Scientists generate two electrons from a single photon

<b>Left: In normal solar cells, the absorption of light generates a single excited electron. The ground state is a so-called singlet, as is the excited state. <br>Right: In the substance DPIBF, the absorption of light generates two excited electrons. The excited state is known as a triplet. </b><br><br><i>Diagram: NREL</i><br>
Left: In normal solar cells, the absorption of light generates a single excited electron. The ground state is a so-called singlet, as is the excited state.
Right: In the substance DPIBF, the absorption of light generates two excited electrons. The excited state is known as a triplet.


Diagram: NREL

If light falls on typical solar cell materials, the absorption of a photon (colloquially: a light particle) causes an electron to become electrically excited. In a solar cell with connected electrodes, the resulting electron / electron hole pair can generate a current. The American scientists have now found a material in which two electrically excited electrons are generated by each absorbed photon. In quantum mechanics, the ground state is called a singlet. The excited state of ordinary solar cell materials is such a singlet, too. The excited state of the substance 1,3 diphenylisobenzofuran (DPIBF) forms a so-called triplet instead of a singlet, a process that is called singlet fission. The effect itself has already been known for more than 30 years, but only very low yields have been observed so far. The research that has now been published is the first documented case in which all of the absorbed photons generated two electrons, i.e. in which a quantum efficiency of 200 % has been achieved. The theoretical maximum of conventional solar cells is 100 %.

The efficiency of 200 % was measured at a temperature of 77 K (-196 °C) and declines as the temperature rises. At 300 K, i.e. about 27 °C, the quantum efficiency drops to 125 %, which is still considerably higher than that of conventional solar cells. There is still a long way to go from this research result to any practical application. For example, it has not been fully explained why the quantum efficiency declines so much. Another issue is the fact that DPIBF absorbs only a limited part of the solar spectrum, but it will provide the basis for further materials with a wider absorption spectrum.

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