▲ Structure of manufactured solar cell (a) and organic thin film using water
Comparison of internal structure before (b) and after (c) alignment [Figure = UNIST]

The separator is a material that determines the efficiency of a solar cell. The research team improved performance and simplified the manufacturing process by adding water to the organic separator.

Professor Kyung-Jin Choi said, “This film can be used for both n-type (nitrogen) and p-type (boron) silicon solar cells by controlling the electrical properties (direction of the electric field) of the organic thin film (ferroelectric),” and “This research also solved the chronic problem of temperature and humidity instability in organic thin films (1,000 hours of operation possible).”

The rear separator of a silicon solar cell serves to prevent photogenerated electrons from recombinating with the positive space. When exposed to sunlight, the photoactive layer (e.g., silicon, perovskite) releases electrons and holes. This prevents these electrons (negatively charged, -) and holes (positively charged, +) from recombining and disappearing. Since the amount of power a battery produces is determined by the quantity of electrons and holes, a separator that effectively prevents their recombination is essential for increasing battery efficiency.

Professor Choi's team improved the efficiency of the separator by adding a trace amount of water to the organic ferroelectric film. When water is added to the organic film, which, like oil, dislikes water, fiber-like organic particles, several microns (μm, 10-6), are aligned into a dense, regular structure. As the microstructure is aligned, the force that attracts electrons and repels holes increases, improving the performance of the separator.

Furthermore, the battery manufacturing process eliminates the need for the expensive process of drilling holes in the separator. Because separators are non-conductive materials, holes must be drilled to create passages for electrons and holes. In contrast, the newly developed separator can easily create holes by simply evaporating and removing the added water.

Professor Choi said, “This research has expanded the silicon solar cell rear separator technology, which was limited to inorganic materials, to organic materials,” and predicted, “Unlike inorganic thin-film solar cells that require expensive vacuum equipment, using organic thin films simplifies the process, allowing for the creation of cost-competitive solar cells.”

This study was published online on the 13th in the world-renowned academic journal in the field of materials, 'Advanced Functional Materials', under the title 'Ambipolar passivated back surface field layer for Silicon photovoltaics'.