| In the context of green and low-carbon development characterized by energy structural adjustments and reduced emissions,Organic solar cells（OSCs）are considered one of the most promising cutting-edge technologies in the field of new energy,owing to their excellent advantages such as flexibility and light weight,easy availability of raw materials,low cost,easily controlled performance,simple process,and energy savings.In recent years,the rapid development of high-performance non-fullerene small molecule acceptor materials has accelerated the continual and remarkable improvement of photovoltaic performance.At the time,the power conversion efficiency of binary organic solar cells based on polymers as donors and non-fullerene small molecules as acceptors had surpassed 18%,breaking the commercial application threshold and ushering in the era of OSCs.The development of novel D-π-A polymer donor materials is critical in OSCs.Among these,the advancement of new electron-donating and electron-drawing units is an effective method for improving the performance of polymer donor materials.In orther words,the use of functional groups and single atoms to tailor the properties of polymers is an expense and practical strategy.Introducing fluorine or chlorine atoms into the D-π-A configuration polymer donor material can modify the polymer donor material’s electron-donating or electron-deficient unit,which has obvious advantages for regulating the spatial configuration,energy level structure,spectrum absorption,and carrier properties of the entire molecule.However,according to the reported literature,there are few investigations on modifying the molecular properties of polymers by tailoring theπ-bridge units of D-π-A polymers.The idea of improving polymer characteristics by modificating a simpleπ-bridge with halogen aroused our interest.Therefore,in this research,a series of fluorinated and chlorinated D-π-A polymer donor materials were made by adding halogen atoms onto the bridge units of classical polymer molecules.These materials have been systematically examined for their molecular structure,photophysical properties,electrochemical behavior,photovoltaic performance,charge transport characteristics,and active layer morphology,among other characteristics.The primary research and results are as follows:1)A series of polymers J52F,J52ClF and J52FF based on benzodithiophene as electron donating unit and benzotriazole as electron accepting unit were synthesized by introducing fluorine atom（F）into theπbridge unit in the polymer backbone.The effect of introducing fluorine atoms with strong electronegativity and a small atomic radius on polymer properties was thoroughly investigated.The results reveal that inserting fluorine atoms lowers the HOMO energy level of the polymer molecule while increasing the open circuit voltage of the device.The intense intra/intermolecular interaction through non-covalent bond contacts such as F-H,F-π,and F-S promotes theπ-πstacking of the polymers and the blend film between the polymers and small molecule acceptors.Also,the strongπ-πstacking property broadens the spectral absorption range of the active layer and enhances the absorption intensity.Furthermore,introducing fluorine atoms to theπbridge units improves the flatness of the polymer backbone and boosts carrier mobility.The hole and electron mobilities of the three fluorinated polymers are 2.93×10-4/2.63×10-4,4.2×10-4/4.09×10-4,4.87×10-4/4.80×10-4cm2V-1s-1,respectively.The devices were prepared by blending with Y6 have excellent photovoltaic performance,among which,the device based on J52FF:Y6 exhibited a power conversion efficiency of 12.89%with a VOC of 0.80V,a JSC of 22.87 mA cm-2,and a FF of 70.44%.When compared to J52 without any fluorine on theπ-bridge unit（PCE=5.6%）,the photovoltaic performance is greatly increased.This study shows that fluorination of conjugated polymer backbones is an efficient method for improving the performance of polymer solar cells.2)A series of polymers PBDB-TCl,PBDB-TClCl and PBDB-TClF based on benzodithiophene as the donor unit and benzodithiophene-4,8-dione as the acceptor unit were synthesized via introducing chlorine atoms into theπbridge unit in the polymer backbone.The effect of chlorine atom substitution with larger van der Waals radii on polymer properties was systematically studied.The results demonstrated that three polymers have deeper HOMO/LUMO energy levels,respectively,Thanks to chlorine atoms with 3d orbitals,it can endow the structure of D-alt-A realize the complete separation of electron clouds,and promote the effective transport of charges.The photovoltaic devices blended with IT-4F have better performance.Among them,PBDB-TClF based on PBDB-TClF with better morphology and suitable aggregation ability showed a higher PCE of 7.62%. |
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