Review—Development History of High Efficiency Silicon Heterojunction Solar Cell From Discovery to Practical Use-solarbe文库

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ECS Journal of Solid State Science and Technology OPEN ACCESS ReviewDevelopment History of High Efficiency Silicon Heterojunction Solar Cell From Discovery to Practical Use To cite this article Mikio Taguchi 2021 ECS J. Solid State Sci. Technol. 10 025002 View the article online for updates and enhancements. This content was downloaded from IP address 123.139.57.166 on 28/06/2021 at 1205 ReviewDevelopment History of High Efﬁciency Silicon Heterojunction Solar Cell From Discovery to Practical Use Mikio Taguchi z Energy System Strategic Business Division, Life Solutions Company, Panasonic Corporation, Japan Silicon heterojunction SHJ solar cells are attracting attention as high-efﬁciency Si solar cells. The features of SHJ solar cells are 1 high efﬁciency, 2 good temperature characteristics, that is, a small output decrease even in the temperature environment actually used, 3 easy application to double-sided power generation bifacial module using symmetric structure. We have developed and actively evolved this SHJ solar cells from early 1990s, and introduced the module equipped with SHJ solar cells named as well-known “HIT ® ” in 1997. Since then, we have produced more than 500 MW of HIT ® per year for over 20 years. Although several companies have entered the market along the way, we are the only company with this scale of production. In this paper, we will discuss the history of the development, the unique feature of this solar cell, the technology development required to fabricate the module using these solar cells, and the efforts made to ensure reliability. By sharing our knowledge and reliability technology we have developed, we hope to accelerate the spread of SHJ solar cells, which are expected to become the next mainstream solar cells. © 2021 The Authors. Published on behalf of The Electrochemical Society by IOP Publishing Limited. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License CC BY- NC-ND, http//creativecommons.org/licenses/by-nc-nd/4.0/, which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email permissionsioppublishing.org. [DOI 10.1149/2162-8777/abdfb6] Manuscript submitted October 12, 2020; revised manuscript received January 19, 2021. Published February 5, 2021. This paper is part of the JSS Focus Issue on Photovoltaics for the 21st Century. The cumulative installed capacity for PV in the world at the end of 2019 reached at least 627 GW, 1 and further increases are expected as we move into the Terawatt era. The majority of PV systems use Si solar cells, which have been in use since around 1970 and were expensive at the time, but now provide electricity cheaper than any other form of energy, depending on the installation environment. In addition to the cost savings of mass production, technological advances in conversion efﬁciency have contributed to lowering the overall cost of the power generation system, and thus the LCOE Levelized Cost of Electricity, by lowering the cost of everything except the PV module, which is known as the BOS Balance of System cost. The technology to improve the conversion efﬁciency of Si solar cells was greatly developed in the 1980s, 2,3 and this was due to the technology of defect engineering at the semiconductor edge so called passivation of the Si surface with SiOx-based insulating ﬁlms. 4,5 It is important to reduce the amount of recombination due to these defects before the electrons and holes are separated by a pn junction and can be extracted as a photo current. The PERC Passivated Emitter and Rear Cell structure, which is now the mainstream technology for high performance Si solar cells, was developed in this period. 6 On the other hand, a high performance Si solar cell has been developed with a different approach. It is a heterojunction HJ solar cell. There are not many reports on amorphous and crystalline heterojunctions before 1980. In 1972, Jayadevaiah reported that the current-voltage curves of a-Si/c-Si heterojunctions showed rectifying characteristics, 7 and in 1975, Brodsky and Döhler reported the hopping conduction in a heterojunction between evaporated a-Si and c-Si through the level near the Fermi level in a-Si. 8 Since these heterojunctions were fabricated with unhydrogenated a-Si ﬁlms, it was clear that a-Si ﬁlms had a large number of localized states, so that it would have been difﬁcult to accurately discuss its potential as a semiconductor device. According to De Wolf et al., 9 the ﬁrst hydrogenated a-Si/c-Si heterostructures were studied in 1974 by Fuhs and coworkers. 10 They tried to combine c-Si and a-SiH technology to produce HJ solar cells. However, they could not attain satisfactory cell efﬁcien- cies. In the 1980s, following the second oil shock, the development of solar cells became active in Japan with the support by NEDO New Energy and Industrial Technology Development Organization as a part of the New sunshine Program under the Ministry of International Trade and Industry. Our group has been developing a-Si solar cells since 1975, and has been actively engaged in research and development, such as the release of the ﬁrst calculator with solar cells in 1980 11 and the world’s highest-level efﬁciency at that time. 12 The technology to form high quality a-SiH ﬁlms, which was cultivated in the development of a-Si solar cells, was the basis for the development of a-SiH/c-Si HJ solar cells. Tandem solar cells, which consist of stacked a-SiH solar cells and crystalline Si solar cells, were studied as a way to increase the efﬁciency of solar cells by using light in the long wavelength region λ 800 nm, where a-SiH solar cells are not sensitive. Hamakawa et al. reported a tandem solar cell that uses a-SiH cells and a-SiH/ polycrystalline silicon HJ cells in 1983. 13 This was the ﬁrst and important results of SHJ solar cells. Following that, Rahman et al. reported a-SiCH/c-Si HJ solar cells using a-SiCH as a window layer, 14 and Mimura et al. reported a Visicon targets using a-SiH/c- Si heterojunctions. 15 In the late 1980s, we were developing thin-ﬁlm polysilicon solar cells with the aim of creating tandem solar cells consisting of stacked a-SiH and thin-ﬁlm polysilicon solar cells. Inﬂuenced by Hamakawa and coworkers’ ﬁndings, we started to develop an a-Si H/c-Si junction formation technique to evaluate the quality of thin- ﬁlm polycrystalline silicon materials for solar cells. In the course of the fabrication and evaluation of a-SiH/c-Si HJ solar cells, we have developed a new structure that featured the introduction of a thin buffer layer of undoped a-SiH between doped a-SiH and wafer. In this structure, the surface recombination was effectively suppressed and the open circuit voltage and ﬁll factor were improved. 16,17 It was found that high performance solar cells can be produced by a simple manufacturing process, and in particular, solar cells with high open-circuit voltage which indicates effective suppression of the surface recombination can be easily obtained. The Heterojunction Solar Cell In SHJ solar cell shown in Fig. 1, the dangling bonds on the c-Si surface are covered with a-SiH. a-SiH is a semiconductor, which z E-mail taguchi.mikiojp.panasonic.com ECS Journal of Solid State Science and Technology, 2021 10 025002 has the same short-range order as c-Si, and a larger band gap than c- Si. While a-SiH and c-Si take good covalent bonds, the remaining dangling bonds are terminated by hydrogen atoms. An energy barrier created at the interface between n-type a-SiH and c-Si, effectively blocks holes. Since a-SiH can passivate the c-Si surface effectively and is itself electrically conductive, silicon heterojunction solar cell successfully extracts photo-generated carriers while minimizing the carrier recombination loss. Accordingly, it is referred to as a “passivated contact” solar cell as well as a tunnel contact structure. 18 In SHJ solar cells, however, such tunneling does not necessarily occur, and the thermionic electron emission model dominates the current transport across the heterojunction interface. 19 Compared to solar cells that have metal-semiconductor contact such as the PERC, the SHJ solar cell shows much lower recombination rate, resulting in higher voltage. This feature of high voltage leads to another feature of small output drop during high temperature operation. In addition, the manufacturing process is simple and can be made at low temperatures below 200 °C, the tolerance for bulk wafer quality variation is high, 20 and thus manufacturing yield both production and characteristic yields is high. The symmetrical structure of the front and back sides of the wafer makes it highly adaptable to thinner wafers and bifacial modules are inherently easy to make. Development History Our motivation was to develop low-temperature emitters that could be applied to thin-ﬁlm polysilicon solar cells up to several microns in thickness. The diffusion process and other common processes are not suitable for such a formation of junctions in thin ﬁlm power generation layers. We attempted to form junctions by depositing a-SiH thin ﬁlms, following the structure of Hamakawa et al. 13 We started with the development of junction formation technology using single-crystal Si, which has well-deﬁned properties. The n-type c-Si was used for the power generation layer because the tandem type was the original objective. Initially, the structure of doped a-SiH ﬁlms deposited directly on c-Si was evaluated with only low FF and Voc, suggesting large reverse leakage currents and large recombination rates at the hetero- interface. We came up with the idea of inserting an undoped a-SiH layer to separate the doped layers. One reason is the high number of defects in the doped a-SiH ﬁlm. Since the doped a-SiH layer has many localized states in the bandgap, the recombination through these localized states is considered to occur with high probability at the a- SiH/c-Si hetero-interface. The other reason was that we thought that boron diffusion from the a-SiH p-layer to the n-type c-Si would occur during ﬁlm deposition by plasma CVD even at low tempera- tures, which would interfere with the doping proﬁle as designed. As a result, the open circuit voltage and FF were improved and the effect of the improved junction characteristics was conﬁrmed. 16 The backside of the cell was then studied. 21 It was conﬁrmed that inserting an i-layer at the n c-Si/n a-SiH interface on the backside also reduced the interfacial recombination, and the basic form of a- SiH/c-Si HJ solar cells was established. The structure is similar to that of a double hetero semiconductor laser, because it is essential for both devices to conﬁne charge carriers to the semiconductor active layer by sandwiching it with wider bandgap layers. A conversion efﬁciency of 14.5 4 mm 2 22 was obtained just one year after the start of the research, and a few years later, by optimizing the formation conditions with the same structure, a conversion efﬁciency of 20 was achieved even for a small area of 1cm 2 , 21 which we believe this is because this structure was inherently superior as a junction formation technique. Figure 1. Silicon heterojunction solar cell. Figure 2. History of development of SHJ solar cell in Panasonic from the beginning of the study to the start of production. ECS Journal of Solid State Science and Technology, 2021 10 025002 After achieving a conversion efﬁciency of 20 in a small area with this structure, we developed the proprietary technology needed for commercialization of our SHJ solar cells, such as adopting silver Ag paste that hardens at low temperatures, and launched the SHJ photovoltaic module HIT ® in 1997. Figure 2 shows our history of the development of SHJ solar cells from the early stages to commercialization. In 2000, we released the ﬁrst bifacial PV module HIT ® Double that utilizes the symmetrical structure of this solar cell. Some examples of the installation are shown in the Fig. 3. In recent years, bifacial modules have been attracting a lot of attention because indirect light can also be used for power generation, but we were almost 20 years ahead of the times. Meanwhile, in RD, we continued our efforts to further improve the conversion efﬁciency of HIT ® . The progress in conversion efﬁciency since 2000 is shown in Fig. 4. The cell sizes here are all practical sizes of 100 cm 2 or larger. Since the basic structure had already been completed, we continued to improve the fabrication of the interface, the Ag paste material to achieve ﬁner lines, and the development of high-mobility TCO Transparent Conductive Oxide materials using the reactive plasma deposition method, and con- tinued to brush up these technologies to explore the possibilities of SHJ solar cells. As a result, we have repeatedly broken the world’s record for conversion efﬁciency of practical-sized 100 cm 2 Si solar cells. 23–26 In addition, since 2008, we have been actively developing thin wafers to reduce Si costs by utilizing the characteristics of SHJ. In 2009, we conﬁrmed that it could achieve almost the same efﬁciency on thin wafers, 25 and as a result of subsequent efforts to improve output on thin wafers, we obtained a conversion efﬁciency of 24.7 in 2013 with the 98 micron thick c-Si wafer. 27 When it came to the stage of obtaining nearly 25 efﬁciency, the loss analysis showed that the shadow loss on the light incident side became signiﬁcant, and in order to obtain a higher conversion efﬁciency, a back-electrode type solar cell was indispensable, and as a result of further accelerated development and improved tech- nology, a high conversion efﬁciency of 25.6 was obtained in 2014. 28 The conversion efﬁciency of 25.0 for a 2 cm square cell, 29 which was the world record for the conversion efﬁciency of Si solar cells at that time, was greatly improved for the ﬁrst time in 15 years in both efﬁciency and size. Stimulated by our results, many institutions have made efforts to further improve their efﬁciency. The current world records for conversion efﬁciency of Si solar cells in practical sizes are held by Hanergy for bifacial structure and Kaneka for back-electrode type structure, both based on heterojunction technology. 30 This is good evidence that this technology is leading the ﬁeld of Si solar cells. Since 2014, we have focused on improving efﬁciency on our production lines, reaching a line average of 22.9. Meanwhile, for back-electrode type structure, we are continuing to develop a lower cost process technology that is more suitable for commercialization by eliminating the use of photo-lithography for the patterning process, which is a different approach from the photoli