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Abstract
Highly conductive transparent anodes based on graphene oxide (GO) functional material mixed with poly(3,4-ethylenedioxythiophene)-polystyrene sulfide (PEDOT:PSS) solution were prepared by spin-coating method, and the conductive properties of the anode mixture were further improved by interface treatment. The square resistance of the hybrid film at 36 nm film thickness was 62 Ω/□ and the transmittance at 550 nm was 89%. OLED devices using optimized GO/PEDOT:PSS hybrid films as anodes have lower turn-on voltages and the highest current efficiencies, with a maximum brightness that is 2.37 times that of pure PEDOT:PSS devices and 2.7 times that of ITO devices. Higher transmittance, conductivity, and better highest occupied molecular orbital (HOMO) level matching after hybrid electrode interface treatment contribute to the performance of GO/PEDOT:PSS hybrid anode OLED devices.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Organic light-emitting diodes (OLEDs) have attracted much attention due to their wide operating temperature range, low power consumption, wide viewing angle, fast response, high contrast, and bright luminescent colors [1,2]. Indium tin oxide (ITO) is currently a widely used transparent electrode in OLEDs since its high optical transparency and electrical conductivity. However, ITO suffers from several major disadvantages: its higher cost due to its scarcity; its higher refraction, resulting in total internal reflection at the ITO/glass and ITO/organic interfaces and ultimately high-power loss of the device. Currently, there are a variety of candidate materials to replace ITO as transparent electrodes, including graphene [3–8], carbon nanotubes [6,9–12], metal nanowires [13–16], and conductive polymers [14–18]. However, the high surface roughness of metal nanowires, the poor uniform dispersion of carbon nanotubes in most solvents and cumbersome synthesis methods, and the low work function of graphene all pose major challenges in using these electrodes. Conductive polymers, especially poly(3,4-Ethylenedioxythiophene)-polystyrene sulfonic acid (PEDOT: PSS) [19] has attracted much attention for organic optoelectronic devices because of their application in flexible devices and cost-effective mass production. In organic photoelectric devices, the HOMO energy level of PEDOT: PSS is higher than the HOMO energy level of ITO, so the energy level barrier between PEDOT: PSS and organic energy level is lower than that between ITO and organic layer. The energy level barrier is low, which is conducive to hole injection. PEDOT: PSS films have high transparency in the visible range, high mechanical flexibility and excellent thermal stability. But PEDOT: PSS solution has low conductivity. PEDOT: PSS solutions typically have a conductivity below 1 Scm-1, significantly lower than ITO. In order to solve this problem, many methods have been adopted to enhance the conductivity of PEDOT: PSS, such as using salt [20], ethylene glycol [21], sodium dodecyl benzene sulfonate (SDBS) [22], dimethyl sulfoxide (DMSO) [23], carboxylic acid or inorganic acid [24–26], etc. As an important precursor for the preparation of graphene, graphene oxide (GO) has a two-dimensional structure similar to graphene, and functional groups such as hydroxyl (-OH) and carboxyl (-COOH) epoxy groups exist on the surface and periphery [22]. In addition, the HOMO and Lowest Unoccupied Molecular Orbital (LUMO) levels of GO are -4.9 eV and -1.3 eV, respectively, which have been applied in organic solar cells, OLED and other organic optoelectronic devices [27–29] as wide-band gap materials. Due to the electronegativity of GO, adding GO to PEDOT: PSS can effectively realize the interaction between GO and PEDOT: PSS. In this paper, we mixed the GO dispersion with the PEDOT: PSS solution, prepared the mixed film by spin coating, and modified the interface with sulfuric acid (H2SO4) and hydrochloric acid (HCl) to further improve the conductivity of the mixed film, reduce the square resistance, and further improve the photoelectric performance of the OLED device. At the same time, we analyzed the mechanism of using acid interface to modify GO/PEDOT: PSS hybrid electrode to improve its conductivity.
2. Experimental
GO was synthesized according to the improved Hummer method [29–31] at a concentration of 0.5 mg/mL. The preparation process of GO is divided into two steps: preparation and purification. The following describes the preparation process: Firstly, 1 g of graphite powder was weighed and placed in a beaker in an ice water bath. With constant stirring, 23 ml of concentrated H2SO4 was slowly added to the beaker. After stirring evenly, 3 g of potassium permanganate (KMnO4) was added to the solution. The solution was heated to 35 °C for 1.5 h. Then, 48 ml of hydrogen peroxide (H2O2) was added to the solution and heated it to 98 °C for 0.5 h. Finally, 140 ml of deionized water (DI) was added while stirring continuously. The purification process involves the following steps: Firstly, the solution mentioned above was centrifuged and washed at a low-speed of 1500 rpm for several times to obtain the supernatant. Then, the supernatant was washed with 5% HCl for 5-8 times and centrifuged repeatedly to remove any unreacted KMnO4 and intermediate products manganese dioxide (MnO2). The solution was washed with DI for more than 5 times. During the process, the clear liquid at the top and the impurities at the bottom were removed. Finally, increase the speed of centrifugation (3000 rpm) and continue washing until there are no obvious solid particles at the bottom of the centrifuge tube. Figure 1 shows the flow chart of the GO synthesis experiment. The PEDOT: PSS aqueous solution was purchased from Heraeus, namely Clevios PH1000. It was dispersed in water at a concentration of 1.0-1.3 wt.%. The weight ratio of PSS to PEDOT is 2.5, which means the repeat unit mole of PEDOT and PSS is 1.8. Therefore, the conductivity is in the range of 0.2-1 Scm-1. Figure 2 displays the chemical structures of GO and PEDOT: PSS. In the OLED devices, the hole transport layer used is N, N-diphenyl-N, N-(1-naphthyl)-1,1-biphenyl-4,4-diamine (NPB), and the light-emitting layer is Tris(8-hydroxyquinoline) aluminum (Alq3), purchased from Ossila Ltd Company. All materials were used directly without purification. Filter the PEDOT: PSS solution with a 0.45-micron water filter, then mix GO and PEDOT: PSS in the following ratios (1%, 2%, 4%, 10%, 20%, 40%, 80%,100%,150%,200%). The mixed solution was ultrasonically oscillated for 1 h. The quartz substrate and ITO substrate were ultrasonically cleaned with acetone, alcohol and deionized water in sequence, then dried in an oven at 120 °C for 2 h, and treated with ozone plasma for 15 minutes. The film was spin coated using a spin coater (KV-SC-1550). Firstly, the solution was spin-coated on the quartz substrate at a speed of 3000 rpm for 30 s to form a thin film. The film was heated and annealed on a heating platform at 120 °C for 20 minutes. Then, the film was cooled to room temperature and immersed in 1 M HCl, taken out and placed on a heating platform at 160 °C. After that, 50 ml of HCl (1 M) solution was dropped on the film and heated for 0.5 h. The film was cooled to room temperature and rinsed three times with DI. Then the film was immersed in 1 M H2SO4, taken out and placed on a heating platform at 160 °C. 50 ml of H2SO4 (1 M) solution was added and dropped on the film, and heated for 0.5 h. The film was cooled to room temperature and rinsed three times with DI. Finally, the film was annealed at 160 °C for about 1 h to remove moisture in the film and change the surface morphology of the film, thus affecting the carrier injection and transport characteristics. The experimental process is shown in Fig. 3.
For the prepared GO/PEDOT: PSS hybrid film and pure PEDOT: PSS film, the film square resistance was tested by 4 probes (RTS-8, 4 probes Tech) and the surface morphology was tested by atomic force microscope (Nano Wizard II). The absorption and transmission spectra of the film was measured by UV-2700 UV-Visible spectrophotometer. Raman spectrum and the thickness of the sample film were tested by Raman spectrometer (DXR2, 785 nm excitation light source) and Step meter (Dektak XT). Ultraviolet photoelectron spectroscopy (UPS) measurements of PEDOT: PSS and different volume ratio of GO/PEDOT: PSS hybrid films with interface treatment (10%,20%,40%,80%,100%) were carried by electron spectrometer-Scienta R4000 VG Scienta using Helium (I) UV source ($h\nu $=21.2 eV). The prepared GO/PEDOT: PSS hybrid film and pure PEDOT: PSS film were used as anodes and placed into the evaporation chamber of the multi-source organic molecule thermal evaporation film system. When the vacuum degree is lower than 5 × 10−4 Pa, the MoO3 for hole injection layer, the hole transport layer NPB for hole transport layer, the Alq3 for light-emitting layer, the BPhen for electron transport layer, the lithium fluoride (LiF) for electron injection layer and the aluminum (Al) for cathode were sequentially evaporated. The light-emitting area of the device is 2 mm × 2 mm, and the electroluminescence (EL) performance (current, brightness, spectrum, etc.) of the device is measured using Keithley 2400 combined with a PR-655 spectrometer. All tests were performed at atmospheric room temperature. The energy levels and device structure were shown in Fig. 4.
3. Results and discussion
Figure 5(a) illustrates the variation in conductivity of a pure PEDOT: PSS film under different annealing temperatures. As the annealing temperature elevates, the conductivity of the film correspondingly increases. When the temperature reaches 130 °C, the conductivity peaks at 29 Scm-1, and no significant change in conductivity is observed with further temperature increase. When GO is mixed with PEDOT: PSS at varying volume ratios, a maximum conductivity of 75 Scm-1 is observed for the prepared hybrid film at a mixing ratio of 40%, as depicted in Fig. 5(b). The results of further interfacial treatment of PEDOT: PSS/GO thin films with different mixing ratios indicate a significant improvement in electrical conductivity. After the interface treatment, the electrical conductivity of the pure PEDOT: PSS film escalates to 2386 Scm-1. The conductivity of the hybrid film GO/PEDOT: PSS (40%) is increased to a maximum of 4032 Scm-1, the sheet resistance of the film is 62 Ω/□, and the film thickness is 36 nm. Compared with the optimized PEDOT: PSS film, the conductivity of the hybrid film increases by a factor of 139, and the transmittance of the hybrid film at 550 nm is 89%, as shown in Fig. 5(c). The electrical conductivity of the GO/PEDOT: PSS hybrid films was enhanced after adding graphene functional materials. This improvement can be attributed to two primary factors. Firstly, the GO sheet, which possesses a substantial number of functional groups such as hydroxyl (-OH) and carboxyl (-COOH) on its surface, displays a high degree of negative charge, as depicted in Fig. 2(a). In the PEDOT: PSS solution, the PEDOT+ chain and the PSS- chain are joined via Coulomb interaction, as illustrated in Fig. 2(b). The PEDOT+ chain is positively charged, while the PSS- chain is negatively charged. The addition of a minor quantity of GO causes the PEDOT+ chains in the blended solution to appear partially disconnected. These disconnected PEDOT+ chains merge with the negatively charged GO sheet, thereby augmenting the conductivity of the film. Secondly, at the GO sheet level, a small quantity of H+ ions are generated. These H+ ions combine with the PSS- chains to form a minor quantity of PSSH, leading to a reduction in the PSS- chains and an increase in the conductivity of the film. With an increasing amount of GO incorporation, the conductivity reaches a maximum. However, further increase in GO content disrupted the Coulomb interaction between the PEDOT+ chains and the PSS- chains. Simultaneously, the insulation properties of the film improve due to the extensive incorporation of GO [28,32], indicating a decline in the electrical conductivity of the hybrid film.
Fig. 5. (a)Conductivity of PEDPT: PSS film at different temperatures (b) Conductivity of hybrid films with different volume ratios after annealing at 130 °C (c) Conductivity and the transmittance at 550 nm of hybrid films with different volume ratio after interface treatment.
In order to explain the substantial increase in conductivity of pure PEDOT: PSS films and GO/PEDOT: PSS hybrid films after interface treatment, we tested the Raman spectra and absorption spectra of the GO/PEDOT: PSS hybrid films (40%) before and after interface treatment, as shown in Fig. 6. In the absorption spectrum, the absorption peaks at 225 nm and 193 nm are the absorption of the benzene ring substituent of the PSS- chain [27,33], it can be seen that after the interface treatment, the absorption intensity of the mixed film at 225 nm and 193 nm decreased significantly, indicating that the PSS- component in the film decreased. Through the measured Raman spectrum, it can be seen that in the spectrum of the film without interface treatment, the strongest symmetrical peak wavenumber is 1425 cm-1, which corresponds to the symmetrical stretching of C = C of the PEDOT+ chain. After the interfacial treatment, the wave number has a blue shift [34–37]and becomes 1428 cm-1, indicating that the lattice structure of the film has changed after the interface treatment.
Fig. 6. (a) Raman spectroscopy and (b) Absorption spectra of hybrid films (40%) before and after interface treatment.
Figure 7 presents the AFM images of PEDOT: PSS and the hybrid film before and after interface treatment. Upon addition of GO to PEDOT: PSS, some negatively charged GO sheets align with the positively charged PEDOT+ chains, thereby detaching a small quantity of PEDOT+ chains from the PSS- chains. Concurrently, the hydroxyl and carboxyl groups of GO ionize a portion of H+, which then combine with a fraction of PSS- to form PSSH (PSS-+H+$\rightleftharpoons$PSSH) [34], having a minimal impact on the film's surface roughness. As illustrated in Fig. 7(a) and (b), the film's morphology remains clustered, with the surface roughness transitioning from 2.1 nm to 2.4 nm. When the hybrid film undergoes acid interface treatment, a substantial quantity of H+ is ionized from the acid (H2SO4$\rightleftharpoons$H++HSO4-, HCl$\rightleftharpoons$H++Cl-) [38], combining with a substantial quantity of PSS- in the hybrid film to form neutral PSSH (PSS-+H+$\rightleftharpoons$PSSH). This process causes the PEDOT+ chains to disengage from the PSS- chains. These PSSH chains can be separated from the membrane matrix and removed through washing with deionized water. After GO/PEDOT: PSS is rinsed with DI three times, the PSS- in PEDOT: PSS will be dissolved in DI, and PEDOT+ will remain on the substrate, thereby enhancing the conductivity of the hybrid film. Additionally, the acid treatment alters the morphology of the PEDOT+ chain. The Raman spectrum of Fig. 6(a) reveals a change in the structure of the PEDOT+ chain after interface treatment. As seen in Fig. 7(a) and (b), the morphologies of the hybrid films prior to interface treatment consist of coil clusters. Post-interface treatment, the structure reorganizes into a linear/extended coil structure rich in PEDOT+ chains, as illustrated in Fig. 7(d). This morphological transformation signifies that the PEDOT+ chain has separated from the PSS- chain, and the structure of the PEDOT+ chain has changed. Consequently, the surface morphology and surface roughness of the hybrid film undergo significant alterations, increasing from 2.4 nm to 4.2 nm. These changes in surface morphology indicate that the structure of the PEDOT+ chains has transitioned from coiled clusters to linear/extended coil structures. After interface treatment, the polymer chains realign to form a linear structure of PEDOT+ rich chains, significantly improving the conductivity of the hybrid film [38–40].
Fig. 7. AFM images of (a) PEDOT: PSS film (b) GO/ PEDOT: PSS film (c) hybrid film with HCL interface treatment and (d) hybrid film with interface treatment.
As shown in Fig. 8, the work function(Ф) and EHOMO of PEDOT: PSS and GO/PEDOT: PSS hybrid films with different volume ratios (10%, 20%, 40%, 80%, 100%) were measured using UPS. Figure 8. (a) showed the binding energy of secondary electron cutoff regions, and (b) showed the Fermi edge (valence band edge) region. The EHOMO and Ф of all samples were calculated from the formulas Φ=hυ-(Ecutoff-EF) and EHOMO=Φ+ (EHOMO edge-EF) according to the UPS spectrum [28,41]. Detailed Φ and Ecutoff dates were showed in Table 1. Compared with 4.787 eV of pure PEDOT: PSS, the Ф of the GO/PEDOT: PSS hybrid film increased to 4.950 eV for GO/PEDOT: PSS (10%). The Ф reaches a maximum of 5.014 eV in GO/PEDOT: PSS (40%). The EHOMO edge-EF was obtained from Fig. 8(b), and the EHOMO was calculated. It can be seen from the EHOMO data in Table 1 that the maximum improvement in EHOMO (HOMO) of the GO/PEDOT: PSS hybrid electrode is 0.223 eV and the minimum improvement is 0.163 eV compared to the pure PEDOT: PSS electrode. By comparing the energy level difference between the hybrid electrode and the HIL layer with that of the ITO electrode, it was found that the energy level barrier between the hybrid electrode and the HIL layer was reduced by a maximum of 0.556 eV. This reduction in the energy level barrier is advantageous for energy level matching and can significantly increase the hole injection efficiency.
Fig. 8. The UPS measurements of PEDOT: PSS and different volume ratio of GO/PEDOT: PSS hybrid films with interface treatment (10%,20%,40%,80%,100%); (a) secondary electron cutoff regions and (b) Fermi edge regions, respectively, Source ($h\nu $=21.2 eV).
Table 1. Ecutoff, Ф, EHOMO of PEDOT: PSS and different volume ratio of GO/PEDOT: PSS hybrid films with interface treatment (10%,20%,40%,80%,100%) measured from UPS spectra.
Utilizing ITO, PEDOT: PSS, and varying ratios of GO/PEDOT: PSS hybrid films as anodes, OLED devices with the structure depicted in Fig. 4(b) were fabricated, and their photoelectric properties were evaluated. Figure 9 displays the measured brightness-voltage curve, current efficiency-current density curve, Normalized EL spectra of OLED devices with different anodes and brightness-voltage spectrum of anode device with or without interface modification. The performance of OLEDs based on various anodes is tabulated in Table 2. Notably, the device incorporating a 40% mixed GO/PEDOT: PSS hybrid film as the anode exhibits a lower turn-on voltage of 2.4 V, while the device with a PEDOT: PSS film as the anode possesses a turn-on voltage of 4.1 V, both of which are beneath the 4.3 V turn-on voltage of the ITO device. The device with a GO/PEDOT: PSS (40%) hybrid film as the anode reaches a maximum current efficiency of 7.0 cd/A, which surpasses the maximum current efficiency of the device with ITO and PEDOT: PSS film as the anode (2.8 cd/A, 2.9 cd/A). Additionally, the maximum brightness of the device with a GO/PEDOT: PSS (40%) hybrid film as the anode reaches 36271 cd/cm2(Fig. 9(a-II)), which is 2.37 times the maximum brightness of 15265 cd/cm2 (Fig. 9(a-I)) achieved by the PEDOT: PSS film as the anode device. As illustrated in Fig. 9(c), the Normalized EL spectra of all devices exhibit the same peak wavelength. This finding indicates that the different anodes have no impact on the recombination zone of OLEDs. Figure 9(d) shows the brightness-voltage diagram of devices based on PEDOT: PSS anode and GO/PEDOT: PSS (10%, 40%, 100%) hybrid anode with or without interface modification. The GO/PEDOT: PSS (40%) hybrid anode device has the largest luminescence enhancement (The maximum luminescence brightness increased from 20223 cd/cm2 for the un-interface modification GO/PEDOT: PSS (40%) hybrid electrode device to 36271 cd/cm2 for the interface modification GO/PEDOT: PSS (40%) hybrid electrode device.), and the PEDOT: PSS anode device has the smallest luminescence enhancement (The maximum luminescence brightness increased from 9804 cd/cm2 for the un-interface modification PEDOT: PSS electrode device to 15265 cd/cm2 for the interface modification PEDOT: PSS electrode device.). The maximum brightness of both GO/PEDOT: PSS (10%) and GO/PEDOT: PSS (100%) hybrid electrode devices increased after interface modification. It can be seen that the luminescence brightness of PEDOT: PSS anode and GO/PEDOT: PSS (10%, 40%, 100%) hybrid anode devices has been enhanced after interface modification.
Fig. 9. (a) Brightness - voltage spectra (b) Current efficiency - current density spectra and (c) Normalized EL spectra of the OLEDs based on different anodes (d) Brightness-voltage spectrum of anode device with or without interface modification.
Table 2. Performances of OLEDs based on ITO and PEDOT: PSS composite electrodes with different GO ratio.
For the OLED device utilizing a GO/PEDOT: PSS (40%) hybrid film as the anode, the HOMO of GO/PEDOT: PSS (40%) is 5.256 eV, which is higher than the HOMO of ITO at 4.7 eV. This situation narrows the energy gap between the HOMO of the hole-injection layer MoO3 and that of anodes, which facilitates hole injection and thus reduces the turn-on voltage of the device. Due to the interface treatment, the GO/PEDOT: PSS (40%) hybrid film exhibits higher conductivity, transmittance and HOMO. Consequently, when compared with a device with a pure PEDOT: PSS film as the anode, the device with a GO/PEDOT: PSS (40%) hybrid film as the anode demonstrates higher current efficiency and brightness.
4. Conclusions
In summary, PEDOT: PSS transparent conductive film and incorporated functional material GO were fabricated, followed by interface treatment. This process improved the conductivity of the hybrid film and reduced its square resistance, with a detailed analysis of the change in conductivity. The conductivity and square resistance of the optimal GO/PEDOT: PSS (40%) hybrid film after interface treatment are 4032 Scm-1 and 62 Ω/□, respectively, and the film exhibits a significant transmittance at 550 nm. OLED devices were constructed using ITO, PEDOT: PSS, and GO/PEDOT: PSS hybrid films as anodes, respectively. The device with the optimized hybrid film as the anode exhibits the lowest turn-on voltage and the highest current efficiency. In comparison to the device with the PEDOT: PSS film as the anode, the current efficiency increases from 2.9 cd/A to 7.0 cd/A, and the maximum brightness experiences a 2.37-fold increase, from 15265 cd/m2 to 36271 cd/m2. The interface treatment of the hybrid film results in higher conductivity, transmittance and HOMO, ensuring superior photoelectric performance for OLED devices.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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