Highly Flexible Polymer Solar Cells Made with Uniform Silver Nanowire Patterned Electrodes on Robust PEN Substrate

The polymer solar cells (PSCs) have been extensively researched in recent years, and have the advantage that they can be applied to flexible devices. Although the brittle indium-tin-oxide (ITO) electrode was employed for flat=panel device, flexible transparent conductive electrodes should apply for the flexible and stretchable next-generation device, exhibiting low sheet resistance, high optical transmittance, low surface roughness, and increased flexibility. Various flexible transparent conductive electrodes, such as conducting polymers, graphene, metal nanowires, and hybrid types, have been proposed to replace ITO, and many studies have been conducted to demonstrate the excellent properties of electrodes using the above materials.<br/>Generally, AgNWs were dispersed with the solvent of isopropanol, it is hard to spread uniformly on the hydrophobic plastic substrate because of the surface tension and its wettability To overcome this problem, a few groups have attempted to confer hydrophilicity to the substrate by using PVA and PEDOT: PSS. However, the aqueous solution of the above polymers disadvantageously forms a non-uniform thin film; thus, the AgNW layer applied to the upper layer tends to be non-uniform. Therefore, it is necessary to study new coating materials that can change the surface properties of PET and PEN. However, all these methods only improve the adhesion between the AgNWs network and the substrate and are not ideal in terms of the overall uniform patterning process owing to the unsolved issue of random AgNWs distribution.<br/>In this study, we successfully fabricate a patterned AgNW-based electrode on a flexible PEN substrate after the UV irradiation/washing of an overcoat layer composed of PEDOT: PSS and a UV sensitizer. To increase the wettability between the PEN substrate and the AgNWs, a stable, optically transparent, hydrophilic poly(2-hydroxyethyl methacrylate) (PHEMA) film is coated as an underlayer for the AgNWs. Furthermore, the conductivity and transmittance of the AgNW layer are optimized by varying the concentration of the AgNW solution. The patterned AgNW electrode achieves a low sheet resistance (Rsh) of 22.6 Ω/square and high transmittance of 92.35% at 550 nm. A polymer solar cell (PSC) was fabricated to evaluate the characteristics of the device employing the flexible electrodes. This PSC not only showed a high power conversion efficiency of 11.20%, similar to that of ITO-based devices, but also excellent mechanical stability, which is difficult to achieve in ITO-based flexible devices