Described herein are ultrasensitive gas sensors based on a vertical-channel organic semiconductor (OSC) diode, along with methods for making such devices, and uses thereof. The organic sensing layer comprises a fused thiophene-based organic polymer that connects top and bottom electrodes to deliver a vertical current flow. The nano-porous top-electrode structure enables the contact between ambient gas molecules and the vertical organic channel. The device has high sensitivity, is easy to process, and has a long shelf life.