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J. Renewable Sustainable Energy 7, 013111 (2015); https://doi.org/10.1063/1.4906915 7, 013111
Outdoor performance of organicphotovoltaics: Diurnal analysis, dependenceon temperature, irradiance, and degradationCite as: J. Renewable Sustainable Energy 7, 013111 (2015); https://doi.org/10.1063/1.4906915Submitted: 06 October 2014 . Accepted: 17 January 2015 . Published Online: 29 January 2015
N. Bristow, and J. Kettle
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efficiency, despite the glass encapsulation. Another approach was adopted by Hauch et al., who
used an additional barrier film to encapsulate flexible Poly(3-hexylthiophene-2,5-diyl):Phenyl-C61-
butyric acid methyl ester (P3HT:PCBM) modules on Polyethylene terephthalate (PET)7 and
showed over 1 year, the performance dropped by only 20% of the original efficiency. Other work
includes that of Krebs who showed how edge sealing plays a significant role in preserving the
OPV from environmental degradation by using glass-fibre reinforced thermosetting epoxy (in this
case, prepreg8) as an edge sealant, which demonstrated a reduction in efficiency of only 65% over
a year.9 In addition there have been a number of inter-laboratory stability tests; Gevorgyan et al.conducted an inter-laboratory monitoring programme of flexible modules of P3HT:PCBM on PET,
which were encapsulated by a barrier film from Amcor Flexibles and studied at different outdoor
locations.10 On average, the performance dropped by 40% of the original efficiency after approxi-
mately 1000 h (�42 day) of outdoor exposure. Few of these reports present data showing how an
OPV performs relative to other modules positioned at the same site or have presented measure-
ments to explain how climatic conditions affect the OPVs performance.
In this work, data are presented for two separate outdoor monitoring campaigns performed
on OPV modules made by the Technical University of Denmark (DTU), supplied as part of
their freeOPV programme.11–13 The focus of this work is benchmarking the performance of
OPV modules relative to c-Si modules and identifying how climatic conditions affect solar cell
performance. For the first time in literature, the temperature coefficient of OPV modules is
reported based on outdoor performance data. Results are also shown for how module degrada-
tion is affected by seasonal variation during the summer and winter.
II. EXPERIMENTAL
A. Organic photovoltaic modules
Roll-to-roll coated OPV modules produced at the DTU were used for the tests. The fabrication
of these modules is reported by Krebs et al.12 and were encapsulated using flexible Amcor packag-
ing barrier foil and epoxy adhesive. The devices had an ITO free structure of Ag-grid/Poly(3,4-
Ag-grid/PET-substrate. The device terminals were connected to easily accessible electric plugs,
which were connected to a switch matrix and Source Measurement Unit (SMU). The modules have
a PET/SiOX barrier layer with a refractive index which is similar to that of glass (�1.57). Prior to
monitoring, the modules were fixed on a platform so that air could circulate behind the panels.
B. Outdoor monitoring setup
The OPV modules were measured in at the School of Electronics, Bangor, Gwynedd,
North Wales, which has latitude and longitude of 53.2280� N, 4.1280� W and is located at low
altitude (20 m above sea level) and 250 m from the Menai Straits (Irish Sea). Long term cli-
matic average temperatures for the winter are 4.7 �C and for the summer are 14 �C. However,
the first of these tests was conducted during the summer of 2013, which corresponded to a pe-
riod of abnormally hot and humid conditions, with an average mean temperature of 16.5 �C and
an average maximum temperature of 20.2 �C. Data are also supplied for a winter measurement
campaign during which climatic conditions were milder than usual with an average mean tem-
perature of 8.8 �C, an average minimum of 3.1 �C, and an average maximum temperature of
11.7 �C. The humidity levels for both summer and winter were very similar: with an average
mean of 79%, an average maximum of 90%, and the average minimum being 61% in summer
and 65% in winter. UV indices were very different with an average of 1.07 in the summer (av-
erage maximum 5.45) and 0.43 in the winter (average maximum 2.38).
The outdoor monitoring system at Bangor University is on the roof of the School of
Electronic Engineering. Two 185 Wp silicon modules (manufactured by Pure Wafer Solar Ltd.,
Swansea) which are monitored using an Egnitec PVMS250 PV measurement system (manufac-
turer Egnitec Ltd., Caernarfon, Gwynedd, UK). The modules are kept at maximum power point
in between periodic current-voltage (IV) sweeps (once every minute) and each has a PT100
013111-2 N. Bristow and J. Kettle J. Renewable Sustainable Energy 7, 013111 (2015)
temperature sensor fixed to its backplane. Current and voltage at the maximum power point
(IMPP, VMPP) and PT100 measurements are taken every 15 s.
The OPV measurement system comprises an 8-channel measurement unit with switch matrix and
two separately adjustable mounting boards on a rack (Figure 1). Each board can have up to nine OPV
modules mounted on it and an IMT GmbH solar silicon reference cell for irradiance measurements.
There are two PT100 temperature measurement channels: one was mounted on a horizontal module
and the other on an inclined module. For the purpose of this experiment, one silicon module and one
OPV mounting board were mounted horizontally and the other silicon module and OPV mounting
board were inclined at 35�, which is the optimum inclination for maximising solar power over a year
in Bangor, Gwynedd. The outdoor measurement setup conforms to the ISOS-O-1 outdoor measuring
protocol.3 The data were analysed using a combination of MySQL, MS Access, and MS Excel.
III. RESULTS
Eight DTU modules were monitored for eleven weeks over the summer of 2013 (start date:
09/07/2013) and for fourteen weeks over winter 2014 (start date: 14/01/2014). For the summer
campaign eight modules were monitored with four mounted on the horizontal rack and four
mounted on the inclined rack. For the winter campaign, only three modules were monitored, all
mounted on the inclined rack.
A. Comparison with c-Si modules
Initially, two contrasting days were used to compare relative OPV performance with that of
c-Si. The 3rd of August (corresponding to a sunny day) and the 17th August (overcast day)
FIG. 1. (a) Schematic of the DTU module used for outdoor performance monitoring showing individual layers in the cell
and (b) outdoor performance monitoring setup established in Bangor, Gwynedd, North Wales, which has a latitude and lon-
gitude of 53.2280� N 4.1280� W.
013111-3 N. Bristow and J. Kettle J. Renewable Sustainable Energy 7, 013111 (2015)
were selected and daily temperature and irradiance is shown in Figs. 2(a)–2(c), allowing us to
draw a comparison between the performance of c-Si and OPV modules under diffuse and direct
irradiation. Optical measurements indicate that the irradiance on the 17th August is
FIG. 2. Diurnal module temperatures of OPV and c-Si modules and ambient temperature for the (a) 3rd and (b) 17th
August 2013 and (c) diurnal irradiance on 3rd and 17th August 2013.
013111-4 N. Bristow and J. Kettle J. Renewable Sustainable Energy 7, 013111 (2015)
predominantly diffuse. As the days were reasonably close to one another, the effects of module
degradation are not significant. On the 3rd August, c-Si module temperature reaches 38.1 �C,
whilst the OPV reaches 32.7 �C, despite the ambient temperature only reaching 21.2 �C; indicat-
ing the maximum variation in temperature between cell and ambient (TDelta) is þ16.8 �C for
the c-Si and þ11.6 �C for the OPV. The OPV and c-Si modules were subjected to similar levels
of ambient cooling, so the data indicate that OPV modules are less likely to heat up under out-
door operation. The variation in heating of the modules is likely to be due to the c-Si absorbing
greater infrared radiation, as silicon absorbs up to 1100 nm.13
The following graphs [Figs. 3(a)–3(h)] show open circuit voltage (VOC), short circuit cur-
rent (ISC), FF, and PCE plotted for the two days. Considering first of all the data for the 3rd
August, this represents a day with much stronger direct irradiation and where daily irradiance
reaches 1000 W/m2. During the afternoon intermittent full sun was observed, leading to cloud
lensing affects. It can be seen that the value for ISC tracks the diurnal irradiance level almost
exactly for both OPV and c-Si modules, although some shading is seen in the c-Si in the early
morning (before 8:00 am). However, FF and VOC show some relative variation in output
between the c-Si and OPV modules. At sunrise, the FF of the c-Si module rises up very quickly
to the maximum value (0.65) and reduces at sunset without much variation during the day,
other than a small gradual decrease to 0.58 at 2:00 pm due to module temperature rise. Some
large spikes are present in the data for Figs. 3(c) and 3(d) due to mismatching with the irradi-
ance data caused by cloud lensing. Conversely, the FF and VOC of the OPV module rise more
slowly to their limiting values after sunrise, indicating that FF and VOC for the OPV modules
are more affected by low irradiance than for the c-Si module. The cloud lensing mismatch is
not as obvious with the OPV modules as the IV measurement sweep takes longer (�8 s) than
the c-Si module (�1 s).
Considering data from the 17th August, where the irradiation levels are much lower (and
mainly diffuse) and ambient temperatures are also slightly lower. As with data for the brighter
day (3rd August), the ISC of both the OPV and c-Si track the diurnal irradiance level very
closely and the shading effects seen on the 3rd are removed due to the higher proportion of dif-
fuse light. For FF and VOC, the c-Si module shows similar trends with the brighter day, rising
close to the maximum value early in the morning and staying reasonably flat thereafter until
sunset. Significantly though, the OPV shows quite large variation between the data obtained on
the 3rd and that obtained on the 17th August; the FF reaches a maximum value of 0.33, sub-
stantially lower than the 3rd August value, and the VOC does not appear to reach the maximum
value, reaching a limit of 3.49 V.
B. Effect of irradiance on OPV performance
The following graphs [Figs. 4(a)–4(d)] show the relationship of ISC, VOC, FF, and effi-
ciency to irradiance for both days (3rd and 17th August). As anticipated, ISC has a linear
FIG. 3. Diurnal c-Si and OPV performance from the 3rd August 2014 (sunny day) showing (a) VOC, (b) ISC, (c) fill factor,
and (d) PCE, with corresponding data from the 17th August (overcast day) showing (e) VOC, (f) ISC, (g) FF, and (h) PCE.
013111-5 N. Bristow and J. Kettle J. Renewable Sustainable Energy 7, 013111 (2015)
relationship to irradiance for both OPV and c-Si modules and the rate in change of ISC as a
function of irradiance is constant for both days. Likewise VOC shows expected behaviour with
a logarithmic relationship to irradiance for both c-Si and OPV modules. However, there is a
FIG. 4. The effect of irradiance on (a) ISC, (b) VOC, (c) FF, and (d) PCE for OPV and c-Si modules on the 3rd and 17th August.
013111-6 N. Bristow and J. Kettle J. Renewable Sustainable Energy 7, 013111 (2015)
clear difference in the low light behaviour of VOC and FF for the OPV and c-Si modules,
observed from the data obtained at low irradiance levels [Figs. 4(b) and 4(c)]. As irradiance is
increased, the c-Si modules reaches close to its limiting value for VOC at low levels of irradi-
ance and this is consistent for both sunny and cloudy days: 90% of its maximum VOC value is
reached at an irradiance of 64 W/m2. However, in the case of VOC for the OPV module it can
be seen that the curves for the two days do not overlay and compared to c-Si, VOC rises much
more slowly with increasing irradiance. Furthermore, on the diffuse day the rate is even slower
and maximum VOC is not reached. On the bright day VOC reaches 90% of its maximum value
at �200 W/m2, whereas on the diffuse day this level is reached at �400 W/m2. This poorer low
light behaviour of the OPV is also observed in the FF characteristics.
Figure 4(d) shows that the PCE dependence on irradiance is determined primarily by VOC
and FF, since ISC is linear with irradiance. As the OPV exhibits poor VOC and FF at lower irra-
diance, the overall PCE at these levels is also low and only reaches a peak at 600 W/m2. As the
low light performance of the c-Si is much better, the peak PCE value is reached at much lower
irradiances (150 W/m2).
Overall, the data from Figs. 3 and 4 suggest that the OPV module is not well suited for
performance under low irradiances, based on the current device architecture. The primary rea-
son for the poorer VOC and FF under low light conditions is the appearance of inflexion
behaviour in the IV curves. The majority of the improvement in VOC and FF with increasing
irradiance is as a result of the removal of the inflexion characteristic in the IV-curve. This
behaviour has been reported in various papers for indoor experiments and several theoretical
explanations have been devised to account for this behaviour.14,15 The most commonly
reported cause is attributed to an energy barrier, caused by a poor carrier transport in one of
the layers or interfaces, which prevents charge extraction from the device, leading to
decreased VOC and FF. These reports have shown the inflexion characteristics can be over-
come by exposing the cells to a combination of illumination, temperature, and with UV light
with wavelengths between 360 and 400 nm.16
The data in Figs. 4(a)–4(d) suggest OPV modules underperform c-Si modules at low irradi-
ance levels as the conditions affecting the photo-annealing (temperature, irradiance, and UV
index) at these times are low. The OPV only reaches its maximum PCE when irradiance and
temperature is at its highest (for example, at 11 am on 3rd August). Therefore, over the course
of a day, the OPV modules are subjected to a “temporal photo-annealing” effect; at early morn-
ing and late evening or other times of low irradiation, the photo-annealing effect shows low in-
tensity, limiting VOC, FF, and thus PCE from the cell, whilst on days such as 10 am–5 pm on
3rd August, the OPV module experiences a high intensity photo-anneal and reaches its maxi-
mum possible PCE. This supports the view of Lilliedal et al. who studied how various anneal-
ing processes affected the cell performance.16 The report showed a “low intensity” treatment
had a small impact on photovoltaic performance after treatment though a “high intensity” treat-
ment had a much stronger effect reducing the time taken for removal of the inflexion points in
the IV characteristics. However, our data indicates that when the modules are left under low
irradiance for a few minutes, the behaviour will re-emerge in the IV curves.
C. Effect of temperature on OPV performance
Figures 5(a)–5(d) show the effect of temperature on OPV performance in outdoor environ-
ments. One issue with using outdoor data to understand how variations in temperature affect
PV performance is that temperature often increases in line with irradiance and the change in
PV performance is often a combination of the two variations. Therefore, the data are filtered
for fixed irradiance ranges (600 6 20 W/m2 and 1000 6 20 W/m2) which allows for the effect
of temperature alone to be studied. Data are shown for a 10 day period so we assume no signif-
icant OPV degradation.
VOC as a function of temperature is shown in Fig. 5(a) and shows the least variation out of
the four performance parameters investigated. It appears to increase slightly with increasing
temperature, but the level of increase is relatively low. ISC is also shown to increase with
013111-7 N. Bristow and J. Kettle J. Renewable Sustainable Energy 7, 013111 (2015)
temperature [Fig. 5(b)]. In OPVs, charge carrier transport in polymers is governed by “carrier
hopping” and is therefore thermally assisted. Mobility and thus charge-carrier transport should
improve with increasing temperature, in agreement with indoor measurements.17 In addition,
temperature increases will lead to more efficient dissociation of electrons and holes. This posi-
tive temperature coefficient of ISC is observed also in c-Si, but here the main contribution is
due to thermally excited intrinsic charge carriers and narrowing of the semiconductor’s bandgap
with increasing temperature. The FF of the OPV also appears also to rise with temperature
[Fig. 5(c)] and this is consistent with other reports and has been attributed to a decrease in se-
ries resistance as shunt resistance remains relatively unchanged with temperature.17
Ordinarily, for temperature coefficient calculations, the PV module is first shaded to lower
the temperature to ambient conditions and as soon as the device is uncovered, it rises in tem-
perature, several I-V curves are acquired at different temperatures.18–20 This is impractical for
OPVs as the temperature variation [see Figs. 2(a) and 2(b)] in an OPV over the course of a day
is relatively low. However, the temperature coefficient can also be obtained through outdoor
performance monitoring. To obtain the temperature coefficient of the OPV and c-Si module,
data were fitted to the following equation:18–20
gG ¼ gref ½1� bref ðTG � Tref Þ�; (1)
where gG is efficiency at an elevated/reduced temperature, (TG), gref is efficiency at the refer-
ence temperature, (Tref ), and bref is temperature coefficient. Due to the strongly temperature-
dependent ISC and FF, the OPV possesses a positive temperature coefficient. Based on Eq. (1),
a temperature coefficient of þ0.007%/K can be extracted for the OPV device whereas a value
of �0.34%/K is obtained for the c-Si module at Bangor University (data not shown). Previous
temperature coefficient studies using the approach adopted by Refs. 18–20 have shown that the
power of c-Si PV modules decreases at a rate of �0.400%/K (Ref. 20) based on outdoor data,
so it shows this approach gives a reasonably close value for c-Si and gives some confidence in
the OPV temperature coefficient value. The absolute temperature coefficient of the OPV is low
compared to that of the c-Si, however the PCE at Standard test Conditions (STC) conditions is
also low (see Table I). Therefore, the temperature coefficient values have been normalised to
FIG. 5. Temperature dependence of (a) VOC, (b) ISC, (c) fill factor, and (d) PCE measured at fixed irradiance of 600 W/
m2 6 10 W/m2 and 1000 W/m2 6 10 W/m2. Linear fitted curves are applied to measure the temperature coefficient of the
module.
013111-8 N. Bristow and J. Kettle J. Renewable Sustainable Energy 7, 013111 (2015)
the individual performance at Standard Test Conditions, STC, (1000 W/m2, 25 �C) to compare
the relative changes seen in OPV modules and c-Si modules. The normalised increased in the
OPV is þ0.014/K whereas the c-Si sees a normalised decrease of �0.028/K (see Table I).
As a comparison with other thin film technologies, CdTe modules possess a measured tem-
perature coefficient of around �0.25%/K (Ref. 21) and a-Si shows a power temperature coeffi-
cient of up to approximately �0.17%/K (Ref. 22), so the OPV appears to be one of the few PV
technologies that possess a positive temperature coefficient.
D. Degradation of OPV modules
The graphs in Fig. 6 show how the performance parameters Isc, VOC, FF, and PCE change
with time. Two sets of data have been acquired; one set follows OPV degradation during the
summer months (July–September 2013) and the second set follows degradation of the same
type of modules during winter (January–April 2014). Data were selected for a fixed irradiance
(600 6 15 W/m2).
Considering first of all the summer data, this data is represented in Figs. 6(a)–6(d) with
data shown from one module (“DTU2”). Overall, 8 modules were tested and showed relatively
similar initial PV performance (PCE� 0.7% 6 0.2%) at 600 W/m2, demonstrating the high pre-
cision and control of modules developed in the freeOPV programme. Out of the 8 modules
tested, they all exhibited one of two degradation patterns. The first degradation pattern sees
TABLE I. Temperature coefficient of OPV and c-Si obtained from the outdoor monitoring campaigns and typical values of
other technologies obtained from literature.
Temperature
coefficient, bref (%/K)
PCE at STC
(1 sun, 25 �C)
Normalised temperature
coefficient (AU/K)a
OPV þ0.007 0.69 þ0.010
c-Si (Bangor) �0.341 12.89 �0.026
c-Si20b �0.400 18.20 �0.022
CdTe20,21 �0.25 No data
a-Si20b �0.165 No data
CIGs20 �0.320 No data
aData is normalised to the performance of each solar cell under STC (25 �C) conditions.bData is averaged from 2 (a-Si) and 9 (c-Si) modules.
FIG. 6. Change in (a) VOC, (b) Isc, (c) fill factor, and (d) PCE over time for two selected “average” modules at an irradiance
of 600( 6 15) W/m2 for summer 2013 (DTU2) and winter 2013-14(DTU35) demonstrating the increase lifetime observed
by cells.
013111-9 N. Bristow and J. Kettle J. Renewable Sustainable Energy 7, 013111 (2015)
steady decreases in all performance parameters (VOC, FF, and ISC), leading to an exponential
decay in PCE with time. The second degradation pattern [shown in Figures 6(a)–6(d)] showed
PCE initially relatively stable for the first 2 weeks and a sharper linear decrease thereafter. In
this case, the degradation falls mostly as a result in FF and ISC decreasing, as VOC remains rela-
tively steady for the initial 2 weeks. This degradation pattern was observed by half of all mod-
ules and showed better overall stability.
Overlaid on the summer degradation data are results from similar cells monitored over a
14 week period over the winter of early 2014. This allows the examination of differences in
degradation between summer and winter. It is clear that the module degradation is greatly
reduced owing to the reduced UV index, temperature and irradiance seen over this period. One
“hero” module exhibits only a 40% drop in PCE over the 14 week period. The average half-life
(t1=2) observed for winter modules was over 9 weeks, compared with a half-life of only 3 weeks
for the summer modules.
Overall, the outdoor lifetime of these cells was much lower than other technologies, This is
because the data obtained in this paper is from roll-to-roll manufactured OPVs and the substrate
used in this case was PET coated with e-beam evaporated SiO2, which acted as a barrier layer;
water vapour transmission rate are estimated at around 10�3 g m�2 d�1g/m2/day, which is
FIG. 7. Microscope photographs (magnification �2.5) showing the edge of an OPV module (a) before and (b) after outdoor
performance monitoring, demonstrating degradation at the module edges. Laser cutting is used to isolate modules, giving
rise to the burnt appearance at the edges.
013111-10 N. Bristow and J. Kettle J. Renewable Sustainable Energy 7, 013111 (2015)
insufficient for maintaining performance outdoors. In addition, the silicon dioxide barrier layer
was the only UV filter applied to these cells. Generally, the cells did not alter significantly in
appearance during ageing; the silver metal grid, electrical contacts, and active layer showing no
obvious changes in colour or appearance. One major source of degradation appeared at the
edge of devices [see Figures 7(a) and 7(b)]. It is very likely that oxygen and water can pene-
trated sideways through the device, leading to chemical degradation of the active layer, interfa-
cial layers, or electrodes.
IV. CONCLUSIONS
The outdoor dependence of temperature and illumination on OPV performance has been
analysed in this paper. The performance was benchmarked against c-Si modules and the OPVs
are shown to exhibit relatively poorer performance under low light conditions. The VOC is
shown to possess a logarithmic dependence on irradiation, but the low light performance of
VOC as well as FF is limited by inflexion points in the IV curves. These can be removed by
photo-annealing at higher irradiance; however, this effect has negative consequences for OPV
performance on overcast days. For the first time in literature, outdoor OPV evaluation has
yielded a temperature coefficient, which was found to be positive of þ0.007%/K was found,
which compared to that of �0.341%/K for c-Si. Overall, the cell degradation outdoors for these
modules appeared very severe and highlights the need for better barrier layers, UV filters and
edge sealants for this technology.
ACKNOWLEDGMENTS
The authors would particularly like to thank Morten Madsen and Frederik Krebs of the
Technical University of Denmark for their advice, support and for supplying the OPV modules
through the generous “freeOPV programme” (http://plasticphotovoltaics.org/free-opv). In addition,
the authors would like to thank the Wales Ireland Network for Innovative Photovoltaic
Technologies (WIN-IPT) project, funded through Interreg IVA, for financially supporting this work
and finally to Egnitec Ltd., Gwynedd, Wales, UK (www.egnitec.com) for supporting the work with
state of the art measurement facilities.
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