What is exciton?

 

There are valence and conduction bands in materials, are atoms, the valence bands are filled. While the conduction band is empty at ground state energy between the valence and conduction, the band is a forbidden band, which is also known as the band. When we provide sufficient energy to the electrons, this energy excites the electron, and the electron jumps from the valence band to the conduction band. It leaves a hole in the valence band. Now, this hole in the valence band and the electron in the conduction band are attracted to each other by the electrostatic force of attraction and make an electron-hole pair. This bound state of the electron-hole pair is known as an exciton.

There are two main types of excitons. Wannier-Mott excitons, which is also known as free, excitons and Frankel excitons, also known as bound excitons. Wannier-Mott excitons are mainly observed in semiconductors, They have a sufficiently large radius that encompasses, many atoms and can move freely throughout the crystal. Franklin are bound excitons are mainly observed in insulators. Bound excitons by contrast have a much smaller radius and are comparable to the size of the unit cell. This makes them localized states which are tightly bound to specific atoms or molecules. That is why they are known as Bound excitons.

 

This image was taken from Kéna-Cohen, S., Strong exciton-photon coupling in organic semiconductor microcavities. Princeton University: 2010.

Summary of Journal paper: Mn Doping in Centimeter-Sized Layered 2D Butylammonium Lead Bromide (BA2PbBr4) Single Crystals and Their Optical Properties

In this study, Drake Beery and coworkers investigated an integrated Triplet Triplet Annihilation Up- Conversion, or TTAUC, Solar cell utilizing Cadmium Solenoid (CdSe) quantum dots as the triple sensitizer solar cell with a 1.5 times enhancement in the short circuit current for the TTAUC devices. Compared to the sum of the individual components, a quadratic to linear photocurrent dependence on the excitation rate was observed, supporting a TTA-UC photocurrent generation mechanism with an eighth value was on par with some of the lowest Ith values reported today, and below solar flux. Finally, less than unity energy transfer yields slow quantum dot regeneration by the redox, mediator and competitive quenching of the quantum dot excited state led to poor device performance.

 

The image was taken from https://doi.org/10.1021/acsaem.9b01765

The previous work done by the Hanson research group introduced this integrated TTAUC, solar cell using a self-assembled by layer architecture of sensitizer acceptor molecular die pairs on a metal oxide surface with the TTAUC mechanism. But possess some limitations including inherent energy losses. When inner system crossing from the singlet to the triple state production costs. Absorption limited to the UV visible range and a low extension coefficient all of which led to their interests and semiconductor quantum dots that had recently been shown to be effective.

For more details : https://doi.org/10.1021/acsaem.9b01765