Design and characterization of alkoxy-wrapped push–pull porphyrins for dye-sensitized solar cellswz

Three alkoxy-wrapped push-pull porphyrins were designed and synthesized for dye-sensitized solar cell (DSSC) applications. Spectral, electrochemical, photovoltaic and electrochemical impedance spectroscopy properties of these porphyrin sensitizers were well investigated to provide evidence for the molecular design.


Optical and Electrochemical Properties in Solution
The UV-Visible absorption spectra of YD20, YD21 and YD22 in THF at 23 ºC are shown in Figure S1.The maximum peaks of the absorption spectra and the molar absorption coefficients of the Soret and Q absorption bands under the same conditions are summarized in Table S1.Porphyrins typically show in the absorption spectra a maximum attributed to π-π* transitions in the range 400-450 nm for an intense Soret absorption band and 500-700 nm for the moderate Q absorption bands.The molar absorption coefficients (10 3 M -1 cm -1 ) for the Soret band of YD20, YD21, and YD22 are 223, 172, and 223, respectively; in respect to the Q bands, the molar absorption coefficients (10 3 M -1 cm -1 ) are 61, 56, and 68 for YD20, YD21, and YD22, respectively.
YD20 and YD22 dyes have virtually the same intense Soret band (at 457 nm and 459 nm, respectively) and a low intense Q band (at 667 nm and 671 nm, respectively).The molar absorption coefficients for YD20 and YD22 dyes are equal (223•10 3 M -1 cm -1 ) for the Soret band and similar values for the Q band (61•10 3 M -1 cm -1 and 68•10 3 M -1 cm -1 , respectively).These results are due to the fact that both porphyrin-dyes have the same acceptor group, an acid group which will attach itself into the TiO 2 nanoparticles.
The Soret band for YD21 dye has low intensity absorption at 463 nm and the Q band for YD21 dye is slightly red-shifted (677 nm) with respect to the other porphyrin sensitizers, YD20 (667 nm) and YD22 (671 nm) dyes.The slightly red-shifted Q band for YD21 dye results are due to an elongation of the π-conjugation and loss of symmetry in the porphyrin structure which is caused by the cyanoacrylic acid.Also, the YD21 dye has the lowest molar absorption coefficient for the Soret and Q bands (172•10 3 M -1 cm -1 and 56•10 3 M -1 cm -1 ) compared with YD20 and YD22 dyes.
The emission data was measured in THF and the wavelengths for emission maxima are shown in Table 1.Similar to the UV-Vis absorption spectra results, the emission is redshifted (703 nm) on the incorporation of a cyanoacrylic acid for the YD21 with respect to the acid group for the YD20 (674 nm) and YD22 (676 nm) dyes.
Table S1.Spectral and electrochemical data for porphyrin dyes.a Dye Absorption max/nm (/10 3 M -1 cm -1 )  The redox potential of these porphyrins in solution was measured by using cyclic voltammetry, which shows well-resolved and reversible electron waves (Figure S2).
The oxidation and reduction potentials of these sensitizers are listed in Table S1.These

Electrochemical Impedance Spectroscopy (EIS) measurements
Acquisition of the EIS measurements were carried out with a Zahner impedance analyzer controlled from a computer with Thales software.The amplitude of the AC signal was 20 mV and its frequency ranged between 1 MHz and 10 mHz.Bias potentials used were comprised between 0 and 0.75 V. Illumination in impedance spectroscopy measurements was provided by a LED source.The power of the LED lamp was adjusted to obtain the same V OC as in J-V curves taken under illumination with a solar simulator at 1 sun irradiation (AM1.5G, 100 mW cm  ).