###### Abstract  
We herein report the effect of the viscosity of a prepolymer solution on the thickness uniformity of silver-nanowire-embedded flexible transparent electrodes. We adopted a model system with all the prepolymer solutions possessing identical physical properties except for the viscosity and then explored the most common prepolymer solutions for silver-nanowire-embedded flexible electrodes. In all experiments, single-step spin coating was conducted to coat the prepolymer solution on silver nanowires. We found that the electrodes were thinner for lower viscosity. However, the thickness ratio between the center and edge was comparable (50-60%) and independent of the prepolymer solution viscosity. This indicates that the viscosity does not determine the thickness uniformity, and that the coating method itself is vital to obtain films with uniform thickness. The flexible electrodes were introduced into organic solar cells. Their device performance was comparable regardless of the position of the electrodes and their thickness. This is because the thickness difference of the flexible electrodes did not affect their transmittance significantly. Thus, we conclude that although different coating approaches are needed to obtain flexible electrodes with high uniformity, the performance of optoelectronic devices on silver-nanowire-embedded flexible electrodes is independent of them. silver nanowire; polymer viscosity; embedding; flexible electrode; thickness uniformity +
Footnote †: journal: Applied Physics Letters  
_Applied Physics Letters_  
deposited on a flexible film was a popular material for flexible transparent electrodes because of its high conductivity and stability as an electrode and because of being common in rigid electronics [6; 7; 8; 9; 10]. However, it cracks easily when stress is applied and is not suitable for flexible electronic devices that have to be bent repeatedly [11; 12; 13]. To realize flexible devices, many candidates have been developed, including graphene, carbon nanotubes, metal nanowires, and organic conductors [14; 15; 16; 17; 18; 19]. Although graphene, carbon nanotubes, and organic conductors are limited by their low area, yield, or electronic performance, metal-nanowire-based flexible transparent electrodes, especially those based on silver nanowire, have displayed high conductivity and high transmittance and facilitated easy manufacturing in a large area at a low cost [20; 21; 22]. Although the excellent performance of silver-nanowire-based transparent electrodes showed their potential as flexible electrodes, their rough surface hindered their application to electronic devices. In particular, their rough surface and sharpness cause them to break through the functional layers and contact the counter electrode, resulting in device shorting [23; 24; 25; 26; 27]. To prevent roughness issues, researchers have embedded conductive silver nanowires into flexible polymers [26; 27]. Because the nanowires form a network inside the polymer, the surface of the electrode is flat, avoiding device shorting. In our previous research, we demonstrated silver nanowires on a flexible film with a root mean squared (RMS) roughness of 8 nm and silver-nanowire-embedded electrodes with an RMS roughness of 1 nm, which improved the reliability of the construction and operation of electronic devices [28]. To embed sharp silver nanowires into a polymer, it is essential to coat a prepolymer with high viscosity on the nanowires and detach it from the substrate after curing. However, the high viscosity can induce a nonuniform thickness of the flexible electrode that can affect the electrode properties and the performance of the final device. In this study, we controlled the viscosity of the prepolymer for embedding nanowires and investigated its effect on the thickness difference in silver-nanowire-embedded transparent flexible electrodes.