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Energy migration and charge formation in conjugated polymers

Zaushitsyn, Yuri LU (2004)
Abstract (Swedish)
Popular Abstract in Swedish

I detta projekt studerar vi reaktioner som är av betydelse för omvandling av ljusenergi till elektricitet eller bränsle. Materialen är nya typer av organiska polymermaterial som kan ge morgondagens energisnåla och miljövänliga solceller, TV- och datorskärmar, ljuskällor, mm. Men för att detta skall inträffa krävs mycken ny forskning för att förstå de processer som ligger till grund för materialens funktion. När ljus tas upp av tex ett solcellsmaterial omvandlas ljusenergin till elektriska laddningar som transporteras genom materialet till en yttre krets där de kan utföra arbete. För att kunna framställa ett optimalt material måste vi studera och förstå mekanismerna bakom alla delstegen från... (More)
Popular Abstract in Swedish

I detta projekt studerar vi reaktioner som är av betydelse för omvandling av ljusenergi till elektricitet eller bränsle. Materialen är nya typer av organiska polymermaterial som kan ge morgondagens energisnåla och miljövänliga solceller, TV- och datorskärmar, ljuskällor, mm. Men för att detta skall inträffa krävs mycken ny forskning för att förstå de processer som ligger till grund för materialens funktion. När ljus tas upp av tex ett solcellsmaterial omvandlas ljusenergin till elektriska laddningar som transporteras genom materialet till en yttre krets där de kan utföra arbete. För att kunna framställa ett optimalt material måste vi studera och förstå mekanismerna bakom alla delstegen från ljusabsorption till hur laddningarna genereras och transporteras. När man tillverkar tex en solcell av ett polymermaterial måste också matallelektroder i form av en tunn metallfilm appliceras på polymeren. Detta kan introducera stora förändringar av polymerens egenskaper i gränsytan. Liksom det är nödvändigt att studera polymermaterialets egenskaper är det nödvändigt att studera dess egenskaper i gränsskiktet metall-polymer, för att få en fullständig bild av de faktorer som bestämmer tex en solcells funktion. Ljusabsorption i polymermaterialet inducerar transport av energi och laddning, processer som sker på femtosekund till nanosekundtidskalan. För att studera dessa processer är femtosekundlaserpulser och ytkänsliga spektroskopiska metoder de ideala verktygen. (Less)
Abstract
The main part of the thesis is concerned with charge photogeneration in conjugated polymers. MeLPPP and POMeOPT films were investigated by transient absorption spectroscopy using different excitation photon energies and light intensities with and without an external electric field. We conclude that electric field-assisted charge pair photogeneration occurs from vibrationally relaxed singlet excitons during their entire lifetime without any intermediate states in MeLPPP. This charge pair formation takes place in this manner even in the absence of an external electric field in POMeOPT. High photon energy excitation opens additional channels of charge pair generation such as dissociation of higher excited states and hot exciton dissociation... (More)
The main part of the thesis is concerned with charge photogeneration in conjugated polymers. MeLPPP and POMeOPT films were investigated by transient absorption spectroscopy using different excitation photon energies and light intensities with and without an external electric field. We conclude that electric field-assisted charge pair photogeneration occurs from vibrationally relaxed singlet excitons during their entire lifetime without any intermediate states in MeLPPP. This charge pair formation takes place in this manner even in the absence of an external electric field in POMeOPT. High photon energy excitation opens additional channels of charge pair generation such as dissociation of higher excited states and hot exciton dissociation in MeLPPP, while in POMeOPT the photon energy has no effect at all. We have suggested two models of charge photogeneration in conjugated polymers: the random site model for MeLPPP type of charge photogeneration and the dissociation site model for POMeOPT. These two models or a combination of them may be useful to address photoinduced charge generation in other conjugated polymers. From optical studies of Stark shift dynamics we estimate the distance between electron and hole in MeLPPP. Charge pairs may be separated by 5 - 8 Å immediately after they are created from neutral excitations and then the average distance increases to ~35 Å during 600 ps, where they can be considered as separated charges. In addition, energy transfer was studied in conjugated polymer (PTOPT, POMeOPT) films by transient absorption anisotropy with ~20 fs time resolution. Computer simulation by an incoherent energy migration model reproduces the experimental kinetics in PTOPT for times longer than ~100 fs. Singlet-singlet exciton annihilation was also observed by transient absorption spectroscopy under high peak excitation intensities at low repetition rate, resulting in a typical nonexponential character of the exciton decay kinetics. Singlet-triplet exciton annihilation was studied by fluorescence spectroscopy at high pulse repetition rates in POMeOPT; the annihilation leads to shortening of the singlet exciton lifetime, while monoexponential fluorescence kinetics behaviour is maintained. Finally, the excited electronic states of a polyfluorene-based co-polymer DiO-PFDTBT have been characterized using optical spectroscopy and semiempirical quantum-chemical (ZINDO) calculations. (Less)
Please use this url to cite or link to this publication:
author
opponent
  • Valkunas, Leonas, Institute of Physics, Vilnius, Lithuania
organization
publishing date
type
Thesis
publication status
published
subject
keywords
magnetic resonance, relaxation, spectroscopy, Kondenserade materiens egenskaper:elektronstruktur, egenskaper (elektriska, magnetiska och optiska), supraledare, magnetisk resonans, spektroskopi, supraconductors, magnetic and optical properties, electrical, energy transfer, Physics, Fysik, Condensed matter:electronic structure, transient absorption spectroscopy, conjugated polymer, charge photogeneration
pages
138 pages
publisher
Yuri Zaushitsyn, Cmem. Phys., Lund University
defense location
Lecture hall A, Chemical Center
defense date
2004-05-28 13:15
ISBN
91-628-6128-X
language
English
LU publication?
yes
id
bc14e0c1-a4cd-4d95-b4f2-691f041bed00 (old id 466954)
date added to LUP
2007-10-13 15:21:24
date last changed
2016-09-19 08:45:04
@misc{bc14e0c1-a4cd-4d95-b4f2-691f041bed00,
  abstract     = {The main part of the thesis is concerned with charge photogeneration in conjugated polymers. MeLPPP and POMeOPT films were investigated by transient absorption spectroscopy using different excitation photon energies and light intensities with and without an external electric field. We conclude that electric field-assisted charge pair photogeneration occurs from vibrationally relaxed singlet excitons during their entire lifetime without any intermediate states in MeLPPP. This charge pair formation takes place in this manner even in the absence of an external electric field in POMeOPT. High photon energy excitation opens additional channels of charge pair generation such as dissociation of higher excited states and hot exciton dissociation in MeLPPP, while in POMeOPT the photon energy has no effect at all. We have suggested two models of charge photogeneration in conjugated polymers: the random site model for MeLPPP type of charge photogeneration and the dissociation site model for POMeOPT. These two models or a combination of them may be useful to address photoinduced charge generation in other conjugated polymers. From optical studies of Stark shift dynamics we estimate the distance between electron and hole in MeLPPP. Charge pairs may be separated by 5 - 8 Å immediately after they are created from neutral excitations and then the average distance increases to ~35 Å during 600 ps, where they can be considered as separated charges. In addition, energy transfer was studied in conjugated polymer (PTOPT, POMeOPT) films by transient absorption anisotropy with ~20 fs time resolution. Computer simulation by an incoherent energy migration model reproduces the experimental kinetics in PTOPT for times longer than ~100 fs. Singlet-singlet exciton annihilation was also observed by transient absorption spectroscopy under high peak excitation intensities at low repetition rate, resulting in a typical nonexponential character of the exciton decay kinetics. Singlet-triplet exciton annihilation was studied by fluorescence spectroscopy at high pulse repetition rates in POMeOPT; the annihilation leads to shortening of the singlet exciton lifetime, while monoexponential fluorescence kinetics behaviour is maintained. Finally, the excited electronic states of a polyfluorene-based co-polymer DiO-PFDTBT have been characterized using optical spectroscopy and semiempirical quantum-chemical (ZINDO) calculations.},
  author       = {Zaushitsyn, Yuri},
  isbn         = {91-628-6128-X},
  keyword      = {magnetic resonance,relaxation,spectroscopy,Kondenserade materiens egenskaper:elektronstruktur,egenskaper (elektriska,magnetiska och optiska),supraledare,magnetisk resonans,spektroskopi,supraconductors,magnetic and optical properties,electrical,energy transfer,Physics,Fysik,Condensed matter:electronic structure,transient absorption spectroscopy,conjugated polymer,charge photogeneration},
  language     = {eng},
  pages        = {138},
  publisher    = {ARRAY(0x9cb9290)},
  title        = {Energy migration and charge formation in conjugated polymers},
  year         = {2004},
}