D-A Structured Organic Small Molecules And Polymers: Design,Synthesis And Photovoltaic Properties

Along with the development of economy and the growth of population,the energy demand of our planet is increasing.However the traditional fossil fuels are limited and non-renewable,and the unrestrained use of them also caused serious environmental problems.Solar energy is considered to be the next generation of ideal energy due to their advantages of inexhaustible,widely distributed,easy to collect and pollution-free.Now silicon based solar cells have been commercialized,but they suffer expensive production cost,large energy payback time and their manufacturing incur environmental pollution.On the other hand,solution-processed organic solar cells(OSCs)have attracted much attention because of their unique advantages of low cost,short energy payback time,light weight,flexibility,and the potential of fabrication through high-throughput and large-area roll-to-roll processing.Bulk heterojunction structure is commonly employed in active layer of organic solar cells,in which electron-rich donor and electron-deficient acceptor are blended to form bi-continuous nanoscale interpenetrating network.The chemical structures and properties of donor materials play a vital role in the performance of OSCs because the donor material takes part in the whole processes of exciton generation to separation.In order to meet the requirements of commercialization,more solution-processed donor materials needed to be designed and synthesized for high efficient organic photovoltaic applications.One feasible approach to obtain high efficient organic donor materials is to design donor-acceptor(D-A)architecture with alternating electron donor and electron acceptor moiety among the organic molecular backbone.The D-A type organic molecules can not only narrow band gap by utilizing intramolecular charge transfer transition between electron donor unit and acceptor unit inside molecules,but also adjust the HOMO and LUMO energy levels through selecting different donor or acceptor moiety.Based on the design concept of D-A type molecules,we introduced acceptor unit with strong electron-withdrawing ability,tetrazine or fluorine substituted benzothiadiazole,into the D-A backbone to lower the HOMO energy levels of the molecules.Four small molecules and two polymers donor materials were synthesized by changing the donor unit,side chains,?-bridge and molecular weight(MW).Their photovoltaic properties were investigated and relatively high power conversion efficiencies(PCEs)were achieved when they used for photovoltaic applications.Meanwhile,we investigated the effect of donor unit,acceptor unit,conjugated length and side chains on absorption,energy levels,mobility and active layer morphology of D-A type organic materials,revealing the correlation between molecular structure and photovoltaic performance.In chapter 2,four D2-A-?-D1-?-A-D2 type small molecules were synthesized using Benzo[1,2-b:4,5-b?]dithiophene(BDT)as central donor unit,thiophene or bithiophene as ?-bridge,tetrazine as electron accepting moiety,bithiophene or terthiophene as end donor unit.The optical and electrochemical properties of synthesized molecules indicate that these molecules can absorb sunlight in a broad spectral range from 300 to 700 nm and show suitable energy levels for efficient exciton dissociation.The introduction of tetrazine unit in these molecules effectively reduces the HOMO energy levels of these molecules,thus increases air stability of these molecules as well as the open circuit voltage(VOC)in photovoltaic devices.It was found that extending the length of ?-bridge and end group result in up-shift of HOMO,which reduced the VOCs.However,the JSCs were remarkably enhanced because of the improvement of absorption property,which offseted the low VOCs.On the other hand,when alkoxy side chains were replaced by alkythienyl side chains,the HOMO energy level was lowered and the band gap became wider but the absorption coefficients increased,and the nanoscale morphology of active layer was greatly improved,therefore the VOCs and FFs were remarkably improved while the JSCs kept similar.Among these four molecules,TBDT(TTz2T)2,which incorporate alkythienyl side chain and long conjugated main chain,provide the best PCE of 6.56% with VOC,JSC and FF of 0.94 V,10.69 mA cm-2 and 65.3%,respectively.BDT(TTzT)2,BDT(TTz2T)2,TBDT(TTzT)2 based devices achieved PCEs of 5.01%,5.29% and 6.10%.In chapter 3,a new low band gap polymer Poly{4,4?-bis(2-ethylhexyl)dithieno[3,2-b:2?,3?-d]silole-alt-5,6-difluoro-4,7-bis(4-hexylthio phen-2-yl)-2,1,3-benzothiadiazole}(PDTS-DTffBT)were synthesized through Stille polymerization using dithieno[3,2-b:2?,3?-d]silole(DTS)as donor unit and fluorinated 4,7-dithien-2-yl-2,1,3-benzothiadiazole(DTffBT)as acceptor unit.Three polymer fractions with number-average molecular weight(Mn)of 9,16,36 kDa were synthesized by controlling the polymerization conditions and the effect of MW of this polymer on the photovoltaic properties was investigated.Combined with UV-vis absorption and electrochemical cyclic voltammetry measurements,it was found that PDTS-DTffBT showed strong and broad absorption from 300 to 800 nm.The introduction of fluorine atom on benzothiadiazole unit effectively reduces the HOMO and LUMO energy levels.As the MW increased from 9 kDa to 36 kDa,the HOMO energy level was up-shifted and the absorption was further red-shifted.PDTS-DTffBT with high MW also provided increased charge mobility,smoother film surface and reduced domain size in morphology of active layer.Both the JSC and PCE went up significantly with increasing MW owing to these MW related properties.The best PCE of 6.40% with JSC of 15 mA cm-2 and FF of 0.63 was achieved by the devices based on the PDTS-DTffBT with Mn of 36 kDa.In chapter 4,aiming at the problem that the VOC of PDTS-DTffBT based PSCs is still low,we employed weaker donor unit BDT instead of DTS unit to polymerize with DTffBT unit,bulk branched 2-octyldodecyloxyl side chain was also employed on the 4,8 position of BDT to enhance solubility.The Mn of the resulting copolymer Poly{4,8-bis-octyldodecyloxybenzo[1,2-b:4,5-b?]dithiophene-alt-5,6-difluoro-4,7-bis(4-hex ylthiophen-2-yl)-2,1,3-benzothiadiazole}(PBDT-DTffBT)was measured to be 46 kDa by gel permeation chromatography.PBDT-DTffBT exhibit good film-casting property and solubility in common organic solvents.UV-vis absorption spectra showed the polymer can absorb sunlight in the range of 350 ~750 nm.The HOMO energy level of polymer was efficently lowered to-5.32 eV by using weaker donor unit BDT instead of DTS unit.As a result,the VOC of PSCs was raised to 0.86 V.Finally,the morphology and phase separation of active layer were optimized by changing the Polymer/PCBM weight ratio,processing solvent,thermal annealing conditions,and DIO additives,which make the JSC and FF of PBDT-DTffBT based PSCs raised to 15.2 mA cm-2 and 0.69,PCE of 9.03% was achieved.