Ex situ spectroscopic analysis was performed on PEDOT thin films transferred to suitable solid substrates (Supplementary section 6). A bipolaron band was observed by UV/vis absorbance, signifying that the PEDOT thin film is in an oxidised state and p-doped (Supplementary Fig. ). Raman spectroscopy also confirmed that the PEDOT thin film is pdoped, with high π-π conjugation and a benzenoid (coiled) configuration to the polymer chain (Supplementary Fig. ), the more stable form when PEDOT is highly doped. An X-rayphotoelectron spectroscopy (XPS) survey spectrum showed the presence of only sulfur, carbon and oxygen (Supplementary Fig. ), indicating that the cerium oxidant, boron and fluorine are not incorporated into the thin film. This implies that the organic TB -anion is not involved in p-doping and SO4 2-is the primary dopant. Further XPS analysis revealed that the doping level was estimated as ~39% (Supplementary Fig. ), in line with previous reports of the upper limits possible of PEDOT doping between 35 -40%. Ex situ conductivity of the PEDOT thin film was determined as 554 (±77) S•cm -1 (Supplementary Section 7.1), comparable to the highest conductivity value reported for a pristine PEDOT:ClO4 -film (400-650 S•cm -1 ) made by conventional electropolymerisation at a solid electrode surface in acetonitrile. The in situ conductivity of a PEDOT thin film and a commercial PEDOT:PSS film drop cast and annealed directly onto a microelectrode array were compared in PBS buffer solution (Supplementary Fig. ). The PEDOT and PEDOT:PSS films had maximum conductivities of ~5.35 S•cm -1 and ~1.2 S•cm -1 , respectively, in their plateau regions, with the conductance window in the PEDOT thin film 0.4 V greater than that of the PEDOT:PSS film (Supplementary Fig. ). The latter is advantageous for OECT devices working at lower potentials to avoid oxidative reactions or biological stress (if the active layer is to be functionalized with cells). Corresponding CVs of each film in PBS buffer solution were obtained using a range of scan rates and potential ranges (Supplementary Figs. and). The PEDOT thin film remains doped over a very wide potential range (in comparison to the PEDOT:PSS film) and therefore has high conductivity even at negative potentials. At high scan rates, the PEDOT thin -film displays an ideal capacitive behaviour due to the polymer films high intrinsic conductivity and wide conductance window.