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Multiple Exciton Generation in Colloidal Nanocrystals

View Article: PubMed Central - PubMed

ABSTRACT

In a conventional solar cell, the energy of an absorbed photon in excess of the band gap is rapidly lost as heat, and this is one of the main reasons that the theoretical efficiency is limited to ~33%. However, an alternative process, multiple exciton generation (MEG), can occur in colloidal quantum dots. Here, some or all of the excess energy is instead used to promote one or more additional electrons to the conduction band, potentially increasing the photocurrent of a solar cell and thereby its output efficiency. This review will describe the development of this field over the decade since the first experimental demonstration of multiple exciton generation, including the controversies over experimental artefacts, comparison with similar effects in bulk materials, and the underlying mechanisms. We will also describe the current state-of-the-art and outline promising directions for further development.

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Models of biexciton formation from a single absorbed photon: (a) a photo-generated hot exciton relaxes by impact ionization, exciting another electron across the band gap; thereby creating a biexciton (b) the direct photogeneration of a biexciton through a virtual exciton or biexciton states [62]; and (c) the photo-generation, by a single photon, of a quantum superposition of all possible excited states, including multi-excitons.
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nanomaterials-04-00019-f005: Models of biexciton formation from a single absorbed photon: (a) a photo-generated hot exciton relaxes by impact ionization, exciting another electron across the band gap; thereby creating a biexciton (b) the direct photogeneration of a biexciton through a virtual exciton or biexciton states [62]; and (c) the photo-generation, by a single photon, of a quantum superposition of all possible excited states, including multi-excitons.

Mentions: Several different theoretical descriptions of MEG have been studied: impact ionization, in which the hot exciton initially created by the absorption of a high energy photon gives its excess energy to one or more valance band electrons so that they are promoted across the band gap; the generation of a biexciton via a virtual exciton or biexciton; and the initial photo-excitation of a superposition of quantum states corresponding to all possible excited states, including multi-excitons. These different models are illustrated in Figure 5 for the simplest case of MEG, where just one additional exciton is created.


Multiple Exciton Generation in Colloidal Nanocrystals
Models of biexciton formation from a single absorbed photon: (a) a photo-generated hot exciton relaxes by impact ionization, exciting another electron across the band gap; thereby creating a biexciton (b) the direct photogeneration of a biexciton through a virtual exciton or biexciton states [62]; and (c) the photo-generation, by a single photon, of a quantum superposition of all possible excited states, including multi-excitons.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5304609&req=5

nanomaterials-04-00019-f005: Models of biexciton formation from a single absorbed photon: (a) a photo-generated hot exciton relaxes by impact ionization, exciting another electron across the band gap; thereby creating a biexciton (b) the direct photogeneration of a biexciton through a virtual exciton or biexciton states [62]; and (c) the photo-generation, by a single photon, of a quantum superposition of all possible excited states, including multi-excitons.
Mentions: Several different theoretical descriptions of MEG have been studied: impact ionization, in which the hot exciton initially created by the absorption of a high energy photon gives its excess energy to one or more valance band electrons so that they are promoted across the band gap; the generation of a biexciton via a virtual exciton or biexciton; and the initial photo-excitation of a superposition of quantum states corresponding to all possible excited states, including multi-excitons. These different models are illustrated in Figure 5 for the simplest case of MEG, where just one additional exciton is created.

View Article: PubMed Central - PubMed

ABSTRACT

In a conventional solar cell, the energy of an absorbed photon in excess of the band gap is rapidly lost as heat, and this is one of the main reasons that the theoretical efficiency is limited to ~33%. However, an alternative process, multiple exciton generation (MEG), can occur in colloidal quantum dots. Here, some or all of the excess energy is instead used to promote one or more additional electrons to the conduction band, potentially increasing the photocurrent of a solar cell and thereby its output efficiency. This review will describe the development of this field over the decade since the first experimental demonstration of multiple exciton generation, including the controversies over experimental artefacts, comparison with similar effects in bulk materials, and the underlying mechanisms. We will also describe the current state-of-the-art and outline promising directions for further development.

No MeSH data available.