# Effects of Intrinsic Factors on the Stability of Silver Nanowire-Based Flexible Electrodes
## 1 Introduction  
In recent years, flexible and transparent electrodes have been developed rapidly due to the urgent demands for flexible and wearable electronic and optoelectronic devices for display, lighting, energy, and sensing. Silver nanowires (AgNW)-based electrodes have been widely used in flexible optoelectronics owing to their high visible light transmittance, low square resistance, good flexibility, strong stretchability, large-area fabrication, and low cost [1, 2, 3, 4, 5, 6]. However, AgNW-based electrodes are susceptible to degradation after long-term operation or under external reactions, such as strong light illumination, humidity, or heat variations. The instability of AgNW electrodes limits their applications. AgNW electrodes can be regarded as composite structures in which a network of silver nanowires with a large surface area is bound to a flexible substrate. Given the structural characteristics, chemical, and physical properties of AgNW electrodes, the factors affecting the stability of AgNW electrodes can be summarized as intrinsic and non-intrinsic. Intrinsic factors arise from the material and structural parameters of AgNW electrodes themselves, including the specific surface area (SSA) and surface density (SD) of silver nanowires, the surface roughness of electrodes, network connections between silver nanowires, thebonding strength of silver nanowires with flexible substrates, and the chemical stability of flexible substrates. There have been many studies showing that attaching graphene oxide (GO) to silver nanowire surfaces can reduce the contact resistance between silver nanowires, which makes the square resistance of AgNW electrodes lower and also improves the stability of AgNW electrodes to some extent [7; 8]. Therefore, the intrinsic factors associated with AgNW electrode structures and surfaces determine the stability level of AgNW electrodes directly. Non-intrinsic factors refer to external conditions and effects, such as temperature, humidity, light illumination, magnetic field, electric field, operating voltage or current, etc. Non-intrinsic factors affect the intrinsic parameters of AgNW electrodes and accelerate the degradation of AgNW electrodes. For example, the content of the silver oxides and silver nitrides on AgNPs' surfaces would increase in ambient conditions or after a long period of high humidity or its effects [9; 10; 11] and the morphology of the contact points between silver nanowires and the resistance of electrodes would change after strong illumination [12; 13; 14]. The effects of external factors on the stability of silver nanowire electrodes have been reported and explored extensively in recent research work [9; 10; 11; 12; 13; 14], and some solutions and methods, such as surface passivation, lowering of surface roughness of AgNW electrodes, have been proposed to improve the stability of the AgNW electrodes [15; 16; 17]. However, there have not been any reports so far that systematically demonstrate the influences of intrinsic factors of AgNW electrodes on their stability. In our work, polyethylene terephthalate (PET) and polyvinyl alcohol (PVA) were selected as flexible substrate materials to prepare PET-AgNW electrodes and PVA-AgNW electrodes, respectively. As commonly used flexible and transparent substrate materials, PET and PVA have excellent physical and mechanical properties, fatigue and friction resistance, and excellent electrical insulation. Compared with PVA, PET has high surface hardness, high heat distortion temperature and long-term use temperature, and high bending strength and elastic modulus. PVA has better mechanical tensile properties than PET, as the tensile strength of PVA increases with the degree of polymerization and alcoholysis. Therefore, both PET and PVA are suitable to be substrates for flexible electrodes.