Rational design and engineering of TiO₂ nanotube photonic crystal for dye-sensitized solar cells

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Rational design and engineering of TiO₂ nanotube photonic crystal for dye-sensitized solar cells

 

Author: Guo, Min
Title: Rational design and engineering of TiO₂ nanotube photonic crystal for dye-sensitized solar cells
Degree: Ph.D.
Year: 2015
Subject: Dye-sensitized solar cells.
Nanotubes.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Applied Physics
Pages: xxiii, 200 leaves : illustrations ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2798169
URI: http://theses.lib.polyu.edu.hk/handle/200/7886
Abstract: Integrating photonic crystal (PC) structures in photovoltaics, as an effective light trapping method, has gained increasing interest and led to significant enhancement of power conversion efficiencies in various types of solar cells. For applications in dye-sensitized solar cells (DSSCs), various approaches, such as, nanopatterning, template-assisted and layer-by-layer assembly techniques, are often used to design and fabricate PCs with various nanostructures. Nevertheless, these methods still suffer from one or more intrinsic drawbacks, such as poor physical contact, non-conductive PC layer, clogging channels, and the photonic bandgap being impossible to be continuously tuned. A novel TiO₂ nanotube PC (NT PC) with freely tunable bandgap was recently fabricated by a current-pulse anodization method, which is expected to be a potential alternative to be applied to DSSCs for significantly enhanced light harvesting. In this thesis, a novel strategy to directly and seamlessly couple the TiO₂ NT PC layer to nanotube based DSSCs by a single electrochemical anodization process is developed at first, which increases the conversion efficiency by over 50%, compared with that of a DSSC without a PC layer. A power conversion efficiency (PCE) of 5.61% is achieved in the nanotube based DSSCs. A theoretical analysis on the optical properties and the photocurrent enhancement of the TiO₂ NT PC coupled nanotube based DSSCs is conducted. In this architecture, the introduction of TiO₂ nanotube PC produces both Bragg mirror effect and Fabry-Perot cavity behavior, leading to a significant enhancement of light harvesting for photons in the photonic bandgap and also at the two band edges. By thorough theoretical study of the effects of the PC structural parameters on the optical properties and photocurrent enhancement, the optimum structural parameters are proposed for the nanotube based DSSCs. In order to further improve the PCE of the TiO₂ NT PC coupled DSSCs, we design another novel architecture, consisting of a thick TiO₂ nanoparticle (NP) layer and a thin TiO₂ NT PC membrane, taking the advantage of large surface area of the TiO₂ NP layer. The bandgap of TiO₂ NT PC is precisely tailored by modulating the anodization parameters. Strategies to enhance the efficiency of the newly designed DSSC and its relation to the selective reflection of the TiO₂ NT PC are discussed and evaluated. In this architecture, the TiO₂ NT PC can offer multi-functionalities of both PC and light-scattering effects, yielding the maximum PCE (6.96%) with an enhancement factor of 39.5% when the tailored TiO₂ NT PC has a reflection peak that best matches the dye absorption peak.
It is also noted that in real application of DSSCs, the photovoltaic output of DSSCs is greatly dependent on the amount of absorbed photons, which is limited by the thickness of the photoanode and the illumination conditions, such as the angle and intensity of the incident light. The photovoltaic performance and efficiency enhancement effect of the TiO₂ NT PC coupled DSSCs under these conditions are studied. It is found that due to more unabsorbed photons through cells based on thin absorbing layer, the introduction of TiO₂ NT PC shows stronger influence on the thin DSSCs, leading to much higher enhancement factors than that of thicker cells. The multifunction of TiO₂ NT PC yields an enhancement of PCE up to 99.1% by using ~2.3μm-thick TiO₂ NT PC, which is even enlarged to 130% at lower incident irradiation intensity of 50mW·cm-2. The optimized structures of TiO2 NT PC coupled photoanodes are proposed. Meanwhile, it is also demonstrated that the use of the TiO₂ NT PC can partially compensate the cosine power loss of DSSC under oblique incidence. By purposely choosing the Bragg position of the NT PC to be at the longer wavelength side of the dye absorption peak, the blue shift of the Bragg position with the tilted incident light results in more overlap with the dye absorption peak, generating a higher efficiency that partially compensates the reduced photon flux due to light inclination. Moreover, the unique structure of the vertically aligned TiO₂ nanotubes contributes an additional scattering effect when the incident light is tilted. As a result, the power output of a DSSC coupled with the NT PC layer shows a much flatter angular dependence than a DSSC without the NT PC. Finally, TiO₂ nanotube aperiodic photonic crystals (APCs) are designed and realized by a simple but highly controllable current-pulse anodization process. These APCs provide unprecedented opportunities for much more versatile photon management, due to increased degrees of freedom in the design and the unique properties brought about by the aperiodic structures as compared to their periodic counterparts. By coupling an APC into the photoanode of DSSCs, we demonstrate the concept of using APC to achieve nearly full-visible-spectrum light harvesting, and achieve a PCE of 7.87%.

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