Jeonju Center, Korea Basic Science Institute , Jeonju, Jeonbuk 54907, Republic of Korea.
Department of Materials Science and Engineering, Korea University , Seoul 02841, Republic of Korea.
ACS Appl Mater Interfaces. 2017 Nov 8;9(44):38695-38705. doi: 10.1021/acsami.7b10234. Epub 2017 Oct 26.
The development of highly efficient flexible transparent electrodes (FTEs) supported on polymer substrates is of great importance to the realization of portable and bendable photovoltaic devices. Highly conductive, low-cost Cu has attracted attention as a promising alternative for replacing expensive indium tin oxide (ITO) and Ag. However, highly efficient, Cu-based FTEs are currently unavailable because of the absence of an efficient means of attaining an atomically thin, completely continuous Cu film that simultaneously exhibits enhanced optical transmittance and electrical conductivity. Here, strong two-dimensional (2D) epitaxy of Cu on ZnO is reported by applying an atomically thin (around 1 nm) oxygen-doped Cu wetting layer. Analyses of transmission electron microscopy images and X-ray diffraction patterns, combined with first-principles density functional theory calculations, reveal that the reduction in the surface and interface free energies of the wetting layers with a trace amount (1-2 atom %) of oxygen are largely responsible for the two-dimensional epitaxial growth of the Cu on ZnO. The ultrathin 2D Cu layer, embedded between ZnO films, exhibits a highly desirable optical transmittance of over 85% in a wavelength range of 400-800 nm and a sheet resistance of 11 Ω sq. The validity of this innovative approach is verified with a Cu-based FTE that contributes to the light-to-electron conversion efficiency of a flexible organic solar cell that incorporates the transparent electrode (7.7%), which far surpasses that of a solar cell with conventional ITO (6.4%).
在聚合物衬底上开发高效的柔性透明电极（FTEs）对于实现便携式和可弯曲的光伏器件至关重要。具有高导电性、低成本的铜因其有望替代昂贵的铟锡氧化物（ITO）和银而受到关注。然而，由于缺乏高效的手段来获得原子级薄、完全连续的铜膜，同时具有增强的光学透过率和电导率，因此目前还没有高效的基于铜的 FTE。在这里，通过在 ZnO 上施加原子级薄（约 1nm）的氧掺杂 Cu 润湿层，实现了 Cu 的强二维（2D）外延。透射电子显微镜图像和 X 射线衍射图谱的分析，结合第一性原理密度泛函理论计算，表明在存在痕量（1-2 原子%）氧的情况下，润湿层的表面和界面自由能的降低是 Cu 在 ZnO 上二维外延生长的主要原因。嵌入在 ZnO 薄膜之间的超薄 2D Cu 层在 400-800nm 的波长范围内表现出超过 85%的理想光透过率和 11 Ω/sq 的面电阻。这种创新方法的有效性通过一个基于 Cu 的 FTE 得到了验证，该 FTE 有助于包含透明电极的柔性有机太阳能电池的光电子转换效率（7.7%），远远超过了使用传统 ITO 的太阳能电池（6.4%）。
