Self-Assembly of Crystalline, Large-Area and Periodicity-Tunable Nanostructured Arrays for Solar Cells Application

晶體自組裝,大面積和週期性可調的納米結構陣列在太陽能電池中的應用

Student thesis: Doctoral Thesis

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Award date14 Jun 2017

Abstract

Nanostructured arrays have attracted extensive research attention due to their unique and excellent optical and electrical properties suitable for semiconductor devices. In this work, we employed a newly-developed, low-cost and simple wet anisotropic etching method to fabricate hierarchical silicon nanostructured arrays. These highly-ordered nanoarrays benefit from the self-assemble polystyrene (PS) nanospheres monolayer yielded by Langmuir-Blodgett (LB) technique and metal-assisted chemical etching (MaCE) approach. Besides Si nanostructures such as nanorods, nanocones and inverted nanopencils, the periodicity-tunable and highly-ordered photoresist nanohole layers were synthesized by combining nanolithography and photolithographic techniques, in which the PS nanospheres act as optical lenses. Further, we employed these periodic nanohole arrays as the photoresist templates to construct the self-assembly of crystalline, large-area and periodicity-tunable anatase TiO2 nanotube arrays on varied surfaces. This way, the fabricated tube geometries, such as the height, pitch, diameter and wall thickness, can be reliably controlled by varying the TiO2 sol-gel precursor concentration, photoresist materials and diameters of PS nanospheres.

Based on Si nanostructures and TiO2 nanotube arrays, the photovoltaic devices (i.e. Si solar cells and perovskite solar cells) were investigated. First, for these three-dimensional nanostructured Si solar cells with superior broadband and omnidirectional light-harvesting properties, three different electrode schemes were designed via photolithography and MaCE methods, namely (i) with nanostructured arrays on contact, (ii) conventional top electrode and (iii) low-platform contact (i.e. the contact region was etched). Specifically, Si nanopencils with excellent photon-trapping behaviors coupled with low-platform contact are found to enable a substantial photovoltaic device performance enhancement, due to the shortened current path and minimized carrier recombination and series resistance with respect to this contact. In terms of perovskite solar cells, the power conversion efficiency (PCE) breakthrough is mainly benefited from the utilization of titanium dioxide (TiO2) mesoporous structure which acts as an electron transporting material (ETM). On the other hand, TiO2 nanotube arrays with the advantages of high surface area and an excellent one-dimensional electron transporting path are expected to be a promising structure. Here, the periodicity and diameter of TiO2 nanotube arrays can be controlled in a large range. The wettability is also improved distinctly in comparison with other conventional titania nanotubes, which facilitates perovskite precursor solution (e.g., CH3NH3PbI3) to fill into tubes spaces. The PCE of subsequently fabricated solar cell devices are also improved due to the huge surface area, one-dimensional transporting path, and enhanced light-trapping. Except for the usual applications of TiO2 as carrier transporting layers, we illustrate that the periodic TiO2 nanotube arrays can show optical grating property with large-area uniformity. All of these not only exhibit a versatile nano-patterning technique for the fabrication of silicon nanostructures and TiO2 nanotube arrays but also provide an insight into the applications of these nanostructures on photovoltaic devices.

    Research areas

  • Polystyrene Nanospheres Monolayer, Si Nanopencils, Nanolithography, Photoresist Templates, TiO2 Nanotube Arrays, Silicon Solar Cells, Perovskite Solar Cells