Example 2.
Referring to
The passivation characteristics and electrical characteristics according to the oxygen content of TiOx were examined.
In manufacturing an ITO/TiOx/CIGS structure according to Experimental Example 1, an Nb-doped TiOx (Nb content: 10 wt %) sputter target was sputtered to deposit TNO (titanium niobium oxide), and the flow rate of oxygen gas in the sputtering gas was adjusted to have an oxygen content (O2/(Ar+O2)) of 0.08 to 0.5%. Subsequently, the deposited TNO was subjected to rapid heat treatment (RTA) at 350° C. and 380° C. (proceeded for 30 minutes in an atmosphere of 0.5% H2 and a vacuum degree of 1 to 10 Torr).
Referring to
Front and rear photocurrent characteristics were examined for thin film solar cells to which TNO having an oxygen content of 0.08% and 0.2% was applied.
Referring to
Subsequently, after the Mo thin film layer (0.5 nm) was additionally applied to the thin film solar cell to which TNO having an oxygen content of 0.08% and 0.2% was applied, front and rear photocurrent characteristics were examined.
Referring to
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2023-0164397 | Nov 2023 | KR | national |
This study was conducted with the support of the Ministry of Science and ICT and the National Research Foundation of Korea [Research project name: Climate change response technology development; Research task name: Development of High-Efficiency Transparent CIGS Photovoltaic Module; Task identification number: 1055001324; Task number: 2019M1A2A2072412].
