Chemically synthesized nanocrystal quantum dots (NQDs) are promising materials for applications in solution-processable optoelectronic devices such as light emitting diodes, photodetectors, and solar cells. Here, we fabricate and study two types of p-n junction photodiodes in which the photoactive p-layer is made from PbS NQDs while the transparent n-layer is fabricated from wide bandgap oxides (ZnO or TiO2). By using a pn junction architecture we are able to significantly reduce the dark current compared to earlier Schottky junction devices without reducing external quantum efficiency (EQE), which reaches values of up to similar to 80%. The use of this device architecture also allows us to significantly reduce noise and obtain high detectivity (>1012 cm Hz1/2 W-1) extending to the near infrared past 1 mu m. We observe that the spectral shape of the photoresponse exhibits a significant dependence on applied bias, and specifically, the EQE sharply increases around 500600 nm at reverse biases greater than 1 V. We attribute this behavior to a turn-on of an additional contribution to the photocurrent due to electrons excited to the conduction band from the occupied mid-gap states.