Proceedings of nanoGe Fall Meeting19 (NFM19)
DOI: https://doi.org/10.29363/nanoge.nfm.2019.015
Publication date: 18th July 2019
Solar cells have deficiencies such as disability in absorbing all the sunlight’s wavelengths. Upconversion is one of the processes which helps solar cells to overcome this problem. However, people still need to improve Upconversion yield to make it applicable in solar cells. Two kinds of molecules are used in Upconversion: Sensitizer and emitter molecules. During this process two low energy photons convert to higher energy one.
Upconversion (UC) systems still need to overcome some challenges. For instance, Oxygen is one of the big limitations for these systems. It can kill triplet before they transfer to the emitter molecule. Moreover, Sensitizer’s absorption needs to improve, and it is also desired to bring Upconversion below silicon band gap.
Previously, we have introduced Violanthrone as the molecule which would be able to prevent this bottle neck [2]. In fact, this molecule’s triplet state is below singlet oxygen. So not only oxygen would not be a problem any more, but also can help the process.
The aim of present going experiments is to use Violanthron-79 as the emitter in the mixture of PbS QDs as the sensitizer. The idea of this project is to make a system with a wide range of absorption which is not oxygen sensitive. Ultimately, we can even make the system better and use different types of QDs to bring UC below the Si band gap.
In this system, the oxygen will not be destructive. Moreover, it would behave as a transistor for triplets. So transferring triplets from QDs to Violanthrone would be faster and more efficient. The other point is about fundamental quantum dots ligands which are oleic acid with a long 18 Carbons chain. According to Nienhaus et.al. [3], we also believe these long ligands will be a big barrier for triplet transferring from QDs to Violanthrone. So ligand exchange has been designed. In terms of Ligand exchange the basic oleic acid (OA) ligands will partially change by Tips tetracene carboxylic acid (Tips-TC). So, it is expected that triplets transfer from QDs to the Tips-TC and then transfer to the Violanthone molecule. Then, Triplet-Triplet annihilation (TTA) happens between two Violanthrone molecule and finally the higher energy photon will emit from singlet state of Violanthrone. It is expected oxygen facilitates triplet energy transfer from Tips-TC to Violanthrone since Violanthrone has lower triplet sate than Oxygen.
We are already using QDs which absorb 846 nm and emit 960 nm as the sensitizer. Ultimately, QDs with the absorption below silicon band gap will be applied.
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