Kaneka presents 29.2%-efficient 2T silicon-perovskite tandem solar cell


From pv magazine Global

Japanese chemical company Kaneka has designed a two-terminal (2T) perovskite-crystalline tandem solar cell using a 145 μm thick industrial Czochralski (CZ) silicon wafer.

The company’s researchers built the cell with an anti-reflection intermediate layer relying on what they call “gentle textured structures” that were applied on the front side of the bottom. This reportedly enables a significant improvement in the typical light confinement effects in perovskite-silicon tandem devices.

Light management technology is mandatory to fully utilize the wide range of the solar spectrum in a solar cell, especially for a 2T tandem structure, since its top and bottom cells are electrically connected in series and required to satisfy the constraints of current matching whereby the respective currents at the operating point are aligned to some extent,” they said. “Because of the large difference in the refractive indices between perovskite and crystalline silicon (c-Si) materials, the optimized intermediate layer as shown acts as an anti-reflection coating to suppress the reflection loss of the infrared light that is utilized in the bottom cell.”

The company controlled the morphology of the gentle texture on both sides of the wafer by using chemical etching. Via atomic force microscopy (AFM), it compared the performance of the two absorbers with the gentle structure with that of similar devices without the structure.
By applying the gentle texture, the photocurrent density of silicon bottom cell increases by over 2% from the reference,” the company explained.
The company fabricated a top cell with a substrate made of indium tin oxide (ITO), an electron transport layer made of buckminsterfullerene (C60), a passivation layer, the perovskite absorber, a self-assembled monolayer, an intermediate layer based on ITO and microcrystalline silicon oxide (μc-SiOx). It then fabricated the bottom cell with n-doped amorphous silicon (a-n:Si) layer, several silicon layers treated with different processes, and a bottom contact made of ITO and silver (Ag). 
The passivation layer was introduced between C60 and perovskite layer and the thicknesses of c-Si wafer and top ITO layer were reduced in this work,” the company explained. “After coating the passivation layer, C 60 was thermally evaporated successively on the perovskite film.”

The tandem device achieved a power conversion efficiency of 29.2%, an open-circuit voltage of 1.929 V, a short-circuit current density of 19.5 mA cm2, and a fill factor of 77.55%. The result, which was confirmed by Fraunhofer ISE Callab, improves on the 28.3% efficiency Kaneka had previously achieved for a device with the same architecture, withe the main differences being the passivation layer and wafer thickness.

This is the highest certified power conversion efficiency of 2T perovskite-silicon tandem solar cells using CZ wafer to our knowledge,” the company said, noting that they are currently considering moving to a four-terminal (T4) configuration for the further development of the device.

The company presented the new cell in “High efficiency perovskite/heterojunction crystalline silicon tandem solar cells: towards industrial-sized cell and module,” which was recently published in the Japanese Journal of Applied Physics.

Kanela has developed in the past what is still the second most efficient solar cell based on crystalline silicon – a 26.63%-efficient crystalline silicon solar cell featuring heterojunction and back-contact technology.

*The headline of the article was updated to reflect that the cell is a T2 device and not a T4 cell, as we previously reported.

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