The Design And Synthesis Of Small-molecular Donors Containing Vinylene And Acetylene For Organic Solar Cells

Organic photovaltics(OPV) has become a research focus because of its high potential for light-weight, large-area, cost-effective and flexible energy harvesting devices. Particularly, solution-processable small molecular donors have received increasing attention due to their promising advantages, including easy tailorability, well-defined structure and thus less batch-to-batch variation.In this thesis, we described the synthesis and photovoltaic response of a series of conjugated dye molecules based on dithienyl-diketopyrrolopyrrole unit(DT-DPP) and vinylene(V) or acetylene(A) bond. The choice of vinylene and acetylene as π-linkage may produce multiple beneficial effects, for instance i) the vinylene and acetylene bond can relieve the steric constraints between neighboring aromatic units effectively, and the fluent cylinder-like π-electron density of acetylene is more insensitive to the backbone conformation. ii) The weak electron withdrawing nature of acetylene due to the sp-hybridized carbon atoms may lead to a deeper HOMO level. iii) Based on design, toxic stannyl intermediates or potentially risky lithiation reactions can be avoided during the preparation.The optical absorption, morphology, hole transport, and photovoltaic properties of these molecular donors are discussed.The present results may provide a useful guide for further development of dye molecules for photovoltaic applications.1) Firstly, we synthesized two vinylene containing building blocks, 2-(4-fluorostyryl)-thiophene(4FBVT) and 2-(1-(4-fluorophenyl)-2,2-diphenylvinyl)-thiophene(4FBDBVT). During the preparation of 4FBVT, the E-isomer was easily purified through recrystallization method. Two D-A-D type donors, DPP(4FBVT)2 and DPP(4FBDBVT)2, which bear 4FBVT and 4FBDBVT as the electron-donating(D) end groups and DT-DPP as the electron-accepting(A) core, were synthesized via regular Stille coupling reaction and C-H actived direct arylation. As an electron donor, DPP(4FBVT)2 showed a power conversion efficiency(PCE) of 2.8% in bulk heterojunction(BHJ) solar cells due to its higher planarity, far superior to DPP(4FBDBVT)2.2) Secondly, 3-alkylthiophene was connected to 9,10-anthracenyl unit, and symmetrically flanked to DT-DPP with acetylene as a π-linkage via successive Sonogashira reactions. The resulting molecular donor DT-DPP(AAn AT)2 was obtained, without involving toxic stannyl compounds or potentially risky lithiation reactions. The characterization of simple BHJ solar cells(ITO/PEDOT:PSS/DT-DPP(AAn AT)2:PC61BM/Al) provides a PCE 4.4% based on the active layer without a processing additive or solvent vapor annealing.3) Thirdly, based on the results of DT-DPP(AAn AT)2, ethynylbenzene was inserted between the DT-DPP unit and electron-donating end groups, and the alkyl chain of DPP was elongated simultaneously. We find out that the ethynylbenzene can interrupt the intramolecular charge transfer in the resulting compound C12DPP(APh AAn T)2, and that the HOMO level is lowered. When C12DPP(APh AAn T)2 was used as the electron donor to blend with PC61 BM in OPV devices, a PCE of 5.2 % was achieved without a processing additive or solvent vapor annealing, the open circuit voltage(VOC) was improved. Meanwhile, fill factor(FF) with a value of ~70% was obtained, which is highest among the reported DPP-based small-molecular donors.4) Finally, a bis-DPPs oligomer, Ph(AAn ATDPP)2 containing acetylene linkage was designed and synthesized. The bis-DPPs structure can enhance the spectral absorption, and thus the short-circuit current(JSC) was improved remarkably. A PCE 5.0% was achieved with a JSC of 13.53 m A cm-2 in BHJ solar cell.