/Al-Doped Ag Electrode for Resonant Light Harvesting in Effi cient Organic Solar Cells

Moreover, to further boost the device effi ciency, a resonant optical cavity can be created between the transparent thin Ag fi lm and an opaque back contact metal electrode, thus trapping light inside the active layer and enhancing its light absorption. [ 22 ] However, Ag follows 3D growth mode: during deposition, Ag atoms fi rst agglomerate into isolated islands, and as the deposition continues, these islands fi nally connect to each other and form a continuous fi lm. [ 30 ] On one hand, a certain threshold of thickness (usually beyond 10 nm) is required to get a continuous and conductive Ag fi lm. Though the conductivity is satisfi ed by a thicker fi lm, the transparency is inevitably compromised. On the other hand, the roughness of a continuous Ag fi lm is large (e.g., root-mean-square (RMS) roughness of 6 nm for 15 nm Ag fi lm), [ 30 ] resulting in additional light scattering loss. Moreover, since most OSCs are only few hundred nm thick, a rough surface could easily result in electrical shorts between electrodes, especially for large-area devices. To address the issues mentioned above, a wetting layer is usually placed underneath the Ag fi lm to promote the formation of continuous fi lms below or around 10 nm. Thus far, various wetting layers have been demonstrated by either dielectric materials (MoO x , [ 22,24,31–33 ] ZnO, [ 34 ] WO 3 , [ 35 ] and TeO 2 [ 36 ] ) or semiconductors/metals (Ge [ 30 ] and Au [ 37 ] ). Semiconductors and metals are lossy in the visible band, leading to the reduced transparency. Besides the use of wetting layers, thin and smooth Ag fi lms have also been achieved by treating the substrate with a layer of molecules to promote the Ag nucleation [ 38 ] or cooling down the substrate temperature during the Ag deposition. [ 22 ] However, it is highly desirable to have an alternative, simpler, and more scalable method to fabricate thin and smooth Ag fi lms in a single step and without special treatment of the substrate or special control of the deposition condition. We recently reported an ultrathin and smooth Ag fi lm without any wetting layer, achieved by doping small amount of Al into Ag during the fi lm deposition. [ 39 ] In this work, we will demonstrate a conductive and transparent Ag based fi lm as thin as 4 nm by adding a wetting layer (Ta 2 O 5 ) underneath the Al-doped Ag. OSCs built on such thin fi lms produce a PCE over 7%. In addition, the Ta 2 O 5 layer functions as an optical A effi cient indium tin oxide (ITO)-free transparent electrode based on an improved Ag fi lm is designed by introducing small amount of Al during co-deposition, producing ultrathin and smooth Ag fi lm with low loss. A transparent electrode as thin as 4 nm is achieved by depositing the fi lm on top of Ta 2 O 5 layer, and organic solar cells based on such ultrathin electrode are built, producing power conversion effi ciency over 7%. The device effi ciency can be optimized by simply tuning Ta 2 O 5 layer thickness external to the organic photovoltaic (OPV) structure to create an optical cavity resonance inside the photoactive layer. Therefore Ta 2 O 5 /Al-doped Ag fi lms function as a high-performance electrode with high transparency, low resistance, improved photon management capability and mechanical fl exibility.