Stability enhancement of normal-geometry organic solar cells in a highly damp condition: A study on the effect of top electrodes

• Efforts to enhance the stability of normal OSCs under damp condition are presented.
• Changing top electrode layers to other materials did not bring much improvement.
• Al/Cu bilayer electrode demonstrated reduced bubbles and low WVTR.
• T80-lifetime of Al/Cu-based normal OSC improved 30-fold compared to Al-based one.
• The lifetime of the proposed normal OSC is comparable to that of inverted OSCs.

We present our efforts to enhance the stability of normal-geometry organic solar cells (n-OSCs), which are generally considered inferior to their inverted-geometry counterparts in terms of stability. Upon the identification of the vulnerability of top electrode/buffer layer interfaces under a humid environment, various top electrode combinations are assessed under an extremely damp condition (27 °C, 90%) for n-OSCs based on a bulk-heterojunction of poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl C70-butyric acid methylester (PCBM70). Based on the experimental results, we propose an Al/Cu bilayer top electrode and demonstrate a 30-fold enhancement in the T80-lifetime values. Our study reveals that the enhanced lifetime with an Al/Cu bilayer electrode can be attributed to its water vapor transmission rate (WVTR), which is significantly lower than that of the Al electrodes typically used in conventional organic solar cells. The enhanced normal OSCs yielded stability comparable to that of the previously reported inverted OSCs.

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