Synthesis?Photovoltaic Performance And Structure-Property-Relationship Of Fused-Ring Electron Acceptors

Organic solar cells(OSCs)with the function of converting solar energy into electricity are an environmentally-friendly technology for renewable energy.OSCs have great commercialization prospects due to their low-cost,light-weight,large-area,flexible and semitransparent devices.However,one of the biggest bottlenecks encountered in the commercialization of OSCs is their low power conversion efficiencies(PCEs).Designing and synthesizing new types of electron donors and acceptor materials is one of the most effective ways to solve this problem.The acceptor-donor-acceptor(A-D-A)-type fused-ring electron acceptors(FREAs)own the advantages of wide absorption range,tunable energy levels,adjustable crystallinity and easily-modified structures.Compared with fullerene acceptors,FREAs have greater potential to achieve high PCEs.Such electron acceptors usually consist of two electron-withdrawing end groups,a fused-ring central core and soluble side chains.The main research content of this thesis is to design and synthesize new symmetrical and asymmetrical fused-ring central cores,and then optimizes the end groups and side chains to build high-performance FREAs,improving the PCEs of OSCs.In the second chapter,we designed and synthesized a seven-fused-ring donor core with a symmetrical structure.By using alkylthiophene or alkylselenophene as side chains and thiophene-fused cyclopentanedione dicyano group derivative(CPTCN)as end group,two FREAs,namely ITCPTC-Th and ITCPTC-Se,were synthesized.The effects of thiophene and selenophene side chains on the absorption,energy levels,crystallization properties,blend film morphologies and photovoltaic properties of FREAs were investigated.With PBDB-T as the polymer donor,the ITCPTC-Th-based device(10.61%)achieved higher PCE than that of ITICPTC-Se-based device(9.02%),indicating that thiophene is better than selenophene as side chain of the FREAs.In the third chapter,we designed and synthesized an asymmetrical six-fused-ring donor core(IDT6)by reducing a thiophene from symmetrical seven-fused-ring compound(IDTT)or incorporating one thiophene into the symmetrical five-fused-ring compound(IDT).By using alkylthiophene or alkylphenyl as side chains and CPTCN or methylated INIC as end groups,three FREAs named IDT6CN,IDT6CN-Th and IDT6CN-M were constructed.By comparing with the symmetrical FREAs(ITCPTC and IDTCN),the differences in the molecular configurations,absorption,energy levels,molecular packing,blend film morphologies and photovoltaic performance of the symmetrical and asymmetrical FREAs were investigated.We found that the dipole-dipole interactions between asymmetrical FREAs enhances the?-?stacking and thus significantly increases the fill factor(FF of77%)and PCE(11.2%)of non-fullerene OSCs.In the fourth chapter,based on the optimal molecule(IDT6CN-M)in the previous chapter,we designed and synthesized another asymmetrical FREA,namely IDT8CN-M,by expanding the conjugation length of the asymmetrical central core of IDT6CN-M.The effects of expanding the conjugation length of asymmetrical core on the absorption,energy levels,intermolecular packing,charge mobility,blend film morphology and photovoltaic properties of asymmetrical FREAs were investigated.It was found that expanding the conjugation length of the asymmetrical structure can further improve the performance of the asymmetrical electron acceptors.The device based on PBDB-T:IDT8CN-M achieved a FF as high as 79%and a PCE of 12.4%,which is significantly higher than that of PBDB-T:IDT6CN-M-based OSCs.In the fifth chapter,we designed and synthesized an asymmetrical isomer(MeIC1)of MeIC which is a high-performance symmetrical seven-fused-ring FREA developed by our group.Through this asymmetrical isomer strategy,the LUMO level and intermolecular packing of MeIC1 were improved compared to MeIC,resulting in an increase of open-circuit voltage(V_oc)and PCE of PBDB-T:MeIC1-based OSCs.Although MeIC1 has no dipole moment,the?-?stacking between molecules can be enhanced by introducing a large volume of stacked units,demonstrating the effectiveness of the asymmetrical isomer strategy.In the sixth chapter,regarding the better?-?stacking and higher electron mobility of asymmetrical FREAs,we designed and synthesized two FREAs called IDT6CN-TM and IDT6CN-4F with better performance for multifunctional applications.Acting as electron transport layer(ETL)and interface layer(IL)of inverted perovskite solar cells(PVKSCs),IDT6CN-TM achieved PCEs of up to 18.4%and 19.8%,respectively.As an electron acceptor,OSCs based on IDT6CN-TM also achieved a PCE of 12.4%.IDT6CN-4F showed slightly lower performance but is also superior to the performance of symmetrical analogs,demonstrating that asymmetrical FREAs are highly efficient versatile materials.In the seventh chapter,we designed and synthesized a symmetrical benzodithiophene(BDT)-fused donor core with BDT substituted by alkoxy and thienyl,respectively.By using methylated CPTCN as end group,two FREAs,namely BTOIC and BTTIC,with near-infrared(NIR)absorption were constructed.The effects of different side chains on absorption,energy levels,crystallization performance,blend film morphology and photovoltaic properties were investigated.The PBDB-T:BTTIC-based device achieved a PCE of 13.18%,which is much higher than that of the PBDB-T:BTOIC-based device,indicating that thiophene as side chain is superior to the alkoxy.In the eighth chapter,by incorporating one thiophene into the core of BTTIC,we synthesized an eight-fused-ring electron acceptor(a-BTTIC)with an asymmetric structure.a-BTTIC shows a obviously red-shifted absorption relative to BTTIC,and the LUMO level is also increased.Compared with PBDB-T:BTTIC-based devices,PBDB-T:a-BTTIC-based devices achieved simultaneous increases in V_oc and short-circuit current(J_sc).This asymmetrical strategy enhanced the PCE of devices up to 13.6%and reduced the energy loss lower than 0.53 eV.It shows that the design of asymmetrical FREAs can effectively improve the device PCE and reduce the energy loss of OSCs.In the ninth chapter,we optimized the end group of BTTIC by adding one methyl or two methyl groups onto CPTCN,and synthesized three FREAs,namely BTTIC-0M,BTTIC-2M and BTTIC-4M.The effects of the number of methyl on exciton binding energy and blend film morphology were studied.BTTIC-2M achieved the best balance between exciton binding energy and morphology.OSCs based on BTTIC-2M achieved a PCE of more than 13%,which is higher than the other two FREAs,indicating that monomethylated CPTCN is better than CPTCN and dimethylated CPTCN.In the tenth chapter,we designed and synthesized a novel9H-indole[1,2-b]pyrazine-2,3-dicarbonitrile-based end group(IPC),and synthesized a resulting FREA BTOIPC.Due to the strong electron-withdrawing ability of IPC,BTOIPC shows a wide absorption range with a suitable LUMO level.The PBDB-T:BTOIPC-based device achieves a 9.3%PCE and a relatively low energy loss,demonstrating that IPC is a potential end group.