Electrical Transport Mechanisms and photovoltaic characterization of
Au/CoPc/p-Si Heterojunctions
H.S.Soliman* ,A.A.M.Farag ,N.M.Khosifan*,M.M.El-Nahass
Physics department,Faculty of Education ,Ain Shams University , Cairo ,Roxy,Heliopolis,Egypt, 11757
*Physics department , Faculty of Education for Girls, Saudi Arabia .
Abstract
Heterojunction devices of Au/CoPc/p-Si were fabricated by growing CoPc
films onto p-type Si using the thermal evaporation technique . Current – voltage
(I-V) measurements were performed over a temperature range of 300 to 393 K .
The parameters and mechanisms of conduction of heterojunction diode are studied
to determine the electrical characteristics of these structures. An analysis of the
dark I-V characteristics of Au/CoPc/p-Si for several temperatures is done to
elucidate the conduction mechanisms and to evaluate
the heterojunction
parameters is presented . The forward current increases exponentially with the
applied voltage in the region
of
V 0.4 V,which
was dominated by the
thermionic emission over the CoPc/p-Si . In the 0.5  V 1 V region , the current
transport is due to the space- charge – limited current controlled by an exponential
trap distribution in the the band - gap of CoPc . A free –carrier concentration of
1.8x1015 cm-3 ,built-in potential of 0.28 V and a maximum barrier field of 2.5x103
V/cm are
estimated from the dark C-V measurements at 1 MHz .The current-
voltage characteristics under illumination have also been investigated and
photovoltaic properties of Au/CoPc /p-Si solar cell are evaluated . It is concluded
that the photo-generated current is due to the efficient dissociation of excitons .
Keywords: CoPc ; conduction mechanism ; photovoltaic ; organic solar cell
1.
Introduction
In the last two decades , effort for finding suitable materials for electronic
applications , have attracted an ever- increasing interest in the investigation of the
electrical and photo-electrical properties of organic compounds . Among these
organic materials are the metal free (H2Pc) and metal substituted phthalocyanines
1
(MPcs ) such as PbPc [1] , FePc [2] ,MgPc[3] , ZnPc [4] and CoPc [5].These
materials are generally p-type semiconductors , and they can easily be sublimed ,
resulting in pure thin films without decomposition[6] . They are used as gas
sensors [ 7] ,for optical recording materials [8] rectifiers [9] and photovoltaic
devices [10] . The effects of coating with Pcs films have also been investigated, on
n-type Si electrodes in aqueous redox solution [11]. The n-type Si electrodes
coated with vacuum deposited CoPc or CuPc were stable, showing a slow decay of
photocurrent and a high photovoltage, as compared to the Schottky Au / n-Si [12].
The electrical properties of CuPc / p-Si have been studied by Antoh et al. [13].
Their results have been explained by a charge transport model in which , the
charge transport is limited by thermionic emission at low current densities .The
same authors have also demonstrated the high stability and reproducibility of the
electrical properties of CuPc / p-Si heterojunction which suggests that these
materials have many useful applications.
The effect of dye coating with chloroaluminium phthalocyanine, AlPcCl /n-Si
was examined by Yanagi et al. [14]. They found that the morphology and
thickness of the deposited AlPcCl, which were controlled by changing substrate
temperature of n-Si, have an important role in increasing the photovoltaic
efficiency of the Au/AlPcCl/n-Si cells .
The transverse current-voltage characteristics of CuPc/Si, PbTe/CuPc/Si
junction have been studied in the dark and under illumination by Lee and Kawai
[15]. They found that the PbTe/CuPc/Si junctions exhibited a strong photovoltaic
response with quantum efficiency of 15.4 % and power conversion efficiency of
0.035% . They concluded that the photocurrent is generated in CuPc layer and the
carriers are separated by the steep incline of the potential near the CuPc/PbTe
interface . CuPc/PbTe multilayer showed a large photoconduction effect in the inplane direction. Heterojunction devices of p-MgPc/n-Si have been studied by Raid
[3] in dark and under illumination. He found that these junctions exhibit rectifying
2
and strong photovoltaic characteristics with power conversion efficiency of
1.05%.
Nguyn Van et al.[16] concluded that the junction parameters ,as the
rectification factor ,the saturation current density and the ideality factor,are
strongly influenced by the dopant ,the thickness and the preparation temperature of
the
polypyrrol
layer
for
metallophthalocyanine
doped
pyrrole/silicon
heterojunction .
The p-n junction solar cells consisting of CuPc and perylene pigments were
studied by Hu and Matsumura[17] . They showed stronger structure and thickness
dependence . As well as the devices with the structure of ITO/CuPc/PV/Ag
possess better properties than the devices with the structure of ITO/PV/CuPc/Au .
In addition, the photo-absorbance near the p-n junction determines all the
properties of the solar cells, which is the active centre of the devices .
Darwish et al.[18] studied the photovoltaic properties of ZnSe/metal free
phthalocyanine heterojunctions deposited on substrates of InP single crystals.
They found that the n-ZnSe/p-H2Pc /p-InP junctions gave stronger photovoltaic
characteristics than that of p-H2Pc/p-InP and ZnSe/p-InP junctions which may be
due to the effective light absorbance of H2Pc layer and the effective charge
separation at the H2Pc/ZnSe interface.
The objective of this investigation is to study the dark and illuminated
current - voltage (I-V) characteristics and the capacitance - voltage (C-V)
characteristics at different temperatures . Moreover ,
the illuminated I-V
characteristics have been investigated . The
obtained from these
results
measurements are analyzed in order to determine some HJ parameters and to
suggest a dominant current transport mechanism .
2. Experimental Details
3
The p-type Si single crystals were used as substrates. Chemical etching of p-Si
was performed with HF:HNO3:CH3COOH(1:6:1) composition for 30 sec. After
etching the silicon wafers were washed with distilled water and isopropyl alcohol
and dried in nitrogen . One side of Si was coated with indium layer as a bottom
electrode. The CoPc layer was deposited by thermal evaporation in a vacuum
better than 10-4 Pa on the other side of p-Si . During deposition, the temperature of
p-Si substrate was kept at 310 K . The deposition rate was about 5 nm/min. and
the thickness of CoPc ranged from 30 to 60 nm . A schematic diagram of the
device is shown in Fig.1 .The current flowing through the sample was determined
using a stabilized power supply and a high-impedance Keithley 617 electrometer .
The I-V characteristics were performed in dark over the temperature range 300 373 K .The temperature was measured directly by means of chromel-alumel
thermocouple connected to hand - held digital thermometer. Capacitance-voltage
measurements were performed at 1MHz , using a computerized capacitancevoltage system consisting of the 410 C-V meter interfaced via model 4108 C-V
interface . The loaded I-V characteristics circuit used to determine the solar cell
parameters under illumination is shown in Fig.2. The incident power density of
light illumination was  20 W/m2 provided by a tungsten lamp through a water
filter .The devices had an effective area of  2.4 mm2 .
3. Results and Discussions
3.1. Current - Voltage Characteristics
Fig. 3 shows the I-V characteristics
of CoPc/p-Si heterojunctions under
forward and reverse bias in the temperature range of 300-393 K. The observed
exponential dependence in the lower voltage range my be due to the formation of a
depletion region between CoPc layer and Si single crystal substrate . The ratio of
the forward current to the reverse current at a certain applied voltage is defined as
the rectification factor
RR . It is evident
4
that the junction exhibits strong
rectifying characteristics showing a p-n diode –like behaviour . The RR value of 
103 is estimated forAu/ CoPc / p-Si heterojunction at 1 V . The series and shunt
resistance are obtained at room temperature by the method stated in [3], shown in
Fig.4 and the other stated in [ 18 ] , shown in Fig.5 .The calculated values of Rs
and Rsh are found to be  2.46 k and 1.7 M  respectively .
From the semilogarithmic plots of the forward current vs forward bias (V 
0.4 V) for a Au/CoPc/p-Si heterojunction in the temperature range of 300-393 K
, shown in Fig. 6. This figure reveals that the forward
current increases
exponentially with bias. Such behavior agrees with rectification characteristics
which are generally described by either the diffusion model, the emission
model, or the recombination model [19]. The data for the narrow potential range
(V  0.4 V) could be fitted to the Schottky equation [20].
I = Is [exp (
eV
)-1]
nkT
(1)
where n is the diode quality factor, k is the Boltzmann’s constant and Is is the
saturation current which can be obtained by extrapolation of the ln I- V portion to
the ln I axis at V= 0 . A reasonably good fit was obtained for the CoPc/p-Si
heterojunction
1.98x10-9 A
, whose parameters are determined to be
1.95  0.02 and
for n and Is at room temperature respectively . These parameters
are a direct indicator of the electrical transport properties of the p-n junction and
reflects the properties of the depletion layer [21] . The deviation of the diode
quality factor n from unity may be attributed to the recombination of electrons
and holes in the depletion region [16,22,23] .
The saturation current , Is is given by
Is = AA* T2 exp (
 e
)
kT
(2)
where A is the effective area of the device , A* is the effective Richaradson’s
constant and  the barrier height for the predominant carriers. It is evident from
5
the above equation that if one plots n (Is/T2) against the reciprocal temperature T , a
straight line relationship will be obtained. The slope of the obtained line equals (
- e /k ) , while the intercept with the vertical axis is n AA*. Fig.5 indicates
clearly a linear behaviour which confirms that the thermionic emission is the
predominant conduction mechanism in the applied voltage range of V  0.4 V .
Analysis of the temperature dependence of Is yields the values 0.50.03eV and
2.3x105Am-2 K-2 for b and A* respectively . This value of A* is in good
agreement with the published value [16,23] and the obtained barrier height agrees
with the value deduced for the thermionic emission. According to the thermionic –
emission theory for crystalline semiconductors, the current flow depends weakly
on the barrier height since it is assumed that electron collisions within the
depletion region can be neglected .
Under relatively high forward voltages another mechanism is suggested to
be operative between 0.4  V  1. A superquadratic dependence of current on
voltage of the form I  Vm ( where m  3 ) was observed is shown in Fig.7 .
Such a power dependence suggests that I is a space – charge limited current
(SCLC) and is characteristic of an exponentially trap distribution [ 24 ] . The
equation for the I-V dependence for a p- type semiconductor with an exponential
distribution of trapping levels is given by :


I  e A N V 
eP k T
o
B
t

l
 V l 1

 d 2 l 1

( 3)
where NV is the effective density of states in the valence band edge, Po the
trap density per unit energy range at the valence band edge ,  the hole mobility
and l= Tt/T where T is the room ambient temperature and Tt is the temperature
6
parameter describing the exponential trapping distribution . The energy of the trap
distribution , kTt , is found to be 0.05 0.001eV at T = 300 K .
Fig.8 shows the dependence of
Au/CoPc/p-Si . heterojunction ,at
ln I on inverse temperature 1/T for
a constant voltages of 0.85 V . In
this
temperature range the characteristics show linear segments . The total trapping
concentration Nt is estimated from the measured slopes to be  8x1016 cm-3 for
0.85 V which is in the same order of magnitude as that obtained by Ahmed and
Collins [24].
.
3.2. Capacitance - Voltage Characteristics
The dark capacitance –voltage characteristics of Au / CoPc/ p-Si
heterojunctions were measured at 1MHz . This frequency is high enough to
neglect the dielectric relaxation process in CoPc [24], by which one can get
information on the depletion region extened in the p-Si side .
Fig 9 shows the (C-2-V) characteristics of the Au/CoPc /p-Si heterojunction
at room temperature .The linearity of this dependence indicates that the junction is
reasonably interpreted by assuming an abrupt heterojunction.. Thus the
capacitance is examined with the aid of the following equation [25]
C-2 =
2( Vb  V  kT / e )
eNA2
(4)
where Vb is the built - in voltage and N the net carrier concentration . According
to this equation ,the
net carriers concentration and the built –in voltage are
obtained from the slope and the intercept as 1.8x1015 cm-3 and 0.280.02V
respectively .
The maximum barrier field that exists at the interface of CoPc/Si was
determined from the following equation [23,26]
Emax 
2Vb
W
(6)
7
where W is the width of the depletion region (=
A
Co
) and Co is the capacitance at
zero bias . The calculated values of Emax is  2.5x103 V/cm .
3.3 I-V characteristics under illumination
The photovoltaic properties of CoPc/n-Si heterojunction diode are determined
by measuring the I-V characterisics using a load resistance and an illumination of
power density of 20 W / m2.The devices had an effective area of 2.4 mm2 .
The I-V characteristics of a Au/CoPc /p-Si heterjunction in the dark and under
illumination. The dark current is small at the negative bias and the behaviour can
be understood by a p-n junction. The barrier at the interface limits the reverse
carrier flow across the junction and the built in potential could be developed. The
photocurrent action spectra for the Au /CoPc /p-Si cells indicate photocurrent
maxima at the Q-band absorption region as shown in Fig. 10 . This spectrum
suggests that the homogeneous CoPc layers contribute primarily to photocarrier
generation ,and consequently sensitize the p-Si electrode in this wavelength region
.However , CoPc layers reduce the photocurrents by an order of magnitude
because of their low conductivity. In addition ,the incident light is absorbed by
CoPc and produces excitons which subsequently dissociate into electrons and
holes due to the internal field created by schottky barrier formed at the interface
.The photocurrent action spectrum in the Q-band region is identical to the optical
absorption spectrum of CoPc ,indicating the generation of photoelectrons ,via a
CoPc exciton intermediate ,followed by electron transfer from CoPc into p-Si
through the potential barrier at the interface .This behaviour is believed to be due
to the difference in the electron affinities between the two semiconductors . In
addition , a relatively high photocurrent peak at about 600 nm is observed in
Fig.11a compared with the absorption spectrum ,shown in Fig.11 b .This may be
8
due to a filtering effect of the light ,which is absorbed by CoPc ,for transverse
photocurrents generated in the CoPc layers. Moreover, it is noteworthy that the
photocurrent in the reverse direction is strongly enhanced by photoillumination
.This behaviour yields useful information on the excitons ,which were effectively
generated in the CoPc layer by the incident photons .
The low hole mobility ,the disorderd dissociation and charge dissipated
recombination in organic materials leads to lower efficiency and fill factor[17]. To
estimate the efficiency of the cell, an illuminated cell was connected to a variable
( from zero to infinity) load resistance . The current through the load resistor and
the voltage across the junction were measured and plotted and are shown in Fig.
12. It can be seen from this figure that the CoPc/Si junction shows the
photovoltaic characteristics with Isc (short-circuit current) of 7.50.1 A , Voc
(open-circuit voltage) of 0.44±0.01, FF (fill factor) of 0.330.01 and  (power
efficiency) of 2.4±0.03 %. Where the devices had an active area of 2.4 mm2 .It
should be mentioned that although the Au/p-Si Schottky cell shows a strong
rectifying behavior its Voc value, which was found to be 0.2 V [26,27], is rather
small compared to that of the Au/CoPc/p-Si cell. This may be due to surface states
barrier which act as a recombination centers [27] . Such a formation of surface
states is repressed by inserting the CoPc layer between p-Si and Au, So that the
barrier height is kept large to produce a higher Voc. In addition, an effect of
intervening layers has been recognized by Yanagi et al. [14] when the n-Si surface
is covered with an insulating SiO2 layer, i.e., constructed in the metal / insulator /
semiconductor (MIS) cell. In general, CoPc is considered to be a p-type molecular
semiconductor rather than an insulator. Therefore, the contact between CoPc and
p-Si should produce an organic / inorganic heterojunction.
4. Conclusion
The dark current – voltage measurements suggest that the forward current
transport in these junctions involve thermionic emission of the electrons from p-Si
9
over the CoPc/Si barrier at low forward bias. Moreover, a space –charge- limited
transport across the CoPc layer dominates due to the exponential distribution of
traps above the valence band in the band gap of the CoPc layer at high forward
bias.
From the capacitance – voltage measurements at high frequency (1MHz),
one can obtain information about the depletion layer extending in the Si side.
These characteristics are interpreted by assuming the abrupt heterojunction model
. Under illumination of  20 W/m2, the significantly high Voc (0.440.01 V), as
compared to that (0.2 V) for the Au/p-Si cell, have suggested that the excitons play
an important role in the primary process of photocarrier generation in CoPc. The
obtained power conversion efficiency of 2.40.03 % is comparatively high
compared to the presently available dye- sensitized solar cells.
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
Figure captions
Fig. 1 Schematic diagram of the Au/CoPc/n-Si/In device
11
Fig.2 Schematic diagram of loaded Au/CoPc/p-Si heterojunction
Fig. 3 I-V characteristics of CoPc/p-Si heterojunction at different temperatures .
Fig.4 junction resistance against applied voltaje at room temperatura.
Fig.5 If –V at room temperature (the inset the voltage drop versus forward current )
Fig.6 Semi-logarithmic plot of forward current againt applied voltage at different
temperatures.
Fig. 7 Semi-loarithmic plots of (Io/T2) versus 1/T
Fig. 8 I-V characteristics at different temperatures (at high voltage bias)
Fig. 9 Semi-logarithmic plots of forward current versus 1/T at higher voltage bias.
Fig. 10 The variation of 1/C2 against V.
Fig. 11 (a) Photocurrent action spectrum for Au/CoPc/p-Si heterojunction
(b) Optical absorption spectrum of CoPc thin film
Fig. 12 Loaded I-V characteristics for Au/CoPc/p-Si heterojunction under illumination
12
p-CoPc film
Top ohmic electrode (Au grid)
Si (p-type single crystal)
Bottom ohmic electrode
(In)
Fig.1
I
h
V
RL
Fig. 2
13
1500
300K
323K
348K
373K
393K
1250
1000
I, A
750
500
250
0
-250
-500
-1.0
-0.5
0.0
0.5
1.0
V,Volt
Fig.3
14
V
1E-3
IF
IF(A)
1E-4
0 .8
1E-5
0 .7
R s = 2.46K 
0 .6
0 .5
V
1E-6
0 .4
0 .3
0 .2
1E-7
0 .1
0 .0
1 .5 x1 0
-4
2 .0 x1 0
-4
2 .5 x1 0
-4
3 .0 x1 0
-4
3 .5 x1 0
-4
4 .0 x1 0
-4
I(A m p )
1E-8
0.0
0.2
0.4
0.6
0.8
1.0
1.2
V(Volt)
Fig.4
7
10
6
10
Rsh=1.7M
5
R j ( )
10
4
10
Rs=2.46K
3
10
2
10
-0.5
0.0
0.5
1.0
V(Volt)
Fig.5
15
1.5
0
10
-1
10
-2
10
-3
current (A)
10
-4
10
-5
10
-6
10
300 K
333 K
363 K
393 K
-7
10
-8
10
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
V(V)
Fig. 6
-12
10
2
-2
Is/T (AK )
-13
10
-14
10
2.5
2.6
2.7
2.8
2.9
3.0
-1
1000/T (K )
Fig. 7
16
3.1
3.2
3.3
3.4
Fig. 8
17
Fig. 9
100
60
2
18
-2
1/C (×10 F )
80
40
20
0
-2.0
-1.5
-1.0
-0.5
bias voltage, V (V)
Fig. 10
18
0.0
0.5
1.0
3.0
(a)
photocurrent (a.u.)
2.5
2.0
1.5
1.0
0.4
0.5
0.6
(m)
19
0.7
0.8
5
(b)
4
abs.(a.u.)
3
2
1
0
0.4
0.5
0.6
0.7
 (m)
Fig.11
20
0.8
0.9
8
7
-6
Current ,I (x10 A)
6
5
4
3
FF = 0.33
 =2.4
Isc=7.5 A
2
Voc=0.44 V
1
A=2.4mm
2
0
0.0
0.1
0.2
0.3
Voltage,V (V)
Fig. 12
21
0.4
0.5
22