Limitations of ultra-thin transparent conducting oxides for integration into plasmonic-enhanced thin-film solar photovoltaic devices

Abstract

This study investigates ultra-thin transparent conducting oxides (TCO) of indium tin oxide (ITO), aluminum-doped zinc oxide (AZO) and zinc oxide (ZnO) to determine their viability as candidate materials for use in plasmonic-enhanced thin-film amorphous silicon solar photovoltaic (PV) devices. First a sensitivity analysis of the optical absorption for the intrinsic layer of a nano-disk patterned thin-film amorphous silicon-based solar cell as a function of TCO thickness (10-50 nm) was performed by simulation. These simulation results were then used to guide the design of the experimental work which investigated both optical and electrical properties of ultra-thin (10 nm on average) films simultaneously deposited on both glass and silicon substrates using conventional rf sputtering. The effects of deposition and post-processing parameters on material properties of ITO, AZO and ZnO ultrathin TCOs were probed and the suitability of TCOs for integration into plasmonic-enhanced thin-film solar PV devices was assessed. The results show that ultra-thin TCOs present a number of challenges for use as thin top contacts on plasmonic-enhanced PV devices: (1) optical and electrical parameters differ greatly from those of thicker (bulk) films deposited under the same conditions, (2) the films are delicate due to their thickness, requiring very long annealing times to prevent cracking, and (3) reactive gases require careful monitoring to maintain stoichiometry. The results presented here found a trade-off between conductivity and transparency of the deposited films. Although the sub 50 nm TCO films investigated exhibited desirable optical properties (transmittance greater than 80 %), their resistivity was too high to be considered as materials for the top contact of conventional PV devices. Future work is necessary to improve thin TCO properties, or alternative materials, and geometries are needed in plasmonic-based amorphous silicon solar cells. The stability of ultra-thin TCO films also needs to be experimentally investigated under normal device operating conditions.

Plasmonic enhancement of amorphous silicon solar photovoltaic cells with hexagonal silver arrays made with nanosphere lithography

“…Although the advantages of plasmonic structures have been well investigated, no plasmonicbased a-Si:H solar cell has been commercialized due to two primary reasons: 1) the effect was demonstrated on unrealistically thin solar cells [21] that if scaled demanded an ultra-thin transparent conducting oxide [22,23] and 2) current fabrication constraints. Recently, work on ultra-thin TCOs has been successful [22], which provides the potential to use this approach on full solar cells.…”

Transparent Conductive Oxides and Their Applications in Near Infrared Plasmonics

“…Large Q factors represent strong ability for local field enhancement. Conventional metals, such as gold and silver, still have large figure‐of‐merit in the infrared range, making them the first choice in surface plasmon . For example, at a wavelength of 1550 nm in the communication band, the Q factor of gold is three orders of magnitude larger than that of TCOs .…”

Characterization of Sub-Bandgap Plasmon Excitations in Transparent Conducting Oxides with Electron Energy-Loss Spectroscopy

“…However, TCOs are known to exhibit collective free carrier excitation (plasmon) resonances at energies below their semiconducting bandgaps, typically in the near infrared (NIR) spectral region. These lead to wide absorption bands centered around the resonant frequencies, reducing the requisite transparency which is detrimental to overall PV performance [1]. Characterizing uniformity of these resonances, how they change in proximity to surfaces and interfaces, whether and how they can be controlled via intentional compositional manipulation could lead to vital information to mitigate such effects.…”