Professor Seung Woo Lee’s group develops a neutral-colored transparent crystalline solar cell

The results were published in Joule, a prominent international journal in the field of energy.

The accomplishment was made possible through a collaboration with UNIST, supported by the KU-KIST School Project.

▲ Professor Seung Woo Lee (left) 

and Kwang Jin Kim, a student in the Integrated Master’s and Ph. D Degree Program (right)

Professor Seung Woo Lee’ group at the KU-KIST Graduate School of Converging Science and Technology successfully developed a neutral-colored transparent crystalline solar cell through collaboration with Professor Kwan Yong Seo’s group from the Ulsan National Institute of Science and Technology (UNIST).

The results of the study were published in Joule, a prominent international journal in the field of energy, on December 10.

- Authors: Kang Min Lee/Nam Woo Kim (first coauthors, UNIST), Kwang Jin Kim (first coauthor, KU), Seung Woo Lee (corresponding author, KU) Kwan Yong Seo (corresponding author, UNIST) and five coauthors (a total of 10 authors)

- Title of Article: “Neutral-Colored Transparent Crystalline Silicon Photovoltaics”

- Journal: Joule (published online on December 10, 2019, https://www.cell.com/joule/fulltext/S2542-4351(19)30538-0).

Solar cells that are transparent and efficient are difficult to produce. A solar cell that absorbs light well has a high efficiency in converting optical energy into electric energy, but good light absorbance means poor light transmittance. That’s why most of the materials used to produce solar cells, such as silicon, are not transparent. However, due to the global warming crisis and the increasing importance of renewable energy, the integration of solar cells into devices and buildings that are closely related to daily living, including automobiles, buildings and airplanes, is becoming more important than ever. This increases the technical significance of making solar cells transparent and neutral in color, like glass.

The conventional method that has been commonly employed to produce a transparent solar cell is to reduce the thickness of the light-absorbing and power-generating layer to that of a thin film. However, just as the thin water layer of a soap bubble produces the colors of a rainbow through interference from light waves, a thin film generates colors. This makes it difficult to produce a neutral-colored glass-like solar cell. Such an optical effect, based on the principles of light waves, is called a nanophotonic phenomenon. The research group designed a novel neutral-colored transparent silicon structure based on the ray-optics theory to avoid the nanophotonic phenomenon and render a colorless transparency. A computer simulation in a time scale of seconds showed that when holes of about 100 μm, much larger than the wavelength of visible light, are periodically arranged on a crystalline silicon having a thickness of 200 μm instead of on a thin film, the nanophotonic diffraction effect is considerably offset, resulting in the transmittance of the light in the entire solar light bandwidth without generating a color.

In addition, the research group employed the Schockley-Queisser (S-Q) theory, an analytical method to analyze the limits of solar cells, to show that the light absorbance of the newly designed neutral-colored transparent silicon solar cell may reach the theoretical limit. Professor Seung Woo Lee’s group is Korea’s only research team that is applying the S-Q theory to the analysis of solar cells and publishing the results in international journals. The research group prepared the theoretical basis to fabricate the neutral-colored transparent silicon solar cell based on the analysis record of the highest efficiency among all existing transparent solar cells. This study has laid the foundation to substitute all transparent glass materials with solar cells and is expected to have a huge ripple effect on solar cell-based renewable energy technologies. The study was supported by the KU-KIST School Project.

[Figures]

[Figure 1] The theoretical design of the neutral-colored transparent silicon substrate. When holes (100 μm) that are much larger than the wavelength of visible light are periodically arranged on the substrate, even a 200 μm-thick silicon substrate that perfectly absorbs solar light can let most of the solar light transmit itself (C) without reflection (A) or diffraction (B).

[Figure 2] A neutral-colored transparent silicon solar cell. The left is a conventional 200 μm-thick crystalline silicon substrate, through which the solar light cannot be transmitted. The right is the neutral-colored transparent crystalline silicon substrate of the same thickness designed by the ray-optics theory.