Synthesis And Photovoltaic Applications Of Novel Conjugated Terpolymers

Bulk heterojunction（BHJ） polymer solar cells（PSCs） are making significant progress in terms of power conversion efficiencies（PCEs） recently. The significant increase in PCEs largely originates from the successful development of new electron donor conjugated donor-acceptor（D-A） polymers. In the way, to further improve the PCE, more excellent D-A polymers need to be designed and synthesized, or improve the defect of the existing excellent D-A polymers, such as the energy levels and band gap. Thus, in the work, we centre on the design and synthesis of D-A polymers to further optimize the performance through the strategy of construct ternary polymer.Firstly, we take advantage of the 6H-phenanthro[1,10,9,8-cdefg]carbazole（PC）donor block and diketopyrrolopyrrole（DPP） acceptor unit to create novel condensed aromatic D-A random conjugated copolymers. Based on the D/A ratio of 1/2,thieno[3,2-b]thiophene（TT） and thiophene（T） were chosen as the bridge to afford polymers PDPP-TT-PC and PDPP-T-PC. Two polymers show excellent light absorption. The polymer solar cells（PSCs） based on PDPP-T-PC achieves the efficiency of 2.0%. After replacing T with TT, slightly enhanced power conversion efficiency（PCE） of 2.2% is obtained from the PDPP-TT-PC-based device due to the better charge mobility of TT. The density functional theory（DFT） calculation results reveal that the incorporation of the axisymmetrical PC molecule with centrosymmetrical DPP unit together induces a large dihedral angle and big backbone torsion, and the more planar TT linkage seems to deteriorate molecular packing compared to the T unit.Next, to resolve the problem dicoverd from last work, a novel D-A donor DTTT was designed and synthesized according to the structure of DPP. The donor use the thieno[3,2-b]thiophene unit as the core and flanked by thiophene at the 3, 6-position of TT, the interactions between the sulfur atoms of T and TT unit（S-S） will achieve a high degree of planarity and facilitates charge transport. Through the random copolymerization and direct arylation scheme, the random polymers（PDTTT-T-DPP₃/7 and PDTTT-T-DPP₄/6） and the alternating polymer（PDTTT-DPP） were synthesized. Although the planarity of DTTT structure is not as good as the DPP molecule, but incorporating the DTTT and DPP molecule help the polymers achieves the high hole mobility, especially for PDTTT-T-DPP₃/7. Polymer thin film transistors（PTFTs） devices based on PDTTT-T-DPP₃/7 obtains a high hole mobility of 0.627 cm2V-1s-1and Space charge limited current（SCLC） measurement also show the high hole mobility of 2.3×10-2cm2V-1s-1. Besides,PDTTT-T-DPP₃/7:PC61BM obtain a PCE of 2.7%.Besides, we present a new strategy that construct alternating Donor1-Acceptor-Donor 2-Acceptor（D1-A-D2-A） terpolymer with two donors and one acceptor to further fine-tune energy levels and optical band gap of the D-A copolymers. The novel terpolymer PBDT-DTffBT-F-DTffBT chooses BDT as electron-rich unit（D1）, and the weaker donor fluorene（F） as D2, whereas difluorinated benzothiadiazole（DTffBT） is selected as the electron acceptor（A）.Moreover, the separate D-A copolymer PBDT-DTffBT and PF-DTffBT also have been prepared for comparison. The lower HOMO level and higher LUMO level were achieved for PBDT-DTffBT-F-DTff BT compared to PBDT-DTffBT, translating to higher open-circuit voltage（Voc） and more efficient exciton splitting and charge dissociation. Thus, device utilizing PBDT-DTffBT-F-DTffBT afford the improved PCE of 2.5%, with a higher Vocof 0.853 V, in spite of PBDT-DTff BT-F-DTffBT possessing the worse light absorption and co-planarity. Therefore, through carefully choosing the suitable donor group, the class of D1-A-D2-A type copolymer would be a promising organic semiconducting material.