In this study, we present a zinc oxide (ZnO) thin film-based p-n type thermoelectric energy harvester that can generate thermoelectric power particularly under plane-vertical temperature gradients. The performance of the proposed p-n type thermoelectric energy harvester, consisting of both n-type Al-doped ZnO (ZnO:Al) and p-type Ag & N co-doped ZnO (ZnAgO:N) thin films, is evaluated by measuring its thermoelectric open-circuit voltage ( Voc ) and short-circuit current density ( Jsc ) for the negative and positive plane-vertical temperature gradients, revealing better thermoelectric performance under larger temperature gradients. Moreover, the thermoelectric principle of the proposed p-n type thermoelectric energy harvester is investigated by comparing the thermoelectric performance of the proposed p-n type energy harvester with those of the other p-i-n type energy harvesters containing differently thick undoped ZnO ( i -ZnO) inter-layers between the ZnO:Al and ZnAgO:N thin films. Besides, the thermoelectric principle is further verified employing the scenario of asymmetric temperature gradients. The thermoelectric principle of the proposed p-n type energy harvester is discussed based mostly on the fundamentals of thermoelectricity that charged carriers diffuse from hot to cold side in thermoelectric materials, suggesting that the thermoelectric power of the proposed harvester results from the diffusion of both the electrons and holes according to the plane-vertical temperature gradients. Overall, this study focuses mainly on the fabrication and characterization of the thermoelectric energy harvester that can generate thermoelectric power under the plane-vertical temperature gradients, including the experimental analysis on its thermoelectric principle.