Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is one of the most widely used conducting polymers owing to its interesting properties, namely high optical transparency when in thin film, high thermal stability, high chemical stability, tailored work function, adjustable conductivity (from 10-3 S/cm to over 4000 S/cm) upon doping strategies, and availability in the form of aqueous dispersions.1
We report on the effect of oxetane compounds as a new class of additives to PEDOT:PSS that promote both substantial conductivity enhancements (in up to three orders of magnitude) and improved resistance to aqueous medium of PEDOT:PSS spin-cast thin films, while keeping high transmittance in the visible range. By investigating different oxetanes, the study allowed to unravel the structural characteristics of the oxetanes which allow to attain the referred improvements in PEDOT:PSS films´ properties. In particular, it was found that the oxetanes polymerize in the PEDOT:PSS acidic medium during the film drying step to yield polyether compounds.2 In case oxetanes contain hydroxyl groups, these can establish covalent cross-linking or hydrogen bonds with negative PSS (Fig. 1), as determined by 1H-NMR. Such features are indicated to be at the origin of both enhanced structural stability and conductivity of the polymer films; the resultant screening effects between charged PEDOT and PSS chains are suggested to cause PEDOT segregation and conformational changes, from benzenoid to quinoid, thus favoring charge transport. To assess the films´ applications as transparent electrodes resistant to aqueous medium, organic photovoltaic devices incorporating one of the PEDOT:PSS:oxetane films (R = 213 /sq) as the cathode, onto which an aqueous dispersion of a low conducting PEDOT:PSS hole transport layer was deposited on top, were fabricated and showed performance similar to that of the reference devices with indium tin oxide (ITO) transparent electrode. In conclusion, the work presents a new class of dopants for PEDOT:PSS which simultaneously allow remarkable conductivity improvements and superior water stability of thin films without involving toxic or harsh conditions (e.g. high temperatures).
Acknowledgements: This work was funded by Fundação para a Ciência e Tecnologia (FCT-Portugal) through Instituto de Telecomunicações (contract no. UIDB/50008/2020) and Centro de Química Estrutural (CQE) at IST (UIDB/00100/2020, UIDP/00100/2020).

[1] N. A. Shahrim, Z. Ahmad, A. W. Azman, Y. F. Buys, N. Sarifuddin, Mater. Adv., 2021, 2, 7118-7138.
[2] S. M. Jorge, L. F. Santos, A. Galvão, J. Morgado, A. Charas, Adv. Mater. Interfaces, 2021, 8, 2100517