Organic conjugated polymers continue attracting considerable attention all over the world in the prospect of producing high-efficient, large-area, and flexible organic electronics with tunable properties. The potential applications of organic semiconductors include organic field-effect transistors (OFETs), organic bulk heterojunction organic solar cells (OPVs), organic light emitting diodes (OLEDs) and organic sensors etc. However, the conjugated polymers with ultra-wide bandgap (E_g>2.2eV) generally exhibit very low power conversion efficiency.

Recently, the research team led by Prof. WAN Xiaobo from Qingdao Institute of Bioenergy and Bioprocess Technology (QIBEBT), Chinese Academy of Sciences (CAS), announced that they synthesized a novel conjugated polymer with ultra-wide bandgap, which displayed a high efficiency in converting sun light into electricity, and also exhibited a very interesting charge transfer behavior.

This novel polymer is based on a key lactam acceptor building block, namely dibenzonaphthyridinedione(DBND), which could be synthesized via the isomerization of isoindigo, a well-known pigment. They found conjugated polymers based on O-alkylated DBND exhibit photoelectric conversion efficiency (PCE) up to 6.32%. Such a high PCE is obtained without additives or annealing process, which shows little decay in a thicker active layer and little sensitivity to the weight ratios of active ingredients. These merits make DBND based polymer a potential material for processable large-area tandem or ternary OPVs. (Chem. Mater., 2016.)

They also discovered that such a wonderful performance is strongly influenced by the backbone fluorination positions. OPV devices based on para-fluorinated DBND polymers show best PCEs up to 6.55%, while those based on ortho-fluorinated polymers only exhibit PCEs less than 2%, although both polymers have the same bandgap, similiar HOMO/LUMO energy levels and torsion angle. These results suggest that the control of fluorination positions on the polymeric backbone may have a profound influence on the outcome of the OSC performance, which should be paid more attention for future design of conjugated polymers for better OSC devices. (Chem. Mater., 2017).

More interestingly, they found the change of the alkylation positions on DBND strongly influences the charge carrier mobility of the corresponding polymers. Although with the side-chain branching point one atom closer to the main chain, N-alkylated DBND polymer shows much higher hole mobility (0.55 cm² V^？1 s^？1), almost 100 times greater than that of O-alkylated isomer. It indicates that the major difference between N-alkylated and O-alkylated conjugated polymers lies in their different polarity, in which higher polarity favors tighter interchain packing, which overwhelms the lower steric hindrance of O-alkylation. (Macromolecules, 2017). 

These works shed lights on the structure-property relationship of conjugated polymers, and might be helpful for the design of novel lactam-containing conjugated polymers in the future. The works are supported by the National Natural Science foundation of China.

(TEXT by WAN Xiaobo)

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