With “Quasimono”, SCHOTT Solar has now developed a new technique for manufacturing full-square high-performance wafers that contain a high monocrystalline share that is significantly more cost-effective than the standard processes used in the past.
The company has succeeded in using a modified conventional VGF process that is generally used to manufacture polycrystalline wafers to produce high-quality, quasi-monocrystalline wafers. In November 2011 Fraunhofer ISE confirmed in Freiburg efficiency of 19.9 percent after performing a measurement commissioned by SCHOTT Solar. In other words, SCHOTT Solar has achieved yet another important success in the area of development and thus made significant progress with respect to its plans to introduce the SCHOTT PERFORM MONO solar module in 2012.
What is so special about the new Quasimono technology that SCHOTT Solar has developed is that it combines the advantages of the Czochralski and VGF processes (See below for an explanation) in a single technique. In this case, the seed crystal is placed on the bottom of the crucible and partially melted. Quasi-monocrystalline growth then takes place by way of the Vertical Gradient Freeze or VGF technique. SCHOTT Solar has thus been able to produce pure monocrystalline wafers from an ingot on occasion. Similar attempts in the past had yielded wafers with a polycrystalline content, but not genuine monocrystals, however. The unique achievement by the employees of SCHOTT Solar lies in the development of a new process engineering technique and new furnace technology. The company’s development team is now working on refining this technique even further.
The 19.9 percent efficiency level confirmed by Fraunhofer ISE comes quite close to the record efficiency level that SCHOTT Solar had achieved only recently with a monocrystalline solar cell. The company has now managed to successfully transfer its high-performance concept, which SCHOTT Solar had developed while collaborating very successfully with Roth & Rau AG (backside passivation) and Schmid Group (front side selective emitters), over to less expensive Quasimono wafers. This research work was partially supported by funds from the German government as part of the “QUASIMONO” research project.
Background information: Standard manufacturing techniques for solar wafers
The more homogeneous the crystal lattice of the wafer, the more efficiently the solar cell will be able to convert sunlight into electricity later on. The so-called Czochralski method is usually used to produce monocrystalline wafers. In this case, a seed crystal is dipped into a crucible that contains molten silicon. While rotating, the seed crystal is slowly drawn up to the top without interrupting its contact with the melt. The crystallizing material then takes on the even crystal structure of the seed crystal. A round “ingot” basically forms as a result of the turning motion. Its monocrystalline structure offers relatively high efficiency; however this technique is actually quite expensive. Furthermore, cutting square wafers also wastes silicon material.
Alternatively, the liquid silicon can be directionally solidified into a block inside the crucible and then sawn into razor-thin square wafers. Nevertheless, this so-called VGF method usually produces a heterogeneous crystal structure, in other words polycrystalline wafers that offer lower efficiency. The new Quasimono method developed by SCHOTT Solar combines the advantages of both processes and thus results in higher manufacturing output, which, in turn, reduces the costs.