More Power from the Twin Pack

The efficiency of today’s solar modules leaves only little room for improvement. Combining thin-film solar modules based on perovskite semiconductors with semiconductors made of copper, indium, gallium and selenium can offer efficiencies of over 30 percent, as the Capitano project proves.

For further increasing the efficiency of solar modules, the use of two light-harvesting active layers in so-called tandem solar modules is an option. These consist of two different types of modules in a layered array that puts the solar spectrum to much better use than any single solar cell. This combination is far more efficient. Multi-junction solar cells’ efficiency could extend beyond 30 percent, in theory. The limit for single-layer silicon solar cells, for example, is 29 percent.

Researchers engaged in the Capitano project are combining thin-film solar modules based on perovskite semiconductors with semiconductors made of copper, indium, gallium and selenium (CIGS). This combination is the key to building remarkably efficient tandem solar cells with all the advantages of thin-film technology and an efficiency factor that could top the 30-percent mark. The Karlsruhe Institute of Technology (KIT), the Schwäbisch Hall-based enterprise NICE Solar Energy, and the Centre for Solar Energy and Hydrogen Research Baden-Württemberg (ZSW) have joined forces in this project with the ZSW acting as coordinator.

Several variants of tandem modules are now available. The perovskite solar cell in the CIGS/ perovskite version converts the light in the visible part of the solar spectrum into electricity. The underlying CIGS solar cell absorbs the light in the near-infrared spectrum that penetrates the perovskite solar cell. Both solar cell types can be manufactured on square-metre large substrates using thin-film processes. Thus, costs could be significantly reduced while maintaining a high degree of efficiency. Promising fields of application include modules for building-integrated photovoltaic solutions.

The Capitano project

Launched in July 2019, the Capitano project is to run for three years with the German Ministry for Economic Affairs and Energy providing around €5.2 million in funding. This project aims to develop cells with higher yet stable efficiency factors, and then combine these cells to make efficient tandem solar modules. The industry partner NICE Solar Energy will assess the possibility and cost of manufacturing these modules on an industrial scale.

To achieve this project’s goals, the ZSW is developing CIGS modules with an adapted bandgap and optimized surface, and investigating semitransparent perovskite solar cells and highly efficient and trans-parent modules. Eager to test industrial processes such as slot-die coating for the perovskite layer, researchers are focusing on producing optimized intermediate layers and transparent contact layers that have been adapted accordingly. The results will flow into efforts to make tandem solar cells and modules that are interconnected in monolithic array. These scientists also want to assess the manufacturing process’s ecological impact.

KIT is doing its part for this project by developing new materials, processes, and prototypes for manufacturing semi-transparent perovskite solar cells and highly transparent modules with an adapted bandgap a high efficiency factor. The institute’s researchers are particularly interested in exploring scalable manufacturing processes such as slot-die coating and gas-phase vacuum deposition. Its scientists are developing a light management concept to improved light yield amid the complex architecture of the tandem solar cells. They have also been tasked to calculate yields.

NICE Solar Energy is providing small CIGS solar modules from its innovation line to underpin the other two partners’ efforts to make tandem solar modules. The enterprise will then assess these tandem modules’ suitability for manufacturing at industrial-scale, 300 megawatts capacity. A cost comparison with single-junction CIGS solar modules is also on the company’s agenda.