Flexible, bifacial absorber and high efficiency solar cells

At present, although the cost of solar energy has been greatly reduced with the continuous updating of technology, it is still more expensive than fossil energy. In addition, although solar panels are "blooming everywhere" and even in oversupply, the solar panel manufacturing industry is still at a low ebb. However, although the innovation momentum of the solar energy market has weakened, there are still many research progresses that have been "released". Overall, industry insiders are still optimistic about the long-term development of the solar energy industry.

　　Flexible solar cells made on glass

　　Traditional solar cells still rely on crystalline silicon technology. A few years ago, the cost of silicon solar panels was $4 per watt. Professor Martin Green of the University of New South Wales in Australia, one of the "leaders" in this research field, once claimed that the cost of silicon solar panels would never be less than $1 per watt. But now, he said: "The cost has dropped to about 50 cents per watt, and it may drop to 36 cents per watt."

　　The U.S. Department of Energy has set a goal of less than 1 cent per watt by 2020, a goal that refers not only to the cost of solar panels, but to the entire solar panel installation system. Green believes that the solar industry has the potential to achieve this goal ahead of schedule. By then, the direct cost of solar energy is expected to drop to 6 cents per kilowatt-hour, which is lower than the cost of supplying energy from a new natural gas power plant. The total cost of solar energy will of course be higher because it includes the cost of facilities to make up for the intermittent characteristics of sunlight, but exactly how much higher depends on factors such as how much solar energy is on the grid.

　　The silicon solar industry is constantly looking for ways to cut costs and increase the energy output of solar panels. In the 1990s, Green's lab created a solar cell with a record-breaking conversion rate, a record that still stands today. To achieve this conversion rate, Green had to use expensive lithography to create fine wires to collect the current provided by the solar cell. But steady technological advances have now enabled scientists to create fine wires using screen printing. Recent studies have shown that screen printing can create wires as narrow as 30 microns, about the same width as Green's wires, but at a much lower cost.

　　Green said this and other technologies combined will hopefully make it cheaper and easier to replicate his high-efficiency solar cells on production lines. Companies have developed technology to make the metal contacts on the front of solar cells, but the design of the electronic contacts on the back is more difficult, but he hopes that a company will come up with a solution.

　　Similarly, the National Renewable Energy Laboratory (NREL) has created a flexible solar cell on a new type of glass: ultra-thin, highly curved glass made by Corning. The thin-film cadmium telluride solar cell they created is the only solar cell currently available that can compete with traditional silicon solar cells in mass production. Right now, such solar cells can only be made in batches (the same is true for silicon solar cells), but being able to make them on a piece of bendable glass offers the possibility that they can be made continuously in a roll-to-roll process (like printing newspapers), thus reducing costs by increasing production.

　　The "two-faced girl" makes the sunlight nowhere to escape

　　Green's former student and colleague Zhao Jianhua, co-founder of China Sunergy, a Chinese solar panel manufacturer, announced last week that he is building a pilot production line for "bifacial" cells that absorb sunlight on both the front and back sides. The basic idea behind these solar cells is that during most of the day, sunlight that falls on the ground between rows of solar panels is reflected to the back of the solar panels, where it is expected to be absorbed and used to increase energy output. The research is particularly applicable to desert areas, where sunlight is highly reflective. Single-sided solar panels can generate 340 watts of electricity; bifacial solar panels are expected to produce up to 400 watts. Zhao hopes that these solar panels will increase energy output by 20% within a year.

　　Such solar panels could be mounted vertically, like a fence, so that one side of the panel absorbs sunlight in the morning and the other side absorbs sunlight in the afternoon, allowing them to be installed on a small piece of land, for example, as noise barriers on highways. And the advantages of this layout strategy are expected to be apparent in places where dust is prevalent. Many places in the Middle East seem to be ideal for such solar panels because, although these places have particularly strong sunlight, frequent sandstorms can reduce energy production. Vertically mounted solar panels will not provide a "home" for dust, so the whole solar system is expected to become more economical.

　　Semiconductor "companion" may double the efficiency of silicon solar cells

　　Longer term, though, Green is betting on silicon. He hopes to greatly increase the efficiency of silicon solar panels by marrying it to one or two other semiconductors. Each of the added semiconductors selectively absorbs parts of the solar spectrum that silicon cannot absorb efficiently and converts them into electricity.

　　Adding a semiconductor is expected to increase the photovoltaic conversion efficiency of solar panels from the current 20% to 25% to about 40%. Adding another semiconductor is expected to increase the efficiency to as high as 50%, which can reduce the installation of at least half of the solar panels. Of course, the main challenge facing this method is to make these semiconductors "marry" well, a challenge mainly caused by the arrangement of silicon atoms in crystalline silicon.