Increasing Electrode Work Function Using a Natural Molecule

Providing sustainability to organic electronics is highly demanded to reduce the negative impact of organic devices on environments and human health upon their disposal. To attain biodegradability and biocompatibility of the electronic devices, utilization of the natural molecules for the device constituents is essential. In this study, it is reported that the adsorption of caffeic acid (CfA), a polar phenylpropanoid that plants bio‐synthesize, universally increases work functions (WFs) of practical electrodes and organic films. Either vacuum‐depositing or spin‐casting CfA films on the electrode materials form a dipole layer with the negative charges on the carboxyl group exposed to the outermost surface. The preferential adsorption of the catechol moiety of CfA onto substrate surfaces drives the molecular orientation, leading to the WF increase up to 0.7 eV. As a consequence, the single‐layer devices with the CfA interlayer facilitate the hole injection in forward bias by a factor of 101–102, which validates the usability of the natural molecule for organic electronics.

configuration (Figure 1). CfA has a permanent dipole of 4.34 D that points toward the catechol. The dipole unit vector is slightly tilted from the molecular long axis (x-axis in Figure 1) by 14.4°. Moreover, the catechol group exhibits preferential adsorption on metal surfaces via hydrogen or coordination bonds between the surface and hydroxyl groups. [43] Combining the abovementioned characteristics of the molecule, the adsorption of CfA on the electrode surface will form a dipole layer with the negative charges at the outermost surface and hence expectedly increase the WF of electrodes.
In this study, we demonstrate that WFs of wide-ranging practical electrodes increase up to 0.7 eV by either evaporating or spin-casting CfA onto their surface. The WF manipulation by vacuum deposition of CfA is effective even for organic solids such as poly (3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and zinc phthalocyanine (ZnPc) films. Quantitative estimation of molecular orientation by Fourier transform IR reflection absorption spectroscopy (FTIR-RAS) confirmed that the carboxyl groups tended to orient along the surface normal. The adsorption of cinnamic acid (CnA), which comprises a phenyl group instead of a catechol (Figure 1), resulted in small and random change in WF of electrodes, suggesting that the preferential adsorption of CfA led by the catechol group. Furthermore, it is demonstrated that the electrodes covered with CfA lower the hole injection barriers, which consequently enhances current density by a factor of 10 1 -10 2 in sandwiched devices comprising ZnPc and poly(3-hexylthiophene) (P3HT) as semiconducting layers. This study shows the potential of phenylpropanoids as electrode modifiers for organic electronics.

Results and Discussion
Firstly, we tested the vacuum deposition of the CfA and CnA molecules under high vacuum (<10 -3 Pa). Figure 2 compares FTIR spectra of bulk CfA and 30-nm-thick film of CfA on an Au substrate (bottom and middle spectra, respectively). The bulk spectrum was measured by transmission mode, whereas the film spectrum was acquired by FTIR-RAS. Overall, the spectral feature appears to be similar for both film and bulk. Characteristic vibrational absorptions of CfA, which are assigned to C = C in-plane stretching (1000-1500 cm -1 ), CO stretching (≈1650 cm -1 ), and O-H stretching (3300-3500 cm -1 ), are clearly observed. Therefore, this result suggests that the chemical structure of CfA molecules is conserved in the evaporated film. The distinct difference in the relative spectral intensity between the two samples may imply a preferential orientation of CfA molecules on the solid surface (discussed later). On the other hand, in the case of CnA (top spectrum in Figure 2), it is clearly seen that the vibrational absorptions are absent in a whole spectral range, indicating that CnA film is not formed on an Au substrate at room temperature. The difference in the film stability between CnA and CfA should originate from their molecular structures: CfA has a catechol group, whereas CnA has a phenyl group (Figure 1). Taking the metal-adsorption ability of catechol [43] into account, the catechol moiety may enable CfA to form the film on solid surface.
Vacuum-deposition of CfA onto various practical electrodes changes their WF. Figure 3a shows the evolution of WF for six electrodes as a function of CfA thickness. WF was measured with Kelvin probe (KP) in air. The monotonic increase of WF is evident. It is noteworthy that the WF universally increases by ≈0.5 eV when CfA of 10 nm covers the substrate surface. Table 1 summarizes the WFs of substrates before and after  One can see the universality of the WF increase using CfA as an electrode modifier. Notably, ΔWF for highly oriented pyrolytic graphite (HOPG) reaches 0.68 eV. We also found that CfA layer increases WFs of even organic films, such as PEDOT:PSS and ZnPc films. As for ITO and SiO x (n-type silicon with naturally oxidized layer), we performed the KP measurements on the CfA films of the larger thickness (the inset in Figure 3a). The WF values are unchanged beyond 10 nm. While there are reports that polar molecules can exhibit the spontaneous orientation polarization that increases surface potential in proportion to the film thickness, [41,44] CfA is not the case.
Since CfA and CnA are known to be soluble in polar solvents, we also examined spin-coating. The WF increase was also achieved by spin-casting the ethanol solution of CfA on Au, Cu, ITO, and SiO x ( Table 2). Again, the WF of these substrates increases by ≈0.5 eV upon the adsorption of CfA on the surfaces except for ITO. It is noted that atmosphere during spin-casting influences ΔWF of ITO. Spin-casting CfA onto an ITO increases WF by 0.26 eV under N 2 atmosphere, while ΔWF decreases to 0.18 eV when CfA is spin-casted under ambient conditions. However, the effect of CfA is rather limited for PEDOT:PSS and P3HT films. This can be due to the diffusion of CfA into the polymer layers which hampers the formation of a dipole layer.
The variation of ΔWF seen in Table 1 may be caused by the difference in wettability of solid surfaces. Figure 4 compares the surface morphology of 10-nm-thick CfA film evaporated on different substrates obtained by atomic force microscopy (AFM). CfA molecules form aggregates with a lateral size of ≈80 nm on ITO, Au, and SiO x . As a consequence, their surfaces are not fully covered with CfA. Root-mean-square roughness (R q ) of the CfA layer are 18.1, 17.8, and 18.9 nm for ITO, Au, and SiO x substrates, respectively. On the other hand, the CfA layer on HOPG is relatively smooth, which indicates that a CfA layer more effectively wet the HOPG surface. R q for the HOPG case is 8.2 nm, which is much smaller than the case of the other substrates. The better wettability of HOPG surface improves a molecular alignment in the CfA layer.
The increase in WF by the adsorption of CfA should be related to a particular alignment of the molecular dipoles not in bulk but in the vicinity of the substrate surface. To raise the electrostatic potential of a solid surface, CfA must form a dipole layer by exposing the negative charges toward the outermost surface. Since the permanent dipole of CfA points toward the catechol group (Figure 1), CfA molecules should thus stand or tilt with preferential adsorption of the catechol moiety onto the solid surfaces, where the molecular long axis (parallel to the x-axis in Figure 1) can be tilted from the substrate surface.
Comparing the FTIR transmission spectrum of bulk CfA and the FTIR-RAS spectrum of a 10-nm-thick CfA film gives information on the molecular orientation in the film (Figure 3b). To discuss the CfA orientation, we firstly focus on the vibrational absorptions at 1650 and 781 cm -1 (labeled as A and B in Figure 3b, respectively). Analysis of a simulated IR spectrum suggests that peaks A and B are assigned to C-H out-of-plane Adv. Mater. Interfaces 2023, 10, 2201800  bending and CO stretching modes, respectively. We noted that the relative intensity of peak B to A was significantly higher for the CfA film on an Au than for bulk CfA. This result indicates that the electric field of p-polarized IR effectively interacts with the CO bond, whereas C-H out-of-plane bending mode is not effectively excited. The FTIR-RAS spectrum of the film implies that the CO group align in the direction parallel to the surface normal and CfA molecule adopt standing or tilting orientation. The universal increase of WF (Tables 1 and 2) suggests that the adsorption geometry should be independent of the substrates. Quantitative analysis of FTIR-RAS spectra, in principle, provides Euler angles of adsorbates on metal substrates [45] if their arrangement is uniform. In the case of the 10-nm-thick CfA interlayer, its arrangement within an entire film is not solely determined: As shown in Figure 3a, WF increases upon the CfA adsorption but saturates around 10 nm. This result implies that the dipole layer is formed near the substrate surface, but the molecules start to arrange in the up-down geometry, where the dipole-dipole interaction is cancelled out, as the nominal thickness of CfA increases. To exclusively determine molecular orientation in the dipole layer, thickness-dependence of FTIR-RAS spectra would be helpful, which we will address in upcoming works. It is inferred that the catechol moiety drives the specific orientation due to its adsorption ability to solid surfaces. To demon strate if the catechol group leads to the molecular orientation of CfA, we measured the change in WF of several substrates after spin-casting CnA that has a phenyl group instead of the catechol moiety. Noticeably, the magnitude of the WF increase is largely suppressed ( Table 3). The signs of ΔWFs for Cu and SiO x are still positive, but their magnitudes decrease to less than half of the CfA case ( Table 2). WFs of Au and ITO even decrease by 0.08 and 0.32 eV after the CnA treatment, respectively. These comparative experiments suggest that the absence of two hydroxyl groups on the phenyl group interrupts the preferential adsorption with the negative charges exposed toward the surface. In the extreme case, the sign of the ΔWF is reversed by the carboxyl group preferentially adsorbed on solid surfaces because the molecular dipoles align with exposing the phenyl group toward the outermost surface, which were indeed observed for Au and ITO. Therefore, it is clearly suggested that the catechol group is necessary to bind the molecule to the surface of the substrates via the coordination bond and/ or hydrogen bonding, resulting in the adsorption geometry in which the carboxyl groups orient toward the outermost surface ( Figure 2).
The increased WFs using a CfA film will promote hole injection into an organic semiconducting layer from the modified   electrode. Therefore, we finally investigated the applicability of CfA to single-layer devices. Figure 5a shows the impact of 5-nm-thick CfA evaporated film on current density-voltage characteristics of Au/ZnPc/Au devices. The CfA layer was inserted between the ZnPc layer and the bottom Au electrode. The effectiveness of CfA is evident: Current density in forward bias increases with the CfA interlayer by a factor of 10, which is attributed to the reduced hole injection barrier at ZnPc/bottom Au interface owing to the larger WF. It is found that the CfA buffer layer has an additional feature. Figure 5b shows FTIR-RAS spectra of CfA films on Au before and after spin-casting various organic solvents onto the film. The spectra do not change by two solvents, chlorobenzene (CB) and chloroform (CHCl 3 ), which are common solvents for preparing semiconducting layers. On the other hand, the CfA film is washed away by polar organic solvents such as acetone and dimethylformamide (DMF). The results suggest that the CfA interlayer can tolerate the formation of a spin-coated film of a polymeric semiconductor such as P3HT from CB or CHCl 3 solutions. Indeed, a P3HT film could be formed on a CfA-coated ITO substrate from the CB solution, which afforded us to fabricate a diode with the structure of ITO/CfA/P3HT/Al by sequential spin-coating and to investigate the impact of CfA on the performance of the solution-processed device. Figure 5c shows the current density-voltage characteristics of the diodes with and without the CfA interlayer. Again, the insertion of CfA increased current density in forward bias by a factor of 10 2 , because of the lowered hole injection barrier owing to the increased WF of an ITO electrode.
Although the direct impact of the WF increase by CfA on the device performance is evident, the hole transport mechanism in the electrode/CfA layer/organic semiconductor junction is elusive. The analyses with photoelectron yield spectroscopy (PYS) and UV-visible absorption spectroscopy on a CfA film of 100 nm (Figure 6a,b, respectively) give the ionization energy (I) and optical gap (E g,opt ). I and E g,opt of the CfA film are found to be 6.30 and 3.3 eV, respectively, which are determined from the intersections between the tangent and baseline of the spectra. Electron affinity (A) is estimated to be 3.0 eV from these parameters. This value contains the influence of exciton binding energy on the energies of the frontier orbitals, because E g,opt was used for the estimation of A. The transport gap, which is determined by ultraviolet photoelectron and inverse photoemission spectroscopy, will give the energy of a free electron in the LUMO. Compared with the WFs of the used electrodes (Table 1), hole injection barriers in the range of 1.2-2 eV exist at the electrode/CfA interfaces, whereas there is no energy barrier at the CfA/organic semiconductor interfaces because I of ZnPc and P3HT is 4.8 and 4.65 eV, respectively, [46,47] if edge-on orientation [48,49] is assumed for both semiconducting molecules deposited on the CfA layer. Despite the high energy barrier at the electrode/CfA interfaces, inserting the CfA interlayer facilitates the current density as shown in Figure 5a,c. Further investigation is necessary to understand the hole transport mechanism across the electrode/CfA layer/organic semiconductor junctions.

Conclusion
This study demonstrates that the adsorption of CfA, which plants bio-synthesize, can increase WFs of the wide-ranging electrodes and organic solids, showing the ability as the electrode modifier. WF increases roughly 0.5 eV on average by either vacuum-depositing or spin-casting CfA onto solid surfaces. Qualitative analysis with FTIR-RAS and the comparative KP experiments on CnA suggest that CfA molecules tilt with the preferential adsorption of the catechol group onto solid surfaces, which leads to the WF increase because the dipole layer is formed by exposing the negative charges on the carboxy group toward the outermost surface. Inserting the CfA interlayer between the organic semiconductors and hole-injection electrode increases the current density of ZnPc and P3HTbased single-layer devices, which verifies the applicability of CfA as an electrode modifier. The results shown in this study encourage us to seek natural molecules being applicable to organic devices. We believe that investigating the potentials of biomolecules as device constituents will open a pathway to realize sustainable organic devices for earth-and humanfriendly practical use.

Experimental Section
Materials: CfA, CnA, ZnPc, and P3HT were purchased from Tokyo Chemical Industry Corp., FUJIFILM Wako Pure Chemical Corp., NARD Institute, Ltd., and Luminescence Technology Corp., respectively. PEDOT:PSS (Clevios P VP. Al 4083) was purchased from Heraeus. The polymer film was spin-casted onto a cleaned ITO substrate at 3000 rpm for 30 s and annealed at 120 °C for 20 min in air. Ethanol, acetone, DMF, CB, and CHCl 3 were purchased from FUJIFILM Wako Pure Chemical Corp. Ag and Fe films were prepared onto cleaned glass substrates by thermal evaporation. Au substrate was purchased from Geomatec Co., Ltd. SiO x and Cu substrates were purchased from Nilaco Corp. Prior to use, all substrates were ultrasonicated in pure water, acetone, and isopropanol for 15 min each. Glass supporting substrates for Ag and Fe electrodes were further cleaned by UV-ozone treatment for 20 min. HOPG was purchased from Crystal Base Co., Ltd. Its clean surface was obtained by exfoliation in air. CfA was vacuum deposited at a rate of 1 Å s −1 on the substrates. For UV-visible absorption spectroscopy and PYS, CfA of 100 nm was evaporated at a rate of 1 Å s −1 onto a cleaned alkali-free glass and the commercial Au substrates, respectively. Nominal thickness and evaporation rate were monitored with a quartzcrystal microbalance. For spin-coating CfA and CnA, they were dissolved in ethanol with concentration of 0.5 wt%. The prepared solutions were spin-casted onto respective substrates at 3000 rpm for 30 s in air.
Device Fabrication and Characterization: The ZnPc single-layer device was constructed on a cleaned Au substrate. CfA film of 5 nm and ZnPc film of 150 nm were sequentially vacuum-deposited at a rate of 1 Å s −1 on the electrode. Evaporating Au of 50 nm completed the device. For the P3HT single-layer device, CB solution of P3HT with a concentration of 30 mg mL −1 was spin-casted at 1000 rpm for 60 s on an ITO substrate covered with a spin-coated CfA layer. The spin-coating process was carried out in a N 2 -filled glove box. The P3HT film was transferred to a vacuum chamber without exposing the sample to air. Evaporating Al of 50 nm then completed the device. J-V characteristics of these devices were measured under vacuum.
Other Characterization: KP measurements were conducted in air with a commercial probe (KP020, KP Technology). The off-null method was used to determine the contact potential difference by the linear interpolation of the output responses at the two backing potentials. [50] WF measurements with KP were carried out for the substrates covered with CfA of various thicknesses. FTIR spectrum of bulk CfA was measured in transmission mode with a spectrometer (FT/IR-6600, JASCO). Powdery CfA was pressed with KBr plates, which were used for the measurement. FTIR-RAS spectra were acquired with the same spectrometer under ambient conditions. The angle of incidence was 85° with respect to the surface normal. The reflected light was detected with a mercury-cadmium-telluride (MCT) detector. The spectra were recorded with accumulation of 128 times at a resolution of 2 cm -1 . Surface morphology of CfA layers was analyzed by AFM (Innova, Veeco) in tapping mode under ambient conditions. UV-visible absorption spectra were collected using a spectrophotometer (V-750, JASCO). The PYS spectra of thin films of caffeic acid and cinnamic acid were measured under a pressure of less than 5 × 10 -3 Pa with a spectrometer (BIP-KV200, Bunkou Keiki).
Theoretical Calculations: Molecular orbital calculations were performed using the GAUSSIAN09 package. DFT calculations of CfA and can molecules were performed using the B3LYP exchange-correlation function and 6-311G(d,p) basis set after structural optimization with the same calculation conditions.