Novel Donor Photovoltaic Materials And Cathode Interfacial Materials Of Polymer Solar Cells

Recently polymer solar cells(PSCs)have attracted extensive attention due to the advantages of light weight,low cost,simple fabrication and flexibility.Developing new photovoltaic materials including donor and acceptor materials is one of the effective strategies to improve the power conversion efficiency of polymer solar cells.Among them,the polymer donors,consisting of alternating donor(D)and acceptor(A)building blocks,have attracted great interests due to the tunable energy levels and band gaps.Moreover,nonfullerene acceptors have achieved important progress in recent years.Using a wide-bandgap(WBG)donor-acceptor(D-A)copolymer donor to match the low-bandgap nonfullerene acceptor is an effective approach to realize high-performance nonfullerene solar cells.On the other hand,ternary PSCs,which incorporate a third component in the binary photoactive layer,such as fullerene derivatives,show unique advantages over conventional binary solar cells.In the thesis,in the aim of developing novel donor photovoltaic materials and cathode interfacial materials for polymer solar cells,we designed and synthesized several polymer donor materials based on lactam units and imidazole-modified fullerene derivative interface materials.The main research contents are as follows:(1)Polycyclic aromatic lactams(PALs)demonstrated great potential in developing effcient D-A copolymers.Benefiting from their strong electronwithdrawing capability and good planarity,PAL-based D-A copolymers usually exhibit deep HOMO levels and high hole mobilities.We developed a tricyclic aromatic lactam unit,thieno[3,2-c]isoquinolin-5(4H)-one(TIQ),which can be viewed as a hybrid of dithieno[3,2-b:2',3'-d]pyridin-5(4H)-one(DTP)and phenanthridin-6(5H)-one(PN).A TIQ-based D-A copolymer PTIQ4TFBT presented a high hole mobility of 1.33 × 10-3 cm2V-1s-1 and exhibited high performance in bulk heterojunction solar cells.PTIQ4TFBT:PC71BM blend films exhibited favorable morphology and balanced charge transport and yielded a PCE of 10.16%(active layer thickness,267 nm),much higher than that of analogy polymers(PDTP4TFBT,8.61%;PPN4TFBT,8.47%).The higher performance of PTIQ4TFBT:PC71BM solar cells originates from enhanced short-circuit current density(Jsc)and fill factor(FF),because of favorable morphology,less bimolecular recombination,and balanced charge transport in the active layer.Moreover,the performance for PTIQ4TFBT:PC7iBM solar cells is less sensitive to active layer thickness than PDTP4TFBT:PC7iBM and PPN4TFBT:PC-7iBM solar cells.Over 8%PCEs can be obtained from PTIQ4TFBT:PC7iBM solar cells when the active layer thickness is over 500 nm.(2)Wide-bandgap D-A copolymers have complementary absorption spectrum with the narrow bandgap non-fullerene acceptor materials.Their blend films can make full use of sunlight in the range of visable and near-infrared spectral radiation and thus improving the Jsc.and PCE of the device.We designed and synthesized two DTP-based wide-bandgap copolymers,PBD and PBD2T.PBD2T with 3-octylthiophene ?-bridges has enhanced ?-? stacking in the solid state and ideal polymer nanofiber structures,and endows the polymer with higher hole mobility.As a result,PBD2T:IT-M cells afforded decent PCEs up to 10.34%,higher Jsc,FF and PCE values than PBD:IT-M cells(8.33%).(3)Fullerenes have been widely employed as acceptor photovoltaic materials and cathode interfacial materials in PSCs due to their unique closed cage structure,high electron affinity and high electron mobility.By using a facile one-pot nucleophilic addition reaction,we synthesized a novel imidazole(IMZ)-functionalized fullerene(C60-IMZ)and applied it as cathode interfacial material in PSCs.By doping C60-IMZ as a third component into P3HT:PC6iBM and PTB7:PC7iBM blend films,respectively,the PCE of the devices reach 3.4%and 5.3%,dramatically higher than that of the control devices.The introduction of imidazole endows C60-DMZ with high surface energy.C60-IMZ doped in active layer may migrate to the surface of ITO cathode via vertical phase separation,consequently forming a cathode interfacial layer,which can lower the work function of ITO cathode and reduce the energy level offsets between ITO and PC6iBM,facilitating electron transport from the active layer to ITO cathode and improving the device efficiency.