Analysis of degradation mechanisms in donor-acceptor
copolymer based organic photovoltaic devices using
impedance spectroscopy
Author
Srivastava, SB, Sonar, P, Singh, SP
Published
2016
Journal Title
Materials Research Express
Version
Accepted Manuscript (AM)
DOI
https://doi.org/10.1088/2053-1591/3/9/096202
Copyright Statement
© 2016 Institute of Physics Publishing. This is the author-manuscript version of this paper.
Reproduced in accordance with the copyright policy of the publisher.Please refer to the journal's
website for access to the definitive, published version.
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99Analysis of Degradation Mechanisms in Donor-Acceptor Copolymer
Based Organic Photovoltaic Devices Using Impedance Spectroscopy
Shashi B. Srivastava
1
, Prashant Sonar
2
and Samarendra P. Singh
1*
1
Department of Physics, Shiv Nadar University, Gautam Buddha Nagar,
Uttar Pradesh, India-201314
2
School of Chemistry, Physics and Mechanical Engineering,
Queensland University of Technology, Brisbane, Australia
(*samarendra.singh@snu.edu.in)
Abstract
The stability of organic photovoltaic (OPV) devices in ambient conditions has been a
serious issue which needs to be addressed and resolved timely. In order to probe the
degradation mechanism in a donor-acceptor block copolymer PDPP-TNT:PC71BM bulk
heterojunction based OPV devices, we have studied current-voltage (J-V) behaviour and
impedance spectroscopy (IS) of fresh and aged devices. The current-voltage characteristic of
optimized fresh devices exhibit a short circuit current (J
sc
) of 8.9 mA/cm
2
, open circuit
voltage (V
oc
) of 0.79 V, fill factor (FF) of 54.6 %, and power conversion efficiency (PCE) of
3.8 %. For aged devices, J
sc
, V
oc
, FF, and PCE was reduced to 42 %, 10 %, 56% and 76% of
its initial value, respectively. The impedance spectra measured under illumination for these
devices was successfully fitted using a CPE based circuit model. For aged devices, the low
frequency response in impedance spectra suggests an accumulation of the photo-generated
charge carriers at the interfaces which impedes the charge extraction at the electrodes and
leads to a significant lowering in fill factor. Such degradation in device performance is
attributed to the incorporation of oxygen and water molecules in devices. An increase in the
recombination resistance associated with the bulk heterojunction active layer indicates a
deterioration of free charge carrier generation and conduction in devices. Such observations
from impedance measurement appear as an electrical signature of the probable physical
mechanisms responsible for degradation of OPV devices.
Index Terms - Capacitance-voltage (C-V) characteristics, charge carrier lifetime, charge
transport, impedance spectroscopy, degradation mechanism, lifetime, organic photovoltaic
device.
I. INTRODUCTION
During the past decade, bulk heterojunction based organic photovoltaic (OPV)
devices have emerged as a potential renewable energy source. OPV devices offer a wide
range of advantages like flexibility, lightweight, low cost and high throughput processing [1].
Optimization of device architectures, improvement in device fabrication techniques and
continuous development of high performing materials have steered improvement in the
power conversion efficiency (PCE) up to >10% [2, 3]. In order to establish OPVs
competitive with conventional silicon photovoltaic technology, the efficiency and life-time of
these devices need to be significantly improved [4, 5].
In OPV devices few degradation mechanisms are known. For example: Diffusion of
oxygen and water molecules into the devices, degradation of interfaces, the active materials,
inter-layers and electrode materials, electrochemical reactions with the organic materials
during device operation and morphological changes [4-7]. However, some of these
mechanisms are interrelated and occurs simultaneously which makes it difficult to identify
exact degradation mechanisms. Further, it is more difficult to quantify effect of individual
degradation mechanism towards the overall degradation of OPV devices. Decay in current-
voltage (I-V) characteristics is typically due to changes in dynamics and recombination
kinetics of charge carriers lead by various degradation mechanisms. In order to identify the
degradation mechanism and their impact on electrical parameters of devices we need a group
of complimentary characterization tools to probe charge carrier transport and their
recombination kinetics in the bulk active layer, and at various interfaces in OPV devices.
Impedance spectroscopy (IS) is an effective tool to study transport and recombination
mechanisms in the bulk, interfaces and electrodes in OPV devices under working conditions
[8]. This technique has been applied to outline the possible degradation mechanisms by
comparing the impedance characteristics in fresh and aged devices. Using impedance analysis
for ageing OPV devices the formation of an electron barrier at the electrode barrier and role
of defects are identified as important degradation mechanisms [9, 10].
In this paper we report an investigation of the degradation mechanism in poly{3,6-
dithiophene-2-yl-2,5-di(2-octyldodecyl)-pyrrolo [3,4-c] pyrrole-1,4-dione-alt-naphthalene}
(PDPP-TNT) and PC71BM based bulk heterojunction inverted OPV devices using
impedance spectroscopy. PDPP-TNT is a conjugated structure having naphthalene and
diketopyrrolopyrrole (DPP) as donor and acceptor moieties in the polymer backbone,
respectively. PDPP-TNT is an interesting hole transporting polymer which has shown
reasonably good performance in organic field-effect transistors (OFETs) and OPV devices
[11]. In this paper, we report a comparative study of internal charge dynamics and
recombination kinetics in fresh and aged PDPP-TNT:PC71BM bulk heterojunction based
inverted OPV cells. These devices were fabricated and characterized under room ambient.
The electrical characteristics and performance of these devices were measured over three
weeks under the ambient condition without encapsulation. We have performed current-
voltage (I-V), capacitance-voltage (C-V) measurements and impedance spectroscopy (IS) to
study charge carrier transport, their accumulation, and recombination kinetics in PDPP-
TNT:PC71BM bulk heterojunction based fresh and aged devices.
II. EXPERIMENTAL DETAILS
Inverted OPV devices were fabricated on patterned indium tin oxide (ITO) substrates
with a configuration of ITO/ZnO/PDPP-TNT:PC71BM/MoO
x
/Ag. The schematic of inverted
OPV devices with the molecular structure of PDPP-TNT and PC71BM is shown in Fig. 1.
ITO coated glass substrates were cleaned using soap solution (2% micro-90) in de-ionized
(DI) water at 50
0
C for 20 minutes. The substrates were rinsed with DI water, and sonicated
in acetone and isopropanol, respectively. Further, these substrates were dried with dry
nitrogen (N
2
) and treated with UV-ozone to induce the hydrophilicity. A sol-gel of ZnO (0.45
M) was prepared using zinc acetate in 2-methoxyethanol and ethanolamine. The prepared
solution was spin coated at 2000 rpm for 60 seconds to deposit ZnO thin film (~ 40 nm) on
the substrate. A drying process was performed on a hot plate at 250
0
C for 15 minutes in
room ambient [12].
The PDPP-TNT was synthesized as described earlier, and PC71BM was
used as it was received from Lumtec, Taiwan. The PDPP-TNT:PC71BM blend layer was
deposited by spin-coating a solution (15 mg/ml) of PDPP-TNT (33 wt%) and PC71BM (67
wt%) in a mixture of chloroform and o-dichlorobenzene (4:1 by volume) at 5000 rpm for 60
seconds on top of ZnO layer. The PDPP-TNT:PC71BM layer was annealed on a hot plate at
60
0
C for 10 min to remove the excess solvent [11, 13]. The thickness of the active layer used
in these devices is ~150 nm. Molybdenum oxide (MoO
x
) of 10 nm thickness, an electron
blocking layer, was deposited on top of the active layer by thermal evaporation. The top
silver cathode (100 nm) was subsequently deposited by thermal evaporation through a
shadow mask under a pressure of ~ 2 × 10
-6
mbar to complete the device, resulting a device
area of approximately 0.09 cm
2
.
Current density-voltage (J-V) characteristics of OPV devices were measured under an
AM1.5G illumination source (1000 Wm
-2
) using a Photo Emission Solar Simulator (Model
#SS50AAA). The light intensity was adjusted with a NREL calibrated Si solar cell. The
Keithley 4200 SCS parameter analyzer was used for the measurement of J-V characteristics
of OPV devices. The impedance analyzer (Autolab PGSTAT-302N) was used for C-V
measurement and impedance spectroscopy. Impedance spectra were recorded by applying a
small voltage perturbation (10 mV) at frequencies ranging from 1 MHz to 1 Hz.
Fig. 1.
The structural design of inverted PDPP-TNT:PC71BM bulk heterojunction solar cells with
molecular structures of the active layer components.
III. RESULTS AND DISCUSSIONS
The J-V characteristics of fresh and aged OPV device, shown in Fig. 2(a) and 2(b)
respectively, were measured under dark and light in room conditions. The observed electrical
parameters of optimized fresh devices were: a short circuit current (J
sc
) of 8.9 mA/cm
2
, open
circuit voltage (V
oc
) of 0.79 V, fill factor (FF) of 54.6 % and power conversion efficiency
(PCE) of 3.8 %. The same for three weeks aged devices were: a short circuit current (J
sc
) of
5.1 mA/cm
2
, open circuit voltage (V
oc
) of 0.71 V, fill factor (FF) of 24.2 % and power
conversion efficiency (PCE) of 0.9 %.
PDPP-TNT
PC71BM