Molecular Design Of D-A Conjugated Polymers Based On Carbazole-bisthiadiazole And Dithienosilole For Polymer Solar Cells

At present, the performance in polymer BHJ solar cells has been gradually improved. The most promising composite structure in polymer BHJ solar cells is a system based on solution-processable conjugated polymer donor and fullerene derivative acceptor materials. PSCs have attracted much attention in recent years, due to their advantages of easy fabrication, simple device structure, low cost, light weight, and solution processability, large area and flexibility. The maximum power conversion efficiency (PCE) of the PSCs has increased quickly to over8%recently. But the efficiency still needs to be improved for the demand of commercialization. Therefore, present studies of the PSCs are mainly focused on increasing PCE of the devices, and the key point to increase the PCE is the design and synthesis of high-efficiency conjugated polymer donor. A kind of ideal conjugated polymer should have broad absorption spectra, ideal highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels and high charge carrier mobility. In order to obtain high performance donor materials, a new electronic acceptor unit and a new electronic donor unit, corresponding donor-acceptor (D-A) conjugated polymers were synthesized.A novel architecture N-dodecyl-carbazole[3,4-c:5,6-c]bis[1,2,5]thiadiazole (CBT) was designed and synthesized based on carbazole, and made up of directly fixing two thiadiazoles on3-,4-and5-,6-position of carbazole to form penta-fused ring, a new type of donor-acceptor (D-A) copolymer poly{(N-dodecyl-carbazole[3,4-c:5,6-c]bis[1,2,5]thiadiazole-alt-4,8-di(2-ethylhexy-loxyl)benzo[1,2-b:4,5-b’]dithiophene)}(PCBTBDT) has been designed and synthesized using and di(2-ethylhexy-loxyl)benzo[1,2-b:4,5-b’]dithiophene)(BDT) as the basic building blocks. The copolymer has a low-lying highest occupied molecular orbital (HOMO) energy level of-5.41eV and a broaden absorption matching well to the main solar photon flux. Without any treatment, the resulting polymer achieved a efficiency of2.0%and relatively high open-circuit voltage (Voc) value of0.77V when blended with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) in a typical bulk heterojunction. Optimized operations, such as thermal annealing and addition of solvent additive1,8-diiodooctane (DIO), can not optimize the performance, but devastate the device efficiency instead. Due to the strong association between chains resulting from the high coplanar structures, a larger scale phase aggregation has been induced by these so-called optimized operations, consequently reduced the heterojunction areas and disrupting the pathway for charge transportation.Two novel copolymers containing dithienosilole (DTS) unit and ketone modified thieno[3,4-b]thiophene (TT) unit, namely, poly{4,4’-bis(2-ethylhexyl) dithieno [3,2-b:2’,3’-d] silole-5,5’-diyl-alt-1-(thieno[3,4-b]thiophen-2-yl)-2-ethylhexan-1-one}(PDTSTT) and poly{4,4’-bis(2-ethylhexyl) dithieno[3,2-b:2’,3’-d]silole-5,5’-diyl-alt-1-(4,6-bis(4-ethylhexylthien-2-yl)thieno[3,4-b]thiophen-2-yl)-2-ethylhexan-1-one}(PDTSDTTT), were synthesized for the application in polymer solar cells (PSCs) by copolymerzation, and the effects of thiophene bridge on the structural geometry and photovoiltaic performance have been investigated. The polymer PDTSTT and PDTSDTTT have the optical bandgaps of1.54and2.02eV, together with low-lying HOMO energy levels of-5.47and-5.37eV, respectively. Molecular geometry simulation result shows that compared with PDTSTT, the insertion of thiophene bridges in PDTSDTTT can well relieve the steric hindrance between the TT and DTS unit, as confirmed by the reduced the dihedral angle between DTS and DTTT unit. This is because the enhanced planarity and increased effective conjugation of the main chain in PDTSDTTT promote the more favorable morphology for charge transportation, although PDTSTT possesses broader absorption band than PDTSDTTT. Under the illumination of AM1.5G,100mW/cm2, the PSC based on PDTSDTTT/PC61BM (1:1wt.%) is demonstrated a power conversion efficiency of1.0%, which is significantly improved in comparison with the device based on PDTSTT/PC61BM (1:2wt.%) under the same experimental condition.