The Design,Synthesis And Photovoltaic Properties Of Star-Shaped Fused-Ring Electron Acceptors

Bulk heterojunction(BHJ)organic solar cells(OSCs)based on nonfullerene acceptors(NFAs)have gained great progresses in recent years.Among various NFAs,the fused-ring electron acceptors(FREAs)have been proven to be the most efficient for achieving high power conversion efficiencies(PCEs).Unlike linear-shaped FREAs,star-shaped FREAs have two-dimensional(2D)conjugated structures,which leads to stronger light absorption and higher isotropy in charge transportation.In addition,the multi-armed conjugated structure could obtain unique molecular assembly and aggregation,which is significantly different from the star-shaped counterparts.However,few examples of star-shaped FREAs have rarely been reported,and their photovoltaic performance is still lagging behind.The unsatisfactory photovoltaic performance of the star-shaped FREAs is primarily ascribed to the following reasons:(1)Most star-shaped electron acceptors possess relatively wide optical bandgaps(the absorption spectra limited to 650 nm),which to a certain extent restricts their solar energy utilization(especially the solar photons of near infrared region);(2)Star-shaped fused-ring cores are scarce probably owing to their tedious and challenging syntheses.With these issues in mind,in this dissertation,we designed and synthesized a series of star-shaped FREAs based on novel star-shaped fused-ring cores with stronger electron-donating ability and extended ?-conjugation via three strategies of modification of fused-ring conjugated arms,central cores and bridging groups,and realized the PCE of 12%in OSCs.The main research contents are summarized as following:1.Trindenotrithiophene(TITT)has stronger electron-donating ability owing to the incorporation of electron-rich thiophene rings.Firstly,we designed and synthesized two new star-shaped FREAs,TITT-6H and TITT-6F,which consisted of TITT as the central core and 3-(dicyanomethylidene)indan-1-one(IC)and fluorinated IC(IC-2F)as end groups,respectively.These two acceptors exhibit strong intermolecular charge transfer absorption in the region of 500-650 nm with the optical bandgaps around 1.9 eV.Blended with a polymer donor PTB7-Th,the OSC devices based on TITT-6H and TITT-6F provide the maximum PCE of 3.87%and 4.26%,respectively.Secondly,we adopted the strategy of modification of fused-ring conjugate arms to construct a new star-shaped fused-ring core,trindenotrithienothiophene(BTCTT)by introducing the thienothiophene unit,and designed a star-shaped FREA BTCTT-6F.Compared with TITT-6F,BTCTT-6F shows narrower optical bandgap of 1.73 eV and more ordered molecular packing because of extended ?-conjugation of BTCTT.Furthermore,when blending with a polymer donor of PM6,the BTCTT-6F-based OSC device achieves a higher PCE of 9.63%with a higher short-circuit current density(JSC)of 14.30 mA cm-2 and a higher fill factor(FF)of 72.40%.2.In order to further decrease the optical bandgap of star-shaped FREA,we adopted the strategy of modification of central units to develop a novel star-shaped fused-ring core,benzotri(thienocyclopentathienothiophene)(BTTCTT)by replacing the central benzene ring of the BTCTT with the benzotrithiophene.Due to its strong electron-donating ability and efficient ?-electron delocalization,star-shaped FREAs based on BTTCTT exhibit narrow optical bandgaps of less than 1.60 eV and high absorption extinction coefficients of more than 3.0 × 105 M-1 cm-1.In addition,we investigated the side-chain isomerization of star-shaped FREAs by changing the substitution positions of the hexyl chains on the phenyl rings.In a comparison with its isomeric counterpart m-BTTCTT-6F with the mata-hexyl-phenyl substituents,p-BTTCTT-6F with the para-hexyl-phenyl substituents shows slightly redshifted absorption and more ordered molecular packing in film state.These inherent advantages of p-BTTCTT-6F result in a higher PCE of 10.39%for the OSC device with p-BTTCTT-6F as acceptor and PM6 as donor,which is improved over that(9.98%)of the corresponding device with m-BTTCTT-6F as acceptor.3.Compared with typical sp3-hybridized C-bridged fused-ring cores,sp2-hybridized N-bridged fused-ring cores could greatly improve the electron-donating capability and facilitate intermolecular ?-stacking.Hence,we adopted the strategy of the modification of bridging moieties to develop a novel star-shaped fused-ring core,benzotri(dithienopyrrole)(BTDTP),and star-shaped FREAs based on BTDTP also exhibit narrow optical bandgaps.Firstly,two star-shaped FREAs,BTDTP-C16-6F and BTDTP-C8C9-6F were designed and synthesized,in which n-cetyl and 1-octylnonyl side chains were introduced on the nitrogen atoms of BTDTP,respectively.BTDTP-C16-6F tends to adopt an edge-on orientation and has low solubility,whereas BTDTP-C8C9-6F contains both face-on and edge-on orientations and exhibit good solubility mainly due to the steric hindrance of the bulky branched chains on the nitrogen atoms.When blending the polymer donor PM6,the BTDTP-C8C9-6F blend film shows less molecular recombination and greatly enhanced and more balanced electron-hole charge transport.Consequently,the PM6:BTDTP-C8C9-6F based OSC device shows a high PCE of 11.42%,higher than the device based on PM6:BTDTP-C16-6F(6.50%).Secondly,we introduced chlorinated IC(IC-2Cl)as terminal group into BTDTP with 1-octylnonyl side chains,and synthesized BTDTP-C8C9-6Cl.Relative to BTDTP-C8C9-6F,the absorption of BTDTP-C8C9-6Cl film redshifted from 691 to 712 nm with the bandgap of 1.56 eV.Paired with the polymer donor PM6,the BTDTP-C8C9-6Cl-based OSC device achieve a comparable PCE of 11.21%in comparison with BTDTP-C8C9-6F.When the polymer donor changed to D18,the BTDTP-C8C9-6Cl based device produces a higher PCE of 12.00%,which is attributed to the enhanced charge mobilities and more balanced electron-hole transport owing to higher crystallinity and more obvious fibrous network morphology in blend film.To the best of our knowledge,the PCE of 12.00%is among the highest values of OSC devices based on star-shaped FREAs.