J Mater Sci: Mater Electron
DOI 10.1007/s10854-007-9152-5
Copper phthalocyanine based Schottky diode solar cells
Suresh Rajaputra Æ Subhash Vallurupalli Æ
Vijay P. Singh
Received: 8 January 2007 / Accepted: 12 February 2007
Springer Science+Business Media, LLC 2007
Abstract Copper phthalocyanine (CuPc)/Aluminum (Al)
Schottky diode solar cells were studied. The thickness of
the CuPc layer was varied from 15 nm to 140 nm. Short
circuit current densities (Jsc) increased with thickness from
0.042 mA/cm2 at 15 nm to 0.124 mA/cm2 at 120 nm
reaching saturation at that level. Open circuit voltages
(Voc) increased from 220 mV at 15 nm to 907 mV at
140 nm. Analysis of the current-voltage characteristics
indicated that tunneling and interface recombination current mechanisms are important components of the current
transport at the CuPc/Al junction.
1 Introduction
Solar cells based on organic semiconductors are of interest
because of their potential as flexible, lightweight and
inexpensive devices. One of the promising devices [1–7],
involves the heterojunction between copper phthalocyanine
(CuPc) and 3,4,9,10-perylenetetracarboxylic bis-benzimidazole (PTCBI). Earlier, we reported [1, 3] the highest
Voc in a single organic heterojunction solar cell involving
CuPc and PTCBI. A high open circuit voltage of 1.125 V
was obtained in an ITO-PEDOT:PSS/CuPc/PTCBI/Al
device structure with a thin PTCBI layer of 7 nm thickness.
Results were interpreted in terms of a modified CuPc-Al
Schottky diode for this thin PTCBI case [3]. Also, earlier,
S. Rajaputra S. Vallurupalli V. P. Singh (&)
Center for Nanoscale Science and Engineering, Department of
Electrical and Computer Engineering, University of Kentucky,
453 Anderson Hall, Lexington, KY 40506, USA
e-mail: vsingh@engr.uky.edu
Kwong et al. [8] reported a high Voc of 870 mV in a CuPc
Shottky diode. To further elucidate the physical processes
in our cells, a more detailed investigation of CuPc/Al
interface was undertaken. Results are described in this
paper.
2 Experimental procedures
Glass/ITO substrates of sheet resistance 4–6 W/Square
(Delta technologies) were sonicated in acetone, then
methanol and then dried under nitrogen flow. A 100 nm
thick buffer layer of PEDOT: PSS (Bayer) was spin-coated
onto a clean ITO substrate at 2000 rpm and subsequently
annealed in vacuum at 100 C for 30 min. Layers of CuPc
(99.995% Aldrich) and Al electrode were deposited by
vacuum evaporation. Devices with CuPc thickness varying
from 15 nm to 140 nm were fabricated; device area was
0.07 cm2.
3 Results and discussion
An illustrative scanning electron micrograph showing the
morphology of the CuPc film of 100 nm thickness deposited on ITO/PEDOT:PSS is shown in Fig. 1; the grain sizes
are in the 20 nm–30 nm range. The surface of this film and
films of other thicknesses were all relatively smooth over
large areas. Fig. 2 shows the x-ray diffraction pattern taken
on the 100 nm thick CuPc film. The peak for the CuPc film
is seen at the 2º positions of 6.92, corresponding to the
a-form as observed also by Forrest et al. [9]. Additional
peaks observed in the pattern were from the ITO substrate.
Absorption spectra of CuPc films of varying thicknesses in
the ultra-violet to visible (UV-Vis) wavelength range,
123
J Mater Sci: Mater Electron
Fig. 1 Scanning electron
micrographs of a 100 nm thick
CuPc film under, (a) 10 K and,
(b) 40 K magnification
1600
Intensity (arb.units)
1400
1200
1000
800
600
400
200
0
10
20
30
40
50
60
70
2θ (degrees)
Fig. 2 X-ray diffraction pattern of a 100 nm thick Cu Pc film on ITO
a CuPc layer and an Al electrode is sketched in Fig. 4; note
that ITO/PEDOT: PSS makes an ohmic contact to CuPc
and the active junction is between CuPc and Al; electron
affinity and ionization potential values used in this diagram
were obtained from the literature [4, 10]. For the purpose of
illustration, an upper bound value of the difference (x)
between the Fermi level and the HOMO level of CuPc was
calculated from the observed Voc value of 0.907 V. In
Fig. 4, the junction barrier from the aluminum side (y), is
given by, y = [(3.6 + 1.7 – x) – 4.06] eV, while the junction barrier from the CuPc side (qVbi), is given by,
qVbi = y – x = [1.24 – 2x] eV. Further, requiring Vbi to
exceed Voc, we must require that x be less than or equal to
shown in Fig. 3 are in conformity with the work in the
literature [8]. The long wavelength peak of 700 nm corresponds to an energy gap of 1.7 eV which is the difference
between the HOMO and the LUMO levels of CuPc [4].
Energy level diagram [3] of a Schottky diode consisting of
Fig. 3 Absorption spectra of CuPc films of varying thicknesses in the
ultra-violet and visible (UV-Vis) wavelength ranges
123
Fig. 4 Sketch of the energy level diagram, in equilibrium, of a CuPcAl Schottky diode. For this sketch, which is not drawn to scale, device
with 140 nm thick CuPc was used as an example
8.0x10
-3
6.0x10
-3
4.0x10
-3
2.0x10
-3
-1.8
-1.5
-1.2 -0.9
0.0
-0.6 -0.3 0.0
-2.0x10
Table 1 Device parameters for CuPc Schottky diodes under dark
conditions
15nm
60nm
80nm
100nm
120nm
140nm
2
J (Amps/cm )
J Mater Sci: Mater Electron
0.6
0.3
Thickness Series Resistance (Rs) Ideality factor (n) Jo (mA/cm2)
0.9
1.2
1.5
1.8
V( volts)
-3
2
J(Amps/cm )
Fig. 5 I–V curves of ITO/PEDOT:PSS/CuPc/Al devices with varying thicknesses under dark conditions
3.0x10
-4
2.0x10
-4
1.0x10
-4
15 nm
60 nm
80 nm
100 nm
120 nm
140 nm
0.0
-0.4
-0.2
0.0
-1.0x10
-4
-2.0x10
-4
-3.0x10
-4
0.2
0.4
0.6
0.8
1.0
1.2
V (volts)
Fig. 6 I–V curves of ITO/PEDOT:PSS/CuPc/Al devices with varying thicknesses under one sun illumination
{[1.24 – qVoc]/2} eV. Thus, substituting 0.907 V for Voc,
we get 0.16 eV as an upper bound value for x.
The current-voltage (I–V) characteristics of ITO/PEDOT:PSS/CuPc/Al solar cell devices of varying CuPc
thicknesses are shown in Figs. 5 and 6 and in Tables 1 and 2,
under dark and ‘‘one sun’’ illumination conditions
respectively. Voc increased with increase in CuPc layer
thickness reaching a value of 907 mV for a CuPc layer
thickness of 140 nm. The overall low values of Jsc are
attributed [3] to exciton diffusion length problems in
organic semiconductors and to the series resistance of a
thin aluminum oxide layer in these devices. As for the
variation in short circuit current density with thickness, we
observed a rapid increase initially but as the thickness of
the CuPc layer reached 120 nm, saturation set in. This
saturation is attributed to the inability of the optically
Table 2 Device parameters for
CuPc Schottky diode solar cells
under ‘‘one sun’’ illumination
Rs
15 nm
6.87 kW/cm2
60 nm
2
100 nm
7.10 kW/cm2
2
7.12 kW/cm
2
8.24 kW/cm
2
8.57 kW/cm
7.7
0.121
60 nm
7.86 kW/cm
18.03
0.127
80 nm
8.45 kW/cm2
17.5
0.135
100 nm
8.31 kW/cm2
17.78
0.149
120 nm
9.32 kW/cm2
15.95
0.142
140 nm
9.41 kW/cm2
18.29
0.126
generated excitons to reach the CuPc/Al interface when the
CuPc film became too thick.
I–V curves were analyzed for extracting the effective
values of the diode ideality factor (n) and the reverse saturation current density (J0). These and other important
solar cell parameters are tabulated in Tables 1 and 2. In our
CuPc/Al cells, the measured value of n is larger than two
and therefore tunneling, recombination-generation currents
in the depletion region and recombination through interface
states at the CuPc/Al junction are expected to play
important roles [11]. Also, we see from Tables 1 and 2 that
In CuPc devices under study, variations in effective values
of n and Jo with thickness are relatively small (less than
40%) while the value of Jsc varies over a much larger range
(factor of three). Thus it appears that as the thickness of
CuPc layer is increased, the junction transport mechanism
(tunneling etc.) is not much altered but the light generated
current is. It is likely therefore that the enhancement in the
Voc with thickness is primarily due to enhanced Jsc and not
due to reduced J0. In other words, the Voc in these solar
cells is not being limited by some fundamental junction
transport factor and reasonably high Voc values (0.907 V
for example) are achievable.
4 Conclusions
CuPc/Al Schottky diode solar cells exhibited higher Voc
values as the CuPc layer thickness was increased, reaching
a value of 907 mV at a CuPc thickness of 140 nm. Jsc, on
the other hand saturated at a CuPc thickness of 120 nm.
Diode ideality factor values (n) ranged from 7.7 to 18.2.
n
Thickness
80 nm
15 nm
Jo
Voc
Jsc
7.66
0.147 mA/cm2
220 mV
0.042 mA/cm2
19.30
0.110 mA/cm
2
360 mV
0.054 mA/cm2
0.127 mA/cm
2
584 mV
0.094 mA/cm2
0.139 mA/cm
2
770 mV
0.114 mA/cm2
2
879 mV
0.124 mA/cm2
907 mV
0.125 mA/cm2
16.08
17.62
120 nm
2
9.11 kW/cm
16.50
0.118 mA/cm
140 nm
8.97 kW/cm2
16.08
0.109 mA/cm2
123
J Mater Sci: Mater Electron
Such high values of n are indicative of the importance of
tunneling and interface recombination mechanisms for the
current transport at the CuPc/Al junction.
Acknowledgements This work was supported in part by a grant
from Kentucky Science & Technology Council Inc. (Grant # KSEF 148-502-02-27).
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