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Ultrafast exciton dynamics at molecu...

Monahan, Nicholas R.

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Ultrafast exciton dynamics at molecular surfaces.

Record Type:	Electronic resources : Monograph/item
Title/Author:	Ultrafast exciton dynamics at molecular surfaces./
Author:	Monahan, Nicholas R.
Description:	97 p.
Notes:	Source: Dissertation Abstracts International, Volume: 77-07(E), Section: B.
Contained By:	Dissertation Abstracts International77-07B(E).
Subject:	Physical chemistry. -
Online resource:	http://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=10011000
ISBN:	9781339469805

Ultrafast exciton dynamics at molecular surfaces.
Monahan, Nicholas R.

Ultrafast exciton dynamics at molecular surfaces. - 97 p.

Source: Dissertation Abstracts International, Volume: 77-07(E), Section: B.

Thesis (Ph.D.)--Columbia University, 2016.

Further improvements to device performance are necessary to make solar energy conversion a compelling alternative to fossil fuels. Singlet exciton fission and charge separation are two processes that can heavily influence the power conversion efficiency of a solar cell. During exciton fission one singlet excitation converts into two triplet excitons, potentially doubling the photocurrent generated by higher energy photons. There is significant discord over the singlet fission mechanism and of particular interest is whether the process involves a multiexciton intermediate state. I used time-resolved two-photon photoemission to investigate singlet fission in hexacene thin films, a model system with strong electronic coupling. My results indicate that a multiexciton state forms within 40 fs of photoexcitation and loses singlet character on a 280 fs timescale, creating two triplet excitons. This is concordant with the transient absorption spectra of hexacene single crystals and definitively proves that exciton fission in hexacene proceeds through a multiexciton state. This state is likely common to all strongly-coupled systems and my results suggest that a reassessment of the generally-accepted singlet fission mechanism is required. Charge separation is the process of splitting neutral excitons into carriers that occurs at donor-acceptor heterojunctions in organic solar cells. Although this process is essential for device functionality, there are few compelling explanations for why it is highly efficient in certain organic photovoltaic systems. To investigate the charge separation process, I used the model system of charge transfer excitons at hexacene surfaces and time-resolved two-photon photoemission. Charge transfer excitons with sufficient energy spontaneously delocalize, growing from about 14 nm to over 50 nm within 200 fs. Entropy drives this delocalization, as the density of states within the Coulomb potential increases significantly with energy. This charge separation mechanism should occur at all donor-acceptor interfaces. My results show that entropy facilitates charge separation and indicate that the density of acceptor states should be a design consideration when constructing organic solar cells.

ISBN: 9781339469805Subjects--Topical Terms:

1981412
Physical chemistry.

Ultrafast exciton dynamics at molecular surfaces.
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100 1 $a Monahan, Nicholas R. $3 3192598
245 1 0 $a Ultrafast exciton dynamics at molecular surfaces.
300 $a 97 p.
500 $a Source: Dissertation Abstracts International, Volume: 77-07(E), Section: B.
500 $a Adviser: Xiaoyang Zhu.
502 $a Thesis (Ph.D.)--Columbia University, 2016.
520 $a Further improvements to device performance are necessary to make solar energy conversion a compelling alternative to fossil fuels. Singlet exciton fission and charge separation are two processes that can heavily influence the power conversion efficiency of a solar cell. During exciton fission one singlet excitation converts into two triplet excitons, potentially doubling the photocurrent generated by higher energy photons. There is significant discord over the singlet fission mechanism and of particular interest is whether the process involves a multiexciton intermediate state. I used time-resolved two-photon photoemission to investigate singlet fission in hexacene thin films, a model system with strong electronic coupling. My results indicate that a multiexciton state forms within 40 fs of photoexcitation and loses singlet character on a 280 fs timescale, creating two triplet excitons. This is concordant with the transient absorption spectra of hexacene single crystals and definitively proves that exciton fission in hexacene proceeds through a multiexciton state. This state is likely common to all strongly-coupled systems and my results suggest that a reassessment of the generally-accepted singlet fission mechanism is required. Charge separation is the process of splitting neutral excitons into carriers that occurs at donor-acceptor heterojunctions in organic solar cells. Although this process is essential for device functionality, there are few compelling explanations for why it is highly efficient in certain organic photovoltaic systems. To investigate the charge separation process, I used the model system of charge transfer excitons at hexacene surfaces and time-resolved two-photon photoemission. Charge transfer excitons with sufficient energy spontaneously delocalize, growing from about 14 nm to over 50 nm within 200 fs. Entropy drives this delocalization, as the density of states within the Coulomb potential increases significantly with energy. This charge separation mechanism should occur at all donor-acceptor interfaces. My results show that entropy facilitates charge separation and indicate that the density of acceptor states should be a design consideration when constructing organic solar cells.
590 $a School code: 0054.
650 4 $a Physical chemistry. $3 1981412
650 4 $a Condensed matter physics. $3 3173567
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690 $a 0611
710 2 $a Columbia University. $b Chemical Physics. $3 3192599
773 0 $t Dissertation Abstracts International $g 77-07B(E).
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791 $a Ph.D.
792 $a 2016
793 $a English
856 4 0 $u http://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=10011000