Metal Ion/Dendrimer Complexes with Tunable Work Functions in a Wide Range and Their Application as Electron- and Hole-Transport Materials of Non-Fullerene Organic Solar Cells

The work functions of electrodes can be modified by adding charge transport layers to have good energy level matching with the active materials for organic solar cells (OSCs). Usually, a certain material gives rise to one definite work function of an electrode. In this work it is demonstrated that complexes of poly(amido amine) (generation 3) (PAMAM) with Cu2+ can continuously tune the work function of indium tin oxide (ITO) in a range of 4-5 eV by controlling the ratio of Cu2+ to PAMAM. PAMAM can lower the work function of ITO from 4.60 to 4.07 eV, while Cu2+-PAMAM can increase the work function. The work function increase depends on the Cu2+-to-PAMAM molar ratio, and the work function can be up to 4.96 eV. The Cu2+ effect is ascribed the Cu2+-caused change in the dipole moment of PAMAM. Moreover, this method can be used to continuously modify the work function of other materials, including Ag, Au, poly(3,4-ethylenedioxythiophene):polystyrene sulfonate, and reduced graphene oxide. In addition, PAMAM and Cu2+-PAMAM are investigated as the charge collection buffer materials of non-fullerene OSCs of poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b: 4,5-b']dithiophene))-alt-(5,5-(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl)benzo[1',2'-c:4',5'-c']dithiophene-4,8-dione))]:3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene). The power conversion efficiency can reach 9.2%, which is comparable to that using conventional charge transport materials.