  The conductivity and stability of specially treated PEDOT:PSS thin films are
investigated. Based on the proposed treatment method, ITO-free PTB7:PC71BM
organic solar cells are fabricated and the electrical properties of the cells
are analyzed. It is shown that by applying a two-step post-treatment method
using methanol and ethylene glycol, the conductivity of the PEDOT:PSS thin film
increases from 0.9 to 1448 S/cm, and at the same time, a significant
improvement of the stability of the layer over time is achieved. It is shown
that after 30 days of aging under high humidity condition, the conductivity
remains above 70% of its initial value, which is a remarkable result compared
with the results reported in the literature. In this paper, important factors
affecting the conductivity and stability of the treated layer are studied in
detail. In addition, the effect of immersion time in methanol on the
conductivity of the layer is also investigated and it is found that dipping
times less than 3 min have no appreciable effects on the improvement of the
conductivity. An ITO-free non-inverted PTB7:PC71BM solar cell is also
fabricated using the proposed post-treated PEDOT:PSS thin films as the
transparent anode. The power conversion efficiency of the resulting cell is
5.8%. The stability of the fabricated ITO-free cells is considerably better
than the stability of the non-treated ITO-free cells or the cells made using
ITO as anode.
