Objective
This project is aimed at investigating and realizing plasmonic structures for use in inorganic thin-film photovoltaics. Properly engineered metal structures - called plasmonics - can localize incident light on a sub-micrometric scale and could therefore be used to enhance solar light absorption in ultrathin semiconductor films. By dramatically reducing the required thickness of the active layer, the use of plasmonics is expected to expand the range and quality of absorber materials that are suitable for photovoltaic devices. In particular, this would enable effective utilization of both low-dimensional semiconductor structures and thin films of earth-abundant, low-cost, and non-toxic absorbers with poor charge transport properties. Plasmonic-based omnidirectional absorbers are also investigated to enhance angular response of multijunction devices.

Background

Sunlight incident on a metal can be converted into propagating surface plasmon polaritons that enable efficient light absorption in extremely thin (10’s -100’s of nanometers thick) photovoltaic absorber layers. The ability to construct optically-thick but physically very thin photovoltaic absorbers could revolutionize high efficiency photovoltaic device designs by acting as a light concentrator. Plasmonic materials allow full light absorption in extremely thin multijunction semiconductor structures as shown in the schematic of Figure 1. These structures use active layers comprised of earth-abundant semiconductor absorbers (e.g. Cu₂O, Zn₃P₂, Si, β-FeSi₂, BaSi2, Fe₂O₃) and low-cost plasmonic metals (Al, Cu). For absorber layers with good surface passivation, the ability to diminish the solar cell base thickness via plasmonic design yields an increase in cell open circuit voltage, in addition to enhancing carrier collection. The ability to do this would open up new approaches to carrier collection from quantum wires and dots that do not rely on inter-dot or inter-wire carrier transfer prior to collection at the device contacts.

Figure 1: Schematic of a plasmonic multijunction solar cell with > 45% efficiency potential

The performance of a solar cell, especially when working under concentrated illumination, is highly sensitive to the angle of incidence of incoming radiation. Solar tracking modules are often used to increase the energy yield from a solar cell throughout the day by tracking the sun across the sky. Typically, such systems require undesired, bulky mechanical parts, cost energy to run, and require maintenance. It would therefore be desirable to integrate the solar tracking functionality into the solar cell without the use of mechanical parts. A planar metallic microcavity structure consisting of a metal–dielectric–metal (MDM) configuration can exhibit an omnidirectional resonance, i.e., a resonance for which light can efficiently couple to cavity modes independent of the incidence angle for a carefully chosen metal-to-metal spacing (equal to about a quarter wavelength - on the order of one hundred nanometers). MDM structures are studied to develop built-in tracking systems that would both enhance cell performance throughout the day and decrease the balance of system cost of the overall photovoltaic generator.

Figure 2: Schematic of a test bed structure consisting of a surface wave interferometer coated with CdSe quantum dots to test plasmonic light absorption. Absorption along the Ag surface is achieved through incident light conversion into surface plasmon polaritons that exhibit mode overlap with the absorber layer.

Approach

This research combines theory, numerical simulation, and experimental work to realize prototype tandem cells using plasmon enhanced light absorption and omnidirectional plasmonic absorbers. Full-field electromagnetic codes based on finite-difference methods are used to design the plasmonic structures and define incidence angle and spectral dependence of the plasmonic light absorption efficiency. During the project, several prototypical solar cells that feature surface plasmon polariton coupling to thin film absorbers – such as the one illustrated in Figure 2 - are being developed including Si vertical and lateral pn junctions, Si nanocrystal and nanowire cells, and Si-based two junction tandem cells. Test cells are used to investigate carrier transport and collection in ultrathin plasmonic solar cells and to assess the potential for plasmon enhanced light absorption and omnidirectional plasmonic absorbers in thin film concentrators.