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

• Formation mechanism of the continuous metal electrodes is investigated.
• The introduction of the MoO3 interlayer can improve the wetting of Ag on substrate.
• A 2 nm MoO3 layer effectively reduces Ag threshold from 11 to 9 nm.
• Good conductivity and transparency are obtained for MoO3 (2 nm)/Ag (9 nm) anode.
• The PCE for MoO3 (2 nm)/Ag (9 nm) anode is comparable to that for ITO anode.

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 MoO3 between the Ag anode and glass substrate is introduced. Two different thicknesses of MoO3 (2 nm or 10 nm) are chosen here, corresponding to the unclosed or closed MoO3 layer, respectively. It is observed that the introduction of the MoO3 interlayer can effectively improve the wetting of Ag on the substrate and reduce the percolation threshold of Ag. When the MoO3 thickness is 10 nm, since the surface energy of Ag (γ=1.25 Jm−2) is higher than that of MoO3 (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 MoO3 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 MoO3 (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 MoO3 interlayer (especially when the MoO3 layer is thin). It may be instructive for the further research of metal thin film electrodes.