Design,Synthesis And Characterization Of A-D-A Small Molecule Donors With "Twisted" Backbones

In all small-molecule organic solar cells（ASM-OSCs）,the crystallization of small molecule donor/acceptor is a main driving force for microphase separation of the ASM blends.Therefore,in this thesis,we focus on crystallization behavior manipulation of small molecule donors and propose the molecular design strategy:“twisting the conjugated backbones to slow down the crystallization rate of the molecules”.A series of acceptor-donor-acceptor（A-D-A）small molecule donors were synthesized with fused ring aromatics,such as benzo[1,2-b:4,5-b’]dithiophene（BDT）and thiopheno[3,2-b]thiophene（TT）,as building blocks to enable the strong intramolecular interactions in solid state.Additionally,the different substituents were introduced to theβ-positions of the central TT units for adjusting the backbone twisting,and thereby the molecular crystallization characteristics.The relationships between molecular structures,crystallization characteristics,morphology of blend films and photovoltaic properties were studied in detail by X-ray diffraction,atomic force microscopy,transmission electron microscopy and OSC devices,and high-performance ASM-OSCs were also fabricated.The main results are as follows:1.Four small molecule donors,i.e.,DRTT,DRTT-OR,DRTT-R and DRTT-T with unsubstituted TT and alkoxy-,alkyl-and alkylthiophene-substituted TT as the central units were designed and synthesized.Compared to DRTT and DRTT-OR,the molecules（DRTT-R and DRTT-T）with more bulky side chains adopt an“twisted”geometry,and their crystallinity in films could be controlled by regulating annealing conditions.As a result,DRTT-R and DRTT-T based blend films（acceptor:F-2Cl）showed appropriate morphology after post-annealing,and the higher power conversion efficiencies（PCEs）of 9.37%and 10.45%were achieved for DRTT-R and DRTT-T based ASM-OSCs,respectively.2.On the basis of DRTT-R and DRTT-T,two new small molecule donors named DRTT-2T and DRTT-TT with alkyldithiophene and alkylthienothiophene as the substitutes on central TT units,respectively,were designed and synthesized to extend the conjugation of side chains.The crystallization onset temperatures(T_c,onset)of the donors and acceptor in films were characterized and a strong correlation between the T_c,onsets of active layer materials and the morphology evolution of the blend films was established across these systems.Consequently,the guiding principles of the device optimization of these ASM systems were proposed.After two-step thermal annealing,DRTT-T:N3 blend films exhibited highly ordered molecular packing and appropriate phase separation,and the ASM-OSCs with the blend films as active layers obtained a high PCE of 13.21%.3.Three small molecule donors named DRTT-x Se（x=2,4 and 6）were synthesized by replacing the thiophene units in the side chains of DRTT-T partially or completely with selenophene units.Compared to DRTT-T,DRTT-x Se（x=2,4 and 6）displayed improved intermolecular interactions and more orderedπ-πpacking,and thereby enhanced hole mobilities.Thus,the ASM-OSCs based on the selenophene-containing donors all exhibited higher fill factors（FFs）,and the devices based on DRTT-6Se:N3 displayed the best PCE among the donors,which was up to 15.03%.