Open-Circuit Voltage Improvement in Hybrid ZnO–Polymer Photovoltaic Devices With Oxide Engineering

We present strategies to improve low open-circuit voltage (V
_oc
) for ZnO-poly(3-hexylthiophene) (P3HT) photovoltaic devices, which are typically ≤0.4 V, but vary among different reports. One factor affecting V
_oc
variability is the ZnO bandgap (E
_g
), which depends on detailed processing conditions. By decreasing the pyrolysis temperature of sol-gel ZnO films, we increased the ZnO E
_g
by 0.14 eV and V
_oc
of corresponding bilayer devices by 0.1 V. This is understood as increased donor-acceptor energy-level offset. Next, we demonstrate significant enhancement in V
_oc
by depositing conformal amorphous TiO
_x
films at the surface of planar ZnO films and ZnO nanorod arrays using a spin-coating method. The TiO
_x
coatings monotonically increased V
_oc
from 0.4 to 0.8 V for devices with increasing TiO
_x
thicknesses from 0 to ≥50 Å. Dark current-voltage measurement reveals that the TiO
_x
coating significantly decreases the reverse-bias current density, leading to an improvement in V
_oc
, in excellent agreement with predictions from the modified ideal diode equation. This is consistent with passivation of ZnO surface defects by TiO
_x
. In short, by varying the solution processing conditions, we modify the bulk and interfacial properties of the metal oxide acceptor, thus leading to systematic improvement in open-circuit voltage.