Study On The Design And Synthesis Of Conjugated Polymers For Organic Solar Cells And Devices Engineering

Organic solar cells(OSCs)have attracted significant attention recently due to their advantages in light weight,low cost,flexible large-area devices by solution processes.In this dissertation,a research has been focused on design and synthesis of conjugated polymers in combination with interfacial and device engineering for high-efficiency OSCs.The effects of side chains and the match between electron-donating and electron-withdrawing units on the properties of the polymers have been investigated.The optimization of single-layer devices in terms of processing and ternary solar cells structure have been explored to evaluate the potential of as-prepared polymers for bulk-heterojunction(BHJ)solar cells.The application of organic small molecules as electron transport materials in inverted devices has also been explored.In chapter 2,four ?-conjugated benzo[1,2-b:4,5-b']dithiophene(BDT)based polymers were synthesized with different electron-withdrawing units,thiazolo[5,4-d]-thiazole(TTz),benzothiadiazole(BT)and pyrrolo[3,4-c]pyrrole-1,4-dione(DPP).The effects of different side chains and the match between electron-donating and electron-withdrawing units on the thermal stability,optical and electrochemical properties of the polymers were investigated.A maximum power conversion efficiency(PCE)of 2.60%was obtained from photovoltaic cells with PBDTT-TTz:PC6iBM(1:2,w/w)blend film as the active layer,with PFN as electron transporting layer(ETL).In chapter 3,four isostructural polymers of BDT/naphtho[1,2-b:5,6-b']difuran(NDF)and benzoselenadiazole(BSe)/BT were developed and evaluated for OSCs.The substitution of one-atom(Se for S)in the accepting units exerts remarkable impact on the optoelectronic properties of polymers.Extended absorption,narrowed bandgap and higher HOMO energy levels were observed for Se-containing polymers in comparison to their S-containing counterparts.Solar cells based on PBDTT-DTBSe/PC71BM blend films deliver the best PCE of 5.40%,with an open-circuit voltage(VOC)of 0.75 V,a short circuit current(Jsc)of 11.23 mA/cm2.and a fill factor(FF)of 64.1%.In chapter 4,4,9-bis(5-bromothiophen-2-yl)-6,7-bis(5-dodecylthiophen-2-yl)-[1,2,5]-thiadiazolo[3,4-g]quinoxaline(TQxT)was designed as a new strong electron-withdrawing unit.Three TQxT based conjugated polymers were further developed by alternating with BDT or NDF.Featuring ultra low bandgaps of 1.0 eV,these polymers exhibit absorption spectra beyond near-infrared(NIR)region.The doping of 9 wt%PBDTT-TQxP into P3HT:PC71BM(1:2)blend,however,leads to a highest PCE of 3.58%for the resultant ternary solar cells,corresponding to?22%improvement in comparison to 2.93%PCE for P3HT:PC71BM binary cells.In chapter 5,polymers containing alkylthio side chain modifying two-dimensional BDT or benzo[1,2-b:4,5-b']difuran(BDF)with different electron-deficient acceptors have been synthesized.The effects of alkylthio side chains on the thermal stability,optical and electrochemical properties of the polymers are investigated.The photovoltaic perfomance of the polymers is systematically studied.The PBDT-DTBTff:PC71BM based devices exhibited a highest PCE of 7.65%,with a Voc of 0.86 V,a Jsc of 13.32 mA/cm2 and a FF of 66.8%.The good performance is attribured to to the polymer's high hole mobility(1.1×10-3 cm2 V-1 s-1),facilitating exciton dissociation and charge transport.The PBDF-DTBTff:PC71BM based devices delivered a best PCE of 5.48%,with the active layer processed from chloroform solutions with 1,8-diiodooctane(3%)as additive and treated with tetrahydrofuran vapor for 30 s in inverted solar cells.In chapter 6,two alcohol-soluble PDI based materials(ca.PDIN and CDIN)were exploited as low-temperature processable and thickness-inert ETLs to effectively enhance the photovoltaic performance of various inverted devices.Besides the decent charge-transporting property,PDIN and CDIN ETLs were manifested to facilitate the face-on orientation of atop BHJ layers as evidenced by GIWAXS analysis,which might benefit from its discotic geometry endowed with strong face-on ?-? stacking in solid state and better compatibility to the constituent organic photoactive components.This study shows the feasibility of modulating BHJ of OSCs by rationally selected ETLs to facilitate exciton dissociation and collection in the devices.Consequently,an enhancement in PCE can be achieved.The PTB7-Th:B-DIPDI based devices with PDIN as ETL achieved?10-14%enhanced PCE than that of the pristine ZnO-based device.To evaluate the potential of CDIN as ETL,different kinds of active layer were investigated.For the representative device with r-PTB7-Th:PC71BM blend film,the PCE was increased from 10.14%to 11.17%when introducing CDIN as ETL.The device PCEs remained over 10%even with the active layer thickness increased upto 380 nm.Under low-temperature processing,the flexible device of r-PTB7-Th/PC71BM obtained a high PCE of 8.40%.