Synthesis And Optoelectronic Properties Of Small Molecule Organic Solar Cells

Over the last ten years,research on materials design and synthesis,device engineering and physics of solution-processed small molecule organic solar cells(OSCs)have achieved great progress.Active layer is the most important part in organic solar cells.Ideal active layer morphology is a bicontinuous interpenetrating network formed by donor(D)and acceptor(A)materials blends with proper domain size and relatively high domain purity.The domain size and the relative domain purity have significant influence on exciton diffusion,exciton dissociation and charge transport.So the power conversion efficiencies(PCEs)and photovoltaic performances of OSCs largely depend on the morphology of the active layer.The morphology of the active layer is strongly related to the inherent properties of donor/acceptor materials and the film formation processing.This paper starts from the design and synthesis of small molecule donor/accepor materials,improves the optoelectronic properties by adjusting the chemical structures of the compounds,deeply investigates the relationship between structure,property and performance,thereby optimizes the active layer morphology and improves the photovoltaic performance.Firstly,we briefly introduced OSCs,including working principles,device structures and performance parameters.Afterwards,we summarized the development of active layer materials.The design strategies of small molecule materials were proposed based on that.In Chapter Two,we developed a series of A-D-A-D-A type small molecule donors named DPPID,DPPBI and DPPTBI.These molecules used diketopyrrolopyrrole as as the central acceptor unit and isoindigo as the terminal acceptor unit.The conjugation length was extended through introducing multiple alternating donor and acceptor units.The number of the donor units was increased by substitution of benzene with thiophene.As a result,the absorption was enhanced and the energy levels and crystallinity were effectively tuned.Among them,DPPTBI belongs to narrow band-gap small molecule.The organic photovoltaic device based on DPPBI achieved an optimal power conversion efficiency(PCE)of 2.36%.In Chapter Three,by changing the side chains and further tuning the skeletons,a series of narrow band-gap small molecule donors were obtained,namely DPPTTC6,DPPTTC8,IDPPTIC8 and IDPPTIEH.The optical band-gaps of these small molecules were less than 1.5 eV.The crystallinity of these small molecules all exhibited differences before and after thermal annealing,indicating that side chains cause different packing.OSCs based on DPPTTC8 achieved PCE of 2.01%.By adding PTB7-Th to the blend film of DPPTTC6:PC61BM,a ternary organic solar cell was prepared and the PCE was increased from 1.06%to 3.78%.In Chapter Four,we further extended the conjugation length by increasing the number of donor and acceptor units.We synthesized a series of D-A-D-A-D-A-D-A-D type medium-sized small molecule donors(BFD,TFD,IDTFD and IDTTFD)via C-H direct functionalization,which simplified the synthesis route and improved the synthesis yield.The absorption ability was enhanced and the HOMO energy level became deeper through improving the electron donating ability of the central core and enhancing the interaction between donor and acceptor units.An optimal PCE of 7.06%was obtained for OSCs based on IDTTFD,which is the highest.In Chapter Five,a novel two-dimensional conjugation donor unit MIDT was obtained by introducing thioether conjugated side chains into the central core of IDTTFD.A series of small-molecule donors(DEIR,DEIN,EHIR and EHIN)were synthesized by coupling with different terminal acceptor units.The optoelectronic properties were systematically investigated and photovoltaic performances were characterized.As a result,an open circuit voltage up to 1.0 V was achieved with low energy loss.This demonstrated that MIDT has great potential as a novel two-dimensional conjugation donor unit.In Chapter Six,a novel kind of small molecule acceptor(A1 and A2)as hybrids of fullerene and non-fullerene acceptors containing C60 and a linear conjugated backbone was obtained.They possess the absorption characteristics of both fullerenes and non-fullerenes,which can effectively broaden the absorption range.Compared with A1,A2 has stronger absorption,higher LUMO levels and higher electron mobility.OSCs were prepared by blending it with a polymer donor,resulting in an optimal PCE of 4.52%.This may pave a new way for small molecule acceptor.In Chapter Seven,two three-dimensional twisting small molecule isomers were designed and synthesized,namely m-PIB and p-PIB.The perylene diimide units were coupled into the central core through different linking sites.The molecular twisting was different,which lead to different intermolecular aggregation and intramolecular planarity.The PCE of 4.27%and 5.95%were achieved for OSCs based on m-PIB and p-PIB without any post-treatments,respectively.This simplifies the fabrication of devices and is beneficial for practical application.