Diffusion-enhanced hole transport in thin polymer light-emitting diodes

Abstract

Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. The transport of holes in polymer light-emitting diodes ͑PLEDs͒ based on poly͑2-methoxy, 5-͑2Ј ethylhexyloxy͒-p-phenylene vinylene͒ ͑MEH-PPV͒ is investigated as a function of layer thickness. For thicknesses smaller than 100 nm, the current in these thin PLEDs is strongly enhanced as compared to the expected space-charge limited ͑SCL͒ current. Applying the standard SCL model to measurements on a PLED with a thickness of only 40 nm results in an apparent increase of the hole mobility of a factor of 40. We show that this strong increase of the hole transport properties in these thin devices originates from the presence of an Ohmic hole contact. For Fermi-level alignment, holes diffuse from the contact into the MEH-PPV, forming an accumulation layer with a width of a few tens of nanometers. Due to the density dependence of the mobility, the hole transport in this accumulation region is strongly enhanced. For the analysis of thin PLEDs, it is therefore essential that both drift and diffusion of charge carriers are taken into account.

Universal Arrhenius Temperature Activated Charge Transport in Diodes from Disordered Organic Semiconductors

“…2 the low-field mobilities h E 0; T of the samples with various thicknesses are shown in an Arrhenius plot. It is observed that the thinnest devices exhibit not only a higher mobility [16] but also weaker temperature activation. This is consistent with the fact that the thinnest devices have the largest concentration of background carriers that have diffused in from the Ohmic contact.…”

“…In order to align the Fermi level, charge carriers diffuse from the Ohmic contact into the device. Because of the density dependence of the mobility the hole transport in the device is strongly enhanced, for devices of only 40 nm thickness even more than 1 order of magnitude [16]. As a result, there is always a background of extrinsic charge carriers present in SCL diodes that modify the transport.…”

Formation of inversion layers in organic field-effect transistors

“…Various temperature, density, and fielddependent mobility models have been reported. 13,[25][26][27][28][29][30] Here, we assume that charge transport is described by variable range hopping in an exponential DOS. 13 The hole mobility then reads…”

Space-charge-limited hole current in poly(9,9-dioctylfluorene) diodes

“…For a complete description, diffusion effects from the contacts, 34 the charge carrier density dependence and field dependence of the mobility need to be taken into account. The occurrence of a SCL current enables us now to also further investigate the densityand field dependence of the hole mobility of PFO.…”