Self-biased ZnO nanowire humidity sensor vertically integrated on triple junction solar cell

This study presents a zinc oxide (ZnO) Schottky humidity sensor. This kind of sensor needs a forward voltage bias to detect electrical changes in relative humidity (RH). Authors replace the external voltage bias for the sensor with the bias from GaInP/GaAs/Ge triple junction (TJ) solar cell. In this study, two independent components, solar cell and humidity sensor, are integrated vertically to a self-biased and small-sized device. The open-voltage (Voc) of TJ solar cell under standard solar irradiation (AM 1.5G) is about 2.5 V to bias ZnO humidity sensor. With this self-bias from TJ solar cell, measured current values of the integrated device are 75.9, 123.8, 181.2, and 222.6 μA for 35%, 50%, 70% and 85% RH under the conditions of 25 °C and AM 1.5G. Moreover, ZnO nanowire (NW) structure is added on the integrated device to enhance RH response due to increasing surface-area-to-volume ratio. The measured RH responses of the device with ZnO NW at 25 °C were 141.2, 268.1, 365.7, and 572.5 μA for 35, 50, 70, and 85% RH, respectively.