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Breakthrough in solar cell research is expected to design a new interface for organic solar cells

Date: 2018-3-20 11:54:10 Visits: 1345

Purdue scientists have recently determined the mechanism of charge separation in organic solar cells. The team claims that it solves a long-standing problem in physics. By understanding how excitons separate, it will help researchers design new interfaces for organic solar cells.

Organic solar cells are made of soft molecules, while inorganic solar cells (silicon crystals) are made of harder materials. Silicon cells currently dominate the solar industry due to the best conversion efficiency, but they are expensive and hard. Compared to other rookie batteries, It may have the potential of lightness, flexibility and cheapness. The dominance of silicon batteries will not be guaranteed at any time in the future, but the disadvantages of organic batteries are the difficulty in generating current, and the energy conversion efficiency and stability are still poor.

An exciton (electron-hole pair) formed at the interface between tetracene molecules (an organic semiconductor) and single-layer WS2 (an inorganic semiconductor). Dissociation of such interfacial excitons is necessary for the function of organic solar cells. (Image provided)

In order to generate an electric current, the two positively charged (electron) and negatively charged (hole) tightly bound particles must be separated. They usually form an exciton (electron-hole pair) together and require another artificial interface to It helps to separate, but usually after the interface, the electrons and holes still attract each other.

According to the structure, organic solar cells can be divided into single or double layers (or heterojunctions). The single layer is the simplest form. The organic electronic material layer is sandwiched between two metal conductive layers. The work function difference creates an electric field at both ends of the organic layer. When the organic layer absorbs photons, it will excite an electron-hole pair. The potential caused by different work functions on the electrode will help the excitons to separate into independent electrons and holes. When electrons are pulled to the positive electrode and holes are pulled to the negative electrode, the voltage and current formed in this process can be used.

However, single-layer organic solar cells actually perform poorly because their quantum efficiency is less than 1% and energy conversion efficiency is less than 0.1%. The main reason is that the electric field between the two electrodes is rarely enough to separate excitons. Electrons recombine more with holes than reach electrodes. In order to solve this problem, multilayer organic solar cells have begun to develop. Two different layers are added between two metal conductive layers. By making the electron affinity and ionization energy more different, the local electric field is large enough. Letting excitons separate is more effective than single-layer solar cells.

However, recently, Libai Huang, an assistant professor of the Department of Chemistry at Purdue University, published a paper in Science Advances and found that the electron hole interface is not a single static state. In fact, electrons and holes can be far away or very close to each other. The farther you go, the greater the chance of a smooth separation. Just as the separated couples are less bound, they are more likely to disappear like wild horses. The farther the electrons and holes are, the faster the movement speed. .

Research on organic solar cells is difficult, but now, understanding how excitons are separated will help researchers design new interfaces for organic solar cells, Libai Huang said, which also means that there are still many materials for solar cells that have not yet been applied. Mature control.

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