Abstract
The conversion solar energy into electricity is one of the most interesting and fascinating topics of research. Of the different molecular photovoltaic devices used to convert solar energy into electricity, dye-sensitized solar cells (DSSCs) are commonly regarded as a promising next generation solar cell technology because they have good performance at 13% with single organic dye and can be made relatively inexpensively without intensive purification and fabrication steps. The sensitizer, one of the key components of DSSC, is responsible for the harvesting of solar light and transfers the energy via electron transfer to a suitable material (e.g. TiO2) to produce electricity. The overall efficiency of the DSSC is determined by the complex processes at the TiO2/Dye/redox electrolyte/Pt interface. Inefficient light harvesting due to narrow absorption spectral range, signified charge recombination with the dye as well as with the redox electrolyte are among main reasons for efficiency deterioration. Therefore, the search of a material with panchromatic absorption that cost a minimum loss at the interface has been one of the crucial research directions in the field of DSSC. The ultimate target is to reach a high conversion efficiency of 15% in DSSCs based on organic dyes, while retaining their stability under standard reporting conditions. The research topic of this thesis focuses on the intentionally design and synthesis of metal-free organic dyes for applications in DSSCs and boost the performances. Specific attention has been paid to the correlation between the molecular structures and physical properties, as well as their performances in DSSCs. In chapter 3, a unique strategy for effectively suppressing charge recombination and enhancing light harvesting in dye-sensitized solar cells (DSSCs) is demonstrated by designing a new dipolar organic dye functionalized with a diacetylene unit, which is capable of undergoing photoinduced cross-linking reaction to generate a hydrophobic polydiacetylene layer. The polydiacetylene layer serves as not only an electrolyte-blocking layer to effectively block the approaching of the oxidized redox mediator and supress the dark current but also light-harvesting role by efficient energy transfer to the dipolar dyes. A 15% efficiency improvement is achieved from monomer dye (JSC = 13.5 mA/cm2, VOC = 0.728 V, FF = 0.73, = 7.17%) to cross-linked dye (JSC = 14.9, VOC = 0.750, FF = 0.74, = 8.27%) under AM1.5 condition. In chapter 4, the influences of heterocyclic π-linkers on photovoltaic parameters of the dye-sensitized solar cells of quinoxaline-based organic dyes integrated in a D-A-π-A configurational framework is systematically investigated. The new sensitizers: EE8, MA177 and MA136 used for this investigation derived from the reported CR147 by replacing thiophene with EDOT, N-methyl pyrrole and furan unit into the π-system. When employed in DSSCs, these dyes displayed a significant difference in light harvesting and aggregation behavior. Best performance is achieved by the EDOT based sensitizer EE8 (JSC = 13.3 mA/cm2, VOC = 0.754 V, FF = 0.70, 8.08%) under AM1.5 condition; 98% of the intrinsic performance of N719 measured under similar condition. While, CR147, MA177 and MA136 resulting 86%, 72% and 22% of the reference N719, respectively. Furan in conjunction to diphenylquinoxaline appears to be more advantageous in forming a more planar geometry for a better charge transfer process. On the other hand, N-methyl pyrrole develop high torsional angle between diphenylquinoxalin and N-methyl pyrrole, triggering the low dye loading, weak light harvesting and high aggregation. Time-correlated single photon counting (TCSPC) fluorescence study indicates that the final outcome of the JSC of the four dyes is not determined by the electron injection step. In Chapter 5, a series of quinoxaline derivative auxiliary acceptors containing dyes that differs only on the quinoxaline ring substituent has been synthesized, characterized, and applied as photosensitizers for DSSC. The dye contains 5-butylthiophene (MA169), 5-butylfuran (MA174), N-methylpyrrole (MA181), thiophene (MA190) and phenyl (CR147) substituent on quinoxaline ring. These heteroatoms generate effective -conjugation forming more planar D-A--A geometry. The structure-performance relationships in DSSCs has been systematically evaluated and found to be strongly dependent on the nature of substituent on the quinoxaline ring. Devices sensitized by the dye MA169 display highest power conversion efficiencies (PCEs) of 8.33% (JSC 16.1 mA/cm2, VOC = 0.770 V, FF = 0.672) (up to 13% improvement from the analogous CR147, and 98% of the intrinsic performance of N719) measured under simulated AM 1.5 sunlight in conjunction with the I- /I3- redox couple, which further achieve over 11% under 0.25 sunlight illuminations. While CR147, MA190, MA174 and MA181 displayed the best efficiency of 7.36%, 4.45%, 3.29% and 2.04% respectively. MA169 exhibits higher performance with outstanding long-term stability as a result of excellent light harvesting, good blocking effect of the triiodide to suppress the dark current. The lower dye loading, smaller driving force for electron ejection and lower charge injection efficiency in MA181 contributes the weak light harvesting and lowers the overall power conversion efficiencies. Hence, 2,3-bis(5-butylthiophene-2-yl)quinoxaline might be a promising auxiliary acceptors in the design of efficient DSSCs materials with improved VOC. In chapter 6, a series of 2,3-bis(5-butylthiophen-2-yl)quinoxaline auxiliary acceptor based sensitizers (such as; MA186, MA197, MA199, MA1102, MA1104, and MA1111) comprising either a thiophene/thieno[3,2-b]thiophene conjugated triphenylamine donor or triphenylamine as the donor, cyanoacetic acid as the acceptor, and either thiophene, bithiophene, EDOT, or diethyl cyclopentadithiophene moiety as -linker from the acceptor side are designed and successfully synthesized. With these structural modifications, the PCE of the DSSC based on the resulting MA-series synthesized dye ranges from 3.65-8.1% and only the EDOT spaced sensitizer, MA1102 performance (7.97%) almost levels with MA169. Unlike MA1102, the sensitizer MA169 show out-of-plane twist conformation, thus, the 3D twisted structure of MA169 is favorable for suppressing dye aggregation, charge recombination, and back-electron transfer. Apparently, the effect of coplannarity of the whole molecular skeleton results unfavorable for the intramolecular charge separation of the D-A--A/D--A--A push-pull system and causes dye aggregation. These MA-series dyes could be a good model for designing efficient sensitizers. Finally, a new sensitizer (MA1107) incorporating 6,7-bis(5-butylthiophen-2-yl)-[1,2,5]thiadiazolo[3,4-g]quinoxaline auxiliary acceptor is designed and systematically synthesized. The new material displays a significant red shift of the absorption spectral coverage into the NIR region, onset about 800 nm. The electrochemical band gap is 1.52 eV. Which indicates that the NIR absorption could be easily achieved by incorporating such auxiliary acceptor into the sensitizer. However, the LUMO position for MA1107 is very close to the TiO2 ECB and lacks of sufficient driving force for electron ejection. Thus, structural modification targeting to tune the LUMO energy level, systematic analysis to fully understand efficiency loss mechanism in the cells or use of a different photoanode material is the ultimate route to success with this chromophore.