Evaporation – Evolar’s choice for perovskite thin-film deposition

Within the solar industry, the race to commercialize perovskite solar cell technology is on. A key factor for success revolves around the choice for the deposition process of perovskite films, as this greatly influences device performance as well as meeting industrial goals, like scalability.

Two main categories exist for perovskite thin-film deposition methods: solution-based processing and dry processing. The first includes methods such as spin-coating, blade-coating, slot die coating, inkjet printing, and spray coating, to name a few. The latter is mostly known for vacuum-based evaporation. While most lab-scale work is performed with simple solution-based processing methods, as the required equipment is low-cost and easy to use, when coating large areas with high performance requirements, dry processing brings significant advantages. That is why at Evolar, the vacuum-based evaporation method is the chosen method to achieve high conformity thin-film deposition of a perovskite layer onto glass and structured surfaces with its turnkey PV Power Booster line solution. An interview with Evolar’s Co-founder and CSO, Lars Stolt, highlights some of the main advantages behind this method.

Advantages of evaporation

High purity
In contrast with solution-based methods, evaporation is carried out in a high vacuum environment. This means that unwanted elements in the process are removed, resulting in very low levels of impurities. “Nothing can contaminate your film,” explains Stolt. “Compared to atmospheric conditions, when operating in vacuum you have 1 billion times lower level of potential impurities,” he adds. The lack of undesired atoms and molecules in the process environment makes it possible for the evaporated source to go straight into the substrate without colliding with other atoms along the way, which could otherwise cause a possible reaction and result in impurities whose effect on the film might be detrimental.

While spin-coating, a solution-based method, is commonly used in the perovskite community for laboratory work, it cannot be scaled to large size. Other solution-based methods, such as spray coating and slot die coating, can in principle be scaled, yet progress seems to be difficult. In contrast, evaporation is a method that can be scaled to large manufacturing production scale. “In industrial production, you want uniform composition in all the substrate area, and to do it as fast as possible to keep the cost low. Evaporation makes this possible,” explains Stolt. Through their R&D line, Evolar has already developed a perovskite evaporation process at mid-scale, with a substrate size of 1.19mx0.63m, and nominal capacity of 20 MW. “We’ve chosen evaporation because we have a strong background using this method and we know it works and can be scaled,” confirms Stolt.

Material efficiency
In contrast with other methods where the sources need special preparation and shaping, evaporation source materials need very little preparation, with no special shaping required. Moreover, in solution methods, additional solvents and chemicals are needed for the process, which are then required to be removed and dealt with. “In evaporation,” notes Stolt, “you only add the things you want in the film.”

Cost efficiency
Evaporation is used in a variety of industrial methods, such as in OLED displays, which are usually deemed as high production cost products. However, evaporated films can also be very cheap. Taking as an example aluminium, a thin film of this element is deposited through the evaporation method inside a bag of potato chips to keep moisture away, being a massively used low-cost method. While the nature of the perovskite technology requires a more specialized process, “We want to do it at a low cost,” remarks Stolt, “We have the expertise to make it advanced and still low cost.”

Long history of evaporation expertise

Evolar’s history of expertise in evaporation started at Solibro, a solar technology company which grew out of the same Uppsala University thin-film solar cell research cluster that later initiated Evolar in 2019. At Solibro, the team successfully commercialized the CIGS thin-film solar cell manufacturing process, scaling it from the lab into a global company with industrial scale production and hundreds of employees before it was acquired by Q-Cells in 2006. While Evolar’s thin-film tandem cells are based on perovskite rather than CIGS, much of the underlying manufacturing process is the same.

The company’s previous thin-film expertise together with a verified technology for a large scale and high-volume production leaves it well positioned to take the next step with perovskite. “Evaporation has worked for other materials on the highest efficiency levels, now we will use the process for perovskite,” says Stolt.