Italian researchers have claimed 26.3% conversion efficiency for what they call an innovative tandem solar cell combining perovskite and silicon materials. They call it innovative since it introduces a graphene layer for the top perovskite solar cell, and for the purpose of the research, it was used on an active area of 1.43 cm².
This research work by the Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA) and Tor Vergata University of Rome, has been published in the Joule journal titled Mechanically Stacked, two-Terminal Graphene-Based Perovskite/Silicon Tandem Solar Cell With Efficiency over 26%.
The 26.3% efficiency was achieved as a result of introduction of a graphene-doped mesoporous electron selective layer for the perovskite top cell. The two-terminal perovskite and silicon tandem solar cells were mechanically stacked. Sub-cells were independently fabricated, optimized and finally coupled by contacting the back electrode of the mesoscopic perovskite top cell with texturized and metalized front contact of the silicon bottom cell. This process reduced optical losses, using a graphene-doped mesoporous electron selective layer led to the efficiency level they have achieved.
"In the past we had already exploited the extraordinary properties of Graphene to improve the efficiency of the Perovskite single junction devices, improving the extraction of charge to the electrodes," explained Professor Aldo Di Carlo, head of the project together with Dr. Mario Tucci of ENEA. "In this case, however, we observed that the addition of Graphene in the layers used for the transport of electrons in the Perovskite device allows to not alter its transparency to red and infrared light, an essential feature for a front cell of a tandem with silicon."
In July 2018, University of Kansas developed a process to extend life of active electrons in graphene which they said offers the possibility to improve efficiency and flexibility of solar cells (see Longer Electron Lifetime Through Graphene).
Earlier this year, Germany's HZB reported a record efficiency level of 29.15% for a perovskite and silicon tandem cell by developing a new contact layer and optimizing another intermediate layer (see Perovskite Tandem Cell 29.15% Efficiency Record).