Organic Zener diodes with a precisely adjustable reverse breakdown from -3 to -15 V without any influence on the forward current-voltrage curve are realized This is accomplished by controlling the width of the charge depletion zone in a pin diode with an accuracy of one nanometer independently of the doping concentration and the thickness of the intrinsic layer The breakdown effect with its exponential current voltage behavior and a weak temperature dependence is explained by a tunneling mechanism across the highest occupied molecular orbital lowest unoccupied molecular orbital gap of neighboring molecules The experimental data are confirmed by a minimal Hamiltonian model approach including coherent tunneling and incoherent hopping processes as possible charge transport pathways through the effective device region