Design And Synthesis Of Highly Efficient Porphyrins For Mesoscopic Solar Cells

Dye-sensitized solar cells (DSCs) are one of the most promising third-generation photovoltaic technologies. Imitating the extraordinary chlorophyll-based photosynthetic light harvesting process has inspired researchers to investigate porphyrin dyes for DSCs. Porphyrins and derivatives take their advantage of availability of several substituent sites and intrinsic spectroscopic properties, presenting a rapaid development in the applicaition in DSCs. In2007, Officer and Gratzel et al. reported a benchmark power conversion efficiency of7.1%based on porphyrin sensitizer, triggered a new hope of highly efficient of porphyrin sensitizers. Then, in2009, Diau and co-workers repotrted series of meso-substituted push-pull porphyrins, and reported outperformed DSCs with power conversion efficiency over10%. Now, the highest efficiencies of DSCs are dominated by porphyrin sensitizers, reaching a remarkable record value of13.0%with cobalt based electrolyte.Hererin, systematic design and synthesis of porphyrin dyes, and investigation of the relationship between porphyrin’s intrinsic property and the DSCs performance have been discussed and presented. Based on the concept of D-「-A, we modified the structure of porphyrin sensitizers from the donor site, porphyrin macrocycle site, acceptor and anchoring group site to optimize the highly efficient porphyrin sensitizers, with a aim to improve the performance of DSCs. The main contents of this thesis are listed as following:A series of alkoxy-wrapped zinc porphyrins LW1-LW4with donor-「-acceptor structure were designed and synthesized for DSCs. By modifying the acceptor with phenyl, thiophene or EDOT, the corresponding porphyrins showed stepwise red-shifted absorption spectra. A rigid conjugation acceptor of LW4was further designed and synthesized, by switching the cyanoacetic acid to carboxyl acid for LW2. Spectral, electrochemical, photovoltage transient decay and impedance measurements were performed to reveal the influence of π-conjugated linkers and anchoring groups upon the optoelectronic features of porphyrins dyes in DSCs;Two new D-π-A porphyrins (coded as LW5and LW6) with extended conjugation units at acceptor were designed and synthesized for DSCs. While keeping a same donor moiety to the well-investigated porphyrin LD14, the introduction of thiophene linker units can red shift the absorption spectra efficiently. The electron injection and oxidized dye regeneration processes were monitored with time resolved photoluminescence and scanning electrochemical microscopy measurements, respectively. LW5and LW6dyes were evaluated in DSCs using [Co(bpy)3]2+/3+as redox couple, showing efficiencies of7.8%and6.1%under standard test conditions, respectively. A detailed investigation with transient photovoltage decay measurement provided important information on factors affecting the principal photovoltaic parameters;Cyclopenta[1,2-b:5,4-b’]dithiophene (CPDT) was introduced as spacer between porphyrin chromophore and cyanoacetic acid to obtain a porphyrin dye (coded as LW9). The resulted novel porphyrin dye exhibits an extended absorption spectra and a split B band at520nm. To make a thoroughly investigation on the influence, spacer unit with biphenyl (LW7) and bithiophene (LW8) were also synthesized. Stepwise red-shifted absorption spectra and consistently decreased energy gap are presented for LW7-LW9. Evaluating these novel porphyrin with DSCs, power conversion efficiency of6.50%under full-sunlight irridation was achieved employing [Co(bpy)3]2+/3+as redox couple for LW9device. Detailed investigation, including time-resolved photoluminescence, transient photovoltage decay, and scanning electrochemical spectroscopy measurements, provides important information on the factors affecting the principal photovoltaic parameters.Pyridine was introduced as the anchoring group for donor-π-acceptor zinc porphyrin dyes (LW11), and successfully utilized as efficient dyes for DSCs. The photo-physical and electrochemical investigations demonstrate that the pyridine ring works effectively as anchoring group for porphyrin sensitizers. The DSCs based on these new porphyrins show an overall power conversion efficiency of～4.0%under full sunlight irradiation. Detailed investigation, including transient absorption spectrum, and transient photovoltage decay, provides important information on the factors affecting the principal photovoltaic parameters.Two hexyl-thiophene groups are attached to the meta-positions of each meso-positions phenyl, with a motivation to increase the light-harvesting ability as well as retarding the aggregation of porphyrins dyes (LW16). For comparison, none-alkyl substituted (LW14) and octyloxy substituted (LW15) porphyrin dyes were also synthesized to fully investigate the influence of porphyrin chromophore modification on the opto-electronic property. Different performance are presented in DSC devices, power conversion efficiency of8.5%is achieved with LW16porphyrins under full sunlight irradiation. At similar condition, it is6.9%and8.2%for LW14and LW15devices. Spectral, electrochemical, transient photovoltaic measurements are well-performed to provide general influence of π-conjugation extension at the meso-position to the optoelectronic features of porphyrins dyes in DSCs.Pyrene-conjugated porphyrin dyes with various π-spacers between the porphyrin chromophore and carboxylic acid have been designed and synthesized, including phenyl (LW17), thiophene (LW18), and2-phenylthiophene (LW19). These novel porphyrins show stepwise red-shifted absorption spectra and consistently decreased oxidation potential. The DSCs based on LW18dye can achieve an overall power conversion efficiency of8.7%under full sunlight irradiation. The result reveals that both photocurrent and photovoltage can be effectively tuned by changing the spacers. Detailed investigation with transient photovoltage decay measurement provides general information on factors affecting the principal photovoltaic parameters.