Design And Synthesis Of Organic Small-Molecule Cathode Interfacial Materials For Organic Photovoltaics

Organic photovoltaic(OPV) cells have been the subject of current research interest due to light weight, mechanical flexibility and the potential to provide a low-cost solution for harvesting solar energy. Among others, cathode interfacial material(CIM) which is placed between the active layer and cathode becomes a critical element in determining the power conversion efficiency(PCE) and durability of an OPV device. As a result, introducing a proper CIM has been shown capable of producing multiple beneficial effects, including(i) facilitating electron extraction in the presence of air-stable cathodes,(ii) blocking exciton and hole transport to the cathode, and(iii) preventing metal diffusion into the underlying active layer during thermal evaporation.Organic small-molecule based CIMs show a number of potentially attractive characteristics. They possess a well-defined chemical structure, ease of synthesis with high purity and low-temperature processability(via thermal evaporation). The thesis mainly focuses on developing a facile approach to high-performance organic small-molecule cathode interfacial materials for OPVs utilizing air-stable cathodes.1)Considering the low stability of the film morphology of bathocuproine(BCP) and 4,7-diphenyl-1,10-phenanthroline(Bphen), we report a novel CIM through an effective and yet simple combination of(2-naphthyl)diphenylphosphine oxide with a 1,10-phenanthrolinyl unit(Phen-Na DPO). The resulting CIM possesses easy synthesis and purification, a high Tg of 116 oC and attractive electron-transport properties(~10-4 cm2 V-1s-1). Ultraviolet photoemission spectroscopy studies reveal that this promising CIM can significantly lower the work function of the Ag metal as well as ITO and HOPG, and facilitate electron extraction in OPV devices. Notably, utilizing a thermally deposited Phen-Na DPO interlayer leads to 7.5% or 8.56% of PCE in the photovoltaic device(ITO/PEDOT:PSS/PTB7:PC71BM/CIM/cathode) with Ag or Al as the cathode, which even compares well with that of the Ca/Al device.2)Organic small-molecule cathode interfacial layer that exhibits resistance to weakly polar solvents(such as toluene and chlorobenzene) plays a crucial role in solution-processed inverted OPV. Introducing diphenylphosphine oxide to the zwitterions provides the new CIM with high solubility in alcohols, but a low solubility in weakly polar solvents(less than 0.1 mg m L–1). UV-vis absorption reveals that the thin film of the new CIM can resist the erosion of weakly polar solvent. The effect of this new CIM on photovoltaic performance, as well as molecular innovation needs to be further studied.In addition, we also report a facile approach to high-performance organic small-molecule electrode transport material(Bi Na-Bi DPO) for organic light-emitting diodes, which shows a slightly higher electron mobility than TPBi.