Film uniformity in PEDOT:PSS coatings: why it matters and how to achieve it
What is PEDOT:PSS and why does it matter?
Poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate), universally written as PEDOT:PSS, has become one of the most important conductive polymers in printed and flexible electronics. Its combination of tunable electrical conductivity, high optical transparency, mechanical flexibility, and aqueous processability makes it the material of choice across a broad range of next-generation device platforms: organic photovoltaics (OPV), perovskite solar cells, organic light-emitting diodes (OLEDs), electrochromic devices, flexible gas sensors, and more1.
Commercially available as a water-based dispersion, PEDOT:PSS is typically deposited as a thin transparent electrode or charge-transport interlayer.
As device architectures scale toward larger substrates for pilot and commercial manufacturing, one parameter becomes critically important: film uniformity.
Figure 1: Chemical composition of PEDOT and PSS structures.
The critical role of coating uniformity
In any multilayer optoelectronic device, PEDOT:PSS functions as a hole-transport or electron-blocking interlayer. Its sheet resistance and optical transmittance are dependent on film thickness. A non-uniform film means spatially varying electrical properties, which translate into non-uniform current extraction, localised recombination losses, and ultimately reduced device efficiency and lifetime across the active area.
Two axes of uniformity must be controlled independently and simultaneously:
Cross-web uniformity (the width direction) is primarily governed by fluid dynamics during coating and drying. Variations here arise from edge effects in the meniscus, non-uniform airflow during drying, or pressure non-uniformities across the coating die. In slot-die processes, any variation in internal manifold and slot pressure distributes directly into the wet-film thickness profile 2.
Longitudinal uniformity (the deposition direction) is governed by coating speed stability, pump pressure fluctuations, and the transient flow behaviour at the start and end of a coating pass. Periodic thickness ripple (so-called “chatter”) can emerge from mechanical vibrations or from instabilities in the wet film before it stabilises on the substrate2.
The challenge of low-viscosity formulations
Standard PEDOT:PSS dispersions are inherently low-viscosity fluids, typically in the range of 5–30 mPa·s — low enough to introduce distinctive coating challenges.
Low-viscosity fluids are prone to ribbing instability, where the wet film spontaneously develops periodic thickness striations in the cross-web direction. Gravitational sagging and meniscus instability at the coating bead also become more pronounced when both surface tension and viscous forces are low. As a consequence, the processing window for stable coating narrows considerably.
Controlling these effects requires precise matching of coating gap, speed, and fluid flow rate, combined with well-engineered drying conditions that lock in the film morphology before redistribution can occur. This is where equipment engineering and design become decisive.
The FOM alphaSC: engineering for precision
The FOM alphaSC is FOM’s sheet-to-sheet slot-die coating platform, designed to bridge the gap between laboratory-scale research and pilot-scale production. It has been adopted by academic groups and industrial R&D labs worldwide and sits behind a growing body of peer-reviewed publications spanning organic solar cells, perovskite photovoltaics, OLEDs, and flexible sensors — applications that demand a level of reproducibility and process control that spin coating, spray coating and blade coating are not well suited to deliver at scale3, 4, 5, 6.
Several design choices built into the alphaSC’s software and hardware are directly responsible for the uniformity results described in this article.
Achieving and assessing uniformity: approach and methodology
To assess the capabilities of the machine, coating trials are performed on 300 × 300 mm² glass and flexible PET substrates using low-viscosity PEDOT:PSS formulations.
Quantifying film homogeneity over a large area requires a systematic measurement strategy. For PEDOT:PSS films, the primary techniques are spectroscopic ellipsometry, four-point probe sheet resistance mapping, and profilometry. In our case, full-area thickness mapping via diamond-stylus-based profilometry confirmed that the process consistently achieves a relative standard deviation below 5% in both the cross-web and longitudinal directions. Thickness measurements are taken at five areas distributed across the surface, as shown in Fig. 2b.
Uniformity is reported as the relative standard deviation of all thickness measurements across the full area. This metric is directly relevant to device performance, since the thickness standard deviation maps one-to-one onto the spatial variation in sheet resistance of the films.
Figure 2: a) distribution of thickness over the 5 points of measure (positions A through E). The corresponding positions are shown on the image on the right. b) Picture of the AlphaSC right after the coating of a PEDOT:PSS layer. Marked in red the 5 positions being measured through profilometry.
No systematic thickness gradient is detected along the machine direction, confirming stable fluid delivery and constant coating speed throughout the pass. Cross-web profiles show consistent flatness, with no evidence of ribbing instability or edge-bead effects within the active coating zone — both common failure modes when coating low-viscosity aqueous dispersions on large substrates.
In device terms: a <5% standard deviation over a 200 nm target thickness means a tolerance below ±10 nm across the entire coated area. For researchers and R&D teams targeting OPV, perovskite, or OLED formats, this level of spatial consistency is a prerequisite for device yield and efficiency uniformity.
Conclusion
Film uniformity in PEDOT:PSS coatings is not a secondary concern but a primary device-performance parameter. Spatial variation in thickness maps directly onto variation in sheet resistance, optical transmission, and charge-transport efficiency — all of which determine device yield and efficiency uniformity in large-area organic electronics.
Using the FOM alphaSC, we have demonstrated that thickness uniformity below 5% relative standard deviation across 300 × 300 mm² substrates is consistently achievable with low-viscosity PEDOT:PSS formulations. This result reflects an integrated engineering approach — precision fluid metering, optimised die geometry, uniform drying, and vibration-isolated substrate transport — all built into the alphaSC by design.
For researchers scaling PEDOT:PSS-based devices, and for R&D teams that need reproducibility across instruments and labs, the FOM alphaSC provides the process control to support that work.
References
1. Fan, X., et al. (2019). PEDOT:PSS for flexible and stretchable electronics: modifications, strategies, and applications. Advanced Science, 6(19), 1900813. https://doi.org/10.1002/advs.201900813
2. Schweizer, P. M., & Kistler, S. F. (Eds.) (2012). Liquid Film Coating: Scientific Principles and Their Technological Implications. Springer.
3. Ávila Ramírez, A. E., van der Laan, D. P., Shah, M. B., Wang, L., Zeglio, E., & Savva, A. (2026). PEDOT:PSS — A key material for bioelectronics. Advanced Science, 13(11), e13480. https://doi.org/10.1002/advs.202513480
https://doi.org/10.1002/advs.202513480
4. Ramos Canabarra dos Santos, T., et al. (2025). Gas sensor based on highly effective slot-die printed PEDOT:PSS@ZnO hybrid nanocomposite for methanol detection. ACS Applied Materials & Interfaces, 17(9), 13065–13073. https://doi.org/10.1021/acsami.4c03131
5. Cai, W., Österberg, T., Ingänäs, O., et al. (2020). Dedoping-induced interfacial instability of poly(ethylene imine)s-treated PEDOT:PSS as a low-work-function electrode. Journal of Materials Chemistry C, 8, 328–336. https://doi.org/10.1039/C9TC05018C
6. Jayaraman, E., et al. (2025). Slot-die coated bulk heterojunction vs layer-by-layer organic photovoltaics: device architecture dependent degradation. APL Energy, 3, 036101. https://doi.org/10.1063/5.0241898
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