Design,Synthesis Of Small Molecule Non-Fullerene Acceptor Materials For Efficient Organic Solar Cells

Organic solar cells(OSCs)are devices that convert light energy directly into electrical energy.They are one of the most promising technologies for humans to use solar energy.Therefore,the development of active layer materials,reduction of raw material costs,simplification of synthesis steps,and high stability of the device are the key to commercialization.Based on the existing highly efficient and stable acceptor structures,this paper uses electron-rich or electron-withdrawing groups as end groups to construct highly efficient and stable active layer acceptor molecules.We develop a series of highly efficient and stable solar cells by changing the end groups,improving the type of chemical bonds in the intramolecular groups,and effective side chain engineering,combined with its high efficiency and low cost,and good film formation.Based on these new materials,this paper further optimizes the molecular structure through basic material characterization and preparation of organic solar cell devices,and enhances device performance,energy level regulation and other methods.Mainly the paper can be divided into the following three parts:(1)The star-shaped small-molecule acceptors were designed and synthesized based on indacenodithiophene(IDT).The benzotrithiophene(BTT)was used as the core,and the A-D-A structure with electron-trapping end groups was constructed to explore its photoelectric properties.The rigid and coplanar structure of BTT and the stronger electron donating ability are beneficial for enhancing absorption and charge transfer,increasing the open circuit voltage and absorbance.Fluorine atom shows strong electronegativity,thus the terminal group has a stronger electronwithdrawing ability after the introduction of the fluorine atoms,which leads to promote the intermolecular interaction.Meanwhile,the introduction of fluorine atoms optimizes the molecular stacking,thereby increasing its short-circuit current and fill factor(FF).We compared these star molecules with the linear molecule IDC8 IC and found that star molecules have higher membrane thermal stability than linear molecules due to their amorphous morphology.Finally,such receptors were applied to OSCs,and it was found that BTTIDC8 IC had power conversion efficiency(PCE)of 6.24%,and fluorinated end-group BTTIDC8IC-F had PCE of 8.24%.(2)A series of small molecule acceptors were synthesized based on perylene diimide(PDI).We introduced Schiff base(-CH=N-)structure into PDI-based receptor molecules for the first time(PDINCB and 2PDINCB).After the introduction of the-CH=N-,PDICCB,which has a carboncarbon double bond(-CH=CH-),exhibits a broadened spectrum and a higher PCE.The reason is mainly due to carbon atoms and nitrogen atoms are different in electronegativity,which improves the short-circuit current and improves the device performance.Further,we synthesize PDICCB,which is catalyzed by the noble metal palladium.PDICCB is high in synthesis cost and only 70% yield,while the synthesis of PDINCB and 2PDINCB is simple,low cost,no precious metal catalysis,and the solvent is ethanol,and the yield is over 90%.This strategy provides an effective strategy for future small molecule non-fullerene receptors.(3)The small molecule non-fullerene acceptor materials were designed based on fluorene.Fluorene as a low-cost electron-rich molecule has been reported on its excellent performance in OSCs.Previously,molecules based on thiophene units attached to both ends of ruthenium have been reported,which are thermally annealed with chloroform solvent.More than 7% of PCE was obtained after treatment and when additives were required.We use hydrazine as the basic unit,and the two ends are connected with a thiophene unit,a 3-position alkylthiophene unit,a 4-position alkylthiophene unit,and the other ends are terminated by 3-(dicyanomethylidene)fluorenone(IC).First,the alkyl side chain engineering can improve the molecular solubility to obtain good film-forming properties.Secondly,the alkyl side chains at different positions can also adjust the optical absorption,molecular energy level and morphology of the photovoltaic material.The PCE of the 3-position alkylthiophene unit was as high as 8.91% with chlorobenzene as the solvent without any post-treatment.Our alkyl side chain engineering has great significance for improving the performance of the device.