Design,Synthesis And Characterization Of Two-dimension-conjugated Polymer Donor Photovoltaic Materials

Polymer solar cells（PSCs）have attracted considerable attention due to their advantages of light weight,simple preparation,solution processing,and flexibility to fabricate large-area devices.At present,it is still the focus to further improve the energy conversion efficiency（PCE）of PSCs.As the key component of the active layer,the development of new and efficient conjugated polymer donor materials is one of feasible strategies for improving the photovoltaic performance of PSCs.In this dissertation,focusing on the molecular design of materials,a series of two-dimension-conjugated polymer donor materials based on benzodithiophene（BDT）units were synthesized by the side chain engineering.The relationship between molecular structure and absorption spectra,molecular energy levels,charge transfer and photovoltaic properties of the polymer donor materials is systematically studied.The main achievements and results are as follows:1.Two kinds of two-dimension-conjugated polymers PBTF-OP and PBFZ-OP were synthesized based on m-alkoxyphenyl-substituted benzodithiophene（BDT-m-OP）donor unit,ester and fluorine-substituted thienothiophene（TT）acceptor unit and benzotriazole（FBTZ）acceptor unit.The effect of the conjugated side chain of the benzodithiophene donor unit changed from alkyl thiophene to m-alkoxy benzene on polymer properties was studied.Compared with polymer PTB7-Th(the optical bandgap was 1.57 eV,the HOMO energy level was-5.30 eV,the hole mobility was 1.11×10^-3 cm² V^-1 s^-1),the optical bandgap of PBTF-OP increased to 1.62 eV,the HOMO energy level decreased to-5.45 eV,and the hole mobility increased to 1.25×10^-3 cm² V^-1 s^-1.The PCE of the device based on PBTF-OP:PC₇₁BM reached 9.0%with V_oc=0.86 V,J_sc=16.4 mA cm^-2,FF=62.2%;Under the same conditions,the PCE of the device based on PTB7-Th:PC₇₁BM was 8.3%with V_oc=0.78 V,J_sc=16.9 mA cm^-2,FF=61.4%.Compared with polymer J52（the optical bandgap was 1.96 eV,the HOMO energy level was-5.21 eV）,the optical bandgap of PBFZ-OP increased to 1.99 eV,and the HOMO energy level decreased to-5.33 eV.When non-fullerene PSCs were prepared using PBFZ-OP as donor and ITIC as acceptor,the PCE of the device reached 10.5%with V_oc=0.91 V,J_sc=18.7 mA cm^-2,FF=61.8%;Compared to the J52:ITIC device reported in the literature(PCE=5.2%,V_oc=0.74 V,J_sc=11.7 mA cm^-2,FF=59.4%),the parameters of V_oc,J_sc,FF and PCE have all been greatly improved.2.Four kinds of wide bandgap conjugated polymers（PBD1,PBD2,PBD3 and PBD4）were designed and synthesized based on m-alkoxyphenyl substituted benzodithiophene（BDT-m-OP）donor units and benzodithiophenedione（BDD）acceptor units,and investigated systematically the effect of different alkyl side chains on the absorption spectra,molecular energy levels,charge transport and photovoltaic performance of the polymers.From PBD1 to PBD4,with the total number of carbon atoms in the alkyl side chain increasing,the absorption spectra showed a slight blue-shift,but the HOMO level decreased in turn.Among them,PBD1 with the shortest alkyl side chain exhibited strong intermolecular aggregation and high hole mobility.With toluene as the processing solvent,the PCE of the devices based on PBD1:IDIC reached 12.3%with V_oc=0.87 V,J_sc=19.6 mA cm^-2,FF=72.3%;and the devices based on PBD2/PBD3/PBD4:IDIC showed the PCE of11.0%,10.3%and 8.9%,respectively.3.Two kinds of wide bandgap conjugated polymers（POBP and PFOBP）were designed and synthesized based on p-alkoxyphenyl-substituted benzodithiophene（BDTP）donor units and benzodithiophenedione（BDD）acceptor units.By introducing two fluorine atoms into the phenyl side chain of the BDT unit,the effects of side chain fluorination on the absorption spectra,molecular energy levels,charge transport,molecular aggregation and photovoltaic performance of the polymer were investigated.Compared with the non-fluorinated polymer POBP,the fluorinated polymer PFOPP possessed a wider optical bandgap,a lower HOMO energy level,a higher molar extinction coefficient and hole mobility.The devices based on PFOPB:IT-4F showed a higher PCE of 11.7%with V_oc=0.89 V,J_sc=20.4 mA cm^-2,FF=64.5%.However,the devices based on POPB:IT-4F showed a lower PCE of 6.2%with V_oc=0.72 V,J_sc=16.8 mA cm^-2 and FF=51.2%.The results show that the fluorination of the phenyl side chain can effectively improve the performance of wide bandgap polymer donor photovoltaic materials.4.Two kinds of wide bandgap conjugated polymers（PBZ1 and PBZ-m-CF₃）were designed and synthesized based on p-alkoxyphenyl substituted benzodithiophene（BDTP）donor units and benzotriazole（FBTZ）acceptor units.By introducing trifluoromethyl substituent into the phenyl side chain of the BDT unit,the effects of the trifluoromethyl group on the absorption spectra,molecular energy levels,charge transport,and photovoltaic performance of the conjugated polymer were systematically investigated.Compared with the polymer PBZ1 without trifluoromethyl substituent,the polymer PBZ-m-CF3 containing trifluoromethyl substituent possessed a wider optical bandgap,a lower HOMO level,a higher extinction coefficient and higher hole mobility.Using toluene as the processing solvent,non-fullerene PSCs were prepared with PBZ1 and PBZ-m-CF3as electron donors and ITIC as electron acceptor,respectively.The devices based on PBZ-m-CF3:ITIC showed the PCE of 10.4%with V_oc=0.94 V,J_sc=18.4 mA cm^-2,FF=60.2%;and the PCE based on PBZ1:ITIC device is only 5.8%with V_oc=0.74 V,J_sc=15.7 mA cm^-2,FF=49.8%.The results show that the introduction of trifluoromethyl group into the phenyl side chain can effectively enhance the performance of wide bandgap polymerdonorphotovoltaicmaterials,andtheBDTunitwith m-trifluoromethyl-p-alkoxyphenyl side chain is a potential donor building block in the molecular design of photovoltaic materials for PSCs.5.Based on alkylbenzene-substituted benzodithiophene donor units and benzodithiophenedione（BDD）acceptor unit,two kinds of wide bandgap conjugated polymers（PBPD-p and PBPD-m）were designed and synthesized.The effect of different alkyl substitution position on the absorption spectra,molecular energy level,charge transport,and photovoltaic performance of the conjugated polymer was studied systematically.Compared to PBPD-p with p-alkylbenzene conjugated side chain(the optical bandgap was 1.82 eV,the HOMO energy level was-5.39 eV,the hole mobility was1.21×10^-3 cm² V^-1 s^-1),PBPD-m with m-alkylbenzene conjugated side chain possessed a wider optical bandgap（1.87 eV）,a lower HOMO level（-5.50 eV）,and higher hole mobility(1.37×10^-3 cm² V^-1 s^-1).The devices based on PBPD-p:IT-4F showed the PCE of6.9%with V_oc=0.81 V,J_sc=15.9 mA cm^-2,FF=53.9%;and the PCE of PBPD-m:IT-4F devices reached 11.9%with V_oc=0.88 V,J_sc=19.7 mA cm^-2,FF=68.7%.6.Two wide bandgap conjugated polymers（PM6-S and PM9）were synthesized based on thiophene with fluorine atom and alkylthio group substituted benzodithiophene units and benzodithiophenedione（BDD）unit.The length of alkyl side chain for PM9 was increased compared to the length of PM6-S,resulting in the improved solubility.The PM9-based devices were processed with non-halogenated solvents and achieved high PCE.Both PM6-S and PM9 have lower HOMO levels,higher hole mobility,and good crystallinity.Using chlorobenzene as the processing solvent,the devices based on PM6-S:IDIC showed the PCE of 11.1%with V_oc=0.92 V,J_sc=17.9 mA cm^-2,FF=67.4%.Using toluene as the processing solvent,the PCE of the devices based on PM9:IT-4F reached 13.4%with V_oc=0.92 V,J_sc=20.6 mA cm^-2,FF=70.8%,it is worth noting that for this blend system,the photovoltaic performance of the devices changes little when the active layer thickness increases from 70 nm to 250 nm and the device area increases from0.04 cm² to 1.00 cm².When the thickness of the active layer varies from 70 nm to 250 nm,the PCE of devices exceeds 11.5%.When the device area increases from 0.04 cm² to 1.00cm²,the PCE exceeds 11.9%.7.The conjugated polymers PM3 was synthesized based on methoxythiophene substituted benzodithiophene and benzotriazole units.PM3 possessed an optical bandgap of 1.96 eV and a HOMO energy level of-5.31 eV.Using toluene as the processing solvent,the PSCs based on PM3:ITIC showed the PCE of 10.7%with V_oc=0.89 V,J_sc=18.9 mA cm^-2,FF=63.5%.