Synthesis And Optoelectronic Properties Of Benzodithiophene Polymer Donor Materials Containing Sulfur Alkyl Substituted Side Chains

In recent years,the power conversion efficiency?PCE?of polymer solar cells?PSCs?has exceeded 18%.Mang of medium/wide bandgap D-A conjugated copolymers based on electron-donating unit?D?benzodithiophene?BDT?and electron-accepting unit?A?benzodithiophene-4,8-dione?BDD?,fluorobenzotriazole?BTZ?or ester group substituted thiophene were widely developed and used to construct high-efficiency PSCs.In this dissertation,three series of new D-A polymer donor materials based on BDT unit were designed and synthesized via adjusting the conjugated side chains of BDT unit and the type of acceptor units and their substituents.The effects of the type of acceptor units,the conjugated side chains of BDT unit and the substituents of acceptor units on the thermal,photophysical,and electrochemical properties and photovoltaic performance of resultant polymer donor materials were investigated in detail.1. A BDT donor unit with 5-alkylthio-3,4-dimethoxythiophene as the conjugated side chain was synthesized and copolymerized with BDD,BTZ and ester-substituted terthiophene?TCE?units,respectively,to obtain three medium/wide bandgaps D-A conjugated polymers PTO-BDD,PTO-BTZ,and PTO-TCE.The effect of the type of acceptor units?BDD,BTZ,and TCE?on the photoelectric properties of the copolymer was systematically studied.The results showed that PTO-BTZ has better solubility and greater absorption coefficient,which is conducive to obtaining high short-circuit current(J_SC);while PTO-BDD and PTO-TCE have lower HOMO energy levels,benefiting to obtain a higher open-circuit voltage?Voc?.The synthesized copolymers were used as the donor material and ITIC was used as the acceptor material to prepare polymer solar cell devices.The PSCs based on PTO-BDD,PTO-BTZ,and PTO-TCE exhibited a PCE of 2.39%,5.32%,and 0.52%,respectively.The main reason for the best performance of PSC based on PTO-BTZ is the most effective charge transfer,higher and more balanced carrier mobility as well as better active layer morphology,which is beneficial for obtaining high J_SC and FF.2. Two BDT donor units with alkylthiothiophene and alkylthiobenzene-thiophene as side chains respectively,were synthesized,and copolymerized with ester group-containing terthiophene A unit?TCE?to obtain two medium bandgap D-A copolymers PTE-TCE and PTP-TCE.By comparing with PTO-TCE,the influence of different side chains on the BDT unit on the photoelectric properties of the polymer donor material was systematically studied.Results showed that the solubility of PTE-TCE and PTP-TCE is greatly increased,and the absorption spectra are red-shifted as compared to PTO-TCE.The PCE of PSCs with the synthesized copolymer as the donor material and ITIC as the acceptor material were10.10%and 8.39%for PTE-TCE and PTP-TCE,respectively.When the acceptor material was changed to DTCFO4Cl,PSCs based on PTE-TCE and PTP-TCE achieved better performance,with an increased PCE up to 10.60%and 9.43%,respectively.The improvement of device performance should be ascribed to the improved the blend film morphology with DTCFO4Cl as the acceptor,resulting in the incease of J_SC,FF and PCE.When Y6 is used as the acceptor material,the PCE of PTP-TCE-based devices is further increased to 11.46%,while the efficiency of PTE-TCE was dropped to 7.36%.3. Two cyanoacetate substituted thiophene A units 3TCN and TCN were synthesized and copolymerized with the alkylthiothienyl substituted BDT unit to obtain two medium bandgap D-A conjugated polymers PTE-3TCN and PTE-TCN,respectively.By comparing with PTE-TCE,the influence of A unit based thiophene with different electron-withdrawing substituents on the photoelectric properties of the copolymer was deeply investigated.The results demonstrated that both polymers had good thermal stability.Compared with PTE-3TCN,PTE-TCN showed better solubility.Due to the enhanced electron-withdrawing ability of the acceptor unit,the optical bandgaps and HOMO energy levels of PTE-3TCN and PTE-TCN were lower than that of PTE-TCE.PSCs based on the two polymers achieved poor device performance with ITIC as the acceptor,but both of them produced a higher V_OC.When the acceptor was changed to DTCFO4Cl,the PTE-TCN-based device showed the best PCE of 1.32%,corresponding to a V_OCas high as 1.09 V.