With their new printing and etching technology, Schmid in Freudenstadt, Germany now manufactures a new type of crystalline solar cell with selective emitters. The “inline Selective Emitter Cell Technology“, in short inSECT, can be easily integrated in the manufacturing process, and increased efficiency by up to 0.7 percentage points is claimed.
Selective emitters, which combine a low resistance of the emitter directly under the contacts (high n-doping) with a somewhat higher resistance in the areas between the contacts (low n-doping). However these concepts always required two separate processes for diffusing the phosphorous in which the other area always had to be covered by a mask. This double doping was not only expensive, it also proved to be susceptible to faults because heating it twice shortened the life span of the charge carriers in the thin crystalline wafers considerably.
The inSECT (inline Selective Emitter Cell Technology) technology also uses the concept of selective emitters but applies it more simply. The concept involves only one proven standard diffusion process instead of carrying it out in two stages. Only one further process step is necessary to reduce the surface between the subsequent contacts down to a depth of 50 nanometers. The high rate of doping with phosphorous atoms is purposefully reduced simply by thinning the material. However there where the contacts are later printed on, it remains intact. For the selective etching process which produces these steps, a mask is again required. However for this purpose wax only needs to be applied to the surface with a special ink jet printer. These contact-free printers are the Schmid Group's own development and achieve a positioning precision of plus/minus 15 micrometer at a printing resolution of 900 dots per inch (dpi). For etching, the Schmid inSECT technology uses a thin solution of hydrofluoric acid, nitric acid and water, which is more gentle on the wafers and also more economical. The concept of selective emitters develops its effect only when the etching depth is precisely met plus/minus a few nanometers over the entire area of the wafer. Once a depth of 50 nanometer is reached, the wafer shines in a gold colour. Sophisticated measuring techniques indicating the correct time to end the etching process are not required. The etching process is followed by immersion in a caustic potash solution. This removes the wax layer of the mask and also the porous silicone. The electrical degree of efficiency is increased in this way from e.g. 16.7 percent using homogenous emitters to 17.4 percent using selective emitters, which corresponds to an increased output of four per cent. This brings benefits to manufacturers: for a gain in efficiency of 0.7 percentage points a surcharge of currently approx. 28 euro cents can be achieved for a standard solar cell with an edge length of six inches. The additional processing stage including investments, consumed materials and amortization costs only 8 euro cents.
Crystalline solar cells.
The Schmid Group's system with ink jet printer, etching and washing is based on proven production systems. It is claimed to be easy to integrate in an inline production line adding only 5 metres to its length – it is not necessary to take out the wafers and process them separately.
Final trials are currently being held with test wafers from customers; serial production is due to start later this year.