| Polymer solar cells (PSC) have attracted an increasing amount of attention in theresearch community due to the potential advantages of PSC over inorganic-basedsolar cells, including low cost, light weight, and fast/cheap roll-to-roll production.PSCs are predicted to yield power conversion efficiency (PCE) up to a commerciallevel, if a suitable low band gap donor material is discovered. To date, tremendousefforts have been focused on the design of building blocks for polymer backbones,very little work has been done toward a coherent molecular design that cansubstantially govern intermolecular self assembly by manipulating structural featuressuch as the length, bulkiness, rigidity, and chirality of the solubilizing groups on thebackbone. We mainly work is the incorporation of functional groups in the polymermolecules, in order to study the effects of polymers with functional groups for activelayer morphology and PCE of solar cells.Firstly, a new liquid crystalline (LC) acceptor monomer2,5-bis[4-(4′-cyano-biphenyloxy)dodecyl]-3,6-dithiophen-2-yl-pyrrolo[3,4-c]pyrrole-1,4-dione (TDPPcbp)was synthesized by incorporating cyanobiphenyl mesogens into diketopyrrolopyrrole(DPP). The monomer was copolymerized with bis(2-ethylhexyloxy)-benzo[1,2-b:4,5-b′] dithiophene (BDT) and N-9′-heptadecanylcar-bazole (CB) donors to obtaindonor-acceptor alternating copolymers poly[4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene-alt-3,6-bis(thiophen-5-yl)-2,5-bis[4-(4′-cyanobipheny-loxy)dodecyl]-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione](PBDTDPPcbp). Andpoly[N-9′-heptadecanyl-2,7-carbazole-alt-3,6-bis(thiophen-5-yl)-2,5-bis[4-(4′-cyano-biphenyloxy)dodecyl]-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione](PCBTDPPcpb)with reduced band gap, respectively. The LC properties of the copolymers, the effectsof main chain variation on molecular packing, optical properties, and energy levelswere analyzed. Incorporating the mesogen cyanobiphenyl units not only help polymerdonors to pack well through mesogen self-organization but also push the fullereneacceptor to form optimized phase separation. The bulk heterojunction photovoltaicdevices show enhanced performance of1.3%for PBDTDPPcbp and1.2%for PCBTDPPcbp after thermal annealing. The results indicate that mesogen-controlledselforganization is an efficient approach to develop well-defined morphology and toimprove the device performance.Then, we have used Stille coupling polymerization to synthesize a series of newdonor/acceptor (D/A) conjugated random copolymers PTBDTDPP, that compriseelectron-rich alkylthienyl substituted benzodithiophene (TBDT) units in conjugationwith electron-deficient2,5-dio[5-(5-cyano-5,5-dimethyl-pentyl)]-3,6–dithiophen-2–yl-pyrrolo[3,4-c]pyrrole-1,4-dione containing cyano functional groups (DPPCN),3,6-Bis-(5-bromo-thiophen-2-yl)-2,5-bis[8-(1,1,3,3,5,5,5–heptamethyl-trisiloxane-3-yl)octly]-3,6-dithiophen-2-yl-pyrrolo[3,4-c]pyrrole-1,4-dione with siloxanefunctional groups (DPPSi) moieties that have complementary light absorption. Byvarying the feed ratio of the monomers (TBDT: DPPSi: DPPCN=1:0.3:0.7,1:0.5:0.5,1:0.7:0.3) were achieving three random copolymers PTBDTDPP1,PTBDTDPP2and PTBDTDPP3. These copolymers exhibited (i) broad visible lightabsorption from500to1000nm and (ii) a low optical band gap that is smaller than1.6eV and a low-lying highest occupied molecular orbital that is deeper than-5.36eV.The surface energy proved that a higher ratio of DPPCN and a lower ratio of DPPSi,which resulted in better film morphology. It is worthy to note that an extremely shortπ-π stacking distance for PTBDTDPP in the solid states, which is an advantage ofcharge carrier transport. As a result, they will be as promising photovolatic materialsfor solar cells. |
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