Efficient Small-Molecule-Based Inverted Organic Solar Cells With Conjugated Polyelectrolyte as a Cathode Interlayer

An efficient organic photovoltaics (OPVs) fabricated by using benzofuran-substituted diketopyrrolopyrrole (DPP)-oligothiophene-3,6-bis(5-(benzofuran-2-yl)thiophen-2-yl)-2,5-bis(2-ethylhexyl) pyrrolo[3,4-c] pyrrole-1,4-dione (DPP(TBFu) 2 ) blended with [6,6]-phenyl-C61-butyric acid methyl ester (PC 61 BM) as the bulk heterojunction composite and a conjugated polyelectrolyte as the electron extraction layer in the inverted device structure is reported. Improved fill factor (FF) but reduced short-circuit current density were obtained from DPP(TBFu) 2 :PC 61 BM-based OPVs processed from chloroform solution with small percentage of solvent processing additive, 1,8-diiodooctane (DIO). The investigation by grazing incidence X-ray diffraction and resonant soft X-ray scattering indicated that the large phase separation in the thin film of DPP(TBFu) 2 :PC 61 BM BHJ composite was induced by DIO, which results in decreased $J_{{\rm SC}}$ .