Extending Lifetimes Of Hot Electrons For More Efficient Solar Cells

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Researchers suggest that bandgap engineering can improve the optical properties of optoelectronic devices.

Semiconductors are the material of choice in electronic devices as they provide control to their electrical properties. This is due to their signatory property known as the bandgap, a region of forbidden electron energies. Electrons with an energy above the top of the bandgap are free electrons, and are free to move through the material. On the other hand, the electrons with energies below the bottom of the bandgap are not, and no electrons with an energy in between can exist in the material.

Researchers from King Abdullah University of Science and Technology have demonstrated that bandgap engineering can improve the performance of optoelectronic devices. 

The bandgap of a semiconductor dictates its optical properties. The energy from any absorbed light is transferred to the material’s electrons. However, the absorption of photons with an energy much larger than the bandgap creates, in turn, much higher energy electrons that are also called “hot electrons.” 

According to the researchers, it is important to understand how hot electrons relax to an energy nearer the top of the bandgap. For example, the efficiency of a solar cell is reduced if this huge excess energy is lost as heat. 

“However, it was extremely difficult, if not impossible, to sufficiently utilize these hot electrons in real light-conversion applications due to their very short lifetimes,” says material scientist Omar Mohammed.

The researchers used interface and bandgap engineering to delay hot electrons and holes relaxation, and to significantly increase their lifetimes.

They studied lead halide perovskite, and fabricated architectures made up of multiple quantum wells: a thin layer of semiconductor sandwiched between light-absorber layers of larger bandgap material. They compared the optical properties of structures in which the wells were all the same thickness and asymmetric structures in which the well widths varied. They used a technique called femtosecond transient absorption spectroscopy combined with theoretical calculations to determine the timescale of the hot electron relaxation.

“This new finding provides a unique strategy on how to significantly slow down the hot carriers cooling in semiconducting materials for their better utilization in solar cells applications,” says postdoc Partha Maity.

The work is described in the journal ACS Energy Letters.


 

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