Nanomanufacturing of multicomponent plasmonic nanogels and interfaces with broadband solar absorption capability

Thin film photovoltaics (PVs) offer much potential in cost-effective harvesting of solar energy through efficient use of the semiconductor materials, typically in the form of thin films of thickness 1-2 microns. However, the efficiency of single junction thin film PVs is significantly lower compared to their wafer-based counterparts due to poor light trapping by such devices. Improving the efficiency of thin-film PVs using plasmonic interfaces is an area of active research. A promising approach is the incorporation of a light trapping layer that consists of noble metal nanoparticles (NPs) onto the PV device. Nanostructured plasmonic interfaces for this purpose have been fabricated by using lithography, vapor deposition, dewetting of thin metal films by ns and fs pulsed lasers and wet chemistry using self-assembled monolayers. However, economical scale up and adaptation of such processes to fabricate interfaces with multiple species/shapes/sizes in a controllable and repeatable fashion are not straightforward. In this work, we show how a nanostructured interface consisting of multiple metals and/or multiple shapes (e.g. sphere, rod) of a given metal can serve as a "broadband antenna" that would trap light in the UV-visible range. Specifically, a network of wormlike surfactant micelles (WLMs) in an aqueous solution is used as a template for producing stable multicomponent suspensions of Au and/or Ag NPs with desired optical properties. Such suspensions, hereafter referred to as plasmonic nanogels (PNGs), exhibit a long shelf life (∼ weeks) and remarkable color uniformity. The structure of the PNGs was studied by small angle X-ray scattering, cryogenic transmission electron microscopy (Cryo-TEM) and rheological experiments. These studies, together with Molecular Dynamics simulations, support a mechanism of self-assembly in which the nanoparticles bridge the micelle fragments to form a stable double network. As evidenced by UV-visible transmission spectroscopy, the shape, size and concentration of the NPs can be varied to tune the optical properties of the PNGs in a way that they absorb radiation over a broadband of wavelengths. Multicomponent PNGs reported in this work have relatively low viscosity and low elastic modulus. Hence they are processable by conventional coating techniques. We employed spin- and dip-coating to produce multicomponent plasmonic interfaces. The structure and optical properties of such interfaces and their integration into PV devices will be discussed.