Photoelectron Spectroscopy Reveals the Impact of Solvent Additives on Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) Thin Film Formation

Abstract

The conductive polymer poly­(3,4-ethylenedioxythiophene):poly­(styrenesulfonate) (PEDOT:PSS) is used in a manifold of electronic applications, and controlling its conductivity is often the key to attain a superior device performance. To that end, solvent additives like Triton, ethylene glycol (EG), or dimethyl sulfoxide (DMSO) are regularly incorporated. In our comprehensive study, we prepare PEDOT:PSS thin films with seven different additive combinations and with thicknesses ranging from 6 to 300 nm on indium-tin-oxide (ITO) substrates. We utilize X-ray photoelectron spectroscopy (XPS) to access the PSS-to-PEDOT ratio and the PSS–-to-PSSH ratio in the near-surface region and ultraviolet photoelectron spectroscopy (UPS) to get the work function (WF). In addition, the morphology and conductivity of these samples are obtained. We found that the WF of the prepared thin films for each combination becomes saturated at a thickness of around 50 nm and thinner films show a lower WF due to the inferior coverage on the ITO. Furthermore, the WF shows a better correlation with the PSS–-to-PSSH ratio than the commonly used PSS-to-PEDOT ratio as PSS– can directly affect the surface dipole. By adding solvent additives, a dramatic increase in the conductivity is observed for all PEDOT:PSS films, especially when DMSO is involved. Moreover, adding the additive Triton (surfactant) helps to suppress the WF fluctuation for most films of each additive combination and contributes to weaken the surface dipole, eventually leading to a lower and thickness-independent WF.

Surface Functionalization with (3-Glycidyloxypropyl)trimethoxysilane (GOPS) as an Alternative to Blending for Enhancing the Aqueous Stability and Electronic Performance of PEDOT:PSS Thin Films

“…Furthermore, the P@GOPS film had a lower PSS:PEDOT ratio (∼2.2:1) compared to P:GOPS films (∼2.9:1). As the films were not immersed in water before the measurements, we believe that some excess PSS was removed from P@GOPS during the spin-coating process due to the absence of GOPS cross-linker in its formulation . But, since the devices operate in water and were washed by immersion in water before electronic characterization, we also performed XPS measurements after immersing the films for 2 h in deionized water (Figures a and S5b).…”

Thermally Induced Phase Separation of the PEDOT:PSS Layer for Highly Efficient Laminated Polymer Light-Emitting Diodes

“…Notably, the S 2p XPS signals from the bare PEDOT:PSS films at 100 and 200 °C are almost identical, whereas the sample with a T anneal of 300 °C exhibits an increase in the intensity of the PEDOT signal along with a shift in the peak positions to a lower BE due to the carbonization of the PSS (top row, Figure b). ,,, While a similar effect is observed for the LTE at 300 °C (bottom row, Figure b), the annealing process has also introduced noticeable variations in both the maximum position of the PSS peak and the relative intensity of the PEDOT peaks at 100 and 200 °C for the LTE. For a more detailed analysis, the XPS spectra have been fitted to the asymmetric S 2p doublets corresponding to S 2p 1/2 and S 2p 3/2 , which can include contributions from both the PEDOT and PSS chains. − In particular, the PSS chains exhibit two kinds of S 2p doublets due to the neutral PSSH on the one hand and the BE shift from the strong localized charge of the PSS – chains on the other . By integrating the fitted lines in Figure b, we can obtain the peak areas for PSSH, PSS – , and PEDOT (which were proportional to the total content of each species) (Table ).…”

“…As mentioned, the dried PEDOT:PSS film consists of a nanosized granular structure with PEDOT- and PSS-rich phases. − In addition, it is known that the work function of a PEDOT:PSS film can be tuned by changing the content ratio of PSS to PEDOT. − Hence, the changes in the work function of the as-fabricated LTEs depending on the T anneal can be elucidated by the XPS results shown in Figure b. Here, the sulfur S 2p XPS signals at binding energy (BE) values of around 160–172 eV for the hole-injecting AI-4083 on glass are presented in the top row, and those of the LTE are presented in the bottom row. ,, In each case, the specific S 2p peaks of PEDOT at around 163 eV and PSS at around 168 eV are well-distinguishable owing to the chemical shifts of the S atoms in the aromatic backbone of PEDOT and the sulfonate groups of PSS. ,,, This makes it possible to estimate the variations in the contents of PEDOT and PSS near the surface. Notably, the S 2p XPS signals from the bare PEDOT:PSS films at 100 and 200 °C are almost identical, whereas the sample with a T anneal of 300 °C exhibits an increase in the intensity of the PEDOT signal along with a shift in the peak positions to a lower BE due to the carbonization of the PSS (top row, Figure b). ,,, While a similar effect is observed for the LTE at 300 °C (bottom row, Figure b), the annealing process has also introduced noticeable variations in both the maximum position of the PSS peak and the relative intensity of the PEDOT peaks at 100 and 200 °C for the LTE.…”

“…Here, the sulfur S 2p XPS signals at binding energy (BE) values of around 160–172 eV for the hole-injecting AI-4083 on glass are presented in the top row, and those of the LTE are presented in the bottom row. ,, In each case, the specific S 2p peaks of PEDOT at around 163 eV and PSS at around 168 eV are well-distinguishable owing to the chemical shifts of the S atoms in the aromatic backbone of PEDOT and the sulfonate groups of PSS. ,,, This makes it possible to estimate the variations in the contents of PEDOT and PSS near the surface. Notably, the S 2p XPS signals from the bare PEDOT:PSS films at 100 and 200 °C are almost identical, whereas the sample with a T anneal of 300 °C exhibits an increase in the intensity of the PEDOT signal along with a shift in the peak positions to a lower BE due to the carbonization of the PSS (top row, Figure b). ,,, While a similar effect is observed for the LTE at 300 °C (bottom row, Figure b), the annealing process has also introduced noticeable variations in both the maximum position of the PSS peak and the relative intensity of the PEDOT peaks at 100 and 200 °C for the LTE. For a more detailed analysis, the XPS spectra have been fitted to the asymmetric S 2p doublets corresponding to S 2p 1/2 and S 2p 3/2 , which can include contributions from both the PEDOT and PSS chains. − In particular, the PSS chains exhibit two kinds of S 2p doublets due to the neutral PSSH on the one hand and the BE shift from the strong localized charge of the PSS – chains on the other .…”

Energy-Level Alignment Governs Doping-Related Fermi-Level Shifts in Polymer Films

“…In the UPS data of the doped films, the tailing states of the HOMO do indeed reach E F (Figure b). In such a way, the highly doped P3HT:F4TCNQ films resemble common conductive polymers such as poly­(3,4-ethylenedioxythiophene):poly­(styrenesulfonate) (PEDOT:PSS), which are frequently used as electrode materials. , In this case, the IE and EA of the adsorbate equal the WF, which can be tuned for P3HT:F4TCNQ by the doping ratio (Figure and Table ). The tunable IE (WF for high dopant loads) is due to the concomitant occurrence of CPX- and IPA crystallites in the P3HT:F4TCNQ films: The CPX crystallites have a larger IE than undoped P3HT and increase the measured VL ( cf .…”