摘要
太阳能电池是一种能将太阳能转变为电能的有效技术途径,发展太阳能电池技术对解决世界能源危机和促进环境保护具有非常重要的作用。目前太阳能电池的主要种类有：硅太阳能电池、薄膜太阳能电池和染料敏化太阳能电池。其中,染料敏化太阳能电池(DSSCs)由于成本低廉、组装简易等特点受到了广泛的关注。就染料敏化太阳能电池的发展现状而言,二氧化钛纳米晶半导体光阳极的性能严重制约着电池的光电转换效率。作为光阳极的重要组成部分,二氧化钛材料的形貌和结构决定了光阳极的电子传输动力学性能。因此,合成新型纳米二氧化钛光阳极材料,研究其电子传输动力学特征,并通过材料微观形貌的控制,以期优化光阳极的电子传输动力学性能,从而提高电池的光电转换性能。
本论文选取了三种不同形貌(纳米晶、纳米棒和纳米管)的质子钛酸盐纳米材料作为研究材料的起始物,对随后合成的二氧化钛结构、形貌及其光电化学性能进行了研究。研究结果显示,质子钛酸盐纳米粒子随着烧结温度的升高,其物相经历了从TiO2(B)到锐钛矿相的变化,且晶粒尺寸不断增大,其中500℃烧结制备出的纳米晶粒具有最好的光电性能。对于大尺寸纳米棒而言(直径约100nm左右、长度为微米级),随着烧结温度的升高,质子钛酸盐物相也从TiO2(B)相逐渐变为锐钛矿相,但转变温度明显升高,且纳米棒在高温烧结后断裂现象明显,其中700℃烧结得到的大尺寸纳米棒具有良好的光电性能。在纳米管的研究中,发现质子钛酸盐纳米管随着烧结温度的升高,除物相由TiO2-B相变为锐钛矿相外,其形貌也发生了显著的变化,即从纳米管转变成实心的小尺寸纳米棒,并随着温度的升高进一步转变为较大的纳米晶粒。光电性能测试显示,600℃烧结得到的纳米材料的光电性能最优。
采用强度调制光电流谱和强度调制光电压谱研究了上述不同形貌二氧化钛的电子传输动力学特征。研究结果表明：对于二氧化钛纳米晶而言,由于其本身结构的局限性和大量的结构缺陷,使得在光阳极膜内晶粒间的电子传输时间长、电子寿命短,导致光生电子复合机率大。大尺寸二氧化钛纳米棒具有一维柱状结构特点,有利于电子的快速传递,显示出电子传输速率快、电子寿命长和光生电子复合率低的优势。由于不同烧结温度导致了纳米管的相结构和形貌发生了显著变化,其相应产物的电子传输过程呈现出不同的动力学特征。其中,在300℃和400℃烧结时得到的纳米管具有TiO2(B)结构和较多的结构缺陷,因此导致其具有电子传输时间长、电子寿命短和光生电子复合严重的特征。而在500℃和600℃烧结时,质子钛酸盐纳米管转变为尺寸较小的二氧化钛纳米棒,其电子传输速率快,电子寿命可达106 ms。以上研究结果表明：具有棒状形貌的二氧化钛纳米材料具有良好的电子传输性能,为后续的复合材料构筑及其光电性能研究提供了良好的研究基础。
在以上研究基础上,本论文优化设计并制备出具有一维结构的二氧化钛复合材料,即将纳米晶均匀负载在大尺寸纳米棒表面,构筑了具有一种双功能结构的复合材料。研究结果表明：该一维复合材料可将纳米晶的高比表面特征和大尺寸纳米棒优良的电子传输性能有机的结合起来。其中,表面负载的二氧化钛纳米晶不仅显著地提高了复合材料的比表面积,也降低了界面电荷转移电阻。同时,由于二氧化钛纳米棒的存在,电子在复合材料中传输时具有快速传输的动力学特征。这使得该复合材料的光电转换效率较单一纳米棒的转换效率提高了五倍之多。
为进一步提高复合材料的比表面,本论文还以二氧化钛纳米管为前驱体制备了小尺寸、高比表面的纳米棒,并进一步构筑了纳米晶／纳米棒复合材料。研究结果表明,该复合材料秉承了一维纳米复合材料的结构优势,不仅具有良好的电子传输特性,而且具有更高的比表面积和更佳的染料吸附性能,其相应的染料敏化太阳能电池的光电转换效率得以明显提高,达到7.87%。
本论文采用具有特定微观结构的纳米复合材料制备染料敏化太阳能电池的光阳极,所用材料能够将纳米晶的高比表面特征和纳米棒优良的电子传输性能有机的结合起来,具有更优的电子传输动力学特征,使相应的光电转换效率得到了明显的提高。本文的研究结果为构筑新型光阳极材料和改善染料敏化太阳能电池的光电性能提供了新思路。
A solar cell is a device that converts the energy of sunlight directly into electricity based on the photovoltaic effect. The development of a low-cost and high-efficiency solar cell is highly significant for utilization of solar energy, which will help solve the serious energy crisis and environmental problem faced by the whole world. Among solar cells, silicon-based solar cell and thin-film solar cell have been commercially available. In recent years, dye sensitized solar cells (DSSCs) are receiving increasing attention as a potential cost-effective alternative due to its low cost and simplicity. Usually, the energy conversion efficiency of the DSSCs is limited by the photoanode performance. As a key material in photoanode, fast electron transfer kinetics of TiO2 is necessary to avoid photoelectron recombination, which usually depends strongly on the micromorphology and crystallographic structure of TiO2. Therefore, it is very valuable to synthesize controllably titania nanomaterials with specific structures and optimize the electron transfer kinetics based on nanotechnology.
In this work, protonated titanate nanocrystallites, nanorods, and nanotubes were used as precursors to fabricate titania photoelectrode materials. We focused mainly on the conversion of the phase structure and micromorphology after calcination at different temperature. In particular, the effect of such conversion on photoelectrochemical properties of the products obtained subsequently was investigated in detail. In the case of protonated titanate nanoparticles, it is found that the transformation of the crystallographic structure occurs from titanate to TiO2-B and anatase with increasing calcination temperature, respectively. Meantime, the nanocrystalline size grows gradually. Among all products, nanocrystallites obtained after calcination at 500℃show the best photoelectrochemical properties. Similarly, the strcture transformation can be also observed from protonated titanate nanorods to TiO2-B nanorods and anatase nanorods with increasing calcination temperature. However, such a phase transformation temperature is obviously higher. Moreover, the rod-like morphology could be destroyed under high calcination temperature. It is noted that nanorods calcined at 700℃show the optimized photoelectrochemical properties. Finally, it is found that titanate nanotubes can convert to TiO2-B nanotubes (300-400℃), anatase nanorods (500-600℃), and anatase nanoparticles (700℃) with increasing calcination temperature. The obtained anatase nanorods after calcination at 600℃present the exellent photoelectrochemical properties.
Intensity-modulated photocurrent and photovoltage spectroscopies (IMPS and IMVS, respectively) were used to investigate the electron transport and recombination processes of the various titania nanomaterials. The results indicate that titania nanocrystallites provide a slow electron transport and a short electron lifetime, which leads to a high electron recombination rate due to the configuration confinement and the existence of abundant surface defects. In the case of titania nanorods with a relatively large size, the one-dimensional structure and single crystallites allow a fast electron transport, a long electron lifetime, and a low electron recombination rate. In respect of nanotubes, the electron transfer kinetics is varied obviously with the change in morphology and crystallographic structure. The limited electron transport and serious electron recombination can be found for TiO2-B nanotubes obtained after calcination at 300℃and 400℃due to the existence of abundant surface defects. The anatase nanorods with a relatively small size obtained at 500℃and 600℃allow a fast electron transfer, and electron lifetime of nanorods obtained at 600℃is long up to 106 ms. It is indicated from above results that the rod-like morphology can provide the optimized electron transfer kinetics, which is beneficial to provide important information for further fabricating one-dimensional titania nanomaterials with enhanced photoelectrochemical properties.
To optimize electron transfer kinetics of titania materials, one-dimensional titania nanocomposites, in which titania nanocrystallites are dispersedly supported on the surface of nanorods, were fabricated based on the above results. It is demonstrated that one-dimensional titania nanocomposites can combine the high surface area of nanocrystallites and the electron transport advantage of nanorods, which may present a long electron lifetime and a low electron recombination rate. As anticipated, the energy conversion efficiency of one-dimensional titania nanocomposites is increased by 5 times as compared to that of nanorods.
To further increase the surface area of titania nanocomposites, nanorods with a smaller size, obtained by calcining titanate nanotubes at 500℃, were used to support titania nanocrytallites to fabricate one-dimensional nanocomposites. It is shown that the charge transfer resistance is reduced and the adsorption of dye is increased for such one-dimensional nanocomposites with the large surface area. Besides, the fast electron transport is also obtained due to the contribution of nanorods. Therefore, the photovoltaic conversion efficiency of the nanocomposites is greatly improved to 7.87 %, the highest value obtained under our experimental conditions in this work.
In summary, titania nanocomposites with desired structure and morphology are used to prepare photoanode for DSSCs. The one-dimensional nanocomposites prepared here have optimized electron transfer kinetics due to the combination of the advantages of nanocrystallites and nanorods, resulting in an obviously improved energy conversion efficiency of DSSCs. The results obtained in this work can provide a potential approach to fabricate new photoanode materials and improve energy conversion efficiency of DSSCs.
引文
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