A Study Of Type Ⅱ Catechol Dyes With D-π-A Structure For Dye-sensitized Solar Cells

As the new generation photovoltaic technology, dye-sensitized solar cells (DSSCs) have obtained worldwide attention in recent decades. The photosensitizer as a light harvesting molecule to inject the photoexcited electron to the conductor band (CB) of titania semiconductor has become one of key factors affecting the photovoltaic performance of DSSCs. And depending the electron-injected pathway, they can be classified into two types, namely, Type I and Type II. Type II catechol dyes are the dyes which can inject electrons to TiO2electrode not only followed by first excited to the lowest unoccupied molecular orbital (LUMO) of dyes, but also directly by photoexcitation of the dye-to-TiO2charge-transfer (DTCT) band, created when dyes anchored on TiO2surface.In this work, two new Type II catechol dyes with D--A structure commonly used in Type I dyes have been successfully synthesized, characterized and applied to DSSCs, respectively using triphenylamine and its derivative with two butoxy chains as the electron-donating part, thiophene as the bridge group, and2,3-dihydroxybenzonitrile as electron-withdrawing part and anchoring group. It was found that higher short circuit current density (Jsc) and incident photon-to-current conversion efficiency (IPCE) has been obtained but no more improvement in total efficiency (1.62%after optimazation on dipping time, amount of CDCA and the thickness of TiCO2film) as open circuit voltage (Voc) and fill factor (ff) lower than that ever reported. We also show that an increase occurred in Jsc and ff of the device as well as optical absorption of the catechol-titania complex further broaden, when adding two butoxyl groups onto the electron-donating part of the dye to rise the highest occupied molecular orbital (HOMO) level and to some extent suppress charge recombination. Furthermore, in this work a cyano group added onto the electron-withdrawing part of the dye not only can stabilize the catechol easily oxidized in the air but can also enhance its acidity and further its absorption on TiO2due to the bidentate catechol chelation to the titania. And electron-withdrawing property of this part was also enhanced to advance the electron injection to TiO2.As to the specific characteristic of Type II catechol dyes, we studied the effects on the photovoltaic performance of DSSCs caused by dipping time of TiO2electrodes in dye bath, namely the dye uptake, and the co-absorption of CDCA, through photovoltaic tests, UV-vis absorption spectra, electrochemical impedance spectra and so on. And contrary change trends were found in Jsc and Voc of DSSCs as dipping time increased, which indicates that recombination still plays a significant role in the interplay between them to suppress device performance, commonly occurred in such Type II DSSCs. So it’s clear that these factors need to be independently controlled when to design more efficient Type II dyes for DSSCs in the future.