Improvement of transparent silver thin film anodes for organic solar cells with a decreased percolation threshold of silver

摘要

Organic solar cells (OSCs) using thermally evaporated Ag thin films as the transparent anode are investigated with poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) films as the active layer. The optimal power conversion efficiency (PCE) of 2.57% is obtained for the device with the 11 nm thick Ag layer, namely the percolation threshold of Ag. Then an interlayer of MoO_3 between the Ag anode and glass substrate is introduced. Two different thicknesses of MoO_3 (2 nm or 10 nm) are chosen here, corresponding to the unclosed or closed MoO_3 layer, respectively. It is observed that the introduction of the MoO_3 interlayer can effectively improve the wetting of Ag on the substrate and reduce the percolation threshold of Ag. When the MoO_3 thickness is 10 nm, since the surface energy of Ag (γ=1.25 Jm~(-2)) is higher than that of MoO_3 (γ=0.06 Jm~(-2)), the Ag-Ag interactions are stronger than the Ag-substrate interactions, weakening the surface-modifying effect. In contrast, since the 2 nm thick unclosed MoO_3 layer may create preferred nucleation sites on the substrate to enhance the lateral growth of Ag film effectively, a very low sheet resistance of 9.32 Ω/□ and a relatively high transmittance are obtained for MoO_3 (2 nm)/Ag (9 nm) anode. The PCE of corresponding OSCs can be improved to 2.71%, comparable to that of ITO-based reference OSCs (PCE of 2.85%). From our results, it is concluded that the performance of the Ag thin film electrode can be improved by decreasing the percolation threshold of Ag with a MoO_3 interlayer (especially when the MoO_3 layer is thin). It may be instructive for the further research of metal thin film electrodes.