Plasmonic nanomeshes: their ambivalent role as transparent electrodes in organic solar cells

Christian Stelling,Matthias Karg,Tobias A F König,Mukundan Thelakkat,Chetan R Singh,Markus Retsch

doi:10.1038/srep42530

Abstract

In this contribution, the optical losses and gains attributed to periodic nanohole array electrodes in polymer solar cells are systematically studied. For this, thin gold nanomeshes with hexagonally ordered holes and periodicities (P) ranging from 202 nm to 2560 nm are prepared by colloidal lithography. In combination with two different active layer materials (P3HT:PC61BM and PTB7:PC71BM), the optical properties are correlated with the power conversion efficiency (PCE) of the solar cells. A cavity mode is identified at the absorption edge of the active layer material. The resonance wavelength of this cavity mode is hardly defined by the nanomesh periodicity but rather by the absorption of the photoactive layer. This constitutes a fundamental dilemma when using nanomeshes as ITO replacement. The highest plasmonic enhancement requires small periodicities. This is accompanied by an overall low transmittance and high parasitic absorption losses. Consequently, larger periodicities with a less efficient cavity mode, yet lower absorptive losses were found to yield the highest PCE. Nevertheless, ITO-free solar cells reaching ~77% PCE compared to ITO reference devices are fabricated. Concomitantly, the benefits and drawbacks of this transparent nanomesh electrode are identified, which is of high relevance for future ITO replacement strategies.

Highlights

Of thin film solar cells has extensively been studied in the past years[23,24,25,26,27,28]
The active layer was in direct contact with the 12 nm thick nanohole array (400 nm period), which was prepared by colloidal lithography
Beside their application as transparent electrodes, the introduction of nanohole arrays as back reflectors in organic solar cells resulted in an enhanced device absorption by coupling of the light to SPP modes in experimental[30] and numerical studies[31,32,33] for ultrathin active layers of P3HT blended with fullerenes

Summary

Introduction

Of thin film solar cells has extensively been studied in the past years[23,24,25,26,27,28]. The enhanced efficiency of such devices can originate from plasmonic enhancement in direct proximity to the nanomesh as well as from an increase in the interfacial area between hole and electron conductor. Beside their application as transparent electrodes, the introduction of nanohole arrays as back reflectors in organic solar cells resulted in an enhanced device absorption by coupling of the light to SPP modes in experimental[30] and numerical studies[31,32,33] for ultrathin active layers of P3HT blended with fullerenes. The reduction in JSC compared to the state-of-the-art electrode ITO was attributed to the reduced transparency

Methods

Results

Conclusion