Recent advancements in sensing technology and wireless communications have accelerated the development of the Internet of Things (IoT) which promotes the usage of wearable sensors. An emerging trend is to develop self-sustainable wearable devices, thus eliminating the necessity of the user to carry bulky batteries. In this work, the development of a flexible piezoelectric energy harvester that is capable of harvesting energy from low-frequency vibration is presented. It was designed with a cantilever structure of PET/AZO/Ag layers in d33 mode which can generate large output voltages with small displacements. Aluminium doped to ZnO (AZO) was chosen due to its low deposition temperature, does not need post-deposition annealing and poling compared to other materials. AZO is also environmentally friendly that will not cause serious pollution during the fabrication process. Two significant design parameters were chosen, namely the effect of the gap between electrodes and the number of interdigital electrodes (IDE) pairs to the output voltage and resonant frequency. These two parameters have been simulated using a finite element simulation tool named COMSOL Multiphysics. The sputtered AZO on PET showed c-axis orientation at 002 peak with 2θ values of 34.45° which indicates piezoelectric behavior. The silver IDE pairs were screen-printed on the AZO thin film. The average measured d33 constant value was 1.8 pC/N. The energy harvester was capable of generating 0.867 Vrms output voltage when actuated at 49.6 Hz vibrations. This indicates that the AZO thin films with printed silver electrodes have the potential to be used as flexible, d33 energy harvesters for wearable sensors.