ZnO　is　one　of　the　most　widely　studied　semiconductors　due　to　its　direct　wide　band　gap　and　high　exciton　binding　energy.　Due　to　its　ease　of　synthesis,　robustness　and　low　cost,　ZnO　has　been　applied　in　a　wide　range　of　devices,　including　nanogenerators,　solar　cells,　and　photodetectors.　In　this　work,　ZnO　nanorods　were　synthesized　in　a　single　step　using　an　aqueous　method　at　temperatures　below　100　degrees　C.　The　nanorods　were　annealed　in　oxygen　and　nitrogen　and　a　p-type　polymer　poly(3,4-ethylenedioxythiophene)　polystyrene　sulfonate　(PEDOT:PSS)　was　spray　coated　onto　the　top　of　ZnO　nanorods　to　form　a　p-n　junction.　The　I-V　characteristics　of　the　device　showed　that　the　annealing　atmosphere　had　a　significant　effect　on　the　rectification　ratio　of　the　device.　Further　analysis　using　Mott-Schottky,　photoluminescence,　and　X-ray　photoelectron　spectroscopy　(XPS)　indicated　that　oxygen　vacancy　concentration　correlated　well　with　the　free　electron　density　in　ZnO　as　well　as　the　rectification　ratio　of　the　p-n　junction　devices.　Devices　made　with　ZnO　nanorods　annealed　in　nitrogen　had　a　better　rectification　ratio　than　oxygen,　representing　a　simple　method　to　improve　p-n　junction　diode　behaviour　through　tuning　the　defect　properties　of　the　nanorods　via　controlled　annealing.