Charge transport across organic heterostructure: Role of interfacial density of states

Abstract

The mechanism of carrier transport across organic-organic hetero-interfaces is crucial for organic devices. For purposes of this study, we choose two hole transport materials 4, 4′, 4″-tris(N-3-methylphenyl-N-phenyl-amino) triphenyl-amine (m-MTDATA) and N, N′-diphenyl-N, N′-bis(1-naphthyl) (1, 1′-biphenyl)-4, 4′diamine. We demonstrate that transport across such interfaces can be controlled by introducing a thin undoped layer, which is chosen to be 10–30 nm of m-MTDATA layer in this particular case. By correlating the low frequency capacitance-voltage and temperature dependence of current density-voltage (J-V) characteristics, we show that the occupation of local density of states (LDOS) on both sides controls the transport at the hetero-interface and that it can be modified by the introduction of interlayer. The thickness of the undoped interlayer effectively acts as a control on carrier concentration at the interface leading to the modification of the effective energy offset, and overlap of the LDOS. The slope of the J-V characteristics in the exponential regime has a temperature dependent term and a purely voltage dependent term. The temperature dependence of J-V characteristics is explained by including field dependence of mobility and tunneling across LDOS. We trace the origin of the offset at the hetero-interface and estimate its values for the different cases consistent with the proposed model of thermally assisted tunneling at the interface.