In the past researches, many PV polymeric backbones incorporate carbazole donor moieties with thiophene/thienothiophene(T/TT) as acceptor units. In the past few years, T/TT acts as Donors to the electron deficiency isoindigo. However, T/TT are also reported to be used as the pi-bridge between two different donor and acceptor units in some research papers. Is it because of carbazole is more electron rich than T/TT and the electron density of T/TT is higher than isoindigo units? If this is the answer, why is that the PV performance of the copolymers is poor when incorporate carbazole (Highest Donor) with isoindigo (Strong electron acceptor)?
Donor and acceptor capacity depends largely on how many lone (non-boding) pairs are available and the extent to which they may be localised (or delocalised) in the lattice. This is what impacts flow. I do not expect how many donors or acceptors there are and the arrangements in which they are to matter. It is the net availability and flow of electrons that matters. You would have to look into the detailed bond chemistry of these interest moieties or functional groups to come up with cogent explanations which could be verified in experimentation. Shalom!!!!!!!!
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