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　　Transformational thinking in organic photovoltaic cell research

　　Organic photovoltaic cells have been touted as a lightweight, low-cost alternative to rigid silicon-based solar panels. In recent years, the conversion efficiency of organic photovoltaic cells has been greatly improved. However, how exactly organic photovoltaic cells convert sunlight into electricity is still a matter of intense debate.

　　Now, a research team from Stanford University in the United States is involved in this topic. The research team disclosed in the journal Nature Materials on November 17, 2013 that the originally recognized working principle was incorrect and that thinking should be focused on material design to improve the performance of organic batteries.

　　Michael McGehee, a professor at the School of Materials Science and Engineering at Stanford University (one of the authors of the paper) said: We all know that organic photovoltaic cells have outstanding performance. The question now is, why are they so outstanding? —The answer remains controversial.

　　Traditional organic solar cells consist of two semiconductor layers made of plastic polymers and other flexible materials. By absorbing photons (particles of light), the battery produces electricity.

　　When the battery draws in light, the photons bounce around the polymer atoms, causing them to spill electrons, leaving behind a hole—scientists call it a hole. The holes and electrons quickly form a combination of excitons (excited electrons). The exciton then splits off and moves independently into a hole created by another photon. The continuous movement of excitons from one hole to another generates an electric current.

　　In this paper, the Stanford team solved a long-debated question—what causes excitons to split.

　　Alberto Salleo, an associate professor in the Department of Materials Science and Engineering at Stanford University, claimed: To generate an electric current, it is necessary to separate excitons and holes - which requires two different types of semiconductor materials. If material B has a greater attraction for excitons than material A, then the excitons will swim toward material B. Theoretically, even if dropped into a material, the exciton would still be bound to the hole.

　　However, the protracted debate centers on how this bound state can be split?

　　Passionate

　　One explanation widely accepted by scientists is the thermal exciton effect theory. The theory is that when an electron falls from material A into material B, it carries extra energy—this extra energy gives the excited electron enough speed to escape the hole.

　　However, the Stanford team's experimental results do not support this hypothesis.

　　Stanford scientists may have solved the long-running debate about how organic photovoltaic cells convert sunlight into electricity, says Stanford's Koen Vandewal. The core of the problem: What exactly causes the separation of electron-hole pairs (excitons)? Possible answer: The natural gradient at the interface between the disordered polymer and the ordered buckyball causes excitons to split, allowing electrons (purple) to escape, creating an electric current.

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