Solar energy creates a positive impact. But how do we use it! (11)

With a worldwide market share of 95%, crystalline silicon still forms the basis for most (PV) solar panels. But the potential for further improvement and cost reduction is enormous. With a 25-year warranty on panels, reliability is already good. The requirements will become even stricter for new applications, such as at sea. At the same time, the already favorable environmental profile can be considerably improved, so that PV systems can increasingly be integrated into the environment in a socially accepted way. The end of silicon technology has often been predicted (for the first time about 40 years ago), but time and again there have been opportunities for further cost reduction, efficiency increase and new applications.

Silicon: cells

Increase in yield is an important theme. Silicon solar cells in the laboratory are now reaching efficiencies that are approaching the fundamental limit of about 29% and the trick is to approach such high values in production as closely as possible. Because much of the low-hanging improvement fruit has already been picked in recent decades, the emphasis is increasingly on tackling the last major remaining loss item: The surfaces and interfaces and in particular the contacts. To this end, advanced ultra-thin layers are being developed that make it possible to meet sometimes seemingly contradictory requirements. Furthermore (partly on the basis of these ultra-thin layers) the market is working on cell designs that enable new high-yield applications. Double acting cells, rear contact cells and bottom cells for tandems.

Silicon: panels

In its most well-known form, a silicon solar panel is a glass plate with, for example, 60 or 72 solar cells connected in series and an aluminum frame behind it. The appearance may be solid black, or with clearly visible dark blue or gray-black cells. Such panels are excellent for the standard applications that we know, but not or not optimal for new, integrated applications and some forms of combined use of space. Micro Energy Holding therefore specialises together with companies on panels, or more broadly: solar energy elements with new designs and materials, based on silicon solar cells. A few examples: double-sided panels for a higher energy yield or vertical installation in combination with agriculture or horticulture or in noise barriers, rear contact panels with shapes and sizes of your choice, in which the cells can be switched in such a way that they optimally deal with inhomogeneous lighting and partial shading (such as on the roof of a car), lightweight panels and panels with other colors or patterns than we are used to.

Thin film: perovskite

Perovskite is a new kid on the block and a big sensation and promise in solar energy research and development over the past 10 years. Perovskite panels have the potential to be produced very cheaply, at high speed and to achieve an efficiency comparable to current panel types. A distinction is made between non-flexible cells on glass and flexible cells on foil and between semi-transparent (two-sided acting) and non-transparent (opaque) cells. This means that a multitude of existing, but especially new applications can be served. Research focuses on making efficient cells, but also on packaging, stability and durability. Important examples of applications being worked on are power generating windows, where translucent perovskite cells are integrated into insulating glazing, and perovskite cells that act as the top cell in a stack with a silicon cell or other thin film cell. Furthermore, perovskite technology in combination with new applications is seen as a prime opportunity for business.

Thin film: panels

Thin films offer unique, different possibilities than silicon. The most imaginative and also very important is to make efficient lightweight solar films on the roll at high speed (roll-to-roll; R2R) and then tailor them for a variety of uses. Easy to say but unfortunately not so easy to do, but very important to work on and in a way the holy grail of global solar energy development. This offers the advantage of more flexibility in shape, size and output voltage of the 'panel' or the foil or the solar power element. Many manufacturers are also working on transparent thin film panels. For these panel developments, CIGS is still often used as a proven and available technology. In addition, packing (encapsulation) and barrier layers are a focus area. The packaging largely determines the reliability and the lifespan of foils and panels, especially with materials that are very sensitive to moisture such as perovskites.

Tandems

Last, but not least, the industry inside and outside Solliance focuses on the development of highly efficient tandems. Hybrid tandems consisting of a silicon bottom cell and a thin-film perovskite top cell or thin-film tandems consisting of a perovskite or CIGS bottom cell and a perovskite top cell.

Of course, optimum use is made of the developments in Silicon cells and panels, Perovskite and Thin Film panels, but the merging of two technologies into one new, tandem technology poses a range of specific challenges. Two important ones: Connecting cells electrically and optically into tandem configurations and then into a tandem panel to be wrapped. There are two fundamentally different concepts for connecting cells, each with advantages and disadvantages. It is possible (simply put) to stack a complete semi-transparent perovskite solar foil on a complete custom silicon panel. The resulting tandem panel has 2 x 2 = 4 electrical connections: a 4-terminal tandem. The second option is to place a perovskite cell on top of each silicon cell such that the stack has two contacts (one to the perovskite and one to the silicon; the two cells are then connected in series as the bottom of the perovskite cell contacts the top of the cell). silicon cell). Those tandem cells are in turn also connected in series to make a panel that then has two connections, just like a normal panel: a 2-terminal tandem.

In particular, many studies focus on optimizing the silicon bottom cell, improving the transparency of the perovskite top cell, module design, stability, two-sided operation and extensive indoor and outdoor testing. An efficiency of 29.5% has now been achieved in the laboratory with a small perovskite-silicon tandem by Oxford PV and the 30% limit is expected to be passed soon. This opens the way to achieving that coveted value with tandem panels in the long term.