ZnO is a n-type semiconductor that has gained a high interest in the field of UV photo detection because of its suitable band gap of 3.37 eV, high excitation energy of 60 meV and low production cost. However, ZnO nanostructures like nanorods and nanowires exhibit persistent photoconductivity (PPC) which has adverse effects on their UV detector applications. On combining with  p-type semiconductors such as Poly(3,4 ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and Poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) polymers, a hybrid inorganic-organic p-n junction can be created to reduce PPC and allow an efficient photo detection in a hybrid photodiode configuration.

  In this study, we fabricated hybrid UV photodiodes with a configuration of indium tin oxide (ITO)/ZnO-nanorod/F8BT/PEDOT:PSS/Ag. The photodiodes were fabricated by spin coating p-type F8BT and PEDOT:PSS polymers above n-type ZnO nanorods, where ZnO nanorods were grown on ITO substrates via a low-cost hydrothermal synthesis method. The precursor amount, speed and time for polymer layers were optimized to 20 µl, 2000 rpm, and 30 s for F8BT and to 20 µl, 3000 rpm, and 50 s for PEDOT:PSS. Four different seeding layer solvents including methanol, ethanol, isopropanol and aqueous ethanol (70%) were considered to grow ZnO nanorods. Scanning electron microscopy (SEM) images of bare nanorods show that the nanorods prepared using methanol, ethanol and isopropanol seeding solvents exhibit a smaller dimeter of ~80-90 nm, while the nanorods prepared using aqueous ethanol exhibits a larger diameter of ~110 nm. The cross-sectional SEM images of the photodiodes show a similar thickness of ~1 µm for all nanorods, and the combined F8BT and PEDOT:PSS polymer layer thickness of ~150 nm. The photoluminescence study on bare nanorods revealed higher surface defects in nanorods prepared using methanol and isopropanol seeding solvents than those prepared using ethanol and aqueous ethanol seeding solvents. We demonstrate that seeding solvents influenced the nanorods diameters, uniformity and surface defects of nanorods, which further influenced the photoresponse properties and figures of merit of the UV photodiodes. All the photodiodes are evaluated based on their rectification ratio, ideality factor (η), responsivity (R), external quantum efficiency (EQE) and response time (Ƭ_decay). The photodiode with ZnO nanorods grown using methanol seeding solvent shows the best performance, in terms of the rectification ratio of  ~640, a T_decay of 108 s, η of 2, an EQE of ~2760%, and a R of ~8.14 A/W at -2 V. These properties can be correlated to the uniform surface coverage of the nanorod on the ITO substrate.