Solution-Processed Zinc Oxide as High-Performance Air-Stable Electron Injector in Organic Ambipolar Light-Emitting Field-Effect Transistors

Electron injection from the source-drain electrodes limits the performance of many n-type organic field-effect transistors (OFETs), particularly those based on organic semiconductors with electron affinities less than 3.5 eV. Here, it is shown that modification of gold source-drain electrodes with an overlying solution-deposited, patterned layer of an n-type metal oxide such as zinc oxide (ZnO) provides an efficient electron-injecting contact, which avoids the use of unstable low-work-function metals and is compatible with high-resolution patterning techniques such as photolithography. Ambipolar light-emitting field-effect transistors (LEFETs) based on green-light-emitting poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) and blue-light-emitting poly(9,9-dioctylfluorene) (F8) with electron-injecting gold/ZnO and hole-injecting gold electrodes show significantly lower electron threshold voltages and several orders of magnitude higher ambipolar currents, and hence light emission intensities, than devices with bare gold electrodes. Moreover, different solution-deposited metal oxide injection layers are compared. By spin-coating ZnO from a low-temperature precursor, processing temperatures could be reduced to 150 degrees C. Ultraviolet photoemission spectroscopy (UPS) shows that the improvement in transistor performance is due to reduction of the electron injection barrier at the interface between the organic semiconductor and ZnO/Au compared to bare gold electrodes.