Design,Synthesis And Properties Of Indeno[1,2-b]Fluorene-based Non-fullerene Small-Molecular Acceptors

In recent years,non-fullerene small-molecule acceptors（NFAs）have gradually replaced traditional fullerenes and their derivatives and become the mainstream in organic solar cell acceptors because of their strong absorption in the vis-NIR region,easy synthesis,low cost,and easy adjustment of energy levels.The NFAs have been developed rapidly and have achieved excellent peformance.Thorough investigation of the relationship between structure and performance is the key to further improve the power conversion efficiency.Abstracts of each part of this thesis are presented as following:Firstly,a series of non-fullerene acceptors based on the indeno[1,2-b]fluorene central moiety,with thiophene or 3-octylthiophene asπ-bridge and either non-fluorinatedorfluorinated2-（3-oxo-2,3-dihydro-1H-inden-1-ylidene）malononitrile as end-capping groups（namely ICBF-O,ICBF,FICBF-O and FICBF）,have been designed and synthesized.The effect of alkylation of theπ-bridge and fluorination of the end-capping groups on the absorption spectra,energy levels,active layer morphology and photovoltaic performance were systematically investigated.Alkylation upshifts the molecular LUMO levels and thereby a high open circuit voltage(V_oc)of 1.06 V was obtained.However,the larger band gap induced by alkylation led to lower short circuit current(J_sc).Fluorinated acceptors display lower V_oc but higher J_sc and fill factor（FF）compared with non-fluorinated acceptors,coinciding with their lower LUMO levels,narrower band gaps and favourable morphology.As a result,the non-fullerene solar cells（NFSCs）based on FICBF showed the highest PCE of 7.41%among these four acceptors.The ICBF based device delivered a comparatively high V_oc of 0.99 V and a PCE of 6.07%.Secondly,two non-fullerene acceptors based on the indeno[1,2-b]fluorene central moiety,withselenopheneorvinylselenopheneasπ-bridgeand2-（5,6-difuoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene）malononitrile as end-capping groups,FICBF-Se and FICBF-SeVi,were synthesized,and the effect of alkylation of theπ-bridges on the absorption spectra,energy levels,active layer morphology and photovoltaic performance were systematically investigated.The introduction of vinylene group led to clearly redshifted and broader absorption and smaller bandgap.FICBF-Se and FICBF-SeVi exhibited optical bandgaps of 1.68 and 1.54 eV,respectively.Solar cells using polymer PBDB-T as a donor and FICBF-Se as an acceptor demonstrated a PCE of 6.25%with a J_sc of 11.61 mA cm^-2.However,the device based on PBDB-T/FICBF-SeVi only delivered a PCE of 3.63%with a J_sc of9.08 mA cm^-2,which should be mainly because the introduction of the vinylene group decreased crystallinity and its active layer morphology showed no distinct phase separation.