A CIGS / perovskite monolithic tandem flexible solar cell

Abstract

The invention is related a CIGS / perovskite solar cell, which comes to the fore in tandem structures in which more than one solar cell is used together in order to exceed the theoretical limit that limits the efficiency in photovoltaic technology.

Description

[0001] A CIGS / PEROVSKITE MONOLITHIC TANDEM FLEXIBLE SOLAR CELL

[0002] TECHNICAL FIELD

[0003] The invention is related to a CIGS / perovskite solar cell, which comes to the fore in tandem structures in which more than one solar cell is used together in order to exceed the theoretical limit that limits the efficiency in photovoltaic technology.

[0004] BACKGROUND

[0005] Recently, there has been a serious trend towards renewable energy sources from fossil fuels in order to meet the increasing energy demand around the world. Photovoltaic devices, which work on the principle of converting solar energy directly into electrical energy, have an important place in the field of renewable energy. However, photovoltaic devices show low efficiency theoretically and practically. In order to use these devices more efficiently, the use of more than one solar cell together has come to the fore and tandem structures have emerged. Recent efforts have gained momentum in the fabrication of tandem structures and the enhancement of efficiency to achieve a more robust production. In this way, it will be possible to benefit from renewable energy more effectively.

[0006] Although silicon (Si)-based solar cells have dominated the field of photovoltaic technology from the beginning, different types of solar cells have also become an important competitor over time. As a result of approaching the efficiency limit in Si- based solar cells, which are classified as the first generation, thin film (second generation) and innovative (third generation) solar cells have also reached high efficiency values, allowing the development of photovoltaic technologies. Copper Indium Gallium Selenide (CIGS), one of the thin film solar cells, has managed to reach high efficiency values. Therefore, with the intense interest of researchers, it has become a candidate solar cell for module production to be used in panels.

[0007] Another solar cell that reaches high efficiency in a short time is perovskite solar cells. Advantages such as low cost and ease of production have made perovskite solar cells attractive. Tandem structures involving the co-production of CIGS and perovskite solar cells, which promise high efficiency and low cost, have been studied recently. In addition, nanostructured solar cells, which are innovative solar cells, have begun to find a place in photovoltaic technology.

[0008] Nanorod-like nanostructures are used in photovoltaic devices because they improve optical properties. Since the production of nanostructures requires more technical knowledge compared to thin film materials, the mentioned cell types (CIGS & perovskite & tandem) are mostly obtained in thin film form. However, by including the use of nanorods, it is possible to go to the next level in terms of efficiency and design, especially in CIGS solar cells and tandem structures.

[0009] AIM OF THE INVENTION

[0010] The purpose of the invention is related to a CIGS / perovskite solar cell, which come to the fore in tandem structures in which more than one solar cell is used together, in order to exceed the theoretical limit that limits the efficiency in photovoltaic technology.

[0011] Another aim of the invention is to use nanorods instead of CIGS thin films in order to take the production of CIGS and perovskite on flexible substrates one step further due to their high absorption properties.

[0012] Another aim of the invention is to achieve superior price / performance criteria with the CIGS nanorod / perovskite tandem structure by obtaining advanced optical and electrical properties with the use of nanorods.

[0013] Another aim of the invention is to provide cost-effective and mass production with the possibility of production on flexible substrates.

[0014] Another aim of the invention is to produce CIGS thin films and nanorods with the band gap of ~1 .1 eV on flexible (PI) substrates pre-coated with a bi-layer Mo thin film.

[0015] Another aim of the invention is that CIGS nanorods obtained with the Glancing Angle Deposition (GLAD) technique exhibit better optical absorption than the equivalent CIGS thin film.

[0016] Another aim of the invention is to form a p-CIGS / n-CdS core-shell conformal structure by coating nanorods homogeneously with an n-CdS thin film and to form a successful pn conformal structure.

[0017] Another aim of the invention is to have CIGS nanorod and thin film solar cells with a band gap of ~ 1 .1 eV and a smooth surface to ensure current compliance. Another aim of the invention is to synthesize Spiro-OMeTAD and PEDOT:PSS materials separately as HTL in order to solve the instability problem of perovskite and to determine the more effective one.

[0018] Another aim of the invention is to synthesize perovskite with a dark color, ideal band gap (~1 .6 eV) and high absorbance.

[0019] Another aim of the invention is to ensure that the Jsc(mA / cm2), Voc(V), FF(%) values obtained from the cells and tandem structure are >15, >0.8, >50, respectively, and to obtain 15-20% and 10-15% efficiency values from tandem structure for rigid and flexible substrates, respectively.

[0020] LIST OF FIGURES

[0021] Figure 1a. SEM image of Mo bi-layer thin film grown at room temperature.

[0022] Figure 1 b. SEM image of Mo bi-layer thin film grown at 300°C temperature.

[0023] Figure 2. Reflectance graph of single-layer and bi-layer Mo films.

[0024] Figure 3a. Schematic representation of the flux generated at low pressure (LPS) sputtering method.

[0025] Figure 3b. Schematic representation of the flux generated at high pressure (HPS) sputtering method.

[0026] Figure 4a. CIGS thin film produced with LPS.

[0027] Figure 4b. CIGS thin film produced with HPS.

[0028] Figure 5. One- and two-step production method of perovskite absorber layer.

[0029] DETAILED DESCRIPTION OF THE INVENTION

[0030] The invention is basically about the production of CIGS nanorod and perovskite thin film solar cells and the creation of the monolithic tandem structure. While CIGS solar cells can be obtained by sequentially coating thin films using the physical production method (sputtering), synthesis of layers in perovskite solar cells is essential. In order to obtain the monolithic structure, direct production of the perovskite cell on the CIGS cell and the intermediate layer ITO is required. During the synthesis of perovskite and other layers, the parts outside the cell surface must be covered with a temperature-resistant material (teflon tape, for example) or used the mask to prevent contamination of the CIGS cell and deterioration of its structure. Otherwise, successful fabrication of CIGS and perovskite cells cannot be achieved. As a result, it becomes difficult to obtain effective efficiency from the monolithic structure produced. In addition to high efficiency, low-cost production is among the priority criteria for solar cell fabrication. Moreover, interest in elastic substrates has also increased over time, as wearable photovoltaic devices and roll printing to enable large-scale production are among the important applications of nanotechnology. Since the elastic (polyimide) PI substrates are transparent, insulating and can also withstand temperatures up to 450°C, they are suitable for use in CIGS photodetectors and photovoltaic devices. For these reasons, PI was used as the flexible substrate.

[0031] During CIGS solar cell fabrication, the first layer usually coated on glass substrate is a molybdenum (Mo) metal conductive thin film. Mo thin film is used as the back metal contact in the CIGS solar cell and helps the CIGS layer adhere better to the substrate. Additionally, another function of this layer is to increase absorption by reflecting the light, that falls on the cell surface and remains unabsorbed after passing through CIGS, back to the CIGS layer. Therefore, the obtained Mo thin film must be a good conductor as well as having high reflective properties. In order to meet all these conditions, two-stage production was carried out by RF sputtering. High pressure (HPS) was first used in order for CIGS to better adhere to the substrate, and then the production was completed by reducing the pressure (LPS). As a result, bi-layer Mo thin film with very low resistivity (~ 14 pQ.cm) and high reflection properties has been successfully obtained. When coating with RF sputtering, inert Ar gas is used to create plasma in the chamber. Flux occurs as a result of the collision of target atoms and Ar ions, which become ionic during plasma formation, and the orientation and density of the flux towards the substrate changes according to the pressure of the gas. When low pressure is used (low pressure sputter - LPS), the target atoms will encounter fewer collisions and can reach the substrate without deviating too much from their direction. In this case, a thin film with high material density and smooth surface is generally obtained. However, in case of high Ar gas, that is, high pressure sputter (HPS), a larger flux will be obtained as the target atoms will undergo more collisions and thus, the atoms will reach the substrate at high angles. This results in obtaining a very dilute thin film with a nanorod-like columnar microstructure.

[0032] Evaporation method is generally used to obtain the absorber layer in standard CIGS thin film solar cells. However, in order to be a practical and effective method and also to enable the production of nanorods, the RF magnetron sputtering method was used in our invention for both thin film and nanorod CIGS structures. A quaternary (four-element) single RF sputtering target (CulnxGa(1-x)Se2 (x=0.7-0.8)) was used as the CIGS source.

[0033] As discussed above, it is possible to obtain thin films with different morphological properties at different pressure values. For thin film solar cells, CIGS thin film production was carried out using the LPS method. CIGS thin film production will be carried out before PVD modification, because the GLAD technique will be activated after modification. CIGS thin film was produced with a thickness of -0.5-1 pm because it can absorb light effectively even at low thickness due to its high absorption coefficient.

[0034] RF sputtering (PVD) system was used to produce CIGS nanorods. However, firstly, it is necessary to increase the angle between the sample holder and the target in the PVD system for the nanorod production. To briefly explain the reason for this, whether the material is two-dimensional (thin film) or one-dimensional (nanorod) depends on the production mode, which develops depending on the kinetics of the reactions between the target atoms reaching the surface and the surface atoms. While thin film is obtained as a result of layer-by-layer coating of target atoms on the surface, nanoclusters must first be formed by the combination of target atoms on the substrate for the nanorod-like nanostructure. After the nanoclusters become stable, the target atoms reach the surface and accumulate on the clusters, forming nanorods / nanowires or other type of one-dimensional nanostructures.

[0035] In order to form nanoclusters during material production using the RF sputtering method, the target atoms must reach the substrate at a high angle (0 > 700) relative to the substrate normal. Atoms arriving at high angles form clusters before they have the opportunity to travel on the substrate surface. Afterwards, the target atoms that reach the surface join the first clusters they encounter, that is, the clusters in front of them act as an obstacle to the progress of these atoms. This phenomenon is called the "shading effect" and is very important for growth of nanorods. This method of obtaining nanorods by creating a high angle between the target and the substrate is called "high angle production" ("GLAD - Glancing" or "Oblique Angle Deposition").

[0036] It has been observed that CIGS nanorods can be produced between 200 nm and 600 nm with the GLAD method. Since nanorods with higher thickness will be likely to coalesce and it is not possible to absorb sufficient light at lower thickness, CIGS nanorods in this range were produced within the scope of the invention. In a conventional CIGS solar cell, n-type CdS thin film is produced on the absorber p-type main layer to both form a pn junction and provide a buffer layer. The thickness of this layer usually has a value in the range of 50-80 nm. Within the scope of the invention, CdS film was produced by RF sputtering method with a thickness of approximately 50 nm. The sputtering method was preferred in order to both obtain superior structural, optical, electrical properties and to enable a more successful conformal coating. CdS growth was carried out at room temperature for a more successful coating of CIGS nanorod and thin film samples. Additionally, low Ar gas pressure (5-10 mT) was preferred for a more compact and smooth thin film.

[0037] The last layers required to complete the CIGS solar cell are i:ZnO / AZO and ITO films, which serve as transparent conductive electrodes. These layers were also produced with low Ar gas (LPS) in RF sputtering system using a single quaternary target. These thin films, also called window layers, must be highly transparent and conductive. Temperature was used in the fabrication of each layer to simultaneously achieve both high transmittance (> 80% in the visible region) and low sheet resistance (-30-80 Q / sq). First, i:ZnO / AZO layers with a thickness of - 500 nm and at 300°C were obtained. A 100 nm thick ITO coating was fulfilled as the upper transparent conductive layer using 200°C temperature.

[0038] All of the absorber perovskite, HTL and ETL layers that make up the perovskite solar cell were synthesized in a glove-box cabinet with Ar gas flow.

[0039] In addition to the unique properties of perovskite solar cells, the stability problem continues due to the effects of external factors (oxygen, moisture, UV, etc.). To solve this problem, hole transport layer (HTL) is used. An ideal HTL should have high hole mobility, be stable, be easily synthesized, have a HOMO energy level compatible with the valence band of the perovskite layer, ensure hole conduction through the perovskite layer, and maintain the stability of the perovskite layer. Within the scope of the invention, Spiro-OMeTAD and PEDOT:PSS materials were used as hole transport layers. With the help of the spin coating device rotating at high speed, the coating was made with a thickness of approximately 300 nm by dropping the material onto the transparent conductive layer forming the interface of the tandem structure. At least one of these materials was used in tandem construction. Doping of Spiro-OMeTAD with Li- TFSI and TBP in chlorobenzene was also carried out.

[0040] In perovskite solar cells, the perovskite material, which is used as the active absorber layer to absorb light, consists of a hybrid organic / inorganic lead or halide- based perovskite component. The production of perovskite solar cells is basically divided into two: solution-based and vacuum-based. The most commonly used production method in the literature is the solution-based one-step and two-step perovskite production method, which is simple, practical and low-cost. Within the scope of the invention, perovskite solution was coated in an inert environment by both one- and two-step production methods. Either method can be used. In both methods, it is essential to drop an amount of perovskite solution on the substrate and spin it at high speed with the help of a spin coating device to obtain a perovskite layer with a thickness of ~ 500 nm. This surface, which is then covered with the solution, is annealed on the hot plate at the optimum temperature. In Figure 5, the one- and two- step production method of the perovskite absorber layer is shown schematically.

[0041] In most perovskite cells, compact or mesoporous metal oxides are used as electron transport layers (ETLs). ETL material should be selected suitable to the energy levels of the perovskite material. Within the scope of the invention, ~ 300 nm thick SnC>2, which is the material used to obtain high efficiency ETL, was used.

[0042] After perovskite solar cell production, ~ 100 nm thick ITO transparent conductive electrode coating was applied before coating the metal contacts. To complete the monolithic structure, a metal (Ag-silver) contact coating was applied as the final layer on the perovskite solar cell.

[0043] To summarize: CIGS / perovskite monolithic tandem flexible solar cell;

[0044] - Flexible substrate made of PI (polyimide) material,

[0045] - Molybdenum (Mo) metal conductive thin film coated on the substrate, used as the back metal contact, helps the CIGS layer to adhere better to the substrate, and increases absorption by reflecting the light that falls on the cell surface and remains unabsorbed after passing through CIGS, back to the CIGS layer,

[0046] - CIGS thin film produced by LPS method,

[0047] - CIGS nanorod produced by applying RF sputtering (PVD) and GLAD method,

[0048] - n-type CdS thin film, due to both creating a pn junction and providing a buffer layer on the absorber main layer,

[0049] - i:ZnO / AZO and ITO layers, which serve as transparent conductive electrodes produced with low Ar gas (LPS) using a single target in the RF sputtering system,

[0050] - As a holetransport layer in tandem structure; at least one of the Spiro-OMeTAD or PEDOT:PSS materials, obtained by dropping on the transparent conductive layer forming the interface of the tandem structure and rotating at high speed, and doped with Li-TFSI and TBP in chlorobenzene,

[0051] - Contains; SnO? as the electron transport layer and a metal (Ag-silver) contact coating as the final layer on the Perovskite solar cell.

Claims

CLAIMS1 . A CIGS / perovskite monolithic tandem flexible solar cell characterized by comprising;- A flexible substrate made of PI (polyimide) material,- A molybdenum (Mo) metal conductive thin film coated on the substrate, used as the back metal contact, helps the CIGS layer to adhere better to the substrate, and increases absorption by reflecting the light that falls on the cell surface and remains unabsorbed after passing through CIGS, back to the CIGS layer,- A CIGS thin film produced by LPS method,- A CIGS nanorod produced by applying RF sputtering (PVD) and GLAD method,- An n-type CdS thin film, due to both creating a pn junction and providing a buffer layer on the absorber main layer,- An i:ZnO / AZO and ITO layers, which serve as transparent conductive electrodes produced with low Ar gas (LPS) using a single target in the RF sputtering system,- At least one of the Spiro-OMeTAD or PEDOT:PSS materials as a hole transport layer in tandem structure obtained by dropping on the transparent conductive layer forming the interface of the tandem structure and rotating at high speed, and doped with Li-TFSI and TBP in chlorobenzene,- An SnO? as electron transport layer,- A metal (Ag-silver) contact coating as a final layer on the perovskite solar cell.

2. The CIGS / perovskite monolithic tandem flexible solar cell according to claim 1 characterized by comprising a successful bi-layer Mo thin film, with very low resistance (~ 14 pQ.cm) and high reflection properties, which was produced in two stages with RF sputtering as high pressure (HPS) was first used and then the production was completed by reducing the pressure (LPS) in order for the CIGS to adhere better to the substrate.

3. The CIGS / perovskite monolithic tandem flexible solar cell according to claim 1 characterized by the CIGS thin film has a thickness of -0.5-1 pm.

4. The CIGS / perovskite monolithic tandem flexible solar cell according to claim 1 characterized by nanorods having a thickness between 200 nm and 600 nm.

5. The CIGS / perovskite monolithic tandem flexible solar cell according to claim 1 characterized by having the CdS thin film with a thickness of 50 nm and produced by the RF sputtering method.

6. The CIGS / perovskite monolithic tandem flexible solar cell according to claim 1 characterized by comprising the i:ZnO / AZO layer with a thickness of - 500 nm.

7. The CIGS / perovskite monolithic tandem flexible solar cell according to claim 1 characterized by comprising a 100 nm thick ITO layer.

8. The CIGS / perovskite monolithic tandem flexible solar cell according to claim 1 characterized by comprising an electron transport layer with a thickness of ~ 300 nm.

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