Preparation And Photovoltaic Properties Of Semiconductive Materials Based On 12H-Dithieno[2',3':4,5]thieno[3,2-b:2',3'-h]Fluorene

Nowadays,ladder-type and planar fused-ring copolymers are two main categories of donor materials for polymer solar cells?PSCs?.The backbone of ladder-type copolymer contains multiple sp³-hybridization carbon atoms,the tetrahedral configuratio is conducive to solution processing,while adverse to intermolecular stacking and photovoltaic performance improvement.In contrast,the planarization fused-ring copolymer have rigid conjugated structure,which favorable for molecular assembly and photovoltaic performance enhancement,while the complicated synthesis,high cost and poor solution processability hider the practical applications.Thus,in this dissertation,considering the characteristics of those two kinds of copolymers,we propose a kind of novel molecular backbone design which comprehending both of their merits,i.e.12H-dithieno[2',3':4,5]thieno[3,2-b:2',3'-h]fluorene?DTTF?.In DTTF backbone,the fluorene unit in the center offers a single sp³-hybridized carbon bridge with two attached bulky side chains,and the thienothiophenes fused at both ends of the fluorene unit would extend the conjugation to allow the intermolecular interaction among polymers for the desirable molecular assembly.Conjugated copolymer donors and small molecule acceptors were synthesized based on DTTF to investigate photovoltaic properties.?1?A novel heptacyclic electron donating building block of 12,12-bis?2-hexyldecyl?-12H-dithieno[2',3':4,5]thieno[3,2-b:2',3'-h]fluorene?DTTF?with well-defined structure was designed and synthesized.Copolymerization of DTTF with a common acceptor 4,7-bis?5-bromo-4-hexylthiophen-2-yl?-benzo[c][1,2,5]thiadiazole?DTBT-C6?affords a novel soluble ladder-type D-A copolymer PDTTF-DTBT-C6.PDTTF-DTBT-C6 exhibits strong intermolecular stacking,ordered molecular packing and orientation.The pure polymer and BHJ blend film can form more tight molecular stacking and reasonable phase separation size on the ZnO layer compared with PEDOT:PSS layer.Therefore,the maximum power conversion efficiency?PCE?of conventional device is only 5.93%,while under the same conditions,the maximum PCE of inverted device dramatically improved to 8.17%.?2?PDTTF-DTfBT-C6?mono-fluorine?and PDTTF-DTffBT-C6?dual-fluorine?were obtained by introducing different numbers of fluorine atom into the DTBT-C6unit based on DTTF.With more fluorine atoms introduced,the intermolecular interaction of each polymer became more intensely,while fluorination had little impact on the polymers'HOMO energy level,which are-5.39 and-5.42 eV for PDTTF-DTfBT-C6 and PDTTF-DTffBT-C6,respectively.The maximum PCE of the devices fabricated by PDTTF-DTfBT-C6 and PDTTF-DTffBT-C6 were 8.26%and 8.98%,respectively,both of which were slightly higher than that of PDTTF-DTBT-C6?non-fluorine?,with PCE as 8.17%.?3?PDTTF-DTnfBT-C6 were selected as host donor,PTB7-Th as guest donor,and they were blended with PC₇₁BM to prepare D1:D2:A ternary PSCs,respectively.The performance of ternary PSCs based on PDTTF-DTBT-C6 was improved,arising from favorable compatibility of two donors.The maximum PCE reaches 9.16%when30%PTB7-Th mixed with PDTTF-DTBT-C6.However,the incompatibility of PDTTF-DTfBT-C6 and PDTTF-DTffBT-C6 mixed with PTB7-Th resulted in unfavorable morphology of the ternary BHJ films,led to decreased performance for all ternary PSCs prepared from PDTTF-DTfBT-C6 and PDTTF-DTffBT-C6,respectively.?4?A series of D-A polymers PDTTF-DTnfBT-EH?n is 0,1 and 2,f represents fluorine atom and EH is 2-ethylhexyl?were obtained by changing the alkyl chain and introducing different numbers of fluorine atoms on DTBT unit based on DTTF.Those three polymers feature similar light-absorbing properties,but the branched alkyl side chain on DTBT reduces the intermolecular interaction of PDTTF-DTnfBT-EH compared with PDTTF-DTnfBT-C6.Unexpectedly,fluorination result in completely opposite orientation of pure polymer and BHJ films were observed.Meanwhile,the HOMO energy level were significantly affected by fluorination,the HOMO energy levels for PDTTF-DTBT-EH,PDTTF-DTfBT-EH and PDTTF-DTffBT-EH were-5.40,-5.45 and-5.55 eV,respectively.The maximum PCE of the devices based on PDTTF-DTBT-EH and PDTTF-DTfBT-EH were 5.06%and 5.73%,respectively.While the device based on PDTTF-DTffBT-EH obtained much higher PCE of 9.12%,attributing to more orderly molecular stacking and suitable phase separation of PDTTF-DTffBT-EH BHJ film.?5?A non-fullerene small molecule acceptor DTTF-IC was obtained by introducing 1,1-dicyanomethylene-3-indanone electron deficient dye group at both ends of DTTF.DTTF-IC gives high optical absorption coefficient about 10⁵ M cm^-1with optical band gap of 1.76 eV.The LUMO and HOMO energy levels are-3.90 and-5.86 eV,respectively.The poor complementarity absorption of DTTF-IC and PTB7-Th,results in lower J_scc for the devices.Moreover,unfavorable BHJ film morphology leads to serious charge recombination loss and lower fill factor.Therefore,the maximum PCE of devices fabricated by DTTF-IC blend with PTB7-Th was only6.10%.