Ionically Gated Small Molecule OPV: Interfacial doping of Charge collector and Transport layer

Abstract

We demonstrate an improvement in the performance of organic photovoltaic (OPV) systems based on small molecules by ionic gating via controlled reversible n-doping of multi-wall carbon nanotube (MWCNT) coated on fullerenes ETL: C60 and C70. Such electric double layer charging (EDLC) doping, achieved by ionic liquid (IL) charging, allows tuning the electronic concentration in MWCNT and in the fullerene planar acceptor layers, increasing it by orders of magnitude. This leads to decreasing the series and increasing the shunt resistances of OPV and allows to use of thick (up to 200 nm) ETLs, increasing the durability and stability of OPV. Two stages of OPV enhancement are described, upon the increase of gating bias Vg: at small (or even zero) Vg the extended interface of IL and porous transparent MWCNT is charged by gating, and the fullerene charge collector is significantly improved, becoming an ohmic contact. This changes the S-shaped I-V curve via improving the electron collection by n-doped MWCNT cathode with ohmic interfacial contact. The I-V curves further improve at higher gating bias Vg due to the raising of the Fermi level and lowering of MWCNT work function. At the next qualitative stage, the acceptor fullerene layer becomes n-doped by electron injection from MWCNT while ions of IL penetrate into fullerene. At this step the internal built-in field is created within OPV, which helps exciton dissociation and charge separation/transport, increasing further the Jsc and the FF (Fill factor). Overall power conversion efficiency (PCE) increases nearly 50 times in CuPc/fullerene OPV with MWCNT cathode. The concept of ionically gated MWCNT-ETL interface is numerically simulated by the drift-diffusion model which allows to fit the observed I-V curves.