Inorganic Interfaces and the Formation of

May 18, 2015 - We present in situ infrared spectroscopy as a powerful tool for the qualitative and quantitative analysis of the charge transfer throug...
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Charge Transfer at Organic/Inorganic Interfaces and the Formation of Space Charge Regions Studied with Infrared Light Sebastian Beck,*,†,‡ David Gerbert,†,‡,§ Tobias Glaser,†,‡ and Annemarie Pucci†,‡ †

Kirchhoff-Institute for Physics, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 227, 69120 Heidelberg, Germany InnovationLab GmbH, Speyerer Strasse 4, 69115 Heidelberg, Germany

‡

S Supporting Information *

ABSTRACT: We present in situ infrared spectroscopy as a powerful tool for the qualitative and quantitative analysis of the charge transfer through the prototypical interface between the organic semiconductor 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) and MoO3 that in organic electronic devices is often used to improve their performance. Due to the different infrared vibrational spectra, charged and neutral species of CBP molecules can be well distinguished, which allows the measurement of the amount of charged species in the vicinity of the interface. The quantitative analysis of CBP thickness-dependent infrared transmission spectra delivered the extension of the space charge region from the interface into the CBP on a nanometer scale. The clear influence of the deposition sequence on these interface properties was clarified by further studies of the inverted layer structures.

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INTRODUCTION In organic electronics the energy level alignment at interfaces between electrodes and organic semiconductors is crucial for the effective barrier heights for charge extraction (organic photovoltaics (OPVs)) and/or injection (organic field-effect transistors (OFETs)/organic light-emitting diodes (OLEDs)) and thus for the overall device performance. Inserting suitable interlayer materials is one approach to modify the energetics at such interfaces. For example, for many years, thin transitionmetal oxide (TMO) layers between organic semiconductors and the metal electrodes have been used to improve device performance.1,2 For these systems the increased efficiency has been explained in terms of a decreased effective hole injection barrier due to a change of the work function of the contact and the energy level alignment at the interface between the TMO and the organic layer.3−6 The origin of these effects and their experimental evaluation as well as their description by theoretical models based on molecular properties such as the density of states of the involved frontier orbitals are topics of recent publications.7−12 Different experimental techniques are used to investigate the evolution of energy levels at the interface and the associated interfacial charge transfer (CT) that is also termed contact doping. In addition to the frequently used UV photoemission spectroscopy (UPS) and Kelvin probe measurements, which measure the potential profile as a consequence of the transferred charges,1,7,13 CT at interfaces also has been investigated with differential-reflection spectroscopy in the UV−vis range.12 Especially for the latter method, the broad spectral features complicate an easy evaluation of these. Very recently, Shallcross et al. showed that X-ray photoemission spectroscopy (XPS) can be used to investigate the extent of contact doping at interfaces of high work function materials, © XXXX American Chemical Society

such as indium tin oxide (ITO) or MoOx-covered ITO and solution-processed thin layers of poly(3-hexylthiophene-2,5diyl) (P3HT).11 Since a better understanding of CT effects on the molecular level seems to be crucial for the optimal device design, further analytical tools may be helpful to address this topic. In this connection, infrared (IR) spectroscopy is especially suited for the nondestructive investigation of CT in organic semiconductor materials as the rich vibrational fingerprint spectrum of organic molecules can be accessed and analyzed not only with respect to the molecular structure and orientation, but also with respect to electrical charging.14−16 During IR spectroscopic measurements, the whole sample is always probed, allowing for the investigation of buried interfaces as well as bulk effects at the same time during film formation. With the help of the well-established densityfunctional theories (DFTs), IR lines can be safely assigned to molecular vibrations, even for complex molecules, which yields extremely valuable information on the molecular level. The good theoretical description is facilitated by the fact that the vibrational excitations represent only comparably small deviations from the ground state. IR spectral information can be obtained from fractions of monolayers already, as has been shown, for example, in the various studies of organic molecules on metal surfaces performed under ultra-high-vacuum (UHV) conditions.14−16 Signals of molecules on metallic substrates appear enhanced by up to about 1 order of magnitude, when compared to molecules on nonmetallic substrates. As we show Received: May 7, 2015 Revised: May 18, 2015

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Figure 1. (a) Chemical structure of CBP. (b) Measured relative transmission spectrum of 7 nm of CBP deposited on silicon. (c) Measured relative transmission spectra of CBP deposited on 24 nm of MoO3 for various CBP layer thicknesses. The dashed red and dotted blue vertical lines indicate the vibrational modes of neutral and positively charged CBP molecules, respectively. (d) Selected relative transmission spectra (black) shown together with the contribution from the cations (blue) and the neutral molecules (red). For more details, see the Supporting Information, Figure S4.

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EXPERIMENTAL METHODS Sample Preparation. All samples were prepared and measured in situ during film deposition in a custom-designed UHV chamber connected to a Fourier-transform infrared (FTIR) spectrometer. After the beam was transmitted through the sample, it was focused onto a liquid nitrogen-cooled mercury−cadmium−telluride (MCT) detector (see the Supporting Information, Figure S1). The spectrometer and detector housing were evacuated to a pressure of 3 mbar to prevent absorption from ambient air. All films under investigation were deposited onto a clean silicon substrate with native oxide by thermal evaporation from quartz crucibles in a stepwise fashion. Both CBP and MoO3 were purchased from Sigma-Aldrich, with purities of 99.9% and 99.99%, respectively, and were used without any further purification. Deposition rates were measured with a calibrated quartz crystal microbalance. Profilometer measurements on neat samples of CBP and MoO3 were performed for thickness calibration. The deposition rates of CBP and MoO3 were kept between 0.2 and 1.0 nm min−1 and between 0.7 and 1.4 nm min−1, respectively. During film deposition, the pressure in the UHV chamber stayed below 1 × 10−8 mbar and the substrate was at room temperature. The IR spectra were measured during film

here, the lower sensitivity due to nonmetallic substrates does not prevent successful IR investigations of monolayers. Herein, we investigate the interface between the ambipolar charge transport material 4,4′-bis(N-carbazolyl)-1,1′-bipheny (CBP; the molecular structure of CBP is shown in Figure 1a) and the TMO MoO3, which is a prototypical example for an interface energetics-modifying contact material. The Fermi level of MoO3 is positioned at 6.86 eV with respect to the vacuum level, just 0.16 eV below its conduction band, while the Fermi level of CBP lies at 4.7 eV, and the highest occupied molecular orbital (HOMO) is positioned at 6.2 eV.1,17 When both layers are in contact, the equilibration of the chemical potential (“Fermi energies”) in both materials leads to the formation of an interface dipole and to band bending of the CBP HOMO toward the Fermi energy. According to this shift, an electron transfer from the CBP molecules to MoO3 occurs, so reduced TMO species are formed at the interface.18 On the CBP side, due to this interfacial charge transfer, a strongly p-doped nearsurface layer in the CBP is generated. The p-doping profile, i.e., the profile of the CBP cation concentration, is studied with IR spectroscopy. B

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The Journal of Physical Chemistry C deposition in transmission geometry, and with a resolution of 4 cm−1. The deposition steps for the stacked samples were performed in immediate succession to minimize contamination. Special care was taken to have extremely precise reference spectra of the bare substrates. Spectral Analysis. Spectral fits to the experimental transmission spectra were conducted with the commercially available software package Scout.19 The optical properties of each material were defined by its dielectric function, which consists of a dielectric background and several Brendel oscillators to account for the vibrational modes of the material.20 All used dielectric functions are assumed to be isotropic according to the random structure of the amorphous layers. Details on the procedure for achieving dielectric functions have been published elsewhere.21

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RESULTS First, we discuss the CBP/MoO3 interface that forms when CBP is deposited on MoO3. This layer structure is used in many applications, e.g., when the metal oxide acts as a holeinjection layer in OLEDs.22−24 In Figure 1c, the experimental relative transmission spectra of CBP on 24 nm of MoO3 for various CBP thicknesses are shown. The transmission spectrum of the underlying MoO3 layer on silicon was used as the reference for all spectra in Figure 1. Various absorption bands arise in the spectra already for very thin layers below 1−2 nm of CBP on MoO3, but their position and relative intensity differ from those of the spectrum of neutral CBP, which is shown in Figure 1b. These bands show an overall agreement with the vibrational spectrum of single positively charged CBP (cation) (see the Supporting Information, Tables S1 and S2).21 With increasing thickness of CBP, the spectrum changes significantly (see also Figure 1d) due to the appearance of new bands that correspond to the spectrum of neutral CBP. The further increase of cation signals also for thicker CBP layers above 1−2 nm indicates the formation of charged CBP molecules that are not in direct contact with the underlying MoO3 layer (which was proven as smooth; see the Supporting Information, Figure S2). It should be mentioned that CBP rapidly covers the chemically active MoO3 surface (see Figure S2). The CT interaction between both the CBP and the MoO3 layer leads to a high amount of charged CBP molecules within the first molecular layer. The further increase of that amount decelerates for the following layers and vanishes for the thickest layers under investigation. Accordingly, the concentration of neutral CBP increases from a low value at the interface to 100% on the top of a thick CBP layer. Incremental spectra, which show the developing absorption features with deposited thickness, confirm this interpretation (see the Supporting Information, Figure S3). All the experimental IR spectra for the different CBP layer thicknesses can be fully described by a superposition of the vibrational oscillators of neutral CBP and CBP cations where the weightings change with the layer thickness (see Figure 1d and the Supporting Information, Figure S4). From such spectral fits we obtained the CBP thickness dCBP (in accordance with quartz-microbalance results) and the number Ncation of charged CBP molecules in the film. We used a density of 1.7 g cm−3 for neutral CBP and CBP cations, and a molar mass of 484.6 g mol−1 for this calculation.25 Figure 2 shows the calculated values for Ncation plotted over dCBP. For the sake of clarity, the symbol size in this figure was chosen in such a way that it accounts for the statistical and experimental errors. The

Figure 2. Left y-axis: number of positively charged CBP molecules, Ncation, per area versus the total layer thickness of CBP, dCBP. The data points were derived by fitting the experimental spectra as a superposition of the spectra of neutral and charged CBP molecules. The dotted line shows the fit with eq 2. Right y-axis: calculated space charge density ρ(z) = ρ0 exp(−z/b) derived from the fit parameters ρ0 and b.

systematical errors of the dielectric functions that were used in the fitting procedure were neglected in the further evaluation since they would simply lead to a rescaling of all data points by the same factor (∼1 ± 0.28), which would not have any influence on the general statement of the presented work. In Figure 2 a strong increase of Ncation for small dCBP and a saturation for bigger dCBP can be observed. With IR spectroscopy the whole layer is always probed, and therefore, the total number of charged molecules in the whole CBP layer is detected. This experimentally obtained value Ncation(dCBP) is the integral of the space charge density ρ(z) (divided by the elementary charge e): Ncation(dCBP) =

1 e

∫0

dCBP

− ρ (z ) d z

(1)

Fitting the Schottky model with a step function for ρ(z) to our data leads to a steep linear increase of the total number of charged molecules until a constant value is reached for a certain layer thickness. Such behavior is in clear contrast to our experimental data, and thus, the Schottky model is not considered further. Instead an exponential decrease of the space charge density ρ(z) = ρ0 exp(−z/b) is assumed, which seems to be a reasonable approximation if compared to recent studies.9,10 Oehzelt et al. numerically calculated the charge density distribution in the space charge region of pentacene on gold, solving Poisson’s equation in an iterative way.9 With the same procedure Wang et al. simulated the charge density distribution in a layer of C60 at the interface between a NaClcovered silver electrode and C60.10 The shapes of the calculated curves for ρ(z) are nearly exponential (as we have numerically checked) and therefore motivate our ansatz.10 Applying an exponential space charge density to eq 1, the total amount of charged molecules in a layer with thickness dCBP is expressed by the relation Ncation(dCBP) = C

ρ0 ⎡ ⎛ d ⎞⎤ b⎢1 − exp⎜ − CBP ⎟⎥ ⎝ e ⎣ b ⎠⎦

(2)

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The Journal of Physical Chemistry C The parameter ρ0 describes the charge carrier density directly at the interface between CBP and MoO3, and b describes the decay width. Equation 2 was used to fit the experimental data of Ncation(dCBP) in Figure 2 with ρ0 and b as fit parameters. The result of this fit with only two free parameters is shown in Figure 2 as a dotted black line. These parameters have reasonable values, and the fit is in good agreement with the experimental curve. For the charge carrier density ρ0 directly at the interface, we obtained 0.2 C m−2 nm−1, and for the decay width b, we obtained 0.64 nm. The charge distribution ρ(z) resulting for the obtained fitting parameters ρ0 and b is shown as a dashed red line in Figure 2. The observed charge transfer and our interpretation are in accord with the well-accepted model of the energy level alignment at the interface between CBP and MoO3 under consideration of an interface dipole and band bending.7,18 Using this terminology, the high space charge density in the first monolayer of CBP for distances z < 1 nm from the interface corresponds to the interface dipole, whereas the measurable number of charged molecules at distances z > 1 nm is an indication of the formation of a space charge region in the CBP layer. It is important to note that, for the CBP cations, the IR oscillator strength is larger than for the neutral molecules. This leads to a higher dc dielectric constant of the CBP cations compared to the neutral molecules as is shown in Figure S5 in the Supporting Information. Therefore, the dc dielectric constant in the CBP layer at the interface where both molecular species exist is also enlarged compared to that of a pure layer of neutral CBP molecules. That means the very high CBP cation density at the interface to MoO3 is not compatible with the usual assumption of a constant dielectric background in Poisson’s equation that is often made in theoretical descriptions.9,10 Since we have demonstrated reasonable IR spectroscopic results for charge transfer from CBP to MoO3 for deposition of organic material onto the metal oxide, a situation where the interface stays smooth upon layer growth, we now introduce the inverted case where interdiffusion occurs and enhances the observed charge transfer. Transition-metal oxides are deposited onto organic layers for certain applications, for example, when transition-metal oxides are used as a charge generation layer.26 We prepared such inverted layer structures by evaporating MoO3 on CBP, as depicted in Figure 3a. Measured relative transmission spectra of this MoO3/CBP interface are shown in Figure 3b. For a good comparison, the wavenumbers of the vibrational modes of neutral CBP are marked with dashed red lines and those of the vibrational modes of CBP cations are marked with dotted blue lines. The transmission spectrum of the silicon substrate was used as the reference for all spectra in Figure 3. If MoO3 is deposited onto the 15 nm thick CBP layer, the vibrational modes of CBP cations, e.g., those at about 1523 and 1575 cm−1, arise in the spectra. The intensities of these spectral features increase with MoO3 coverage and saturate at a thickness of about 10 nm of MoO3. Additionally, the vibrational modes of the neutral CBP, e.g., the mode at 1506 cm−1, decrease in intensity with metal oxide deposition. These results give evidence that also for this inverted layer sequence CT takes place between CBP and the transition-metal oxide. Incremental spectra (see the Supporting Information, Figure S6) confirm this interpretation. In addition to the similarities to the CBP/ MoO3 structure discussed above, there are also important differences among the spectra.

Figure 3. (a) Cross-section models of the layer structure of reference and sample measurements. (b) Measured relative transmission spectra of MoO3 deposited on 15 nm of CBP for various MoO3 thicknesses as given. The dashed red and dotted blue lines indicate the vibrational modes of neutral and positively charged CBP molecules, respectively.

Figure 4 compares the experimental relative transmission spectra of CBP doped with MoO3, MoO3 deposited on CBP,

Figure 4. Measured relative transmission spectra of 15 nm of CBP on 24 nm of MoO3 (red), 24 nm of MoO3 on 15 nm of CBP (blue), and 15 nm of CBP doped with 30 vol % MoO3 (black).21

and CBP deposited on MoO3 in the fingerprint region.21 All layers contain approximately 15 nm of CBP. For the stacked layers also the amount of deposited MoO3 is identical and corresponds to a nominal layer thickness of 24 nm. Clearly, as the main difference, the intensities of the absorption bands of the CBP cations are much stronger in the inverted structure when compared to the CBP/MoO3 structure, indicating the presence of approximately 3 times more CBP cations in the inverted structure. An increased interface area between CBP and MoO3 induced by an enlarged surface roughness of the D

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deposited on 14 nm of CBP, result of the fitting procedure for the spectra of CBP deposited on 24 nm of MoO3, and description of the experimental setup. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b04398.

underlying CBP compared to MoO3 can be excluded as a reason for the greater extent of CT between both materials. AFM measurements of a 5 nm thick CBP layer on silicon revealed a root-mean-square (RMS) roughness of 1 nm, which is only slightly higher than the RMS roughness of MoO3 on silicon of 0.2 nm (see the Supporting Information, Figure S2). However, the larger number of charged CBP molecules in the spectra of the inverted structure can be understood in the following way: For low coverage, the evaporated MoO3 diffuses into the organic layer, forming a p-doped area in CBP that leads to spectral features that correspond to those of cations in doped layers.21 The diffusion of MoO3 in this system and also for the deposition of MoO3 on a polymeric host has already been observed.27,28 The whole range of interdiffusion corresponds to a strongly doped region within the CBP layer where electronic CBP states are depopulated. Therefore, in this region the Fermi level shifts to higher energies compared to a pure layer, as observed for intentionally doped CBP layers.17 Besides the formation of a doped interlayer, also the interfacial CT should occur, but because of the reduced difference in Fermi-level energy between the MoO3 and the unintentionally doped CBP layer, the extent of interfacial CT should be reduced regarding the system without a doped interlayer. After the deposition of about 10 nm of MoO3, the amount of charged CBP molecules saturates, which means that further deposited MoO3 does not diffuse into the CBP layer anymore and the interfacial charge transfer is also completed.

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*E-mail: [email protected]. Present Address §

D.G.: Physikalisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany. Author Contributions

S.B. and T.G. performed the experimental work, data analysis, and experimental planning. D.G. assisted in sample preparation and IR spectroscopic measurements. The project was conceived, planned, and supervised by T.G. and A.P. The manuscript was written by S.B., T.G., and A.P. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank M. Scherer for his assistance with the AFM measurements. Financial support from the BMBF (Bundesministerium für Bildung und Forschung) via the MESOMERIE Project (Grant FKZ 13N10724) is gratefully acknowledged. S.B and T.G. thank the Heidelberg Graduate School for Fundamental Physics (HGSFP).

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CONCLUSION With IR spectroscopy we have shown that CBP deposition onto smooth MoO3 leads to a high concentration of CBP cations directly at the interface and also a measurable amount of cations separated from it in the organic layer. The charged CBP molecules do not need to be in direct contact with the MoO3 and can be found up to 2 nm away from the interface (Figure 2), thus forming a space charge region. By fitting the experimental spectra that were measured in situ during CBP deposition as a superposition of the vibrational oscillators of neutral CBP and CBP cations, the space charge distribution in CBP could be determined. The results are consistent with theoretical models and photoelectron spectroscopy findings.9,18 The inversion of the deposition sequence makes obvious the significantly differing amounts of CT in the two systems CBP/ MoO3 and MoO3/CBP which arise due to the different interdiffusion behaviors. This difference is important for both device functionality and efficiency and has to be considered already in the device design. The presented work shows in situ IR spectroscopy as a powerful tool for the quantitative characterization of CT through interfaces between organic semiconductors and inorganic contact materials. This nondestructive method represents a complementary approach to access CT effects at device interfaces on a molecular level. The information achieved by IR studies can serve to verify theoretical models of this CT, so it is extremely valuable for the precise design of electronic and optoelectronic devices.

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AUTHOR INFORMATION

Corresponding Author

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

Tables of the vibrational modes of neutral CBP and positively charged CBP, AFM measurements of silicon, CBP on silicon, MoO3 on silicon, and CBP on MoO3 on silicon, incremental IR spectra of CBP deposited on 24 nm of MoO3 and MoO3 E

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