After decades of research, the efficiency of the prevailing silicon cells is fast approaching the practical limit of around 27 per cent. As it stands, there is but one technology that promises further increases to well over 30 per cent – the tandem solar module. It consists of solar cells made of different materials layered on top of one another. The different active layers work together to increase efficiency by jointly making use of a wider solar spectrum than each single solar cell does on its own.
One group of materials, metal-organic perovskites, holds great promise for tandem solar modules. “Some compounds in this class of materials exhibit excellent optical and electronic properties and are abundantly and inexpensively available on Earth,” says Jan-Philipp Becker, head of ZSW’s Photovoltaics: Materials Research department.
Excellent adaptation to the solar spectrum
Conventional silicon PV cells would appear to be the obvious choice for the bottom solar module. However, an even more interesting proposition is to use thin-film technologies exclusively. The bottom module can also be made of perovskite or of CIGS, which is the case in ZSW’s module. CIGS is a mix of materials – copper, indium and gallium vapor-deposited onto a rigid or flexible substrate in a selenium atmosphere. The institute’s experts and industry partners had developed this technology and ramped it up for mass production in earlier projects. CIGS’s spectral absorption can be tailored to a perfect fit for the tandem composite.
See also: KIT: Perovskite stacks increase energy yield
ZSW’s tandem solar module has an area of nine square centimetres and achieves 21.1 per cent efficiency. This prototype also features scalable component architecture suitable for industrial manufacturing. The best performance attained to date with tandem solar modules made of perovskite and CIGS is just slightly higher at 22 per cent. ZSW has already achieved an efficiency level of 26.6 per cent with this combination of materials in smaller laboratory cells.
Several steps to success
It took multiple stages of technical development to achieve these values. First, the researchers optimized the submodules. The top perovskite half-module not only has to be highly efficient; it must also be semi-transparent to allow enough light to pass through to the bottom module. They made several improvements to this end, including developing more transparent electrodes and enhancing the passivation of boundary layers.
The tandem structure in all prepared cells and modules outperformed individual cells or modules in terms of efficiency. This clearly demonstrates the superiority of tandem solar modules. ZSW now aims to further scale up and develop tandem thin-film technology in a joint effort with interested industry clients.
Tandem solar modules – efficient, light and flexible
Tandem solar cells that pair perovskite with CIGS offer benefits beyond high efficiency. This thin-film technology can also be deposited on plastic or steel films to make light, flexible modules that lend themselves to many more use cases. They can be installed in the conventional way in solar parks and be seamlessly integrated into vehicles and on factory roofs that cannot handle heavy loads.
Also interesting: Germany secures top spot in electric mobility
Recent studies point to even more benefits: For one, it costs less to generate power with thin-film PV modules. Also, PV production consumes less material and energy to leave a better environmental footprint.
ZSW partnered with the Karlsruhe Institute of Technology (KIT) in this research initiative, which was part of the recently completed CAPITANO project funded by the German Federal Ministry of Economic Affairs and Climate Action (BMWK). (mfo)
