A-?-D-?-A Type Small Molecule Electron Acceptors: Design,Synthesis,and Photovoltaic Performance

Organic solar cells have been intensively studied in recent years,due to their advantages as low-cost,light weight and large area roll-to-roll printing.In this dissertation,we focus on the material design and synthesis of small molecule donor to achieve high efficiency all-small-molecule organic solar cells(ASM OSCs).On one hand,by changing the orientation of alkyl chains on the ? bridges,the phase separation morphology of active layer is optimized,and as a result,the fill factor and power conversion efficiency(PCE)of the devices are significantly enhanced.On the other hand,by adjusting the conjugate degree of ? bridges,deeper HOMO levels of the molecule could be obtained,and result in a higher Voc of devices.Firstly,based on the classical A-?-D-?-A type small molecule donor structure,we designed and synthesized two novel type small molecule donor with various isomers as “inward” BIHTR and “outward” BOHTR,to reveal the effects of orientation of alkyl chains on ? bridges on the phase separation morphology and photoelectric properties.Finally,12.3% PCE is achieved,which presents a record efficiency for single junction ASM OSCs.Further research finds that the “inward” orientation of the alkyl chains on ? bridges could effectively adjust the molecule planarity and ?-? stacking,and then produce a favorable “interpenetrating network” phase separation structure after blended with Y6.The optimized morphology is benefit for carrier transportation and extraction,which could reduce the bimolecular recombination and trap-assistant recombination in devices,finally significantly enhance the device fill factor and PCE.Secondly,based on the classical A-?-D-?-A small molecule donor structure,we designed and synthesized three small molecule donors with various conjugate degree of ? bridges as B-T-CN,B-TT-CN and B-DTT-CN to explore the influence of conjugate degree of ? bridges on their photoelectric properties and phase separation morphology,and devices Voc is improved from 0.90 V to 0.96 V by expanding the conjugate degree.Further research find that enhancement of conjugate degree of ? bridges assists to reducing the HOMO level,which could explain for the improvement of Voc.However,the phase separation morphology and molecule stacking are also unexpectedly changed together with the enhancement of conjugated degree of ? bridges,resulting in more D/A interfaces,severer bimolecular recombination and trap-assistant recombination,which are the reasons for lower fill factor,short circuit current and PCE of devices.This dissertation focus on modification of the ? bridges of A-?-D-?-A type small molecule donors to adjust their photoelectric properties and phase separation morphology,which provides a novel method for PCE improvement of ASM OSCs.