Organic doped/undoped interface based diode structure: Distinct mechanisms underlying forward and reverse bias

摘要

Organic semiconductor diodes fabricated using doped/undoped (high-low) homojunction has the potential of providing controlled and high quality current density-voltage (J-V) characteristics based on majority carrier transport. We study mechanisms of transport underlying such characteristics both in forward and reverse bias regimes of typical doped/undoped homojunction organic diodes fabricated using 4,4′,4″-tris(N-3-methylphenyl-N-phenyl-amino) triphenylamine (m-MTDATA). We study the J-V characteristics over a wide temperature range (200-300 K), and by varying the intrinsic layer thickness between 10 and 100 nm. The forward bias current, before entering into the space charge limited regime, is exponential over several orders of magnitude with the slope being temperature independent for all intrinsic layer thickness down to 10 nm. The reverse bias characteristics, on the other hand, are highly sensitive to the thickness of the intrinsic layer. While the forward bias is controlled by tunneling at the homojunction interface, the reverse bias is controlled by the interface of cathode (aluminum in this case) and the intrinsic layer. We show that the reverse current is due to Fowler-Nordheim tunneling across a barrier height, which is temperature independent but is sensitive to the layer thickness of the intrinsic layer. The origin of the thickness dependence of barrier height (0.45-0.72 eV for 10-20 nm) is attributed to the change in background carrier concentration in the intrinsic layer due to diffusion of carriers from the highly doped side. The results clearly show that the diffusion length of the majority carriers is approximately 1 nm and is comparable to the nearest neighbor jump distance.