Nonlinear Optics, Quantum Optics, Vol. 32, pp. 13–20
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Charge Carriers Generation in Thin Polymer
Films by Weak External Influences
A.N. LACHINOV1, R.B. SALIKHOV2, A.A. BUNAKOV2 AND A.R. TAMEEV3
1
Institute of Molecular and Crystal Physics, Russian Academy of Sciences, Ufa, Russia
2
Bashkir State Pedagogical University, Ufa, Russia
3
A.Frumkin Institute of Electrochemistry, Russian Academy of Sciences, Moscow, Russia
The paper considers the features of the charge transport near to the threshold of the transition of the thin poly(diphenylenephthalide) films both
from insulator into the high-conductivity state, induced by a small
uniaxial pressure. Two independent methods were used to measure a drift
mobility of charge carriers in the polymer films. The electric field and temperature dependences of hole and electron mobility were explored. It was
also shown that the effect of uniaxial pressure leads to increase both hole
and electron mobility. It is concluded that charge carrier transfer in thin
polymer films is probably explained by the hopping mechanism through
traps.
Keywords: poly(diphenylenephthalide), time-of-flight technique, current-voltage
characteristics, charge carriers mobility, space-charge limited conduction
INTRODUCTION
Phase transition the insulator to metal type in some polyheteroarylenes thin films was observed earlier [1]. The transition can be induced by
both abnormally small electric field [2] and pressure [3]. The transition
take place in the wide band organic dielectric with band gap ∼ 4.2 eV [4].
Despite of the event well known more than ten years the mechanism
of the band structure transformation is unknown. Because of this fact
it will be useful to study charge transport peculiarities in pretransition
area. Two experimental techniques were offered to solve this aim: a
13
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LACHINOV, et al.
current-voltage (I–V) characteristics method and a conventional time-offlight (TOF) technique. The estimations of mobility were made on the
basis of the analysis of I–V characteristics within the framework of the
spase-charge limited conduction (SCLC) model. To measuring of charge
carriers mobility in polymer films by the different methods a lot of papers
were devoted [5–7]. The investigation of mobility dependence on value of
an applied electric field and temperature allowed to reveal, how the carrier charge transfer in polymer film samples is carried out. We assumed
that the data comparison obtained by these independent methods allows
us to correct the charge transport model in pretransition area of the
transition like dielectric to metal, induced by uniaxial pressure.
EXPERIMENTAL
Sandwich-type specimens were prepared for the TOF experiments. At
first a charge transport layer was deposited onto aluminum substrate by
spin coating from a solution of poly(diphenylenephthalide) (PDP) in
cyclohexanon. The layer was dried overnight at room temperature, then
it is situated in a drying case during 30 minutes at temperature 60°–
100°C. The layer thickness, which varied between 6 and 9 µm, was
determined using a micro-interferometer (MII-4). Further PDP film
sequentially was coated by a 0,1–0,2 µm thick charge generation layer
from a selenium or phtalocyanine and an upper semitransparent electrode from aluminium. Both the generation layer and the top electrode
were deposited by thermal evaporation in a vacuum of (1–2) ⋅10−5 Torr.
The I–V characteristics measurements were carried out on a setup that
consisted of a uniaxial mechanical pressure controller, pressure transducer and a measuring circuit for the I–V characteristics recording. The
pressure was varied in the interval 0–1.5 MPa. Test specimens were metal
– polymer – semiconductor sandwiches. The thin polymer PDP films
were prepared by spin coating on glass substrates with transparent
conductive layers of SnO2. The top Al electrode was deposited onto the
polymer film surface by vacuum evaporation. The contacting area was
of s ≈ 2 mm2. The thickness of the polymer films were set by the
concentration of the solution and had values from 0,8 to 1,5 µm.
The current transients were recorded with a Tektronix 340A digital
oscilloscope. The impulse from the N2-laser (wavelength 337.3 nm, pulse
CHARGE CARRIERS GENERATION IN THIN POLYMER FILMS
15
width 10 ns), transiting through a semitransparent Al electrode, hitted on
generation layer of selenium or phtalocyanine. Some measurings were
carried out with the help of xenon valve by pulse duration 20 ns. There
was a photogeneration of charge carriers in generation layer under activity of an applied electric field. Depending on a direction of an external
field the narrow package of holes or electrons was injected in a polymer
film. This package drifted through a film up to an opposite Al electrode.
The current transients were measured in a small signal mode, obeying
q ≤ 0,05CsV and RC << tT, where q is the total injected charge, Cs is the
sample capacity, C is the capacity of a measuring circuit, R is the load
resistance, V is the applied voltage and tT is time of charge carriers
drift up to a collector electrode. The measurings were carried out in a
temperature interval from 260 K up to 393 K.
In a kinetics of a typical transient a short initial spike, then a site of a
plateau followed by a detained part of a current, slowly waning up to
zero, were observed. The intersection point of the asymptotes which have
been plotted through a plateau and a detained part, determines an
arrival time of forward front of a package of charge carriers to an
opposite electrode, i.e. width time tT. A drift mobility of charge carriers m
calculated under the formula:
m = d / (F ⋅ tT ),
(1)
where F is the applied electric field and d is the thickness of a film.
RESULTS AND DISCUSSION
The typical oscillogram of a current transient in test specimen is shown
in Figure 1. The similar kinetics, reference for a normal (non-dispersive)
transport in TOF experiment, was observed in all interval of change of an
electric field. In figure the arrow indicates transit time of charge carriers,
which corresponds to an intersection point of the asymptotes which
have been carried out through a site of a plateau and a detained part of a
transition current.
The field dependences of the hole and electron mobility in coordinates
lgm − F 0.5 are shown in Figure 2. The hole mobility grows with increasing
of an electric field. The electron mobility is smaller than the hole mobility
and its quantity feeblly depends on an electric field. The temperature
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LACHINOV, et al.
FIGURE 1
Typical oscillogram of a current transient.
FIGURE 2
Electric field dependences of hole () and electron (■) drift mobility at 260 K.
dependences of the hole and electron mobility at different values of an
applied electric field are shown in Figure 3. It is visible, that an increase
in the applied electric field decreases a dependence of charge carriers
mobility from temperature. Besides the hole mobility increases with
temperature more strongly in comparison with the electron mobility.
As the field dependences of hole mobility are linearized in coordinates
lgm − F 0.5, they can be described with the help of the equation:
CHARGE CARRIERS GENERATION IN THIN POLYMER FILMS
17
FIGURE 3
Hole (, ⌬) and electron (■, ◆) drift mobility vs. temperature at different electric fields:
6 × 104 V/cm (■, ⌬), 3 × 105 V/cm () and 5 × 105 V/cm (◆).
  E − b ⋅ F 1/ 2
m = m0 ⋅ exp  −  0
kT
 

 ,
 
(2)
where m0 is the mobility at absence of traps, k is Boltzmann constant, E0 is
the activation energy in a zero field, b is the Poole-Frenkel coefficient and
T is the temperature. In case of a hole carriers it has appeared equal
1.3 ⋅ 10−4 eV(sm/V)1/2 at temperature 260 K. If coefficient b to interpret
within the framework of the Poole-Frenkel model, b = [e3/(pee0)]1/2. The
calculation gives b value equal 4.1 ⋅ 10−4 eV(sm/V)1/2. As it is visible,
the experimentally found b value in about 3 times is less, hence, the
Poole-Frenkel model not quite adequately interprets the obtained field
dependences of mobility. Charge carrier transfer in thin polymer films,
most likely, is explained by the hopping mechanism through traps,
instead of the Poole-Frenkel model as it was shown in [8].
Typical I–V characteristics taken from the 1-µm thick polymer films in
the subthreshold area under different pressures are shown in Figure
4.The I–V characteristics were treated within the SCLC model [9], which
provides information on localized states in the energy gap and allows one
to explain the shape of the I–V curves.This model implies that, at low
voltages (up to U1), the I–V characteristic is well described by the Ohm’s
law. At U1, the equilibrium concentration of thermionic free electrons
becomes comparable to the concentration of injected charges:
J ≈ en0 m
U
,
L
(3)
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LACHINOV, et al.
FIGURE 4
Current-voltage characteristics of the polymer film at different pressures: (◆) 600 and (●)
1400 kPa (excess pressure relative to atmospheric value).
where J is the current density, e is the charge of an electron, n0 is the
equilibrium concentration of free charges, m is the electron mobility,
and L is the film thickness. Next, the I–V curve obeys the trap-related
quadratic law
J ≈ hee0 m
U2
,
L3
(4)
where e is the relative permittivity, e0 is the dielectric constant, and h is a
constant taking into account the degree of trap occupation. The first
quadratic portion of the curve is followed by the region where the current
rises nearly vertically (Figure 4). This voltage range is treated as that
where the occupation of traps reaches a maximum. Since the occupation
of traps becomes maximal when the imref crosses trap levels in the energy
gap, the injected charge density and, accordingly, the current in the
polymer increase ubstantially. As the voltage rises further, the I–V curve
is described, as a rule, by trap-unrelated quadratic law (Figure 4).
From the above formulas, it is possible to estimate the electron mobilities m in a transition point U1 from Ohm’s law to trap-related quadratic
law at different values of applied pressure:
m=
JL3
.
hee0U12
(5)
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CHARGE CARRIERS GENERATION IN THIN POLYMER FILMS
TABLE 1
Effect of uniaxial pressure on some injection model parameters.
P (kPa)
m ⋅ 10−5 (cm2/ (V⋅s))
U1 (V)
600
1.1
2.6
1400
2.5
2.1
h
0,41
0,28
P, uniaxial pressure; m, electron mobility; U1, transition point from Ohm’s law to
trap-related quadratic law; h, constant taking into account the degree of trap occupation.
At calculations the value of the relative permittivity of PDP films started
equal 3.4. Below in Table 1 the values of voltages U1 and basic parameters
of model are submitted: m – an electron mobility, h – a constant taking
into account the degree of trap occupation.
From the tabulated data, it follows that an increase in the pressure
increases the carrier mobility. This process is likely to be associated
with the increase in the trap concentration and the decrease in their
occupation. The comparison of electron mobility values obtained from
estimated calculations within the framework of SCLC model with
electron mobility values measured by TOF technique displays, that they
are approximately identical and equal 10−5–10−6 cm2/Vs. The electron
mobility grows with increasing of an applied uniaxial pressure.
To confirm the obtained estimations measurings of charge carriers
mobility in the same samples were carried out by TOF technique at
absence of uniaxial pressure and at pressure in 1.5 MPa. It has appeared,
that the activity of pressure gives in an increase of mobility from values
4.1 ⋅ 10−6 up to 8.8 ⋅ 10−6 cm2/Vs for holes and from 3.0 ⋅ 10−6 up to
6.0 ⋅ 10−6 cm2/Vs for electrons.
The obtained results shows that an increase of the pressure leads to an
increase of hole and electron drift mobility. This fact was proved by both
used different methods. In PDP films the transition in a high-conductivity state can be initiated not only pressure, but also electric field. In this
connection, apparently, in both cases the close physical mechanisms of
charge transport in the pretransition area can take place. The results
analysis allows to make preliminary conclusion that charge carrier
transfer in thin polymer films is probably explained by the hopping
mechanism through traps.
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