Oligothiophene-Based Phosphonates for Surface Modification of Ultraflat Transparent Conductive Oxides

The self‐assembly of electroactive organic molecules on transparent conductive oxides is a versatile strategy to engineer the interfacial energy‐level alignment and to enhance charge carrier injection in optoelectronic devices. Via chemical grafting of an aromatic oligothiophene molecule by changing the position of the phosphonic acid anchoring group with respect to the organic moiety (terminal and internal), the direction of the main molecular dipole is changed, i.e., from parallel to perpendicular to the substrate, to study the molecular arrangement and electronic properties at the organic–inorganic interface. It is found that the observed work function increase cannot solely be predicted based on the calculated molecular dipole moment of the oligothiophene‐based phosphonates. In addition, charge transfer from the substrate to the molecule has to be taken into account. Molecular assembly and induced electronic changes are analogous for both indium‐tin oxide (ITO) and zinc oxide (ZnO), demonstrating the generality of the approach and highlighting the direct correlation between molecular coverage and electronic effects.


1.
Synthesis of T5-tP/iP Organic Synthesis. All solvents and other starting materials were commercial and used as received. The NMR spectra were recorded on a 500 MHz Bruker AVANCE II 500 spectrometer at room temperature. 1 H-NMR and 13 C-NMR spectra were referenced to 7.26 ppm and 77.16 ppm, respectively, for CDCl3 and 5.32 ppm and 53.8 ppm, respectively, for CD2Cl2. UPLC-MS was performed on a Waters UPLC Acquity equipped with a Waters LCT XE Mass Detector for UPLC-HRMS, with Waters Alliance systems. 11.24 mmol) in THF (10 mL) was added dropwise and the mixture was stirred for 1 h at this temperature. Bu3SnCl (3.35 mL, 12.37 mmol) was added at -78 °C. After stirring for 4 h at this temperature, the mixture was allowed to warm to room temperature overnight. The S3 solvent was removed under reduced pressure, the residue was diluted with CH2Cl2 (150 mL) and washed with NH4Cl-solution (100 mL). The organic layer was dried over MgSO4, filtered and the solvent was removed under reduced pressure. The resulting brownish oil was subjected to column chromatography (AcOEt) to afford 1.80 g (27%) of compound 2. 1   0.487 mmol) and 5-stannyl-bithiophene 2 (1.800 g, 3.043 mmol) are suspended in toluene (50 mL). CuCl (54 mg, 0.548 mmol) was added at room temperature and the mixture was refluxed for 2 h. After cooling to room temperature 2 M aqueous KF-solution (25 mL) was added and the mixture was allowed to stir at this temperature for 1 h. The reaction mixture was filtrated over Celite and diluted with AcOEt (100 mL). The organic phase was washed with brine (100 mL) and was dried with MgSO4. After removal of the solvents under reduced pressure the residue obtained was subjected to column chromatography (CH2Cl2 : MeOH = 100 : 2) to afford 1.12 g (66%) of 4 as an orange powder. 1   added to 3-diethoxyphosphoryl-thiophene 5 (6.14 g, 27.88 mmol) in THF (75 mL) at -78 °C.

Synthesis of T5-tP
The reaction mixture was allowed to stir at this temperature for further 2 h. Bu4SnCl (18.91 mL, 69.70 mmol) dissolved in THF (50 mL) was added and the reaction was stirred for 4 h at -78 °C. The pH was adjusted to 7 by addition of a Na2HPO4/NaH2PO4-buffer (125 mL).
The mixture was extracted with AcOEt (3x70 mL). The combined organic phases were washed with brine (200 mL) and dried over MgSO4. The solvent was removed under reduced pressure and the resulting brownish oil was subjected to column chromatography (petroleum ether : AcOEt = 10 : 1) to afford 13.50 g (61%) of compound 6. 1

Binding of PAs to ultraflat TCOs
The molecules' binding to the ITO/ZnO metal oxide surfaces was evaluated by O 1s core level fitting analysis ( Figure S1). The fitting was carried out with energetic positions corresponding to two binding modes (stable configurations) of the PA's oxygen to the substrate, namely bidentate and tridentate binding.

Optical absorption (UV/Vis) spectroscopy
Optical absorption (UV/Vis) spectroscopy (Perkin-Elmer Lambda 900 spectrometer) was performed to estimate the surface coverage N/A (i.e., molecules/nm 2 ) of oligothiophenebased PA monolayers on transparent conductive oxides (TCOs). For this purpose, the newly synthesized quinquethiophene (T5) with the PA group in internal position (T5-iP) fulfills the requirements to be a suitable reference molecule: i) the T5 moiety absorbs in the UV/Vis region (see Figure S2a); ii) the main transition dipole moment µT of T5 is polarized parallel to its long axis, [1] thus, after forming a monolayer of T5-iP on TCOs, µT lies approximately parallel to the substrate surface, rendering it suitable for quantitative measurements via UV/Vis spectroscopy.

UV/Vis in solution
In order to check for any influence of the PA group on the optical properties, T5 and Assuming a linear relationship between absorbance and molar concentration c of the absorbing molecules (c < 10 -3 mol/l), the Beer-Lambert law [2] for the absorbance A (= absorption, neglecting any contribution of reflection and scattering) can be written as: where ε is the extinction coefficient (molar absorptivity), and d is the optical path length (= thickness of quartz glass cuvette, d = 10 mm). In turn, σ = c•d corresponds to the molecular S9 coverage of a thin film, giving an estimation of the surface coverage of T5-iP-modified (transparent) surfaces via UV/Vis spectroscopy.
In Figure S2c, the absorbance (extinction) at the peak maximum (409 nm) was plotted vs. the concentration c of the T5-iP molecules in solution. The linear fit gives an extinction coefficient of εsol = (3.3 ± 0.3) × 10 3 m 2 /mol for T5-iP, whereas for T5 it amounts to εsol = (4.3 ± 0.9) × 10 3 m 2 /mol (linear fit not shown here). The obtained values for the extinction coefficients coincide within their measurement uncertainties. It can be concluded, that the PA group has only small influence on the absorption of the T5-iP molecule and therefore the assumption can be made, that the influence of the PA group binding to the ITO surface can be neglected.

UV/Vis of T5-iP multilayers
In a second step, T5-iP multilayers were evaporated in UHV onto ITO substrates (Thin UV/Vis absorption spectra of each ITO were collected before and after multilayer deposition, and the spectra of the bare ITO were subtracted from those of the corresponding T5-iP multilayers on ITO. It is noteworthy that the obtained differential spectra show negative absorbance in the wavelength regions around the molecule's main absorption (i.e., between λ = 300 − 370 nm and λ > 550 nm). This is mainly due to the fact that the UV/Vis experimental setup used in the present study only allows for the detection of transmitted light S10 and, therefore, any changes of light reflection and scattering are neglected. We assume that the reflectivity at the T5-iP/ITO interface with respect to the bare ITO (i.e., T5-iP layers instead of ambient air) is constantly decreased for each sample. As it is obvious from Figure S2b, the T5-iP molecule in solution does not absorb in the wavelength region around λ = 600 nm. Therefore, a constant shift along the y axis was applied for each differential spectrum to bring the observed minimum at around λ = 600 nm (e. g., see black curve in Figure S3a) to zero absorption. The as-processed differential UV/Vis absorption spectra of the T5-iP multilayers are reported in Figure S3a. The additional negative absorption in the UV region of the thickest T5-iP multilayer (d = 25.5 nm) cannot be unambiguously accounted for, since there occurs an overlap with the absorption tail of the molecule (see Figure S2b). However, it was neglected at this stage, because this region does not contribute to the maximum absorbance (which is the determined value needed for further calculations).

S11
It can be seen that the absorbance maximum of the thinnest layer (d = 1.6 nm) lies at 409 nm, which corresponds to the value found for the molecules in solution (see Figure S2b).
This finding indicates that at this layer thickness there is no significant molecule-molecule interaction that could influence the absorption behavior of the molecules. With increasing thickness the maximum of the absorbance exhibits a red shift due to increasing molecular interaction. In Figure S3b, the maximum absorbance is plotted against the film thickness. The linear fit proves the linear behavior also for the solid T5-iP multilayers. The obtained extinction coefficient ε* = 2.16 × 10 -3 nm -1 can be further used to determine the thickness of T5-iP monolayers on TCOs (see below).

Estimation of surface coverage of T5-iP-modified ITO
The linear behavior between absorbance and layer thickness in the case of T5-iP multilayers supports the previous assumption that the PA group does not have a strong influence on the absorption behavior, even after PA binding to the metal oxide surface, and that the extinction coefficient in solution εsol can be used to determine the molecular coverage σ of the T5-iP monolayers on TCOs.
At this point, it has to be considered, that the molecules also bind to the backside of the sample, i.e. the glass substrate, during the wet-chemical monolayer preparation (see experimental section for preparation details). Therefore, our first assumption to roughly estimate the surface coverage of T5-iP-modified ITO is that the absorbance of the molecules on the backside of the glass substrate ( ̅ glass) contributes equally to the overall absorbance ( ̅ tot):

S12
This assumption is based on our preliminary work with other phosphonic acid derivatives, where we found that the maximum surface coverage is very similar on most oxide surfaces (ZnO, ITO, SiO2, glass, etc.). [3] A second assumption accounts for the difference between the extinction coefficient of the molecules in a bound monolayer ( ) and of those in solution ( In the present study, we obtained for the T5-iP-modified ITO a maximum absorbance of Considering ITO surface unit-cell dimensions of 24.79 Å × 14.32 Å, [4] this corresponds to 1.9 molecules/unit-cell, which is in good agreement with the assumption that ITO can theoretically bind up to 2.0 molecules/unit-cell (based on the most densely packed geometry in agreement with steric hindrance of the T5-iP molecules).
The estimation of the surface coverage via UV-Vis absorbance was cross-checked via the XPS signal of the In 3d core level before and after surface modification (see Figure S4). In the S13 case of T5-ip on ITO, the In 3d signal was attenuated by about 22 % after surface modification. This attenuation roughly corresponds to a molecular coverage of ⁓ 2 molecules/unit-cell, which is in good agreement with the value obtained by UV-Vis absorbance. Figure S4. XPS In 3d core level of unmodified and T5-iP-modified ITO.
Using the extinction coefficient ε* obtained from the T5-iP multilayers (see section 3.2), the layer thickness d of the T5-iP monolayer on ITO can be estimated to be: The estimated thickness of 1.3 nm for a T5-iP monolayer is in agreement with the theoretically expected value of the free-molecule T5-iP equilibrium geometry (i.e., in our calculations the perpendicular distance from the lowest oxygen to the highest hydrogen in the T5-iP molecule amounts to about ~ 1 nm). The small discrepancy might be due to the fact that the phosphonate bonds for the T5-iP monolayer are forced to be highly strained as the T5 moiety has to tilt on the ITO surface (see discussion in the main text). Thus one can reasonably assume that the overall monolayer thickness is larger than the size of the molecule. showing no preferential orientation of the molecule.