Tunable Energy-Level Alignment in Multilayers of Carboxylic Acids on Silver

Precise energy-level alignment between a metal electrode and an organic semiconductor is required to reduce contact resistance and enhance the efficiency of organic-semiconductor-based devices. Here, we introduce monolayer-thick charge-injection layers (CILs) based on aromatic carboxylic acids that can induce an energy-level shift in subsequent layers by up to 0.8 eV. Through a gradual chemical transformation of the as-deposited molecules, we achieve a highly tunable energy-level shift in the range of 0.5 eV. We reveal that the work function and energy-level positions in the CIL increase linearly with the density of induced dipoles. The energy-level position of subsequent layers follows the changes in the CIL. Our results thus connect the energy-alignment quantities, and the high tunability would allow precise tuning of the active layers deposited on the CIL, which marks a path towards efficient charge-injection layers on metal electrodes.

Figure 1

Molecule and CILs employed in this study. (a) Chemical structure of BDA. (b) Partially and fully deprotonated BDA has a similar structure but largely differing properties compared to intact BDA. (c) STM images of distinct CILs considered in this study. Degree of deprotonation, $\mathcal{D}$_, is given for each phase. Quadrangles mark the BDA unit cells (note that there is no unit cell for the ${\alpha }_{(001)}$ phase). Density of BDA molecules, $n_{\mathrm{BDA}}\mathrm{per}\mathrm{n}{\mathrm{m}}^2$, of every CIL is given in the bottom-right corner.

Figure 2

Photoelectron-spectroscopy analysis of the BDA CILs. (a), (b) $\mathrm{C}$ 1s spectra measured by synchrotron radiation with an energy of 420 eV from samples comprising a full layer of BDA deposited on $\mathrm{Ag}(001)$ substrates, that is, ${\alpha }_{(001)}$ and ${\dot{\alpha }}_{(001)}$ CILs. Peak components are marked by the associated chemical states; the full description and peak parameters for all CILs are given in the Supplemental Material, Secs. 1 and 2 [38]. Vertical lines mark the shift of the peaks discussed in the text. (c), (d) $\mathrm{O}$ 1s spectra (670 eV) of ${\alpha }_{(001)}$ and ${\dot{\alpha }}_{(001)}$ CILs. (e) Wide spectra and details of the Fermi edge measured on samples with ${\alpha }_{(001)}$ and ${\dot{\alpha }}_{(001)}$ CILs used for the determination of the WF. Full data for all CILs are given in the Supplemental Material, Sec. 3 [38].

Figure 3

(a) Degree of deprotonation of carboxyl groups obtained during gradual annealing of the sample with 0.8 monolayer (ML) of BDA on $\mathrm{Ag}(001)$ up to 200 °C. After each annealing step, heating is turned off and the spectra are measured. Black line indicates the temperatures at which the degree of the deprotonation only slightly changes with temperature; we associate this region with the ${\beta }_{(001)}$ phase. (b) Change of work function as a function of the degree of deprotonation. Red horizontal line marks the measured substrate work function (4.36 eV). Two last points are given in gray as there is a loss of BDA molecules due to desorption.

Figure 4

(a)–(c) Calculated unit cells for the $\alpha$, $\dot{\alpha }$, and $\delta$ phases. Cyan, black, red, and white spheres represent $\mathrm{Ag},\mathrm{C},\mathrm{O}$, and $\mathrm{H}$ atoms, respectively. The orange circle marks an extra hydrogen atom in the $\alpha$ phase. (d)–(f) Averaged electron-density difference over the x-y plane and charge-transfer functions for the $\alpha$, $\dot{\alpha }$, and $\delta$ phases, respectively. Black horizontal line represents the calculated center of a dipole [see Eq. (3)]. (g)–(i) Charge-density redistribution in the $\alpha$, $\dot{\alpha }$, and $\delta$ phases, respectively, induced by the interaction between the silver substrate and the molecular layer. Red arrow marks the shift of the deprotonated carboxylic group towards the silver substrate.

Figure 5

Dependence of work function on the position of the $\mathrm{C}$ 1s component associated with the phenyl rings for the BDA CILs marked at each point.

Figure 6

Photoelectron-spectroscopy analysis of the second layer of BDA on the CILs. (a), (b) $\mathrm{C}$ 1s spectra measured by synchrotron radiation with an energy of 420 eV from samples comprising 0.7 ML of BDA deposited on the ${\alpha }_{(001)}$ and ${\dot{\alpha }}_{(001)}$ CILs, respectively. Peak components are marked accordingly. Full description and peak parameters are given in the Supplemental Material, Secs. 1 and 2 [38]. (c), (d) $\mathrm{O}$ 1s spectra (670 eV) of 0.7 ML of BDA deposited on the ${\alpha }_{(001)}$ and ${\dot{\alpha }}_{(001)}$ CILs, respectively. Vertical lines mark the shift of the peaks discussed in the text. Positions of the second-layer peaks as a function of the CIL carbonyl $\mathrm{O}$ 1s position (e), the CIL phenyl $\mathrm{C}$ 1s position (f), and the second-layer $\mathrm{O}$ 1s position (g). Gray point for the ${\delta }_{(001)}$ phase marks an incompletely formed layer. Energy scale is the same on both axes in each panel. Blue value denotes the slope of the linear fit (black line).

Figure 7

Summary of energy-level positions determined for the studied CILs in an energy-level diagram; vacuum level, $E_V$; Fermi level, $E_F$; $\mathrm{C}$ 1s core-level positions of BDA in the CIL and the second layer; and WF shifts, $\mathrm{\Delta }\phi$, induced by the CILs are shown. Blue lines present the measured work functions with respect to the bare substrate (mean literature values [53] are taken for the substrate). Red and green lines mark the positions of the $\mathrm{C}$ 1s peak of BDA in the CILs (red lines) and second layers (green lines).

Figure 8

Fitting of the $\mathrm{O}$ 1s XPS spectra measured during 2500-min-long x-ray exposure. Peaks associated with the ${\alpha }_{(001)}$ and ${\dot{\alpha }}_{(001)}$ CIL (the first layer) and the second layer are marked accordingly.

Figure 9

(a) Shift of the second-layer $\mathrm{O}$ 1s peak as a function of the degree of deprotonation of the CIL. Variable degree of deprotonation is achieved during the isostructural ${\alpha }_{(001)}$ to ${\dot{\alpha }}_{(001)}$ transformation. (b) Work-function change determined from the shift of the second-layer $\mathrm{O}$ 1s peak as a function of electric dipole density, $n_{\mathrm{dip}}$, during the isostructural ${\alpha }_{(001)}$ to ${\dot{\alpha }}_{(001)}$ transformation. Red line is a linear fit of the initial region, and blue curve is the full model given by Eq. (6).