Phosphor-doping enhanced efficiency in bilayer organic solar cells due to longer exciton diffusion length

Abstract

We fabricated bilayer organic solar cells (OSCs) in the structure glass/ITO/PEDOT:PSS/PtOEP:MEH-PPV/C70/Al, where MEH-PPV was doped with platinum octaethylporphyrin (PtOEP). Enhanced exciton diffusion length (LD) is realized via converting generated singlet excitons to triplet excitons. Investigation based on transfer matrix simulations reveals that it is the extended exciton LD of the doping donor layer that leads to the short-circuit current density (Jsc) and power conversion efficiency (PCE) improvement, when compared with those of the OSCs with a non-doping donor layer. As a result of the increased LD, Jsc and PCE increase by 30% and 42% respectively for a device with 5wt% PtOEP-doped 25nm-thick donor layer. Meanwhile, by doping with phosphorescent bis(1-phenyl-isoquinoline)(acetylacetonato)iridium(III), the reduction in open-circuit voltage and the comparable Jsc are shown due to its higher HOMO level and higher LUMO level, leading to the decrease of PCE. It demonstrates that doping a polymer with a suitable phosphorescent molecule is an important approach to be considered to increase the exciton LD.