  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.
