Synthesis And Optoelectronic Properties Of Small Molecule Acceptor Based On PDI And ITIC

Organic solar cells(OSCs)have attracted a lot of attention due to their light weight,low cost,solution processing and their potential applications in flexible devices.Generally,the active layers of OSCs are composed of a polymer donor and a fullerene derivative acceptor.However,fullerene derivatives also have several shortcomings,such as limited visible light absorption,difficulty in tuning the energy level.Therefore,the study of non-fullerene acceptor materials is of great significance for the development of OSCs.In recent years,the devices based on non-fullerene small molecule acceptors have made great breakthroughs in the power conversion efficiency(PCE),fused-ring electron acceptor of ITIC and perylene diimide(PDI)derivatives are the most common non-fullerene acceptors.The rational design of the nonfullerene acceptors is mainly based on strategies that aim to tune the energetics and crystalline behaviors.Mainstream design strategies include side-chain engineering,backbone modification,and end-capping group manipulation.This thesis starts from the design and synthesis of small molecule acceptor materials,optimizes the molecular structure through a series of engineering strategies,and deeply studies the relationship between structure and performance,thus achieving better photovoltaic performance.Firstly,we briefly introduced OSCs,including working principles,device structures,photovoltaic parameters and interlayer materials.Afterwards,the development of donors and acceptors in the active layer are summarized.Taking this as a starting point,the main ideas of this paper are designed.In Chapter 2,a new family of PDI oligomers,T-PDI and H-PDI,were designed and synthesized via dehydration condensation reactions with two PDI monomers linked together at bay-and imide-positions.The as-cast T-PDI based PSC gave a PCE of3.50%,with a V_OC of 0.837 V.Since this non-homologous PDI derivative is inefficient and the V_OC of this AA-linked acceptor is quite low,in Chapter 3 three novel star-shaped S/Se-annulated PDI small molecule acceptors(SMAs)with triphenylamine as the core were developed,namely TPA-PDI,TPA-PDI-S and TPA-PDI-Se.The TPA-PDI-Se-based device achieves 6.10%efficiency with V_OC over 1.00 V.In Chapter 4,in order to further improve the V_OC and device efficiency,we developed two SMAs based on benzodithiophene dithiophene(BDTT)as core,namely BPT and BPT-S.The device based on BPT-S yields a PCE of 8.28%with V_OC as high as 1.02 V.On this basis,in Chapter 5,we designed and synthesized two isomerized SMAs,namely BPT-Se and BPT-Se1.The BPT-Se1-based device achieves a high PCE of9.54%,an excellent FF(73.2%)and a high V_OC(1.059 V).It is worth mentioning that73.2%of FF is the highest value for PDI-based OSCs to date.In Chapter 6,we synthesized two SMAs with IDT as the core,and alkylthiophenyl and alkoxythiophenyl as the side chains,namely IDTC and IDTO.The IDTO-based device achieved 10.02%PCE.In Chapter 7,we developed a SMA of IDTPC by replacing the hexyl phenyl on IDT with a hexyl side chain.The IDTPC-based OSC displays a PCE as high as 12.2%.Importantly,IDTPC-based devices exhibit properties that are insensitive to active layer thicknesses from 70 to 505 nm,which still gives a PCE over 10.0%with the active-layer thickness of 400 nm.In Chapter 8,a novel SMA of MeIC with a methylated thiophene-fused end group was developed.Compared to the unmethylated counterpart(ITCPTC),MeIC exhibits a higher LUMO level,stronger crystallinity and broader absorption in the 520-740 nm wavelength range.The MeIC-based OSC achieves a PCE of 12.54%,which is significantly higher than that of the device based on ITCPTC(11.63%).In order to improve the J_SCC of device,in Chapter 9,a SMA of ITC-2Cl with a chlorinated thiophene-fused end group was synthesized.In comparison with the unchlorinated counterpart(ITCPTC),the introduction of Cl improves not only the electronic properties by redshifting the absorption spectra and deepening the LUMO energy levels,but also the molecular packing and thus thin-film morphology.The ITC-2Cl-based device yields a significantly higher PCE(13.6%)than that of the ITCPTC-based device(12.3%).Compared to the ITCPTC system,ITC-2Cl-based devices reduce radiated and non-radiative recombination losses,resulting in lower energy losses.In Chapter 10,three isomers of brominated thiophene-fused end groups were designed and synthesized,and condense them separately with IDTT to obtain three SMAs,namely ITC-2Br,ITC-2Br1 and ITC-2Br2.The three isomers show significant differences in absorption,electrochemical and crystallization properties.Among them,the ITC-2Br-based device achieves an PCE of 13.1%.In Chapter 11,we designed and synthesized a new SMAs,namely m-MeIC,which is proved to be effective with different band-gap polymer donors including wide band-gap J71,medium band-gap PBDB-T and low band-gap PCE-10.Photovoltaic devices based on J71,PBDB-T and PCE-10 achieves PCEs as high as 12.08%,10.93%and8.34%,respectively.