The Design And Synthesis Of Small Molecule Acceptor Materials Based On N,S-Heterocycles For Organic Solar Cells

Organic solar cells(OSCs)possess versatile features such as semi-transparency,light weight,and flexibility,endowing them with great potential for applications in flexible solar panels and building-integrated photovoltaics.Therefore,OSCs have received widespread attention from the scientific research community.The development of novel active layer materials is an important way to improve the photoelectric conversion efficiencies(PCEs)of OSCs.With the development of non-fullerene electron acceptors(NFEAs),PCEs of OSCs are quickly increasing.At present,PCEs of OSCs based on NFEAs have exceeded 20%.This paper mainly focused on designing and synthesizing a series of novel fused cores and central units.At the same time,the paper combined side-chain engineering and end-group engineering to explore the relationship between the chemical structure of NFEAs and photovoltaic performance.Benzodithiophene(BDT)as an electron-donating unit has been widely used in polymer donors and exhibites excellent photovoltaic performance.In chapter 2,two NFEAs were synthesized using the S,N-heteroacene ring as fused core with the BDT unit and the thiophenepyrrole ring.Both NFEAs showed strong spectral absorption,narrow optical bandgaps,and high electron mobility.Even though HOMO(the highest occupied molecular orbital)energy level offsets between the donor and acceptor were less than 0.10 e V,OSCs still exhibited efficient charge transfer.In the system,the highest PCE of OSC was approaching15%.These results showed that the N,S-heteroacene ring was a very potential unit to construct efficient NFEAs,which provides an important guide for the design of efficient NFEAs.In order to further explore the effect of embedding multiple electron-deficient groups in fused core on the photovoltaic performance of NFEAs.In chapter 3,a dodecacyclic-fused core with three electron-deficient units was designed and synthesized.Three NFEAs with dodecacyclic-fused core were obtained by end-group engineering.They all showed strong absorption spectral in range of 600-900 nm.With increasing the electron-deficient ability of end groups,absorption edges of three NFEAs gradually red-shifted,HOMO and LUMO(the lowest unoccupied molecular orbital)energy levels gradually decreased.In the system,the highest PCE of OSC was close to 12%.These results indicated that designing the multiplefused aromatic core with multiple electron-deficient units was a promising strategy to obtain high-performance NFEAs.This work provides a novel design idea for the design of efficient NFEAs.In chapter 4,a series of non-fused-ring small molecule acceptors(NFRAs)were designed and synthesized with bisthiadiazole carbazole(DBT)as the central unit,which effectively solved the problem of difficult synthesis in fused-cores.Through the analysis of the single crystal structure of NFRAs,the effects of alkyl chains and end groups in NFRAs on the photoelectric properties were further explored.With the growth of alkyl chains on the central unit and the increase of electron clouds density in end group,π-π stacking distance between molecules decreased.In the system,the highest PCE of the OSC attained nearly 11%.This work has certain reference significance for explaining the relationship between the structure and properties of NFRAsIn chapter 5,a series of NFRAs with asymmetric benzotrithiophene(BTT)as the central unit were designed and synthesized.The study found that alkyl chains in small molecule acceptors located on two sides of the main chain skeleton.With the extension of terminal π-conjugation,the molecular dipole moments of NFRAs increased,and the exciton dissociation efficiencies were also significantly improved.The highest PCE of OSCs reached 11.6%.These results suggested that enlarging the molecular dipole moment was an effective way to improve device exciton dissociation efficiency and photovoltaic performance.In chapter 6,in order to further improve absorption spectral of NFRAs in the near-infrared region,the N,S-five-membered heterocycle with excellent planarity and rigidity was used to construct NFRAs.Four NFRAs were obtained by changing the position of alkoxy group in thiophene and the electron-deficient properties of end groups.Compared with DX-O,DX-I showed narrower optical bandgaps.For OSCs based on DX-I,the EQE response edges were both exceed 1100 nm,indicating potential applications of NFRAs in semitransparent devices and near-infrared photodetectors.However,the PCEs based on DX-I were only about 3%,due to the poor energy level matching between PTB7-Th and DX-I.The PCEs based on DX-O obtained about 9%,due to better energy level matching between the PBDB-T and DX-O.