Modification of PEDOT:PSS towards high-efficiency OLED electrode via synergistic effect of carboxy and phenol groups from biomass derivatives

Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonic acid) (PEDOT:PSS) emerges as a promising transparent conducting electrode to replace indium tin oxide (ITO) in the field of optoelectronic devices. It is a pending issue that how to achieve high-conductivity PEDOT:PSS to meet the requirement as device electrode. In this work, two biomass-derived modifiers of PEDOT:PSS, i.e., protocatechuic acid (PA) and gentianic acid (GtA), were investigated. The resultant conductivity was respectively estimated as 2022 S/cm (PA-treated) and 2270 S/cm (GtA-treated), in comparison with that of the ordinarily prepared film (∼0.9 S/cm). By analyzing the PEDOT/PSS ratio, phase morphology, as well as benzenoid/quinoid conformation, crystalline structure, the mechanism on conductivity enhancement was systematically elucidated. Based on the density functional theory, the synergistic effect of carboxy and phenol groups was verified as the origin of excellent doping-efficiency of these two biomass-derived modifiers. What is more, an ITO-free OLED device was successfully fabricated, where the PA or GtA-treated PEDOT:PSS was employed as transparent conducting electrode. Surprisingly, a remarkable improvement with regard to device performance was observed, in spite of the not good enough conductivity relative to ITO. Using the PA/GtA-modified electrodes, the external quantum efficiency (EQE), current efficiency, and power efficiency of OLED devices is elevated by 50–300% at the brightness of 100 cd/m2 for electronic display application. To reveal the underlying mechanism, the energy levels and the light extraction were examined and then ascribed to the paradox. The findings present a possible strategy to construct high-efficiency OLEDs relying on organic transparent electrodes and a promising application of biomass-derived chemicals.