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

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.