**2. Materials and Methods**

An aqueous dispersion of PEDOT:PSS has the concentration of 10.2 mg/mL and the nominal weight ratio of PEDOT to PSS of 1:2.5. The PEDOT:PSS dispersion was mixed with various additives at controlled molar concentrations and stirred for 24 h at room temperature. PEDOT:PSS films were prepared on a clean glass or silicon substrate by spin-coating at 3 krpm for 60 s and annealed on a hot-plate at 140 ◦C for 10 min in air. The absorption spectra of the films were measured using a Jasco V-670 UV-Vis-NIR spectrophotometer. The sheet resistances ( *R*sh) of the films were measured using 4-point probes in a colinear arrangemen<sup>t</sup> with a spacing of 1 mm and an Agilent 34450A digital multimeter. The resistance ( *R*) in a unit of Ω, obtained by dividing the voltage difference in the inner probes by the current flowing between the outer probes, was converted to the sheet resistance ( *R*sh) in a unit of Ω per square ( Ω/sq.) by multiplying a correction factor (π/ln2) for very thin films [44]. The film thickness (*d*) was measured by using an Alpha-step 500 surface profilometer. The electrical conductivity ( *σ*dc) was calculated with the sheet resistance and thickness of PEDOT:PSS films ( *σ*dc = 1/ *R*sh*d*). At least four different points were tested to ge<sup>t</sup> the average values. X-ray photoelectron spectra (XPS) were obtained under ultra-high vacuum (7 × 10−<sup>9</sup> mbar) by using Sigma Probe (Thermo VG Scientific, East Grinstead, UK) with a monochromatic Al-K α X-ray at 15 kV and 100 W. Survey scan of XPS was performed at 50 eV for pass energy and 1.0 eV for step size. A high-resolution scan was performed at 20 eV for pass energy and 0.1 eV for step size. The XPS peaks were fitted with the Avantage program and calibrated with C1s (284.5 eV) as reference. Scanning electron microscopic imaging was conducted using S-3400N (Hitachi, Tokyo, Japan).
