We designed highly efficient porphyrin sensitizers with two phenyl groups at meso-positions of the macrocycle bearing two ortho-substituted long alkoxyl chains for dye-sensitized solar cells; the ortho-substituted devices exhibit significantly enhanced photovoltaic performances with the best porphyrin, LD14, showing J(SC) = 19.167 mA cm(-2), V(OC) = 0.736 V, FF = 0.711, and overall power conversion efficiency η = 10.17%.
Co-sensitization of two or more dyes with complementary absorption spectra on a semiconductor film
is an effective approach to enhance the performance of a dye-sensitized solar cell (DSSC). Porphyrin
sensitizer YD2-oC8 showed outstanding photovoltaic performance co-sensitized with an organic dye to
cover the entire visible spectral region, 400–700 nm. To promote the light-harvesting capability beyond
700 nm, a porphyrin dimer (YDD6) was synthesized for a co-sensitized system. We report a systematic
approach for engineering of molecular co-sensitization of TiO2 films in a cocktail solution containing
YD2-oC8, an organic dye (CD4) and YDD6 in a specific molar ratio to optimize the photovoltaic
performance of the device. The resulting device showed panchromatic spectral features in the IPCE
action spectrum in the region 400–700 nm attaining efficiencies of 75–80%; the spectrum is extended to
the near-IR region attaining 40–45% in 700–800 nm region, giving JSC/mA cm
2 ¼ 19.28, VOC/mV ¼
753, FF ¼ 0.719, and h ¼ 10.4% under standard AM 1.5 G one-sun irradiation. This performance is
superior to what is obtained from the individual single-dye devices and the two-dye co-sensitized
systems. The shifts of TiO2 potential upon dye uptake and the kinetics of charge recombination were
examined through measurements of the charge extraction (CE) and intensity-modulated photovoltage
spectroscopy (IMVS), respectively. Five co-sensitized systems were investigated to demonstrate that
suppression of dye aggregation of YDD6 in the co-sensitized film is a key factor to further improve the
device performance
Porphyrins bearing a polyaromatic or a heterocyclic group are prepared to study their fundamental and photovoltaic properties. Solar cells sensitized with a fluorene-modified porphyrin outperform other dyes in the series, reaching ~90% efficiency of N719 dye.
New π-extended porphyrin dyes YD26-YD29 with long alkoxyl chains at the ortho positions of the meso-phenyls, and meta di-tert-butylphenyl-substituted porphyrins YD12-CN, and YD13-CN were synthesized for dye-sensitized solar cells, and their optical, electrochemical and photovoltaic properties were investigated and compared with those of YD12 and YD13. The absorption spectra of YD26-YD29 showed a slight red shift of Soret bands and blue shift of Q bands as compared to the meta-substituted porphyrins due to the electron-donating effects of dioctyloxy substituents at the ortho-positions of the meso-phenyl rings. Replacement of the carboxyl with a cyanoacrylic acid as the anchoring group results in significant broadening and red shifts of the absorptions, which is due to the strong electronic coupling between the pull unit and the porphyrin ring facilitated by the C≡C triple bond. The electrochemical studies and quantum-chemical calculations (DFT) indicated that the ortho alkoxy-substituted sensitizers exhibit lower oxidation potential, i.e. a higher HOMO energy level, and their HOMO-LUMO gaps are comparable to the meta-substituted analogues. The photovoltaic measurements confirmed that the ortho-octyloxy groups in the two meso-phenyls of YD26 and YD27 play a significant role in preventing dye aggregation thereby enhancing the corresponding short-circuit current density and open-circuit voltage. The power conversion efficiency (η) of YD26 is 8.04%, which is 11% higher than that of YD12, whereas the efficiency of YD27 is 6.03%, which is 135% higher than that of YD13. On the other hand, the poor performance of YD28 and YD29 is due to the floppy structural nature and limited molecular rigidity of the cyanoacrylic acid anchor.
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