ARTICLE IN PRESS
Solar Energy Materials & Solar Cells 90 (2006) 798–812
www.elsevier.com/locate/solmat
Characterization of high-photovoltage
CuPc-based solar cell structures
V.P. Singha,, B. Parsarathya, R.S. Singha, A. Aguileraa,
J. Anthonyb, M. Payneb
a
Electrical and Computer Engineering Department and Center for Nanoscale Science and Engineering,
University of Kentucky, Lexington, KY 40506, USA
b
Chemistry Department, University of Kentucky, Lexington, KY 40506, USA
Available online 10 May 2005
Abstract
Organic solar cells of the configuration ITO/PEDOT:PSS/CuPc/PTCBI/Al (Indium Tin
Oxide/poly(3,4-ethylenedioxythiophene): polystyrene sulfonic acid/Copper phthalocyanine/
3,4,9,10-perylenetetracarboxylic bisbenzimidazole/Aluminum ) were investigated. A high open
circuit voltage (V OC ) of 1.15 V was obtained when the PTCBI layer was 7 nm thick. Lower Voc
values were observed for the same structure with silver, copper and gold electrodes instead of
aluminum. However, short-circuit current density (J SC ) with these electrodes was much higher
(4 mA/cm2) than in the case of aluminum (0.12 mA/cm2). Incorporating a 10 nm thick CdS
interlayer between PTCBI and aluminum resulted in an increase in current density to 0.3 mA/
cm2. Results were interpreted in terms of a modified CuPc/Al Schottky diode for the thin
PTCBI case and a CuPc/PTCBI heterojunction for the thick PTCBI case. Also, the formation
of a thin, protective aluminum oxide layer under the aluminum electrode was postulated. For
devices with silver, copper and gold electrodes, absence of this protective layer was thought to
be the cause of a relatively lower V oc and higher J SC .
r 2005 Elsevier B.V. All rights reserved.
Keywords: CuPc/PTCBI solar cells; Organic semiconductors; Electron transport modeling
Corresponding author. Tel.: +1 859 257 3243; fax: +1 859 257 3092.
E-mail address: vsnigh@engr.uky.edu (V.P. Singh).
0927-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.solmat.2005.04.016
ARTICLE IN PRESS
V.P. Singh et al. / Solar Energy Materials & Solar Cells 90 (2006) 798–812
799
1. Introduction
Organic solar cells are of interest because of their potential as flexible, lightweight
and inexpensive devices that can power portable electronics, from personal gadgets
to space exploration probes [1,2]. Copper phthalocyanine (CuPc) has been used
extensively as an absorber in organic solar cells due to its marked photovoltaic effect
and photoconductivity characteristics [2–5]. Under ‘‘one Sun’’ illumination, Tang [3]
achieved an open circuit voltage (V OC ) of 450 mV and a short circuit current density
(J SC ) of 2.3 mA/cm2 in CuPc-based devices. Later, Peumans et al reported a V OC of
480 mV and J SC of 4.2 mA/cm2 in this system [4]. Also, using a triple junction and a
silver electrode, this group [5] achieved a V OC of 1.2 V and J SC of 4.5 mA/cm2. Even
though there has been substantial progress in these devices, short circuit current
density remains much lower than in thin film solar cells based on inorganic
semiconductors [6–9], and the mechanisms of carrier generation and transport are
not as well understood [7–9]. Clearly, an improved understanding of device physics
and novel device designs are needed for further advances in organic solar cells.
In this article, we report on the characteristics of a single-heterojunction CuPc/
PTCBI/Al device structure that exhibited a V OC of 1.15 V and a J SC of 0.125 mA/
cm2. Also, variations on this basic structure, including a variety of electrode materials
and incorporation of CdS, were investigated with a view toward understanding the
physics of device behavior. Experimental procedures and results are presented in
Sections 2 and 3 and an interpretative discussion is offered in Section 4.
2. Experimental procedures
Fig. 1 shows the configuration of our basic experimental device. ITO-coated glass
substrates (Delta Technologies Inc., Rs ¼ 4–6 O/Square) were sonicated in acetone,
in methanol and dried under nitrogen flow. To aid CuPc deposition and the
formation of an ITO/CuPc ohmic contact, a thin (3 nm) buffer layer of PEDOT:PSS
(Aldrich) was spin-coated on ITO and subsequently annealed in vacuum at 100 1C
for 15 min. Layers of CuPc (99.995%) and PTCBI, as well as the metallic electrode
were thermally evaporated under vacuum (106 Torr). Aluminum, silver, copper and
gold electrodes were thermally evaporated through a mask with 1/8-inch diameter
holes, resulting in multiple devices, each with an area of 0.079 cm2. Electrical
characterization was performed with an automated I–V tester, solar simulator and
an HP 4192A LF impedance analyzer. Scanning electron microscopy was done with
a Hitachi S-900 FE-SEM.
3. Results
3.1. Cell characteristics
Current–voltage (I2V ) characteristics of the cell of Fig. 1, in dark and under ‘‘one
Sun’’ illumination (100 mW/cm2), are plotted in Fig. 2. A V OC of 1.15 V was
ID
80791
Title
Characterizationofhigh-photovoltageCuPc-basedsolarcellstructures
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