| The rapid growth of advanced flexible optoelectronic and photovoltaic devices urgently requires the development of flexible transparent conducting electrodes (FTCEs) with high-performance, including not only good optoelectrical properties, high flexibility and stability but also low-cost, simple fabrication method and good compatibility to the large-scale process. The conventional single-layer transparent conducting oxide, usually indium tin oxide (ITO) cannot be competent as qualified electrode, due to the high-cost, mechanical brittleness and the unavoidable heating process to improve the electrical performance. Currently, the electrodes based on metal nano-wire, nano-mesh, carbon nanotube and graphene, are regarded as the most promising candidates and get extensive study. Unfortunately, the aforementioned candidates suffer different technique issues, the irregularly stacked metal nonowires with high surface roughness cause unavoidable susceptibility to oxidation and thus degradation of stability, while the carbon nanotube and graphene based electrodes are still struggled in searching a low-cost and effective fabrication method to touch the theoretically predicted high performance. Thus, it is not realistic for these electrodes to realize scale-up manufacture easily to replace ITO in the near future.With the simple and durable architecture, mechanical flexibility, well-matured manufacture technique and easy scale-up process, the hybrid electrode with a structure of thin metal film sandwiched between oxide (oxide-metal-oxide, OMO), is a potential candidate. But the tradeoff between optical transmittance and electrical conductivity is unavoidable unless the absolutely ultrathin continuous metal layer on oxide substrate can be achieved. However, because of the intrinsic three dimensional growth of metal on heterogeneous oxide substrate, it is difficult to realize the ultrathin metal film on oxide. Up to now, there is no report related to improve the wettability of Cu on oxide. Once the low-cost Cu was chose to replace Ag as inserted metal in OMO structure, the transmittance perform a further degradation, and thus Cu thin film based electrodes are not attractive to the research field of FTCEs.In this research, one effective method was developed to improve the Cu wettability on ZnO. By introducing trace amount of O2 and N2 during the deposition, the ultrathin smooth and continuous oxygen-doped Cu(O) and oxygen-doped Cu(N) thin films was fabricated. Further, the Cu(O) thin films were verified to be a good wetting layer for Cu growth on oxide. Based on these ultrathin Cu(O), Cu(N) and Cu(O)/Cu films, OMO electrodes with highly flexibility, stability and optoelectrical properties were developed, and using these novel Cu-based electrodes replace the traditional Cu thin film electrode and single-layer ITO as the front electrode of organic solar cell, the efficiency of devices has been effectively improved. The main work of this study is followed:Firstly, by introduce trace amount of oxygen, ultrathin continuous weakly oxidized Cu (Cu(O)) film was produced at low thickness. This was achieved through the successful suppression of nanoscopic cluster coalescence with limited Cu oxidation during the very early stages of Cu growth. The ZnO/Cu(O=5.0%)/ZnO electrodes exhibits good optoelectrical performance (an average transmittance of 83% over the visible spectral range of 400-800nm and a sheet resistance of 13.0Ωsq-1)and strong oxidation resistance. A flexible organic solar cell employing the Cu(O)-based FTCE exhibited a high power conversion efficiency (PCE) of 7.5%, outperforming efficiency of the same device that used a traditional Cu film based electrode and ITO electrode.Seondly, considering the high oxidation affinity of Cu, and thus the unavoidable technical issues of target poisoning and the difficulty with precise control of oxidation in the large-size Cu(O) films, by replace O2 with inert N2, the effect of minimal nitrogen dose on the Cu thin film evolution was studied. It was verified that less than 1 at% nitrogen in Cu can still contribute to the early formation of continuous Cu(N) film. N atoms adsorbed on Cu clusters may enhance the adhesion of Cu(N) clusters to ZnO surface by disturbing the cohesion between Cu atoms, it will promote the lateral expansion of the evolving Cu(N) clusters along the substrate surface and early formation of continuous film. A low-cost, simple, highly reliable and realistic method for preparation high-performance FTCEs was developed, and the prepared Cu(N) based electrode show better performance than pure Cu based electrode. The organic solar cell (OSC) using the Cu(N)-based electrode exhibited a power conversion efficiency (PCE) of 7.1%.At last, insert 1 nm-Cu(O) thin film as wetting layer between ZnO substrate and top Cu film, an continuous Cu(O)/Cu hybrid film was firstly reported, and it was found that the top Cu film perform a kind of quasi-epitaxy growth on Cu(O) wetting layer. Due to the reduction of impurity concentration in sandwiched Cu film, the ZnO/Cu(O)/Cu/ZnO electrode show further improved optoelectric performance which is better than ITO performance, the average transmittance in the visible wavelength is higher than 85%, sheet resistance is as low as 11Ω sq-1. Further, the organic solar cell (OSC) adopt the ZnO/Cu(O)/Cu/ZnO as window electrode exhibited a record power conversion efficiency (PCE) of 7.7%.In a word, here one effective way was reported to fabricate ultrathin metal films on oxide substrate. What’s more, mechanism behind the wettability improvement of metal on heterogeneous oxide was explored. Using the ultrathin continuous Cu as the sandwiched metal layer of OMO electrode, a kind of electrodes with high optoelectronic performance was successfully developed, and thus provided a promising solution for a simple fabrication of high-quality FTCEs on flexible polymer substrate. And by using these novel FTCEs as window electrode, high-efficiency organic solar cells were realized. |
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