Charge density at the contacts of symmetric and asymmetric organic diodes

The organic diode, or metal-organic-metal (MOM) structure, is constituent key building block of organic-devices. The physical understanding and performance evaluation of these devices usually require proper modeling and simulation of the metal-organic structure. A topic of major concern in the simulation of the MOM structure, although frequently mishandled, is the selection of proper boundary conditions at the metal-organic interface. In this work, we determine the boundary conditions for the charge density at the metal-organic contact. Symmetric and asymmetric organic diodes with unipolar and bipolar conduction are analyzed. Using experimental current-voltage curves, an analytical method to determine the value of the charge density at the contacts is proposed. In single-carrier diodes, we observe that the charge concentration at the interface due to injection follows a power-law function of the current in metal-organic contacts in drift-dominated transport. This boundary condition is the way to introduce the contact effects in models. The contact affects the other regions (e.g., the bulk) as a boundary condition. This boundary condition for the charge density keeps information about the limited recombination velocity at the contacts and the contribution from space charge limited conduction (SCLC) in the bulk. In diffusion-dominated transport, at low bias close to the diode’s built-in voltage, the charge density at the contact is almost constant with the current. The complete relation between charge and current for injecting electrodes, extracted from the analysis of single-carrier diodes, can be used as boundary condition in bipolar devices.