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.