Increased efficiency from new type of silicon solar cell
Researchers at ECN have developed a simple method for increasing the efficiency of crystalline silicon solar cells by 6 percent in relative terms. By using a different type of silicon they managed to reduce electrical losses at the front of the cells substantially, thus making a new generation of industrial solar cells possible.
At present, crystalline silicon solar cells are made predominantly from p-type silicon wafers. Silicon is a ‘semiconductor’ manufactured in two varieties. The particles in the silicon that conduct the charge (the ‘charge carriers’) are either positive or negative. The positively charged particles are created by mixing the silicon with a tiny amount of boron, yielding what is known as ‘p-type’ silicon. The negatively charged particles are created by mixing the silicon with a small quantity of phosphorus, thus creating ‘n-type’ silicon. Researchers at ECN have shown – as described in a published article – that much higher cell efficiencies can be achieved by making solar cells from n-type silicon wafers and subjecting them to a special process (see below). In their project they used inexpensive fabrication steps used in the normal industrial production process, such as screen printing of metal contacts.
Major efficiency gain
The researchers have also developed a simple, new method, which they have patented, for substantially reducing electrical losses at the front of n-type solar cells, by ‘passivating’ the surface on which the light falls. This prevents charge carriers at the surface diffusing (recombining) and therefore no longer contributing to the electrical current from the cell. The new technology retains more charge carriers, thus increasing cell efficiency.
Efficiency varies from one type of wafer to another. The efficiency gain with n-type cells made from monocrystalline wafers, compared with that of current p-type cells, is substantial – about 1 percent in absolute terms – while the fabrication process is similar to the normal p-type process in terms of cost and complexity (figures based on in-house comparison at ECN). This may not seem like a major gain, but with the current standard 15-17 percent conversion efficiency of solar cells a gain of 1 percent in absolute terms yields a relative gain of 6 percent. In the case of n-type cells made from multicrystalline wafers the gain depends on the wafer quality, but it would seem to be comparable with that of monocrystalline wafers.
A cross-section of the new cell, showing the principle involved. The green layer (with its dual function of anti-reflection coating and passivating layer) is where the patented breakthrough was achieved. The fundamental difference lies in the semiconductor polarities: the new cells have an n wafer and p+ emitter, whereas ‘normal’ silicon solar cells have a p wafer and n+ emitter.
A maximum efficiency of 18.3 percent was achieved with the monocrystalline cells (average 17.9 percent); 16.4 percent was achieved with multicrystalline wafers. In both cases the researchers used standard 125mm industrial wafers.
Higher efficiency from more impure material
Paul Wyers, unit manager ECN Solar Energy, on the advantages: “We believe that n-type cell technology offers important advantages for commercial solar cell production, for example in that you can achieve higher efficiencies with less pure material. That could be important to companies wanting to employ new lower grades of silicon feedstock, i.e. cheaper material. But this cell technology also has advantages across the board, which apply to any quality of silicon and wafer, such as better conversion of short-wavelength sunlight into electricity. Industrial n-type cells based on simple, cheap production methods such as screen printing therefore have the potential to develop into a major new item on the solar power market.”
As the fabrication process uses the same kind of manufacturing equipment as currently used to make p-type silicon cells, the process can be implemented in industry quickly, possibly just by making minor modifications to existing production lines. ECN intends to further develop and commercialise various types of solar cells based on this technology in the short and medium term.
Cheaper solar power
Solar power could develop into the main source of sustainable energy. Solar cells convert sunlight directly into electricity. This type of energy conversion is regarded as one of the main applications in the worldwide sustainable energy system of the future. The new type of solar cell, combined with developments in the CrystalClear Project, where cells are glued instead of soldered, and the technology for making extremely thin solar cells which can be made into solar panels up to eight times faster than at present, using the recently developed pilot line, are contributing to ECN’s ambition to reduce the cost of solar power by improving the conversion efficiency of cells and allowing cheaper materials to be used. In the Netherlands, electricity from solar cells costs about twice as much as the consumer price of electricity from the grid, currently costing 50 cents per kilowatt hour. This should go down to 25 cents by 2015 and 10 cents or less by about 2030.
Information:
ECN researcher Valentin Mihailetchi et al., Appl. Phys. Lett. 11 February 2008, Volume 92, Issue 6, p. 063510. The article can be downloaded from the ECN website.
Contact:
Jan Bultman
ECN Solar Power
Tel. 0224 - 564786
bultman@remove-this-part-ecn.nl
