Electrical measurements of ß-cobalt phthalocyanine single crystals

ELSEVIER

Physica B 215 (1995) 217 221

Electrical measurements of [3-cobalt phthalocyanine single crystals A.S. R i a d a, A.E. E 1 - S a m a h y b'*, S . M . K h a l i l b aPhysics Department, Faculty of Science, Minia University, Minia, Egypt b physics Department, Faculty of Science, Alexandria University, Alexandria, Egypt

Received 12 January 1994; revised 13 December 1994

Abstract A detailed study of current as a function of voltage at different temperatures, coupled with that of the dependence of current on sample thickness at a given voltage and temperature has been made in the C*-direction of [3-cobalt phthalocyanine (13-CoPc) single crystals using gold electrodes. Results show that at low voltages the conduction process is ohmic, while at high voltages space-charge-limited conduction is present. Traps with a density of 5.2 x 1025 m 3 located at 0.34 eV above the valence band edge have been observed. The voltage at which the transition from ohmic to space-charge-limited behaviour takes place has been found to be independent of the temperature. The results are interpreted in terms of the extrinsic nature of ohmic conduction in 13-CoPc single crystals. The thickness dependence in the square-law region has been found to confirm the L-3 law.

1. Introduction The study of the temperature dependence of ohmic and space-charge-limited (SCL) currents in organic semiconductors is capable of providing considerable insight into the mechanism of charge transport and carrier trapping in these materials [1-4]. The group of organic molecular solids known as phthalocyanines have been the subject of such investigation. These materials are of interest, partly on account of their unusually high chemical and thermal stabilities and also because of their structural similarity to a number of biologically important materials in particular chlorophyll and haemoglobin [5]. Many investigations have dealt with the phthalocyanine compounds such as metal*Corresponding author.

free phthalocyanine (H2Pc) and its metal derivative CuPc [6, 7]. However, CoPc has received considerable less attention. 13-cobalt phthalocyanine has a base centred monoclinic structure; the large area surfaces are (00 1) faces and the needle axis is the b-axis [8]. In this paper, DC conductivity measurements in the C*-direction in crystals of 13-CoPc are reported with the view to understand the role of charge carriers, injected from the electrode and generated in the bulk, towards understanding the conduction mechanism in 13-CoPc single crystals.

2. Experimental technique Cobalt phthalocyanine powder (CoPc) obtained from Eastman Kodak Ltd., New York, was purified

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218

A.S. Riad et al. 'Physica B 215 (1995) 217 221

0.5

0.4

0.3 o

0.2

0.1

0.0 290

i

i

i

340

390

440

T(K)

Fig. 1. Temperature dependence of the Seebeck coefficient for p-type CoPc.

by vacuum sublimation in a two-zone electrically heated tube furnace before growing crystals using a method previously described [2]. The material to be sublimed was maintained at a temperature of 660 K. Purified nitrogen gas at 5 torr was used as entrainer gas. Sublimed CoPc was deposited in the form of ]3-phase in the cooler zone of the furnace regulated to a temperature of approximately 620 K. Single crystals of the [3-form of CoPc tend to grow as needles and are extremely brittle, making it difficult to cut and impossible to work mechanically. For this purpose, gold electrodes were vacuum deposited onto the as grown (00 1) faces of a given specimen. Typical dimensions of the used crystals were 1 - 2 c m length, 0 . 5 - 2 m m width and 0.15-0.40ram thickness. The samples were then mounted onto an electrically heated copper disk, the temperature of which could be held constant to within 1 K over the range from 290 to 430 K. Temperatures were measured using two NiCr-NiAI thermocouples, one attached to the substance disk very close to the crystal under test and the other to the front face of the mounted crystal. No remarkable differences between the readings of the thermocouples were noticed in the temperature range of interest which could be read out on a Keithley 871 digital thermometer display. Electrical measurements were carried out using a stabilized power supply and a Keithley 610C electrometer. Capacitance measurements were made at 1 kHz using a Tinsley 6471 LCR DataBridge. In order to eliminate the possibility of photoelectric effects which have

been widely reported in phthalocyanines [3, 6], the samples were placed in a darkened chamber at least 24 h before measurements were made and also shielded from all incident light. The Seebeck effect in CoPc was investigated by measuring the small voltage AV which developed across (00 1) faces of specimens bearing gold electrodes when a temperature difference AT was established across them. This experiment was carried out to establish the sign of the majority carriers and to ascertain whether abrupt changes in this occurred as the temperature of the crystal was increased. Results varied slightly from crystal to crystal, but the sign of the Seebeck coefficient was always positive over the entire temperature range indicating that the charge carriers were positive. Therefore CoPc single crystal is a p-type material which is in agreement with that reported in the literature [9]. The smooth curve of Q = AV/AT versus T is shown in Fig. 1.

3. Results and discussion Fig. 2 shows a set of current density voltage curves at different temperatures ranging from 293 to 425 K. There were two distinct regions for each characteristic: at lower voltages, the slope of log J versus log V plot was approximately equal to 1, while at higher voltage levels above a well-defined transition voltage, Vt the slope was ~ 2. Hence CoPc, in common with other phthalocyanine, exhibits ohmic conductivity at low applied voltages and SCL conductivity governed by a single dominant level at high voltages [10]. Similarities between SCL conductivity in this material and that observed in [3-NiPc single crystals [2] are probably due to similarities in band structure [11-13] and clearly their mode of conduction are the same. The current density for both ohmic and SCL regions are given by the familiar expression [-2, 14]:

(J-V)

V Jn = Po e/.t ~

(1)

and 9 V2 JscL = ~ ~ Z-~ 0,

(2)

A.S. Riad et al./ Physica B 215 (1995) 217-221

219

lO.2

'E .,<

d~

'E ~o-3

,01 10'

,

I

i

i

,,,tJ

I

102

I

I

I

I III

103

V ( volt )

Fig. 2. Current density-voltage characteristics for t3-CoPc single crystal at various temperatures. Crystal thickness 2.5× 10-4 m.

where Po is the thermally generated hole concentration, e the electronic charge, ~ the hole mobility, L the crystal thickness and 0 the fraction of the total carriers which are free. Following the theory of Lampert [14], Eq. (2) is valid for the injection of one type of carrier only in the presence of a single discrete trapping level. The temperature-independent transition voltage, Vt, at which the current converts from ohmic to SCL behaviour implies a sample which is extrinsic [2]. To account for the observed behaviour of the dark current as a function of temperature for p-type 13-CoPc single crystals, a simple hole trapping

I

10-/*

I

L(m)

I

I

5xI~

-4

Fig, 3. Thicknessdependence of ohmic and SCL current densities at 100 and 400 V respectively,for different thicknesses of 13-CoPcsingle crystals at 293 K. model with acceptors has been assumed. In this model hole traps (positive when occupied) and acceptors (neutral when occupied) sites are assumed to exist at a level located at E, above the valence band edge. It is assumed that the density of hole trapping sites N t i s much greater than the density of acceptor sites Na. The density of free holes responsible for ohmic conduction, in the extrinsic case, is given [2] by Po = Nv(Na/Nt)exp(- Et/kT),

(3)

where Nv is the effective density of states in the valence band, N a the density of acceptor sites and

220

A.S. Riad et al. /Physica B 215 (1995) 217 221

,°-i

,°I "~E
--

0

0

•

10-3 . -

,o-'l 2.5

2.0

3.0

3.5

O03/r )K-' Fig. 4. Semilogarithmic variation of ohmic and SCL current densities with reciprocal temperature for 13-CoPc single crystal. The applied voltages are: (©) 50; (0) 100 for ohmic behaviour and (A) 400; ( × ) 500 V for SCL conduction. Crystal thickness 2.5x 10-4 m.

the concentration of trapping centers. A similar model for electron trapping levels has been successfully applied to the dark conductivity results of 13-HzPc [1] and I3-NiPc [2] single crystals. The electron trapping factor, 0, for SCL conduction is given [14, 15] by Nt

0 = (Nv/Nt)exp(

- Et/kT).

(4)

According to Eqs. (1) and (2) the transition voltage, V,, is given by

8e e )

V, = ~ ~ L 2

.

(5)

Our measurements of the sample capacitance as a function of crystal thickness yielded a permittivity of 2.98 × 10 11 Fro- 1 which is in good agreement with the available literature value 3.11 x 10-11 for 13-CuPc single crystals. The above value of e will be used later in the following analysis. Fig. 3 shows the dependences on crystal thicknesses of current density at room temperature for both ohmic and SCL regions. Each point corresponds to a different crystal. The ohmic dependence (slope ~ - 1) indicates that good contacts had been applied. Further, the relatively small scatter of points about this line implies that the density of conduction holes was relatively constant from crystal to crystal. The slope of ~ - 3 in the square-law region verifies that single carrier SCL conduction dominated by a single trap level is occurring. Fig. 4 shows current density measurements in the temperature range 293 425 K at different voltages. The activation energies for both ohmic and SCL regions were 0.34 _+ 0.02 eV. The activation energy in the SCL region corresponds to a single dominant hole trapping level situated above the valence band edge. That the activation energies in the ohmic and SCL regions are the same adds further support to the hypothesis of the presence of a single dominant hole trapping level associated with an acceptor level at the same energy separation from the edge of the valence band [2, 16]. It should be mentioned that other trapping states may also be present, but their density is such that they are incapable of dominating the statistics. Abdel-Malik and Cox [2] reported hole traps in 13-NiPc single crystals at 0.64 eV with trap density 3 x 1018 m -3 above the valence band edge. The method of analysing their experimental results is different from that used in the present investigation. Values of ( # N v / N t ) at different voltages in square-law region, which have been obtained by extrapolating each of the lines in Fig. 4 to l I T = 0 have been found to be approximately identical. The value has been estimated to be ~ 1.94x 10 3 m 2 V - 1 s - 1. We assume the effective density of states in the valence band to be of the order of the density of molecules, approximately 1027 m 3 [3, 17] and p to be 1 × 10 - 4 m 2 V - 1 s- 1. The latter value is the approximate hole mobility in H2Pc at 300 K [5]. Then, the value of N t is found to be

A.S. Riad et al./Physica B 215 (1995) 217 221

(5.2 _+ 0.3)× 10 zs m -3, which is in good agreement with the values reported in literature [-2, 10].

4. Summary and conclusions Current density voltage measurements on 13CoPc single crystals have shown characteristics typical of other phthalocyanines; namely ohmic conduction at low applied voltages and spacecharge-limited (SCL) conductivity at higher voltages. Furthermore, results obtained within the SCL region suggest the existence of a trap distribution dominated by a single trap level. The transition voltage, Vt, between the ohmic and SCL conduction was found to be temperature independent indicating that the conduction mechanism in 13-CoPc is extrinsic. It has further been found that, within the limit of our experimental error, the thermal activation energies for ohmic and square-law regions are identical. The value for each region has been found to be 0.34 + 0.02 eV which suggests a trap located at this energy distance above the valence band edge.

221

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