摘要
染料敏化纳米晶TiO_2太阳能电池因为价格便宜、工艺简单、可制成大面积、形状多样化等优点而成为可再生能源研究领域的一个热点。本论文成功地构筑了染料敏化TiO_2纳米晶光电化学池,利用表面光电压和光电流谱对其进行了光生电子传输的机理研究,利用瞬态光电压谱定量的研究了光生电子在TiO_2层的传输时间和衰减寿命,为制备高效率的染料敏化纳米晶太阳能电池提供了实验和理论基础。主要内容包括:
采用类丝网印刷(Screen-printing)方法制备了纳米晶TiO_2多孔膜电极,以明星染料联吡啶钌络合物(N3)作敏化剂,成功地组装了染料敏化TiO_2纳米晶光电化学池原型器件,并对其光电性能进行了测试。原型器件的短路光电流为9.08 mA/cm~2,开路光电压为620 mV,填充因子为56 %,最大的输出功率为3.15 mW/cm~2,光电转换效率为4.8 %。结合吸收光谱和表面光电压谱,讨论了光电化学池的光电流产生机理:N3的敏化作用使光电化学电池在300~700 nm的紫外可见区均有光电流响应。这大大拓宽了TiO_2纳米晶的光电流响应区间。
使用磺酸基铝酞菁(Al(OH)PcSn)对TiO_2纳米晶多孔膜电极进行敏化,研究了其在红光区的光电响应。光电响应波长扩展到750 nm;在685nm处单体的Q带产生较强的光电流和光电压;吸收光谱中观察到的二聚体峰没有光电流响应。结合吸收光谱和表面光电压谱,研究了Al(OH)PcS_n的敏化机制。
采用四羟基苯基卟啉(THPP)和四羧基苯基卟啉(TCPP)分别对TiO_2纳米晶多孔膜电极进行敏化。TCPP与TiO_2之间的作用强于THPP与TiO_2的作用,因此,TCPP吸附量大于THPP。在不同光照方向下的光电流响
It is an important research subject for utilization of the solar energy.Dye-sensitized nanocrystalline TiO_2 mesoporous solar cell (DSSC) is agood competitor due to its simple-fabrication, low-cost andhigh-efficiency. In 2003, the M. Gratzel group of EPFL (Switzerland)reported 10.6 % light-to-electrical energy conversion efficiency, whichdraws more attention in the world for the research of DSSC.Dye-sensitized nanocrystal TiO_2 mesoporous film electrode is the keypart of DSSC, which is mesoscopic network structure, formed by theinternnected nanosized TiO_2 particles. The network structure ensuresthe electron transfer among it freely. Due to the small size of thenanosize particles in the nanocrystal TiO_2 film electrode, a high spacecharge layer can not be formed on the interface between the TiO_2surface and electrolyte. This results in no energy barrier. So, thesurface modification of TiO_2 film electrode and research on theone-dimension TiO_2 nanocrystal electrode are an important project forthe research of DSSC. Dye is also a key part of DSSC, which is like light
capture antenna of DSSC. Its function was to absorb solar light so as toexcitated electron of the ground state to the excited state. It is also oneof the key factors of determing the light-to-electrical energy conversionefficiency. The electrolyte in this thesis is composed of 0.1 M LiI (Acros),0.01 M I2 in acetonitrile and 3-methyl-2-oxazolidinone (99.9 %, Aldrich)(volume ratio, 1:1). The counter electrode is Pt electrode.In our dissertation, we have fabricated successfully the model ofthe DSSC. The mesoporous electrode was fabricated with TiO2nanocrystalline, nanobelt and nanorod. We also sensitized the TiO2electrode with Ru complex dye (N3), aluminum phthalcynines(Al(OH)PcSn), porphyrin derivatives and Nafion?. At last, weresearched the photo-electrochemical conversion property ofone-dimensional TiO2 cell. The main contents are following:(1)Titanium oxide is safety, non-poisonous and inexpensivesemiconductor materials, which has extensive application in the field ofphotocatalysis and photoelectrical conversion. But, it has photoelectricalactivity in the UV region (<400 nm)because the band gap of the TiO2is ca. 3.2 eV. So, extending the photoresponse of TiO2 towards thevisible and infrared region is the pursued target of the reseachers. Wefabricated the nanocrystal TiO2 mesoporous electrode using thedoctor-blade method. Ru complex dye (N3)was used to sensitize theTiO2 electrode. At last, the model device of DSSC was successfullyfabricated. The cell model generated a short-circuit photocurrent of 9.08mA/cm2, a open-circuit photovoltage of 620 mV, a fill factor of 56 % and
a maximal power output of 3.15 mW/cm2 under the light intensity of 65mW/cm2. The efficiency is 4.85 %. This data were approaching theefficiency of 7 % reported by Gr?tzel group in 1991. At the same time,surface photovoltage spectra were used to study the photoelectricalresponse of the model device. Combined with absorption spectra,electrochemistry, photoelectrochemistry and the theory of the chargetransfer, we discussed the generation mechanism of the photocurrent.These work provided experimental and theory foundation for theresearch of the organic dye-sensitized nanocrystal TiO2 mesoporouscell.(2)So far, Ru complex dye (N3)-sensitized TiO2 cell had the bestphoto-electrical energy conversion efficiency, so N3 dye was thought assuperstar dye. But it had some flaws: synthesis complexity, expensiveprice, only sensitization in the visible region and little molecularextinction coefficient. It was necessary to reseach the sensitization oforganic dye because organic dye was well complementarity for Rucomplex dye in the red and infrared region. TiO2 nanocrystal electrodeswere senstitized by aluminum phthalcynines (Al(OH)PcSn) modifiedwith sulfonate group. It was found that in the red region thephotochemical response of TiO2 electrode was obvious. The results ofthe surface photovoltage spectra and the photocurrent action spectraindicated that in the red region the monomer of aluminum phthalcynineswas the major factor to determine the photochemical response of TiO2electrode. The dimer had weak photochemical response. Nanoporous
TiO2 electrodes were sensitized by different porphyrin derivatives:terahydroxyphenyl porphyrin (THPP) and teracarboxylphenyl porphyrin(TCPP). The results showed that the interaction between TiO2 andTCPP was stronger than that between TiO2 and THPP. Thephotocurrent responses of the photoelectrochemical cell under differentillumination direction were studied. Calculated from the current-voltagecurve of photoelectrochemical cells, the overall light-to-electric energyconversion yield (η) of TCPP-sensitized cell was 0.13%, while theconversion efficiency of THPP-sensitized cell was 0.06%.(3)Based on the photo-electrical property of nanocrystal TiO2mesoporous photoelectrical cell and the mechanism of photo-generatedcharge transfer, we designed a further interfacial engineering about thenanocrystalline TiO2 mesoporous electrode. TiO2 photoelectrode wassensitized by N3 dye and perfluorosulfonic sodium Nafion?. As a result,the Nafion? modified TiO2 photoelectrode exhibited enhancedphotocurrent (about 15%) and high conductivity compared to thosewithout Nafion?. After Nafion? modification, Li+ in the liquid electrolytecould ionic-exchange with Na+ in Nafion and the anion was excludedaway from Nafion layer because Nafion was a kind of cation exchangefilm. This resulted in a potential drop in Helmholtz layer and a negativeshift of the conduction band of TiO2 (vs. vacuum level) underillumination and in turn increased the driving force for thephoto-generated electron injection and enabled the electron transfermore effective. An energy model was founded. We got the
light-to-electric energy conversion efficiency of 4.9%.(4)It is important merit for DSSC that photo-electrical material hasdiversity. The TiO2 electrodes are fabricated with TiO2 nanoribbons andnanorods, respectively, which both match the structure of TiO2-B. Thedye-sensitized solar cells made from TiO2 nanoribbon electrodes havebetter photoelectrical energy conversion efficiency (η) of 2.2 % thanthose made from TiO2 nanorod electrodes, which have η of 1.8 % underthe xenon lamp intensity of 50 mW/cm2. The rise time and decaylifetime of the photovoltage transients are 0.37 and of 9.32 ms for thenanoribbons cells, respectively, while those are 1.39 and 6.11 ms for thenanorods cells, respectively. The reason is that only saturated Ti (IV)atoms exist in the TiO2 nanoribbons surface while there are theunsaturated coordination Ti (III) atoms in the TiO2 nanorods surface,which could trap the photo-gennerated electrons. So the TiO2nanoribbons cells have better charge collection efficiency and lowerinterfacial charge recombination. The surface photovoltage spectraindicate that photo-generated electron transfer rate in the nanoribbonscells is stronger than that in the nanorods cells.
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