Investigation of Photovoltaic Properties of Polymer Solar Cells with Fluorene-based Polyelectrolytes as the Interlayer

A series of conjugated polyelectrolytes (CPEs) based on fluorene named 6,6'-(2-phenyl-9H-fluorene-9,9-diyl)bis(N,N,N-trimethylhexan-1-aminium) bromide (PFB-Br), 6,6'-(2-(thiophene-2-yl)-9H-fluorene-9,9-diyl)bis(N,N,N-trimethylhexan-1-aminium) bromide (PFT-Br), and 6,6'-(2-([2,2'-bithiophen]-5-yl)-9H-fluorene-9,9-diyl)bis(N,N,N-trimethylhexan-1-aminium) bromide (PF2T-Br) were synthesized and applied as the interlayer to investigate how the backbone structure influence the photovoltaic properties. The ionic functionality of CPE accumulates on the ZnO surface, owing to the existing interactions between the ionic groups and the ZnO. The CPE backbone is shifted toward the ZnO surface. Thus, there is a formation of interface dipole via the re-organization of the ionic side chains and hydrophobic backbone. However, incorporation of the electron-rich moiety, thiophene, or bithiophene, in the polymer backbone interfere with the electron transport at the cathode interface. As a result, the power conversion efficiency (PCE) of polymer with thiophene and bithiophene backbone structure was decreased compare with PFB-Br, which has electron-affinity property than PFT-Br, and PF2T-Br.