Solar panel efficiency continues to improve as costs fall

Solar panels are available in various configurations. Besides the familiar panels, there are bifacial solar panels and solar panels with cells for which all of the contacts are on the back.

Independent of its configuration, we can distinguish between three categories of PV technology: based on crystalline silicon wafers, based on thin-film solar cells or based on a stack of individual solar cells such as a tandem.

Silicon PV: heading for the practical limits

Over half a century old and still the most widely used type of solar panel with 95% of the global market share: crystalline silicon, derived from the Latin word ‘silex’, or flint. This has the suffix -ium because it resembles a metal. More than half of the world’s solar panels contain TNO technology. In the last decade, the efficiency of silicon solar cells which are manufactured on a large scale has increased significantly to over 20%.

In the coming years, TNO and its partners want to move closer to the expected maximum efficiency which is possible in large-scale, cost-effective production, estimated to be around 26%. We will do this by developing and applying functional layers to the silicon wafers in order to create complete solar cells. These are ultra-thin layers which can be as thin as a few nanometres (a thousandth of a micrometre). We develop these advanced technologies together with companies and other knowledge institutions in our solar lab and are at the international forefront in this field.

Thin-film modules: flexible PV application

The flexibly-applicable alternative to silicon is thin-film PV solar cells and modules. Silicon solar cells are currently around 150 micrometres (0.15 millimetres) thick, whereas thin-film is one to three micrometres. This approach also involves very different materials which can be applied so thinly that they make flexible PV modules possible.

TNO mainly focuses on two materials: Copper Indium Gallium Selenide (CIGS) and the new perovskite. CIGS solar modules are already produced on a large scale. Due to its flexibility, thin-film PV is also easier and more widely applicable than silicon. Furthermore, the modules generally have better aesthetics due to being uniform in colour. TNO is focusing on developing technologies for integrating thin-film solar cells into all kinds of products with different shapes and colours (customized products) and in packaging technologies that result in a high reliability for these products.

For its developments in thin-film PV technology, TNO collaborates in the thin-film PV lab with industry, universities and other research institutions. A great deal of research is done within the Solliance partnership in which TNO is a main partner.

Tandems: from lab to production

By combining silicon with thin-film solar cells – but also by using combinations of thin-film solar cells – much better performance can be achieved because the energy from sunlight is better utilised. TNO is working on so-called tandem constructions, which can achieve efficiencies up to about 1.4 times those of single solar cells. The better performance means that less PV surface area is needed, which is essential to a densely populated country like the Netherlands. Due to this higher yield, the system costs (with cables and support structures for the generated electricity) are comparatively low, resulting in lower kWh costs. In addition to improving the efficiency of solar panels and reducing the costs, the aim here is to scale up production processes and achieve a long lifespan for the tandem modules. Having explored and experimented in the lab, we are now moving to testing under real outdoor conditions.

Double-sided solar panels

Standard solar panels capture sunlight on one side. TNO was one of the pioneers in the development of bifacial panels, which are now used widely. Depending on the reflection of the surroundings, called albedo, these panels can provide 10-20% higher energy yield than their single-sided variant. Due to this higher yield and because the manufacturing costs are comparable, double-sided solar panels result in lower kWh costs when applied in the right PV system design. Examples include applications on large roofs, on agricultural land on which crops can grow or cattle can walk between the rows of panels, on inland bodies of water or at sea.

Back contact solar panels

Together with partners in science and industry, we are working on back contact silicon solar cells and modules. In this type of cell and module, the electrical interconnection of the cells takes place on the back, as a result of which the metal covering at the front is largely or even entirely absent and the metal patterns are hardly visible, if at all. Because the electrical connections are all on the back, there are no shading losses and resistance losses can be minimised. With industrial partners that include manufacturers of materials, equipment, cells and modules, we are working to further optimise this technology and the resulting products for building integration and cars, for example.