Perovskite Solar Cells: 'Big Mac' Architecture Reaches 27.3% Efficiency and Promises to Exceed 30%
Perovskite-based solar cells continue to represent one of the most promising technologies for the future of photovoltaics. In addition to being lightweight and inexpensive to produce, they can also be deposited on flexible substrates, offering application possibilities that are difficult to achieve with traditional silicon panels.
A new study from the Helmholtz-Zentrum Berlin (HZB), published in the journal Joule, shows how it is possible to improve both efficiency and operational lifespan through a new multi-junction architecture.
The German research group has developed a monolithic "triple-junction" solar cell, composed of three different perovskite materials, each characterized by a different band gap. The overlap of the three absorbers allows for the utilization of different portions of the solar spectrum, increasing the amount of energy converted compared to single-junction cells and bringing this technology closer to performance levels exceeding the physical limits already reached by silicon.
Image credits: Laura Canil / HZB
However, creating such a structure requires numerous functional layers, each with a specific role in charge transport. Steve Albrecht, head of the HZB Tandem Perovskite Solar Cells Department, compared this configuration to a "Big Mac," where the three "buns" represent the absorber layers and the "fillings" correspond to the intermediate materials that allow the device to function correctly. The interface between the central cell and the lower one was the main focus of the study.
In traditional perovskite cells, the polymer PEDOT is generally used as a material for hole transport. Although widely used, this material introduces optical losses and tends to degrade during continuous operation, especially when used alongside the more delicate tin- and lead-based perovskites, which are particularly sensitive to oxygen and moisture.
To overcome this limitation, the researchers experimented with an alternative solution based on a self-assembled monolayer (SAM), made up of organic molecules that can spontaneously organize themselves into an extremely thin layer. However, initial tests showed that the SAM alone did not guarantee sufficiently efficient transport of electric charges.
The solution came by introducing an ultra-thin layer of graphene oxide (GO) beneath the SAM. This GO/SAM combination improved both the morphological and electronic characteristics of the interface, promoting hole transport and reducing optical losses. According to the authors of the study, the double layer also helps create a more effective chemical barrier that protects the delicate tin-lead perovskite from oxidation and environmental degradation phenomena.
Integrating the new interface into the triple cell has allowed for a conversion efficiency of 27.3%, one of the highest values reported so far for this category of photovoltaic devices entirely based on perovskite. At the same time, a significant improvement in stability has also been recorded: after over 770 hours of continuous operation, the device maintained more than 90% of its initial efficiency, setting a new benchmark for this particular architecture.
According to the HZB team, the potential for improvement is not yet exhausted. By further optimizing the quality of the individual perovskite layers and the intermediate films, this architecture could exceed the 30% efficiency threshold, bringing perovskite multi-junction cells closer to performance that could make them increasingly competitive in future high-efficiency photovoltaic applications.