Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), commonly known as PEDOT:PSS, stands out as a crucial material in the realm of Electronic Transport, particularly for applications in organic electronics. Despite its widespread use, the intricate relationship between its microstructure and electronic transport properties remains a subject of intense research. Traditional methods for structural analysis, such as electron or X-ray techniques, often fall short due to the low-density contrast within PEDOT:PSS films. However, advancements in synchrotron radiation and resonant Soft X-ray Scattering (rSoXS) have opened new avenues for detailed multi-scale structural investigations, providing unprecedented insights into electronic transport phenomena within these films.
Delving into the Microstructure of PEDOT:PSS for Enhanced Electronic Transport
Wide-angle Grazing Incidence X-ray Scattering (GIXS) emerges as a powerful technique to probe the molecular arrangement in PEDOT:PSS films, especially concerning aggregation and crystallization, which are pivotal for efficient electronic transport. Analysis using GIXS allows us to observe how the addition of ethylene glycol (EG) as a co-solvent during film preparation influences the molecular packing. It’s crucial to note that ideally, these dispersion co-solvents should be effectively removed from the film after formation through annealing and vacuum treatments, ensuring they don’t interfere with the intrinsic electronic transport properties of PEDOT:PSS.
Figure 2a from the original study, along with Supplementary Fig. 1, vividly illustrates the alterations in wide-angle scattering patterns as EG co-solvent concentration is varied. A notable enhancement in the π-stacking scattered intensity at a scattering vector (Q) of 1.83 Å⁻¹ is observed with increasing EG content. This is further substantiated by a sharpening of the peak width, indicating a growth in coherence length from approximately 1 to 3 nm. This change suggests either an increase in the size of crystallites or aggregates, or a higher degree of order within these aggregates, both of which are beneficial for improved electronic transport pathways. Furthermore, the data suggests a tendency for π-stacked aggregates to adopt a face-on texture, aligning with previous research in the field.
To further explore the meso-scale structure, which significantly impacts electronic transport at a larger dimension, Carbon-edge rSoXS is employed. This technique leverages the contrast between PEDOT and PSS components within the film to reveal domain morphology. By determining optical constants from PEDOT:Cl and Na:PSS films, researchers can effectively isolate the signals relevant to the polymeric species, crucial for understanding electronic transport at the molecular level.
Figure 2b presents the rSoXS profiles obtained in transmission geometry at two key energies: the pre-edge energy (270 eV) and the C 1s to π* resonance (285.1 eV). The scattering intensity at low Q values (Q⁻¹), corresponding to larger features (>600 nm), remains consistent across different incident energies, indicating it primarily arises from vacuum contrast, i.e., film roughness. However, a scattering feature in the Q≅0.1 nm⁻¹ range is significantly enhanced at resonance. This enhancement is attributed to the material contrast between PEDOT and PSS, making it a valuable indicator of the bulk meso-scale domain morphology, directly influencing the bulk electronic transport properties. The upturn at the highest Q values is mainly due to X-ray fluorescence.
Previous studies suggest a film morphology characterized by closely packed PEDOT-rich domains within a PSS-rich matrix. To quantify how the average meso-scale domain size and purity—factors directly affecting electronic transport—vary with EG concentration, the rSoXS features are analyzed using a Lorentz plot fitting method. The peak intensity scales linearly with EG content, suggesting a substantial increase in domain purity or volume fraction, which is vital for enhancing electronic transport pathways. The peak position, as shown in Figure 2c, indicates domain spacing and reveals that particle size increases from 16 to 42 nm with increasing EG concentration, consistent with other reported size scales. These findings collectively point towards an increase in film heterogeneity with EG addition, rather than a homogenizing effect, suggesting a refined domain structure more conducive to electronic transport.
Compositional analysis using transmission Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy corroborates these structural findings. NEXAFS reveals minimal variation in average film composition, confirming the effective removal of EG from the films. The PEDOT to PSS content ratio is found to be around 1:1.5 to 1:1.8 by weight, slightly lower than the initial ratio of 1:2.5, but without a consistent trend with EG concentration. The increased scattering intensity is therefore attributed to enhanced contrast between scatterers, confirming domain purification due to the co-solvent, which is a key factor in optimizing electronic transport.
Combining NEXAFS and rSoXS data allows for an estimation of the absolute PEDOT concentration in both PEDOT:PSS-rich phases and the surrounding PSS-rich matrix (Figure 2c). The analysis reveals that neither phase is entirely pure. As EG content increases, the PEDOT concentration in the gel particles increases mildly, while it decreases in the matrix. This suggests that within these domains, PEDOT and PSS chains may alternate, rather than forming pure PEDOT crystals, influencing the mechanisms of electronic transport.
Ionic Transport Dynamics and its Interplay with Electronic Transport
To gain a holistic understanding of transport phenomena, it is essential to consider ionic transport alongside electronic transport. Ionic transport in PEDOT:PSS films was investigated using a one-dimensional ‘moving front’ experiment. This method tracks the visible light transmission changes in an electrochromic film as it undergoes doping/de-doping via lateral ion injection from an electrolyte junction. By monitoring the time-dependent position of this moving front, the drift mobility of ions can be determined, providing crucial insights into the ionic component of the overall transport behavior, which is indirectly linked to electronic transport properties especially in mixed conductors.
Figure 3a illustrates the experimental setup, while Figure 3b shows the temporal characteristics of the spatial moving front profiles. The effective mobility of potassium ions (K⁺) is found to decrease with increasing EG concentration (Figure 3c), likely due to the meso-scale structural changes induced by EG.
Interestingly, a detailed analysis of the moving front profiles reveals a more complex picture. The derivative of these profiles indicates the presence of not one, but two moving fronts: a ‘leading’ front and a ‘lagging’ front (Figure 3b). The lagging front becomes more prominent with higher EG content. Spectro-temporal analysis further elucidates the nature of these fronts. By monitoring the time-resolved ion motion through local microstructure using absorption spectroscopy, it is observed that the leading front is associated with ion movement through disordered PEDOT regions (likely in the PSS-rich matrix), while the lagging front corresponds to ion penetration into aggregate-rich domains within the PEDOT:PSS cores. This differential ionic transport behavior is intricately linked to the microstructure and consequently, influences the overall electronic transport.
Figure 4 details the spectro-temporal characteristics, showing how the absorption spectra evolve as ions move through the PEDOT:PSS film. The persistence of vibronic absorption features in EG-rich samples (Figure 4g) further confirms the presence of the lagging front and highlights the role of aggregates in dictating effective ion mobility, and by extension, affecting the mixed ionic-electronic transport properties of PEDOT:PSS.
Implications for Electronic Devices: Balancing Electronic and Ionic Transport
The interplay between electronic and ionic transport in PEDOT:PSS films has profound implications for device performance, particularly in Organic Electrochemical Transistors (OECTs). Figure 5a demonstrates the significant improvement in electrical conductivity with the addition of EG, reaching a plateau beyond 10 vol%. However, ionic mobility decreases with increasing EG content.
Interestingly, Figure 5b reveals that neither maximum electrical conductivity nor maximum ionic mobility alone yields optimal OECT performance. The highest transconductance (gm) is achieved at 5 vol% EG, indicating a crucial balance between efficient electronic and ionic transport is necessary for superior device operation. Formulations with 0 vol% and 50 vol% EG show significantly lower gm, despite having either the highest ionic mobility or highest electrical conductivity, respectively.
This underscores that for optimal device performance in mixed conducting materials like PEDOT:PSS, it is not sufficient to simply maximize either electronic or ionic transport in isolation. Instead, a delicate balance and synergistic interaction between these two transport mechanisms, dictated by the film’s microstructure, is paramount. The morphological changes induced by EG, summarized schematically in Figures 5c and 5d, directly influence both electronic and ionic pathways, ultimately determining the overall device efficiency.
Conclusion: Microstructure as the Key to Optimizing Electronic Transport in PEDOT:PSS
This comprehensive analysis underscores the critical role of microstructure in governing both electronic and ionic transport in PEDOT:PSS films. By employing advanced techniques like synchrotron radiation-based X-ray scattering and moving front experiments, we have gained deeper insights into how co-solvents like EG modify the film morphology and consequently, the transport properties. The findings highlight that optimizing electronic transport in PEDOT:PSS for advanced electronic devices requires a nuanced approach that considers the interplay between meso-scale domain structure, ionic mobility, and electronic conductivity. Future research should focus on further tailoring the microstructure of PEDOT:PSS to achieve an even finer balance between these properties, paving the way for next-generation organic electronic devices with enhanced performance and efficiency.
