Synthesis And Photovoltaic Properties Of Organic Small Molecule Electron Donor Materials Based On Diketopyrrolopyrrole Units

This paper systematically expounds the development of organic solar cells,and emphatically introduces the application of intramolecular non-covalent bond action and diketopyrrolopyrrole(DPP)acceptor unit in the design of small molecule photovoltaic materials.In order to solve the shortcomings of OSCs such as the current open circuit voltage,carrier mobility and low energy conversion efficiency of small molecule photovoltaic materials based on DPP acceptor units.The non-covalent interactions between intermolecular and intermolecular were obtained by introducing strong electronegativity atoms into donor materials.It effectively optimizes the molecular energy level,promotes the ?-? stacking between the molecules and self-assembly performance,and ultimately improves the short-circuit current,the open circuit voltage,the carrier mobility and the energy conversion efficiency of the photovoltaic devices based on the donor materials.Four organic small molecule donor materials were designed and synthesized.The structures of these small molecules were characterized by 1H-NMR,13C NMR and Time-of-flight mass(MS)spectra.The photophysical,thermal stability,electrochemical and photovoltaic properties of the small molecules had been investigated by UV-vis spectra,thermogravimetric analysis(TGA)and cyclic voltammetry(CV).Furthermore,the photovoltaic performances of organic solar cells based on these small molecules were fabricated.The key researches are listed as follows:1?A class of A-D-A type conjugated small molecules based on DPP acceptor units,namely DPh(DPP)2 and DFDT(DPP)2,are designed and synthesized.The fluorinated benzene(DFPh)and fluorinated bithiophene(DFDT)were used as the central donor cores and the DPP units as acceptor uints,respectively.Meantime,the thermal stability,molecular crystallinity,photophysical,electrochemical,molecular planarity and photovoltaic properties of fluorinated small molecules were systematically studied.It is found that the introduction of F atoms in the core of the A-D-A type small molecule can effectively regulate the molecular energy level,improve the accumulation between molecules and the self-assembly of the molecules,and can effectively improve the open circuit voltage,short circuit current and filling factor of the devices,and thus obtain better photoelectric conversion efficiency.DFPh(DPP)2,with weak electron donor core,shows a lower highest unoccupied orbital(HOMO)energy level,and thus a higher open circuit voltage(0.97 V)is obtained,while DFDT(DPP)2 with a stronger electron giving longer conjugate unit shows a broader UV visible absorption and a better ?-? stacking.After optimization,the DFDT(DPP)2/PCB7iM-based device presented a higher PCE of 7.87%with Voc of 0.86 V,Jsc of 12.6 mAcm-2,and FF of 72.6%.Notably,the PCE of 7.87%is one of the highest PCEs reported for the A-D-A type SMs-based on DPP as terminal acceptor groups.These primary studies indicated that the incorporating fluorine atom on rigidity skeleton central units is an efficient approach to improve photovoltaic performance of A-D-A type SMs.2.Two narrow band-gap small molecules with D(A-Ar)2 framework,namely DMPh(DPP-Py)2 and DFPh(DPP-Py)2,were designed and synthesized for high-performance small molecule organic solar cells(SMOSC),in which the 1,4-dimethoxybenzene(DMPh)and 1,4-difluorobenzene(DFPh)were employed as rigid donor cores,respectively,and the pyrenere(Py)unit is selected as terminal-capping groups on an electron-deficient diketopyrrolopyrrole(DPP)-based linear backbone.The impacts of the fluorine-sulfur(F…S)atoms and oxygen-sulfur(O…S)atoms noncovalent interaction on their absorption spectra,molecular energy levels,morphological properties,hole mobilities and photovoltaic properties were investigated thoroughly.The fluorinated DFPh(DPP-Py)2 possess a relatively lower-lying HOMO energy level,better miscibility of the blend with PC71BM,as well as higher mobility in comparison with those of the methoxyled DMPh(DPP-Py)2.As a consequence,the OSCs devices based on DMPh(DPP-Py)2 and DFPh(DPP-Py)2 exhibited PCEs of 5.47%and 7.54%,respectively.Obviously,the device based on DFPh(DPP-Py)2 presented a better performance,which should be ascribed to the improved simultaneously Vo,of 0.77 V,Jsc of 15.3 mA cm-2,and FF of 64%.The results indicated that the choice of the fluorination designation on the molecular backbone is an effective approach to develop D(A-Ar)2 type small molecule donors for highly efficient solar cell applications.The result of photovoltaic performances of organic solar cells based on these small molecules show that the molecular design is crucial to achieve organic small molecular photovoltaic materials with good stability and high photovoltaic efficiency.This study to explore the DPP based structural unit of the molecular structure of small organic molecules on the material of the photovoltaic performance,to explore the influence of the way to enhance the efficiency of the photovoltaic(PV)of organic small molecule materials and the new type of high-efficiency organic small molecular photovoltaic material has positive practical significance.