Organic light-emitting diodes (OLEDs) are widely used in displays and lighting due to their active luminescence, rapid response, high contrast, and flexibility. However, the current standard transparent electrode, indium tin oxide (ITO), is rigid, limiting the development of truly flexible OLEDs. Flexible electrodes are crucial for next-generation display devices that conform to curved surfaces or even human skin. The challenge lies in finding materials that combine high electrical conductivity and stretchability—a property not found in traditional metal or metal oxide electrodes. Current approaches focus on incorporating conductive materials like silver nanowires, carbon nanotubes, and graphene into flexible substrates. Silver nanowires are particularly promising due to their high conductivity and light transmittance; however, compatibility issues between nanowires and polymer substrates limit long-term stability. Highly conductive polymers offer an alternative, but conventional ones like polyaniline, polythiophene, and PEDOT:PSS have limitations in conductivity for OLED applications. While techniques exist to improve PEDOT:PSS conductivity and flexibility, the search for superior materials continues. Polybenzodifuranedione (PBFDO), a self-doped conducting polymer with high conductivity and suitable work function, is a promising candidate to replace ITO. However, the high viscosity and freezing point of commercial PBFDO solutions hinder high-quality film formation via spin coating due to intermolecular hydrogen bonding. This research addresses this challenge by modifying the PBFDO solution to improve its processability and ultimately create a high-performing flexible OLED.

The literature highlights the limitations of ITO as a transparent electrode in flexible OLEDs, leading to the exploration of alternative materials such as silver nanowires, carbon nanotubes, and graphene. While silver nanowires show promise, their integration with polymer substrates poses challenges to long-term stability. Conductive polymers, particularly PEDOT:PSS, have been investigated, but improvements in conductivity are needed for effective OLED applications. Various methods, including acid treatment, plasticizer addition, and topological supramolecule approaches, have been used to enhance the properties of PEDOT:PSS. The emergence of PBFDO as a highly conductive, n-type self-doped polymer with a suitable work function presents a potential solution; however, its high viscosity and freezing point, due to intermolecular hydrogen bonding in solution, present significant challenges in film formation.

The researchers modified a commercial PBFDO solution by adding n-butanol at a 1:2 volume ratio to the PBFDO mother liquor. The addition of n-butanol was intended to disrupt the intermolecular hydrogen bonds between PBFDO molecules and DMSO solvent molecules. This reduced both the solution's viscosity and its freezing point, improving its spin-coating processability. The modified solution was filtered through a hydrophilic PTFE membrane to remove any particulate matter and heated at 120 °C for 10 min to remove residual solvents. The modified PBFDO solution was then spin-coated onto glass or PEN substrates in multiple layers to achieve the desired thickness (15-65 nm). After each PBFDO coating, the substrate was washed with ethanol and heated at 120 °C for 15 minutes before applying the subsequent layer. PEDOT:PSS (PH 4083) was then spin-coated, followed by a chlorobenzene solution of Super Yellow (SY) to create the light-emitting layer. Finally, CsF (1 nm) and Al (100 nm) were sequentially deposited via vacuum evaporation to complete the device structure. The devices were characterized using various techniques: Differential Scanning Calorimetry (DSC) and rheometry for solution properties, step profilometry and UV-Vis-NIR spectroscopy for film thickness and transmittance, ellipsometry for refractive index, atomic force microscopy (AFM) for surface roughness and work function (using SKPM), Fourier Transform Infrared (FTIR) spectroscopy for hydrogen bonding analysis, ultraviolet photoelectron spectroscopy (UPS) for work function, X-ray diffraction (XRD) for crystallinity, four-probe resistivity testing for electrical conductivity, and a color luminance meter and spectrometer for device performance (current-voltage, luminance-voltage, luminous efficiency-luminance curves and electroluminescence spectra). For flexible device fabrication, PEN substrate was used instead of glass. Blue OLEDs were also fabricated by replacing SY with PFSO10.

FTIR analysis confirmed the presence of hydrogen bonding in both the unmodified PBFDO solution and the resulting film. The addition of n-butanol led to a shift in the hydrogen bonding peak, indicating interaction with both DMSO and PBFDO. DSC showed a significant reduction in the freezing point from -1 °C to -27 °C upon addition of n-butanol. Rheological measurements showed a decrease in viscosity of approximately 4 mPa·s at similar temperatures. The modified solution displayed improved wettability on the substrate, allowing for better film formation. The PBFDO films demonstrated high optical transmittance (up to 91% at 15 nm thickness), low surface roughness (2.06 nm), and high conductivity (up to 1271 S cm⁻¹ at 65 nm thickness). The work function of the PBFDO film was determined to be approximately 5.3 eV using both SKPM and UPS. XRD indicated highly regular molecular stacking. OLED devices fabricated with the PBFDO anode exhibited a low threshold voltage of 2.6 V and a maximum luminous efficiency of 12.8 cd A⁻¹, comparable to ITO-based devices. The lower refractive index of PBFDO (1.23-1.40) compared to glass (1.51) minimized light loss from total internal reflection, improving light extraction. Flexible devices on PEN substrates demonstrated satisfactory performance with a luminous efficiency of 7.7 cd A⁻¹. Devices without PEDOT:PSS showed improved performance compared to ITO-based devices due to the reduced hole injection barrier. Long-term stability tests indicated comparable lifetime of the PBFDO-based devices compared to ITO-based devices. Blue OLEDs were fabricated using PFSO10 with comparable performance to ITO-based devices.

The successful integration of PBFDO as a transparent electrode in OLEDs demonstrates the potential of highly conductive polymers for flexible electronics. The strategic addition of n-butanol effectively addressed the processing challenges associated with the high viscosity and freezing point of the commercial PBFDO solution. The excellent optical and electrical properties of the PBFDO films, coupled with their appropriate work function, contributed to the high performance of the resulting OLED devices. The comparable performance of PBFDO-based devices to ITO-based devices, including in flexible configurations, showcases the potential of PBFDO to replace ITO as a transparent electrode in flexible OLED applications. The lower refractive index of PBFDO, compared to ITO and glass, contributes to efficient light extraction, further improving device performance. The ability to fabricate high-performance blue OLEDs using PBFDO further expands the versatility of this material. The study's findings contribute to the advancement of flexible and efficient OLED technologies.

This study successfully demonstrated the use of a modified PBFDO solution for fabricating high-quality, highly conductive polymer films suitable for use as transparent anodes in OLEDs. The modification, involving the addition of n-butanol, significantly improved the solution's processability and enabled the fabrication of OLEDs with performance comparable to those using ITO anodes. The results highlight the potential of PBFDO as a promising replacement for ITO in flexible OLED technology. Future research could focus on further optimizing the PBFDO formulation, exploring different polymer combinations for enhanced device performance, and investigating long-term stability under various operating conditions.

While the PBFDO-based devices showed excellent performance comparable to ITO devices, the maximum luminance was lower due to the lower conductivity of PBFDO compared to ITO at the same thickness. The flexible devices on PEN substrates showed lower efficiency compared to glass-based devices, potentially due to lower light transmittance and light extraction efficiency of PEN. The study focused primarily on the device performance and didn't extensively explore the long-term stability or the mechanical durability of the flexible devices under various stress conditions.