Role of Electron Blocking Layer in Performance Improvement of Organic Diodes

Abstract

Charge-carrier mobilities in organic diodes based on an anthracene-containing poly(arylene-ethynylene)-alt-poly(p-phenylene-vinylene) generally known as AnE-PVstat, stacked with an electron blocking thin layer of NPB(N, N′-bis (1-naphythyl)-N, N-diphenyl-1,1′-biphenyl-4,4′-diamin) of various thicknesses are investigated through current density-voltage (J-V), capacitance-frequency (C-w), conductance-frequency (G-w) and impedance-frequency (Z-w) experiments in conventional structures of ITO/AnE-PVstat/NPB/Al. Analysis of J-V, C-w and G-w results show that current-density, capacitance and conductance of the active layer decrease with increasing NPB thickness and permit determination separately of a hole mobility of the polymer of the order of $\sim10^{ - 4} \,{\hbox{cm}}^{2}\, {\hbox{V}}^{ - 1} \,{\hbox{s}}^{ - 1}$. This value is less than the global mobility (7 × 10⁻⁴ ${\hbox{cm}}^{2} \,{\hbox{V}}^{ - 1} \,{\hbox{s}}^{ - 1}$) obtained without the blocking layer, which probably includes electrons and holes mobilities and indicates that NPB absorbs the majority of electrons. Analysis of impedance spectroscopy results shows that the impedance (Z) and parallel capacitor (C_p) decrease and parallel resistance (R_p) increases with increasing NPB thickness layer. All these results clearly confirm the role of NPB as a blocking layer for the electrons.