Japan-based chemicals company, Kaneka, has reported the design of a two-terminal (2T) perovskite-crystalline tandem solar cell using a 145 μm thick industrial Czochralski (CZ) silicon wafer. The cell has an anti-reflection intermediate layer relying on what Kaneka calls “gentle textured structures” that were applied on the front side of the bottom, which 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,” the scientists said in their work. “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.”
Kaneka controlled the morphology of the 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 team 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 scientists 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 cm−2, and a fill factor of 77.55%. The result, which was reportedly confirmed by Fraunhofer ISE Callab, improves on the 28.3% efficiency Kaneka had previously achieved for a device with the same architecture, with 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.