We have studied the piezoelectric potential output of a ZnO wire/belt functionalized with p-type oligomer (2,5-Bis(octanoxy)-1,4-bis(4-formyl phenylene vinylene) benzene) (OPV2) when it was deflected by an atomic force microscope (AFM) tip in contact mode. In comparison to the ZnO wire/belt without oligomer coating, an extra positive voltage peak was observed prior to the appearance of a negative potential peak. The paired positive and negative voltage peaks are the results of tip contact to the stretched and the compressed side of the wire/belt, corresponding to the positive and negative local piezoelectric potential, respectively. The p-n junction between OPV2 and ZnO serves as a "diode" that controls the flow of current. When the nanowire/nanobelt is first bent by the AFM tip, the diode is reversely biased and the piezoelectric charges are stored in the ZnO wire/belt. As the AFM tip further bends the wire/belt, the local piezoelectric potential is continuously accumulated to a value that is large enough to break through the diode. Then the free charges from the external circuit can flow in and neutralize/screen part of the piezoelectric charges, resulting in a positive pulse in the output signal. When the AFM tip continues to scan to reach the compressed side of the ZnO wire/belt, the p-n junction is forwardly biased. Neutralizing/screening the residual and the newly created piezoelectric charges leads to the flow of current from the tip to the ZnO wire/belt, resulting in a negative voltage pulse. This study supports the charging and discharging model proposed for the piezoelectric nanogenerator.