Design Of Asymmetric Dithieno[3,2-b:2',3'-d]pyrrol Fused-Ring Electron Acceptors For Highly-Efficient Organic Solar Cells

Organic solar cells(OSCs)have made remarkable progress under the concerted efforts of new material development and device technology.The innovation of non-fullerene electron acceptors has opened a new avenue for OSCs.Especially,acceptor-donor-acceptor(A-D-A)structured fused-ring electron acceptors(FREAs)have become the major driving force in promoting their power conversion efficiencies(PCEs).In the early years,rapid development of FREAs was initiated by taking advantage of various molecule engineering on fused-ring electron-donating cores,electron-withdrawing end groups and side chains.In recent two years,FREAs design is facing some bottlenecks,with the performance of most new A-D-A FREAs losing to that of the state-of-the-art Y6-series ones.The success of Y6 has also turned the attention of researchers from relatively simple material design to more careful correlation study on the molecular structures and their optoelectronic properties.This thesis focuses on molecular design of new asymmetric FREAs for high-efficient organic photovoltaic devices.The strategies include extension of conjugated backbone for conformation control,coordination side chain engineering on donating cores and accepting end groups,and modulating the electrostatic potential molecular surface of the acceptors.The correlation between acceptor structure and their optoelectronic properties is thoroughly investigated,which elaborates as follows.Firstly,on the basis of excellent electron-donating unit as called dithieno[3,2-b:2',3'-d]pyrrol(DTP),we constructed three DTP fused-ring cores with different conjugation lengths to screen the optimal DTP-fused donating core for developing high-efficient FREAs for OSCs.By adopting the classic indacene(IND)like structure,three asymmetric FREAs(ca.IPT-2F,IPTT-2F and IPTTT-2F)have been developed by fusing DTP unit with thiophene,thieno[3,2-b]thiophene and dithieno[3,2-b:2',3'-d]thiophene,respectively.The molecular conformation of these FREAs can be subtly tuned by extending the donating cores with thiophene rings,where IPT-2F and IPTTT-2F adopted S-shape molecular conformation while IPTT-2F presented as C-shape one.Indeed,the blends of polymer donor PBDB-T with either IPT-2F or IPTTT-2F reveal less trap-assisted recombination and better microphase separation compared with that with IPTT-2F.Decent power conversion efficiency(PCE)values of 14%and 12.3%were achieved for IPT-2F-and IPTTT-2F-based OSCs,respectively.Our results indicate the S-shaped conformation of asymmetric NFAs is advantageous to the morphology control in active layer for more efficient OSCs.Next,based on excellent core IPT from IPT-2F,we further investigated the synergistic effect of double side chain engineering on the design of asymmetric FREAs,i.e.,N-alkylation of DTP block and the halognation on the electron-withdrawing end groups.Linear octyl and branched 2-butyl-1-octyl chain(BO)were adopted while fluorination and chlorination used for end-capped groups,respectively.Four FREAs have thus been developed including IPT-4F,IPTBO-4F,IPT-4Cl and IPTBO-4Cl.The inherent features of the larger Cl atom and longer C–Cl bond markedly extend the backbone stacking area and thus enhance molecular aggregation,while bulky BO chains exert stronger steric-shielding effect on backbone stacking.Consequently,IPTBO-4Cl shows properly weakened intermolecular interaction for balanced molecular aggregation.IPTBO-4Cl when blended with a PM6 polymer donor delivers a highest power conversion efficiency(PCE)of 15%.Expectedly,fluorinated IPT-4F bearing shorter C8chains outputs a good PCE nearing 15%.By contrast,IPT-4Cl and IPTBO-4F with either excessively strong or weak aggregation result in relatively low photovoltaic performance.Our results indicate that both side chain and the terminal group engineering are equally important and need combinational consideration in FREA design.Furthermore,we conducted molecular engineering to modulate the electronegativity on IPT core to systematically reveal its impact on the optical absorption,energy levels,molecular packing and photovoltaic performance of the resulted FREAs.A series of asymmetric FREAs(ca.IPTO-4F,IPTEH-4F,IPT-4F and IPTCl-4F)are designed by substituting IPT core with side-chains including 2-ethylhexyloxy,2-ethylhexyl,hydrogen and chlorine atoms.Enhancing side-chain's electron-withdrawing capability was found to narrow the optical bandgap but dowshift the lowest unoccupied molecular orbital(LUMO)level of the FREAs,yielding a trade-off between J_SC and V_OC in OSCs.Three modified FREAs are found to deliver inferior photovoltaic performance compared with IPT-4F.It should be noted direct side chain engineering on IPT core can effectively adjust their light absorption and molecular energy level but may render negative effect on photovoltaic performance.Finally,we conducted side chain engineering on IPT core to regulate both electrostic potential molecular surface and the conformation of four phenyl groups.The spatial conformation and interaction of side chains were analyzed to understand the FREAs'properties and the resultant photovoltaic performance.Four para-hexyphenyl groups in IPTBO-4Cl were changed to meta-hexyloxy phenyl groups to generate IMO for further modification.Three new FREAs(ca.IMOF,IMOCl,and IMOM)were thus developed by introducing either F or Cl and methyl onto meta-alkoxyphenyl groups of IMO.Affected by steric hindrance of the modifying groups,three FREAs exhibited a pie-shaped spatial arrangement as the dominant molecular conformation,while a cross-shaped form is found for IMO.The pie-shaped conformation can help the FREAs to build compact and ordered molecular stacking.In particular,the electronegative F atoms in IMOF can promote a F…F interaction with polymer donor PM6.Excitingly,the OSCs be blending IMOF with PM6 exhibted low sensitivity to device treatment processing including additives use and thermal annealing.The resultant as-cast device can achieve up to 13.9%PCE without any device post-treatment.Our study shows that the intermolecular interaction and side-chain conformational locking induced by F atom are beneficial to optimize the molecular aggregation of FREAs and thus phase separation morphology in the active layers.Our study provides good guidance in the design of highly-efficient FREAs for future OSC applications.