New Hampshire, United States [Photovoltaics World magazine] With new printing and etching technology, Schmid in Freudenstadt, Germany says it can now manufacture 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 increases efficiency by up to 0.7%. The new technology was announced at the EUPVSEC show and conference in Hamburg, Germany.
Like many insects, solar cells with selective emitters “see” blue and UV light better and the degree of quantum efficiency lies at 0.86, which means the losses are more than halved. The electrical degree of efficiency is increased in this way from 16.7% using homogenous emitters to 17.4% using selective emitters, which corresponds to an increased output of 4%. This brings benefits to manufacturers; for a gain in efficiency of 0.7 percentage points, a surcharge of approximately 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 of only 8 euro cents.
The Schmid Group’s system with ink jet printer, etching, and washing is based on the company’s existing systems. Schmid says it is easy to integrate in an inline production line, adding only 5m to its length; it is not necessary to take out the wafers and process them separately. In Freudenstadt, final trials are being held with test wafers from customers; serial production is due to start later this year.
The high rate of doping with phosphorous atoms is purposefully reduced simply by thinning the material; however where the contacts are later printed on, it remains intact. Viewed under the microscope, this thinning out is visible as a small lower-level step next to the contacts. For the selective etching process that 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. The 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).
A favorable aspect is that the porous silicon acts as a reflective layer for light, and the wafer changes color as the etching process progresses. Once a depth of 50 nm is reached, the wafer shines in a gold color, providing a simple method of visual control. Sophisticated measuring techniques indicating the correct time to end the etching process are not required anyway. The correct dosing of acid and duration of exposure are sufficient to achieve the correct etching depth. The etching process is followed by immersion in a caustic potash solution, which removes the wax layer of the mask and also the porous silicone. The deeper space between the contacts of the selective emitter is then finished.
But directly on the surface, where the phosphorous atoms have penetrated, the n-doping is considerably higher. Here, every tenth atom is a phosphorous atom. This ensures that the transition resistance between the semiconductor and the metal contacts, which are subsequently applied by screen printing, is as low as possible. But this high concentration of phosphorous atoms impairs the silicon crystal to such a degree that nearly all charge carriers on this layer recombine before they reach the contacts. The topmost 50 nm of a crystal solar cell is, therefore, known as the “dead layer” (i.e., it is of no use for producing electricity).
The new process approach developed by Schmid is said to avoid these problems.
(Read more at Photovoltaics World.)