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摘要
Quinoxaline derivatives show great potential in recent organic photovoltaics, not only as polymer acceptors for bulk heterojuction (BHJ) solar cells but also as molecular sensitizers for dye-sensitized solar cells (DSSCs). This work focuses on the effect of pi-linkers on photovoltatic performances of D-A-pi-A quinoxaline-based sensitizers used for DSSCs. The extension of pi-linkers is one of the viable tactics to improve the molar absorption coefficient and red-shift the absorption peak, which is beneficial to light harvesting. With respect to IQ4, a series of quinoxaline sensitizers 1Q6, 1Q7, and IQ8 were synthesized on the basis of a promising building block of 2,3-diphenylquinoxaline with g-linker modification. Dye IQ8, with an additional thienyl unit near the anchor group, shows little change in absorption spectra and energy levels, while in IQ6 and IQ7, the additional thienyl group close to the donor group obviously red-shifts the absorption band and positively shifts the HOMO levels. In the series of sensitizers, their adsorption amounts on the TiO2 surface are slightly decreased by introduction of a thienyl unit near the donor part and/or the introduction of alkyl chains. Their photovoltaic performances are well evaluated by the electron collection length values (L-col), first-principles calculations, the conduction band edge (E-CB), and the fluctuation of electron density or charge recombination rate in DSSCs. Instead of the electron injection efficiency (Phi(inj)), the low charge collection efficiency (Phi(col)) of IQ6, IQ7, and IQ8 results in their unsatisfactory incident photon-to-current conversion efficiency (IPCE) plateaus. Also the difference of V-oc among these dyes mainly arises from the fluctuation of TiO2 electron density, which is closely related to the recombination resistance. Upon increasing the thiophene number, the electron collection lengths of 1Q6, 1Q7, and 1Q8 based DSSCs become shorter, which dramatically decreases their photocurrent with an unbeneficial preferable photovoltaic performance. As demonstrated, it is essential to have a judicious design on pi-linker modification for high-performance D-A-pi-A quinoxaline-based sensitizers.
