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Nanostructured materials and their charge transport properties in photoanodes of dye sensitized solar cells.

机译：染料敏化太阳能电池光阳极中的纳米结构材料及其电荷传输性能。

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摘要

Since the big progress of dye sensitized solar cells (DSCs) by adopting TiO2 nanoparticles for a photoanode in 1991, DSCs have been intensively studied as an alternative to conventional Si-based solar cells. As a main component of DSCs, a photoanode composed of a nanostructured semiconducting oxide network plays a significant role in determining performances of DSCs in terms of light harvesting efficiency (LHE) and charge collection efficiency related to charge transport and recombination. Nanomaterials with various morphologies, such as particles, rods and tubes have been fabricated and investigated to improve performances of DSCs. Among them, submicrometer-sized aggregates of nanocrystallites have demonstrated to be promising as a photoanode of DSCs for higher power conversion efficiency. Such hierarchical structures make it possible to have both high specific surface area for dye molecule adsorption and internal light scattering within the photoanode, leading to a much enhanced LHE. This work focused on the surface modification and charge transport characterization of such hierarchically structured photoanodes. First, a core-shell configuration was fabricated by atomic layer deposition (ALD) process, which was achieved by depositing ultrathin TiO2 layer on inner surface of ZnO aggregate film in which the TiO2 shell was anticipated to act as a chemical and energy barrier. Although the ALD-TiO2 coating failed to improve chemical stability of the ZnO aggregate against to an acidic dye solution due to the ultrathin thickness ( 1 nm), the ALD-TiO2 shell layer effectively suppressed charge recombination at the interface. As a result of the reduced charge recombination, Voc, and FF of DSCs were increased, leading to 20 % enhancement of power conversion efficiency. Second, effects of annealing temperatures on ALD-TiO2 coated aggregates of ZnO nanocrystallites were investigated in terms of sintering behavior and charge transport. 350 °C as the maximum temperature was typically used to preserve the specific surface area of ZnO aggregates. Nitrogen sorption analyses revealed that the ALD-TiO2 layer improved thermal stability of ZnO aggregates at high temperature, ALD-TiO2 coated ZnO aggregates retained the same specific surface area even annealed at 450 °C. The higher annealing temperature resulted in an improved crystallinity, resulting in the highest charge transfer resistance when annealed at 450 °C. As a result, the DSC with photoanode made of ALD-TiO2 coated ZnO aggregates annealed at 450 °C showed the highest Voc and FF, with a little reduced Jsc and thus, the highest power conversion efficiency. Third, charge transport properties such as electron lifetime, chemical diffusion coefficient and diffusion length of ZnO nanorod aggregates were investigated. Electrochemical impedance spectroscopy (EIS) was used to characterize charge transport properties, and it was found that increased crystal sizes and widened necks with higher annealing temperature reduced charge diffusion resistance (Rd), and increased diffusion length from 50 microm to 140 microm. As a result, the power conversion efficiency increased 25 %. Lastly, TiO2 nanoparticles were added into TiO2 aggregates with different ratios; 10 wt% and 20 wt% nanoparticle. As a result, diffusion resistance was found to be reduced and the corresponding diffusion length was increased by filling the bottlenecks between adjacent aggregates with nanoparticles, while additional reduction of diffusion resistance was not observed when the amount of the added TiO 2 nanoparticles increased from 10 to 20 wt%, indicating that there is a saturation point for charge transport. Even though there was no significant impact of the added nanoparticles on electron lifetime despite of the increase of surface area, admixing TiO2 aggregates with TiO2 nanoparticles improved Jsc and eventually leaded to the enhanced efficiency by 30 % as a result of the improved diffusion length and increased surface area.
机译：自1991年通过将TiO2纳米粒子用于光阳极以来，染料敏化太阳能电池（DSC）取得了长足的发展，人们已对DSC进行了深入研究，以替代传统的硅基太阳能电池。作为DSC的主要成分，由纳米结构的半导体氧化物网络组成的光阳极在决定DSC的性能方面起着重要作用，其光收集效率（LHE）和与电荷传输和重组相关的电荷收集效率均如此。已经制造并研究了具有各种形态的纳米材料，例如颗粒，棒和管，以改善DSC的性能。其中，已证明亚微米尺寸的纳米微晶聚集体有望作为DSC的光阳极，以实现更高的功率转换效率。这样的分级结构使得可以具有高的比表面积用于染料分子吸附和在光阳极内具有内部光散射，从而导致大大提高的LHE。这项工作集中于这种分层结构的光阳极的表面改性和电荷传输表征。首先，通过原子层沉积（ALD）工艺制造核-壳结构，这是通过将超薄的TiO2层沉积在ZnO聚集膜的内表面上实现的，其中TiO2壳有望充当化学和能量屏障。尽管由于超薄厚度（<1 nm），ALD-TiO2涂层未能提高ZnO聚集体对酸性染料溶液的化学稳定性，但ALD-TiO2壳层有效地抑制了界面处的电荷复合。由于电荷重组减少，DSC的Voc和FF增加，导致功率转换效率提高20％。其次，从烧结行为和电荷传输的角度研究了退火温度对ALDO-TiO2包覆的ZnO纳米微晶聚集体的影响。通常使用最高温度350°C来保持ZnO聚集体的比表面积。氮吸附分析表明，ALD-TiO2层提高了ZnO聚集体在高温下的热稳定性，ALD-TiO2包覆的ZnO聚集体即使在450°C退火后仍保持相同的比表面积。较高的退火温度可改善结晶度，在450°C退火时可产生最高的电荷转移阻力。结果，在450°C退火的带有ALD-TiO2涂层的ZnO聚集体制成的带有光阳极的DSC表现出最高的Voc和FF，Jsc几乎没有降低，因此具有最高的功率转换效率。第三，研究了ZnO纳米棒聚集体的电荷迁移性质，如电子寿命，化学扩散系数和扩散长度。电化学阻抗谱（EIS）用于表征电荷传输性质，发现随着较高的退火温度，晶体尺寸的增加和颈的变宽会降低电荷扩散电阻（Rd），扩散长度从50微米增加到140微米。结果，功率转换效率提高了25％。最后，将TiO 2纳米颗粒以不同的比例加入到TiO 2聚集体中。 10重量％和20重量％的纳米颗粒。结果，发现通过用纳米颗粒填充相邻的聚集体之间的瓶颈，降低了扩散阻力，并且相应的扩散长度增加，而当TiO 2纳米颗粒的添加量从10增加至10时，未观察到扩散阻力的进一步降低。 20wt％，表明存在电荷传输的饱和点。尽管增加了表面积，尽管添加的纳米粒子对电子寿命没有显着影响，但由于改善了扩散长度并增加了TiO2纳米粒子的掺入量，TiO2纳米粒子与TiO2纳米粒子的混合改善了Jsc，最终使效率提高了30％。表面积。

著录项

  • 作者

    Park, Kwangsuk.;

  • 作者单位

    University of Washington.;

  • 授予单位 University of Washington.;
  • 学科 Nanotechnology.;Engineering Materials Science.
  • 学位 Ph.D.
  • 年度 2011
  • 页码 145 p.
  • 总页数 145
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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