Impedance spectroscopy measurements provide resistance, conductance and quality factor which play key roles to find optimum thickness of electron transport layer (ETL) in organic devices. The optimized thickness of hydrochlorosilole in an electron only device was determined to be 7 nm by the minimum contact and bulk resistances along with the maximum conductance and quality factor. We confirm that the optimized thickness from the electron only device works as the optimum condition for optical emission for multi-layer light emitting diodes which contain hydrochlorosilole. (c) 2012 Elsevier B.V. All rights reserved.