(color online) Device structure and performance of flexible transparent ZnO diode. (a) Conceptualized structure diagram, which features two-terminal configuration. (b) Fabrication procedure of field-effect diode. (c) Optical transmittance spectra of devices on glass and PEN substrates with transmittance over 80% in a visible spectral range. The insets show the photographs of these two devices. (d) Current–voltage (I–V) characteristics of field-effect diode with a high rectification ratio of while flat and bent. Insets show the photographs of device under test.^[88]