Excellent power generating properties by using the hit structure

Excellent power-generating properties by using the HIT structure Daisuke Ide, Mikio Taguchi, Yukihiro Yoshimine, Toshiaki Baba, Toshihiro Kinoshita, Hiroshi Kanno, Hitoshi Sakata, Eiji Maruyama and Makoto TanakaAdvanced Energy Research Center, SANYO Electric Co., Ltd., JapanCorresponding e-mail: daisuke.ide@ABSTRACTWe are developing HIT solar cells with high conversion efficiency, which was achieved the world’s highest conversion efficiency of 22.3% in a practical size solar cell in July 2007. We have four main approaches to reducing power-generating cost: improve the conversion efficiency, apply the HIT structure to a thin wafer, improve the temperature coefficient, and apply HIT solar cells to a bifacial solar module. Using these approaches, we have achieved the remarkably high conversion efficiency of 21.4% due to a high Voc of 0.739 V with an 85-μm cell, which was measured at SANYO. A thinner Si wafer brings not only high Voc but also generating more output power at high temperature for a better temperature coefficient. We have confirmed that the HIT structure is suitable for use in thinner wafers, allowing us to reduce power-generating cost.1. INTRODUCTIONGlobal demand for solar energy is increasing by an annual rate of more than 50% from 2006 to 2007. The world production volume of solar modules reached a level of more than 3,700 MW per year in 2007. To meet this demand, SANYO has decided to increase the annual production capacity of its HIT (Heterojunction with Intrinsic Thin-layer) solar modules to over 600 MW by FY2010.In order to further expand the use of solar modules, it is necessary to reduce the power-generating cost. We have developed four approaches to do this. With our first approach, i.e., raising the conversion efficiency, we plan to raise cell conversion efficiency to more than 23% in the laboratory by the end of 2010. Therefore, we must simultaneously improve open circuit voltage (Voc) and short circuit current density (Jsc) by more than 2% each. The second approach of using a thinner Si wafer applies to HIT solar cells. The third approach of improving the temperature coefficient generates more output power in high temperatures. The fourth approach of applying HIT solar cells to a bifacial solar module, which has been commercialized as the "HIT Double TM," can be used in a wide range of applications.2. STRUCTURE OF THE HIT SOLAR CELLIn the HIT solar cell structure shown in Figure 1, an intrinsic (i-type) amorphous silicon (a-Si) layer followed by a p-type a-Si layer is deposited on a randomly textured n-type CZ crystalline silicon (c-Si) wafer to form a p/n heterojunction. On the opposite side of the c-Si wafer, intrinsic 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 by using a screen-printing method. The temperature for all of the processes used to fabricate the heterojunction interface of this HIT structure is below 200 ℃.Fig. 1 Schematic view of a HIT solar cellBy inserting the high-quality intrinsic a-Si layer, the defect level on the c-Si surface can be effectively reduced, and a high Voc can be obtained [1].The excellent c-Si/a-Si hetero interface of the HIT structure leads to an extremely high Voc of more than 0.72 V. The higher Voc allows not only high conversion efficiency but also a good temperature coefficient, which excels over those of diffused c-Si solar cells and results in higher output power even athightemperatures.3. APPROACHES FOR REDUCING THE POWER-GENERATING COSTWe plan to use the following four approaches to reduce the power-generating cost of HIT solar cells: (ⅰ) improving the conversion efficiency(ⅱ) apply the HIT structure to a thin wafer(ⅲ) improving the temperature coefficient(ⅳ) apply HIT solar cells to a bifacial solar module (ⅰ) Improving the conversion efficiencyWe are aiming to achieve a high efficiency of more than 23% in our laboratory by the end of 2010. As a result of our progressive studies, we achieved a new world record of 22.3% conversion efficiency (Voc: 0.725 V, Jsc: 38.9 mA/cm2, FF: 0.791, total area: 100.5 cm2, confirmed by AIST) in July 2007, as shown in Figure 2. The Voc, Jsc and Fill Factor (FF) were simultaneously improved compared to those of the previous 21.8% HIT solar cell.Furthermore, we will simultaneously improve the Voc and Jsc by more than 2% each, to achieve our target of conversion efficiency.Fig. 2 I-V characteristics of the HIT solar cell that was confirmed to have the world’s highest conversion efficiency of 22.3% for a practical size cell (> 100 cm2) We are focusing on the following techniques for obtaining high conversion efficiency with the HIT structure:(a) Improving the HIT structure by enhancing thea-Si/c-Si heterojunction properties for a higherVoc(b) Improving the grid electrode for a higher Jscand, FF(c) Reducing the absorption loss of the TCO layerfor a higher Jsc.(a) Improving the HIT structureThe 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.(b) Optimizing the grid electrodeFor a higher Jsc and FF, the grid electrode simultaneously requires a lower resistance and finer lines for a larger aperture.The grid electrode of the HIT solar cell is 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 viscosity of the silver paste and the parameters in the screen-printing process 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.(c) Reducing the absorption lossThe HIT structure uses the TCO on the surface tocollect 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. Figure 3 shows the internal quantum efficiency (IQE) spectra of HIT solar cells with the newly developed TCO and those with our previous TCO. These TCO films were deposited on HIT solar cells fabricated with the same thickness. 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 [4]. Our efforts to reduce the absorption in TCO include the application of high-quality TCO.0.00.20.40.60.81.0800900100011001200Wavelength (nm)I Q E Previous TCONew TCOFig. 3 IQE spectra of HIT solar cells, with improved TCO transparency in the long wavelength region(ⅱ) Applying the HIT structure to a thin waferFig. 4 A HIT solar cell on two blocks (85-μm -thick c-Si wafer)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 4 shows a picture of a HIT solar cell with an 85-μm-thickc-Si wafer. We can 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.The J-V curve of the HIT solar cell on an 85-μm- thick c-Si wafer is shown in Figure 5. 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, theJsc is 37.3 mA/cm 2, and the FF is 77.6%. These values were measured under standard conditions at SANYO. With regard to parameters, as the thickness of the c-Si wafer decreases, the Voc increases because of an extremely low surface recombination velocity due to the excellent passivation on the c-Si surface. Meanwhile, the Jsc decreases due to insufficient long wavelength light confinement in the Si wafer. The Jsc value with the 85-μm-thick wafer decreased about 3% in comparison with that of a 200-μm-thick wafer. Those values corresponded approximately to the values calculated using SUNRAYS [5].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.Fig. 5 The J-V curve of a HIT solar cell with an 85-μm - thick c-Si wafer, which was measured at SANYO (measurement conditions: AM 1.5, 1 SUN, 25.0 ℃, cellsize (aperture size): 103.3 cm 2)(ⅲ) Improving the temperature coefficientGenerally, an excellent temperature coefficient for the conversion efficiency enables high performance inoutdoor use, which leads to a number of user benefits. As shown in Figure 6, 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 [6][7]. 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 cost by further improving the Voc, such as by thinning the wafer.Fig. 6The relationship between the temperature coefficient of conversion efficiency and the Voc of solar cells(ⅳ) Applying HIT solar cells to a bifacial solar moduleFig. 7 Bifacial HIT modules commercialized as the HIT Double TM (Left: Front view, Right: Back view)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. Figure 7 shows photographs of a bifacial HIT module.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 mounting 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, for example, to the roof of a carport or a bus stop shelter (see Figure 8).Fig. 8 A horizontal mounting example of the HIT Double TM on a bus stop shelter in JapanFig. 9 A simulation of the annual amount of output power by the dependence on the mounting tilt angle [8] Module power: 10 kW, Site: Osaka, Japan, latitude:35 deg, Ground reflectance: 0.3, Module height fromthe ground: 2 m, Facing direction: southFigure 9 shows a simulation of the annual amount of output power by the dependence on the mounting tilt angle. The output power of the bifacial module, as shown by the red line, is higher than that of the monofacial module, as shown by the blue line. We effectively increased the output power of the bifacial solar module in perpendicular mounting (tilt angle: 90 deg.) over that of the monofacial HIT module. The fluctuation in the output power of the bifacial solar module is also smaller than that of the monofacial solar module. For example, the output power in a perpendicular mounting decreases only 10% relative to that of the horizontal mounting in the bifacial module. This allows the HIT Double TM to be used in a wide range of applications.4. SUMMARYWe have developed four approaches for reducing the power-generating cost.With our first approach, i.e., raising the conversion efficiency, we plan to raise the cell conversion efficiency to more than 23% in the laboratory by the end of 2010. To do this, we must simultaneously improve the Voc and Jsc by more than 2% each.The second approach is to use thinner Si wafers. This applies to HIT solar cells, which exhibit a high potential for use on thinner c-Si wafers. We obtained a high conversion efficiency of 21.4% and an extremely high Voc value of 0.739 V (AM 1.5, 1 SUN, 25 ℃, cell thickness: 85 μm: cell size (AP): 103.3 cm2: measured at SANYO). The Voc increases as the thickness of the c-Si wafer decreases, due to the extremely low surface recombination velocity with excellent passivation on the c-Si surface.The third approach is to improve the temperature coefficient to generate more output power in high temperatures by further improving the Voc, such as by thinning the wafer and improving HIT structure.The fourth approach is to apply HIT solar cells to a bifacial solar module, which has been commercialized as the "HIT Double TM," which is used for a wide range of applications for practically generating electricity.Consequently, improving the power-generating cost of HIT solar cells will make it possible to expand the use of solar cells to achieve a society that does not depend on fossil fuels.REFERENCES[1] M. Taguchi et al., “Improvement of the ConversionEfficiency of Polycrystalline Silicon Thin Film Solar Cell”, Fifth PVSEC, 1990, pp. 689-692.[2] M. Tanaka et al., “Development of New a-Si/c-SiHeterojunction Solar Cells: ACJ-HIT (Artificially Constructed Junction-Heterojunction with Intrinsic Thin-Layer)”, Jpn. J. Appl. Phys. 31, 1992, pp.3518-3522.[3] E. Maruyama et al., “Sanyo’s Challenges to theDevelopment of High-efficiency HIT Solar Cells and the Expansion of HIT Business”, Fourth WCPEC, 2006, pp. 1455-1460.[4] Y. Tsunomura et al., “22%-Efficiency HIT SolarCell”, Seventeenth PVSEC, 2007.[5] R. Brendel, “SUNRAYS: A Versatile Solar Cell RayTracing Program for the Photovoltaic Community”, Twelfth EC Photovoltaic and Solar Energy Conf., 1994, p. 1339.[6] S. Taira et al., “Temperature Properties ofHigh-Voc HIT Solar Cells”, Renewable energy, 2006, pp. 115-118.[7] A. Terakawa et al., “High Efficiency HIT SolarCells and the Effects of Open Circuit Voltage on Temperature Coefficients”, Fifteenth PVSEC, 2005.[8] H. Sakata et al., “20.7% Highest Efficiency LargeArea HIT™ Cell”, Twenty-eighth IEEE PVSC,2000.。