Bio-inspired networks for optoelectronic applications

Abstract

Modern optoelectronics needs development of new materials characterized not only by high optical transparency and electrical conductivity, but also by mechanical strength, and flexibility. Recent advances employ grids of metallic micro- and nanowires, but the overall performance of the resulting material composites remains unsatisfactory. In this work, we propose a new strategy: application of natural scaffoldings perfected by evolution. In this context, we study two bio-inspired networks for two specific optoelectronic applications. The first network, intended for solar cells, light sources and similar devices, has a quasi-fractal structure and is derived directly from a chemically extracted leaf venation system. The second network is intended for touch screens and flexible displays, and is obtained by metalizing a spider’s silk web. We demonstrate that each of these networks attain an exceptional optoelectonic and mechanical performance for its intended purpose, providing a promising direction in the development of more efficient optoelectronic devices.

Introduction

The advent of optoelectronics in the 1950s has depended on the development of transparent conductors¹. Such electrically conducting and optically transparent materials play a critical role in numerous important optoelectronic devices such as photovoltaic sensors, solar cells, photo diodes and lasers, all of which harvest or emit light. However, high optical transmission and good electrical conductivity are mutually limiting requirements since electrical carriers invariably scatter photons, and require a compromise. One successful approach had been to exploit the fundamental property of any carrier plasma, electromagnetic transparency above its plasma frequency. This idea led to the development of an entire class of materials (metal-oxides), which dominate the field today, such as the ubiquitous indium tin oxide (ITO), a metal with the plasma frequency in the infrared range, and thus transparent in the visible range². However, while metal-oxides often have desirable electro-optical properties, they are also brittle, and this deficiency limits their usefulness in many practical applications. To address these challenges, new approaches have been recently devised, based on metallic micro- and nanoscaffoldings (such as wire and nanowire grids^{3,4,5,6,7,8,9,10}, nanoparticles^11,12 and so on) and even using atomic-scale scaffolds such as graphene^13,14. Structures of this kind do in fact improve mechanical flexibility, but their electro-optical performance has not yet been sufficiently high.

In this work, we propose a new strategy, based on adopting ingenuous network designs readily found in nature. Indeed, micro- and nanoscaffolds of exceptional properties occur naturally in many biological systems, generated by a genetic code of a given organism, and self-assembled during the organism development^15,16,17. These structures perform various functions, such as a support for mechanical cellular integrity (for example, nanoscopic cellular cytoskeletons)¹⁸, nutrients distribution via microfluidic channels (for example, leaf or insect wing venations)^19,20, light management (for example, the Moth eye)^{Ag and ITO film depositions}

Sputtering (AJA International. ATC Orion 8, USA) was used to deposit Ag and ITO films. The SSW and LV structures were placed in vacuum chambers, and the sputtering was performed at ~50 °C to avoid damaging the samples.

Performance measurements

Morphologies of samples were characterized in a SEM system (JEOL JCM-5700, Tokyo, Japan), and by employing an optical microscope (MA 2002, Chongqing Optical & Electrical Instrument Co.). The sheet resistance of samples was measured by employing the standard van der Pauw method. Four contacts were deposited at the corners of a square sample (2 × 2 cm), numbered clockwise 1–4, and I–V data were recorded with the Keithley 2400 Source meter. Sheet resistance was calculated from the usual formula: R_ab,cd=4.5(R_12,34+R_34,12+R_14,23+R_23,14)/4, with R_ab,cd=U_ab/I_cd and where U_ab is the voltage drop between ‘a’ and ‘b’ contacts, produced by current I_cd flowing from contact ‘c’ to contact ‘d’. The second method, employed when the van der Pauw could not be used (for example, while the sample was stretched) was the two-probe method. In this method, two parallel electrodes (sputtered narrow silver lines of length L) were placed on the surface of the sample. Sheet resistance was calculated from R_s=R*L/W, where R is the measured resistance and the sample width is W. A modified two-probe method was also used for the directed current transport in the LV network, in which one electrode was a narrow line, and the other was a circular silver contact. In this case, a simple resistance between the electrodes was measured. To test the flexibility of SSW and LV networks, the structures were transferred to elastic substrates, for example, PDMS (Polydimethylsiloxane) and PET (polyethylene terephthalate). A lab-made stretching and bending system was used, with an automatic control and a data acquisition system. Optical transmittance was measured in an integrating sphere system (Ocean Optics, USA). These measurements were normalized to the absolute transmittance of the substrate (glass, PET or PDMS).