A Cost-Effective Strategy to Modify the Electrical Properties of PEDOT:PSS via Femtosecond Laser Irradiation

Abstract

Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) is a commonly used conductive polymer in organic optoelectronic devices. The conductivity and work function of the PEDOT:PSS are two important parameters that significantly determine the performance of the associated optoelectronic device. Traditionally, some solvents were doped in PEDOT:PSS solution or soaked in PEDOT:PSS film to improve its electrical conductivity, but they damaged the integrity of PEDOT:PSS and reduce the film’s work function. Herein, for the first time, we use femtosecond laser irradiation to modify the electrical conductivity and work function of PEDOT:PSS film. We proposed that the femtosecond laser irradiation could selectively remove the superficial insulative PSS, thereby improving the electrical conductivity of the film. The femtosecond laser-irradiated PEDOT:PSS film was further employed as a hole injection layer within cutting-edge perovskite light-emitting diodes (PeLEDs). A maximum luminosity of 950 cd/m² was obtained in PeLEDs irradiated by femtosecond laser light in thin films, which is five times higher than that of the controlled device. Moreover, the external quantum efficiency of the devices was also increased from 4.6% to 6.3%. This work paved a cost-effective way to regulate the electrical properties of the PEDOT:PSS film.

1. Introduction

Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) is a kind of conductive polymer material composed of polymerized PEDOT and insulative PSS. The solubility of PEDOT in the intrinsic state is very poor. By combining PEDOT with PSS, the solubility of PEDOT can be greatly improved, and stable PEDOT:PSS suspension can be formed. By spinning a PEDOT:PSS suspension onto a glass or silicon wafer, we can obtain PEDOT:PSS films that have excellent flexibility, light transmittance, heat resistance, and machinability. In organic solar cells (OSCs), organic light-emitting diodes (OLEDs), and other organic optoelectronic devices, it can function as a hole injection layer or a hole transport layer [1,2,3]. Although PEDOT:PSS film has many unique advantages, its electrical conductivity is still limited due to the existence of insulative PSS. Further improving the conductivity of PEDOT:PSS thin films has been a subject of intense research [4]. At present, adding a certain number of acids [5,6,7], organic reagents [8,9,10], or ionic liquids [11,12] to dope the PEDOT:PSS solution is the main conditional method to improve its conductivity. Alternatively, post-dipping the PEDOT:PSS film in alcohols [13], acids [14], or polar solvents [15] is also an efficient way. However, both the doping and post-dipping process inevitably introduce chemical components, such as acids and polar solvents, which make the preparation process more complex and lengthy, and are also not environmentally friendly. To remove the above shortcomings, PEDOT:PSS films were irradiated in a vacuum with UV light by the researchers, and their conductivity was successfully enhanced [16,17,18]. They attributed the main reason for the improvement of conductivity to the decomposition of the chemical structure of the PEDOT:PSS polymer chain and the reconfiguration of the conductive pathway induced by crosslinking. However, the entire UV irradiation process takes a minimum of 10 h, which is time-consuming. In addition, the vacuum environment required by ultraviolet irradiation also limits its large-scale application. Therefore, it is imperative to develop new methods to efficiently and environmentally improve the conductivity of PEDOT:PSS films.

Recently, Yun and colleagues presented a highly effective, solvent-free method for enhancing the conductivity of PEDOT:PSS films by employing a continuous-wave laser with a wavelength of 1070 nm [19]. The scientists showcased that the conductivity of pristine PEDOT:PSS films could be enhanced significantly, up to a thousand times, through the utilization of laser light exposure. The difference between the absorption ratio of photons between PEDOT and PSS during the utilization of laser technology enables the laser to selectively heat and expand the PEDOT core. This process disrupts the insulating PSS nanoshell, a phenomenon that underscores the potential of this technique. The reconfiguration of the insulating PSS nanoshells, resulting in the consolidation of PEDOT subdomains, creates a better transport route and consequently increases the conductivity of the PEDOT:PSS film. However, the work function of the PEDOT:PSS film was reduced during the above process, sacrificing the important benefits that the PEDOT:PSS film can provide as a hole injection layer. It is of utmost importance to devise effective and solventless approaches for enhancing the conductive of electricity and work performances of thin films composed of PEDOT:PSS.

Femtosecond laser has also been proven as an efficient and environmentally friendly method in many fields due to its strong light and electrical field, which can ionize, dissociate, and modify the organics [20,21,22]. For instance, some scholars have studied the phenomenon of ionization and dissociation of CF₂Br₂, CH₂Br₂, CH₃I, methanol molecules, cyclohexane molecules, and other molecules under the strong femtosecond field [23,24,25]. With the help of time-of-flight mass spectrometry, the process of chemical bond breaking and ion fragmentation in the parent molecule under the action of a femtosecond laser can be observed.

In this study, the PEDOT:PSS film was prepared by spin-coating and then irradiated by a femtosecond laser. We conducted tests on the electrical conductivity and optical transmittance to evaluate the potential of femtosecond laser processing in enhancing the characteristics of the PEDOT:PSS film, thereby improving its performance as a transparent electrode. X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and Raman spectrum were conducted to explain the property variation in the film. To investigate the effect of femtosecond laser on the hole injection efficiency of PEDOT:PSS, we fabricated perovskite light-emitting diodes (PeLEDs) utilizing a PEDOT:PSS film as the hole injection layer (HIL). Ultraviolet photoelectron spectroscopy (UPS) was used to reveal the enhancing luminescence mechanism of PeLEDs.

As a cost-effective and efficient approach for enhancing the function of PEDOT:PSS films, femtosecond laser processing offers an environmentally friendly solution by eliminating the need for solvents. Moreover, the processing efficiency is high, and it can be completed within 48 s. The femtosecond laser’s narrow beam width and short period of time enable it to deliver an immediately elevated power level, leading to the induction of states with significant deviations from equilibrium and phase transitions in materials that do not involve changes in temperature. The region-specific warming can be achieved by freezing the thermal effects produced during ultrafast laser manipulation in close proximity to the location of initial energy deposition. Therefore, precise positioning processing can be achieved by removing a 3 nm thick PSS layer, which reduces damage to the PEDOT layer. After processing, the oxidation degree of the PEDOT layer increases, effectively solving the problem of reduced work function during the processing by Yun.

2. Materials and Methods

The solution of PEDOT:PSS (PH1000) was from Heraeus Co., Ltd. (Hanau, Germany), while transparent quartz glass, used as the substrate for the PEDOT:PSS film, was sourced from Nanjing Jianaite Technology Co., Ltd. (Nanjing, China) with dimensions of 20 mm × 20 mm × 1 mm. 4-F-PMAI was produced through the combination of 4-fluorophenylmethylamin (obtained from Sigma-Aldrich, Shanghai, China) and an equivalent amount of aqueous hydrogen iodide (Stable state, sourced from Sigma-Aldrich). The mixture was stirred continuously at 0 °C (in an ice bath). The removal of water was achieved through rotary evaporation, followed by dissolution of the remaining substance in ethanol and subsequent purification using rotary evaporation. 4-F-PMAI underwent recrystallization three times from ethanol within a nitrogen-filled glove box, subjected to gradual cooling on a heated surface followed by dehydration under vacuum conditions. PbI₂, CsI, PbBr₂, CsBr, and LiF were procured from Alfa Aesar, while TPBi was purchased from Lumtec. The CsPbI₃ precursor solution was formulated by combining equal amounts of PbI₂ and CsI in dimethylformamide (Sigma-Aldrich) with a solution having a molarity of 0.15. Subsequently, the CsPbI₃ precursor solution was enriched with a 4-F-PMAI molar proportion equivalent to 60%.

First, the transparent quartz glass is submerged in solutions of ethanol and acetone for a duration of 15 min. It is then dried on a constant temperature hot plate at 150 °C for 15 min, and subjected to a 15 min treatment in a UV-ozone processor, resulting in a clean glass substrate. Afterward, fix the clean glass substrate to the rotating collimator with the rotation speed set at 3000 rpm and the duration set at 45 s. Subsequently, 120 μL of the PEDOT:PSS solution was drawn up with a pipette and uniformly dispersed onto the glass substrate, and the spin coater was started to prepare wet PEDOT:PSS film, which was then taken out and baked on a thermostatic hot plate at 140 °C for 15 min to vaporize the solvent, PEDOT:PSS film could be finally obtained after it was naturally cooled to room temperature.

The femtosecond pulse laser used in this experiment was purchased from Newport Corporation (Irvine, CA, USA), the wavelength of the laser was 1035 nm, the maximum power was 40 W, the repetition frequency was 1 MHz, the pulse width was 300 fs, the focal length was 100 mm, and the focal spot diameter was 20 μm. Initially, the prepared PEDOT:PSS film is placed in a horizontal orientation on the loading platform of the system used for femtosecond laser processing. The optical path is adjusted to focus the femtosecond laser vertically onto the film surface. Then the laser processing path was introduced into the integrated software, and the appropriate fluence, defocusing amount, and scan speed were selected to match it. Afterward, the film underwent femtosecond laser irradiation. In this experiment, the laser processing path consisted of a transverse array measuring 20 mm × 20 mm, with a gap between rows measuring 10 μm, in order to guarantee complete irradiation coverage of the entire sample. After the irradiation, the film could be taken out for subsequent testing and application. The schematic illustration of the PEDOT:PSS films preparation process and the subsequent femtosecond laser irradiation process are shown in Figure 1.

A solution of PEDOT:PSS was applied to an ITO/transparent base material made of glass by revolving deposition at a speed of 3000 revolutions per minute for a period of 45 s, followed by drying the PEDOT:PSS layer on a hot plate at a temperature of 140 °C for 15 min. After annealing, samples were separated into two categories. Some of the samples were irradiated by femtosecond laser while the others were not. The sample was then spin-coated with a solution of CsPbI₃ precursor at 3000 rpm for 35 s. After annealing the CsPbI₃ perovskite film, we applied thermal deposition to sequentially evaporate TPBi, LiF, and Al layers onto the film. The thicknesses of these layers were 1.2 nm, 40 nm, and 100 nm, respectively. The structure of the tested PeLEDs is shown in Figure 2. The device area was 4 mm². PeLEDs were measured using an angle-dependent luminance and QE measurement system for luminescent devices (QE-R, Enlitech, Shanghai, China).

3. Results and Discussions

As shown in Figure 1, PEDOT:PSS (PH1000) coatings were fabricated onto the surface with a transparent quartz glass with a size of 20 mm × 20 mm × 1 mm using the spin coating method and then irradiated by femtosecond laser (λ = 1035 nm, pulse width = 300 fs and repetition frequency = 1 MHz) with scan speed (v) of 1000 mm s⁻¹. The full irradiation of the sample took 48 s.

A four-point probe system was used to test the variations in sheet resistance of PEDOT:PSS films with respect to the defocusing amount (D) and fluence (F). The results are shown in Figure 3a. The results show that the pristine PEDOT:PSS film had a relatively poor electrical conductivity with a sheet resistance of 448,020 Ω/sq. At F = 127 mJ cm⁻². When using focused laser processing, the PEDOT:PSS film exhibited a slight increase in resistance per unit area of a sheet, reaching 483,862 Ω sq⁻¹.When using defocusing laser processing, the resistance per unit area of a sheet of PEDOT:PSS film changed little, and gradually approached the pristine value with the increase of defocusing amount. At F = 254 mJ cm⁻², when using focused laser processing, the resistance per unit area of a sheet of PEDOT:PSS film increased to 594,337 Ω sq⁻¹. When using defocusing laser processing, the resistance per unit area of a sheet of PEDOT:PSS film first decreased and then returned to the pristine value with the increase of defocusing amount. The lowest sheet resistance was obtained at a defocusing amount of 0.4 mm, which was 73,565 Ω sq⁻¹. At F = 381 mJ cm⁻², when using focused laser processing, the resistance per unit area of a sheet of PEDOT:PSS film increased significantly, reaching 626,873 Ω sq⁻¹. When using defocusing laser processing, the resistance per unit area of a sheet of PEDOT:PSS film decreased first and then returned to the pristine value with the increase of defocusing amount, showing a similar trend.

As expected, focused laser processing could result in an increase in resistance per unit area of a sheet of the PEDOT:PSS film. This might be attributed to the fact that the degree of damage to the PEDOT layer increases with an increase in femtosecond laser fluence. Defocusing treatment with a femtosecond laser can allow light to be evenly distributed on the surface of the PEDOT:PSS film, thereby reducing thermal damage and material structural changes caused by local focusing. It also helps to remove some defects in the PEDOT:PSS films, such as particles, voids, or uneven PSS distribution. In addition, when a 1035 nm femtosecond laser is irradiated onto the PEDOT:PSS film, PEDOT exhibits high absorption capability towards photons. Excitation of carriers and dispersion of the conjugated backbone take place in chains of PEDOT, and the conformation of PEDOT tends to transition from irregular clusters to regular linear clusters, which often benefits the migration rate of carriers [26]. In the electro-acoustic coupling stage after carrier excitation, the PEDOT core effectively transfers the high levels of heat energy it generates to the adjacent PSS shell, which is in closest proximity to the core, leading to the selective removal of PSS. This enhances the conductivity of the film.

By properly calibrating the relationship between F and D, it is possible to effectively reduce the sheet resistance of the film after femtosecond laser irradiation. At F = 254 mJ cm⁻² and D = 0.4 mm, the sheet resistance of the film dropped clearly, to 73,565 Ω sq⁻¹, which was only 1/6 of the pristine value. Therefore, we pay a lot of attention to the effect of the above optimized F’s and D’s on the properties of the PEDOT:PSS brane in the sequel of this paper.

In order to demonstrate the changes in transmittance of the PEDOT:PSS films, we measured the transmittance spectrum of the samples before and after they were irradiated with a femtosecond laser using a UV-VIS-NIR spectrometer (UV3600 I Plus, Shimazu, Kyoto, Japan). The transmittance spectrum for the tested PEDOT:PSS film can be seen in Figure 3b. In the visible range (400~780 nm), the total transmittance of the laser-treated PEDOT:PSS film was marginally superior to that of the untouched PEDOT:PSS film, which showed a transmittance of 88.9% at 550 nm and 85.3% at 780 nm. The substrate used is made of glass, the main component of which is SiO₂. The enhancement of transmittance in the longer wavelength region is more noticeable with the addition of SiO₂ to PEDOT-PSS. In the film composed of SiO₂ nanoparticles and PEDOT-PSS, the homogeneous blending of these components minimizes the intermolecular associations within PEDOT-PSS. Therefore, the reduction in the obstacle of energy required for activation facilitates polymer chain reorientation, resulting in a diminished level of anisotropy observed in the SiO₂/PEDOT-PSS film. This results in lower absorbance (increased transparency) in the long-wavelength range [27]. Previous work by Wu showed that the transmittance of PEDOT:PSS films is mainly determined by their thickness [14]. Thus, the thickness of PEDOT:PSS film before and after femtosecond laser irradiation was tested by a spectroscopic ellipsometer (Horiba UVISEL) to explain the reason why the transmittance of PEDOT:PSS film changed. As shown in Figure 3c, the thickness of pristine PEDOT:PSS film was 152.554 ± 2.431 nm, which became 149.636 ± 3.114 nm after the irradiation, indicating that femtosecond laser irradiation slightly reduced the film thickness of PEDOT:PSS (about 3 nm). In this way, the transmittance is increased. It can be concluded that femtosecond laser irradiation is a potential method to enhance the optoelectronic characteristics of films. Interestingly, the femtosecond laser irradiation process in this work only spent 48 s, which is 1/750 of the ultraviolet irradiation process [16], indicating its ultrafast processing feature.

The molecular weight of PEDOT is relatively smaller, whereas that of PSS is relatively larger. The main structure of PEDOT is a ring structure which makes it relatively stable, while PSS has some single bonds such as the C-C single bond and C-H single bond, which make it less stable than PEDOT. According to the previous study [28], PEDOT and PSS are closely combined in aqueous solution by ionic bonds to form spherical double-layer PEDOT:PSS colloidal particles, with the inner part being conductive, but hydrophobic PEDOT and the outer shell being insulating by hydrophilic PSS. Therefore, the surface of the PEDOT:PSS film was mainly composed of a layer of PSS with several nanometers [23]. Damaging the PSS layer at the PEDOT surface may be an effective way to enhance the conductive properties of the PSS film. The thickness of the tested PEDOT:PSS film in Figure 3c demonstrates that the PEDOT:PSS film was thinned by femtosecond laser irradiation. To elucidate the mechanism behind the improvement in conductivity of PEDOT:PSS films, it is necessary to confirm that the thinning of PEDOT:PSS films is induced by a reduction in PSS at the surface. Thus, 2D surface morphology images and phase images of the samples were characterized using an atomic force microscope (AFM, Brooke Dimension Icon). Figure 3a,b depict the characteristic surface structure of the PEDOT:PSS film prior to and following exposure to femtosecond laser radiation. There were some nanoparticles with a diameter of several hundred nanometers on the pristine film (Figure 4a). After femtosecond laser irradiation, the nanoparticles increased on the surface, and thus did the roughness of the film (Figure 4b). The calculated Rq and Ra values for the pristine film were 1.82 nm and 1.24 nm, respectively, which increased to 2.58 nm and 1.63 nm after femtosecond laser irradiation. According to the findings, femtosecond laser exposure led to an increase in surface roughness of the PEDOT:PSS film. This may be induced by the removal of the PSS.

In the spin coating process, because of the variation in molecular weight between PEDOT and PSS, phase separation of PEDOT:PSS solution will occur in the vertical direction under the action of centrifugal force, so the surface of the final formed PEDOT:PSS film is mainly a layer of PSS. According to U. Lang et al., the bright region in the phase image of the PEDOT:PSS film is dominated by PEDOT, while the dark region is dominated by PSS [29]. Thus, the phase images of PEDOT:PSS film were also tested by AFM as shown in Figure 4c,d. The brightness of the phase image is increased in the irradiated film compared to the pristine film, suggesting a reduced PSS at the film surface.

To further verify the reduction of PSS, the XPS spectrum of the samples was measured by an X-ray photoelectron spectrometer (Thermofischer Escalab 250 XI, Thermo Fisher Scientific Inc., Carlsbad, CA, USA), which represented the S2p fine scans of PEDOT:PSS films both pre- and post-femtosecond laser irradiation. In Figure 5 there are two signal bands. The higher band around 168 eV is ascribed to the presence of sulfur atoms within the PSS, while for the lower band, around 164 eV, the presence of sulfur atoms in the PEDOT is responsible for this phenomenon [30]. The relative content between PEDOT and PSS can be reflected by the area of the peaks formed by the corresponding peaks and the abscissa. After femtosecond laser irradiation, the film exhibited an increase in the peak intensity ratio between PEDOT and PSS, from 0.464 to 0.476. This indicates that the relative content of PEDOT increased while that of PSS decreased, a finding that was also corroborated by AFM test results. In addition to the changes in intensity, the positions of these peaks also changed. After femtosecond laser irradiation, the peak associated with the sulfur atom in PEDOT experienced a slight shift from 164.1 eV to 164.2 eV, while the peak associated with the atom of sulfur in PSS shifted slightly from 167.9 eV to 168 eV, both have increased by 0.1 eV. According to the results of O1s fine scans, the peak around 533 eV matched with the oxygen atom present in PEDOT, while the peak around 531.5 eV corresponded to the oxygen atom in PSS. After femtosecond laser irradiation, the peak associated with the oxygen atom within PEDOT did not change significantly, while the peak associated with the oxygen atom within PSS moved from 531.5 eV to 531.4 eV, which decreased by 0.1 eV. After laser treatment, the PEDOT-enriched core arises from the matrix of PSS; however, the overall mass of the PSS matrix remains unchanged. Our laser therapy preserves the film’s thickness and the PSS content. Given the insulating nature of PSS, augmenting the electrical conductivity of the PEDOT:PSS film can be enhanced by adjusting the proportion between PEDOT and PSS.

The Raman spectrum of the samples was tested by a confocal Raman spectrometer (Andor SR-500 I, Oxford, UK) to evaluate the above effects. Both the pristine and irradiated films showed a main band at 1436 cm⁻¹ corresponding to the Cα = Cβ(−O) symmetrical stretching vibration [31], whereas the location of the main band was slightly different. The curve fits of the Raman spectra, as shown in Figure 6a,b, indicate that the irradiated films exhibit subband redshifts. These shifts correspond to neutral PEDOT structural vibrations. The band centered at 1431 cm⁻¹, which corresponded to the neutral PEDOT structure vibration [32], shifted to 1428 cm⁻¹ after femtosecond laser irradiation. Ouyang et al. suggested that the redshift of the main band frequently accompanies the transition from the benzoid to the quinoid resonant structure of the PEDOT [33]. In the quinoid harmonious configuration, π-electrons that have undergone conjugation are more easily reallocated across the entire chain of PEDOT than in the benzoid resonant structure. Thus, the irradiated PEDOT:PSS film has a higher conductivity. The band centered at 1450 cm⁻¹ in the pristine samples, which were assigned to oxidized PEDOT structures [34], kept unchanging during the irradiation process. However, the band area ratio (oxidized:neutral) in the pristine samples was 0.133, whereas it nearly tripled to 0.360 after femtosecond laser irradiation, indicating the oxidization of PEDOT. Such an increase in the oxidized state implies that the PEDOT polymer strand has a larger charge carrier concentration, which is also beneficial for the conductivity.

The work function Φ is another important indicator of PEDOT:PSS film of which the minimum energy needed to transfer an electron from the interior of a material to its outer surface is denoted. The work function Φ of the samples was tested by an ultraviolet photoelectron spectrometer (Thermo Fisher Scientific Escalab Xi+, Thermo Fisher Scientific Inc., Carlsbad, CA, USA) so as to further investigate the variation of work function Φ of PEDOT:PSS film after femtosecond laser irradiation. The calculation of this value work function Φ can be determined using the formula provided below.

$$\Phi =E_{Vac}-E_{Fermi}=h\nu -E_{Cutoff}$$

(1)

where hv is the energy of the incident He I source with the value of 21.22 eV; E_Vac represents the vacuum energy level; E_Cutoff corresponds to the secondary electron cutoff edge, and E_Fermi corresponds to the Fermi edge. According to the results shown in Figure 7a,b, the E_Cutoff value of the irradiated film is shifted from 16.76 eV to 16.58 eV compared to the pristine film. Upon calculation, it has been determined that the pristine PEDOT:PSS film possesses a work function Φ of 4.46 eV. After femtosecond laser irradiation, this value increased by 0.19 eV to 4.64 eV. Lin and colleagues inferred that transition from the coiled configuration of the benzoid structure to either the straight-line or expanded-coil arrangement of the quinoidal configuration resulted in an increase in the work function of the PEDOT:PSS film [35]. The Raman spectrum in Figure 5a,b also showed the same conformation transformation. Thus, femtosecond laser irradiation femtosecond laser irradiation has the potential to enhance Φ of PEDOT:PSS film. It can also reduce the energy barrier height at the boundary of the connection between hole injection layer and layer responsible for transporting holes, thereby facilitating carrier crossing over this barrier and improving photoelectric device performance.

For optoelectronic devices such as PeLEDs, either extreme conductivity or high work function is preferred in the hole injection layer. A hole injection layer that exhibits a higher work function can provide improved alignment with the highest occupied molecular orbital (HOMO) of perovskite materials. This reduces the energy barrier, facilitating carrier crossing and consequently enhancing the efficiency of photoelectronic devices. To assess the contribution of PEDOT:PSS film subjected to femtosecond laser irradiation to optical emission, we fabricated PeLEDs with ITO/PEDOT:PSS/CsPbI₃/TPBI/LiF/Al structures, as shown in Figure 2. The energy maps of PeLEDs with pristine and irradiated PEDOT:PSS films are shown in Figure 8a,b, respectively. The increase in E_cb from 4.46 eV to 4.64 eV after femtosecond laser irradiation reduces the energy barrier between the PEDOT:PSS layer and the CsPbI₃ layer. This enhances hole injection in the HIL and enables better-balanced electron-hole pair injection.

Figure 9a exhibits the phenomenon of the electroluminescence (EL) spectrum of the PeLEDs assessed under optimal luminosity conditions. These devices had the deep red light emission function with a peak wavelength of 688 nm, which would not be affected by the femtosecond laser irradiation, and the CIE coordinate (0.72, 0.27) of the devices also remained the same after the irradiation. Figure 9b shows that the resistance per unit area of a sheet of PEDOT:PSS film decreased after femtosecond laser irradiation, so correspondingly, the current density of the PeLEDs using irradiated PEDOT:PSS film. The maximum luminosity of the device with irradiated PEDOT:PSS film is five times larger than that of the pristine device, up to 950 cd/m² at 6 V, which also reveals the enhanced electrical conductivity exhibited by the irradiated film of PEDOT:PSS. In comparison to the device that utilized the pristine PEDOT:PSS film, the efficiency of converting the external quantum of the device featuring the irradiated PEDOT:PSS film experienced an increase, moving from 4.6% to 6.3% (Figure 9c), and no shift occurred in the EL peak position of the device. The increase in EQE value is due to the excitation of carriers in the PEDOT chain and the delocalization of the conjugated main chain when femtosecond laser irradiates the PEDOT:PSS film. The conformation of the PEDOT chain changes from irregular clusters to regular linear clusters, thereby enhancing its conductivity. Moreover, femtosecond laser treatment can lead to selective removal of PSS, changing the interface contact between PEDOT and the dynamic stratum, making it more compact, and enhancing the ability of charge tunneling through the interface. In addition, femtosecond laser treatment can increase the oxidation level of PEDOT, leading to an increase in carrier concentration, and further improving EQE. It indicates better energy level matching between the PEDOT:PSS layer and the perovskite layer, which enhanced the hole injection ability in HIL.

4. Conclusions

In summary, a proposal was made for a strategy that utilizes femtosecond laser irradiation to enhance the characteristics of thin films composed of PEDOT:PSS, which is cost-effective and decreases not just the film sheet resistance by nearly ten times, in addition to enhancing the film work function. Given the relatively poor stability of PSS and the fact that the conductive PEDOT core is buried in the insulative PSS shell, rapid femtosecond laser irradiation in 48 s can selectively remove the superficial PSS. This exposes the conductive PEDOT to the surface, thereby enhancing the electrical conductivity of the film. The rise in the work function could potentially be ascribed to the heightened oxidation of PEDOT and its conversion from a benzoid to a quinoid resonant configuration following exposure to femtosecond laser irradiation. Finally, irradiated PEDOT:PSS slender films are further employed as hole injection layers for PeLEDs. A maximum luminosity of 950 cd/m² has been achieved in PeLEDs with irradiated thin films, which is five times higher than that of controlled devices. Moreover, the device’s EQE also increased from 4.6% to 6.3%. This work paves an economical method to regulate the electricity conductive characteristics of PEDOT:PSS slender films for applications in advanced optoelectronic devices. It is worth mentioning that in this paper, there is no aging test for the processed devices, so the life of the devices and stability of this processing method are not actually discussed, which are directions of future research.