The Synthesis Of Conjugated Small-Molecular Donors Containing Difluorenylamine As End Groups For Organic Solar Cells

Nowadays, utilizing fossil fuels such as coal, oil, and gas causes a mass of environmental problems, and furthermore their reserves are running out. The need to develop renewable energy sources has become urgent. Organic solar cells(OSCs) gradually become a research focus because of the potential for lightweight, low cost and flexible large-area devices. Particularly, significant progresses have been made in the synthesis and processing of small molecules donors in OSCs due to their potentially high purity, well-defined and tunable molecular structure.In this context, a series of small molecules with dithienyldiketopyrrolopyrrole(DT-DPP) as the core and difluorenylamines as electron-donating units are reported. The resulting optical, energy levels, thermal, mobility and photovoltaic properties were investigated.Firstly, two molecules DT-DPP(FBN)2 and DT-DPP(FBN-A)2 using DT-DPP as the core and difluorenylphenylamine as electron donors were synthesized. The structure difference between them is that DT-DPP(FBN)2 linked to DT-DPP with single bond while DT-DPP(FBN-A)2 with acetylene. There are no apparent melting peaks or crystallization peaks observed in DSC curves between 30 oC and 250 oC, indicating both of them are amorphous. In comparison with DT-DPP(FBN)2, DT-DPP(FBN-A)2 with acetylene as linkage showed a deeper HOMO level. DT-DPP(FBN)2 showed a power conversion efficiency(PCE) of 1.0%(Voc = 0.76 V, Jsc = 3.71 m A cm-2, FF = 35.47) in bulk heterojunction(BHJ) solar cells(ITO/PEDOT:PSS/donor:PC61BM/PFN/Al) while DT-DPP(FBN-A)2 provided a PCE of 1.71%(Voc = 0.92 V, Jsc = 5.08 m A cm-2, FF = 36.52).Then, difluorenylnaphthylamine was introduced as a new end group. Compared with DT-DPP(FBN-A)2, the molecule DT-DPP(FNN-A)2 showed a higher Tg and hole mobility. Intersetingly DT-DPP(FNN-A)2 provided a PCE up to 2.58%(Voc = 0.96 V, Jsc = 6.06 m A cm-2, FF = 44.32) in the device structure(ITO/PEDOT:PSS/donor:PC61BM/PFN/Al).At last, DT-DPP(FFBN-A)2 and DT-DPP(FFNN-A)2 were synthesized. Introducing fluorine atoms in such materials produced only a slight impact in the oxidation potential and absorption spectrum. DT-DPP(FFBN-A)2 and DT-DPP(FFNN-A)2 showed a PCE of ~1.6% in the OPV devices(ITO/PEDOT:PSS/donor:PC61BM/PFN/Al).Further studying the photovoltaic responses of the above materials are ongoing.