Wafer scale microwire solar cell with 21.1% efficiency

Scientists from the Kangwon National University in South Korea have designed a radial junction solar cell based on tapered microwire (TMW) and have increased its size from the lab scale to the wafer scale in order to demonstrate the feasibility of conventional fabrication process.

“We think our microwire solar cells have great potential to compete with commercial cells,” the research project's corresponding author, Handom Um, told pv magazine. “The fabrication technologies we used were the conventional semiconducting processes such as photolithography, reactive ion etch, doping process, and atomic layer deposition.”

Crystalline silicon TMW solar cells are considered a potential alternative to conventional solar cells as these devices require thinner silicon wafers instead of the industry standard 160 µm thick wafers. “This could reduce manufacturing capital expenditure by 48% and module cost by 28%,” the Korean group claims.

The researchers used reactive-ion etching (DRIE) to build a black crystalline silicon TMW array on a 6-inch wafer. This array has a low reflection of incident light and minimized impedance mismatch. “The TMW array considerably enhanced the light absorption over the entire wavelength region from 300 to 1,100 nm,” they explained, adding that the p–n junction was formed radially to separate the absorbed
photons. “A surface passivation layer was also essential for minimizing the surface recombination of minority carriers in the TMW array as they have a large surface area.” They used a surface passivation layer made of aluminum oxide (Al2O3) with a thickness of 10 nm.

The solar cell built with this configuration achieved a power conversion efficiency of 18.7%, an open-circuit voltage of 598 mV, a short-circuit current of 39.8 mA cm⁻², and a fill factor of 78.4. The same cell built without the Al2O3 layer reached an efficiency of 16.1%, an open-circuit voltage of 568 mV, a short-circuit current of 37.3 mA cm⁻², and a fill factor of 75.8.

“Our c-Si TMW solar cell with effective Al 2 O3 passivation and radial junction efficiently separated and collected the generated multiple carriers,” they emphasized. “As a result, we have observed over 100% internal quantum efficiency (IQE) in our c-Si TMW solar cell in the UV region.”

The research team built the cell on a 4-inch wafer scale by applying a microgrid electrode with a uniform square pattern array with a width of 3–4 µm and a spacing of 400 µm. This cell achieved a power conversion efficiency of 21.1% thanks to significantly improved open-circuit voltage and fill factor, which reached 608 mV and 80.3, respectively. The academics said this enhancement can be explained by the PV performance trend according to the area ratio of the edge electrode.

They described the device in the paper “Wafer-Scale Radial Junction Solar Cells with 21.1% Efficiency Using c-Si Microwires,” which was recently published in Advanced Functional Materials. “To the best of our knowledge, our work showed the highest power conversion efficiency among the previously reported MW-based radial junction solar cells,” they concluded. “In addition, there is more room for improving the efficiency of c-Si TMW solar cells.”