Synthesis And Properties Of The Polymers Containing Benzimidazolone Or Dithienobenzoselenodiazole Derivatives

Polymer solar cells(PSCs),combined with a high-performance non-fullerene small molecular electron acceptor,represented by IDTIC and Y6,and a wide band-gap conjugated polymer electron donor,have achieved rapid development.Polymer electron donor materials are usually constructed by alternating copolymerization of electron-donating units(D)and electron-withdrawing units(A).Among them,the A unit with proper electron-deficient property can enable a deeper HOMO(highest occupied molecule orbital)energy level,and the large and coplanar fused-ring structure can enable high hole mobility.Therefore,4,7-dibromo-1,3-bis(2-ethylhexyl)-5,6-difluoro-1,3-dihydro-benzo[d]imidazol-2-one(IO)and dithieno[3',2':3,4;2'',3'':5,6]benzo[1,2-c][1,2,5]selenodiazole(DTBSe)have been designed and synthesized,and the corresponding conjugated polymers have been synthesized by using these A units and used as the polymeric donor materials in PSCs.The relationship between the chemical structure and optical properties,electrochemical properties and photovoltaic properties of these polymer donor materials were studied in more detail.1.The polymer PBIO2F was designed and synthesized with IO derivative as the A unit,and 4,8-bis(5-(2-ethylhexyl)-4-fluorothiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene(FBDTT)as D unit.It is found that the HOMO energy level of PBIO2F is as low as-5.94 e V,which could be attributed that the steric hindrance of the IO unit causes a distorted backbone of the polymer.Therefore,the hole mobility of the device prepared with the PBIO2F:Y6 blend film is only 8.77×10^-5cm²V^-1s^-1.Moreover,too low HOMO energy level of the polymeric donor material results in impossible hole transfer from the acceptor to the donor,which also seriously depresses photovoltaic performance of the PSCs based on the PBIO2F:Y6 blend film.As a result,its photoelectric conversion efficiency(PCE)is only 0.08%.Subsequently,a terpolymer,named as TB,was designed and synthesized with the IO derivative as the second A unit(A₂),and the benzodithiophene-dione derivative(BDD)as the main A unit(A₁).The HOMO energy level of the terpolymer TB is elevated to-5.59 e V which well matches the HOMO energy level of Y6,and TB also possesses better complementary absorption spectrum with Y6,which all contribute to high PCE values of PSCs.The PCE value of the PSCs with TB as the electron donor and Y6 as the electron acceptor reached 11.45%,with a J_scvalue(short-circuit current density)of 25.20 m A cm^-2,a V_oc(open-circuit voltage)value of 0.80 V,and a FF(fill factor)value of 56.78%.The results indicate that it is efficient strategy to obtain efficient polymeric donors by ternary copolymerization as well as selecting appropriate D units and A units.2.A new fused-ring unit,named as dithieno[3',2':3,4;2'',3'':5,6]benzo[1,2-c][1,2,5]selenodiazole(DTBSe),was designed and synthesized,which was used as A unit and copolymerized with the BDTT and fluorine-substituted BDTT derivatives to obtain the polymers PSe and PFSe with different molecular weights respectively.It is found that the polymers PFSe-L and PFSe-H containing fluorine atoms have deeper energy levels than PSe,better nanophase separation,more effective exciton separation,higher and balanced hole and electron mobility compared with PSe without fluorine atoms.Therefore,the PSC based PSe:Y6 possesses the lowest PCE of 8.00%with a V_ocof 0.77 V,a J_scof 20.85 m A cm^-2and a FF of 49.85%.In addition,PFSe-H with high molecular weight possesses stronger and more orderly intermolecular?-?stacking compared with PFSe-L with low molecular weight,which is conducive to obtaining higher J_scand FF.Therefore,the PSC based on PFSe-H:Y6 shows the highest PCE of 13.45%with a V_ocof 0.83 V,a J_scof 25.32 m A cm^-2and a FF of64.00%,but the PSC based on PFSe-L:Y6 shows lower PCE of 9.91%with a V_ocof0.86 V,a J_scof 22.21 m A cm^-2,and a FF of 51.89%.This work provides a new electron-withdrawing unit DTBSe,which shows a potential to construct high-performance polymer donors.The results also indicate that fluorination strategy and high molecular weight are key factors to determine photovoltaic performance of polymeric donor materials.