TFSA doped interlayer for efficient organic solar cells

Organic solar cells based on bis(trifluoromethanesulfonyl)amide (TFSA, [CF₃SO₂]₂NH) doped poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT) were fabricated to investigate the effect of molecular doping. By replacing poly(3,4-ethylenedioxythiophene) (PEDOT): poly(styrenesulfonate) (PSS) with a thin TFSA layer, we have found more efficient charge injection at anode/active interface enhanced photovoltaic performance. The doping effect is confirmed by photoemission spectroscopy that the Fermi level of doped PCDTBT shifts downward to its HOMO level and results in higher carrier concentration. The reduced injection barrier also evidented by impedance spectroscopy that the real impedance of the TFSA doped PCDTBT solar cell decreases more than 50%. Using the molecular doping approach, the overall power conversion efficiency (PCE) was largely increased from 4.70% to 5.98%. Our results suggest that TFSA functions not only as a surface doping molecule, but also an anode interfacial layer to replace the conventional PEDOT:PSS.