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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.

No MeSH data available.


Trion formation in a QD provides a charge capable of receiving and dissipating the energy liberated in the recombination of an exciton, resulting in an enhanced single-exciton recombination rate.
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nanomaterials-04-00019-f004: Trion formation in a QD provides a charge capable of receiving and dissipating the energy liberated in the recombination of an exciton, resulting in an enhanced single-exciton recombination rate.

Mentions: As discussed above, a signature of MEG is, at low pump fluences, a rapid signal decay on the sub-nanosecond time-scale corresponding to fast Auger recombination of multi-excitons, a process inaccessible to single-excitons. However, fast Auger recombination can also occur when a trion is formed within the QD, i.e., an excitation involving three charge carriers—see Figure 4. Trions can form when a hole or electron is trapped on the QD surface for a time longer than the period between excitation pulses. Its geminate charge is thus already present within the QD when an exciton is formed following photon absorption during a subsequent pump pulse. The unpaired electron or hole residing within the QD is able to receive the energy liberated in recombination, enabling fast Auger recombination of the remaining electron–hole pair. This process can produce a sub-nanosecond decay in the signal even at low pump fluences and thus can resemble the signature of MEG. However, several careful [27,28,29] studies have shown that sufficient stirring or flowing of the sample can refresh the QDs within the excitation volume in between pump pulses, preventing the formation of trions and thus of this misleading component to the signal.


Multiple Exciton Generation in Colloidal Nanocrystals
Trion formation in a QD provides a charge capable of receiving and dissipating the energy liberated in the recombination of an exciton, resulting in an enhanced single-exciton recombination rate.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

nanomaterials-04-00019-f004: Trion formation in a QD provides a charge capable of receiving and dissipating the energy liberated in the recombination of an exciton, resulting in an enhanced single-exciton recombination rate.
Mentions: As discussed above, a signature of MEG is, at low pump fluences, a rapid signal decay on the sub-nanosecond time-scale corresponding to fast Auger recombination of multi-excitons, a process inaccessible to single-excitons. However, fast Auger recombination can also occur when a trion is formed within the QD, i.e., an excitation involving three charge carriers—see Figure 4. Trions can form when a hole or electron is trapped on the QD surface for a time longer than the period between excitation pulses. Its geminate charge is thus already present within the QD when an exciton is formed following photon absorption during a subsequent pump pulse. The unpaired electron or hole residing within the QD is able to receive the energy liberated in recombination, enabling fast Auger recombination of the remaining electron–hole pair. This process can produce a sub-nanosecond decay in the signal even at low pump fluences and thus can resemble the signature of MEG. However, several careful [27,28,29] studies have shown that sufficient stirring or flowing of the sample can refresh the QDs within the excitation volume in between pump pulses, preventing the formation of trions and thus of this misleading component to the signal.

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.