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Photovoltaics: What Happens to Excess Energy?

​In a solar cell, the absorption of photons with energy exceeding the absorption threshold results in excess energy. But how is this excess energy dissipated? Researchers from IRAMIS answered this question by developing concepts that could facilitate the description of other semiconductors.
Published on 10 April 2017
​Most solar photons absorbed by a solar cell transfer more energy than necessary to the electrons in the semiconductor, transforming them into "hot" electrons. This excess energy is quickly dissipated in favor of the vibrations of the semiconductor network (phonons). The ultimate goal of scientists is to recover the totality of this energy before its dissipation.

Japanese researchers have studied the "relaxation" of hot electrons in gallium arsenide in experiments using angle-, time- and energy-resolved photoemission spectroscopy.

A team from LSI (Irradiated Solids Laboratory) was able to interpret their results successfully with a new model developed in collaboration with Paris 6 University. The model provides an ab initio description of the electron-phonon coupling and allows for calculation of the lifetime of electrons with very high precision. The physicists showed that the relaxation of electrons unfolds in two stages. The first being very swift, of a few tens of femtoseconds (10-15 s), and corresponds to the diffusion without energy loss. The second is ten times slower and relates to the transfer of energy from the electrons to phonons. 

These results were achieved as part of a collaboration between IRAMIS, Paris 6 University, La Sapienza University of Rome, and Osaka University in Japan.

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