Since its discovery in the 1990s, PEDOT:PSS has been widely used as an interlayer in organic-based electronic devices, such as polymer PVs and LEDs, to reduce the ITO roughness and facilitate the hole collection/injection between the polymers and the ITO.146 Low conductivity PEDOT:PSS was first developed with a conductivity of 1 to , about three orders of magnitude lower than that of ITO (). While the electrical conductivity is not the most critical factor for a thin interlayer between ITO and organic active layer(s), the relatively high resistivity of PEDOT:PSS severely limits its application as a stand-alone electrode. Several methods have been successfully developed to improve the electrical conductivity of commercialized PEDOT:PSS. Some polar organic molecules with a high boiling temperature, such as dimethylsulfoxide (DMSO),13,147,148 ethylene glycol (EG),149,150 diethylene glycol,151 and sorbitol,17,152 have been mixed into the PEDOT:PSS aqueous solutions, which can enhance the film conductivity by more than one order of magnitude without affecting the transparency and high work function. For example, Na et al. reported highly conductive PEDOT:PSS layers with an average conductivity of by adding 5% DMSO to Baytron PH500 solution, compared to a film conductivity of without DMSO.13 Using such a modified PEDOT:PSS layer to replace the bottom ITO electrode, the ITO-free P3HT:PCBM device showed an open-circuit voltage () of 0.63 V, short-circuit current density () of , fill factor (FF) of 53.5%, and of 3.27% under 1 sun simulated AM1.5 solar illumination, which was comparable with the ITO-based reference cell () [see Fig. 7(a)]. The authors also explored the flexibility of the PEDOT:PSS films and its based device on PET substrates: as shown in Fig. 7(b), the resistance of the PEDOT:PSS film on a PET substrate remained nearly constant after 2500 bending cycles, while the resistance of conventional ITO films on PET substrate increased times after the bending, which led to a dramatic degradation in device efficiency. The ITO-based cell almost completely degraded after 75 cycles of bending, while the PEDOT:PSS-based devices showed nearly the same efficiency after 300 bending cycles.13 Similarly, flexible OLEDs with PEDOT:PSS as the anodes have been reported by several research groups.12,153,154 Wang et al. successfully demonstrated a flexible white OLED using a PEDOT:PSS anode with good device performance (power efficiency of at the brightness of was achieved) and mechanical flexibility (power efficiency reached after 100 bending cycles).155 Some other techniques have also been demonstrated to further increase the PEDOT:PSS film conductivity. Kim et al. reported conductivities of for PEDOT:PSS films with the addition of 6 vol% EG, which were further increased to with solvent post-treatment (immerse the PEDOT:PSS films into EG and dry afterward).37 Formic acid was also used to treat PEDOT:PSS for high conductivities: the highest conductivity of up to was successfully achieved with the formic acid treatment, approaching that of ITO.156 According to these reports, the OPV cells based on the highly conductive PEDOT:PSS TEs exhibited the same efficiency as their ITO counterparts, which suggests their promising applications in optoelectronic devices.