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Ultrafast Photoinduced Processes in Core and Core–Shell Quantum Dots for Solar Cell Applications “Tiny Crystals for Big Applications”

Qenawy, Mohamed LU (2015)
Abstract (Swedish)
Popular Abstract in Swedish

Balansen mellan våra energibehov och den energi som konsumeras är ej längre i jämvikt. Därför är jakten på alternativa energikällor nödvändig. Hållbara energikällor erbjuder ett bra alternativ till fossilbränslen. Bland många typer av hållbara energikällor erbjuder solenergi möjligheten för mer energi än dagens konsumtion. Solceller är de smarta verktyg som omvandlar ljusets fotoner till användbar elektricitet. Utvecklingen av solceller har redan genomgått några generationer där två viktiga faktorer styr marknaden. Solcellernas verkningsgrad samt produktionskostnaderna för att tillverka dem är hörnstenar inom solcellsmarknaden. Tredje generationens solceller försöker optimera verkningsgrad mot... (More)
Popular Abstract in Swedish

Balansen mellan våra energibehov och den energi som konsumeras är ej längre i jämvikt. Därför är jakten på alternativa energikällor nödvändig. Hållbara energikällor erbjuder ett bra alternativ till fossilbränslen. Bland många typer av hållbara energikällor erbjuder solenergi möjligheten för mer energi än dagens konsumtion. Solceller är de smarta verktyg som omvandlar ljusets fotoner till användbar elektricitet. Utvecklingen av solceller har redan genomgått några generationer där två viktiga faktorer styr marknaden. Solcellernas verkningsgrad samt produktionskostnaderna för att tillverka dem är hörnstenar inom solcellsmarknaden. Tredje generationens solceller försöker optimera verkningsgrad mot kostnad via billiga material som fortfarande är tillräckligt ’smarta’ för att skapa laddningsbärare när ljus absorberas och därefter genererar elektricitet. Samtidigt som elektricitet genereras så är det också ett mål att övervinna termodynamikens effektivitetsgräns. Nanomaterial har framkommit som byggstenar för att samla solenergi i tredje generationens solceller. Bland dessa så har kvantprickar visat sig vara starka kandidater eftersom de har goda egenskaper som hög extinktionskoefficient, justerbar absorption samt möjligheten att generera och samla flera excitoner via en enda högenergetisk foton.

Beteendet hos både fotoexciterade elektroner och dito hål bestämmer den generella verkningsgraden hos en solcell baserad på kvantprickar. Denna avhandling presenterar en systematisk studie av den ultrasnabba foto-aktiverade elektrondynamiken inom kvantprickmaterial för solceller. Dessa processer inkluderar laddningsöverföring, excitonmigration, fällnivåers laddningsbärarinfångning och deras verkan på solcellers prestanda. De studerade materialen börjar med konventionella enkla CdSe-kärnkvantprickar och fortsätter med Cd1−xSe1−yZnxSy-gradient-kärn–skal-kvantprickar (CS). Det sistnämnda används för att förbättra optik- och apparatur-prestandan. Elektroninjektionen från CdSe till ZnO-nanotrådar observerades först att hända väldigt snabbt (några ps). Denna snabba elektroninjektionen uppmuntrar oss att studera möjligheten att injicera multipla elektroner från en kvantprick under höga excitationsförhållanden. Vi visade att det uppstår en konkurrens mellan elektoninjektion och Augerrekombination.

Jämfört med elektroner så har fotoinducerade hål större sannolikhet att fångas. Men sådana typer av fälltillstånd kan ibland vara radiativa med långa livstider, upp till några 10-tals mikrosekunder, när de befinner sig i oleinsyratäckta CdSe-kvantprickar. I detta scenario är hålinjektion i p-typ-kvantpricksolceller mindre effektivt (<10 %) jämfört med elektroninjektion i motsvarande n-typ-kvantprickar. Det påverkas ytterst av fällor på ytan som skapats av länkmolekylutbytesprocessen. Hålinjektionen kan då förbättras genom att passivera fällorna på ytan genom att använda kärn–skalstrukturer. Utöver elektron- eller hål-injektion kan excitonmigration också ske via FRET (Förster resonance energy transfer, Förster-resonansenergiöverföring). Vi upptäckte att FRET mellan kvantprickar kunde möjliggöra användning av ljusabsorption via de indirekt kopplade kvantprickarna i QD-sensitiserade metalloxidanoder (MO). Inom välstrukturerade blandningar av kvantprickar med olika storlekar är energiöverföring även tydligare. Vi kunde experimentellt observera FRET-processen i slumpmässigt arrangerade kvantprickar av olika storlekar samt i tandemstaplade QD-lager via tidsupplösta metoder och steady state-spektroskopi. Teoretiska simuleringar där en dipolfördelningsmodell introducerades för kopplingsberäkningar överensstämmer väl med experimentella resultat. För att minimera effekten från defekter på ytan och förbättra fotostabiliteten hos solceller gjorda av kvantprickar så studerade vi kärn–skalkvantpricksystem där laddningsbärarinfångning i ytfällor kan vara väl passiverat av skalmaterial som har förbättrade optiska egenskaper och apparaturprestanda. Här används en halvledare med bredare bandgap som en sköld runt den aktiva kärnan i gradienttillväxt, känt som Cd1−xSe1−yZnxSy-gradient-kärn–skal-kvantprickar. Sådana kvantprickar erbjuder högre fotostabilitet, bättre kvantutbyte för fluorescens samt mindre ytdefekter där materialen möts jämfört med de konventionella stegvisa kärn–skalkvantprickarna. Först karaktäriserade vi våra CS-kvantprickar via steady state-spektroskopi tillsammans med HR-TEM-bilder för att fastställa dimensionerna av våra kvantprickar och utvärdera tjockleken på det skyddande skalet. Sen användes XRD och EDX för att karaktärisera den kemiska sammansättningen samt kristallstrukturen.

Fotodynamiken hos dessa CS-kvantprickar i fotovoltaiska system studerades också. Vi trodde först att elektroninjektionen från den aktiva kärnan till MO av n-typ visade relativt stort exponentiellt beroende av tjockleken på skalet jämfört med stegvisa kärn–skalkvantprickar. Vi fastställde att det högsta elektroninjektionutbytet (~80 %) kan uppnås när skalet har en tjocklek upp till 1,3 nm. Ett sådant skal tillåter också hög ytpassivering vilket ger optimala förhållanden för laddningsuppsamling i solceller. Till slut integrerade vi vår kunskap om beteendet av elektroner och hål för att förklara solcellsprestanda enligt kärn–skalstrukturer. Vi bekräftade att hålfällor är den kritiska faktorn för verkningsgraden hos solceller gjorda av kvantprickar. Fällornas påverkan kan gottgöras väl genom att använda optimala kärn–skalstrukturer. (Less)
Abstract
The balance between our demands of energy and the energy we are consuming is not in equilibrium anymore. Therefore, the search for other energy resources is indispensable. Sustainable energy sources offer the alternative to the fossil fuels. Within many types of sustainable energy sources, solar energy offers more than the total global energy consumption. Solar cells are the smart conversion tools to harvest the incident photons and create electricity out of these photons. Solar cells have passed through different generations where two important factors control the solar cells market. The solar cell efficiency and price play the cornerstones in the solar cell marketing. Third generation of solar cells aims to maximize the price–performance... (More)
The balance between our demands of energy and the energy we are consuming is not in equilibrium anymore. Therefore, the search for other energy resources is indispensable. Sustainable energy sources offer the alternative to the fossil fuels. Within many types of sustainable energy sources, solar energy offers more than the total global energy consumption. Solar cells are the smart conversion tools to harvest the incident photons and create electricity out of these photons. Solar cells have passed through different generations where two important factors control the solar cells market. The solar cell efficiency and price play the cornerstones in the solar cell marketing. Third generation of solar cells aims to maximize the price–performance equation by using cheap materials without compromising efficiency. Nanomaterials have emerged as the promising building blocks to harvest the solar light in the third generation of the solar cells. Among them, quantum dots (QDs) can be used as viable candidate due to their superb features such as high extinction coefficient, a tunable absorption edge, and the possibility to generate and collect multiple excitons by using single, high energy photon. Both the behaviors of photoexcited electrons and holes determine the overall efficiency of QD based solar cells. This thesis presents a systematic study of the ultrafast photoinduced charge dynamics in QD solar cell materials including the charge transfer, exciton migration, carrier trapping and their influence on real solar cell performance. The materials investigated start with conventional neat core CdSe QDs and extend to gradient Cd1-xSe1-yZnxSy core–shell (CS) QDs. The latter are used to obtain improved optical and device performance. The electron injection from CdSe into ZnO nanowires were first observed to be very fast (few ps). This fast electron injection encourages us to study the possibility to inject multiple electrons from a QD under high excitation conditions. We revealed that a competition between electron injection and Auger recombination occurs.

Compared with electrons, the photoinduced holes are more likely to be trapped. However, such trap states sometimes can be radiative with long lifetime up to tens of microseconds in oleic acid capped CdSe QDs. In this scenario, the hole injection in p-type QD solar cells are proved to be less efficient (<10%) compared with electron injection in n-type counterparts. It is highly affected by the surface trapping sites induced by the linker exchange process. The hole injection can then be improved by passivating the surface trap sites using core shell structures. Besides electron or hole injection, exciton migration can also occur via Förster resonant energy transfer (FRET). We found that FRET between QDs would enable to make use of the absorption of light by the indirectly attached QDs in QD-sensitized metal oxide (MO) anodes. In well-organized multi-sized QD mixtures, the energy transfer is even more pronounced. We experimentally observed the FRET process in randomly arranged multi-sized QD assembly and tandem stacked QD layers by using time-resolved and steady-state spectroscopies. Theoretical simulations where dipole distribution model was introduced for coupling calculations complies well with the experimental results. In order to minimize the effect of surface defects and improve the photostability of QD solar cells, we investigated the core–shell QD system where the surface trapping of carriers can be well passivated by shell materials with enhanced optical properties and device performance. Herein, a wider band gap semiconductor is employed as a shield shell around the active core in gradient growth, known as gradient Cd1-xSe1-yZnxSy CSQDs. Such QDs offer higher photostability, higher fluorescence quantum yield, and less interfacial defects than the conventional step-like CSQDs. We first characterized the gradient CSQDs using steady-state optical spectroscopy and HR-TEM images in order to determine their dimensions and to evaluate the shell thickness. Then XRD and EDX were used to characterize the chemical composition and the crystal structures.

The photodynamic of these CSQDs in photovoltaic systems was also studied. We first found that the electron injection from the active core to n-type MO showed relatively larger exponential shell thickness dependence compared with step-like CSQDs. We established that the highest electron injection efficiency (~ 80%) can be found with shell thickness up to 1.3 nm. Such shell also allows high surface passivation providing optimal conditions for charge collection in solar cells. Finally, we integrated our knowledge about the electron and hole behaviors to explain the solar cell performances according to the core–shell structure. We confirmed that the hole trapping is the critical factor for QD-sensitized solar cell efficiency. The trapping can be well repaired by using optimal core–shell structure. (Less)
Please use this url to cite or link to this publication:
author
opponent
  • Professor Kamat, Prashant, Professor of Science Radiation Laboratory and Department of Chemical & Biomolecular Engineering, University of Notre Dame, Indiana, United States
organization
publishing date
type
Thesis
publication status
published
subject
keywords
Quantum dots, core–shell, electron injection, hole injection, hole trapping, exciton migration, solar cells, ultrafast dynamics, time-resolved spectroscopy.
pages
174 pages
publisher
Division of Chemical Physics, Department of Chemistry, Lund University
defense location
lecture hall C, at the Center of Chemistry and Chemical Engineering, Getingevägen 60, Lund
defense date
2015-03-13 09:15
ISBN
978-91-7422-388-0
language
English
LU publication?
yes
id
e8429083-a838-485c-9fbb-42d5a2f35bec (old id 5051679)
date added to LUP
2015-02-25 12:04:05
date last changed
2016-09-19 08:45:04
@misc{e8429083-a838-485c-9fbb-42d5a2f35bec,
  abstract     = {The balance between our demands of energy and the energy we are consuming is not in equilibrium anymore. Therefore, the search for other energy resources is indispensable. Sustainable energy sources offer the alternative to the fossil fuels. Within many types of sustainable energy sources, solar energy offers more than the total global energy consumption. Solar cells are the smart conversion tools to harvest the incident photons and create electricity out of these photons. Solar cells have passed through different generations where two important factors control the solar cells market. The solar cell efficiency and price play the cornerstones in the solar cell marketing. Third generation of solar cells aims to maximize the price–performance equation by using cheap materials without compromising efficiency. Nanomaterials have emerged as the promising building blocks to harvest the solar light in the third generation of the solar cells. Among them, quantum dots (QDs) can be used as viable candidate due to their superb features such as high extinction coefficient, a tunable absorption edge, and the possibility to generate and collect multiple excitons by using single, high energy photon. Both the behaviors of photoexcited electrons and holes determine the overall efficiency of QD based solar cells. This thesis presents a systematic study of the ultrafast photoinduced charge dynamics in QD solar cell materials including the charge transfer, exciton migration, carrier trapping and their influence on real solar cell performance. The materials investigated start with conventional neat core CdSe QDs and extend to gradient Cd1-xSe1-yZnxSy core–shell (CS) QDs. The latter are used to obtain improved optical and device performance. The electron injection from CdSe into ZnO nanowires were first observed to be very fast (few ps). This fast electron injection encourages us to study the possibility to inject multiple electrons from a QD under high excitation conditions. We revealed that a competition between electron injection and Auger recombination occurs.<br/><br>
Compared with electrons, the photoinduced holes are more likely to be trapped. However, such trap states sometimes can be radiative with long lifetime up to tens of microseconds in oleic acid capped CdSe QDs. In this scenario, the hole injection in p-type QD solar cells are proved to be less efficient (&lt;10%) compared with electron injection in n-type counterparts. It is highly affected by the surface trapping sites induced by the linker exchange process. The hole injection can then be improved by passivating the surface trap sites using core shell structures. Besides electron or hole injection, exciton migration can also occur via Förster resonant energy transfer (FRET). We found that FRET between QDs would enable to make use of the absorption of light by the indirectly attached QDs in QD-sensitized metal oxide (MO) anodes. In well-organized multi-sized QD mixtures, the energy transfer is even more pronounced. We experimentally observed the FRET process in randomly arranged multi-sized QD assembly and tandem stacked QD layers by using time-resolved and steady-state spectroscopies. Theoretical simulations where dipole distribution model was introduced for coupling calculations complies well with the experimental results. In order to minimize the effect of surface defects and improve the photostability of QD solar cells, we investigated the core–shell QD system where the surface trapping of carriers can be well passivated by shell materials with enhanced optical properties and device performance. Herein, a wider band gap semiconductor is employed as a shield shell around the active core in gradient growth, known as gradient Cd1-xSe1-yZnxSy CSQDs. Such QDs offer higher photostability, higher fluorescence quantum yield, and less interfacial defects than the conventional step-like CSQDs. We first characterized the gradient CSQDs using steady-state optical spectroscopy and HR-TEM images in order to determine their dimensions and to evaluate the shell thickness. Then XRD and EDX were used to characterize the chemical composition and the crystal structures.<br/><br>
The photodynamic of these CSQDs in photovoltaic systems was also studied. We first found that the electron injection from the active core to n-type MO showed relatively larger exponential shell thickness dependence compared with step-like CSQDs. We established that the highest electron injection efficiency (~ 80%) can be found with shell thickness up to 1.3 nm. Such shell also allows high surface passivation providing optimal conditions for charge collection in solar cells. Finally, we integrated our knowledge about the electron and hole behaviors to explain the solar cell performances according to the core–shell structure. We confirmed that the hole trapping is the critical factor for QD-sensitized solar cell efficiency. The trapping can be well repaired by using optimal core–shell structure.},
  author       = {Qenawy, Mohamed},
  isbn         = {978-91-7422-388-0},
  keyword      = {Quantum dots,core–shell,electron injection,hole injection,hole trapping,exciton migration,solar cells,ultrafast dynamics,time-resolved spectroscopy.},
  language     = {eng},
  pages        = {174},
  publisher    = {ARRAY(0xaf77170)},
  title        = {Ultrafast Photoinduced Processes in Core and Core–Shell Quantum Dots for Solar Cell Applications “Tiny Crystals for Big Applications”},
  year         = {2015},
}