Synthesis And Characterization Of Wide Bandgap Polymer Donors Based On Carboxylate Substituted Thiophene

In recent years,acceptor-donor-acceptor(A-D-A)-type small molecule acceptor materials with medium or narrow bandgap have developed rapidly.To pursue maximized photon harvesting,wide bandgap(WBG,optical bandgap(E_g^OPT>1.8eV)donor polymers have complementary absorption with this type of acceptor materials receive more and more attention.Along with the improvement of organic solar cells(OSCs),easy accessibility of the photoactive materials is of importance to meet the commercial applications of OSCs.Carboxylate substituted thiophene units can be easily synthesized from cheap raw materials,and they can efficiently lower the highest occupied molecular orbital(HOMO)levels of polymers,which is beneficial to achieve higher open-circuit voltage(V_oc)of OSCs.In this thesis,we synthesized three series of wide bandgap polymers with carboxylate substituted thiophene units as building blocks.The relationships between the structure parameters such as conjugated backbone,solubilizing side chains and substituents and the physicochemical properties,film aggregation structures and device performance were studied in detail.The main results are as follows:1.Carboxylate side chains and vinylene linkers were introduced into polythiophene derivatives,and two polymers named PBT and PTT were synthesized.Compared to poly(3-hexylthiophene)(P3HT),both polymers displayed deeper HOMO energy levels,lowerE_g^OPTand shorter ?-? stacking distances.PBT exhibited relatively superior charge transport property compared to PTT,owing to its more ordered intermolecular packing in solid state.PSCs based on PBT:PC₇₁BM and PTT:PC₇₁BM both showed V_oc above 0.8 V,which was higher than that of P3HT-based PSCs(0.5-0.6 V).Owing to the better hole transport property and nano-fibrillar film morphology,PBT-based PSCs exhibited a superior photovoltaic performance with power conversion efficiency(PCE)of 6.25%,in comparison to that of PTT-based PSCs.2.A series of conjugated polymers based on[2,2?-bithiophene]-4,4?-dicarboxylate(DCBT)and benzo[1,2-b:4,5-b']dithiophene(BDT)units,i.e.,PBDTEH-M,PBDTEH-E,PBDTEH-H,PBDTO-E,PBDTO-H and PBDT-OD were synthesized.The effect of the structure of alkyl side chains on the solubilities,film aggregation structures and photovoltaic performance of the polymers,were studied comprehensively.The devices based on PBDTO-H with linear side chains exhibited the best photovoltaic performance(PCE=8.21%),ascribed to the well-ordered molecular packing and superior morphology with good interpenetrating network and fiber-like nano-structures.3.Five wide-bandgap polythiophene derivatives,i.e.,PDCBT,PDCBT-F,PDCBT-Cl,PDCBT-2F and PDCBT-2Cl were synthesized.The effect of the chlorination on photophysical and charge transport properties,molecular packing,and film microstructures,were studied comprehensively in comparison to the fluorination.The introduction of fluorine or chlorine atoms reduced the HOMO energy levels of polymers,leading to higher V_oc in the OSCs.Owing to the proper phase-separated morphology with favorable molecular packing,the device based on PDCBT-Cl:ITIC-Th1 exhibited efficient exciton dissociation and charge collection,thereby displayed the best PCE up to 12.38%,which is the best photovoltaic performance for polythiophene derivatives reported so far.