The effect of barrier performance on the lifetime of small-molecule organic solar cells

Solar Energy Materials & Solar Cells 97 (2012) 102–108

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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat

The effect of barrier performance on the lifetime of small-molecule organic solar cells a ¨ Martin Hermenau a,n, Sylvio Schubert a, Hannes Klumbies a, John Fahlteich b, Lars Muller-Meskamp , a a Karl Leo , Moritz Riede a b

Institut f¨ ur Angewandte Photophysik, Technische Universit¨ at Dresden, 01069 Dresden, Germany Fraunhofer-Institut f¨ ur Elektronenstrahl- und Plasmatechnik, 01277 Dresden, Germany

a r t i c l e i n f o

a b s t r a c t

Available online 5 October 2011

In this work, we use different encapsulations to protect vacuum-evaporated small molecule organic solar cells with a simple p-i-i-stack for lifetime studies. Our devices use ZnPc and C60 as active materials. Lifetimes (T50) in a range from 300 h for un-encapsulated devices to 4000 h for glassencapsulated have been observed. We use a model to distinguish between the water vapor transmission rate (WVTR) of the barrier and an additional WVTR of the aluminum top electrode. For all observed devices a loss of 50% of initial efficiency is observed when 10 mg m  2 water entered the device. The losses are related to a reduction of short circuit current density only, whereas open circuit voltage and fill factor remains unaffected. We relate this to an interaction of the water molecules with C60. & 2011 Elsevier B.V. All rights reserved.

Keywords: Organic solar cell Small molecule Lifetime Degradation WVTR Encapsulation

1. Introduction Organic solar cells are an exciting approach for efficient, lowweight and low-cost photovoltaic devices to tackle the challenge of the rising demand for a clean electricity supply. The clear advantage of organic materials used in photovoltaics compared to inorganic materials is the possibility to manufacture modules on low-cost and light-weight flexible substrates, e.g. in a roll-to-roll process [1,2]. In addition to efficiency and cost, device lifetime is an important factor for the commercialisation of organic solar cells. To use such low-cost production processes, it is necessary to replace glass substrates by flexible substrates like PET or PEN films. Other positive effects of flexible substrates are for example the low weight, less needed space and better transportability compared to glass or other rigid substrates. The drawback of those materials is the comparably high permittivity for water and oxygen through the substrate, while glass is considered to be impermeable [3,4]. A strong influence of water and oxygen on the degradation of both polymer and small molecule organic solar cells was shown in a previous work [5,6]. Thus, unless more stable organic materials are found, the devices need a proper protection against atmosphere. In addition to the high sensibility of most organic materials to oxygen and moisture, encapsulation is a challenge to the commercialization of organic photovoltaic modules.

n

Corresponding author. E-mail address: [email protected] (M. Hermenau).

0927-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2011.09.026

In this work, we discuss longtime measurements of p-i-i stacked organic solar cells with small molecule materials vacuum deposited on glass substrates. To reveal the effect of barrier performance, we seal those devices with different barrier foils and compare their behavior with the behavior of un-encapsulated and glass-encapsulated solar cells.

2. Materials and methods 2.1. Solar cell architecture and materials The devices presented in this article have a simple p-i-i structure, using a doped hole-transport layer on top of the semitransparent bottom contact, followed by intrinsic active layers and an undoped electron-transport and exciton-blocking layer. They were fabricated on glass substrates with a 95 nm thick, pre-structured layer of indium-tin-oxide (ITO; Thin film devices, USA) which acts as a semitransparent bottom contact. We use a p-doped layer of BF-DPB (N, N0 -ððdiphenyl-N, N0 -bisÞ 9; 9-dimethyl-fluoren-2-ylÞ-benzidine, Sensient AG) as hole transport layer. Fluorinated C60 (C60F36) acts as p-dopant [7]. The main photovoltaic active layer is formed by a 30 nm thick bulk hetero-junction of ZnPc (zinc phthalocyanine) and C60 in a 1:1 weight ratio, sandwiched between 5 nm ZnPc and 30 nm C60 for additional absorption. Assumed densities for the active layers are 1.34 g cm  1 for ZnPc and 1.54 g cm  1 for C60, respectively. As electron transport and exciton blocking layer, we use 6 nm BPhen (bathophenanthroline; 4,7-diphenyl-1,10-phenanthroline). The top contact is formed by 100 nm aluminum evaporated

M. Hermenau et al. / Solar Energy Materials & Solar Cells 97 (2012) 102–108

103

Fig. 1. Overview over device architecture (left) and layer structure (right).

Table 1 List of devices and encapsulations as well as initial IV characteristics calculated from JV curves measured under simulated sun light (Optopolymer 16S–150V.3, Germany) with an intensity of 100 mW cm  2, corrected by spectral mismatch. The showed JV measurement values are presenting each device out of four with the best efficiency. The WVTR values are calculated for the mean measurement conditions (T¼45 1C; rH ¼5.5%). PET triple stands for a PET film coated with 100 nm ZTO þ 500 nm organic layer þ 100 nm ZTO. For the glass encapsulation, permeation of water and oxygen occurs only through the glue. General

JV characteristics

Device Encapsulation

JSC WVTR (g m  2 d  1) (mA cm  2)

VOC

FF (%) Z (%)

A B C D E

– – PET Melinex ST 504 PET þ 100 nm ZTO PET triple

6.9 6.9 0.43 0.02 1:4  103

7.52 7.57 7.95 8.06 8.19

0.53 0.53 0.54 0.54 0.54

52.7 52.4 52.2 52.1 51.8

2.12 2.12 2.23 2.25 2.28

F

PET þ 245 nm ZTO

3:7  104

8.21

0.54

52.2

2.30

G

Glass

3:9  105

7.75

0.53

53.4

2.20

at a base pressure of 1:7  106 mbar with an average rate of 0.2 nm s  1. The layer sequence is shown on the right side of Fig. 1. Further details on the preparation process have been described earlier [6]. On each substrate four single devices are placed with an area of 6.44 mm2 each, defined by the overlap of ITO and aluminum. 2.2. Device encapsulation For sealing the glass substrate and the top barrier, we use a commercially available UV curing epoxy glue (Nagase Chemtex, XNR5516ZHV-B1). This glue is thermally stable upon 90 1C and UV sensitive at 365 nm and below. In previous work, we showed that for comparable devices, water is more critical for the long-term stability compared to oxygen [6]. Thus, we concentrate on the water vapor transmission rates (WVTR). Different types of encapsulations were applied to the organic devices (Table 1). The devices D and F were encapsulated using a permeation barrier film based on a reactively sputtered zinc-tinoxide (ZTO) single layer barrier on a PET substrate (Melinex ST504, 125 mm for device D and Melinex 400 CW, 75 mm, for Device F) [8,9]. Device E was encapsulated using an all-in-vacuum coated multilayer barrier that consists of two 100 nm thick ZTO barrier layers and a 500 nm thick SiOxCyHz plasma polymer interlayer [10]. The WVTR values are usually given for 38 1C and 90% relative humidity, measured with a MOCON test. We apply a common procedure to calculate those values for the measurement conditions (45 1C and 5.5% relative humidity) we use in our experiments [11].

For the unprotected devices and the glass encapsulation we measured the WVTR using an electrical calcium test and applied the same procedure [12,13]. 2.3. Aging setup For exposing all devices to similar conditions, we use a selfmade set-up which has already been described before [14]. The main advantages are the possibilities to set certain values for temperature and light intensity and keep those values constant over long measurement times. For illumination, we use white LEDs with an emission spectrum in the range from 400 to 700 nm. The intensity is controlled by changing the driving current of the LED and kept constant during the overall experiment. The specific intensity for each solar cell is determined by measuring the first value for JSC under the LED and comparing this value to the one measured under an artificial sun (Optopolymer 16S–150V.3, Germany), which is set to 100 mW/cm2 with taking the spectral mismatch into account. Because of the lenses, two LEDs are used to illuminate the four devices on each substrate, and can therefore set the intensity on two devices to a certain value while the remaining devices get a constant but undefined intensity. This is done to investigate the effect of intensity, we use 100 mW cm  2 and 500 mW cm  2 for device numbers one and four on each substrate. The relative change of the intensity over time is measured using a Si photo-diode and JSC is corrected for relative deviations, which are typically smaller then 5% over 1000 h. Temperature is set to 4573 1C and monitored. Relative humidity is measured using a commercially available USB sensor (pcsensor.com) and ranges from 0.1% to 15.9% with an average value of 5.5 72.7%. These conditions are set according to the ISOS measurement protocol ISOS-L-1 [15]. 2.4. Stretched exponential decay (SED) To evaluate the datasets and eliminate scattering in the solar cell characteristics, which usually occurs in long-time experiments over hundreds or thousands of hours, we use a fitting function. The so called Stretched Exponential Decay (SED) successfully used for the description of degradation dynamics of for organic light emitting devices (OLED) [16,17]. The function is described by:  b ! JSC t ¼ A  exp  ð1Þ JSC,0 t Here, JSC is the short circuit current density at time t, JSC,0 is the short circuit current density at the beginning of the experiment, t is the time and A, b and t are fit parameters. The SED describes a

104

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time-dependent decay probability, for example a reduction of reactive molecules after a certain degradation took place. JSC is divided by JSC,0 for normalization. Here we have to note that not the first, but the second measurement point after starting the measurement (which means: after switching the LEDs and the heating on) is used because the heating usually needs several minutes to reach 45 1C and stabilize there. Therefore, especially VOC and FF change between first and second measurement. At the second data point, which is recorded 1 h after start, target temperature is reached and therefore the measurement values are used for normalization. In this work, we use JSC for describing the degradation kinetics, since this is the only factor which is affected by device deterioration. For measurement curves like those shown in this paper, we use 1, 1 and 1000 as starting values for A, b and t, respectively. A is fixed at 1, because no initial burnin effects are visible for our measurement curves. From the fitted parameters we can derive the device lifetime following the recommendations of the ISOS protocol [15]:   1=b 0:8 T 80 ¼ t  ln ð2Þ A

Since only JSC is affected by device deterioration, we concentrate on it for the discussion of the long-term behavior of our devices. 3.2. Intensity dependence In previous work, a strong intensity dependence of device deterioration for encapsulated solar cells was discovered. There, the conclusion was that it is the number of extracted charge carriers which is a measure for degradation [19]. Since the possibility to apply different intensities in the aging set-up is given, device degradation at four different intensities for all measured devices is compared. As can be seen from Fig. 3, the intensity does not have an effect on device aging on this short-term device deterioration. For an intensity range from 24 to 478 mW/cm2, JSC shows a similar decay. A similar behavior is seen for all other devices. Even the

1.0

where T80 gives the time which is needed to deteriorate device efficiency to 80% of its initial value. For calculating T50, 0.8 is replaced by 0.5 in Eq. (2).

0.8

3. Results and discussion

0.6

3.1. Lifetime results Analysis of lifetime data is started with device A, which was without any encapsulation. In Fig. 2A, long-term development of JSC, FF, VOC and efficiency is shown. Since the FF and VOC remain unchanged over the complete measurement range of almost 1200 h, JSC dominates the long-term behavior of the efficiency. After 1000 h, a 90% loss of the initial JSC is visible. T50 is around 300 h, T80 only 100 h. This behavior is shown in more detail in Fig. 2B, where the evolution of the JV-curves is depicted. All curves are fixed at VOC. Reverse-direction saturation and the slope in forward direction are unchanged as well. Thus, without any S-kink behavior, FF is constant while JSC decreases rapidly. A similar behavior is present for all devices independent of the encapsulation used.

0.4

0.2

0.0 0

250

500

750

1000

1250

Fig. 3. Behavior of JSC for the un-encapsulated device A at four different intensities.

1.2

20

1.0

15

time / h

Efficiency Voc Isc FF

0.8

0.6

10

5

0.4

0

0.2

-5

0.0 0

250

500

750

1000

1250

-10 -0.50

5 100 200 400 600 800 1000 1200

-0.25

0.00

0.25

0.50

0.75

Fig. 2. (A) Time development of the JV-characteristics for the un-encapsulated device A at an intensity of 1 sun. (B) JV-curve evolution of the same device over time.

M. Hermenau et al. / Solar Energy Materials & Solar Cells 97 (2012) 102–108

3.3. Comparison of different barriers and calculation of water amount passing the encapsulation In previous work, we found that the degradation of device with an almost identical layer structure is much faster in the presence of water, compared to pure oxygen [6]. There, the lifetime of the device in a dry oxygen environment was about 6–7 times higher than in a nitrogen atmosphere saturated with water vapor. Similar behavior was reported by another group on pentaceneC60 solar cells [18]. This work showed a very fast decay of photovoltaic properties in the presence of water vapor, and a slow degradation when oxygen is present. Thus, the degradation showed in this paper is dominated by water and we can focus on discussing the water permeation properties of the different encapsulation materials. The decay of JSC of all devices is shown in Fig. 4. To overcome measurement fluctuations, all curves are fitted using the SED function in formula (1). We see a good coherence of measurement data and fit functions. Furthermore, a remarkable decrease of degradation speed is visible especially for device G, but also less pronounced for device F. Those two devices are protected by the strongest encapsulants, glass and the PET 245 nm ZTO. The other five devices with no or weak barrier materials, resulting in WVTRs from 0.01 to 10 g m  2 d  1, show an almost similar behavior with only small differences. Since the WVTRs are known for all barrier materials, one is able to estimate the number of water molecules or the mass of the water that crossed the barrier, entering the photovoltaic device,

using the formula: mH2 O ¼ WVTR  time area

ð3Þ

which gives the water amount per square meter. Fig. 5 shows the normalized short circuit current density in dependence of the amount of water that penetrated through the encapsulation. It is visible that for devices with a weak or no encapsulation, several grams of water penetrating the device, moving those curves to the right side in our plot. With increasing barrier performance, meaning a decreasing WVTR, the total mass of water, accumulated over the entire measurement time of 1125 h, decreases. Thus, the curves in Fig. 5 are shifted left for weaker barriers. For the two devices G and F, which are protected by glass and the best barrier film (PET with 245 nm ZTO), the mass of water is too low to completely deteriorate

1.0

0.8 normalized JSC

degradation of the glass-encapsulated devices shows no intensity dependence, which is in contrast to previous work. The reason for this deviating behavior is the use of another glue in this work, which leads to a much weaker WVTR for the glass-encapsulated device. Another reason is the replacement of the unstable holetransport material MeO-TPD by BF-DPB, which increases the stability in the absence of oxygen and moisture. Thus, one can conclude that there are other degradation processes dominant when devices are not well protected against the permeation of water and oxygen. This extrinsic degradation is not or only slightly accelerated by increased illumination intensity.

105

-4

4.7∗10 -2 -1 gm d

0.6

0.4

0.2

0.0 1E-7

without without PET Melinex PET 100 ZTO PET triple PET 245 ZTO glass 1E-5

-5

3.9∗10

-3

1.4∗10 1E-3

0.02

0.43 6.9

0.1

10

1000

water amount / gm-2 Fig. 5. In this figure, JSC is plotted over the accumulated water amount, calculated with the WVTR of the barrier. The numbers in the graph are the WVTR values for the different devices.

10000 T50 T80

0.1

500

1000

1500 2000 time / h

2500

24 5

M el in ex w w ith ith ou ou t t

PE T

10 0

tri pl e

ZT O

PE T

100

0.04 0

PE T

without without PET Melinex PET 100 ZTO PET triple PET 245 ZTO glass

1000

PE T

lifetime / hours

normalized JSC

ZT O

gl as s

1

3000

50 10-5

10-4

10-3 10-2 10-1 100 WVTR / g m-2d-1

101

102

Fig. 4. (A) JSC behavior over time for all measured devices at an intensity of approximately 100 mW cm  2. The grey lines are the SED fit curves for all devices. (B) shows the lifetimes over WVTRs. The lifetime data for T80 are represented by blue diamonds, for T50 in black squares. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

M. Hermenau et al. / Solar Energy Materials & Solar Cells 97 (2012) 102–108

JSC during measurement time. The interesting feature in this graph is that despite all curves having an offset in the X direction, their shape is similar as well as the slope in the region where JSC is reduced rapidly. This leads us to the following conclusion: For the different barrier materials, it takes different amounts of water getting into the devices, where it causes a reduction of solar cell performance. The lower the WVTR, the smaller is the initial amount of water to start device degradation. Once the reduction of JSC has started, the degradation speed (related to the amount of water) is identical for all devices. The implication we draw from that behavior is that there exists a second barrier in the device with a mean WVTR. This hinders water in weakly encapsulated devices to quickly damage the device. On the other hand, for low WVTR encapsulants, this additional barrier is much smaller then the one on top and would thus not act as an additional resistance for water, once it has crossed the outer barrier. 3.4. The top contact as additional barrier To explain this model in detail, we draw an analogy to an electrical series connection [20]. The WVTR acts as conductance value, since it gives the mass of water which is transmitted per day and square meter. To calculate the so-called water-resistance R, the inverse value is taken: R¼

1 WVTR

ð4Þ

Taking into account that the existence of a second barrier is proposed, the total resistance is simply the sum of the two series resistances. Therefore, the total WVTR is given by the following equation: 1 1 1 ¼ þ WVTRtotal WVTR1 WVTR2

ð5Þ

This procedure is possible with the assumption that both barriers are separated by gas volume, to be specific filled with nitrogen. Since the degradation of the device is completely represented by a loss in JSC, one can conclude that degradation takes place in the active materials, namely ZnPc and C60. Thus, the two layers on top are the possible additional barriers. The

water amount / g m-2

100

10

1

B A

WVTRAl / g m-2d-1 without second barrier 10-1 10-2 10-3 8∗10-4 10-4

BPhen layer is very thin, so this neglects its effect as barrier and we propose that the aluminum layer with a thickness of 100 nm is the second barrier layer for water diffusion. Thus, we identify WVTR1 with WVTRbarrier and WVTR2 with WVTRAl. With the further assumption that all devices reach a similar state of degradation after a certain amount of water diffused through barrier and aluminum layer, we are able to find the value of WVTRAl that gives us a good agreement of measurement data and theory. In Fig. 6A, we show the calculated mass of water in the organics reached at T50 as a function of the WVTRBARRIER and for different values for WVTRAl in the range from 10  1 to 10  4 g m  2 d  1. For the calculation without a second barrier (black squares), the calculated water mass is different for the samples, protected by differently strong barriers, and covers several orders of magnitude. This number gives the amount of water which crosses the main barrier and reaches the upper side of the aluminum contact. By adding a WVTR value of 10  1 g m  2 d  1 (red circles) for the aluminum layer, we see that data points are getting closer to a common level but still differ significantly. With assuming a WVTRAl of 10  3 g m  2 d  1 (pink triangles) we see that T50 is reached when 1271 mg m  2 water crossed the two barrier layers. This value is independent from WVTRBARRIER. With a lower value of 10  4 g m  2 d  1 (dark blue triangles) the discrepancy is increasing. With a further optimization regarding minimized standard deviation 1 of mH2O, we find a value of 8  104 g m2 d . For this barrier, we calculate an accumulation of 1071 mg m  2 water in the organic layers when T50 is reached. This leads to the conclusion that, assuming that a certain amount of water is needed to deteriorate JSC to 50% of its starting value and the aluminum layer with a 1 thickness of 100 nm has a WVTR of around 8  104 g m2 d . One can then take this value to modify the X-axis of Fig. 5, we get a convergence of all curves, with a slight difference for sample G with the glass encapsulation (see Fig. 6B). Comparing the water that crossed barrier layers, the value of 10 mg m  2 is in the range of those Cros et al. reported for ‘‘classical’’ structures [21]. A dramatic improvement of lifetime was seen by using an inverted structure, pulling the lifetime and therefore water mass that entered the device until T50 was reached from 20 mg to 1140 mg. Cros et al. attributed this improvement to the exchange of the aluminum electrode by a silver electrode with a lower WVTR.

1.0

0.8

C

D 0.1

normalized JSC

106

0.6

0.4

F E 0.01

G

0.2

1E-3

1E-5 1E-4 1E-3 0.01 0.1 1 10 100 WVTRBARRIER / g m-2d-1 (@ T=45°C, rH=5.5%)

0.0 1E-5

without without PET Melinex PET 100 ZTO PET triple PET 245 ZTO glass

1E-4

1E-3 0.01 water amount / g m-2

0.1

1

1

Fig. 6. (A) The graph shows the change of accumulated water at T50 by assuming different numbers for WVTRAl and a clear convergences for 8  104 gm2 d at around 1 10 mg m  2. (B) Decay of JSC over water amount calculated by using the WVTR of the encapsulation and WVTRAl of 8  104 gm2 d . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

M. Hermenau et al. / Solar Energy Materials & Solar Cells 97 (2012) 102–108

As visible from Fig. 5, less water (around 6 mg m  2) is needed for a comparable device degradation in the case of glass encapsulation, and the shape of device F with the best barrier film also shows differences compared to the other films. This might be explained by additional, long-term degradation effects which are not directly related to the accumulated water in the devices or to the WVTR of the aluminum layer. Edge-in diffusion, as reported by other groups might be more relevant over long times, especially since our devices have only small areas [22–24]. Other chemical or physical processes might occur. A hint for this is the reduction FF over time from 58% to 55% after 2750 h for device G.

4. Conclusions To analyze the degradation behavior of small-molecule organic solar cells encapsulated with different barrier films a stretched exponential decay function is used. The influence of the WVTR is expressed by an increase of operational lifetime for better barriers. To describe the water permeation into the organic layers, a two-barrier model is used and next to the encapsulation film the aluminum top electrode is identified to work as second permeation barrier. A relevant diffusion of water molecules through the aluminum is observed, most likely through pinholes. The WVTRAl 1 is determined to be around 8  104 g m2 d at a mean temperature of 45 1C and a mean relative humidity of 5.5%, assuming that for all encapsulations except glass, simply the accumulated water (that entered through the surface of the device) in the organic layers is dominant for device degradation. In the case of glass encapsulation, other effects occur that cause a more rapid degradation on the water scale. Diffusion of water through the edges might play a more important role. This leads to the misalignment of our data. 1 We are aware that 8  104 g m2 d is a rather low value for such a thin aluminum layer and would maybe even be enough to fulfill barrier requirements for simple applications where no long lifetimes are necessary. Reasons for this might be the very low roughness of the organic layer, on which the aluminum was evaporated, and the atmospheric conditions, especially the very low mean relative humidity. With a correction to typically used humidities this value increases around one to two orders of magnitude [25]. Also the observed areas are rather small. Literature values for cross comparison are not available. Thus, more work is necessary to study the impact of water on the materials in organic electronic devices. With this barrier value for the top contact and the different WVTR values for different barriers, we obtain a water mass of around 10 mg m  2, which is reaching in the organic layers until T50 of the photovoltaic device is reached. In the case of doped small molecule materials, the ability to move the electron transport layers, which are more sensitive to water and oxygen, away from the permeable top contact would be a possibility to increase device lifetimes. Another option is a reduction of the WVTR of the top electrode, especially by avoiding or closing pinholes. With the help of these findings, one is able to conduct accelerated aging experiments for well-encapsulated organic solar cells by using low-barrier films. Calculation of lifetimes for better encapsulations is then possible, as long as ambient gases remain the dominant reason for degradation. A separation of intrinsic degradation effects, which became visible for the glass encapsulated device, and extrinsically dominated degradation, as we showed for all other devices, is possible. Furthermore, one can use these results to evaluate different structures and/or materials with respect to their resilience against water.

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Acknowledgments ¨ The authors would like to thank the Bundesministerium fur Bildung und Forschung in the frameworks of the InnoProfile project (03IP602), the OPEG project (13N9720), the OPA project (13N9872) and the R2Flex project (13N11059). Further thanks for technical assistance and sample preparation are addressed to ¨ Sven Kunze, Carsten Wolf, Tobias Gunther and Caroline Walde.

References [1] M. Riede, T. Mueller, W. Tress, R. Schueppel, K. Leo, Small-molecule solar cells—status and perspectives, Nanotechnology 19 (42) (2008) 424001–424012. doi:10.1088/0957-4484/19/42/424001. [2] N.S. Sariciftci, Plastic photovoltaic devices, Materials Today 7 (9) (2004) 36–40. doi:10.1016/S1369-7021(04)00400-6. [3] C.J. Brabec, J.A. Hauch, P. Schilinsky, C. Waldauf, Production aspects of organic photovoltaics and commercialization of devices, MRS Bulletin 30 (January 2005) 50–52. [4] J.A. Hauch, P. Schilinsky, S.A. Choulis, S. Rajoelson, C.J. Brabec, The impact of water vapor transmission rate on the lifetime of flexible polymer solar cells, Applied Physics Letters 93 (10) (2008) 103306-1–103306-3. doi:10.1063/ 1.2975185. [5] M. Joergensen, K. Norrman, F.C. Krebs, Stability/degradation of polymer solar cells, Solar Energy Materials and Solar Cells 92 (7) (2008) 686–714. doi:10.1016/j.solmat.2008.01.005. [6] M. Hermenau, M. Riede, K. Leo, S. Gevorgyan, F.C. Krebs, K. Norrman, Water and oxygen induced degradation of small molecule organic solar cells, Solar Energy Materials and Solar Cells 95 (5) (2011) 1268–1277. doi:10.1016/ j.solmat.2011.01.001. [7] R. Meerheim, S. Olthof, M. Hermenau, S. Scholz, A. Petrich, N. Tessler, ¨ O. Solomeshch, B. Lussem, M. Riede, K. Leo, Investigation of C60F36 as lowvolatility p-dopant in organic optoelectronic devices, Journal of Applied Physics 109 (10) (2011) 103102-1–103102-6. doi:10.1063/1.3590142. ¨ [8] J. Fahlteich, M. Fahland, W. Schonberger, N. Schiller, Permeation barrier properties of thin oxide films on flexible polymer substrates, Thin Solid Films 517 (10) (2009) 3075–3080. doi:10.1016/j.tsf.2008.11.089. ¨ [9] J. Fahlteich, W. Schonberger, M. Fahland, N. Schiller, Characterization of reactively sputtered permeation barrier materials on polymer substrates, Surface and Coatings Technology 205 (2011) S141–S144. doi:10.1016/j. surfcoat.2011.01.045. [10] M.F.u.N.S.J. Fahlteich, S. Barth, All-in-Vacuum deposited transparent multilayer barriers on polymer substrates, Vakuum in Forschung und Praxis: Best of Glass (2011) 30–34. ¨ ¨ [11] P. Hulsmann, D. Philipp, M. Kohl, Measuring temperature-dependent water vapor and gas permeation through high barrier films, Review of Scientific Instruments 80 (11) (2009) 113901-1–113901-6. doi:10.1063/1.3250866. [12] S. Schubert, H. Klumbies, L. Mueller-Meskamp, K. Leo, Electrical calcium test for moisture barrier evaluation for organic devices, Review of Scientific Instruments 82 (2011) 094101-1–094101-8. doi:10.1063/1.3633956. [13] R. Paetzold, A. Winnacker, D. Henseler, V. Cesari, K. Heuser, Permeation rate measurements by electrical analysis of calcium corrosion, Review of Scientific Instruments 74 (12) (2003) 5147–5150. doi:10.1063/1.1626015. [14] M. Hermenau, K. Leo, M. Riede, Comparison of different conditions for accelerated ageing of small molecule organic solar cells, Proceedings of SPIE 7722 (77220K) (2010) 772220K-10. doi:10.1117/12.853834. [15] M.O. Reese, S. Gevorgyan, M. Joergensen, E. Bundgaard, S.R. Kurtz, D.S. Ginley, D.C. Olson, M.T. Lloyd, P. Morvillo, E.A. Katz, Consensus stability testing protocols for organic photovoltaic materials and devices, Solar Energy Materials and Solar Cells 95 (2011) 1253–1267. doi:10.1016/j.solmat.2011. 01.036. [16] C. Fery, B. Racine, D. Vaufrey, H. Doyeux, S. Cina, C. Fery, S. Cina, Physical mechanism responsible for the stretched exponential decay behavior of aging organic light-emitting diodes, Applied Physics Letters 87 (21) (2005) 213502-1–213502-3. doi:10.1063/1.2133922. [17] R. Meerheim, K. Walzer, M. Pfeiffer, K. Leo, Ultrastable and efficient red organic light emitting diodes with doped transport layers, Applied Physics Letters 89 (6) (2006) 061111-1–061111-3. doi:10.1063/1.2268354. [18] H.B. Yang, Q.L. Song, C. Gong, C.M. Li, The degradation of indium tin oxide/ pentacene/fullerene/tris-8-hydroxy-quinolinato aluminum/aluminum heterojunction organic solar cells: by oxygen or moisture? Solar Energy Materials and Solar Cells 94 (5) (2011) 846–849. doi:10.1016/j.solmat. 2010.01.006. [19] M. Hermenau, S. Scholz, K. Leo, M. Riede, Total charge amount as indicator for the degradation of small molecule organic solar cells, Solar Energy Materials and Solar Cells 95 (5) (2011) 1278–1283. doi:10.1016/j.solmat.2010.09.031. [20] G.L. Graff, R.E. Williford, P.E. Burrows, Mechanisms of vapor permeation through multilayer barrier films: lag time versus equilibrium permeation, Journal of Applied Physics 96 (4) (2004) 1840–1849. doi:10.1063/1.1768610. [21] S. Cros, R. de Bettignies, S. Berson, S. Bailly, P. Maisse, N. Lemaitre, S. Guillerez, S. Bertho, N. Lemaˆıtre, Definition of encapsulation barrier

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M. Hermenau et al. / Solar Energy Materials & Solar Cells 97 (2012) 102–108

requirements: a method applied to organic solar cells, Solar Energy Materials and Solar Cells 95 (1) (2011) 65–69. doi:10.1016/j.solmat.2011.01.035. ¨ [22] E. Voroshazi, B. Verreet, A. Buri, R. Muller, D. Di Nuzzo, P. Heremans, Influence of cathode oxidation via the hole extraction layer in polymer: fullerene solar cells, Organic Electronics 12 (5) (2011) 736–744. doi:10.1016/ j.orgel.2011.01.025. [23] F.C. Krebs, Air stable polymer photovoltaics based on a process free from vacuum steps and fullerenes, Solar Energy Materials and Solar Cells 92 (7) (2008) 715–726. doi:10.1016/j.solmat.2008.01.013.

[24] T. Jeranko, H. Tributsch, N.S. Sariciftci, J. Hummelen, Patterns of efficiency and degradation of composite polymer solar cells, Solar Energy Materials and Solar Cells 83 (2–3) (2004) 247–262. doi:10.1016/j.solmat.2004.02.028. [25] C.L. Huisman, A. Goossens, J. Schoonman, Photodoping of zinc phthalocyanine: formation, mobility, and influence of oxygen radicals in phthalocyanine-based solar cells, Journal of Physical Chemistry B 106 (41) (2002) 10578–10584. doi:10.1021/jp025671t.