Perovskite semiconductors efficiently convert sunlight into electrical energy; they are also inexpensive and extremely lightweight. A team at HZB has developed a triple-junction solar cell comprising different perovskite semiconductors, with a novel bilayer of graphene oxide (GO) and a self-assembled monolayer (SAM) as the hole conductor. This bilayer significantly increases both efficiency and long-term stability. The efficiency of the novel perovskite triple-junction solar cell is 27.3% and shows hardly any decline even after more than 770 hours of operation. The study has been published in the renowned journal Joule.
Perovskite-based photovoltaic cells can achieve high power conversion efficiencies (PCEs). Combining two or more different perovskite semiconductors with different band gaps in a multi-junction solar cell allows the solar spectrum to be harnessed even more effectively, thereby increasing efficiency. All-perovskite multi-junction solar cells potentially offer very low manufacturing costs, low weight, and the option of mounting them on flexible substrates.
The HZB team used three different perovskite absorbers with different band gaps. Stacking these on top of one another creates a highly complex monolithic perovskite triple-junction solar cell. The main focus of the work was on the interlayer between the middle and rear perovskite subcells. 'You can imagine it like a Big Mac, where the three buns are separated by different fillings like meat, salad or cheese. Here, that would be the filling between the middle and bottom buns,' explains Prof. Dr Steve Albrecht, head of the Department of Perovskite Tandem Solar Cells at HZB. The rear absorber layer consists of a tin-lead-based perovskite semiconductor with a low bandgap. The interaction between this perovskite layer and the hole transport layer is considered the most important factor in improving efficiency and stability. Usually, the polymer PEDOT:PSS is used as the hole transport layer; however, this results in losses due to absorption processes, and the PCE degrades quickly.
'We then systematically investigated the influence of various hole-conducting layers on the properties of tin-lead perovskites, and thus also on the novel triple-junction solar cells,' says Dr Philipp Tockhorn, group leader at HZB. 'We had already established self-assembled monolayers (SAMs) as hole-conducting contact layers in lead-based perovskite solar cells with great success, so it made sense to use these in tin-lead perovskites and triple-junctions as well,' says Kevin Prince, co-first author of the study and a postdoc at HZB. SAMs consist of large organic molecules that arrange themselves spontaneously into a monolayer. However, SAMs alone do not work very well in these tin-lead perovskites, as the transport of hole charges is inefficient. 'We therefore experimented with additional layers beneath the SAM layer to act as a kind of substrate,' explains Yeonghun Yun, co-first author and postdoc in the team. Eventually, they discovered that a layer of graphene oxide (GO) beneath the SAM layer improves the interface both morphologically and electronically, thereby enabling more efficient charge transport.
The team incorporated the GO/SAM bilayer into a perovskite triple-junction solar cell in place of a conventional PEDOT:PSS layer. This significantly reduced optical losses. 'We achieved an efficiency of 27.3% with the triple-junction solar cells, which is one of the highest values for this technology,' says Albrecht. Furthermore, triple-junction solar cells with GO/SAM also proved their worth in continuous operation, retaining over 90% of their original efficiency even after 770 hours, setting a new stability record for this solar cell architecture. 'Our analyses suggest that, with improvements to the quality of the individual perovskite layers and interlayer films, the efficiency of this architecture could be increased to over 30%,' says Albrecht.
