Stability enhancement of ITO-free non-inverted PTB7:PC₇₁BM solar cell using two-step post-treated PEDOT:PSS

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.