Facile transfer fabrication of transparent, conductive and flexible In₂O₃:Sn (ITO) nanowire arrays electrode via selective wet-etching ZnO sacrificial layer

Highlights

• In₂O₃:Sn nanowires array (ITO NWs) was grown by a vapor transport method.

• Transparent/flexible ITO NWs electrode was fabricated by etching/transfer process.

• Flexible solid-state dye-sensitized solar cell with 1.1% efficiency was fabricated.

Abstract

We report a facile transfer fabrication of tin-doped indium oxide (In₂O₃:Sn) nanowires array (ITO NWs) electrode with high transparency and bending stability via selective wet-etching of a sacrificial ZnO layer. The ITO NWs are grown on ZnO (200 nm)/Si substrate by using a vapor-liquid-solid (VLS) method and then subsequently it is soaked in basic solution (1 M NaOH) to dissolve ZnO under-layer selectively, which enables to detach the ITO NWs electrode from the substrate without damage. We found that open-structure and porous nature of the ITO NWs with ITO nanoparticles support facilitate the NaOH solution penetration into the ZnO layer, thus enabling rapid peeling off the ITO NWs, even for a large sized substrate. The fabricated ITO NWs electrode on the PET substrate showed good transparency, conductivity and flexibility, eventually enabling to fabricate a flexible solid-state dye-sensitized solar cell with a satisfactory performance.

Introduction

Transparent conducting electrodes (TCEs) are a crucial and basic component in a variety of optoelectronic devices, such as thin-film solar cells [1], [2], [3], organic light emitting diodes (OLEDs) [4], [5], and flat panel displays [6], [7], touches screens [8], [9], [10]. Recently, carbon nanotubes (CNT) [11], [12], graphenes [3], [13], metal grid or metal nanowires [14], [15], [16], conducting polymers [17], [18] with attractive mechanical, electrical and optical properties have been extensively studied. Among them, CNT and, more recently, graphene have attracted significant attention and have shown potential use in organic light emitting diodes and solar cells [19], [20]. However, their conductivity and transparency are highly sensitive to the fabrication process and are still need to be improved.

On the other hand, despite of the scarcity of indium and the brittleness of the material (tensile fracture strain ~1%), tin-doped indium oxide (ITO) is the most widely used transparent conducting materials due to their excellent optical transparency and high electrical conductivity [19], [20], [21]. Recently, with the need of flexible and wearable electronic devices, the use of ITO is challenged due to its brittleness leading to a failure of the device under pressure, physical stress and/or bending. Accordingly, there has been a need for alternative processing methods to overcome these issues. Recently, Jiang et al., reported an interesting work on orderly aligned nanowires (NWs) can efficiently release the bending stress in the film through the gap between nanowires adjustment [22]. Using ZnO nanowires array electrode, they demonstrated a flexible dye-sensitized solar cell with a good bending stability. Therefore, nanowire morphology should be preferable in crack resistance due to its efficient release of bending stress and can be a feasible way to fabricate flexible ITO-based TECs.

Here, we report a facile transfer printing process to fabricate ITO NWs array electrode with good transparency and bendability. ZnO film was introduced as a temporary holder for the ITO NWs growth and eventually it was etched away to detach the ITO NWs from the substrate by elective wet-etching in NaOH solution. The fabricated ITO NWs array electrode shows a much better transmittance than the commercialized ITO film/PET. Finally, we demonstrated a flexible solid-state dye-sensitized solar cell using the fabricated ITO NWs electrode.