Researchers at Tokyo City University recently developed flexible perovskite/silicon tandem solar cells by fabricating perovskite solar cells atop bendable thin-crystalline silicon solar cells. By reducing the thickness of the silicon substrate to approximately 60 µm, applying microtexturing to its surface, and incorporating a low-refractive index-doped layer, the team produced a flexible silicon heterojunction solar cell with an efficiency exceeding 21%.
Subsequently, by optimizing the self-assembled monolayer processing conditions on the microtextured surface and constructing an inverted perovskite solar cell on the flexible SHJ, they achieved 26.5% efficiency for the flexible perovskite/silicon tandem solar cell.
To make the 21.1%-efficient bottom cell as thin as possible, the research group used a technique known as potassium hydroxide (KOH) etching, which is a wet chemical etching process used to create cavities in silicon. It was used to texture the cell wafer on the back side, while the front side was micro-textured and rounded.
The scientists also deposited a protective layer made of silicon nitride (SiNx) film on both sides of the wafer by chemical vapor deposition (CVD). The wafer was then cut into 5 cm square pieces, and the SiNx film along the outer edges was removed using a laser.
The top cell was fabricated with a self-assembled monolayer made of MeO-2PACz, which is also known as [2-(3,6-Dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid, a glass substrate coated with indium tin oxide (ITO), a perovskite absorber with an energy bandgap of 1.68 eV, an electron transport layer (ETL) made of buckminsterfullerene (C60), a tin oxide (SnO2), another ITO layer, an anti-reflective coating based on magnesium fluoride (MgF2), and a silver (Ag) metal contact.
Tested under standard illumination conditions, the tandem cell achieved a power conversion efficiency of 26.5%, 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 result represents the highest efficiency ever recorded for a flexible perovskite-silicon tandem solar cell, according to the research group.
Going forward, the researchers intend to enhance the current in the bottom cell with a more precise design of the back-reflection structure. They feel that to further enhance the conversion efficiency, future research should involve improving current matching by applying a bifacial heterojunction to the bottom cell. Conducting detailed evaluations of the bending properties and durability of this tandem solar cell is also required, according to the team.
