Investigation of the effect of conjugated oligoelectrolyte as a cathode buffer layer on the photovoltaic properties

The spontaneous rearrangement of side chain on FFiF, FPiF, and FPF backbone during the spin coating. [Display omitted] •Conjugated oligoelectrolytes are used as a cathode buffer layer for PSCs.•The arrangement of side chains affects the work function of Al.•The PCE was increased from 2.76% to 3.37% by FPF modified Al. A series of conjugated oligoelectrolytes consisting of fluorene and phenothiazine, 6,6',6″,6‴,6′′′′,6′′′′′-(9H,9′H,9′′H-[2,3′:6′,2′′-terfluorene]-9,9,9′,9′,9′′,9′′-hexayl)hexakis(N,N,N-trimethylhexan-1-aminium)bromide (FFiF),6,6′,6′′,6′′′-((10-(4-(trimethylammonio)butyl)-10H-phenothiazine-3,7-diyl)bis(9H-fluorene-9,9,2-triyl))tetrakis(N,N,N-trimethylhexan-1-aminium)bromide(FPiF) and 6,6',6”,6”'-((10-hexyl-10H-phenothiazine-3,7-diyl)bis(9H-fluorene-9,9,2-triyl))tetrakis(N,N,N-trimethylhexan-1-aminium) bromide (FPF), are utilized as the cathode interfacial layer in conventional-type polymer solar cells based on P3HT and PCBM. Insertion of these interfacial layers between P3HT/PCBM active layer and the metal cathode increases overall power conversion efficiency from 2.76% (reference device) to 2.81%, 3.04%, and 3.37% of devices with FPiF, FFiF, or FPF-coated Al, respectively, primarily due to the reduction of the metal work function through the introduction of conjugated oligoelectrolytes (COEs). This result comes from the difference moiety (i.e., phenothiazine or fluorene) and arrangement of sides chains with quaternary ammonium bromide (QAB) on trimer backbones. The conformation of FFiF is expected to be not exactly in a zigzag arrangement, while the zigzag conformation is expected for the alternated oligomer based on phenothiazine and fluorene (FPiF and FPF). Therefore, the number of accumulated QAB in the side chains of FPiF at the surface of cathode will be smaller than that of FFiF or FPF. The larger amount QAB groups gathering near the cathode layer leads to the bigger in reduction of metal work function. Additionally, FPF coated Al shows the lower work function than that of FFiF coated Al, even though FFiF contains more QAB groups accruing at cathode layer. Thus the device with FPF modified-Al displays the best performances among the devices.