A typical
organic light-emitting diode is made of several functional
organic semiconducting layers including a hole transport layer (HTL),
an emissive layer (EL) comprising host and dopant molecules, and an
electron transport layer. The charge carrier hopping from the HTL
into the host–dopant EL is known to control the device performance.
Thus, a clear understanding of charge transport physics at the HTL/EL
interface is essential to guide selection of organic materials for
achieving optimal device performance. In this paper, we report experimental
and theoretical studies of electrical characteristics of a variety
of HTL/EL junctions. We show that charge transport characteristics
at an organic–organic junction vary, depending on the energy
level alignments of the dopant to that of the host and HTL. In addition,
the dopant concentration is found to dictate the device electrical
characteristics. Detailed theoretical studies show that carrier transport
across the junction is charge-injection-limited at low dopant concentration
(≤2 wt %), while a space-charge-limited injection, that is,
Ohmic junction, is obtained at higher dopant concentration.