Meng Fanying——Thin-film solar cells need to break through the bottleneck of process materials

Environmental and energy issues have always been hot issues of international concern. Human demand for sustainable energy has become very urgent. The Chinese government has put forward specific development goals for new energy in the "12th Five-Year Plan". In the field of new energy, solar photovoltaic power generation occupies an important position.

The photovoltaic industry is developing rapidly, and the application of photovoltaic products is diverse, which provides development opportunities for thin-film solar cells. However, thin-film solar cells still face great challenges: there is a lack of sufficient understanding of the physical science problems of some compound materials, and the process technology needs to be innovated and improved. New materials and new structures are also needed to replace expensive and toxic materials, especially compound thin-film batteries. The integration of key precision equipment and processes needs to be further studied and developed.

Thin-film solar cell production sets new record

Solar photovoltaic power generation technology has developed rapidly in recent years. In the past 10 years, the annual growth rate of global photovoltaic cell and module production has exceeded 35%. According to Solarbuzz's latest annual photovoltaic market report, the global photovoltaic market installed capacity reached 18.2GW in 2010, an increase of 139% over 2009. In terms of output value, the global revenue of the photovoltaic industry reached US$82 billion in 2010, an increase of 105% over 2009. In 2010, the top five photovoltaic market countries in the world were Germany, Italy, the Czech Republic, Japan and the United States, accounting for 80% of the global installation volume. The photovoltaic markets in Japan and the United States grew rapidly, with annual growth rates of 101% and 96% respectively.

In general, more than 100 countries in the world contributed to the rapid growth of the photovoltaic industry in 2010. In addition, in terms of output, the global solar cell output reached 20.5GW in 2010, and the cell output of mainland China and Taiwan accounted for 59% of the global total output. In terms of manufacturer shipments, Suntech Power and JA Solar tied for first place, followed by First Solar of the United States. Among them, thin-film solar cells accounted for 13.5% of the global total solar cell output, which is also a new record for thin-film cell output in the photovoltaic market.

At present, the main types of thin-film solar cells in the photovoltaic market are CdTe thin-film cells, silicon-based thin-film cells and CIGS thin-film cells. These thin-film solar cells can be divided into rigid (i.e. glass substrate) cells and flexible (stainless steel or polyester film substrate) cells according to the substrate material. Among them, the advantages of flexible batteries are foldable, light weight and not easy to break. Compared with conventional crystalline silicon solar cells, these thin-film batteries use very few materials. The thickness of the thin film material that constitutes the solar cell does not exceed 5 microns, while the thickness of the crystalline silicon cell is about 180 microns to 200 microns. Various thin-film solar cells have occupied a place in the photovoltaic market with their unique advantages and gradually mature process technology, and the growth rate is very fast.

Process and materials are the biggest bottlenecks in large-scale production

Thin-film solar cells have attracted much attention due to their cheap substrate materials (such as glass, stainless steel, polyester film), flexibility, adjustable bandgap width, and low module temperature coefficient. Their application scale in the photovoltaic market has gradually expanded, accounting for more than 13% of the market share in 2010. Various thin-film batteries have some bottleneck problems that have not yet been solved.

At present, the manufacturing process of silicon-based thin-film solar cells, whether single-junction, double-junction or triple-junction cells, is plasma-enhanced chemical vapor deposition (PECVD). The vacuum chamber is basically cleaned with fluoride (SF6 or NF3), and the exhaust is fluorine-containing gas. Dutch scientist Rob van der Meulen et al. published an article in Progress in photovoltaics: Research and Applications, No. 19, 2011, stating that the greenhouse effect caused by this fluorine-containing gas on the environment is 17,200 to 22,800 times higher than that of CO2. Solar photovoltaic power generation has always been considered green electricity, so green processes should also be used in the manufacturing process. For silicon-based thin-film battery manufacturers, there is a solution to find new cleaning gases to replace the current fluoride and reduce greenhouse gas emissions. Another solution is to increase the conversion efficiency of the battery as soon as possible. Only when the stable efficiency reaches 12% to 16% can it be possible to compensate for the impact of greenhouse gas emissions on the environment. In addition, due to the low efficiency of silicon thin-film modules (5% to 7%), the area occupied per unit of power generation in photovoltaic system applications is almost twice that of crystalline silicon modules (efficiency is 13% to 15%). Correspondingly, the cost of BOS will also increase. It can be seen that improving the photoelectric conversion efficiency of silicon-based thin-film modules is an important condition for winning the market.

In recent years, more people have begun to pay attention to CIGS thin-film solar cells. The successful case of mass production is Japan's SolarFrontier (formerly Showa Shell), whose technology and equipment are independently developed. The technical route of its CIGS thin film is sputtering plus post-selenization treatment, and the core is post-selenization treatment. Since the selenization process uses toxic gases, the design of selenization equipment is very important, and there is no professional selenization equipment supplier on the market. In addition, the problem encountered in the preparation of CIGS thin films using evaporation technology is how to achieve uniformity and reliability of large-area thin films, including the uniformity and reliability of the microstructure, optics, electricity and thickness of the thin film. In addition to the above technical difficulties, the raw material In is also one of the focuses of discussion. In is a precious metal, and the abundance of materials on the earth is small, which may limit the large-scale production of such batteries. This puts high demands on the development of new materials. PV-tech.com reported in May 2011 that Swiss scientists prepared CIGS batteries with a conversion efficiency of 18.7% on flexible plastic substrates. The main advantage of flexible batteries is that roll-to-roll processes can be used to reduce manufacturing costs. The above figure shows the product schematic diagram of Global Solar Company in the United States. The main obstacle to the industrialization of flexible batteries is how to prepare thin films with excellent performance under low temperature conditions.

First Solar in the United States has strictly blocked CdTe thin-film battery technology, and other companies that want to enter this field face many difficulties in the process of mass production.

At present, most of China's thin-film battery companies are in the field of silicon-based thin-film, and some companies have begun to enter the field of CIGS and CdTe thin-film batteries. The common practice of these companies is to import key equipment and raw materials from abroad. Due to the lack of core technology, the problems encountered are that the compatibility of various equipment in the entire production line is not ideal, and the integration of equipment and process is not high, resulting in low product yield, difficulty in reducing costs, and weak market competitiveness.

Three types of thin-film batteries advance in parallel, each with its own advantages

The current photoelectric conversion efficiency of ordinary crystalline silicon solar cells is about 16% to 18%, and the highest laboratory efficiency is 25% (area 4cm2). The highest laboratory efficiency of double-junction silicon-based thin-film solar cells is 11.9% (1.227cm2), and the product efficiency is 8% to 9%; the highest laboratory efficiencies of CIGS cells and CdTe cells are 20.1% (area 0.503mm2) and 16.7% (area 1.032cm2), respectively, and the product efficiencies of these two compound cells are 7% to 11% and 8% to 11%, respectively.

We know that solar cells must be packaged into modules when they are used. When crystalline silicon cells are packaged into photovoltaic modules, due to the limitations of packaging materials (such as glass, EVA, etc.), there are optical and electrical losses, and the photoelectric conversion efficiency of the module will be reduced by about 1 to 3 percentage points. However, due to the high integration of thin-film batteries, the process from battery to module is almost completed simultaneously, and there is almost no loss in conversion efficiency. It can be seen that high-quality thin-film photovoltaic modules can achieve the performance of crystalline silicon photovoltaic cells. Silicon materials account for 30% to 50% of the total cost of crystalline silicon cells, and the cost of batteries is severely limited by the quality and quantity of upstream materials in the industrial chain. In comparison, thin-film batteries have unique development space due to their use of less raw materials, high process integration, low cost, and flexibility.

Silicon-based thin film solar cells

Commercial silicon-based thin-film solar cells have single-junction, double-junction and triple-junction structures, with photoelectric conversion efficiencies of 5% to 6%, 6% to 8% and 7% to 10% respectively. The main production units include United Solar System in the United States, VHF-technologies in Europe, Kaneka, Sharp, Sanyo, Mitsubishi and other companies in Japan, and Tianwei Thin Film, ENN, Prole New Energy, Nantong Johnson, Changzhou Changyuan, Quanzhou Golden Sun, Shenzhen Tuori, Tianjin Jinneng, Zhejiang Chint, Hefei Royalstar and others in China.