Synthesis and characterization of conjugated oligoelectrolytes based on fluorene and carbazole derivative and application of polymer solar cell as a cathode buffer layer

It has been previously reported that the performances of bulk heterojunction polymer solar cells (PSCs) can be improved by incorporation of alcohol/water-soluble conjugated polyelectrolytes as cathode interlayers. Herein, we investigate the effect of three pi-conjugated oliogoelectrolytes based on fluorene and carbazole derivatives with quaternary ammonium bromide called 6,6',6aEuro(3),6aEuro ',6aEuro(3)aEuro(3),6aEuro(3)aEuro '-(9H,9'H,9aEuro(3)H-[2,3':6',2aEuro(3)-terfluorene]-9,9,9',9',9aEuro(3),9aEuro(3)-hexayl)hexakis(N,N,N-trimethylhexan-1-aminium) bromide (FFiF), 6,6',6aEuro(3),6aEuro '-((9-(6-(trimethylammonio)hexyl)-9H-carbazole-3,6-diyl)bis(9H-fluorene-9,9,2-triyl))tetrakis (N,N,N-trimethylhexan-1-aminium) bromide (FCiF), and 6,6',6aEuro(3),6aEuro '-((9-hexyl-9H-carbazole-3,6-diyl)bis(9H-fluorene-9,9,2-triyl))tetrakis(N,N,N-trimethylhexan-1-aminium) bromide (FCF) as a cathode buffer layer between the active layer and the Al electrode on the photovoltaic properties of conventional type PSCs. The best power conversion efficiency (PCE) of PSCs with the FCF layer is 3.25%, which increases by 24% compared to that of PSCs without any cathode buffer layer (2.62%). The PCE improvement is due to enhancement of the short circuit current, the fill factor, and the open circuit voltage, simultaneously. Interface dipole formation between the active layer and the Al cathode was proposed to explain the improved performances.