Large-area soft-imprinted nanowire networks as light trapping transparent conductors.

Jorik Van De Groep,Rene A J Janssen,Marc A Verschuuren,Albert Polman,Martijn M Wienk,Dhritiman Gupta

doi:10.1038/srep11414

Abstract

Using soft-imprint nanolithography, we demonstrate large-area application of engineered two-dimensional polarization-independent networks of silver nanowires as transparent conducting electrodes. These networks have high optical transmittance, low electrical sheet resistance, and at the same time function as a photonic light-trapping structure enhancing optical absorption in the absorber layer of thin-film solar cells. We study the influence of nanowire width and pitch on the network transmittance and sheet resistance, and demonstrate improved performance compared to ITO. Next, we use P3HT-PCBM organic solar cells as a model system to show the realization of nanowire network based functional devices. Using angle-resolved external quantum efficiency measurements, we demonstrate engineered light trapping by coupling to guided modes in the thin absorber layer of the solar cell. Concurrent to the direct observation of controlled light trapping we observe a reduction in photocurrent as a result of increased reflection and parasitic absorption losses; such losses can be minimized by re-optimization of the NW network geometry. Together, these results demonstrate how engineered 2D NW networks can serve as multifunctional structures that unify the functions of a transparent conductor and a light trapping structure. These results are generic and can be applied to any type of optoelectronic device.

Highlights

Using soft-imprint nanolithography, we demonstrate large-area application of engineered twodimensional polarization-independent networks of silver nanowires as transparent conducting electrodes
While this process can be applied to wafer-scale processing, here the stamped pattern consists of 40 square networks (2 × 2 mm[2] each), with each square containing a 30 nm high network with different NW width and pitch
Our analysis shows that optimized NW networks outperform the sheet resistance and transmittance of ITO

Summary

Introduction

Using soft-imprint nanolithography, we demonstrate large-area application of engineered twodimensional polarization-independent networks of silver nanowires as transparent conducting electrodes These networks have high optical transmittance, low electrical sheet resistance, and at the same time function as a photonic light-trapping structure enhancing optical absorption in the absorber layer of thin-film solar cells. A wide variety of geometries have been proposed, including random nanowire meshes[7,9,10,11,12], percolated films13,14, 1D (nano) imprinted gratings[4,5,15], nanogratings interconnected with mesoscale wires[16], self-assembled microstructures[17], as well as NW-graphene hybrid structures[18] These nanoscale and multiscale geometries can be designed to provide improved optoelectronic performance relative to ITO, achieving concurrent improvements in both optical transparency and electrical conductivity. All these geometries are either strongly polarization dependent (limited or no light trapping for other polarization) or allow no control over the network geometry

Methods

Results

Conclusion