Table Of ContentSpringer Theses
Recognizing Outstanding Ph.D. Research
Raphael Schlesinger
Energy-Level
Control at Hybrid
Inorganic/Organic
Semiconductor
Interfaces
Springer Theses
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Raphael Schlesinger
Energy-Level Control
at Hybrid Inorganic/Organic
Semiconductor Interfaces
Doctoral Thesis accepted by
the Humboldt University of Berlin, Germany
123
Author Supervisor
Dr. Raphael Schlesinger Prof. NorbertKoch
Institut für Physik Institut für Physik
Humboldt-Universität zuBerlin Humboldt-Universität zuBerlin
Berlin Berlin
Germany Germany
ISSN 2190-5053 ISSN 2190-5061 (electronic)
SpringerTheses
ISBN978-3-319-46623-1 ISBN978-3-319-46624-8 (eBook)
DOI 10.1007/978-3-319-46624-8
LibraryofCongressControlNumber:2016953641
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To my wonderful family and the memory
of my dear mother
’
Supervisor s Foreword
Itwasonlyafewyearsagothatresearchonelectronicandoptoelectronicmaterials
startedtoseriouslyaddress“hybridmaterialapproaches”comprisingbothinorganic
andorganicsemiconductors.Thisapproachisdrivenbyabest-of-two-worldswish
to overcome the individual weaknesses of each material class and to create syn-
ergistic functionality. For instance, inorganic semiconductors stand out for their
high charge carrier mobility, whereas organic ones feature very strong light-matter
coupling and easy tunability of the fundamental energy gap. However, to build
efficient devices, one needs to know the nature of the interaction between the
components and be able to control their interfacial energy-level alignment, since
unfavorable alignment would impede function. While considerable understanding
has been gained for metal/organic contacts and organic/organic heterojunctions
over the past two decades, the study of hybrid organic/inorganic semiconductor
interfaces is just beginning.
Raphael Schlesinger entered this evolving new field of hybrid materials by
choosing the technologically important transparent conductive oxide ZnO as
inorganic component and paired it with various organic molecular semiconductors
with appropriate energy levels. He used mostly photoemission and X-ray absorp-
tion techniques to unravel the interaction between the materials and to explore the
energy-level alignment at hybrid interfaces. The experiments show that one typi-
callyfacesscenariosinwhichthemolecularlevelsaresubstantiallyoffsetfromthe
inorganicbandedges.Whileimportantforsomefunctions,e.g.,chargeinjectionor
excitondissociationinexcitonicsolarcells,thistypeofalignmentissuitedforjusta
limited range of potential applications. Therefore, he then focused on using inter-
layersofstrongmoleculardonorsoracceptorstodeliberatelymodifytheinterfacial
energy-level alignment by tuning the ZnO work function, a method previously
developed for metal electrodes.
Raphaelprovesthatthisapproachalsoworksathybridsemiconductorinterfaces.
He recognizes the importance of contact-induced band bending—a feature not
relevant for metal surfaces—within the inorganic compound. The range of exper-
iments reported in this thesis indicates that within physical limits, one can control
vii
viii Supervisor’sForeword
the interplay of charge transfer from depletion regions and interface dipoles. This
offers new degrees offreedom to change not only the energy-level alignment but
also the interfacial charge densities on both sides of the contact. By investigating
multiple donor and acceptor molecules, Raphael identifies the limits of
adsorption-induced band bending. He highlights the important influence of gap
statedensityofstatesinthiscontextandisable,forthefirsttime,toevidencesuch
states by employing ultra-high sensitivity photoelectron spectroscopy.
Using various strong molecular donor and acceptor molecules, the experiments
demonstrateadynamicrangeofworkfunctiontuningfrom2.2to6.4eVforZnO,
ayetunsurpassedrange.Thesevaluescanbeconsideredrecordvalues,whicheven
allowZnOtomimicthefunctionalbehavior,intermsofworkfunction,ofanything
between alkali metals and molybdenum oxide, the two known extremes in the
material world. Further experiments were devised showing that it is now possible,
even for wide gap organic semiconductors, to realize all ranges of energy-level
alignment between the limiting Fermi-level pinning regimes. Thus, the field has
taken a major technological leap forward, while remaining rooted in sound scien-
tificinsightandunderstanding.Withthesetoolsathand,onecanimaginedesigning
ultimately low electron- or hole-injection barriers at hybrid interfaces, optimizing
thephotovoltaicgap,orfacilitatingefficientFörsterresonantenergytransferacross
the interface, for which this work also includes proof-of-principle experiments.
Berlin Prof. Norbert Koch
June 2016
Abstract
Hybrid inorganic organic systems (HIOS) are promising candidates for future
(opto)electronic devices by taking advantage of the complementary beneficial
properties of two different material classes. To design efficient HIOS devices,
understanding and controlling HIOS energy-level alignment is a key factor.
Therefore, the present thesis investigates the energy-level alignment at HIOS
interfaceswithaparticularfocusonhowtomodifyit.Fortheinorganicpartwithin
HIOS, ZnO is chosen. Modification of the energy-level alignment is enabled by
employing thin molecular donor or acceptor interlayers between the HIOS com-
ponents with the aim of tailoring interface dipoles and band bending as well as to
tunetheworkfunction(/)ofZnO.Inturn,theinterlayermodifiestheenergy-level
alignment between ZnO and the organic semiconductor HIOS constituent. This
topical complex is addressed using ultraviolet photoelectron spectroscopy (UPS),
X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine
structure (NEXAFS) spectroscopy as the main experimental techniques.
First,theenergy-levelalignmentand/modificationcausedbythedepositionof
donor and acceptor molecules onto different ZnO faces is investigated. This part
will cover a range of four different acceptor and two donor molecules, demon-
strating / reduces down to 2.2 eV and / increases up to 6.4 eV. Second, the
concomitant adsorption-induced band bending will be thoroughly investigated.
Itsdifferent behaviorfordonorandacceptoradsorption,butalsothatupwardband
bendingisrestrainedbygapstatedensityofstates(GDOS)andisthereforelimited
to only a few 100 meV will be a focus of this work. Further, the dependence of
band bending and interfacial energy-level alignment on the ZnO doping concen-
tration with GDOS present will be elucidated. Third, HIOS energy-level readjust-
ment is demonstrated by exploiting the huge / modifications made possible.
Therefore,three different prototypical organicsemiconductors (OSC) onZnOwith
andwithoutdonororacceptorinterlayerswillbeinvestigated.Upwardenergy-level
readjustments of the OSC by 1.7 eV are shown and downward readjustments by
1.3 eV.
Theaforementionedenergy-levelreadjustmentsallowfacilitatingultimatelylow
electron- and hole-injection barriers at HIOS interfaces depending on whether
ix
x Abstract
donor or acceptor interlayers are used. Moreover, by using an OSC, whose gap
matches that of ZnO, an energy-level readjustment is realized, in which the ZnO
band edges are in resonance with the highest occupied and lowest unoccupied
molecular states. It is shown that such a resonant HIOS interface features highly
efficient, non-quenched energy transfer, which is usually inhibited by unfavorable
energy-level alignment.
(cid:1) (cid:1)
Keywords Hybrid inorganic organic systems Zinc oxide Energy-level
(cid:1) (cid:1) (cid:1)
alignment Interfaces Band bending Photoelectron spectroscopy
