Highly Twisted Dianchoring D-π-A Sensitizers for Efficient Dye-Sensitized Solar Cells

Two new organic dyes-BPDTA and BTTA-possessing dual D-π-A units have been synthesized, characterized, and employed as efficient sensitizers for dye-sensitized solar cells. The two individual D-π-A, which are based on (E)-3-(5'-(4-(bis(4-(hexyloxy)phenyl)amino)phenyl)-[2,2'-bithiophen]-5-yl)-2-cyanoacrylic acid unit (D21L6), are connected directly between phenylene or thiophene within linear π-conjugated backbone to constitute a highly twisted architecture for suppressing the dye aggregation. The new dianchoring dyes exhibited pronounced absorption profile with higher molar extinction coefficient, which is consistent with the results obtained from density functional theory (DFT) calculations. The theoretical analysis also indicated that the charge transfer transition is mainly constituted of HOMO/HOMO-1 to LUMO/LUMO+1 that were found to be located on donor and acceptor segments, respectively. Theoretical calculations give the distance between two binding sites of 19.50 Å for BPDTA and 12.04 Å for BTTA. The proximity between two anchoring units of BTTA results in superior dye loading and, hence, higher cell efficiency. The BTTA-based device yielded an optimized efficiency of 6.86%, compared to 6.61% for the BPDTA-based device, whereas the model sensitizer D21L6 only delivered an inferior performance of 5.33% under similar conditions. Our molecular design strategy thus opens up a new horizon to establish efficient dianchoring dyes.