Energy Levels of Molecular Dopants in Organic Semiconductors

The energy level alignments between hosts and dopants dictate many key physical processes such as charge transport and exciton formation, and thus the functionality and performance of organic semiconductor devices. Therefore, design and fabrication of high‐performance organic devices require knowledge of the energy offsets between hosts and dopants. Due to a typically low dopant concentration used in devices such as organic light‐emitting diodes and sometimes overlapping density of states, it is generally not possible to measure directly the energy offset in a doped system by photoemission technique. Here it is demonstrated that the energy offset between a host and a dopant in a doped system equals to that of its reciprocal heterojunction system. This opens the door for facile data collection. These directly measured offsets are also shown to be the same as the detrapping activation energies extracted from variable temperature charge transport modeling analysis. The energy level alignments between hosts and noncharge transfer dopants, charge transfer p‐type donors, and charge transfer n‐type acceptors are shown to exhibit the universal energy alignment rule for organic–organic heterojunction interfaces.