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

Abstract

We demonstrate the controlled n-doping in small molecule organic photovoltaic (OPV) systems by ionic gating of multi-wall carbon nanotube (MWCNT) coated fullerenes: C 60 and C 70 . Such electric double layer charging (EDLC) doping, achieved by ionic liquid (IL) charging, allows tuning the electronic concentration in the acceptor layers, increasing it by orders of magnitude. This leads to decreasing both the series and shunt resistances of OPV and allows to use thick (up to 200 nm) electron transport layers, increasing the durability and stability of OPV. Two stages of OPV enhancement are described, upon increase of gating bias: at small (or even zero) V g the interface between 1 porous transparent MWCNT charge collector with fullerene is improved, becoming an ohmic contact. This changes the S-shaped I-V curve and improves the electrons collection by a MWCNT turning it into a good cathode. The effect further enhances at higher V g due to raising of Fermi level and lowering of MWCNT work function. At next qualitative stage, the acceptor layer becomes n-doped by electron injection from MWCNT and ions penetration into fullerene. At this step the internal built-in field is created within OPV, that helps exciton dissociation and charge separation/transport, increasing further the I sc and the F F (Filling factor). Overall power conversion efficiency (PCE) increases nearly 50 times in classical CuPc/fullerene OPV with bulk heterojunction photoactive layer and MWCNT cathode. Ionic gating of MWCNTfullerene part of OPV opens a new way to tune the properties of organic devices, based on controllable and reversible doping and modulation of work function.