Fluorene-Based Conjugated Polyelectrolytes as Interlayers for Organic Photovoltaic Cells

In this study, we explored the effect of interlayers between ZnO layer and active layer on the performance of inverted organic photovoltaic cells by employing fluorene-based conjugated poly-electrolytes (CPE) with different side chains. Poly[9,9′-bis(6″-N,N,N-trimethyl- ammoniumhexyl)fluorene-alt-1,4-phenylene dibromide] (P1) and poly[(9,9′-bis(4-(6-N,N,N-trimethyl- ammoniumhexyloxy)phenyl) fluorene-2,7-diyl)-alt-1,4-phenylene dibromide] (P2) were synthesized and used as interlayers in inverted organic photovoltaic cells. We fabricated organic photovoltaic devices with the following structure: ITO/ZnO/CPE/active layer/MoO₃/Ag. The CPE was applied between the ZnO layer and active layer, which comprised poly[4,8-bis-(5-(2-ethylhexylthio)thiophen- 2-yl)benzo[1,2-b:4,5-b′]-dithiophene] (PBDTT-S) and PC₇₁BM. We confirmed the changes in the work functions of ZnO using ultraviolet photoelectron spectroscopy. The measured work functions of ZnO were –4.32 and –4.21 eV when P1 and P2 were used as the interlayer, respectively; these values are slightly higher than that of bare ZnO (–4.42 eV). One of the devices fabricated with P2 showed a maximum power conversion efficiency of 4.37%, which is higher than that of the device without a CPE interlayer (3.76%).