Optimization of ZnO Nanorod Array Morphology for Hybrid Photovoltaic Devices

Hybrid inorganic oxide/conjugated polymer photovoltaic devices using ZnO nanorod arrays (NRAs) instead of planar films as the electron-transport layer exhibit significant improvements in performance that have been attributed to increased heterojunction surface area, although the relationship has not been quantitatively established. Here, we independently measure the surface area of ZnO NRAs and quantify its effect on the performance of ZnO NRA/poly(3-hexylthiophene) (P3HT) photovoltaic devices. We find that a device utilizing a vertically aligned 180 nm ZnO NRA exhibits an similar to 2.7x enhancement in the short-circuit current compared with that of a bilayer device, in excellent agreement with the increase in surface area. In addition, we show that a subtle difference in the NRA morphology can impact P3HT crystallinity in the photoactive region. Improved P3HT crystallinity leads to an similar to 25% enhancement in the short-circuit current for devices with the same surface area. On the basis of these findings, we modify the NRA growth to introduce more spacing between nanorods and create a ZnO NRA/P3HT device with a high short-circuit current density of 2.91 mA/cm(2). These results indicate that, although increased surface area is the most important factor to improving photocurrent and efficiency, other factors, such as ZnO NRA morphology and P3HT crystallinity, also impact the performance of ZnO/P3HT photovoltaic devices.