Effectively Modulating The Structures And Photovoltaic Performance Of DPP-based Molecular Donors By Introducing Ethylenic And Acetylenic Linkages

Recent years,encouraging progress has been witnessed in the organic solar cells(OSCs)research,and the power conversion efficiency(PCE)has already broken through 12%.The great improvement of PCE is mainly attributed to the photoactive materials with high photovoltaic performance,including electron donors and electron acceptors,and the modulation of active layer morphology.Compared with polymer electron donors,small molecular donors possess various advantages,such as,defined chemical structures and molecular weight,no batch-batch variations,and easier preparation and purification,which make small molecular donors more and more attractive to the researchers.Among the small molecular donors,diketopyrrolopyrrole(DPP)based small molecular donors has been one of the main part,mostly due to their simple structures,facile preparation and high performance.In this thesis,we emphasize on the environmanetally friendly preparation of small molecular donors.Additionally,we discover the prepared donors achieve > 5% of power convertion efficiency in simple device structure: ITO/PEDOT:PSS/Active layer/Al,without solvent additives or solvent vapor annealing.Firstly,we designed two building blocks: E-thienyl-vinyl-thiophene(TVT)and Ebenzofuryl-vinyl-thiophene(BFuVT),and then used the building blocks as electron donating side-groups to deign small molecular donors DPP(TVT)2 and DPP(BFuVT)2.After carefully studying the absorbance,CV and thermal property,we empirically concluded that introducing ethylenic linkages could reduce the steric hindrance between the aromatic units beside the ethylenic linkage and could form more planar structures and thus improve the solid state stacking.Because of the improved planar structures the absoance was obviously enhanced and bathochromic-shifted,and moreover,the hole mobilities were dramatically improved.Finally,OSCs devices based on DPP(TVT)2 and DPP(BFuVT)2 provided PCE of 3.01% and 4.06%,without solvent additives and solvent vapor annealing.Secondly,considering the intrinsic disadvantages of ethylenic linkage based materials,e.g.i)difficult separation of cis-isomer and trans-isomers produced during Wittig-Horner reactions;ii)need of using lithiation reactions at extremely low temperature and organotin reagent,which is highly dangerous and environmentally unfriendly,we further designed a series of acetylenic linkages based small molecular donors.Introducing acetylenic avoids using lithiation reactions and organotin reagents.Additionally,due to the sp hybridization of carbon atom in acetylenic linkage with higher s orbital electron ratio than that of sp2 hybridized carbon atom in ethylenic linkage,acetylenic linkage based small molecular donors possess deeper HOMO level and consequently provide high Voc in OSCs devices.Finally,acetylenic linkage based small molecular donor TAAnAPhA-C12 DPP and TAAnANaA-C12 DPP gave PCEs of 5.07% and 5.36%,respectively.Moreover,due to improved thermal stability,the OSCs based on TAAnANa A-C12 DPP kept a PCE of 5.15% after thermal annealing at 80 oC.Thirdly,we simplified and designed three molecules with TVT unit and anthracene unit as electron donating group,containing ethylenic and acetylenic linkages,and compared with two reported anthracene and DPP based molecules.Due to simplified molecular structures,relevant small molecular donors can be prepared through less steps and combined with near 90% overall yields.Besides,introducing ethylenic and acetylenic linkages dramatically reduced the steric hindrance and faciliatate almost fully coplanar configurations,and thus enhanced and bathochromic-shifted the absorbance,as well as improved the solid state stacking and improved the hole mobilities.Finally,anthracene and DPP based small molecular donor An(C8DPP)2 provided a PCE of 5.26% in simple device structure,without any uses of sovent additives and solvent vapor annealing.As an conclusion,introducing ethylenic linkages into molecules can effectively reduce the steric hindrance and inprove the molecular planarity,but combining with intrinsic drawbacks: i)cis-isomers always exist in the reaction products,which increases the purification difficulty and reduces the general yields;ii)need to use lithiation reactions at extremely low temperature and organotin reagents in the preparing process.Additionally,introducing acetylenic linkages can not only reduce the band-gap by releasing the steric hindrance,but also deepen the HOMO level and facilitate the acetylenic linkages based molecules to provide higher Voc in OSCs.It’s also worth to note that,all the OSCs device using ethylenic and acetylenic linkages based molecular donors can avoid the use of solvent additives and solvent vapor annealing and reach high photovoltaic performance.