Synthesis And Photovoltaic Properties Of The Polymers Based On S-tetrazine

There are two design strategies to improve polymer photovoltaic performance.On the one hand,adjusting the conjugated main chain can achieve the purpose of optimizing the photovoltaic performance of polymers.On the other hand,optimizing the alkyl side chains of conjugated polymer can also improve the photovoltaic performance of polymers because it tune the intermolecular interaction between polymer chain,improve the miscibility of polymer and acceptor,controll the domain size of blend film and the processability of polymer.This dissertation mainly focuses on the development of s-tetrazine-based polymer.s-Tetrazine is used as a electron-withdrawing moiety?A?,which is copolymerized with different electron-donating moieties?D?to obtain a series of D-A conjugated polymeric photovoltaic materials,and the effects of the backbone and side chains of the conjugated polymers on the photophysical,electrochemical and photovoltaic properties of the polymers have been studied in detail.Three polymer photovoltaic materials of PTTTz-HD,PTTTz-OD and PTTTz-DT have been synthesized by using s-tetrazine as A unit and a difluoro-substituted tetrathiophene derivative as D unit with 2-hexyldecylalkyl,2-octyldodecyl and2-decyltetradecyl as side chains respectively.The results show that PTTTz-HD with the shortest alkyl chain exhibits the weakest intermolecular?-?stacking.PTTTz-OD with an appropriate length of alkyl side chain?2-octyl dodecyl?possesses the strongest intermolecular interaction and the most regular molecular stacking,which is in favour of better carrier transport.Therefore,the PSC based on PTTTz-OD/PC71BM exhibits the best power conversion efficiency?PCE=5.10%?.The Voc of the three fullerene PSCs are higher than 1.0 V since the HOMO?highest occupied molecular orbital?levels of three polymers are all lower than-5.56 eV.Among them,the PSC based on PTTTz-DT exhibits a Voc?open circuit voltage?of 1.04 V,which is the highest Voc in the currently reported PSCs with s-tetrazine-based polymers as donor materials.However,the extremely low HOMO level of the three polymers does not match the HOMO level of the high performance non-fullerene receptor,which limits its application in non-fullerene PSCs.The polymer PHTTz has been designed and synthesized by using s-tetrazine as the A unit and a difluoro-substituted hexanthiophene derivative with two alkyl side chains?hexyl and 2-octyl dodecyl?as the D unit.Moreover,the3,3'-difluorodithiophene unit in PHTTz was substituted by benzo[1,2-difluorodithiophene]dithiophene?BDT?derivativewith2-ethylhexyl-3-fluorothiophene as side chain to synthesize the polymer PBDT-dTTz.The results show that PHTTz with hexylthiophene unit possesses higher HOMO level than PTTTz-OD,and its intermolecular interaction is weakened.The blend film with PC71BM possesses more suitable microphase separation,more effective exciton dissociation,higher hole mobility.Therefore,the PSC based on PHTTz/PC71BM exhibits higher J_sc(short circuit current density,11.54 mA cm^-2),FF?68.8%?and PCE?6.69%?.Moreover,the ternary PSC based on PHTTz/PC71BM/ITIC exhibits more efficient exciton dissociation and complementary absorption spectra,which leads to a higher PCE value?7.88%?than the binary PSCs,which is also the highest PCE value in s-tetrazine-based polymers.However,compared with PHTTz,the absorption spectrum of PBDT-dTTz is blue-shifted and the intermolecular interaction is very weak.The HOMO level of PBDT-dTTz is low-lying?-5.58 eV?,which can not well match the HOMO level of high-performance non-fullerene acceptor.Therefore,its PSC based on PBDT-dTTz/PC₇₁BM shows poor photovoltaic performance with a low J_sc value of 6.61 mA cm^-2 and a PCE value of 4.04%.