超过22%的HIT电池

23rd European Photovoltaic Solar Energy Conference, 1-5 September 2008, Valencia, SpainOVER 22% EFFICIENT HIT SOLAR CELLHiroshi Kanno, Daisuke Ide, Yasufumi Tsunomura, Shigeharu Taira, Toshiaki Baba, Yukihiro Yoshimine, Mikio Taguchi, Toshihiro Kinoshita, Hitoshi Sakata, and Eiji Maruyama Advanced Energy Research Center, Sanyo Electric Co., Ltd, Kobe 651-2242, Japan Hiroshi.Kanno@ ABSTRACT: For reducing the power-generating costs, the following approaches are discussed; (i) enhancing the conversion efficiency, (ii) utilizing a thin wafer for the HIT cell, (iii) improving the temperature coefficient, and (iv) applying HIT solar cells to bifacial solar modules. Furthermore, we have exhibited a high potential for use on thinner c-Si wafers. The high conversion efficiency of 21.4% and an extremely high Voc of 0.739 V were obtained with 85-μm- thick HIT cell. The benefits of using HIT solar cells for bifacial solar modules are also described. Keywords: Heterojunction, Photovoltaic, High-Efficiency 1 INTRODUCTION By developing the high-quality i-type a-Si layer, the defect level on the c-Si surface can be pronouncedly reduced, and high Vocs can be obtained [1]. The excellent c-Si/a-Si hetero interface of the HIT structure leads to a outstandingly high Voc of over 0.72 V. Higher Voc allows for not only high conversion efficiency but also en excellent temperature coefficient, which excels over those of diffused c-Si solar cells. The feature results in higher output power even at operating temperatures. Sanyo has improved conversion efficiencies of HIT solar cells as shown in Fig. 2. Recently, we have reached 22% in April 2007 and shortly after that we renewed the record to 22.3% in July 2007. The key technologies for the improved conversion efficiency of HIT solar cells are following (i) to (iv). And each item is described in terms of reducing power-generation costs of HIT solar cells. (i) Enhancing the conversion efficiency (ii) Utilizing a thin wafer for the HIT cell (iii) Improving the temperature coefficient (iv) Apply HIT solar cells to a bifacial solar moduleWith increasing demands of the high quality solar cells from all over the world, Sanyo Electric plans to expand production of HIT solar cells up to over 600 MW by 2010. At the same time, we increase the pace of development to offer technical advantages into our future products. We have originated the high-efficiency solar cell structure known as Heterojunction with Intrinsic Thinlayer (HIT) structure, and been developing the quality of it. Recently we had updated the world’s highest conversion efficiency of 22.3 % with a practical-sized solar cell in June 2007. Also using the high-efficiency HIT cells, we have newly achieved a record conversion efficiency of 20.6% for a R&D prototype module, which had been certified by Advanced Industrial Science and Technology (AIST). The developed technologies are steadily being transferred to production. In this paper, the high efficiency technologies and characteristics of HIT solar cells are described. 2 FEATURES OF HIT SOLAR CELLSThe HIT solar cell is primarily characterized by its high conversion efficiency. Also it is appreciated for its excellent temperature coefficient, potentiality for the use of a very thin Si wafer, and application possibility for bifacial solar modules. As shown in Fig. 1, an intrinsic (i-type) amorphous silicon (a-Si) layer followed by a ptype a-Si layer is deposited on a randomly textured ntype CZ crystalline silicon (c-Si) wafer to form a p/n heterojunction. On the opposite side of the c-Si wafer, itype and n-type a-Si layers are deposited to obtain a Back Surface Field (BSF) structure. On both sides of the doped a-Si layers, Transparent Conducting Oxide (TCO) layers as an antireflective coating, and metal grid electrodes are formed. It is a matter of significance that the temperature for all of the processes forming the heterojunction interface can be below 200 ℃.Grid electrode p-type a-Si: ~ 0.01 µm i-type a-Si~ 0.01 µm24.0Conversion efficiency (%)22.3%22.0 20.0 18.0 16.0 14.0(Jul. 2007)22.0%(Apr. 2007)R&D prototype ( ~ 100 cm2 )96 98 00 02 04 06 08 10 YearFigure 2: Progress of the conversion efficiencies of HIT solar cells developed in R&D. 3. REDUCING THE POWER- GENERATING COSTS 3-1. Enhancing the conversion efficiency We target a high conversion efficiency of over 23% in our R&D by the end of 2010. As a result of our aggressive studies, we have achieved a record high conversion efficiency of 22.3% (Voc: 0.725 V, Jsc: 38.9c-Si (CZ, n-type)n-type a-Si: ~ 0.01 µmTCO(Transparent Conductive Oxide)Figure 1: The structure of a HIT solar cell113623rd European Photovoltaic Solar Energy Conference, 1-5 September 2008, Valencia, SpainmA/cm2, FF: 0.791, total area: 100.5 cm2, certified by AIST) in July 2007, as shown in Fig. 3. All the PV parameters such as Voc, Jsc and Fill Factor (FF) have been improved compared to those of the previous record of the 21.8% HIT cell. It is noted that over 2% increases of its Voc and Jsc indicate the potential of HIT solar cell for the targeted 23% efficiency.The high Voc of the HIT solar cell is achieved by the effective passivation of c-Si surface defects with a high-quality intrinsic a-Si layer. The following fabrication processes are being used in the development of these solar cells: ・ Cleaning the c-Si surface using low-cost wet cleaning processes before a-Si deposition ・ Depositing a high-quality intrinsic a-Si layer by chemical vapor deposition ・Maintaining low plasma and thermal damage to the c-Si surface and heterojunction while fabricating the a-Si, TCO layers and grid electrode. By using these processes, the carrier recombination is decreased, resulting in fewer localized states in the intrinsic layer and interface of the heterojunction [2]. We have decided to target a Voc value of more than 0.74 V by unifying these techniques to achieve our conversion efficiency goal (see Fig. 4). (b) Optimizing the grid electrode For a higher Jsc and FF, the grid electrode also requires a lower resistance and finer lines for a larger aperture. Grid electrodes of HIT solar cells are made of silver paste with high conductivity. The spreading area of the paste must be eliminated to lower the optical shadowing loss, and the height of the paste must be improved to reduce the resistance loss. We optimized the silver paste and the process parameters to achieve that performance. We have achieved a 78% width in the grid electrode with half the spreading area of previous HIT solar cell processes [3]. We have also simulated the relationship between aspect ratio and conversion efficiency, it will be a 1.6% increase in conversion efficiency relative to a conventional HIT solar cell with 21.8% efficiency to improve the aspect ratio by more than 1 without a spreading area, where the aspect ratio is equal to the cross-sectional area divided by the width of the exclusive spreading area squared [4]. (c) Reducing the absorption loss HIT structure uses the TCO on the surface to collect the generating carriers. A lower level of optical absorption loss and a higher electrical conductivity for the TCO layer will lead to a higher Jsc and FF. The optical absorption loss in the TCO is mainly caused by free carrier absorption. The deposition conditions of the TCO have been optimized to obtain a high-quality TCO layer with high carrier mobility. The new TCO shows better sensitivity in the long wavelength region (>1,000 nm) of the IQE spectra. Furthermore, the high FF value (0.791) in the HIT solar cell using the new TCO suggests high conductivity for high carrier mobility [5]. 3-2. Utilizing a thin wafer for the HIT cell The wafer accounts for about half the cost of a solar module. We have been developing thinner HIT solar cells to reduce the power-generating cost. The HIT solar cell has a front and back symmetrical structure, which is suitable for thinner Si wafers. Figure 5 shows a picture of a HIT solar cell with an 85-μm-thick c-Si wafer. We see no sagging because the effect of thermal stress is suppressed by its symmetrical structure. This shows high potential for HIT solar cells with a thinner c-Si wafer.Figure 3: I-V characteristics of the HIT solar cell certified by AIST, which shows the world’s highest conversion efficiency of 22.3% for a practical size cell (> 100 cm2). As an approach for a further high-quality product, we have been developing module technologies for maximizing long-term stability and module efficiency taking advantage of HIT cell’s excellent features. As a result, we have demonstrated the very high conversion efficiency with a R&D prototype module. The module efficiency has reached 20.6 %, certified by AIST in December 2007. This achievement also endorses the future superiority of HIT modules. We have been focusing on the following techniques for obtaining high conversion efficiency with the HIT structure: (a) Improving the HIT structure by enhancing the a-Si/c-Si heterojunction properties for a higher Voc, (b) Improving the grid electrode for a higher Jsc and, FF, and (c) Reducing the absorption loss of the TCO layer for a higher Jsc. (a) Improving the HIT structureTarget0.74 0.72 0.70High-quality a-Si layersVoc (V)0.68 0.66 0.64Low-damage process during deposition of a-Si layers Cleaning process for c-Si surface500 1000 1500 2000 25000~ ~~ ~Minority carrier lifetime (µsec)Figure 4: Vocs of HIT solar cells as functions of minority carrier lifetimes.113723rd European Photovoltaic Solar Energy Conference, 1-5 September 2008, Valencia, SpainFigure 6 depicts the relationship between the Iscs, Vocs and conversion efficiencies of HIT solar cells as functions of various HIT cell thicknesses. The values are normalized by that of 165-μm-thick HIT cell. It is clearly shown that with decreasing thickness, Voc is increased, by contrast, Isc is decreased. The increase of Voc accounts for an extremely low surface recombination by the excellent passivation on the c-Si surface. Meanwhile, decreased Isc results from the insufficient optical absorption in thin c-Si wafers. This fact suggests that when the surface recombination is ideally reduced, the conversion efficiency only suffers minimal decrease even at 85-microns-thick The J-V curve of the HIT solar cell on an 85-μmthick c-Si wafer is shown in Fig. 7. We obtained a high conversion efficiency of 21.4% with this HIT solar cell. The Voc is an extremely high value of 0.739 V, the Jsc is 37.3 mA/cm2, and the FF is 77.6% [6]. These values were measured under standard conditions in-house. The Jsc value with the 85-μm-thick wafer decreased about 3% in comparison with that of a 165-μm-thick wafer. Those values corresponded approximately to the values calculated using SUNRAYS [7]. These results suggest that the HIT structure is suitable for thinner Si wafers with high conversion efficiency. However, the light confinement design is a very important factor for further improving the Jsc in a thin HIT cell structure. This will enable HIT structure to reduce the wafer cost while maintaining high conversion efficiency.40 35 30 25 20 15 10 5 0 0Current density (mA/cm)2Eff.: 21.4% Voc: 0.739 V Jsc: 37.3 mA/cm2 FF: 0.7760.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Voltage (V)Figure 7: The J-V curve of a HIT solar cell with an 85-μm- thick c-Si wafer. (in-house measurement: AM 1.5, 1 SUN, 25.0 C-degrees, cell size (aperture size): 103.3 cm2) 3-3. Improving the temperature coefficient Generally, an excellent temperature coefficient for the conversion efficiency enables high performance in outdoor use, which leads to a number of user benefits. As shown in Fig. 8, the temperature coefficient mainly depends on the Voc of the solar cell. Therefore, the HIT solar cell with its high Voc exhibits an excellent temperature coefficient [8,9]. The temperature coefficient of the HIT solar cell with 0.736 V is -0.23 %/ºC. This is comparable to that of an a-Si solar cell, which is generally said to have an excellent temperature coefficient. Consequently, improving the temperature coefficient helps to reduce power-generating costs by further improving the Voc, such as by thinning wafers.Figure 5: A HIT solar cell using 85-μm-thick c-Si wafer. Note that no sagging is seen.1.04 Isc Voc EffRelative value to 165-m m-thick cell1.021.000.980.96 50 100 150 200 HIT cell ThicknessFigure 6: The Iscs, Vocs and conversion efficiencies of HIT solar cells as functions of various HIT cell thicknesses. The values are normalized by that of 165μm- thick HIT cell.Figure 8: Temperature coefficients of conversion efficiencies as functions of the Vocs of solar cells 3-4. Applying HIT solar cells to a bifacial solar module The front and back symmetrical structure of the HIT solar cell, as shown in Figure 1, allows it to be applied to a bifacial solar module using a transparent back sheet such as glass. When we install this module, scattered light and light that is reflected from the ground are incident on the solar module from the backside. In our experiments, the output power of a bifacial solar module was higher by around 20% than that of a monofacial HIT solar module. That value depends on 113823rd European Photovoltaic Solar Energy Conference, 1-5 September 2008, Valencia, Spainmounting conditions, such as a mounting angle, a ground reflectance, and an amount interval of a module. Perpendicular mounting allows us to apply it, for example, to a fence, a door, or a handrail. Horizontal mounting allows us to apply it. As shown in Fig. 9, the bifacial solar modules are applied for a skylight.4. SUMMARY We have described approaches for reducing the power-generating costs. We have been focusing on the following techniques for obtaining high conversion efficiency with the HIT structure: (a) Improving the HIT structure by enhancing the a-Si/c-Si heterojunction properties for a higher Voc, (b) Improving the grid electrode for a higher Jsc and, FF, and (c) Reducing the absorption loss of the TCO layer for a higher Jsc. For utilizing thinner Si wafers, we have exhibited a high potential for use on thinner c-Si wafers. We obtained the high conversion efficiency of 21.4% and an extremely high Voc of 0.739 V (in-house measurement: AM 1.5, 1 SUN, 25 C-degree, cell thickness: 85 μm: cell size (AP): 103.3 cm2). The Voc increases with decreasing thickness of the c-Si wafer due to the extremely low surface recombination with the excellent passivation on the c-Si surface. Applying HIT solar cells to a bifacial solar module has been commercialized as the "HIT Double," which is used for a wide range of applications for practically generating electricity. Consequently, improving the power-generating costs of HIT solar cells will make it possible to strengthen its superiority in this field.Figure 9: HIT Double used for a sunlight in Kobe, Japan Figure 10 shows output power by the dependence on the mounting tilt angle. The output power of the bifacial modules is higher than those of the monofacial modules (standard module). We effectively increased the output power of the bifacial solar modules with increasing title angles over those of monofacial HIT modules. At a tilt angle of 60 degrees, the increase of output power relative to that the monofacial module is 25%. Meanwhile the relative increase is 17% at a tilet angle of 30 degrees. This allows the HIT Double to be used in a wide range of applications. It is, however, noted that the amount of increase of output power with bifacial solar modules greatly depends on installation environments.REFERENCES[1] M. Taguchi et al., “Improvement of the Conversion Efficiency of Polycrystalline Silicon Thin Film Solar Cell”, Fifth PVSEC, pp. 689-692, 1990. [2] M. Tanaka et al., “Development of New a-Si/c-Si Heterojunction Solar Cells: ACJ-HIT (Artificially Constructed Junction-Heterojunction with Intrinsic Thin-Layer)”, Jpn. J. Appl. Phys. 31, pp. 3518-3522, 1992. [3] E. Maruyama et al., “Sanyo’s Challenges to the Development of High-efficiency HIT Solar Cells and the Expansion of HIT Business”, Fourth WCPEC, pp. 1455-1460, 2006. [4] S. Taira et al., “Our Approaches for Achieving HIT Solar Cells With More Than 23% Efficiency”, 22nd EU-PVSEC, pp. 932-935, 2007. [5] Y. Tsunomura et al., “Twenty-two percent efficiency HIT solar cell”, Solar Energy Materials & Solar Cells, 2008 in press. [6] D. Ide, et al., "Excellent power-generating properties by using the HIT structure", 33rd IEEE PVSEC, to be published, 2008 [7] R. Brendel, “SUNRAYS: A Versatile Solar Cell Ray Tracing Program for the Photovoltaic Community”, Twelfth EC Photovoltaic and Solar Energy Conf., p. 1339, 1994. [8] S. Taira et al., “Temperature Properties of High-Voc HIT Solar Cells”, Renewable energy, pp. 115-118, 2006. [9] A. Terakawa et al., “High Efficiency HIT Solar Cells and the Effects of Open Circuit Voltage on Temperature Coefficients”, Fifteenth PVSEC, 2005.Ave. Power generation (a.u.)140 120 100 80 60 40 20 0 30° 45° Tilt Angle 60°Figure 10: Comparison of output power between standard and bifacial modules with various mounting tilt angle. Values are normalized at that of a HIT standard module with a tilt angle of 30 degrees. Ground reflectance: 0.4, Module height from the ground: 3.28 feet, Facing direction: south. Date: May 22 - June 21.1139。