Conductive polymer poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is one of the most attractive materials for transparent conductive electrodes, energy storage and electrochromic devices for smart window application. In this work, we investigated the effect of various chemical treatment on flexible transparent PEDOT:PSS electrode. The optoelectronics properties, electrochemical supercapacitor performance, and electrochromic behavior was determine. Based on the findings, the benzenesulfonic acid (BSA) shows high optical transmittance of 95 % and high conductivity with sheet resistance of 97 Ω/□, which demonstrate an areal capacitance of 0.58 mF/cm2 higher than the previous report. In addition, flexible transparent electrode shows excellent mechanical properties when folded for 9000 cycles at bending radius of 1 mm. In addition, when potential applied into all solid-state flexible PEDOT:PSS, the transmittance changeof 22 % at 630 nm showed promising electrode for electrochromic devices.