【作者】 付国柱；

【导师】 左禹；

【作者基本信息】 北京化工大学 ， 材料科学与工程， 2015， 博士

【摘要】 染料敏化纳晶太阳能电池(DSSC)由于其优异的性能、环境友好和成本低,而且具有广阔的商业应用前景等优点,在新能源领域备受关注。目前,提高DSSC光电转换效率和稳定性逐渐成为研究的热点,而对DSSC电池中重要组件的纳晶半导体光阳极的研究显得尤为重要。目前的难点在于如何解决Ti02纳晶光阳极薄膜形貌与粒径晶型的可控控制,膜层结构优化,电子复合的抑制；如何实现能带结构调整促进材料自身特性改善和结构与性能协同效应的发挥,有效提高光电转换效率。本论文利用密度泛函数理论的第一性原理计算Sr2+、La3+、Cr3+、Sn4+金属离子掺杂的Ti02纳晶能带结构的变化,分析掺杂对DSSC光电性能的影响。研究发现Sr2+、La3+、Cr3+掺杂的Ti02纳晶能带结构带隙宽度变窄,Sn4+掺杂的带隙宽度略变宽；Sr2+和La3+掺杂均表现出p型掺杂特性,但是La3+费米能级进入价带的深度比Sr2+掺杂要浅；Cr3+掺杂表现出n型半导体的特性,Sn4+掺杂属于等电子杂质掺杂。分析表明：Sr2+、La3+、 Cr3+金属离子掺杂使得电子空穴的分离更有效,有利于光电性能。本文通过水热法制备了系列TiO2纳晶与不同金属离子掺杂的Ti02纳晶及其制备的薄膜光阳极,然后对组装成的DSSC进行了光电性能的测试。首先以钛酸四丁酯为原料,采用水热法合成Ti02纳晶和Sr2+掺杂的Ti02纳晶胶体,并通过涂覆法将其制备成DSSC光阳极薄膜。表征显示：Ti02纳晶的平均粒径为20nm,Sr2+掺杂后的Ti02纳晶平均粒径为15 nm,均呈现多孔网状结构。Sr2+掺杂有利于更多锐钛矿型Ti02纳晶的生成,证实Sr原子成功进入Ti02的晶格中,改变其能带结构。Sr2+掺杂有利于光电转换效率的提高,Sr2+掺杂量为1moL%时,光电转换效率达到7.92%,比未掺杂的光阳极提高了7.61%。主要归因于如下几个方面：首先,金属离子掺杂Ti02纳晶的可控合成方法,可实现Ti02晶型和粒径的可控调控——适量Sr2+掺杂的Ti02纳晶薄膜形貌、粒径大小与晶型有利于光电性能提高；TiO2/TiO2/Sr-TiO2纳晶薄膜结构的光阳极是结构优化的膜层结构,掺杂的Sr2+仅分布于薄膜电极表层有利于光电转换效率：其次,导致Ti02平带电位负移,提高了光电压；再者Sr2+掺杂增加了界面阻抗,抑制了界面电子复合,使电子传输速率增加；从而提高短路光电流密度Jsc、填充因子FF,以及光电转换效率η。这也证实Sr2+掺杂的有效性,与理论计算结果基本一致。采用水热法制备La3+掺杂的Ti02纳晶,通过涂覆法获得TiO2/TiO2/ La-TiO2纳晶薄膜结构的光阳极。La3+的掺杂有利于锐钛矿型Ti02纳晶的生成,更重要地是La3+掺杂量大于0.8%时产物为理想的锐钛矿型。适量的La3+掺杂可以使Ti02的平带电位负向移动0.029V,提高其开路光电压；同时,通过交流阻抗分析表明La3+掺杂能够有效阻止Ti02光阳极与电解质之间的界面复合,也可以提高其光电压。适量的La3+掺杂的可以使界面电子复合减小、电荷收集效率大幅提高(68.38%提高到93.69%)以及电子传输速率更快。因此,La-TiO2纳晶光阳极DSSC电池的光电转换效率在掺杂量为0.8 moL%时为7.25%,与未掺杂Ti02薄膜的相比提高了16.94%,使DSSC电池性能得到全面改善。本文还合成了Cr3+、Sn4+掺杂的Ti02纳晶胶体。随着Cr3+掺杂量的提高,锐钛矿型Ti02纳晶比例增加,超过0.8%的掺杂量后,Ti02纳晶完全转变成有利于光电转换效率提高的锐钛矿型。Sn4+的掺杂却有利于TiO2纳晶金红石型的转化和棒状结构形成。当掺杂量为0.75 moL%时,形成长径约为100～200 nm的100%金红石型Ti02纳晶纳米棒。然后将其组装成Cr3+、Sn4+掺杂的TiO2/TiO2/Cr-TiO2纳晶、TiO2/TiO2/Sn-TiO2纳晶结构光阳极的DSSC。通过电化学性能分析表明Cr3+掺杂使得费米能级负移,增加了界面阻抗以阻碍界面复合,从而提高DSSC的开路光电压。当Cr3+掺杂量为0.1 moL%时,Cr-TiO2纳晶光阳极DSSC的光电转换效率η可达6.54%,与未掺杂的相比提高了7.40%。而Sn-TiO2纳晶光阳极DSSC的性能均有所下降,当掺杂量为1.5moL%时光电转换效率达到了最低5.54%,与未掺杂的降低了29.07%。上述研究表明：研制的TiO/TiO2/ Mmetal-ions-TiO2纳晶三层光阳极有助于光电转换效率提高；建立的结构与性能关系,可揭示光电转换效率提升的作用机理。界面阻抗RW、电子寿命τn、电荷收集效率ηcc是DSSC光电性能提高的关键所在。更多还原

【Abstract】 With the high energy conversion efficiency, low cost and environment friendly characteristics, dye-sensitized solar cells (DSSC), have proven to be extremely versatile in commercial applications. Research in enhanced performance and stability of the nano-crystals TiO2 photo-anode of DSSC has also been promising. The challenges at present are the optimization of interface characteristics of the TiO2 photo-anode thin film and its particle size and pattern, the improvement of TiO2 electrode material and structure, suppress of electronic recombination as well as adjusting TiO2 electronic band structure, and improving the synergistic effect of its structural properties so that the efficiency of the photoelectric conversion can be improved.The paper takes use of the first-principle theory of Density Functional Theory to calculate the electronic band structure of TiO2 nanocrystals doped with different metal ions and their influence on photoelectric performance. Furthermore, the bandgap structures of Sr2+、La3+、Cr3+、Sn4+doped TiO2 nanocrystals.were calculated by First principle theory in DFT, and the results show that all these four metal ions have led to the change of the electronic band structures of TiO2 nanocrystals. After the doping of Sr2+、La3+ into TiO2 nanocrystals, the Fermi level has entered the valence band of TiO2 and represents p-type semiconductor property. Fermi level entered the valence bond band of the doping of La3+but the depth of the entrance is shallower than Sr2+which contributes to the light absorption to make light absorption. After the doping of Cr3+, its Fermi level goes through the impurity energy level, showing the property of n-type semiconductor. The doping of TiO2 nanocrystals into the Sn4+belongs to the doping isoelectronic impurity. According to the factors listed above, we can speculate that the photoelectric conversion performance of TiO2 nanocrystalsdoped in Sr2+、La3+、Cr3+ will be improved. The separation of electron hole is more effective.TiO2 and different metal ions doped TiO2 nanocrystals were synthesized by hydrothermal method using tetrabutyl titanate as precursor, and were used to prepare thin film anode of DSSC. The representations show that the size of TiO2 nanocrystals is about 20nm while the size of TiO2 nanocrystals doped with Sr2+is 15 nm. Both of them show a porous mesh structure and the doping of Sr2+promotes the generation of anatase-phase. Sr atom has been doped into the crystal lattice successfully, led to the change of the electronic band structure. The experimental results show the doping of Sr2+can improve the performance of photoelectric. When the doping amount of Sr2+raised upto 1moL%,η raised from 7.61% to 7.92%. These reasons are attributed to several aspects:first of all, the synthesis parameters, doping amount and film structure which be accurately controled by the appropriate doping of Sr2+is suitable for the photoelectric property of DSSC; after this has been applied on DSSC, the test result confirms the optimized anode structure is composed of TiO2/TiO2/Sr-TiO2. The test results of Mott-Schottky indicate that the doping of Sr2+ which contributes to the increase of photo-voltage. The test results of Mott-Schottky indicate that the doping of Sr2+leads to the negative shifts of the flat-band which contributes to the increase of photo-voltage. The analysis result shows that the doping of Sr+increases the resistance, suppresses the electronic interfacial recombination of the electronic interfacial and promotes the electronic transport velocity. The lifetime of electron has been increased and the electric charge collection efficiency has been improved. Thus the photocurrent density Jsc, the impact factor (FF) and photoelectric conversion efficiency η have all been improved. These experimental results are identical with the theoretical calculation results basically.Nanocrystalline TiO2 doped with La3+was synthesized by hydrothermal method and then the photoanode with the structure of TiO2/TiO2/La-TiO2 was prepared. The analysis results show that La3+doping promotes the generating of anatase phase TiO2, and Anatase phase was obtained when the volume of addition is more than 0.8%. The La atom has been successfully doped into the lattice of TiO2. After the doping of La3+, the flat band potential of TiO2 negatively moves for 0.029V and increases the Voc. The doping of La3+ increases the resistance between TiO2 photo-anode and the electrolyte and impedes the electronic interfacial recombination between them. Moderate doping of La3+ can promote the electronic transport velocity and decrease the electronic interfacial recombination which can raise the electric charge collection efficiency up to 93.69% from 68.38%. On the basement of the above factors, the photoelectric conversion efficiency of La-TiO2 Nano-crystals photo-anode DSSC can reach its maximum value of 7.25% when the doping volume reaches to 0.8moL%. It has raised 16.94% compared to the DSSC without being synthesized by the thin film doping with TiO2. The performance of DSSC is totally improved.The colloid of TiO2 Nanocrystals doped with Cr3+and Sn4+has also been synthesized by hydrothermal method. The analysis results show that as the doping volume of Cr3+being improved, TiO2 nanocrystals of the anatase phase increase its proportion and it will reach 100% after the addition volume is more than 0.8%. While the doping of Sn motivates the formation of rutile phase TiO2 nanocrystals. When the doping volume reaches 0.75moL%, it will be totally turned into rutile phase TiO2 nanorods with a length of 100-200 nm. After that, the photo-anode with structure of TiO2/TiO2/Sn-TiO2 was synthesized, and the results show that the photoelectric conversion efficiency of TiO2-0.1moL% Cr has raised 7.40% compared with those without doping, while on the other hand, the properties of DSSC with Sn-TiO2 photoanode obviously drop down. The tests show that the doping of Cr3+can negatively shift the Fermi level which then improves the voltage of open circuit of DSSC and increases the resistance. Thus it finally impedes the electronic interfacial recombination. The analysis results show that when the doping volume of Cr3+ is 0.1moL%, the time of electrical transmission is short, the electron diffusion coefficient is high and the rate of electrical transmission is fast. The electronic life time of it is short and there is a large amount of depletion which makes differences of the short circuit current density and increase the collection efficiency of the charge. Development of the three layers TiO2/TiO2/Mmetal-ions-TiO2 nanocrystals photoanode helps to improve the photoelectric conversion efficiency; relationship established in structure and performance may reveal the mechanism on the photoelectric conversion efficiency. The Rw、τn、ηcc, are the significant keys to the improvement of the photoelectric performance of DSSC.更多还原