Study On The Photovoltaic Performance Of Polymer Donors Based On Difluorobenzoxadiazole And Its Derivatives

Recently,acceptor-donor-acceptor type non-fullerene acceptor materials have developed rapidly.Designing and synthesizing donor polymers that match this type of non-fullerene acceptor are of great significance to further promote the development of organic photovoltaics.Polymers with difluorobenzothiadiazole(ff BT)or difluorobenzoxadiazole(ff BX)as acceptor units and oligothiophene as donor units have higher crystallinity and carrier mobility.In addition,these polymers show a wide optical bandgap(1.60?1.65 e V),and the active layer blended with a narrow bandgap non-fullerene acceptor can achieve a wide spectral response.The aggregation structures of this type of polymers can be tuned by modifying side chains on?-position of thiophene units.Compared with alkyl side chains,alkylthiophene as side chain can promote intermolecular?-?stacking.Given the above analysis,in this paper,we studied the aggregation structure of polymers based on ff BT or ff BX acceptor units and oligothiophene donor units with alkylthiophene as the side chain in solution and film,and film morphology and photovoltaic performance of the blend film based on these polymers and non-fullerene acceptor.The main results are as follows:(1)The conjugated polymer Pff BT-2T with ff BT as donor unit and oligothiophene containing alkylthiophene side chain as acceptor unit was blended with the non-fullerene acceptor ITIC(1.59 e V),IT-4F(1.50 e V),Y6(1.33 e V)and F8IC(1.26 e V)to prepare active layers.Compared with Pff BT-2T:PC₇₁BM system(photo-current response:300?760 nm),all of the Pff BT-2T:nonfullerene systems exhibited broader photo-current response.Among these blend films,Pff BT-2T:F8IC blend system presented the widest photo-current response extending to 976 nm.Owing to the wide photo-current response and the proper morphology of the active layer,Pff BT-2T:F8IC based photovoltaic device achieved a short-circuit current density reaching 20.51 m A cm^-2,which was much higher than that of Pff BT-2T:PC₇₁BM based device.However,Pff BT-2T:F8IC based device showed a low open-circuit voltage(0.66 V).Thereby,the power conversion efficiency(PCE)of Pff BT-2T:F8IC(8.29%)was only slightly higher that of Pff BT-2T:PC₇₁BM(7.91%).(2)Conjugated polymers Pff BX-2T and Pff BX-TT with alkylthiophene as the side chain,ff BX as the acceptor unit and oligothiophene as the donor unit,were synthesized.Both polymers exhibited temperature-dependent aggregation behavior in solution.Pff BX-2T and Pff BX-TT possessed lower highest occupied molecular orbital(HOMO)energy levels in comparison to that of Pff BT-2T.Therefore,the photovoltaic devices based on Pff BX-2T:F8IC and Pff BX-TT:F8IC obtained higher open-circuit voltages up to 0.74 V and 0.76 V,respectively.Compared to Pff BX-2T:F8IC based devices,devices based on Pff BX-TT:F8IC exhibited more efficient exciton separation and charge transport,as well as weaker charge recombination.As a result,Pff BX-TT:F8IC based device achieved a higher PCE of10.12%,while the PCE of photovoltaic devices based on Pff BX-2T:F8IC was 8.72%.