Effect of Different Dye Baths and Dye-Structures on the Performance of Dye-Sensitized Solar Cells Based on Triphenylamine Dyes

Triphenylamine dyes were designed and synthesized as photosensitizers for dye-sensitized solar cells (DSSCs). Different substituted phenylene units, 2,2';5',2''-ter-thiophene (TT) and dithieno[3,2-b;2',3'-d]thiophene (DTT) serve as the π-spacers - the electron acceptors use cyanoacrylic acid or rhodanine-3-acetic acid units. A detailed study on the relation between the dye structure, and photophys., photoelectrochem. properties and performance of DSSCs is described here. By substituting the phenylene group with electron-withdrawing units as π-spacers or replacing the cyanoacrylic acid with rhodanine-3-acetic acid units as electron acceptors, bathochromic shift of absorption spectra is achieved. Significant differences in the redox potential of these dyes are due to small structure changes. The different dye baths for semiconductor sensitization have a crucial effect on the performance of the DSSCs due to the different absorbed amt., absorption spectra and binding modes of anchored dyes on the TiO2 surface in various solvents. From optimized dye bath and mol. structure, TPC1 shows a prominent solar-to-electricity conversion efficiency (η), 5.33% (JSC = 9.7 mA/cm2, VOC = 760 mV, ff = 0.72), under simulated AM 1.5 G irradn. (100 mW/cm2). DFT showed the electron distribution and the intramol. charge transfer (HOMO→LUMO) of the dyes. From the calcn. results of the selected dyes, the authors can also find the cyanoacrylic acid unit better than the rhodanine-3-acetic acid unit as electron acceptor. Also, the electron-withdrawing groups on phenylene units as π-spacers show the neg. effect on the performance of the org. DSSCs.

Influence of π-Conjugation Units in Organic Dyes for Dye-Sensitized Solar Cells

Ulf Anders Hagfeldt, Peng Qin

Two org. dyes with the general structure donor-conjugated chain-acceptor (D-π-A) were studied as sensitizers for nanocryst. TiO2 solar cells. The electron donor and acceptor groups were pyrrolidine and cyano acrylic acid, resp. The conjugated chain of 2-cyano-3-{5-[2-(4-pyrrolidin-1-ylphenyl)vinyl]thiophen-2-yl}acrylic acid contains one Ph ring and a thiophene unit and is therefore denoted PT, while for 2-cyano-3-{5-[2-(5-pyrrolidin-1-ylthiophen-2-yl)vinyl]thiophen-2-yl}acrylic acid the Ph ring is replaced by a 2nd thiophene unit (TT). Solar-to-elec. energy conversion efficiencies under simulated AM 1.5 irradn. (1000 W m-2) of 2.3% were obtained for solar cells based on PT but of

2007

A triphenylamine dye model for the study of intramolecular energy transfer and charge transfer in dye-sensitized solar cells

Ulf Anders Hagfeldt, Ming Liu

A novel dye (2TPA-R), contg. two triphenylamine (TPA) units connected by a vinyl group and rhodanine-3-acetic acid as the electron acceptor, is designed and synthesized successfully to reveal the working principles of org. dye in dye-sensitized solar cells (DSSCs). 2TPA and TPA-R, which consist of two TPA units connected by vinyl and a TPA unit linked with rhodanine-3-acetic acid, resp., are also synthesized as refs. to study the intramol. energy transfer (EnT) and charge transfer (ICT) processes of 2TPA-R in CH2Cl2 soln. and on a TiO2 surface. The results suggest that the intramol. EnT and ICT processes show a pos. effect on the performance of DSSCs. However, the flexible structure and less-adsorbed amt. of dye on TiO2 may make it difficult to improve the efficiency of DSSCs. This study on intramol. EnT and ICT processes acts as a guide for the design and synthesis of efficient org. dyes in the future.

2008

Characterization of the Interface Properties and Processes in Solid State Dye-Sensitized Solar Cells Employing a Perylene Sensitizer

Ulf Anders Hagfeldt

A perylene sensitizer, ID176, was recently reported which performs much better in solid state dye-sensitized solar cells than in those using liq. electrolytes with iodide/tri-iodide as the redox couple. Here, a characterization is presented of the sensitizer and of the TiO2/dye interface by UV-visible absorption and fluorescence spectroscopy, spectroelectrochem., photoelectron spectroscopy, electroabsorption spectroscopy, photoinduced absorption spectroscopy, and femtosecond transient absorption measurements. The absorption spectrum is reported of the sensitizer red-shifted by addn. of lithium ions to the surface due to a downward shift of the excited state level of the sensitizer, which is of the same order of magnitude as the downward shift of the titanium dioxide conduction band edge. Results from photoelectron spectroscopy and electrochem. suggest that the excited state is largely located below the conduction band edge of TiO2 but that there are states in the band gap of TiO2 which might be available for photoinduced electron injection. The sensitizer was able to efficiently inject into TiO2, when a lithium salt was present on the surface, while injection was much less effective in the absence of lithium ions or in the presence of solvent. In the presence of the hole conductor 2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9'-spirobifluorene (spiro-MeOTAD) and LiTFSI, charge sepn. was monitored by the emergence of a Stark shift of the dye in transient absorption spectra, and both injection and regeneration appear to be completed within 1 ps. Regeneration by spiro-MeOTAD is therefore several orders of magnitude faster than regeneration by iodide, and ID176 can even be photoreduced by spiro-MeOTAD.

2011