Synthesis And Properties Of Photovoltaic Materials Based On Thiophene-Benzene-Thiophene And Isoindigo

In the last century,the development of solar cells has undergoned enormous variation,and a remarkable progress has been achieved.Tremendous efforts have been devoted to exploiting organic photovoltaic cells(OPVs)because of their potential for low-cost,flexible,and light-weight devices.In this dissertation,a series of organic small molecules and conjugated polymers have been designed and synthesized by incorporating thiophene-benzene-thiophene(TBT)as electron-donating units and isoindigo(ID)as electron-withdrawing units.The relationship between the structure of these materials and their aggregation,absorption spectra,energy gaps,microphase seperation with fullerene derivatives,carrier mobility and photovoltaic properties has been fully investigated.1 ? An organic intermediate I4,incorporating isoindigo as the core and thiophene-benzene-thiophene are two arms,has been synthesized by Stille coupling reaction.Subsequently,two photovoltaic materials S6 and S7 with A1-D-A2-D-A1 structure have been synthesized by adopting Knoevenagel condensation and introducing 1,3-indandione and 3-ethylrhodanine as the terminal groups respectively.The introduction of 1,3-indandione and 3-ethylrhodanine as the terminal groups have lowed HOMO energy levels(highest occupied molecular orbital),so the OPV devices based S6 and S7 showed a high open circuit voltage(Voc)over 1.0 V.Compared with S7 with 3-ethylrhodanine as the terminal groups,S6 with 1,3-indandione as the terminal groups shows stronger interchain interaction,broader absorption spectrum,higher absorption coefficient,lower band gap.The S6-based OPV device showed higher short-circuit current density(Jsc)of 8.49 mA cm-2 than that of the S7-based OPV device(2.37 mA cm-2).As a result,the S6-based PSC exhibited a power conversion efficiency(PCE)of 4.70%,which is almost seven times of that of the S7-based OPV device(0.66%).2 ? To investigate the length of the alkyl side-chains on the photovoltaic performance,three isoindigo-based polymers with different-length side chains on the thiophene-benzene-thiophene(TBT)(electron-donating)moieties and isoindigo(electron-withdrawing)moieties have been designed and synthesized,in which the total side chains in a constitutional repeating unit are fixed to C48H100.With extending the alkyl chains on TBT units and shortening the alkyl chains on ID units,the polymers were named PTBTEH-IDHD,PTBTHD-IDEH and PTBTOD-IDB.Theresults indicate that the polymer with longer branched side chains on TBT units and shorter linear side chains on isoindigo units shows stronger interchain interaction,higher lightharvesting capacity in 1,2-dichlorobenzene solution and thin film,lower band gaps,more appropriate microphase separation with PC71 BM,higher hole mobility and better photovoltaic performance.As a result,the bulk heterojunction(BHJ)device based on PTBTOD-IDB with 2-octyldodecyl on TBT moieties and n-butyl on isoindigo moieties exhibited a power conversion efficiency(PCE)of5.29%,which is almost eight times as high as that of PTBTEH-IDHD with2-ethylhexyl side chain on TBT moieties and 2-hexyldecyl side chain on isoindigo moieties.This study not only fully investigates the length effect of the alkyl side chains on the photovoltaic performance,but also thoroughly avoids the negative deviation that the different carbon atom number of the side chain in a constitutional repeating unit leads to solubility and morphology variance.