Design,Synthesis And Photovoltaic Performance Of Novel Imide Based Electron Acceptors For Organic Solar Cells

Organic solar cells?OSCs?,as one effective way to solve the environmental pollution and energy crisis,have attracted extensive attention from researchers due to the unique advantages of light weight,low cost,solution processable and potential fabrications for large-area flexible devices.In recent years,power conversion efficiencies?PCEs?of OSCs have been greatly improved benefiting from the continuous development of the active layers,interface and active layer morphology modifications,and device structure optimization.Currently,the highest PCE of single-junction PSCs obtained is over 16%.The active layer,as the core of OSCs,mainly includes electron-donating materials and electron-withdrawing materials.The development of the donor materials with various kinds of bandgap has been quite mature.And the dominant fullerenes derivatives exhibit many intrinsic shortcomings,such as narrow and weak absorption in the visible region,difficulty in level tunability and purification,high production cost,poor photo-thermal stability,limiting the further breakthrough of device performance.In recent years,benefiting from the molecular structure design and optimization,great breakthroughs have been made in non-fullerene small molecule and polymer acceptors.Herein,we developed a series of novel cross-conjugated small molecules and low bandgap conjugated polymer acceptors based on perylene diimide?PDI?and naphthalene diimide?NDI?for the active layers of OSCs,and studied the influence of the molecular structure on the molecular absorption,energy levels,aggregation,and the morphology of the active layers.In chapter 2,to combine the advantages of both the acceptor-donor-acceptor?A-D-A?-and PDI-based acceptors,we developed three novel cross-conjugated small molecular acceptors?SMAs?that characterized with benzo[1,2-b:4,5-b']dithiophene?BDT?as core,three different electron-deficient end-capped groups?EGs?and PDI flanked with thiophene and acetylene,respectively,which exhibit the similar isotropic charge transport property to those of fullerenes derivatives.The cross-conjugated SMAs exhibited a red-shifted absorption in comparison with most SMAs based on PDI,a tunable energy levels and absorption by modifying the terminal groups.The devices based on the series of SMAs exhibited a best PCE of 6.06%,when blended with polymer donor PTB7-Th.In chapter 3,we successively introduced thiophene and cyclopenta[2,1-b:3,4-b']dithiophene?CPD?units in the vertical and horizontal direction of our original molecule in the chapter 2,respevtively,to decrease the twisted angle between EG and BDT moiety and further improve the absorption of SMAs at long wavelength,thus enhance short-circuit current density(J_SC)of devices.The resulting new compound PDI-CPD-IT exhibited a rigid and planar conjugated structure,wide and strong absorption band with approximately two times maximum absorption coefficient compared with the SMA PDI-BDTT-IT without CPD unit.Energy levels,absorptions,and morphology of the active layers were well regulated through the further modifications of the EGs and the alkyl side chains on the CPD unit.When matched with polymer donor PBDB-T,the devices based on PDI-CPD-ITFFS that modificated by the short alkyl side chain and difluorinated EG echibited the best PCE of 9.76%,with an excellent J_SC of18.35 mA cm^-2,FF of 70.86%.These findings disclose that the novel molecular design strategy of cross-conjugation for the NFAs is promising for efficient PSCs.In chapter 4,we applied the quinone resonance and donor-acceptor?D-A?design strategies to construct narrow band gap conjugated polymers,which show an absorption edge over 1000nm.We also found that introducing chlorine atoms on the quinone resonant structure can effectively lower the HOMO energy levels and loose aggregations between polymer chains,which is benefit for charge transport.In addition,ternary conjugated polymers with the quinone structure were also designed and synthesed to further improve the morphology of the blend film.When blended with PTB7-Th,all-PSCs based on these polymer acceptors gave best PCE of2.38%.In chapter 5,we developed two novel PDI based conjugated polymers,PITN-11 and PITN-812,and investigated parity of carbon atom numbers of the branched chain on the two nitrogen atoms.We found that PITN-812 with even number of C atoms exhibited a better crystallization and wider absorption,which is beneficial for light-harvesting ang charge transfer.Therefore,all-PSCs based on PTB7-Th:PITN-812 exhibited a better PCE of 2.16%.