Abstract
Poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is known for its potential to replace indium–tin oxide in various devices. Herein, when fabricating finger-type PEDOT:PSS electrodes using conventional photolithography, the cross-sectional profiles of the patterns are U-shaped instead of rectangular. The films initially suffer from non-uniformity and fragility as well as defects owing to undesirable patterns. Adding a small amount of hydrolyzed silane crosslinker to PEDOT:PSS suspensions increases the mechanical durability of PEDOT:PSS patterns while lifting off the photoresist. To further improve their microfabrication, we observe the effects of two additional oxygen (O2) plasma treatments on conventional photolithography processes for patterning PEDOT:PSS, expecting to observe how O2 plasma increases the uniformity of the patterns and changes the thickness and U-shaped cross-sectional profiles of the patterns. Appropriately exposing the patterned photoresist to O2 plasma before spin-coating PEDOT:PSS improves the wettability of its surface, including its sidewalls, and a similar treatment before lifting off the photoresist helps partially remove the spin-coated PEDOT:PSS that impedes the lift-off process. These two additional processes enable fabricating more uniform, defect-free PEDOT:PSS patterns. Both increasing the wettability of the photoresist patters before spin-coating PEDOT:PSS and reducing its conformal coverage are key to improving the photolithographic microfabrication of PEDOT:PSS.
Highlights
On one hand, finger-type indium– tin oxide (ITO) electrodes [12], which mitigate the absorption of light via ITO, have been fabricated by patterning flat ITO electrodes
We attempted to fabricate finger-type electrodes using PEDOT:PSS; the films suffered from non-uniformity and fragility as well as defects owing to undesirable patterns
We aim to tackle the aforementioned problems, i.e., the non-uniformity and fragility of the films as well as defects owing to undesirable patterns, and to improve the quality of photolithographic PEDOT:PSS patterns
Summary
Advancing terahertz (THz) technology [1,2] requires developing many functional quasi-optical components for THz devices such as phase shifters [3,4,5,6,7,8], filters [9], phase gratings [10], and polarizers [11]; most of these device components use indium– tin oxide (ITO) or metal electrodes, the transmittance of which is not necessarily high enough for THz devices.To improve transmittance characteristics, on one hand, finger-type ITO electrodes [12], which mitigate the absorption of light via ITO, have been fabricated by patterning flat ITO electrodes. Materials with higher transmittance in the THz region such as graphene [13] are actively being attempted to replace ITO For this purpose, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) [14,15,16,17,18,19] is a promising candidate, and patterning PEDOT:PSS as a microfabrication strategy has been well studied. PEDOT:PSS exhibits high transmittance in the THz region, and because of its solution-process capability, it can be patterned using photolithography. Under these circumstances, replacing the finger-type ITO electrodes with patterned PEDOT:PSS ones would attain even higher transmittance in the THz region. We attempted to fabricate finger-type electrodes using PEDOT:PSS; the films suffered from non-uniformity and fragility as well as defects owing to undesirable patterns
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