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

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.

Automated summary

In this study, PEDOT:PSS thin films with seven different additive combinations were prepared and their properties were analyzed. It was found that the work function of the thin films became saturated at a thickness of around 50 nm, and thinner films exhibited lower work function due to incomplete coverage on the ITO substrate. The PSS-to-PSSH ratio showed better correlation with the work function than the commonly used PSS-to-PEDOT ratio. Solvent additives dramatically increased the conductivity of the PEDOT:PSS films, especially when DMSO was involved. Additionally, the surfactant Triton helped to suppress the work function fluctuation and weaken the surface dipole, resulting in a lower and thickness-independent work function.