Design And Synthesis Of Organic Small Molecules Photovoltaic Materials And Devices Characterization

Solution-processed organic solar cells(OSCs)with bulk heterojunction(BHJ)sructures have attracted much attention because of certain inherent advantages,including simple device structure,light-weight,environmental friendly,and capability to be fabricated into semitransparent and flexible devices.OSCs mainly contain electrodes,organic active layer and the interlayer between them.Organic active layer is the blend of donor and acceptor materials,which determined the power conversation efficiency(PCE)of the OSCs directly.The acceptors mainly include fullerene and non-fullerene small molecule with A-D-A structure.At the beginming of OSCs reaserch,fullerene was a widely used acceptor material,and has obtained above 11%PCE.However,the PCE of fullerene-based OSCs is hard to improve because of its narrow absorption spectrum and difficulty to tune its LUMO energy level.In recent years,a series of non-fullerene small molecule acceptors with A-D-A structure were designed and synthesized,the corresponding OSCs gave a PCE as high as 14%.As a result,these non-fullerene small molecules have repaleed traditional fullerene materials and become a new generation of acceptor materials.In this work,we made some structural modification based on these small molecules in order to obtain good device performance for future application.Comared to the polymer,the small molecule possesses well-defined molecular weight,which can show good reproducibility in device performance and provide reliable analyses of molecular properties and device performance relationship.Considering the good photovoltaic performance of small molecule acceptors,the solution-processed organic small molecular donor materials have also attracted great attention,and have otained above 11%PCE in the fullerene-based OSCs.Therefore,the all small molecule organic solar cells(SM-OSCs)were fabricated in order to take dual advantages of both organic small molecular donors and non-fullerene small molecular acceptors.However,the PCEs of these SM-OSCs are still lower than non-fullerene polymer solar cells(PSCs).Consistant efforts should be made to design and synthesize suitable organic small molecular donors to combine with the non-fullerene small molecules with high performance to construct a higher efficiency in all-small-molecule system.The main achievements of this thesis are as follows:(1)A new non-fused small molecule acceptor Ph-IC was designed and synthesized for PSCs applications,with alkoxy benzene as a central building block,IC(1,1-dicyanomethylene-3-indanone)as electron-withdrawing end groups,and thiophene as π-bridges.Ph-IC possesses good thermal stability and excellent solubility.According to the DFT calculation,the electron acceptor Ph-IC may exist O-H and S-H non-covalent interactions caused by alkoxy substituent on the central benzene and exhibit a good coplanar backbone,which is favorable for high mobility.The XRD result also indicates the crystalline nature of Ph-IC.Thus,the non-fused Ph-IC exhibits an electron mobility of 1.84×10-4 cm2V-1s-1,which is in the same order of magnitude of some fused small molecule acceptor.Furthermore,Ph-IC exhibits a narrow optical bandgap of 1.62 eV.When using Ph-IC as acceptor and a wide-bandgap polymer PBFTZ as donor,the PSC devices show a maximum PCE of 5.68%.(2)A new small molecule acceptor with(E)-5-(2-(5-(2-ethylhexyl)thiophen-2-yl)vinyl)-thiophene-2-yl(TVT)functional group asπ-bridge to link indacenodithiophene(IDT)core and 3-(dicyanomethylidene)indan-1-one(IC)terminals,named ITVT,was synthesized and applied in polymer solar cells(PSCs).Benefited from the conjugated TVT side groups,the small molecule ITVT shows a strong absorption at the short wavelength range(300-500 nm).In addition,ITVT exhibits a low optical band gap(Egopt)of 1.48 eV and a medium electron mobility of 1.85×10-4 cm2 V-1·s-1.A wide band gap polymer PBZ was chosen as donor to fabricate an inverted BHJ PSCs because of the complementary absorption and matched molecular energy levels.The as-cast PSCs yield a power conversion efficiency(PCE)of 5.84%with an open-circuit voltage(Voc)of 0.96 V and a short-circuit current density(Jsc)of 14.22 mA cm-2 under the illumination of AM 1.5 G at 100 mW cm-2.(3)A new organic small molecule donor P2TBR was synthesized and applied in the SM-OSCs.P2TBR uses non-fused ring thiophene-2,5-bis((2-ethylhexyl)oxy)benzene-thiophene(TBT)as core,2D-conjugated benzo[1,2-b:4,5-b’]dithiophene(BDT)unit as n spacers and 3-ethylrhodanines as terminal groups.Unlike other fused ring small molecules,the non-fused ring TBT unit could obtain both planar conjugated building block and good solubility according to the non-convalent interaction.P2TBR shows strong absorption,low-lying HOMO energy level,high hole mobility and good thermal stability.The optimized P2TBR:IDIC film has a face-on and edge-on co-existent texture and three dimensional(3D)charge pathways,which is far better than that of the face-on orientated blend film without any treatment.A notable power conversion efficiency(PCE)of 11.5%and a satisfactory fill factor(FF)of 70.1%is achieved.This efficiency is among the top PCEs for non-fullerene SM-OSCs.Overall,the exciting results not only demonstrate that P2TBR is a promising candidate of p-OS donor materials in SM-OSCs,but also give a guideline that the non-fused ring unit with simple structure and easy modification can be used as building blocks in high performation p-OS donors.