Impact of Interfacial Molecular Conformation and Aggregation State on the Energetic Landscape and Performance in Organic Photovoltaics

Handle URI:
http://hdl.handle.net/10754/621923
Title:
Impact of Interfacial Molecular Conformation and Aggregation State on the Energetic Landscape and Performance in Organic Photovoltaics
Authors:
Ngongang Ndjawa, Guy Olivier ( 0000-0001-7400-9540 )
Abstract:
In organic photovoltaics (OPVs) the key processes relevant to device operation such as exciton dissociation and free carriers recombination occur at the donor-acceptor (D-A) interface. OPV devices require the bulk heterojunction (BHJ) architecture to function efficiently. In these BHJs, D-A interfaces are arranged in three dimensions, which makes molecular arrangements at these interfaces ill defined and hard to characterize. In addition, molecular materials used in OPVs are inherently disordered and may exhibit variable degrees of structural order in the same BHJ. Yet, D-A molecular arrangements and structure are crucial because they shape the energy landscape and photovoltaic (PV) performance in OPVs. Studies that use well-defined model systems to look in details at the interfacial molecular structure in OPVs and link it to interfacial energy landscape and device operation are critically lacking. We have used in situ photoelectron spectroscopy and ex situ x-ray scattering to study D-A interfaces in tailored bilayers and BHJs based on small molecule donors. We show preferential miscibility at the D-A interface depending on molecular conformation in zinc phthalocyanine (ZnPc)/ C60 bilayers and we derive implications for exciton dissociation. Using sexithiophene (6T), a crystalline donor, we show that the energy landscape at the D-A interface varies markedly depending on the molecular composition of the BHJ. Both the ionization energies of sexithiophene and C60 shift by over ~0.4 eV while the energy of the charge transfer state shifts by ~0.5 eV depending on composition. Such shifts create a downward energy landscape that helps interfacial excitons to overcome their binding energies. Finally, we demonstrate that when both disordered and ordered phases of D coexist at the interface, low-lying energy states form in ordered phases and significantly limit the Voc in devices. Overall our work underlines the importance of the aggregation and conformation states of molecular materials at and near the D-A interface in determining the operation and performance of OPV devices. This work shows that the role of D-A interfaces in complex BHJ devices can be unraveled through careful experimental design and by in depth characterization of planar heterojunction bilayer devices recreating model interfaces.
Advisors:
Amassian, Aram ( 0000-0002-5734-1194 )
Committee Member:
Laquai, Frédéric ( 0000-0002-5887-6158 ) ; Mohammed, Omar F. ( 0000-0001-8500-1130 ) ; Salleo, Alberto
KAUST Department:
Physical Sciences and Engineering (PSE) Division
Program:
Materials Science and Engineering
Issue Date:
25-Nov-2016
Type:
Dissertation
Appears in Collections:
Dissertations

Full metadata record

DC FieldValue Language
dc.contributor.advisorAmassian, Aramen
dc.contributor.authorNgongang Ndjawa, Guy Olivieren
dc.date.accessioned2016-12-04T12:53:34Z-
dc.date.available2016-12-04T12:53:34Z-
dc.date.issued2016-11-25-
dc.identifier.urihttp://hdl.handle.net/10754/621923-
dc.description.abstractIn organic photovoltaics (OPVs) the key processes relevant to device operation such as exciton dissociation and free carriers recombination occur at the donor-acceptor (D-A) interface. OPV devices require the bulk heterojunction (BHJ) architecture to function efficiently. In these BHJs, D-A interfaces are arranged in three dimensions, which makes molecular arrangements at these interfaces ill defined and hard to characterize. In addition, molecular materials used in OPVs are inherently disordered and may exhibit variable degrees of structural order in the same BHJ. Yet, D-A molecular arrangements and structure are crucial because they shape the energy landscape and photovoltaic (PV) performance in OPVs. Studies that use well-defined model systems to look in details at the interfacial molecular structure in OPVs and link it to interfacial energy landscape and device operation are critically lacking. We have used in situ photoelectron spectroscopy and ex situ x-ray scattering to study D-A interfaces in tailored bilayers and BHJs based on small molecule donors. We show preferential miscibility at the D-A interface depending on molecular conformation in zinc phthalocyanine (ZnPc)/ C60 bilayers and we derive implications for exciton dissociation. Using sexithiophene (6T), a crystalline donor, we show that the energy landscape at the D-A interface varies markedly depending on the molecular composition of the BHJ. Both the ionization energies of sexithiophene and C60 shift by over ~0.4 eV while the energy of the charge transfer state shifts by ~0.5 eV depending on composition. Such shifts create a downward energy landscape that helps interfacial excitons to overcome their binding energies. Finally, we demonstrate that when both disordered and ordered phases of D coexist at the interface, low-lying energy states form in ordered phases and significantly limit the Voc in devices. Overall our work underlines the importance of the aggregation and conformation states of molecular materials at and near the D-A interface in determining the operation and performance of OPV devices. This work shows that the role of D-A interfaces in complex BHJ devices can be unraveled through careful experimental design and by in depth characterization of planar heterojunction bilayer devices recreating model interfaces.en
dc.language.isoenen
dc.subjectdonor-acceptor interfaceen
dc.subjectEnergetic Landscapeen
dc.subjectorganic small moleculesen
dc.subjectbilayersen
dc.subjectbulk heterojunctionen
dc.subjectorganic solar cellsen
dc.titleImpact of Interfacial Molecular Conformation and Aggregation State on the Energetic Landscape and Performance in Organic Photovoltaicsen
dc.typeDissertationen
dc.contributor.departmentPhysical Sciences and Engineering (PSE) Divisionen
thesis.degree.grantorKing Abdullah University of Science and Technologyen_GB
dc.contributor.committeememberLaquai, Frédéricen
dc.contributor.committeememberMohammed, Omar F.en
dc.contributor.committeememberSalleo, Albertoen
thesis.degree.disciplineMaterials Science and Engineeringen
thesis.degree.nameDoctor of Philosophyen
dc.person.id113327en
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