This document summarizes a study on how cathode composition influences the lifetime of organic photovoltaic cells. The study found that cells using a Ca/Ag cathode reached operational lifetimes of 2400 hours under standard sunlight illumination at 25°C, while cells with Al or LiF/Al cathodes degraded much more quickly. At an accelerated aging temperature of 70°C, the efficiency of cells with a Ca/Ag cathode remained over 1% after 600 hours, whereas the performance of cells with Al or LiF/Al cathodes dropped significantly over that time period. The results suggest that cathode composition plays a key role in the stability and longevity of organic solar cells.

1. SPIE Newsroom
10.1117/2.1200606.0307
Cathode composition greatly
influences the lifetime of organic
photovoltaic cells
R´emi de Bettignies and St´ephane Guillerez
Organic solar cells using Ca/Ag as cathode can reach operational life-times
of 2400h under one sun illumination, at 25◦C, in a controlled
atmosphere.
Recent progress in the field of organic solar cells1 suggests
that industrially viable systems for small handheld devices
and large-scale power generation are conceivable.2 Photovoltaic
cells based on flexible, lightweight plastics promise both low
production costs and large conformable devices.3,4 Although
improved performance of polymer solar cells is essential
to validate the field, lifetime is still the key obstacle to
commercialization. Accordingly, lifetime studies are needed to
develop efficient organic photovoltaics.5 We report such studies
incorporating varying cathode composition and thickness
conducted at the Atomic Energy Commission National Institute
for Solar Energy in France (CEA–INES).
The bulk heterojunction solar cells we describe consist
of a blend of methanofullerene[6,6]-phenyl C61 butyric acid
methylester (PCBM) and poly(3-hexylthiophene) (P3HT)
sandwiched between two electrodes. The anode was transpar-ent
indium tin oxide, and the cathode was either LiF/Al or
Ca/metal. The best power conversion efficiencieswere obtained
for cells based on the (1:1, w/w) P3HT:PCBMratio, with LiF/Al
cathodes (0.28cm2 area) and post-production thermal annealing.
Following calibration using a monocrystalline silicon solar cell,
this setup yielded power conversion of 4.1%for AM1.5 (the stan-dard
spectrum of sunlight at the earth’s surface), 100mW/cm2.
The short-circuit current density, Jsc , was 10.9mA/cm2, and the
fill factor (FF) reached 0.64.
One method of studying and predicting the lifetime of
polymer solar cells is accelerated lifetime measurement,3 which
assesses device performance at elevated temperature but
otherwise normal conditions. Using this method, we studied the
influence of the cathode on the aging and stability of solar cells.
Figure 1. This graph presents efficiency data on up to 150h of
aging for P3HT:PCBM solar cells using three types of cathodes—
Al (squares), LiF/Al (circles), and Ca/Ag (stars)—under AM1.5,
100mW/cm2 illumination, at 70◦C.
The study was carried out under an inert atmosphere and at a
temperature of 70◦C imposed by the illumination. During the
degradation process, all of the parameters that can be extracted
from an I(V) curve were recorded, including Jsc, FF, open circuit
voltage (Voc ), efficiency (η), serial resistance (Rs), and shunt
resistance (Rsh ). We focus here on Jsc as a key parameter in
degradation.
We monitored the degradation processes of solar cells with
different cathodes for more than 600h. We extracted and
recorded all photovoltaic parameters from I(V) curves every
hour. The cells were kept in short circuit during the experiment,
even when no I(V) curve was recorded. Figure 1 shows the
variation in the four photovoltaic parameters.
Continued on next page

2. 10.1117/2.1200606.0307 Page 2/2
SPIE Newsroom
Figure 2. Shown is the variation in photovoltaic parameters (Voc, Jsc ,
FF, and efficiency) of P3HT:PCBM (1:1 w/w) solar cells using Ca/Ag
as cathode (20/200nm) under AM1.5, 100 mW/cm2 illumination, at
70◦C.
Three types of cells were tested for aging: one with a simple
Al cathode, one with a LiF/Al cathode known to enhance solar
cell performance, and one with a Ca/Ag cathode. As we did not
observe any change in the absorption spectra of the P3HT:PCBM
films, i.e., the polymer did not visibly degrade, we assume that
the nature of the cathode plays a preponderant role in the long
lifetime of organic solar cells. The cells used for this experi-ment
purposely represent average power conversion efficiency.
Initially, the cells performed similarly, regardless of cathode
composition. When exposed to AM1.5, 100 mW/cm2 illumina-tion
at 70◦C, however, the behavior of the cells began to differ
dramatically. As shown in Figure 1, the Ca/Ag-based cathode
is more stable than the Al-based cathodes, even when LiF is
introduced between the polymer and the metal. At 65◦C, the
efficiency of the Ca/Ag cathode solar cells is still greater than 1%
after 600h of continuouswork. Fromt=1h to 600h of illumination
at 70◦C, Voc dropped to less than 15% and Jsc to less than 10%; FF
was still greater than 30% (see Figure 2).
Using the thermal acceleration factor of 4 between 25 and
70◦C, obtained from recent publications,6,7 we can estimate an
operational lifetime of 2400h for a Ca/Ag cathode solar cell
under one sun illumination at 25◦C if we remove all traces of
oxygen and water by encapsulation.
This study shows that cathode composition affects the
stability of polymer solar cells. The aging process appears to
consist in cathode aging followed by polymer aging.Accelerated
lifetime measurements on different cathodes indicate the work
that must be done to fully understand the aging process of
organic solar cells.
Author Information
R´emi de Bettignies and St´ephane Guillerez
Atomic Energy Commission National Institute for Solar Energy:
Research, Development, and Innovation
Le Bourget du Lac, France
R´emi de Bettignies graduated in electronics and solid-state
physics from the Ecole Superieure d’Electricit´e (Sup´elec) in 2000.
He received his PhD in 2003 from the University of Angers in the
organic photovoltaic cells group of J.M. Nunzi.He subsequently
joined the Laboratory of Organic Components at CEA-Saclay,
where he studied model organic solar cells and aging processes.
References
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Physique 3, pp. 523–542, 2002.
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c 2006 SPIE—The International Society for Optical Engineering