Fluorene conjugated polymer/nickel oxide nanocomposite hole transport layer enhances the efficiency of organic photovoltaic devices

A nanocomposite layer comprising the conjugated polymer poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl)fluorene] (PFN) and nickel oxide (NiO _x) has been employed as the hole transport layer (HTL) in organic photovoltaics (OPVs) featuring PBDTTBO-C₈ and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC71BM) as the active layer. The optimal device incorporating the PFN:NiO_xnanocomposite as the HTLs displayed a power conversion efficiency (PCE) to 6.2%, up from 4.5% for the corresponding device incorporating pristine NiO_xas the HTL layer: A nearly 40% improvement in PCE. X-ray photoelectron spectroscopy (XPS) was used to determine the types of chemical bonding, ultraviolet photoelectron spectroscopy (UPS) to measure the change in work function, and atomic force microscopy (AFM) to examine the morphology of the composite layers. The growth of nickel trioxide, Ni₂O₃, in the PFN:NiO_xlayer played a key role in producing the p-doping effect and in tuning the work function, thereby improving the overall device performance.