Synthesis And Photovoltaic Applications Of Copolymers Based On Novel Carbazole Structure

Over the past few decades, as a renewable resource while minimizing detrimental effects on the environment, photovoltaic cells （PCs） which harvesting energy directly from sunlight have received a great deal of attention from both academic and industrial laboratories. Bulk heterojunction （BHJ） polymer solar cells （PSCs） based on composites of an electron-donating semiconducting conjugated polymer and an electron-accepting fullerene offer significant promise over conventional inorganic materials for the numerous advantages in terms of low-cost, light-weight, ease of processing, large-area, and flexibility. The PSC commercial applications come true, are related to the materials in the PSC active layer:How to design and synthesize polymers with a broad absorption spectrum, a narrow band gap, and a high carrier mobility to improve the efficiency, is currently a research priority of the PSC. Carbazole and its derivatives have a wide range of potential applications in the field of optoelectronic materials. Design and synthesis of some new carbazole derivatives to improve, enhance intramolecular pi electron delocalization and the ability of the charge transport is a significance study. The details are given follows:1. An original strategy to construct a new D-A integrated structure by directly imposing "pull" unit on the "push" moiety to form fused rings architecture has been developed and poly{N-alkyl-carbazole[3,4-c:5,6-c]bis[1,2,5]thiadiazole-alt-thiophene}（PCBTT） with D-A integrated structure, where two1,2,5-thiadiazole rings are fixed on carbazole in3-,4-and5-,6-position symmetrically and thiophene is used as bridge, has been synthesized. The interaction between pull and push units has fine-tuned the HOMO/LUMO energy levels of the resulting copolymer to cover the solar flux from300-750nm. It is worthy to note that thanks to the fused five rings inducing the intrinsic and strong intermolecular interaction, an extremely short Ï€-Ï€ stacking distance of0.32nm has been achieved for PCBTT in solid states, even for powder sample, which is the shortest face stack for conjugated polymer reported to date. Additionally, an obvious intramolecular charge transfer and energy transfer from donor units to acceptor units have been detected in this D-A integration. An relatively high open-circuit voltage of～0.7V in PCBTT:PCBM （w/w=1/2） solar cells is achieved due to the low-lying HOMO energy level of PCBTT.2. Several novel conjugated copolymers PTBDTCBT and PDTSCBT are prepared by alternating copolymerzation of N-alkyl-carbazole[3,4-c:5,6-c] bis[1,2,5] thiadiazole （CBT） with alkylthienyl substituent benzodithiophene （TBDT） and dithenosilole （DTS）, respectively. The energy levels and molecular geometry of all of the CBT-based polymers are compared by the theoretical calculation and experimental observation. It has been found that the band gap and energy levels of all of the CBT-based polymers are well modulated by various building blocks, and the molecular geometry of polymers vary with the block structures as well. Among these CBT-based polymers, PDTSCBT shows the lowest band gap （1.53eV）, which matches the solar flux well, but the low degree of crystallinity and absence of preferential alignment of the Ï€-Ï€ stacking result in a relative low PCE of1.52%. However,2-D structure endows PTBDTCBT with favorable molecular packing to achieve the PCE of1.66%under illumination （AM1.5G,100mW·cm-2） without considerable optimization, although its band gap is larger than PDTSCBT. These results indicate that a good balance between energy levels and molecular microstructure arrangement is crucial to the performance improvement of photovoltaic with CBT-based polymers.3. Random polymerization, a new design strategy for developing high efficiency organic photovoltaic （OPV） materials, has received much attention recently. In our study, we report the synthesis, characterization, and photovoltaic properties of a series of6H-phenanthro[1,10,9,8-cdefg]carbazole （PC） and benzothiadiazole （BT） based donor-acceptor random copolymers. By varying the feed ratio of the monomers（PC:T:BT=3:4:1,2:3:1,1:2:1,1:3:2,1:4:3）, random copolymers PPC-T-BT₃PC, PPC-T-BT₂PC, PPC-T-BT₁PC, PPC-T-BT₁/2PC, and PPC-T-BT₁/3PC with tunable optical and electronic properties were prepared. And the corresponding alternating copolymer （PPCDTBT） was prepared by dibromo-PC monomer copolymerizing with5,5-（4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole）（DTBT）. All of the copolymers studied exhibit good thermal stability, broad absorption to meet the solar flux well, and relatively deep HOMO energy level for better open-circuit voltage, especially in PPC-T-BT₁/2PC （-5.36eV） and PPCDTBT （-5.38eV）. The PSC based on PPC-T-BT₂PC:PC61BM blend （the highest one in random copolymers） and PPCDTBT:PC61BM blend shows a close power conversion efficiency （PCE）（1.9%vs2.3%）. With the advantages of easy synthesis and convenient molecular modulation, the random polymer is promising in developing low band gap polymers for high efficiency photovoltaic.4. Recently diketopyrrolopyrrole （DPP）-based polymers are gaining significant attention in the research community due to their excellent properties such as high stability, weather fastness, large extinction coefficient, and electron-deficient nature. In our study, we used a DPP unit as the acceptor block, and the PC unit as the donor segment. By combining DPP and PC we expect to create a novel condensed aromatic push-pull conjugated backbone system with wide absorption spectrum and good charge transporting ability. By varying the feed ratio of the monomers （PC:T:DPP=1:3:2,1:2:1,2:3:1）, random copolymers PPC-T-DPP₁/2PC, PPC-T-DPP₁PC, PPC-T-DPP₂PC with tunable optical and electronic properties were prepared. which possesses balanced optical absorption from the UV to near IR region, The PSC based on PPC-T-BT₂PC:PC6,BM （w/w=1/2） blend （the highest one in random copolymers） give the best performance （PCE=2.00%）. After add1-Cholonaphthalene, the short-circuit current was enlarged, PCE can reach2.20%.