IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 57, NO. 6, DECEMBER 2010
3109
Origin of Radiation Induced Damage in Organic
P3HT:PCBM Based Photocells
Roderick A. B. Devine, Clay Mayberry, Ankit Kumar, and Yang Yang
Abstract—Organic semiconductor photocells based upon
P3HT:PCBM (1:1 by weight) have been subjected to X irradiation. The carrier lifetime has been determined using a pulsed
optical method with continuous light to generate an open circuit
voltage bias. No effect is observed on the carrier lifetime up to 300
krad (SiO2 ). However, changes in the open circuit voltage are
observed and we argue that these result from compensation of the
internal field which results from the built-in potential. The origin
of the field modifications must lay in the presence of radiation
induced trapped charge near the organic semiconductor/anode
and/or cathode interfaces.
Index Terms—Carrier lifetime, organic semiconductors, photovoltaic cells.
I. INTRODUCTION
S
OLAR panel arrays reserve a place of prime importance in
the field of space based communications, guidance and observation. The basic materials of choice are inorganic semiconand power conversion
ductors such as GaAs and
efficiencies up to 35.8% and 16% respectively have been obtained. In the former case the cells are typically triple junctions
based upon single crystal substrates whereas the latter are thin
film. An important parameter to be taken into account however,
is the “specific power” available from an array quoted in Watts
per kg since one would like to minimize the launch payload associated with each element and launch cost (typically $10 K per
kg). A typical 5 kW source may weigh 100 kg and have a launch
cost of $1 million. One thus understands better the reasoning
behind the use of specific power (SP) as a criterion for judgment of a photocell array’s utility. In Fig. 1 we show a plot comparing SP for Si based cells as a function of the cell efficiency.
This data is taken from a 2005 publication [1]. We note that the
lines are extrapolations drawn so as to emphasize that for any
chosen efficiency the specific power of thin film cells is superior to that of crystalline cells. This is despite the fact that single
crystal substrate based cells are in general more efficient than
thin film cells. There is a rapidly growing interest in building
“nano-satellites” (CubeSats [2]) which have the peculiarity that
they are ultra-small as compared to their “usual” counterparts
Manuscript received April 27, 2010; revised June 25, 2010 and August 13,
2010; accepted August 14, 2010. Date of publication October 04, 2010; date
of current version December 15, 2010. This work was supported by the Air
Force Office of Scientific Research (Dr. Charles Lee) under Contracts AFOSR09RV01COR and AFOSR-FA9550-09-1-0610.
R. A. B. Devine and C. Mayberry are with the Electronic Foundations Group,
AFRL/RVSE, Kirtland AFB, NM 87117 USA.
A. Kumar and Y. Yang are with the Department of Materials Science, University of California, Los Angeles, Los Angeles, CA 90095 USA.
Digital Object Identifier 10.1109/TNS.2010.2068577
Fig. 1. Specific Power as a function of cell efficiency for a series of Si based
photo-cells: a-Si is amorphous Si whilst X-Si is crystalline.
and have limited functional capacity. However, these devices
can be rapidly assembled and deployed individually or in large
numbers. In such small volumes it is hard to imagine the stowage
of rigid or semi-rigid solar panels and there is a good case to
be argued for inflatable “power spheres” [1] which could be
rapidly assembled using flexible photo-voltaic materials such as
solution depositable organic semiconductors. Though presently
such cells have efficiencies 5% [3] they are expected to have
extremely large specific powers. Conceivably, then, future systems may fly with solar arrays based upon organic semiconductors. For this to become a reality, one of the key questions remaining unanswered is how such materials will hold up against
the rigors of space radiation.
In a previous publication [4] we began a study of organic
photocells based upon a blend of p-type poly 3-hexyl thiophene
(P3HT) and n-type [6,6]-phenyl-C61-butyric acid methylester
(PCBM) (1:1 by weight ratio). X-irradiation was observed
to degrade almost all the critical photo-cell parameters (open
, short-circuit current,
, power concircuit voltage,
version efficiency, etc). In particular, it was concluded that
radiation damage resulted in a reduction in photo-generated
carrier lifetime. We have re-visited the question and in particular performed pulsed optical measurements which enable
us to directly access the carrier lifetime. The results of these
measurements, which are described and reported here, and in
particular our interpretation of the data are at odds with our
initial conclusions.
II. EXPERIMENTAL
The photocells used in the experiments to be described were
identical to those outlined in our earlier publication [4]. They
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IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 57, NO. 6, DECEMBER 2010
Fig. 2. A schematic representation of the sample geometry for visible light and
X-ray exposure.
were formed in a layer structure and sandwiched between a glass
substrate and a thin, glass cover slide as shown schematically
. Devices to be irradiin Fig. 2. Cell areas were
ated were mounted on the sample holder plate of an ARACOR
4100 X irradiation system, the cell face touching the sample
plate being the one which is traditionally exposed to visible light
to produce the photo-excited carriers. Since this face could no
longer be exposed, light was brought in from the side using a
quartz fiber as shown in Fig. 2. The beam width emerging from
the width of the photocell. Prior to irradithe fiber was
were made to confirm that
ation, some measurements of
the orientation of the fiber with respect to the photocell surface
did not play a role i.e. this was a crude beam uniformity check.
was detected whether exposure was carNo difference in
ried out “side on” or from the top surface. It is probable light
guiding and multiple reflection effects within one or both of the
glass encapsulation slides in the “side on” mode explains this
effect. It has not, however, been confirmed.
The X-ray attenuation in the top cover slide was measured
via a Si photodiode and a bare cover glass and it was found
that 46% of the incident dose actually reached the organic cell
structure. As a general rule the W target of the ARACOR was
run at 50 kV/5 mA resulting in a dose rate at the organic film
per minute-total doses were accumulated
of 2.9 krad
up to 300 krad
. We have performed experiments with
per minute to 0.73 krad
dose rates in the range 2.9 krad
per minute to ascertain whether or not a dose rate effect
was present. None was observed for this dose rate range. For
the present experiments we concentrated only upon the open
.
was induced by the continuous light
circuit voltage,
source (150 W halogen lamp) and was measured directly using
pulses
a high input impedance Keithly digital voltmeter. 2
of light from the Xenon flashlamp were superimposed on the
continuous light level and induced a small variation in the open
. The flash level was adjusted so that typcircuit voltage,
, i.e. a small perturbation. Flash rates
ically,
were 600 Hz. The
pulses induced by the flash lamp
were recorded using a Tektronics model TDS 640 digitizing oscilloscope. Multiple-pulses were accumulated to permit noise
averaging.
III. RESULTS
Two types of experiments were performed, one set in which
value using the conthe photocell was “biased” to a chosen
tinuous light source and the sample then irradiated up to an ac,
being monitored
cumulated X-ray dose 300 krad
Fig. 3. Typical experimental V versus time after light pulse plots for a
photocell biased to V = 0:29 V(1) and 0.45 V (o).
at different total doses. In the second set, the cells were optically
value and then the carrier lifetime meabiased to a chosen
sured as a function of accumulated dose by adding a small
component induced by the 2
light pulse. Before performing
these measurements, however, the cells were calibrated for their
.
optical response,
A. Calibration of the Photocells
In the first instance the photogenerated carrier lifetime was
. This was set by adjusting the
determined as a function of
continuous light level to obtain the desired value. Note that
the lamp was not calibrated in terms of its optical output and
emission spectrum. Typical pulsed open circuit voltage relaxand 0.29
ation curves are shown in Fig. 3 for
V. The zero is offset arbitrarily. As will be justified later, the
curves were fitted to a simple exponential decay to determine
the carrier lifetime, . was determined over the range of values
. The results are shown for a variety of
samples in Fig. 4. Note that for small
where the photo-generated carrier density is small the carrier lifetime is very long.
The values shown in Fig. 4 are consistent with those obtained
by other researchers [5] although the latter performed measurevalues from 0.41 V to 0.6 V which apments over a range of
for the P3HT:PCBM (1:1) system.
proached the maximum
( 0.62 V). Furthermore, they used a monochromatic, pulsed
light source. It is not presently known if this has a particular
effect.
B. Effects of X-Irradiation on the Open Circuit Voltage,
To complete measurements reported previously [4] we biased
cells optically at three different starting
values: 0.34 V,
0.40 V and 0.44 V and then irradiated them out to accumulated
.
was measured at each dose
X-ray doses of 300 krad
point. The data is shown in Fig. 5. Though we have tried various
dose dependent functions to fit the observed experimental behavior, none of the obvious forms such as an exponential apply.
An alternative representation may be instructive. In Fig. 6 we
where the
plot
value at 220
is chosen simply because for one of
DEVINE et al.: ORIGIN OF RADIATION INDUCED DAMAGE IN ORGANIC P3HT:PCBM BASED PHOTOCELLS
Fig. 4. Photo-generated carrier lifetime as a function of open-circuit voltage
as determined from pulsed optical relaxation measurements. The open circuit
voltage was established using continuous light “bias”.
Fig. 5. Effect of accumulated X-ray dose on the open circuit voltage of cells
initially biased at V = 0:34 V , 0.40 V and 0.44 V. The data for V =
0:59 V was obtained previously [4].
3111
0
Fig. 6. Normalized value, V (220 krad(SiO ))=V (pre irradiation)
for the four devices shown in Fig. 5 irradiated with different initial V values.
Fig. 7. An alternative representation of Fig. 6 showing that the absolute value
of the variation of the open circuit voltage after 220 krad (SiO ) with respect
to the pre-irradiation value increases as V (pre irradiation) decreases.
0
the data sets this was the highest dose used. It would appear that
increases as the initial
value decreases.
the reduction in
In Fig. 7 we show the absolute value of
as a function of
. Despite significant
scatter it is clear from this representation that for the same
increases as
radiation dose
decreases.
C. Effects of X-Irradiation on the Carrier Lifetime,
For the samples irradiated at three different initial
values
(0.34 V, 0.4 V and 0.44 V) we systematically measured the carrier lifetime at each accumulated radiation dose. The results are
from
shown in Fig. 8 together with the initial curves
Fig. 4. It would appear that although
decreases with radiation dose, remains invariant. This result is contrary to our previous assumption [4] that radiation induced defects which could
scatter carriers and so reduce the carrier mobility/lifetime.
Fig. 8. Variation of the carrier lifetime plotted as a function of V in unirradiated (1; ) and irradiated ( ; ; ) photocells. In the former case variations of
V were induced by variation of the light intensity from the continuous halogen
lamp. The radiation induced V variation points are taken from (Fig. 5). Within
experimental error we conclude that does not vary with radiation dose even
though V does.
r
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IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 57, NO. 6, DECEMBER 2010
IV. DISCUSSION
In our earlier work [4] we measured the effects of X irradiation on P3HT:PCBM photocells by examining the current density/voltage (J(V))curves of the cells in the dark and under illumination. From these curves we determined the open circuit
and other cell parameters.
voltage, the short circuit current
In order to try to correlate these macroscopic variables with microscopic physical changes we examined the J(V) curves in the
, where the photo-inneighborhood of the built-in potential,
duced current can be written [9]:
(1)
where
is the photo-induced carrier mobility and L the
the derivative
sample dimensions. In the region of
is linear and equal to
so that one can
product. This was done in [4] and it was found
deduce the
so that
to be X ray dose dependent. Approximating
it was therefore concluded that varied with radiation
dose. Clearly an independent verification of this result was
desirable and this has been undertaken.
A. Justification of the Determination of
As mentioned in the introductory paragraphs, we determined
the carrier lifetime assuming that it is described by the relax. This assumption is justified following the work
ation of
of Shuttle et al. [5] using first order kinetics:
(2)
where
is the small change in the number of photo-generated
is a pseudo-first order rate constant. Solution
carriers and
of this equation is clearly exponential so that we can justifiably
pulse.
determine from the exponential relaxation of the
From their work Shuttle et al. [5] determined that followed a
:
relationship with
(3)
with
and
. For our unirradiated samand
. These
ples (Fig. 4) we determine
results may suggest that even though we find an exponential re, the and factors may depend
lationship between and
considered and/or on exact details of
upon the region of
cell manufacture (processing, for example). The important conclusion that we extract with relation to our irradiation studies is
that when
varies we anticipate a change in since there is
relationship.
a
B. Factors Affecting
The “exact” origin of the open circuit voltage appears to remain ill defined and a topic of considerable discussion [6]–[8].
There is a consensus that for a one Sun irradiation intensity it
where
(exmay be expressed as
pressed in eV) is the energy difference between the highest occupied molecular orbital (HOMO) of the donor type polymer
and the lowest unoccupied molecular orbital (LUMO) of the
electron acceptor. Note that energy levels are expressed in terms
of their value below the vacuum level. For the case in point,
and
so
. The experithat the difference is 0.9 eV leading to
mental maximum value is 0.62 V [6]. A more physical definition
has the form [10]:
of
(4)
where P is the dissociation probability of the e-h pairs into free
is the
carriers, is the Langevin recombination coefficient,
density of states in the conduction band and G is the generation
rate of electron-hole pairs (i.e. the photo-generated carriers).
This expression anticipates such effects as the role of light intensity (through G). The carrier lifetime varies as the density of
free carriers [10] so that we anticipate that will increase as
decreases and the carrier density decreases.
C. Interpretation of the Radiation Effects in P3HT:PCBM
The dose independent lifetime data shown in Fig. 8 when
confronted with (4) cannot explain the experimentally measured
behavior. From (4) one anticipates that
variation will be
intimately linked to variation of as indeed one observes in the
unirradiated case as exemplified by Fig. 4. The observed variawould presumably require increase in the recombitions in
nation coefficient, and/or variation of the photo-induced carrier density (10) which is not substantiated by the constancy of
the measured carrier lifetime, . Radiation induced trap generathrough enhanced recombition would lead to reduction in
nation. This has indeed been observed in the case of amorphous
Si solar cells subjected to energetic proton radiation, electron irradiation and X-rays [11] and correlated with the ionizing component of the radiation. To our knowledge no relaxation time
measurements have been performed to try to correlate with any
changes in for this case. It should be underlined that the amorphous structure contains large numbers of strained Si-Si bonds
and Si-H terminations which could in part be responsible for
the radiation induced charge trapping. Apparently, any analogous radiation induced modification of the network structure of
the P3HT:PCBM blend is not significant for the radiation doses
).
we have studied (up to 300 krad
The variation of
with accumulated X-ray dose shown in
Fig. 5 is therefore inconsistent with our anticipations based upon
(4). What, then, is the explanation? Examination of (4) reveals
that although we have eliminated such phenomena as radiation
induced carrier lifetime variations, we have not taken account
. The built-in
of effective variations in the energy difference
and drives the carrier
potential is directly associated with
separation resulting in
. One possible explanation of the obwith dose could therefore be that raserved variation of
diation induces charges which trap in the structure and compensate for the effect of the built-in potential. These charges
would have to be located such that they could not give rise to
photo-generated carrier scattering/relaxation. In this case one
might invoke the anode/cathode-organic interfaces where such
effects are known to be possible [12]. Such a model would, however, not provide a logical justification for the results shown in
Fig. 7 which appear to indicate (though the data set is limited in
DEVINE et al.: ORIGIN OF RADIATION INDUCED DAMAGE IN ORGANIC P3HT:PCBM BASED PHOTOCELLS
size) that the magnitude of the compensated
component increases as the bias
level decreases. In a “simple” radiation
generated charge/interfacial trapping model we see no reason
would be relevant.
why the magnitude of the bias
Finally, we remark that the ohmic character of the anode/
cathode-organic film contacts would appear to be highly rele[7]. Perhaps, under irradiation,
vant in the determination of
the nature of this contact changes though again, one sees no
would be relevant.
reason why the magnitude of the bias
There are clearly many unanswered questions of a physical nature concerning the effects of radiation on organic based photocells which require further study.
V. CONCLUSIONS
We have re-examined the origin of radiation effects in
P3HT:PCBM based organic photocells. We observe radiation
induced reduction of the open-circuit voltage but without
concomitant variation of the carrier lifetime. This is in contradiction with our earlier analysis [4]. A consistent picture would
be radiation induced charge trapping near the anode/cathode
interfaces giving rise to a compensating electric field opposing
the effects of the built-in potential but without increasing the
carrier relaxation rate. Independent evidence for this charge
buildup is required.
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