Fingerprint of Charge Redistribution in the Optical Spectra of Hybrid

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Fingerprint of Charge Redistribution in the Optical Spectra of Hybrid Inorganic/Organic Semiconductor Interfaces Tino Meisel, Mino Sparenberg, Marcel Gawek, Sergey Sadofev, Björn Kobin, Lutz Grubert, Stefan Hecht, Emil J.W. List-Kratochvil, and Sylke Blumstengel J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03580 • Publication Date (Web): 30 May 2018 Downloaded from http://pubs.acs.org on May 30, 2018

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The Journal of Physical Chemistry

Fingerprint of Charge Redistribution in the Optical Spectra of Hybrid Inorganic/Organic Semiconductor Interfaces †

Tino Meisel,

†

Mino Sparenberg,

Lutz Grubert,

‡

‡

Stefan Hecht,

Marcel Gawek,

†

Sergey Sadofev,

Emil List-Kratochvil,

¶

†

‡

Björn Kobin,

∗,¶

and Sylke Blumstengel

†Institut

für Physik, Humboldt-Universität zu Berlin ‡Department of Chemistry & IRIS Adlershof, Humboldt-Universität zu Berlin ¶Institut für Physik, Institut für Chemie & IRIS Adlershof, Humboldt-Universität zu Berlin E-mail: [email protected]

Phone: (030)2093-7825

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Hybrid structures combining conjugated organic molecules and inorganic semiconductors hold the promise to merge the better of two worlds. To achieve optoelectronic functionality exceeding that of the individual constituent, both the electronic and the optical properties of the hybrid interface must meet certain requirements. Charge redistribution occurring upon deposition of conjugated organic molecules on semiconductor surfaces modulates the electrostatic potential at the hybrid interface. Here we show at the example of ZnO-based hybrid systems that even minuscule charge redistribution is accompanied by a profound modication of the excitonic absorption of the semiconductor. The changes in the optical spectra are detected in real time by dierential reectance spectroscopy (DRS) during the deposition of the molecules. Appropriate modelling of the spectra yields the magnitude of the change of the electrostatic potential. Our ndings provide insight into the subtle interplay between optical and electronic properties at hybrid interfaces which is essential to design structures with truly superior function.

Introduction The electronic structure at the interface between inorganic semiconductors and conjugated organic materials is of crucial importance for the function of optoelectronic devices such as photovoltaic cells or light-emitting diodes. When the two types of materials are brought into contact charge redistribution across the heterointerface can occur which modulates the electrostatic potential at the hybrid interface. This eect is benecial or detrimental depending on the desired function of the heterointerface. Charge transfer to a monolayer of a molecular acceptor or donor is very useful to tune the work function of the semiconductor. 1,2 In this way, charge carrier injection or extraction barriers have been adjusted. On the other hand, in hybrid structures relying on resonant excitonic coupling and Förster-type resonant energy transfer (FRET) across the heterointerface, charge redistribution is expected to corrupt the 2


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function. First, the resonance condition might not be met any longer when charge transfer modies the optical properties of the energy transfer partners. Second, the electric eld associated with the redistributed charges reduces the transition dipole moment of the excitons in the vicinity of the interface and may even cause exciton dissociation rendering excitonic coupling thus inecient. Therefore, a "non-interacting" interface where the electronic and optical properties of the constituents remain unchanged is asked for. The examples show that the knowledge about ground state charge transfer and the related changes in the optical properties are essential to understand and optimize the function of hybrid inorganic/organic semiconductor systems. The electronic structure at interfaces is typically obtained by photoelectron spectroscopy (PES). These experiments yield information about the energetic positions of occupied and unoccupied states, the formation of interface dipoles and the magnitude of adsorption induced changes in band bending, the latter being indicators of charge redistribution at the heterointerface. As additional information, not accessible by PES, the evolution of the optical spectra in the course of the interface formation is needed. Here we report that dierential reectance (DR) spectra recorded in

situ and in real time

during the deposition of molecules on the ZnO surface allow tracking of the changes in the optical spectra induced by interfacial charge redistribution. We show that the electric eld associated with the modulation of the electrostatic potential causes a strong modication of the excitonic absorption of ZnO. Modelling of the DR spectra considering a Wannier-Mott exciton in an electric eld yields the magnitude of the eld strength. Consequently, the charge-transfer induced modication of the electrostatic potential and the related changes in the optical spectra are obtained simultaneously. The reection spectra are recorded

in situ

and in real time in the course of the deposition of the organic layer on the ZnO surface. The DR signal is dened as

R(d) − R(0) ∆R = R R(0) 3


(1)


where R(0) is the reection spectrum of the pristine semiconductor surface recorded prior to the deposition of the molecules, while R(d) refers to the reection spectra of the surface covered by a molecular layer with the thickness d. Since the dierence spectrum is measured

in situ, DRS is highly sensitive and allows detecting the signatures of even a fraction of a molecular monolayer. 3 The present work is organised as follows: The rst section discusses the DR spectra of a "non-interacting" ladder-type quaterphenyl (L4P)/ZnO(0001) interface where ground state charge transfer is absent. The exciton energy of L4P is in resonance with that of ZnO and ecient FRET has been observed from a ZnO quantum well to an adjacent thin layer of a spiro-derivative of L4P. 4 Such hybrid systems can be used as building blocks for hybrid light-emitting diodes where charge carriers are injected via the inorganic part while light is emitted from the organic layer after the energy transfer step. Previous PES performed at the interface between the spiro-derivative of L4P and ZnO(0001) has revealed, that the energy level alignment is of type-II due to the small electron anity of the molecules (see Figure 1 a). 2 Deposition of the molecules leads to a minuscule reduction of the work function φ by 0.2 eV attributed to a mechanism analogous to the "push-back" on metal surfaces. 5,6 Ground state electron transfer from the ZnO substrate to the molecular adsorbate can be ruled out at such an interface. The second section presents DR spectra of a prototypical example for an "interacting" interface, namely 2,2-(peruoronaphthalene)-2,6-(diylidene) dimalononitrile (F6-TCNNQ) deposited on ZnO(0001) where ground state charge transfer is present. F6-TCNNQ is a strong electron acceptor and substantial charge redistribution at the interface with n-type ZnO has been revealed previously by PES. 7 The extracted energy level diagram of the interface (see Fig. 1 b) shows that the related change in the electrostatic potential leads to a huge increase of the work function of ZnO(0001) by ∆φ = 2.4 eV . 7 At this interface, a strong dispersive signal appears in the DR spectra in the near band edge region of ZnO. A model is developed explaining the emergence of this characteristic feature by the electric 4


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eld eect on the excitonic absorption of ZnO. Analysis of the spectra yields an estimate of the electric eld and thus the change in band bending at the ZnO surface induced by the adsorption of F6-TCNNQ. In the third section a hybrid system with a not a priori known electronic structure, namely 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA)/ZnO(0001) is investigated. The exciton energies of the two constituents are in resonance so that dipole-dipole mediated excitonic coupling is expected to be eective. 8 The DR spectra recorded in the course of the deposition of NTCDA on ZnO feature also the characteristic ngerprint indicative of a charge-transfer induced modulation of the electrostatic potential. We discuss the origin as well as the consequence on the eectiveness of short-range dipole-dipole mediated FRET across NTCDA/ZnO(0001) interfaces. (a) ZnO(000૚ഥሻ

(b) ZnO(000૚ሻ F6‐TCNNQ

spiro‐L4P

 4.3 CBM ‫ܧ‬ி 3.4

3.0

4.1

1.8

2.4

 LUMO 3.9 CBM

3.25

‫ܧ‬ி

HOMO

3.4

6.3

after before

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


LUMO

2.2

VBM

2.0 1.0

2.3 HOMO

VBM

Figure 1: Schematic energy level diagrams of the spiro-L4P/ZnO(0001) interface (a) and the F6-TCNNQ/ZnO(0001) interface (b) derived from previously reported PES experiments. 2,7 All values are given in eV. The attachment of the spiro groups to L4P decreases the optical gap of L4P slightly by 70 meV, which is in the order of the error of the PES measurements. The energy level alignment at the interface with ZnO is therefore not expected to dier noticeably between L4P and spiro-L4P. The positions of the the valence band maximum (VBM) of ZnO and the highest occupied molecular orbital (HOMO) of the organic adsorbates are determined by PES. The position of the conduction band minimum (CBM) of ZnO is obtained by adding the exciton binding energy to the optical gap. In case of the molecules, the optical gaps are given due to a lack of knowledge of the exciton binding energies. The positions of lowest unoccupied molecular orbital (LUMO) are indicated by the shaded area since their exact position is not known. The faint grey lines and values on the ZnO side in (b) indicate the levels before F6-TCNNQ deposition.

5



Methods Pristine ZnO surfaces with a dened termination were produced overgrowing either ZnO(0001) or ZnO(0001) wafers (CrysTec GmbH, Germany) with a thin (60 nm) epitaxial ZnO layer employing radical-source molecular beam epitaxy. 9 Subsequently, the samples are transferred under UHV conditions to the organic molecular beam deposition (OMBD) chamber (base pressure 1 · 10−8 mbar) where the molecules are deposited and simultaneously DR spectra recorded. F6-TCNNQ was purchased from Novaled GmbH and NTCDA from Sigma Aldrich. The synthesis of the L4P is reported elsewhere. 10 The molecules were evaporated from Knudsen-type eusion cells and the layer thickness and growth rate monitored by a quartz crystal micro balance. The OMBD chamber is equipped with a DRS set up using a Xenon lamp as excitation source. The light is coupled into a Y-bre and focused by a lens system under normal incidence onto the sample surface. The reected light is collected by the same lens system, coupled back into the bre and recorded by an Ocean Optics USB400 spectrometer. The reection spectra are taken in real time while the molecular layer is growing. The growth rate is kept at ca. 0.1 nm/min to allow for suciently long acquisition times and to guarantee a satisfactory signal to noise ratio. A fraction of the incident light is coupled out by a beam splitter and led to a second Ocean Optics USB400 spectrometer in order to obtain reference lamp spectra and to perform a drift correction. To verify the change of the workfunction upon deposition of F6-TCNNQ on ZnO, Kelvin probe measurements have been performed with an Omicron AFM/STM/KPFM controlled by a Matrix system. Additionally, the morphology of the deposited organic layer has been investigated by AFM. The measurements were carried out

in situ at a base pressure of 10−9

mbar. The optical functions of the ZnO wafers overgrown with an epitaxial ZnO layer were determined

ex situ by ellipsometry measurements (carried out by Sentech GmbH). Absorption

spectra of reference samples of the molecules deposited on inert and transparent substrates are recorded with a Shimadzu UV-2101 PC spectrometer. Absorption spectra of the F66


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TCNNQ anions in dichlormethane solution are obtained applying cyclic voltammetry to reduce the molecules. Details on spectroelectrochemistry are reported in the SI.

Results and Discussion Non-interacting" Interface: L4P/ZnO(0001). A scheme of the DRS experiment is depicted in Figure 2 a. Figure 2 b shows the evolution of the DR spectra while the L4P layer is growing on a ZnO(0001) surface. Apart from a slight red-shift occurring in a thickness range between 0.5 nm and 1 nm, the spectral shape remains largely unchanged up to a nal thickness of 2.3 nm. Before starting a more detailed discussion, it is worthwhile to obtain a qualitative understanding of the DR spectra. The present system can simply be modelled as a three-layer-system consisting of an innitely thick ZnO layer on one side, an organic layer of thickness d and vacuum on the other side. The ZnO wafers are only one-side polished so that reections from the backside are negligible. Furthermore, atomic force microscopy (AFM) images taken in situ immediately after the deposition reveal a smooth L4P layer morphology [see Supporting Information(SI)]. Assuming that the L4P layer thickness is much smaller than the wavelength of light d λ, the DR spectra are approximately 11

8πd ∆R ≈− Im R λ

1−b org 1−b ZnO

(2)

with b org and b ZnO being the complex dielectric functions of the organic layer and of ZnO, respectively. It is apparent from the equation that in spectral regions where b ZnO does not vary signicantly, i.e. below and well above the ZnO band edge, the DR spectra resemble the imaginary part 2,org of b org = 1,org +i2,org of the adsorbate layer. For comparison, the absorption coecient spectrum of isolated L4P molecules embedded in a PMMA matrix is shown in Figure 2 b. The 0-0 transition is located at 3.35 eV. Two vibronic progressions with vibrational energies of 170 meV and 80 meV are visible. All these features can also be 7



recognised in the DR spectra. The absorption edge of ZnO overlaps with the 0-0 transition of L4P modifying the DR spectra mostly in this spectral range. A rst glance over the DR spectra indicates therefore already that the optical properties of the L4P layers are not substantially modied neither due to intermolecular interactions nor by interaction with the ZnO surface. (a) 0.04

(c)

0.6 nm 0.9 nm 1.1 nm 1.3 nm 1.6 nm 1.9 nm

F6-TCNNQ

DRS

0.03

0.02

(b)

L4P

0.5 nm 1.0 nm 1.4 nm 1.8 nm 2.0 nm 2.3 nm

0.2 nm 0.6 nm

0.004

0.4 nm 0.9 nm 1.1 nm

0.002 0.000

0.5 nm ref.

1.0 0.5 0.0

0.00 0.006

DRS

0.11 0.10 0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.00 1.5

Absorbance

DRS

0.01



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3.2

3.3

3.4

3.5

3.6

3.7

3.8

F6-TCNNQ

1.0

F6-TCNNQ2-

F6-TCNNQ-

0.5 0.0

2.4

2.6

2.8

3.0

3.2

3.4

3.6

Energy [eV]

Energy [eV]

Figure 2: (a) Schematic depiction of the geometry of the in situ DRS experiment. (b) DR spectra of the L4P/ZnO(0001) interface for increasing thickness of the organic layer (top panel). The dashed line in the lower panel represents the absorption coecient spectrum extracted from the DR spectrum at a L4P layer thickness d = 0.5 nm, the solid line shows a reference spectrum of L4P molecules dispersed in PMMA. (c) DR (top panel) of the F6-TCNNQ/ZnO(0001) interface and reference DR spectra of F6-TCNNQ on Al2 O3 (middle panel) for increasing thickness of the organic layer. Absorbance spectra of neutral F6-TCNNQ molecules as well as their singly and doubly charged anions, generated in an electrochemical cell, all diluted in DCM (lower panel). To extract the absorption coecient spectrum of L4P on ZnO from DRS and to allow for a quantitative comparison with the spectrum of the isolated molecules in an inert matrix, the DR spectra are tted using the transfer matrix method for the calculation of the reection spectra R(d). The complex refractive index n bZnO = nZnO + iκZnO of ZnO is taken from ellipsometry measurements (see SI) while that of the molecular layer is simulated by the oscillator model. To account for inhomogeneous broadening, the line shapes of the molecular vibronic transitions are modelled by Voigt proles using the approximate expressions for the real and imaginary part of the dielectric function b org suggested in Ref. 12 The DR spectra 8


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are tted assuming six oscillators to account for the vibronic transitions. Apart from an increased inhomogeneous broadening, the extracted absorption coecient spectrum of a thin layer of L4P on ZnO bears close resemblance with that of the isolated molecules (see Figure 2 b). The slight red-shift of 40 meV of the L4P spectra (the full set of absorption coecient spectra derived from the the t of the DR spectra is reported in the SI) with increasing layer thickness is due to the gas-to-crystal shift. The almost perfect reproduction of the DR spectra obtained by using the optical functions of the individual components in the transfer matrix calculations conrms that neither the ZnO nor the L4P optical properties are changed when the two materials are brought in contact. Signicant charge redistribution can thus be ruled out at this interface. This conclusion is in full accordance with the previous PES measurements discussed above. 2 The behavior is expected due to the low electron anity of L4P. 13

"Interacting" Interface: F6-TCNNQ/ZnO(0001). Already a rst glance over the DR spectra of F6-TCNNQ on ZnO(0001) in comparison to the reference spectra on a transparent and inert substrate (Al2 O3 ) reveals a completely dierent behaviour (see Figure 2 c). The reference DR spectra feature a broad unstructured peak centred around 2.7 eV corresponding to the absorption of the molecule. The inhomogeneous broadening of F6-TCNNQ in the thin lm is somewhat larger than the solution spectrum (see Figure 2 c) due to a higher degree of disorder. The feature is also visible in the DR spectra of the F6-TCNNQ/ZnO(0001) interface and can also here be assigned to the F6-TCNNQ absorption since ZnO is transparent in this spectral range. The slight blue shift might be due to a dierent packing of the molecules and a dierent dielectric environment. There are three additional sharp features in the spectral range between 3.2 eV and 3.4 eV which are absent in the reference spectra. In contrast to the broad peak related to the molecular absorption, these features are present already at the lowest coverage and do not grow further in intensity upon deposition of organic material. These features can not be reproduced in simulations of the DR spectra in the frame 9



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of the simple three-layer-system consisting of a ZnO half space on one side, a F6-TCNNQ layer of thickness d with an oscillator at 2.7 eV accounting for the molecules absorption, and vacuum on the other side (see SI). Since it is known from previous PES measurements 7 as well as from Kelvin probe force microscopy (KPFM) measurements (see SI) that substantial charge redistribution occurs at the F6-TCNNQ/ZnO(0001) interface, charged F6-TCNNQ molecules can form, though their presence could not be detected by PES, presumably since their concentration is too low. 7 Nevertheless, it could be possible that the DRS features in the spectral range between 3.2 and 3.4 eV are due to the formation of charged molecules. Figure 2 c shows absorption spectra of the neutral molecules, the radical anion F6-TCNNQ− and the radical dianion F6-TCNNQ2− in solution (see SI for details on the spectroelectrochemistry). Indeed, F6-TCNNQ2− possesses a broad absorption peak in the spectral range of interest. Therefore we expanded the three-layer-system by introducing a fourth layer of F6-TCNNQ2− molecules between ZnO and neutral F6-TCNNQ. However, also with this model, the three sharp DR features can not be produced (see SI). We therefore assign these features to a modication of the dielectric function of ZnO due to electronic interaction with F6-TCNNQ. The conjecture is based on the results of the above cited PES experiments (see Fig. 1 b) which suggest that the large increase of the work function ∆Φ = 2.4 eV of ZnO (0001) induced by adsorption of F6-TCNNQ is due to two phenomena, namely a change in band bending ∆ΦBB in ZnO and the creation of an interface dipole ∆ΦID 7

∆Φ = ∆ΦBB + ∆ΦID .

(3)

The contribution of the band bending to the total work function change is ∆ΦBB ≈ 1 eV according to PES (see Fig. 1 b). This experimental nding is in agreement with densityfunctional theory calculations which show that the work function change is largely due to surface band bending at typical doping levels of ZnO in the range of 1023 ...1024 m−3 . 14,15

10


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Furthermore, the calculations predict a negligible charge transfer to the acceptor molecules at such doping levels which explains that no clear signatures of charged F6-TCNNQ molecules are resolvable in the DR spectra. The measured ∆ΦBB corresponds to an electric eld

F (z = 0) ≈ 3.7 · 107 V/m at the ZnO surface which, within the Schottky approximation, drops linearly over the space charge region of a thickness z ≈ 55 nm. These values are obtained assuming a donor density of ND ≈ 3 · 1023 m−3 and a static dielectric constant of 8.12, which is appropriate for the present ZnO. The electric eld present in the space charge region modies the shape of the excitonic absorption edge of ZnO and causes, as will be shown in the following, the characteristic DR features between 3.2 eV and 3.4 eV. The fact that the characteristic DR features are almost fully developed at the lowest coverage (0.6 nm) further supports the conjecture since also the maximal work function change is already obtained at a similar coverage (ca. 0.8 nm). 7 To demonstrate the eect of ∆ΦBB on the DR spectra we proceed in the following way: (i) We approximate the linearly decreasing electric eld by step functions by subdividing the space charge region into thin layers of thickness zi in which the electric eld Fi is constant (see Figure 3 a). (ii) We calculate the electric eld dependent 2,ZnO (Fi ) spectra for each layer and via a Kramers-Kronig transformation the p ZnO (Fi ) is calculated. (iii) We apply corresponding 1,ZnO (Fi ). Subsequently n bZnO (Fi ) = b the transfer matrix method to calculate the DR spectra of the multilayer system consisting of N ZnO layers with dielectric functions b ZnO (Fi ) and a F6-TCNNQ layer described by the oscillator model embedded between an innitely thick ZnO layer with b ZnO (F → 0) on one side and vacuum on the other side. Within the eective mass approximation, the treatment of the Wannier-Mott exciton in an electric eld is formally equivalent to the problem of the hydrogen atom in an electric eld (see Figure 3 b). The characteristic parameter is the ionisation eld FI corresponding to a potential drop of the eective Rydberg R (exciton binding energy) over the exciton Bohr radius a. The eect of an electric eld on the absorption line shape can be understood as follows: At small eld strengths F FI , the electric eld leads to a widening of the Coulomb 11



‫ܨ‬

0.6

(c)

‫ܨ‬

F/FI:  0.5 1 

0.6

‫ݖ‬

ZnO

‫ݖ‬

ZnO

0.2  2 

(d) F(0)/FI = 1.7

0.4

DRS

(a)

1.55 1.45

0.2

1.3

0.4



(b)

0.0

V F > 0 V/m r

0.05

0.2

DRS

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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F = 0 V/m

0.00

0.0 3.0

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3.6

3.7

3.0

3.1

3.2

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3.6

3.7

Energy [eV]

Energy [eV]

Figure 3: (a) Schematic depiction of the electric eld distribution in the near surface region of ZnO according to the Schottky approximation (left) and electric eld distribution used in the calculation of the reection spectra (right). (b) Schematic depiction of the Coulomb potential of the ZnO Wannier-Mott exciton with and without applied electric eld. (c) Calculated electric eld dependence of the near band edge κ spectrum of ZnO. The depletion layer is subdivided in ve layers of equal thickness in which the electric eld is assumed to be constant. (d) DR spectra of the F6-TCNNQ/ZnO(0001) interface for dierent values of band bending and thus dierent ratios F (0)/FI . The depletion layer width and the eld at the surface F (0) are calculated within the Schottky approximation assuming a doping concentration of 3·1023 m−3 (upper panel). Experimental DR spectrum of the F6-TCNNQ/ZnO(0001) interface for a organic layer thickness d = 1.4 nm (lower panel). The dotted lines serve as guides for the eye.

12


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potential well causing a slight red-shift of the excitonic transition energy (second order Stark eect). As the eld strength increases F < FI , the rim of the Coulomb well further lowers so that the bound levels start to mix with continuum levels. This causes a blue shift and a broadening of the excitonic transition which is, nally, at F ≈ FI completely smeared out into a continuum. For ZnO, the ionisation eld is FI ≈ 2.6 · 107 V/m which is comparable to F (z = 0). Therefore, a strong modication of the absorption line shape is expected. To calculate 2,ZnO (Fi ) in the vicinity of the band gap we follow the approach proposed by Blossey. 16,17 The model is based on the assumption that there is only one conduction band and one valance band, the bands are isotropic and parabolic and the electron and hole in the nal state interact only via Coulomb interaction. Furthermore, exciton-phonon coupling is neglected. These approximations, though not strictly valid for ZnO, are acceptable for the scope of the present work which aims at the demonstration of the principle behaviour of the DR spectra. A more accurate description would take into account that ZnO possesses three non-degenerate valance bands giving rise to three excitonic transitions, termed A, B and C excitons. Under the present excitation geometry, only A and B excitons are excited, which can, however, not spectrally be resolved at room-temperature due to their small splitting (6 meV). The conduction band of ZnO possesses a moderate nonparabolicity which does aect the spectra mostly in a spectral region well above the ZnO optical gap. Furthermore, the small phonon replica at 70 meV above the excitonic absorption feature (see κZnO derived from ellipsometry measurements in the SI) will not be reproduced by the simulations. To account for line broadening we perform a convolution with a Lorentz function with Γ = 25 meV to match best the experimental κZnO spectrum obtained from ellipsometry measurements. The calculated κZnO (Fi ) depicted in Figure 3 c illustrate nicely the described eld dependence. At F ≥ FI , the excitonic feature vanishes and Franz-Keldysh oscillations typical for bandto-band transitions arise. The results of the simulations are depicted in Figure 3 d in comparison to an experimental DR spectrum of F6-TCNNQ/ZnO(0001). To be consistent, we account also for a slight 13



downward band bending at the pristine ZnO(0001) surface which amounts to ca. 0.2 eV for UHV prepared samples according to PES measurements. 7 This corresponds to a space charge layer width of z ≈ 25 nm and F (0) ≈ 1.65 ·107 V/m. Like R(d), R(0) is obtained by the transfer matrix method. The reectivity R(d) of the F6-TCNNQ/ZnO interface is calculated for dierent values of band bending and thus dierent F (0) and space charge layer widths. The F6-TCNNQ layer is described by one oscillator whose parameters (resonance energy, line width, oscillator strength) were obtained by tting the reference DR spectra. F6-TCNNQ2− molecules are not considered, rst because their contribution to the DR spectra would be much weaker than that caused by the modication of the absorption line shape of the ZnO band edge due to band bending, and second because their formation is highly unlikely. 18 The comparison of the calculated DR spectra with an experimental spectrum (see Figure 3 d) shows a surprisingly good agreement considering the approximations made in the simulations. The principal features observed in the experimental spectrum are also visible in the simulated spectra. They are well explained by the changes in the absorption line shape at the ZnO band edge: In the presence of an electric eld, the absorption below the band edge increases leading to a positive DR signal at 3.28 eV. The blue shift of the excitonic absorption peak produces the dispersive signal with a minimum at ca. 3.3 eV and a maximum at 3.33 eV. The features at higher energy are due to Franz-Keldysh oscillations. The absolute values of the DR signals are somewhat larger than in the experiment. It should be noted here that the signal height sensitively depends on the line broadening Γ of the ZnO excitonic resonance. We did not vary Γ in the simulations but kept it at the value which matches best the ellipsometry data. Furthermore, residual reections from the back side of the substrate also lead to a decrease of the signal height. The close agreement between experimental and calculated DR spectra found for the eld distribution predicted by the PES experiments conrms our interpretation of the DR spectra. The presented analysis can thus be used to obtain an estimate of the modication of the electric eld distribution at the semiconductor surface induced by the molecular adsorbate. We which to point out that a quantitative de14


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termination by DRS is possible. It requires, however, either a more comprehensive modelling of the electric eld dependence of the optical functions of the semiconductor or a beforehand experimental determination of the same by electroreectance measurements.

Application to NTCDA/ZnO(0001). Now that we have identied the characteristic ngerprint which a charge-transfer-induced change in band bending leaves in the DR spectra of inorganic/organic semiconductor heterointerfaces, we can apply DRS to study a hybrid system with so far unknown interfacial electronic structure, namely the aforementioned NTCDA/ZnO(0001) system (see Fig. 4). The reference DR spectra of NTCDA deposited on a transparent and inert KBr substrate illustrate the imaginary part of the dielectric function

b org of the molecular layer (see Fig. 4 b). At the lowest coverage corresponding approximately to a monolayer of lying NTCDA molecules, the spectrum resembles the monomeric absorption spectrum of NTCDA. As the coverage increases, the molecules start to aggregate and intermolecular interactions transform the spectrum into the bulk spectrum of NTCDA. 8 The features related to the molecular absorption can also be recognised in the spectra of the hybrid NTCDA/ZnO(0001) interface in the spectral range below the ZnO optical gap, i.e. between 3 eV to 3.3 eV (see Fig. 4 a). Immediately evident is also here the presence of the characteristic ngerprint of a charge-transfer induced change in band bending at the ZnO surface induced by the deposition of the molecules. NTCDA possesses an electron anity of ca. 4 eV 19 which is considerably larger than that of L4P and close to the work function of ZnO (see Fig. 1). This large electron anity is apparently sucient to cause electron transfer from ZnO to NTCDA increasing the band bending in ZnO. In contrast to the DR spectra of the F6-TCNNQ/ZnO(0001) interface, the intensity of the charge-transfer related feature does not saturate even at a nal nominal NTCDA thickness of 3.2 nm. This behaviour points at a dierent growth behaviour of the two molecules: While F6-TCNNQ grows in a Stranski-Krastanov mode 20 with the formation of a wetting layer, the growth of 15



NTCDA is Volmer-Weber-like 20 without the formation of a closed wetting layer (see SI). The low signal strength in the spectral region of the NTCDA absorption in the hybrid DR spectra is a further indication of such a three-dimensional growth scenario on ZnO. The present charge transfer process at the NTCDA/ZnO interface is qualitatively dierent from that at the PTCDA/ZnO interface where previous PES and DR experiments point at the formation of PTCDA anions. 21 PTCDA has as central unit perylene (instead of naphthalene) causing a red-shift of the absorption spectrum. The electron anity of PTCDA (4.1 eV) 22 is, however, quite similar to that of NTCDA. The formation of the PTCDA anions was ascribed to charge transfer and hybridisation with localised gap states due to Zn interstitials acting as shallow donors. These point defects were believed to be present in high concentration in the wet-chemically produced ZnO thin lms used in that study. The increase of the work function of ZnO by 1 eV due to deposition of PTCDA was found to be entirely due the formation of an interface dipole created by the charged molecules. In contrast, on the present epitaxial ZnO surfaces, we could not detect signatures of the formation of molecular anions or a hybridisation neither by PES 7 (F6-TCNNQ) nor DRS (F6-TCNNQ, NTCDA). A simple estimation within the Schottky depletion approximations predicts that a minuscule charge transfer of 0.02 electrons per molecule suces to produce the observed change in band bending in ZnO of the present doping level. 1 Such low concentration of charged molecules is well below the detection limit of DRS and PES. Finally, we discuss the consequences of the present nding on the eectiveness of dipoledipole mediated FRET across such hybrid interfaces. Ecient pumping of L4P excitons via FRET from ZnO has been demonstrated previously. As shown in this study, ground state charge transfer at the hybrid interface is negligible and the optical spectra of the energy transfer partners remain unchanged. Nevertheless, the radiative output of L4P in the hybrid structure has been found to be very low due to dissociation of L4P excitons by excited state electron transfer from the L4P LUMO to the ZnO conduction band at the hybrid interface having a type-II energy level alignment (see Fig. 1 a). 4 A high luminescent yield has only 16


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0.020

(a)

NTCDA

0.2 nm 0.6 nm 1.1 nm 2.3 nm 3.2 nm 3.8 nm

DRS

0.015

0.010

0.005

0.000 0.05

0.2 nm 0.6 nm 1.0 nm

(b)

0.04

DRS

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.6 nm 2.3 nm 3.5 nm

0.03 0.02 0.01 0.00

3.0

3.1

3.2

3.3 3.4 Energy [eV]

3.5

3.6

Figure 4: DR spectra of the NTCDA/ZnO(0001) (a) and reference DR spectra of NTCDA on KBr(001) (b) for increasing thickness of the organic layer. The thickness of NTCDA is a nominal thickness since the growth is three-dimensional (see AFM image in the SI). been achieved by properly tuning the energy levels into a type-I alignment via insertion of a molecular donor layer between the ZnO QW and L4P. 2 Selecting a molecule with a higher electron anity like NTCDA as energy transfer partner for ZnO is expected to mitigate the problem of interfacial exciton dissociation. However, as the present work shows, the large electron anity of NTCDA causes ground state electron transfer from the n-type ZnO to the molecular adsorbate. The associated electric eld, being comparable to the ionisation eld of the ZnO exciton, causes the formation an exciton dead layer at the ZnO surface. Dipole-dipole mediated FRET requiring close proximity (≤ 5 nm) of the interacting exciton species is therefore expected to be very inecient since the ZnO excitons can not reach close proximity to the organic layer.

17



Conclusions We have demonstrated that

in situ DRS is a simple alternative method to reveal very sen-

sitively charge redistribution at inorganic semiconductor surfaces induced by adsorption of conjugated organic molecules. Even minuscule electron transfer from ZnO to an organic adsorbate layer leads to a profound change in band bending at the semiconductor surface causing a substantial modication of its optical spectra in the near surface region. The modications of the spectra are explained by an electric eld eect on the excitonic absorption. The simulation of the DR spectra provides an estimate of the eld distribution in the near surface region and thus the magnitude of band bending in ZnO induced by the molecular adsorbate. We nd that the induced electric eld is comparable to the ionisation eld of the ZnO exciton, which has profound consequences, for example, on the eectiveness of dipole-dipole mediated excitonic coupling across such hybrid interfaces. This highlights the importance of the knowledge of the subtle interplay between the electronic structure and optical properties of inorganic/organic hybrid interfaces for the design of functional hybrid structures with a performance going beyond the single component.

Author contributions: TM and MS carried out the experiments. MG performed the simulations of the DR spectra. SS provided the ZnO substrates, BK and SH provided L4P and commented on the manuscript. The spectroelectrochemistry of F6-TCNNQ was performed by LG. TM, MS, MG, SS and ELK participated in the discussion and interpretation of the results and commented on the manuscript. SB supervised the work and wrote the manuscript.

Acknowledgement This work was supported by Sfb 951 (DFG). The authors thank SENTECH Instruments GmbH for performing the ellipsometry measurements of ZnO.

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Supporting Information Available Refractive index of ZnO determined by ellipsometry, AFM characterization of NTCDA, L4P and

in situ

KP-FM measurements of F6-TCNNQ, absorption coecient κ of L4P derived

from DRS, absorbance spectra of neutral and ionized F6-TCNNQ in solution, and simulations of DRS spectra assuming a non-interacting interface of F6-TCNNQ/ZnO.

References (1) Schlesinger, R.; Xu, Y.; Hofmann, O. T.; Winkler, S.; Frisch, J.; Niederhausen, J.; Vollmer, A.; Blumstengel, S.; Henneberger, F.; Rinke, P. et al. Controlling the work function of ZnO and the energy-level alignment at the interface to organic semiconductors with a molecular electron acceptor.

Phys. Rev. B 2013, 87, 155311.

(2) Schlesinger, R.; Bianchi, F.; Blumstengel, S.; Christodoulou, C.; Ovsyannikov, R.; Kobin, B.; Moudgil, K.; Barlow, S.; Hecht, S.; Marder, S. et al. Ecient light emission from inorganic and organic semiconductor hybrid structures by energy-level tuning.

Nat. Commun. 2015, 6, 6754. (3) Proehl, H.; Nitsche, R.; Dienel, T.; Leo, K.; Fritz, T. In situ dierential reectance spectroscopy of thin crystalline lms of PTCDA on dierent substrates.

Phys. Rev. B

2005, 71, 165207. (4) Bianchi, F.; Sadofev, S.; Schlesinger, R.; Kobin, B.; Hecht, S.; Koch, N.; Henneberger, F.; Blumstengel, S. Cascade energy transfer versus charge separation in ladder-type oligo(p-phenylene)/ZnO hybrid structures for light-emitting applications.

Appl. Phys. Lett. 2014, 105, 233301. (5) Greiner, M. T.; Helander, M. G.; Tang, W.-M.; Wang, Z.-B.; Qiu, J.; Lu, Z.-H. Universal energy-level alignment of molecules on metal oxides.

19

Nat. Mater. 2012, 11, 7681.



Page 20 of 22

(6) Winkler, S.; Frisch, J.; Schlesinger, R.; Oehzelt, M.; Rieger, R.; Räder, J.; Rabe, J. P.; Müllen, K.; Koch, N. The Impact of Local Work Function Variations on Fermi Level Pinning of Organic Semiconductors.

J. Phys. Chem. C

2013, 117, 2228522289.

(7) Schultz, T.; Schlesinger, R.; Niederhausen, J.; Henneberger, F.; Sadofev, S.; Blumstengel, S.; Vollmer, A.; Bussolotti, F.; Yang, J.-P.; Kera, S. et al. Tuning the work function of GaN with organic molecular acceptors.

Phys. Rev. B 2016, 93, 125309.

(8) Slootsky, M.; Liu, X.; Menon, V. M.; Forrest, S. R. Room Temperature FrenkelWannier-Mott Hybridization of Degenerate Excitons in a Strongly Coupled Microcavity.

Phys. Rev. Lett. 2014, 112, 076401. (9) Sadofev, S.; Schäfer, P.; Fan, Y.-H.; Blumstengel, S.; Henneberger, F.; Schulz, D.; Klimm, D. Radical-source molecular beam epitaxy of ZnMgO and ZnCdO alloys on ZnO substrates.

Appl. Phys. Lett. 2007, 91, 201923.

(10) Kobin, B.; Grubert, L.; Blumstengel, S.; Henneberger, F.; Hecht, S. Vacuumprocessable ladder-type oligophenylenes for organic-inorganic hybrid structures: synthesis, optical and electrochemical properties upon increasing planarization as well as thin lm growth.

J. Mater. Chem. 2012, 22, 4383.

(11) McIntyre, J.; Aspnes, D. Dierential reection spectroscopy of very thin surface lms.

Surf. Sci. 1971, 24, 417 434. (12) De Sousa Meneses, D.; Gruener, G.; Malki, M.; Echegut, P. Causal Voigt prole for modeling reectivity spectra of glasses.

J. Non-Cryst. Solids 2005, 351, 124 129.

(13) Kobin, B.; Grubert, L.; Mebs, S.; Braun, B.; Hecht, S. Gradual Fluorination of Laddertype Quarterphenyl.

Isr. J. Chem. 2014, 54, 789795.

(14) Stähler, J.; Rinke, P. Global and local aspects of the surface potential landscape for

20


Page 21 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


energy level alignment at organic-ZnO interfaces.

Chem. Phys. 2017, 485-486, 149

165. (15) Xu, Y.; Hofmann, O. T.; Schlesinger, R.; Winkler, S.; Frisch, J.; Niederhausen, J.; Vollmer, A.; Blumstengel, S.; Henneberger, F.; Koch, N. et al. Space-Charge Transfer in Hybrid Inorganic-Organic Systems.

Phys. Rev. Lett. 2013, 111, 226802.

(16) Blossey, D. F. Wannier Exciton in an Electric Field. I. Optical Absorption by Bound and Continuum States.

Phys. Rev. B 1970, 2, 13821391.

(17) Blossey, D. F. Wannier Exciton in an Electric Field. II. Electroabsorption in DirectBand-Gap Solids.

Phys. Rev. B 1971, 3, 13821391.

(18) Winkler, S.; Amsalem, P.; Frisch, J.; Oehzelt, M.; Heimel, G.; Koch, N. Probing the energy levels in hole-doped molecular semiconductors. Mater.

Horiz. 2015, 2, 427433.

(19) Chan, C. K.; Kim, E.-G.; Brédas, J.-L.; Kahn, A. Molecular n-Type Doping of 1,4,5,8Naphthalene Tetracarboxylic Dianhydride by Pyronin B Studied Using Direct and Inverse Photoelectron Spectroscopies.

Adv. Funct. Mater. 2006, 16, 831837.

(20) Bauer, E. Phänomenologische Theorie der Kristallabscheidung an Oberächen. I.

Z.

Kristallog. 1997, 110, 372. (21) Gruenewald, M.; Schirra, L. K.; Winget, P.; Kozlik, M.; Ndione, P. F.; Sigdel, A. K.; Berry, J. J.; Forker, R.; Brédas, J.-L.; Fritz, T. et al. Integer Charge Transfer and Hybridization at an Organic Semiconductor/Conductive Oxide Interface.

C

J. Phys. Chem.

2015, 119, 48654873.

(22) Krause, S.; Casu, M. B.; Schöll, A.; Umbach, E. Determination of transport levels of organic semiconductors by UPS and IPS.

21

New J. Phys. 2008, 10, 085001.



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Fingerprint of Charge Redistribution in the Optical Spectra of Hybrid

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