Fine film with a big effect

The anti-reflection coating of crystalline solar cells gives them their familiar shimmery blue appearance. Without this coating, the cell would remain gray like polysilicon, the source material from which it is made. The front-side coating of cells, which is produced using various methods in every cell production facility in the world, has auxiliary, yet significant functions besides its anti-reflective effect. These include the passivation of unsaturated silicon bonds on the cell surface, and the passivation of the bulk material. These act to increase the cell’s efficiency. Thus, this wafer-thin film, which is typically only around 80 to 90 nanometers (nm) thick, is a multi-function film in the true sense of the word. The film is applied in a vacuum using various methods. In recent years in particular, the production technology has undergone rapid development to attain high-volume production platforms. And this development is far from over, both from a production standpoint and a technology standpoint.
The history of front-side passivation goes back to the 1980s and ’90s, when the first commercial solar cells were still being coated with titanium oxide (TiO). These films were well-suited for optical surface enhancement, but ill-suited for surface passivation, which was evident from the low efficiency of the cells. This prompted the search for new materials and vapor deposition techniques that combined both requirements. In addition to silicon oxide (SiO), the other main material to gain footing was silicon nitride, which is precipitated out in a high vacuum in plasma processes. With a refractive index of approximately 2.0 to 2.2, silicon nitride has very good prerequisites for an effective optical anti-reflective surface coating. The optimal film thickness is determined by the wavelength of the incident light (?/4) in the refractive index of the film. In the continuous spectrum of sunlight, the maximum anti-reflection is adjusted to a wavelength of approximately 600 to 700 nm. Photons that strike the solar cell at this wavelength are reflected only minimally at the optical boundary surface to the cell. Passivation of the silicon wafer surface in turn requires free hydrogen atoms; these reduce the unsaturated silicon bonds at the cell surface that lead to undesired recombinations in the cell. These surface recombination currents reduce the cell efficiency accordingly.
Silicon nitride coatings are produced using plasma processes that are divided into two main camps: those that use physical vapor deposition (PVD) and those that use chemical vapor deposition (CVD). Both use plasmas for film deposition, however, in the PVD process, purely physical impact processes are used to grow the film, whereas in the PECVD (plasma enhanced CVD) process, chemical precursor substances are broken down in the plasma. This creates the free atoms needed for growing the film, which then reform bonds on the surface of the substrate, forming the film in the process.
CVD processes have become commonplace in practical applications and make use of a number of different plasma frequencies. This significantly influences the energetics and dynamics of film growth through different percentages and energies of atoms and ions in the plasma, which in turn results in different film properties and deposition rates. As a result, the required film properties can be adjusted and “tuned” in a targeted manner. Alongside anti-reflection and surface passivation, the film must possess other properties such as a high degree of transparency and bulk passivation of crystal errors in the area near the surface. And of course, throughput and homogeneity of film thickness and refractive index play a crucial role in this step of the process.

Batch and inline platforms

In modern mass production of solar cells, there are two main production platforms. SiNA systems from PV specialist Roth & Rau, based in Saxony, Germany, use a so-called inline concept, in which the cells are processed in a vacuum under several plasma sources and are thereby continuously coated. This concept uses a special microwave plasma technique with an excitation frequency in the gigahertz range. The process enables very gentle coating of the solar cell surface. The carriers are automatically loaded and unloaded at the infeed and outfeed stations located at the beginning and end of the machine, respectively. A second generation of the system has been manufactured (see graphic) since 2010. Owing to its modular construction, the system is highly flexible and can therefore accommodate a number of different production throughputs. Today, 2400 cells per hour is standard.
The system concept from Centrotherm from Baden-Württemberg, Germany, by contrast, is based on static coating in a tunnel oven. The cells are placed in “boats” and are then exposed to the plasma in an oven for coating. This is a batch system, in which each batch is coated separately. The Centrotherm system works with a significantly lower excitation frequency, in the kilohertz range, resulting in high-energy atom and ion states in the plasma and thus in a rather more compact, dense film.
With the machine concept, the inline approach from Roth & Rau seems to be gaining the upper hand, since the latest machine generation from Centrotherm has also made the switch to an inline concept, although the coating process and plasma frequency remain largely unchanged.

New cell concepts

As cell technology and efficiencies of crystalline solar cells continue to advance, the backside of the solar cell is increasingly gaining attention. Here as well, the coating technology plays a crucial role. In modern, high-efficiency cell concepts, the classic aluminum back surface field, which is based on the electrical shielding of minorities in the cell material, is being replaced by a dielectric, surface-passivating layer or multilayer structure. The higher efficiencies often result from the use of dual layers of aluminum oxide and silicon nitride. At the most recent Intersolar, for example, record efficiencies were announced and presented by Q-Cells, among other companies, with a cell efficiency of 19.5 percent on multicrystalline cells, in which the passivated back surface played an important part. In the electrical contacting of the dielectric multilayer structure, modern laser technology is used for further processing to create the finished solar cell. The advancement of the SiNA platform from Roth & Rau to two-sided coating on the MAiA (Multiple Application Inline Apparatus) production system seems to be a perfect fit for this cell concept, which could become the standard in just a few years. That’s because a MAiA system can perform both top and bottom passivation in a single process step. Egbert Vetter, head of Research and Development at Roth & Rau, comments: “We are convinced that we are going in the right strategic direction with our MAiA concept and will be able to provide highly productive manufacturing equipment for the PV industry.” Development, which began in the ’80s with the deposition of titanium oxide in simple vacuum systems, is far from over and seems to be about to enter a new and exciting chapter.
Wilhelm Stein, Stein Engineering & Consulting