Dye-Sensitized Titanium Dioxide Solar Cells ― Studies on Redox Couples, Ruthenium and Thienyl Dyes

• Main Authors: Ying-Chan Hsu, 徐英展
• Other Authors: Kuo-Chuan Ho
• Format: Others
• Language: en_US
• Published: 2005
• Online Access: http://ndltd.ncl.edu.tw/handle/07900644406890669069

博士 === 國立臺灣大學 === 化學工程學研究所 === 94 === The object of this thesis is to investigate the behavior of the solid-state redox couples and the performance of Ru complexes or thienyl molecules used as sensitizers in the dye-sensitized solar cells (DSSCs). At first the N3 sensitizer is used as a model compound in a TiO2 DSSC to find out the factors that affect the photovoltage, photocurrent, and power conversion efficiency. The photovoltage is influenced by the photoinjected electron and the dark current caused from the recombination of electrons with I3- ions through TiO2/electrolyte and FTO/electrolyte interfaces. Thus, the photovoltage can be raised by increasing the injected electron and decreasing the recombination at the interface. The photocurrent induced from radiant power is affected by the value of incident photon-to-current conversion efficiency (IPCE) that is generated from energetic excitation of the sensitizer. The high absorption coefficient of the sensitizer extending to near infrared and long lifetime of photoinjected electron are necessary to achieve high photocurrent. The lifetime and transport of electrons in the TiO2 film can be estimated by transient photovoltage and photocurrent measurement. From the empirical model for a DSSC derived from the equivalent circuit and the reduction current, the behavior of the photocurrent-voltage curve can be predicted quantitatively. To fabricate a high-voltage, solid-state TiO2 DSSC, indium hexacyanoferrate (InHCF), was initially chosen as the redox couple. Since redox reaction of InHCF involves the K+ insertion/extraction, the KCl-saturated poly(2-acrylamido 2-methylpropanesulfonic acid) (K-PAMPS), a K+-conducting solid polymer electrolyte (SPE), was also incorporated to the cell. The imperfect N3 dye/InHCF contact by the SPE incorporation and slow solid-state diffusion of K+ in InHCF should be mainly responsible for the poor efficiency. Using a dye-incorporated SPE was found to improve the contact and could attain a regenerative cell. The imperfect dye/redox couple contact can be further improved by directly coating the lithium-doped redox couple, such as InHCF or cupric hexacyanoferrate (CuHCF), onto the dye/TiO2 surface. However, the poor contact between the counter electrode and dye/TiO2 electrode cause the charge-transfer resistance at the interface and voltage loss to enlarge. The slow reductive reaction between oxidized dye and Li ion-doped InHCF or CuHCF also plays an important role in determining photocurrent and efficiency. Besides, large charge-transfer resistance of CuHCF results in a low regenerating rate of oxidized N3 sensitizer and a low efficiency. Ruthenium(II) complexes with a new low pi* phenanthrenyl ligand (TAPNB) were obtained from Prof. Jiann T’suen Lin in Institute of Chemistry of Academia Sinica. The spectroscopic and electrochemical measurements showed that the excited states of those complexes matched the conduction band of titanium dioxide. The overall power conversion efficiencies of the solar cells utilized these new complexes as sensitizers for TiO2 films were less (under 1%) than that of N3-sensitized cell. Although the open-circuit voltage was similar to that of N3-sensitized cell, the short-circuit current was about one order lower. Such outcome may be attributed to the short lifetime of photoinjected electrons and less amount of dyes adsorbed due to the steric congestion of the complex. When NCS ligand was replaced by pyridyl ligand, the energy of metal-to-ligand charge transfer (Ru(II) → TAPNB) increased and resulted in blue shift of the absorption band. When carboxylic acid anchor was replaced by acetyl ester, the weaker interaction between the semiconductor and the ligand led to diminishing amount of the complex adsorbed and less photocurrent was detected. Four organic sensitizers based on benzothiadiazole and benzoselenadiazole chromophores with 2-thienyl-2''-cyano acrylic acid anchors were obtained from Prof. Jiann T’suen Lin in Institute of Chemistry of Academia Sinica and incorporated in DSSCs. When the phenylene linker is replaced by a thiophene unit, the improvements in donor property and coplanarity causes a red shift in absorption spectrum. However, the coplanar geometry can neither enhance the charge separation nor decelerate the recombination. The result is proved by the transient measurements. With the phenylene linker, the quantum yield is greatly improved and the cell efficiency approaches 4%. It may be due to the twisted nonplanar structure, which decelerates the recombination of charges. The organic sensitizers that contain diphenylamine donors and cyano acrylic acid acceptors bridged through an aromatic linker and a benzothiadiazole or benzoselenadiazole fragment ensure charge-transfer and facilitate charge separation. The conversion efficiency of a DSSC using the benzothiadiazole dye can reach as high as 4%.