Greater Efficiency Through Thick and Thin

Pete Singer
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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 Schmid Group says it developed inSECT at its technology center in Freudenstadt and has brought it to a stage of industrial maturity. The 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 nm.

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).
 
For etching, the Schmid inSECT technology uses a thin solution of hydrofluoric acid, nitric acid, and water. In contrast to a two-stage diffusion process, it is considerably 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.
The trick: the acid first produces only fine pores in the upper layer of the wafer. As this porous layer increases in depth, it acts as a membrane, which becomes increasingly thicker and stops the inflow of fresh acid and the outflow of silicon. The result: the etching process stops as it gets deeper (i.e., it virtually controls itself).

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.
 
Schmid explains the basics behind the new technology: Solar cells, like the human eye, are UV blind. The so-called quantum efficiency drops at a wave length of under 600 nm–this corresponds to the color yellow-orange. With a violet blue wavelength of 400 nm at the border to the invisible UV range, it lies at around 0.67. This means that the solar cell in this short wave spectral range does not convert almost a third of the photons into charge carriers. The photons still produce pairs of charge carriers and holes in the semi-conducting material, but before these reach the electrodes where they can drive electric consumers, some of them recombine, producing only unwanted heat.
 
The cause of this UV-blindness has been known to crystalline solar cell manufacturers for decades: in the conventional manufacturing process, the approximately 200 micron-thick silicon wafer is first doped with boron atoms and is thus p-conductive. In the next process step, phosphorous atoms are diffused into the wafer at 850°C and convert a thin zone on the surface into an n-conductor, which acts as an emitter. The solar cell is thus nothing other than a large diode. The n-doping of the emitter loses intensity the deeper it gets. At a depth of 300 to 500 nm, the number of phosphorous atoms is the same as the number of boron atoms: approx. 1016 phosphorous atoms or boron to every 10²² of silicon atoms/cm³.

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).
 
Concepts for solving this problem have been available since the 1970s–so-called 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). But 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 new process approach developed by Schmid is said to avoid these problems.

(Read more at Photovoltaics World.)

 

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