A Simple Approach to Fabricate an Efficient Inverted Polymer Solar Cell with a Novel Small Molecular Electrolyte as the Cathode Buffer Layer

A novel small-molecule electrolyte, 1,1'-bis(4-hydroxypropyl)-[4,4'-bipyridine]-1,1'-diium bromide (V-OH), containing a mixture of PTB7:PC₇₁BM has been designed and synthesized as a cathode buffer layer for inverted polymer solar cells (iPSCs). The molecular structure of this new compound comprises a viologen skeleton with hydroxyl group terminals. While the viologen unit is responsible for generating a favorable interface dipole, the two terminal hydroxyl groups of V-OH may generate a synergy effect in the magnitude of the interface dipole. Consequently, the devices containing the V-OH interlayer exhibited a power conversion efficiency (PCE) of 9.13% (short circuit current = 17.13 mA/cm², open circuit voltage = 0.75 V, fill factor = 71.1%). The PCE of the devices with V-OH exhibited better long-term stability compared to that of the devices without V-OH. Thus, we found that it is possible to enhance the efficiency of PSCs by a simple approach without the need for complicated methods of device fabrication.