Synthesis And Properties Of Polymer Photovoltaic Materials Based On Pyridothiadiazole And Triazole

In recent years,exploration of new polymers for high performance polymer solar cells?PSCs?is one key research in this field.The conjugated polymers based on pyrido[3,4-c]thiadiazole?PT?and triazole derivatives have been designed and synthesized in this work,and the photovoltaic performance of the polymers has been optimized by introducing halogen atoms to reduce HOMO energy levels,tuning the length of the alkyl side chain to controll solubility,molecular aggregation and compatibility with electron acceptor material.The main research contents are shown as follows:1.Two novel ternary conjugated polymers PDTTT-DFBT and PDTTT-FBT were designed and synthesized by introducing PT as a strong electron-withdrawing?A?unit,alkylthiophene as an electron-donating?D?unit,difluorobenzothiadiazole or fluorobenzothiadiazole as the second A unit.The polymer show strong absorption in the waveband between 300 nm and 800 nm in solution and film,which indicates that these polymers are low bandgap polymers with broad absorption spectra.The polymer PDTTT-DFBT exhibits a redshift absorption spectrum and a smaller?-?stacking distance than PDTTT-FBT.The bulk heterojunction PSCs were prepared by using the polymers as electron donors and PC₇₁BM?fullerene derivative?as electron acceptor without any additives and post-treatment.The photoelectric conversion efficiency?PCE?values of the PSCs based on PDTPT-DFBT and PDTPT-FBT are 2.56%and3.14%,respectively.All the PSCs showed high open circuit voltage(V_oc)over 0.90 V because their HOMO?highest occupied molecular orbital?energy levels of both polymers are lower than-5.40 eV,which is relatively high V_occ values in low bandgap polymer photovoltaic materials.2.Four polymers named as PTZ-F-O,PTZ-OD,PTZ-F-OD and PTZ-Cl-OD have been designed and synthesized by using 1H-1,2,4-triazole as A units and benzo[1,2-b:4,5-b']dithiophene?BDT?derivatives as D units.On the one hand,two kinds of halogen atoms?F and Cl?with strong electronegativity were introduced to BDT derivatives in order to depress LUMO?lowest empty molecular orbital?and HOMO energy levels,broaden the absorption spectra,and reduce bandgap of the polymers.On the other hand,alkyl chains with different lengths were introduced on1H-1,2,4-triazole units to improve solubility of the polymer.The V_occ values of the PSCs based on these polymers are higher than 1.0 V for their low-lying HOMO energy levels.These polymers show low photovoltaic performance because their wide bandgaps and narrow absorption spectra lead to low short-circuit density(J_sc),and the PSCs based on PTZ-F-O,PBDTZ-F-OD,PTZ-F-OD and PTZ-Cl-OD only showed the PCE values of 2.94%,1.62%,1.74%and 2.01%respectively.3.A new conjugated polymer PBDT-F-TIZ was designed and synthesized by using 4H-1,2,4-triazole derivative as A units and BDT derivative as D units.It is the first time 1,3,4-triazole derivative is used for photovoltaic polymers.A polymer PBDT-F-TZ based on 1H-1,2,4-triazole derivative as A units was also synthesized for comparison,and the effects of molecular regularity on photovoltaic performance of the polymers were studied.The results indicate that the polymer PBDT-F-TIZ with more regular molecular structure possesses better thermostability,more redshifted absorption,more orderly molecular aggregation,which result in better photovoltaic performance.Therefore,the PSC based on PBDT-F-TIZ shows higher PCE?2.42%?than that of the PSC based on PBDT-F-TZ?1.68%?.