Elsevier

Thin Solid Films

Volume 515, Issue 3, 23 November 2006, Pages 1138-1141
Thin Solid Films

Measurements and modeling of the DC temperature dependence of electrical conductivity in thin films of lead phthalocyanine

https://doi.org/10.1016/j.tsf.2006.07.040Get rights and content

Abstract

Many of the phthalocyanines exhibit p-type conductivity, and electrical conductivity through thin films of these materials having ohmic contacts show space-charge-limited conductivity (SCLC) dominated by trap levels located within the band gap. In the present work evaporated thin films of lead phthalocyanine with ohmic gold electrodes were prepared, which showed two distinct regions in the dependence of current density J on applied voltage V. At low voltages sample conductivity was ohmic, changing at higher voltages to a square-law dependence of J on V, which is indicative of SCLC dominated by trap levels located at a single discrete energy level. The results of temperature measurements indicate three distinct regions, in each of which the hole concentrations are controlled by different activation energies. A simple model is proposed in which a single trap level is located at the same energy spacing Et from the valence band edge as a single acceptor level. This predicts three different temperature ranges, two of which correspond to those covered by the experimental results. The experimental results indicate a trap level located at an energy Et = 0.36 eV above the valence band edge and a thermal band gap Eg = 1.51 eV. Using the proposed model together with data from the experimental J–V characteristics, an acceptor concentration of 4.85 × 1019 m  3 and a trap concentration of 5.18 × 1025 m 3 are indicated. Measurements of mobility based on this model yield a value of 2.6 × 10 4 m2 V 1 s 1, which is in close agreement with previous work.

Introduction

Phthalocyanines are organic semiconductors with interesting electrical, optical and chemical properties. The crystal structure of these materials resembles that of chlorophyll, and hence their photoconductive and photovoltaic properties have attracted a great deal of attention [1], [2], [3]. Because phthalocyanines can easily be synthesized using relatively simple chemical reactions many metal phthalocyanines such as CoPC, PbPc and others have been produced with various energy gap and electrical characteristics. The structure of lead phthalocyanine consists of a central metal atom (Pb) surrounded by 4 benzene rings. The unit cell constant of triclinic PbPc are given as a = 1.3123 nm, b = 1.6131 nm, c = 1.2889 nm, α = 94.22°, β = 96.20° and γ = 114.19°. During the last two decades considerable effort has been directed towards the realization of various devices, such as organic light emitting diodes (OLEDs) [4] and organic thin film transistors (OTFT) [5]. Furthermore the response of phthalocyanines to various gases makes them a potential material for use as an active layer in the production of gas sensors [6], [7]. Of particular interest is the works of Burr et al. [8] who have used lead phthalocyanine in the fabrication of gas sensitive FETs, which are reported to be sensitive to NO2 at concentrations of less than 20 ppb. DC electrical measurements are of vital importance in determining the various semiconductor parameters. Current density-voltage characterization is one of the most important tools for observation of material behaviours under carrier injection. The current density-voltage characteristics of p-type lead phthalocyanine thin films have been previously reported elsewhere [9], [10]. Although most phthalocyanines are known to be p-type semiconductors there is little published work concerning conduction specifically in p-type semiconductor, especially those incorporating acceptor and trap levels. Many results concerning both the mobility and the energy gap are widely contradictory, in the former case by many orders of magnitude. For example the value of mobility reported by Waclawek and Zabkowska-Waclawek [11] indicates 3.7 × 10 4 m2 V 1 s 1 for lead phthalocyanine while the same parameter was measured by Ahmad and Collins [10] as 6.05 × 10 10 m2 V 1 s 1. The purpose of this paper is to introduce a model, which incorporates, both trap and acceptor levels in a p-type material and to verify both mobility and energy gap of lead phthalocyanine using temperature measurements.

Section snippets

Experimental

In order to study the dependence of current density on temperature, thin film sandwich samples of lead phthalocyanine were subjected to a temperature variation under vacuum conditions. The method of production of devices is reported in detail elsewhere [12], [13]. Samples were deposited onto Corning 7059 borosilicate glass that had been cleaned prior to deposition by ionic bombardment for a period of 5 min. Gold electrodes were used, as these are known to provide ohmic contacts to most

A proposed model for conduction in p-type phthalocyanines with traps and acceptors

In sublimed films, there are a large number of smeared localized states, of which some are occupied and others are not. In our investigations we assume these localized states to consist of traps and acceptors and located at an energy Et above the valence band edge. Furthermore we assume the density of trapping sites Nt is much greater than the density of the acceptor sites Na. This model is essentially similar to that proposed by Barbe and Westgate [15] for n type phthalocyanines. Introducing

Results and discussion

A typical room temperature current density (JV) characteristic is shown in Fig. 1. The low voltage section of the curve is of approximate slope unity on the logarithmic plot, while the slope in the higher voltage section is approximately 2. This type of behaviour has been observed previously on similar devices [9], while the two regions correspond to ohmic conduction and space-charge limited conductivity (SCLC) respectively. In order to study the temperature effects, samples were subjected to

Summary and conclusions

A model has been proposed to account for the conduction mechanism in p-type triclinic lead phthalocyanine. The experimental data based on this model indicate that the current density increases with the reciprocal of temperature and furthermore shows three distinctive regions with hole concentration which are controlled by a different activation energy in each case. An activation energy of Et = 0.36 eV and a thermal band gap of Eg = 1.51 eV have also been measured using this model. The current

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