Current Status and Future Trends of Amorphous Silicon Solar Cells Masahiro Sakurai Toshiaki Sakai 1. Introduction tion Roadmap for Year 2030 (PV2030)” which is a long- term strategy for technical development from 2004 The Kyoto Protocol for preventing global warming through 2030. Here, it is assumed that by the 2030, was adopted at the Conference of Parties III (COP3) of photovoltaic power generation will provide approxi- The United Nations Framework Convention on Cli- mately one-half of the electric power for household-use mate Change held in Kyoto, Japan in December 1997. (approximately 100 GW as a photovoltaic power gener- This Protocol established numerical targets for reduc- ating system), and therefore, technical innovation and ing greenhouse gas emissions in 2010 for each of the system use have been studied in order to improve participating industrialized countries. In February economic efficiency and to expand applicability. 2005, the Kyoto Protocol was enacted and full-scale Meanwhile, the production of solar cells has in- efforts began to reduce carbon dioxide (CO2) emissions, creased dramatically since 1997, and the produced and subsequent efforts to introduce new energy having capacity of 288 MW in 2000 has undergone a more a low impact on the environment are expected to than 6.1-fold increase to 1,759 MW by 2005. Japan become invigorated. accounts for 833 MW or approximately 47 % of the Among the types of new energy, photovoltaic power total production quantity. Figure 1 shows the share of generation systems emit no CO2 while generating total solar cell production by country. Of the solar cells power, and their widespread use is highly anticipated. produced in 2005, crystalline silicon (Si) solar cells The Japanese government is promoting adoption of accounted for 84 %, and thin film Si accounted for new energy by adopting: approximately 14 %, but due to the problem of deple- (1) measures involving power generation (q expand- tion of Si raw material, the share of amorphous silicon ed adoption by the public sector, and w technical (a-Si) and CIS (copper-indium-selenium) solar cells is development to promote lower cost, higher effi- expected to increase. ciency, etc.) Fuji Electric began research and development on (2) measures involving heat (q comprehensive plans a-Si solar cells in 1978. In 1980, Fuji successfully for local governments to introduce new forms of developed and sold the world’s first commercial solar energy, w promotion of the Biomass Nippon Total cells for use in calculators. Since 1980, Fuji has been Strategy, and e promotion of the introduction of contracted to perform research under the Sunshine biomass-derived fuel as transportation fuel, etc.) Program administrated by the Agency of Industrial and other measures in order to achieve the projected Science and Technology that belongs to the Japanese level of adoption of new energy in 2010 as established by the Investigative Committee on Natural Resources Fig.1 Solar cell production share (data from 2006 PV News) and Energy. In particular, to achieve the goal of photovoltaic 900 Other Japan 833 power generation of 4,820 MW by the year 2010, 800 302 MW Europe USA 17.2 % USA various assisting businesses (such as the Program for 700 154 MW Other 8.8 % Japan 602 Infrastructural Development of Introduction of Resi- 600 833 MW 47.3 % dential PV System and Field Test Project on Photovol- 500 470 Europe taic Power Generation System for Industrial and Other 400 470 MW 26.7 % 364 314 302 300 Applications) and preferential governmental policies 251 200 171 193 have accelerated the introduction of photovoltaic power 154 129 135 121 139 140 Production quantity (MW) 103 100 84 100 75 86 generation. Moreover, the New Energy and Industrial 61 55 23 33 Department Organization (NEDO Technical Develop- 0 20002001 2002 2003 2004 2005 ment Organization), an independent administrative Year agency, has developed the “Photovoltaic Power Genera- 90 Vol. 52 No. 3 FUJI ELECTRIC REVIEW Ministry of International Trade and Industry, and has structure known as SCAF (series-connection through developed a-Si solar cells that provide electric power. apertures formed on film). In order to connect adjacent In 1993, Fuji Electric was the first in the world to cells in series, holes for series connections are formed achieve a 9 % conversion efficiency with a large-area a- on the edges of a module, and metal electrodes formed Si solar cell (30 cm × 40 cm) that uses a glass on both surfaces of the plastic film substrate are substrate. connected via these holes to enable series connections. The commercialization of a-Si solar cells, however, Figure 2 shows the series-connection structure of Fuji would involve the batch processing of solar cells having Electric’s a-Si solar cell. glass substrates, and problems arise involving the long A SCAF-structure film-substrate cell is subdivided manufacturing time, substrate conveyance and han- into several rectangular solar cells known as unit cells. dling when processing large quantities of large-area So that the module output voltage is at a practical substrates (estimated to be approximately 1 m2) with a voltage level, it necessary to connect these unit cells in vacuum apparatus, and therefore these solar cells were series, and a series-connection structure was realized poorly suited for mass production. Consequently, by connecting metal electrodes of adjacent unit cells based on previously acquired technical expertise, Fuji via the series connection holes provided in the film Electric began developing a manufacturing process substrate. With the SCAF structure, due to the capable of simultaneously achieving high productivity formation of several current collector holes having high and low cost since 1994, and as of October 2004 has resistance, the electric current generated by the unit been selling a newly developed a-Si solar cell having a cells avoids generating a voltage loss at the front ITO plastic film substrate. electrodes and instead flows to the metal electrodes having low resistance, and this current flow is also 2. a-Si Solar Cell with Film Substrate connected via series connection holes formed at the cell edge to the metal electrodes of adjacent unit cells. 2.1 Structure of cell having plastic film substrate This structure enables the number of series connec- The greatest advantage in using a flexible sub- tions to be changed by modifying the pattern, and strate such as plastic film is that a roll-to-roll process enables a single solar cell to be capable of providing a can be utilized. By placing an entire roll of the voltage level suitable for the particular application. substrate in the vacuum apparatus, problems relating to substrate conveyance can be avoided and automa- 2.2 Manufacturing process technology tion simplified, thereby enabling the configuration of The SCAF structure solar cell manufacturing pro- processes suitable for large-scale mass-production. cess is based on a roll-to-roll processing method. The Plastic film is electrically non-conductive and process consists of the steps of: q using a substrate therefore can be used to fabricate integrated series- pre-processing apparatus to form holes in a roll-shaped connected structures, however, because it has a low film substrate by a mechanical punching method, w heat resistance characteristic, there are problems such using a sputtering method and an electrode forming as significant expansion and shrinkage of the sub- apparatus to form metal electrodes on the film, and e strate, and connective structures and processes require using a stepping-roll method with a layer forming innovative designs. Fuji Electric manufactures solar apparatus to fabricate sequentially an a-Si layer cells using a special film having a high heat resistance (plasma CVD: chemical vapor deposition method), characteristic as the plastic substrate material, and front ITO electrodes (sputtering method), and backside then forms metal electrodes, an a-Si layer and front electrodes (sputtering method). Multiple layer forma- ITO electrodes on top of the plastic film substrate. tion chambers are contained within a single vacuum Fuji Electric has developed a novel solar cell container, and step e is devised such that, by stepping the substrate and then stopping the substrate inside a Fig.2 SCAF series-connection structure layer formation chamber, each layer formation cham- Series connection hole Current collection hole Fig.3 SCAF-structure a-Si solar cell manufacturing process Hole punching apparatus Electrode forming apparatus Front ITO electrode CVD a-Si layer Metal electrode Laser scriber Plastic film substrate Stepping roll film deposition apparatus Laser scribed line Backside electrode Current Status and Future Trends of Amorphous Silicon Solar Cells 91 ber can operate as an independent chamber. mentally friendly. Figure 3 shows the process for manufacturing a Figure 4 shows the results of a field test of Fuji SCAF-structure a-Si solar cell and the appearance of a Electric’s a-Si/a-SiGe tandem solar cells in the Yokosu- stepping roll layer forming apparatus. ka area of Japan. In the figure, efficiency is shown as the ratio of power generation in the case where it is 3. Application to a Photovoltaic Generation assumed that the solar cell operates with the energy System conversion efficiency of standard conditions, compared 3.1 Advantages of film-type solar cell modules Fig.5 Metal-integrated PV module and its specifications Solar cell modules that use a SCAF-structure cell with a plastic film substrate have the following advantages. (1) Lightweight: A film-type solar cell weighs 1 kg/m2, which is less than 1/10th the weight of a conven- tional solar cell. (2) Flexibility: The structure, which does not use glass, can be installed even on curved surfaces, and has excellent designability. (3) High output voltage: The use of an original series- connection structure makes it possible to obtain Ideally suited for a gymnasium or easily a high output voltage to which an inverter other building having an arched roof.