| As a new-generation photovoltaic technology,organic solar cells(OSCs)have great potential in solving the energy crisis and alleviating environmental pollution.In recent years,with the rapid development of A-DA’D-A(A and D represents electron-donating and-accepting units,respectively)type nonfullerene small-molecule acceptors and device optimizations,the energy conversion efficiencies(PCEs)have exceeded 19%.At present,the efficiencies of all polymer solar cells have reached over 18%,thanks to the development of polymerized small-molecule acceptor strategies.A-DA’D-A type smallmolecule acceptors are considered to be one of the most promising candidate photovoltaic materials,due to their high absorption coefficient,easy adjustment of energy levels and bandgaps and easy tunable chemical structures.The end A units have strong electron accepting ability,which can enhance intramolecular and intermolecular interactions,as well as charge separation and transport.By introducing halogens on the end groups,the energy levels and bandgaps of molecules can be adjusted.However,due to the isomeric effect caused by the mono-halogenated IC end group,the absorption spectrum,molecular packing and charge transfer characteristics of materials can be adjusted by adjusting the substitution positions on the end groups.In addition,small molecules containing brominated end groups are often used as intermediates for preparing dimers and polymer acceptors,which can synthesize new small-molecule acceptors and polymer acceptors by connecting them with different linkage units.This thesis focuses on the effects of bromine substitution sites and linkage units on the optical,electrochemical,and photovoltaic properties of smallmolecule and polymer acceptors.A series of small-molecule and polymer acceptors were designed,and the following results were achieved:1.Two small-molecule acceptors BTAIC-mix-Br and BTAIC-5-Br were synthesized using 2-(5(6)-bromo-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile(IC-mix-Br)and 2-(5-bromo-3-oxo-2,3-dihydro-1Hinden-1-ylidene)malononitrile(IC-5-Br)as end-group A unit,benzotriazole pyrrolodithiophene as the DA’D fused ring unit and 2-hexyldecyl as mediumlength chains.Compared with BTAIC-mix-Br,BTAIC-5-Br exhibits a relatively narrowed bandgap and an upshifted LUMO energy level.Using PBDB-T as the donor,the two devices showed efficiencies of 10.35% and10.16%,respectively.2.Introduction of electron-donating thiophene and selenophene groups at the end groups was able to attenuate the electron conjugation of end IC groups and upshift the LUMO energy levels,thus increasing the open-circuit voltage of device.Two small-molecule acceptors of BTAIC-5-S and BTAIC-5-Se were synthesized by following Stille coupling reaction using BTAIC-5-Br as an intermediate.Compared with BTAIC-5-Br,the LUMO energy levels of BTAIC-5-S and BTAIC-5-Se were upshifted to-3.68 e V and-3.70 e V,respectively.Blend with PBDB-T,the BTAIC-5-S and BTAIC-5-Se based devices achieved open-circuit voltages of 0.949 V and 0.927 V and efficiencies of 8.66% and 7.57%,respectively.3.By using the polymerized small-molecule acceptor strategy,polymer acceptors of P(BTAIC-mix-S),P(BTAIC-5-S)and P(BTAIC-5-Se)were synthesized using BTAIC-mix-Br and BTAIC-5-Br as small-molecule acceptor building blocks and thiophene and selenophenol as linkage units,respectively.Compared with P(BTAIC-mix-S),P(BTAIC-5-S)exhibited a narrower bandgap and lower LUMO energy level.Blended with PBDB-T,P(BTAIC-5-S)showed an efficiency of 6.32%,P(BTAIC-mix-S)achieved an efficiency of4.71%.The results indicated that the regio-regular polymer acceptor achieved better device performance compared to the regio-random counterpast.P(BTAIC-5-Se)only showed 0.42% efficiency,possibly due to its relatively low molecular weight.After purification of P(BTAIC-mix-S)by thin-layer chromatography,the efficiency increased to 8.00%,indicating that reducing the molecular weight distribution might be beneficial for improving device performance. |
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