Design,Synthesis And Properties Of Small-Molecule Donor Materials Based On Triphenylamine-Benzothiadizole Asymmetrical System

Organic solar cells (OSCs) have attracted considerable research interest due to their prominent merits, such as low cost, light weight, and simple technology. At present, solution-processed bulk heterojunction (BHJ) organic solar cells based on small molecules have demonstrated remarkable achievements, with power conversion efficiencies (PCEs) in excess of 10%.To date, the molecular architecture of most small-molecule photovoltaic donor materials can be classified into symmetrical and asymmetrical systems. In general, the molecular configuration of the monomer of most polymer photovoltaic materials is D-A asymmetrical system. D-A type molecules own effective intramolecular charge transfer (ICT) efficiencies, but theses type of molecules generally suffer from an insufficient lighe-absorption range.Therefore, it is highly desired to design novel asymmetrical molecules by taking advantages of the versatility of structural modifications in such systems, allowing donor materials to extend the spectral responses, and thus an enhanced power conversion efficiency.In this paper, a series of D-A1-A2 type asymmetrical donor molecules are reported. In these molecules, the triphenylamine (TPA) is selected as an electron-donating moiety, 2,1,3-benzothiadizole with oxyalkyl chain (BTO) and 2,1,3-benzothiadizole with pairs of flurine atoms (BTF) as two different electron-acceoting (A1) moieties, which constitute a fundamental diplor D-π-A1 structure with benzene, styrene, ethylbenzene and 2-phenylacrylonitrile as π-linkage respectively; meanwhile in order to broden the light-absorption range, incorporting p-cyanophenyl end-capped with thiophene as another electron-accepting (A2) groups and hence afforded D-A1-A2 typed TPABTOCN and TPABTFCN series, amounting to 8 organic small-molecule photovoltaic materials.We carefully investigated the effects that the structural variation including different substituents on benzothiadizole and different π-linkage on their photophysics and optoelectronic properties by density functional theory (DFT) calculations, UV-visible absorption spectrum, cyclic voltammetry, and experiments on solution-processed bulk heterojunction photovoltaic devices. The results indicated that:1) two series of molecules introduing benzothiadizole achieved satisfactory VOC of higher than 0.9V due to their low HOMO energy levels.2) BTF series introducing dual electron-withdrawing fluorine atoms to the BT core effectively further reduce the HOMO energy levels and an slightly enhanced VOC value was obtained.3) BTO series introducing alkoxy to the BT core improve their solubility in chloroform solvent and form good film morphology in solid state, thus effectively ehanced the short-circuit current density and fill factor.4) π-bridge mainly affect their HOMO energy levels, with less impact on LUMO energy levels; materials with 2-phenylacrylonitrile own lower HOMO levels, therefore, solution-processed bulk heterjunction organic solar cells utilizing TPACNBTFCN as donor material attained higer VOC of 1.01 V; TPACNBTOCN based device through solvent vapour annealing obtained higher Jsc and VOC simultaneously, fllowed by an optimaum PCE of 3.03%, JSC=9.48 mA·cm-2, VOC= 1.00 V, FF= 0.32.