1 Introduction

Transparent electrodes play significant roles in various optoelectronic devices, including solar cells, displays, and light-emitting diodes.^[1] Indium–tin oxide (ITO) is a commonly used transparent electrode.^[2] However, indium is expensive and the mechanical brittleness of ITO makes it unsuitable for flexible and foldable electronics applications.^[3] Thus, extensive studies have targeted alternative conductive materials, such as graphene, carbon nanotubes, and conductive polymers, as potential replacements for ITO.^[4] Among these alternatives, poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) has emerged as a promising material across a wide range of industrial applications.^[5]

PEDOT:PSS has a superior transparency to ITO and is exceptionally flexible and chemically stable. Recently, it has widely been studied for an efficient protection and hole transport layer in hybrid perovskite solar cells.^[6] In addition, PEDOT:PSS is also considered to be a significant material for the commercialization of hybrid perovskite solar cells.^[7] However, pristine PEDOT:PSS films show low conductivities, typically less than 1 S cm⁻¹, because the insulating PSS-rich domains isolate the conductive PEDOT-rich domains.^[8] To address this issue, polar organic solvents, such as dimethyl sulfoxide (DMSO) and ethylene glycol (EG), can be added to the PEDOT:PSS aqueous solution prior to film deposition.^[9] For example, adding DMSO and EG increases the conductivities from a few to 400 and 700 S cm⁻¹, respectively.^[10] In addition, the post-treatment of PEDOT:PSS films with acids is also effective in improving their conductivities.^[11] Kim et al. demonstrated a remarkable improvement to 4380 S cm⁻¹ using H₂SO₄ treatment.^[12]

The post-treatment of PEDOT:PSS films with H₂SO₄ leads to the phase separation of PEDOT and PSS and the reduction of excess unbound PSS. In detail, PEDOT:PSS films exhibit a Coulombic interaction between the positive charge of PEDOT and negative SO³⁻ charge of the PSS unit (Scheme 1a). Treatment with highly concentrated H₂SO₄ induces autoprotolysis that produces H₃SO₄⁺ and HSO₄⁻ ions. They can bind to the negative and positive charges of PSS and PEDOT, respectively, leading to the stabilization of the phase separation of PEDOT and PSS. In addition, the unbound hydrophilic PSS can be washed by the treatment. Consequently, post-treatment facilitates the nanofibrillar formation of PEDOT:PSS in crystallographic and morphological structures (Scheme 1b).^[12] In this process, the coiled benzoid conformation of the PEDOT transforms into the quinoid structure that favors a linear conformation as the concentration of H₂SO₄ increases.^[13] This structural transformation leads to an increase in conductivity owing to the improved charge-transport pathway by dense π–π stacking of PEDOT chains.^[14]

Previous Raman studies have extensively investigated the effect of various material treatments on the structural properties of PEDOT:PSS by identifying specific vibrational modes and peak shifts in Raman spectra.^[15] However, there has been an apparent lack on the microstructural homogeneity of the films, which is crucial to achieve high electrical conductivities in PEDOT:PSS system. In this study, we employed Raman map analysis to unveil detailed spatially resolved information regarding the molecular structures and crystallinities of PEDOT:PSS films, which were treated with various concentrations of H₂SO₄. Our Raman heat maps successfully show that the microstructural uniformity is correlated to the conductivities of the films across the entire measurement area (20 × 20 µm²). These results offer valuable insights for evaluating and optimizing the performance of other conductive thin films.

2 Results

The electrical properties of pristine and H₂SO₄-treated PEDOT:PSS films were investigated. Figure 1 shows the thicknesses and conductivities of PEDOT:PSS films post-treated with different concentrations (0%, 20%, 40%, 60%, 80%, and 100%) of H₂SO₄. The thicknesses of the films treated with 60%, 80%, and 100% H₂SO₄ were 63, 27, and 23 nm, respectively. The film thicknesses decreased with increasing H₂SO₄ concentrations because PSS was separated from PEDOT by H₂SO₄ and washed away by the deionized water, which increases the density of PEDOT in the films.^{[11, 16]} However, the thicknesses of the films treated with 20% and 40% H₂SO₄ were 97 and 101 nm, which were thicker than that of the pristine film (84 nm). This suggests that the observed result is related to the swelling due to the hydrophilic nature of the excess PSS. In addition, water molecules are easily adsorbed on the sulfonate groups of the PSS shell, causing further swelling.^[17] Thus, only a small amount of PSS was removed with low concentrations (≤40%) of H₂SO₄ treatment, and the residual excess PSS in the film caused water uptake and volumetric expansion.^[18] Post-treatment at higher concentrations increased the hydrophobicity of the film, which suppressed the absorption of water. In addition, the effect of H₂SO₄ post-treatment on the surfaces of the films was confirmed with atomic force microscopy (AFM) images (Figure S1, Supporting Information). The phase separation of PEDOT and PSS caused by acid treatment was observed in the surface morphology of the film. As PSS was eliminated by H₂SO₄ (40–80% concentrations), agglomerated particles of PEDOT became visible on the surface.^[19] At an H₂SO₄ concentration of 100%, the film had a relatively smooth surface.

The conductivities of the PEDOT:PSS films treated with 0%, 20%, 40%, 60%, 80%, and 100% concentrations of H₂SO₄ were 0.5, 0.8, 145, 1046, 3323, and 4358 S cm⁻¹, respectively. The conductivities increased with the increase of H₂SO₄ concentration, indicating a more linear π-conjugated PEDOT:PSS structure. At higher H₂SO₄ concentrations (≥80%), the delocalization of PEDOT chains and structural rearrangement into nanofibrils increased the charge-carrier mobility, leading to improved electrical conductivities.^[20] In contrast, the films treated with lower concentrations of H₂SO₄ (≤40%) exhibited lower conductivities. This implies that these lower concentrations of H₂SO₄ are not enough for optimal generation of linearly stacked PEDOT:PSS domains. It leads to the increase of the π–π stacking distance between adjacent PEDOT chains by the swelling, resulting in the lower conductivities.^[21]

Figure 2a shows UV–vis absorption spectra with an absorption band corresponding to the PEDOT oxidation state at ≈800 nm, which is associated with a polaron formation of the PEDOT chain.^[22] Interestingly, the films treated with a high H₂SO₄ concentration (≥80%) showed an increased absorbance at ≈800 nm, presenting a notable enhancement in the polaron density. Furthermore, the intense absorption peak observed at ≈225 nm is attributed to the π–π* transition of the aromatic ring of PSS.^[23] Thus, the absorbance decrease after the treatment with high H₂SO₄ concentrations shows the phase separation of PEDOT:PSS and the decrease in the residual PSS density.

Figure 2b shows the Raman spectra of PEDOT:PSS thin films post-treated with H₂SO₄. The Raman peaks at 1252, 1365, and 1412 cm⁻¹ are assigned to the C_α─C_α’ inter-ring, C_β─C_β’, and C_α═C_β symmetrical stretching vibrational modes of PEDOT, respectively.^[24] The peaks at 1094, 1529, and 1562 cm⁻¹ are the vibrational modes of PSS, showing that Raman intensity of the peak decreases markedly with H₂SO₄ treatment concentrations above 40%. This result shows a significant reduction in the density of the residual excessive PSS domains, and the removal of most unbound PSS results in higher conductivities.^[25]

The band at 1412 cm⁻¹ (C_α═C_β stretching vibrational mode) shows prominent differences in intensity with the H₂SO₄ concentrations. Recent Raman studies on PEDOT have shown that this vibrational mode is sensitive to the linearity of the backbone, which is correlated to the delocalization of π-conjugated electrons in PEDOT.^[14] In other words, the central frequency and full-width at half maximum (FWHM) of the C_α═C_β Raman mode are sensitive to the distribution of structural isomers with different conjugation lengths. Moreover, uniformity plays a critical role in achieving high electrical performances in devices based on PEDOT:PSS films. Thus, to measure the spatial uniformity of the thin film, we constructed Raman maps of the central frequency and FWHM of the C_α═C_β stretching vibrational mode at 1412 cm⁻¹.

Figure 3 shows the Raman central frequency heat maps of the peak at 1412 cm⁻¹ in the PEDOT:PSS films treated with various concentrations of H₂SO₄. They were acquired at 400 microspots (20 × 20 µm²). The pristine film untreated with H₂SO₄ has an average central frequency of 1412 cm⁻¹, indicating the dominance of the coiled conformation (Figure 3b).^[24] A significant redshift was observed with 20% H₂SO₄ at 1407.3 (± 0.04) cm⁻¹ (Figure 3c). This implies that the PEDOT chains have the highest level of structural entanglement, which still suppresses the development of efficient charge-carrier pathway (Figure 1). However, at higher H₂SO₄ concentrations (≥40%), the peaks shift to higher frequencies with increasing concentrations (Figure 3d–g), indicating a transition to an extended coil or linear conformation.^[26] In this regime, the thiophene rings of PEDOT are nearly coplanar, showing that the π-conjugated electrons are highly delocalized throughout the chain, which improves the conductivity.^[27] Interestingly, the films showed uniform Raman shifts over the entire area, corresponding to each H₂SO₄ concentration. This suggests that the initial structural homogeneity cannot be significantly affected by the H₂SO₄ treatment.

Figure 4 shows Raman FWHM heat maps, which visually provide the distributions of structural conformations induced by the H₂SO₄ post-treatment. In Figure 4b–d, the widths of the films treated with 20% and 40% H₂SO₄ were uniformly measured to be ≈79 (± 0.05) and ≈77 (± 0.06) cm⁻¹, respectively, which are broader than that of the pristine film (≈74 (± 0.05) cm⁻¹). A broader FWHM indicates a larger distribution of isomers or crystalline disorder.^[28] Consequently, these findings present reduced crystallinities compared to that of the pristine film, which can be attributed to the swelling of PSS. Because the films treated with low H₂SO₄ concentrations (≤40%) still contain excessive PSS domains, they can cause further structural disorder by absorbing water.

However, above 40% concentrations, the linewidths correspondingly become narrow as the concentration increases (Figure 4e–g). Notably, the narrowest linewidth was measured to be 69.6 (± 0.06) cm⁻¹ at 100% H₂SO₄. In this configuration, the smallest amount of PSS acts as a counter ion to acid-treated PEDOT, resulting in highly crystallized nanofibrils and, thus, enhanced conductivities.^[12] Our Raman FWHM maps successfully demonstrate that the quinoid transformation of PEDOT:PSS occurs uniformly across the entire measurement area by H₂SO₄ treatment, as evidenced by the narrower Raman widths observed.

Figure 5 shows the representative central frequencies and FWHMs, as well as the standard errors averaged over the area, which are very small, indicating the high spatial uniformities in 20 × 20 µm² in all the samples. The central frequencies were measured to be 1412.3 (± 0.04), 1407.3 (± 0.03), 1420.7 (± 0.04), 1421.1 (± 0.05), 1433.9 (± 0.05), and 1438.6 (± 0.06) cm⁻¹ at the H₂SO₄ concentrations of 0%, 20%, 40%, 60%, 80%, and 100%, respectively. Compared to the pristine film, the 20% concentration of H₂SO₄ exhibited the frequency downshift, showing the fragmented and insufficient π-conjugation delocalization of PEDOT. The highest frequency is observed at 100% H₂SO₄, representing extended coil or linear conformations with higher carrier mobilities compared with that of the coiled conformation of pristine PEDOT:PSS.^[20] The widths for H₂SO₄ concentrations of 0%, 20%, 40%, 60%, 80%, and 100% are 74.0 (± 0.05), 79.2 (± 0.05), 76.6 (± 0.06), 73.1 (± 0.05), 72.9 (± 0.04), and 69.6 (± 0.06) cm⁻¹, respectively. The increased linewidth at 20% clearly shows the structural disorder and swelling of the PEDOT:PSS domains by the low-concentration H₂SO₄ treatment. Notably, the central frequency and width plots show similar decreasing patterns as the concentration increases to over 40%. The narrowest linewidth is observed at the H₂SO₄ concentration of 100%, suggesting a highly improved crystallinity. The H₂SO₄-induced higher carrier mobilities and crystallinities contribute to the enhanced conductivities.^{[15, 28]}

3 Conclusions

In summary, we investigated the effect of H₂SO₄ post-treatment on the structure of PEDOT:PSS and the electrical properties using Raman map analysis. The conductivity of the PEDOT:PSS thin film was significantly enhanced from 0.5 to 4358 S cm⁻¹ by treatment with H₂SO₄. The highest frequency and narrowest width of the C_α═C_β Raman bands were observed with 100% H₂SO₄ treatment, which indicated that the PEDOT chains were delocalized. In addition, an improved crystallinity was noted over the entire measurement area. However, the use of low H₂SO₄ concentrations (<40%) resulted in broad Raman peaks and low conductivities because of disordered and swelled coil structures of PEDOT:PSS. Based on the high significance of the PEDOT:PSS system microstructures, comprehensive Raman map analysis can be easily utilized to clarify microstructural properties of π-conjugated polymers. In addition, Raman map analysis for PEDOT:PSS layer can be employed to improve the performance of next-generation perovskite solar cells and other advanced electronic devices.

4 Experimental Section

Conflict of Interest

The authors declare no conflict of interest.