Record-shattering flexible solar cell with 26.5% efficiency developed in Japan
Researchers at the Tokyo City University have created a highly efficient and bendable tandem solar cell.
The innovative design, which combines a semi-transparent inverted perovskite top cell with a flexible thin-film silicon heterojunction (HJT) bottom device, has reached a power conversion efficiency of 26.5%. As per the researchers, this is the highest-ever recorded conversion efficiency for a flexible perovskite-silicon tandem solar cell.
“Perovskite/silicon tandem solar cells are a novel class of solar cells that have recently attracted increasing attention due to their notable efficiency; however, they inherently suffer from loss of flexibility,” said the researchers in a new study.
“In this study, we develop flexible perovskite/silicon tandem solar cells by fabricating perovskite solar cells atop bendable thin-crystalline silicon solar cells.”
Tested under standard illumination conditions, the cell also demonstrated an open-circuit voltage of 1.83 V, a short-circuit current density of 17.9 mA/cm², and a fill factor of 81.%.
This development could be beneficial for a wide range of applications, particularly in building-integrated photovoltaics (BIPV) and other areas requiring lightweight and flexible solar energy solutions.
Achieving flexibility in bottom cell
To achieve the cell’s flexibility, the research team employed a technique known as potassium hydroxide (KOH) etching to significantly thin the silicon wafer of the bottom HJT cell, which itself has an efficiency of 21.1%.
This wet chemical etching process textured the wafer’s back side, while the front side received micro-texturing and rounding. A protective silicon nitride (SiNx) film was applied to both sides using chemical vapor deposition (CVD) before the wafer was cut into 5 cm squares and prepared for integration.
The top perovskite cell incorporates several advanced materials, including a self-assembled monolayer of MeO-2PACz ([2-(3,6-Dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid) on an indium tin oxide (ITO) coated glass substrate.
It features a perovskite absorber with a 1.68 eV energy bandgap, a buckminsterfullerene (C60) electron transport layer, tin oxide (SnO2), another ITO layer, a magnesium fluoride (MgF2) anti-reflective coating, and a silver (Ag) metal contact.
According to Ryosuke Ishikawa, the corresponding author of the research, the thinning of silicon is crucial for creating stable and flexible HJT cells, making them ideal for lightweight applications like BIPV.
“Stable and flexible silicon heterojunctions can be fabricated by thinning silicon,” he told pv magazine.
Further research for enhanced efficiency
Now, the research team is focused on further enhancing the tandem cell’s efficiency. Their future efforts will concentrate on improving the current matching between the top and bottom cells, with plans to explore the application of a bifacial heterojunction to the bottom cell and refine the back-reflection structure.
“To further enhance the conversion efficiency, future research should involve improving current matching by applying a bifacial heterojunction to the bottom cell,” said the team, as reported by pv magazine.
Furthermore, the researchers intend to conduct thorough evaluations of the bending capabilities and long-term durability of this promising flexible tandem solar cell.
This breakthrough could propel the development of versatile and high-performance solar energy technologies.
Earlier, researchers at the Ningbo Institute of Materials Technology and Engineering (NIMTE) at the Chinese Academy of Sciences found a new way to stick different layers of tandem solar cells and achieved an efficiency of 24.6%.
They used a high-polarity solvent to prevent the clustering of the self-assembled monolayer (SAM) adsorption, which effectively separated the process from the dissolution step.