Rational design of three-dimensional silicon nanostructures for cost-effective photovoltaics


Student thesis: Doctoral Thesis

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  • Fei XIU


Awarding Institution
Award date15 Jul 2014


Efficient light harvesting in silicon photovoltaic devices has drawn increasing attention for improving their energy conversion and reducing the cost in recent years. One promising approach for the light absorption enhancement is utilizing the light trapping properties of three-dimensional (3-D) nanostructures. Besides, photo-carrier collection efficiency of 3-D nanostructures is also enhanced due to shortened diffusion length, demonstrating promising application in photovoltaics. In this dissertation, a facile wet-chemistry-only fabrication scheme for the controllable hierarchy of highly regular, single-crystalline and high aspect ratio Si nanostructures with different geometrical morphologies is presented. Importantly, systematic investigation of the omnidirectional photon trapping properties of as-made 3-D nanostructures and their applications in photovoltaic devices has also been discussed. In Chapter 2, we demonstrate the controllable hierarchy of highly regular, single-crystalline nanorod, nanopencil and nanocone arrays with tunable geometry achieved over large areas (>1.5 cm x 1.5 cm) by using an [AgNO3 + HF + HNO3/H2O2] etching system. The etching mechanism has been elucidated to originate from the site-selective deposition of Ag nanoclusters. Different etch anisotropies and aspect ratios can be accomplished by modulating the relative concentration in the etching system. Minimized optical reflectance is also illustrated with the fabricated nano-arrays. Overall, this work highlights the technological potency of utilizing a simple wet-chemistry-only fabrication scheme, instead of reactive dry etching, to attain three-dimensional Si nanostructures with different geometrical morphologies for applications requiring large-scale, low-cost and efficient photon trapping (e.g. photovoltaics). Chapter 3 discusses a more systematic investigation to further explore and understand the light coupling, propagation and absorption nature of these nanoarrays, assisted with the optical simulations. It is found that optical properties of these nano-arrays are predominantly governed by their geometrical factors comprising the structural pitch, material filling ratio (base-diameter-to-pitch) and aspect ratio (pillar-height-to-base-diameter), etc. Notably, along with the proper geometrical design, the inverted nanopencil arrays can achieve excellent broadband and omnidirectional light harvesting properties comparable to the nanocone counterparts. Specifically, the fabricated nanopencils with both ~1.27 and ~0.6 um pitches can suppress the optical reflection below 5 % over a broad wavelength of 400 to 1000 nm for the angle of incidence between 0° and 60°. These findings not only offer additional insight into the light trapping mechanism in these complex 3D nanophotonic structures, but also provide efficient broadband and omnidirectional photon harvesters for the next-generation cost-effective nanostructured photovoltaics. In Chapter 4, the realization of solar cell devices textured with various nanostructures will be presented and their photovoltaic performances are investigated. Here, two doping methods (monolayer molecular doping and phosphorus-spin-on-oxide doping) are introduced in this chapter for n+/p junction formation. Results in this chapter suggest that nano-textured surface of these solar cells acts as an effective antireflective absorbing layer, which contributes to a great improvement of short circuit current density (Jsc) and energy conversion efficiency of the devices. Particularly, the inverted nanopencil-textured cell device shows the best omnidirectional PV performance as compared with other nanostructures with the same pitch and diameter, which is consistent with their optical performance studied in Chapter 3. Results in this chapter can provide fundamental design guidelines for developing nanostructure-textured crystalline silicon solar cells in the future study. Finally, the Chapter 5 summarizes the work demonstrated in this dissertation and suggests the future follow-up work.

    Research areas

  • Design and construction, Silicon solar cells, Photovoltaic cells, Nanostructured materials, Materials