Researchers from Switzerland's Federal Laboratories for Materials Science and Technology (EMPA) and École Polytechnique Fédérale de Lausanne (EPFL) have designed a four-terminal tandem mini-module based on perovskite and copper, indium, gallium and selenium (CIGS) with an aperture area of around 2 cm2, and a geometric fill factor of over 93%.
Processing sequence of flexible NIR-transparent perovskite mini-module. Image from RRL Solar
The team reports that the key to efficient flexible perovskite-CIGS tandem modules is the development of near-infrared (NIR) transparent perovskite solar modules on a flexible polymer foil. To achieve these results, the researchers had to overcome the challenges of laser patterning on flexible substrates to realize the first all-laser scribed monolithically interconnected NIR-transparent perovskite mini-modules on polymer film. The perovskite mini-module used in the tandem panel was fabricated on a flexible polyethylene napthalathe (PEN) substrate mounted to a glass substrate in a p–i–n device architecture. This configuration, according to the research team, shows reduced absorption in the NIR region.
The scientists used thermally evaporated buckminsterfullerene, a molecule which is also known as C60, for the fabrication of the electron transport layer (ETL). To protect the underlying layers from ion damage during the sputtering of transparent conductive oxide (TCO), they placed a tin(IV) oxide (SnO2) buffer layer on top of the C60 by atomic layer deposition (ALD). They also applied P2 scribing to create a pathway to interconnect the adjacent subcells and P3 scribing to isolate the rear electrode between the subcells.
The mini-module has a reported power conversion efficiency of 10.8% and an average transmittance of over 75% in the NIR range, which makes it suitable for tandem applications. The team said that the light-harvesting potential of the NIR-transparent mini-module can be improved by utilizing a thicker wide-bandgap (WBG) perovskite absorber with good optoelectronic properties.
The perovskite module was combined with a CIGS module with an efficiency of 15.1%. “The flexible 4T perovskite-CIGS mini-module performs at an efficiency of 18.4% and outperforms both the perovskite and CIGS stand-alone mini-module,” the research group stated. “An absolute efficiency gain of 7.5% and 3.3% is achieved with respect to the perovskite and CIGS stand-alone mini-modules.”