Synthesis And Photovoltaic Performance Of Non-fullerene Acceptors Based On New Cores

In recent years,non-fullerene acceptors?NFAs?have attracted considerable attention because of their better intrinsic advantages than fullerenes derivatives,such as tunable energy levels,adjustable absorption properties and flexible structural modification.Up to date,small molecules based on perylene diimide?PDI?and A-D-A type?such as ITIC?are considered to be the most successful cases in NFAs..Among them,the NFAs based on A-D-A type show higher photovoltaic performance than fullerene derivatives.Accordingly,two kinds of small molecular NFAs based on new central core have been designed and synthesized,and their photovoltaic properties have been studied.A novel asymmetric small molecule B-4PDI was designed and synthesized by using the electron-deficient pyridine ring as the core,four perylene diimide as four arms,in which four thiophenes were used to connect with the core at 2,3,5,6 sites and the arms at the bay positions.B-4PDI shows strong absorption in the waveband between 300 nm and 650 nm,which is complementary with the absorption spectrum of PTB7-Th?500-800nm?.The HOMO?highest occupied molecular orbital?and LUMO?lowest unoccupied molecular orbital?energy levels of B-4PDI are-6.05 eV and-4.11 eV respectively,which meet the energy requirements for charge transport.Although the twisted B-4PDI molecule can impede the aggregation of PDI itself,the phase separation of PTB7-Th:B-4PDI blend is serious,which lead to the bad performance of charge transfer.The weak electron mobility(1.18×10^-44 cm² V^-11 s^-1)results in the lower J_scc and FF.In addition,the larger difference(?E_HOMO?D-A?=0.85 eV)of HOMO between donor PTB7-Th and acceptor B-4PDI which caused greater energy loss(E_loss),and the highest PCE is only 3.32%.This is the highest PCE based on PDI asymmetric NFAs.A novel electron acceptor i-IHIC who is the isomer IHIC was designed and synthesized,through combined two cyclopentane[1,2-b:4,5-b']dithiophene as the central core and connected with 3-?dicyclomethylene?indene-1-one?IC?as the terminal group.Compared with IHIC,isomerized i-IHIC own a more coplanar molecular conformational structure,but the arrangement of molecular ends was changed,which leads to the weakening of?-?stacking between molecules;The absorption edgeband was blue shifted from 896 nm to 753 nm,but there is a stronger absorption in the 300 nm-500 nm.The optimal PCE of PSCs based on i-IHIC:PTB7-Th was 5.31%,V_occ was 0.93V,J_scc was 12.78%mA/cm² and FF was 44.43%.The optimal PCE of PSCs based on i-IHIC:PBDB-T was 6.72%,V_occ was 1.01 V,and J_scc was 12.16 mA/cm²,FF was 54.74%.The photovoltaic performance of i-IHIC was poorer than that of IHIC,ascribed to the blue shift of i-IHIC absorption spectrum that leads to the weak complementarity between i-IHIC absorption spectrum and donor PTB7-Th or PBDB-T spectrum.The another reason was that the compatibility of i-IHIC with donor PTB7-Th or PBDB-T was terrible,and caused a large phase separation in the blend,which hindered the effective dissociation of exciton.Besides,PBDB-T:i-IHIC showed stronger bimolecule recombination than PTB7-Th:IHIC,which made the PBDB-T:i-IHIC blend show lower J_scc and FF.