Effects of multi anchoring groups of catecholamine polymer dyes on the electrical characteristics of metal free dye-sensitized solar cells: A comparison study

Poly epinephrine and poly dopamine were synthesized through a new manner in an alkaline solution containing tris(hydroxymethyl)aminomethane (THAM) buffer with pH = 8.5 under nitrogen atmosphere to make the dyes with very strong adhesion and efficient electron injection by direct dye-to-TiO2 charge transfer. These dyes were used for fabrication of metal free dye sensitized solar cells. Photovoltaic characteristics were obtained for cells made by utilization of two dyes and it was established that the poly epinephrine dye exhibited larger current density and energy conversion efficiency than the poly dopamine dye. This is attributed to the strong attachment of the poly epinephrine dye to the surface of the TiO2 nanoparticles due to the presence of three anchoring OH groups on the back bone of the polymer. Under simulated AM 1.5 G solar light (100 mW cm(-2)), a poly epinephrine-sensitized cell exhibited a short circuit current density of 3.63 mA cm(-2), and overall power conversion efficiency of 0.41%, which is almost 60% larger than that of the poly dopamine dye-sensitized cell (0.26%). UV-Vis absorption spectra of the dyes were recorded and it was shown that absorption by poly epinephrine has extended over a broad range of wavelengths from UV up to mid-range of visible region and has a peak centered at 420 nm. However the range of absorption wavelengths for poly dopamine is limited to the initial part of visible region and the peak wavelength occurs at 257 nm. Moreover, optical band gaps of the dyes were obtained from wavelengths of the absorption edges of the respecting UV-Vis absorption spectra (2.25 and 3.32 eV for poly epinephrine and poly dopamine respectively). HOMO and LUMO energy levels of the dyes were estimated from cyclic voltammograms. Electrochemical impedance spectroscopy was applied to study of the electrical properties of the cells including poly epinephrine and poly dopamine dyes. (C) 2013 Elsevier Ltd. All rights reserved.