Christian Stelling

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

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.

Atomic force microscopy with nanoelectrode tips for high resolution electrochemical, nanoadhesion and nanoelectrical imaging

Multimodal nano-imaging in electrochemical environments is important across many areas of science and technology. Here, scanning electrochemical microscopy (SECM) using an atomic force microscope (AFM) platform with a nanoelectrode probe is reported. In combination with PeakForce tapping AFM mode, the simultaneous characterization of surface topography, quantitative nanomechanics, nanoelectronic properties, and electrochemical activity is demonstrated. The nanoelectrode probe is coated with dielectric materials and has an exposed conical Pt tip apex of ∼200 nm in height and of ∼25 nm in end-tip radius. These characteristic dimensions permit sub-100 nm spatial resolution for electrochemical imaging. With this nanoelectrode probe we have extended AFM-based nanoelectrical measurements to liquid environments. Experimental data and numerical simulations are used to understand the response of the nanoelectrode probe. With PeakForce SECM, we successfully characterized a surface defect on a highly-oriented pyrolytic graphite electrode showing correlated topographical, electrochemical and nanomechanical information at the highest AFM-SECM resolution. The SECM nanoelectrode also enabled the measurement of heterogeneous electrical conductivity of electrode surfaces in liquid. These studies extend the basic understanding of heterogeneity on graphite/graphene surfaces for electrochemical applications.

Surface plasmon modes of nanomesh-on-mirror nanocavities prepared by nanosphere lithography

Metal-insulator-metal (MIM) structures show great potential for numerous photonic applications due to their ability to confine light energy to volumes with deeply sub-wavelength dimensions. Here, MIM structures comprising hexagonal gold nanohole arrays were prepared by nanosphere lithography. Angle-resolved UV-vis-NIR spectroscopy revealed a series of narrow, dispersive and non-dispersive modes, which were attributed to the excitation of surface plasmon polariton (SPP) modes. Applying finite-difference time-domain (FDTD) simulations and analytical diffraction phase-matching theory all resonances can be ascribed to only two SPP modes traveling at the outer gold surface and in the gap layer sandwiched between two metal films. Metamaterial resonances, as reported in the literature for similar structures, are not needed to fully explain the reflectance spectra. Bragg scattering of the symmetric gap SPP mode results in a gap resonance, which is insensitive to the angle of incidence over a broad angular range. The spectral position of this flat band can be controlled by tuning the grating period of the nanohole array as well as the thickness and the refractive index of the dielectric gap.