Degradation of Flexible, ITO-Free Oligothiophene Organic Solar Cells

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Degradation of Flexible, ITO-free Oligothiophene Organic Solar Cells Ludwig Bormann, Frederik Nehm, Luisa Sonntag, Fan-Yu Chen, Franz Selzer, Lars Mueller-Meskamp, Alexander Eychmueller, and Karl Leo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02363 • Publication Date (Web): 24 May 2016 Downloaded from http://pubs.acs.org on May 31, 2016

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ACS Applied Materials & Interfaces

Degradation of Flexible, ITO-free Oligothiophene Organic Solar Cells Ludwig Bormann,∗,† Frederik Nehm,† Luisa Sonntag,‡,¶ Fan-Yu Chen,† Franz Selzer,† Lars Müller-Meskamp,† Alexander Eychmüller,‡,¶ and Karl Leo∗,†,§ †Institut für Angewandte Photophysik, Technische Universität Dresden, George-Bähr-Str. 1, 01069 Dresden, Germany ‡Physikalische Chemie, Technische Universität Dresden, Bergstraße 66b, 01062 Dresden, Germany ¶Cluster of Excellence Center for Advancing Electronics Dresden (CFAED) §Fellow of the Canadian Institute for Advanced Research (CIFAR) E-mail: [email protected]; [email protected]

Abstract We investigate the degradation of organic solar cells based on oligothiophene (DCV5TMe) small molecule donor and the acceptor C60 . Two different flexible, transparent bottom electrode types are employed: a transparent metal electrode (TME) and silver nanowires (AgNWs). They exhibit high optical transparency up to 86 % and a sheet resistance as low as 12 Ω/. Power conversion efficiencies of 7.0 %, 5.7 %, and 7.2 % on TME, AgNWs, and indium tin oxide (ITO, reference) are reached, respectively. The solar cells are protected against moisture ingress utilizing a flexible alumina thin-film, exhibiting water vapour transmission rates down to 3 × 10−5 g m−2 d−1 at 38 ◦C and 90 % relative humidity (RH). Implementation of this ultra-barrier as top and bottom encapsulation enables fabrication of fully-flexible devices. A decrease in PCE to 80 %

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of initial values is observed after (1000 ± 50) h on flexible encapsulated TME but only (20 ± 5) h on AgNWs in a climate of 38 ◦C/50 % RH. Degradation in AgNW-based devices is attributed to electrode decomposition.

Keywords Organic Solar Cells, Silver Nanowires, Degradation, Oligothiophene, Transparent Metal Electrode, Flexible, Encapsulation

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Introduction

Organic solar cells (OSCs) attract much attention in current research due to high power conversion efficiencies (PCEs) over 10 %. 1,2 They allow flexible, 3 light-weight, semi-transparent 4 devices processable on large area with roll-to-roll technologies. 5,6 For creating competitive organic photovoltaic devices, several aspects have to be considered. The transparent electrode needs to provide high conductivity, low absorptivity, and good flexibility. As a commonly used transparent electrode, indium tin oxide (ITO) provides good optical and electrical properties 7 but suffers brittleness and high production cost. 8 Many approaches have been made to replace ITO with flexible and cheaper technologies being similar in optical and electrical perfomance. An ultrathin evaporated or sputtered metal layer sandwiched between two dielectrics is a proven method employed as top and bottom electode in organic devices. 9–11 The subjacent dielectric layer hereby serves as seed layer for the metal film, creating smooth and continuous layers at low metal thickness. 12 Another approach utilizes metal nanowires arranged as a random percolation network on flexible substrates. Nanowires made of copper 13–15 or silver 16–18 exhibit outstanding electrical and optical performance. However, when used as bottom electrode in organic thin-film devices, the high network surface roughness causes device shunts. Therefore, planarization is 2


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needed which has been succesfully demonstrated by using poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate), metal oxides, or small molecules. 18–21 A novel approach buries silver nanowires (AgNWs) in a nonconductive polymer employing a peel-off process using e.g. polyimide or the "Norland Optical Adhesive 63" (NOA). 22–25 In practical applications, organic devices require a protection against moisture ingress. Water vapour transmission rates (W V T R) in the range of 1 × 10−5 g m−2 d−1 are needed to guarantee long device lifetime. 26 Moreover, the barrier needs to be flexible and transparent at least on one side of the device. Glass is an excellent moisture barrier but lacks flexibility and is heavy. Though thin glass can be made flexible to some degree, such substrates remain brittle, posing risks in coating machines as well as consumer products. Atomic layer deposited metal oxide thin-films provide low W V T Rs, flexibility, light weight, and high transparency at the same time. 27–30 With these properties, they are a promising alternative to glass. This study makes an effort to combine all key aspects of competitive OSCs. A transparent metal electrode (TME) and silver nanowires are employed as two types of flexible bottom electrode. Atomic layer deposited thin-films of alumina (AlOx ) serve as ultra-barrier against moisture ingress. Electrode and barrier are tested in view of their flexibility properties. A material system incorporating a methylated oligothiophene derivative (DCV5T-Me) and C60 as small molecule donor and acceptor is used in a bulk heterojunction n-i-p architecture for efficient charge carrier generation. We carry out a lifetime study under AM1.5G illumination in a climate of 38 ◦C and 50 % relative humidity (RH). Flexible devices on the two electrode technologies are also compared to glass-encapsulated counterparts on ITO.

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Figure 2: Transmission of electrode types used in this study. a) Deposited on glass substrates. b) AgNW electrodes after transfer into NOA. c) TME deposited on a pPEN/AlOx barrier substrate, NOA/NW35 laminated onto a pPEN/AlOx barrier substrate. as seed layer and diffusion barrier for noble metal atoms. Gold provides a high surface free energy for the actual silver electrode. 125 µm thick planarized polyethylene naphtalate (pPEN) serves as polymer substrate and is precoated with 20 nm AlOx as moisture barrier. Silver nanowires with a diameter of 90 nm (NW90) and 35 nm (NW35) are used due to their promising properties. 16,18,32–35 Planarization is carried out utilizing a concept recently shown by Nam et al. 25 and succesfully adopted by other groups. 23,24,32 AgNW networks – deposited on glass substrates – are covered by the UV-curable polymer NOA. After peeling off this layer from the glass substrate, NOA serves as 50 µm thin, flexible substrate with AgNWs buried under the polymer surface. In contrast to the TME, the pPEN/AlOx barrier substrate is implemented separately because barrier deposition cannot be included in the AgNW electrode fabrication process. The NOA/AgNW electrode is laminated onto an additional pPEN/AlOx substrate using a UV-curable barrier glue (tesa SE, Germany) which contains a latent getter against lateral moisture diffusion and resulting edge degradation. Transmission spectra of NW90, NW35 and TME electrodes on different substrates are shown

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in Figure 2 and values of sheet resistance (RS ) as well as averaged transmission over the visible part of the spectrum (Tvis ) are summarized in Table 1. The TME electrode exhibits low RS of (12.0 ± 0.5) Ω/ and low Tvis of only (61 ± 6) % and (58 ± 5) % on glass and pPEN/AlOx , respectively. On glass substrates (Figure 2 a)), a drop in transmittance below 400 nm can be attributed to the absorption of p-doped BF-DPB with an optical band gap of 3 eV. 36 With pPEN/AlOx as substrate (Fig. 2 c)), the absorption edge of pPEN at 390 nm is covering BF-DPB absorption. An interference pattern is visible above 500 nm resulting from thin-film interference between substrate and TME. Although (6 to 7) nm of silver is suitable for creating continuous and conductive layers on glass, 10,12 9 nm were chosen to have reproducible and lower RS values. Table 1: Sheet resistance RS and transmittance Tvis in the visible part of the spectrum ((390 to 800) nm) for ITO, TME, NW35, and NW90 electrodes on various substrates. In the case of AgNW electrodes on pPEN/AlOx , the NOA substrate is included. Glass

Type ITO TME NW35 NW90

RS (Ω/) 26 12.0 ± 0.5 17.5 ± 0.5 14.5 ± 0.5

84 ± 2 61 ± 6 82 ± 3 86 ± 1

NOA pPEN/AlOx Tvis (%) – – – 58 ± 5 80 ± 6 77 ± 7 84 ± 2 79 ± 4

Sheet resistance of AgNW electrodes is determined to (17.5 ± 0.5) Ω/ and (14.5 ± 0.5) Ω/ for NW35 and NW90, respectively. No change in RS is observed before and after the NOA coating. AgNW electrodes on glass exhibit Tvis of (82 ± 3) % and (86 ± 1) % for NW35 and NW90, respectively. These values are similar or better as compared to ITO. However, utilizing flexible substrates reduces transmittance by 2 % when embedding AgNWs in NOA and by another (3 to 5) % after lamination on pPEN/AlOx barrier substrates. Transmittance below 390 nm is cut off due to pPEN absorption, as seen for TME samples on pPEN. The planarization with NOA decreases the roughness of silver nanowire networks from initially (40 ± 15) nm down to (1 to 2) nm (see SI S1) which is suitable for implementation into organic thin-film devices. Comparing all electrode technologies within this study, the AgNW electrodes on glass exhibit 6


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provide sufficient flexibility as reported elsewhere. 25 Accordingly, a flexibility study is carried out with TMEs on pPEN/AlOx substrates (see SI S2). Resistance is measured after bending at a certain Rb . After 10 bendings, Rb is decreased and bent again, et cetera. The resistance change is smaller than 10 % down to Rb of 12.5 mm. Smaller radii lead to AlOx cracking which causes TME failure. However, when laminating another pPEN substrate on top of the pPEN/AlOx /TME stack, the breakdown radius could be shifted below 8.5 mm. As the layer is positioned closer to the neutral plane during bending, the strain is much smaller as compared to a location at substrate surface. 28 This scenario is also more realistic when implemented in an OSC which is preferably encapsulated from both sides. These investigations show very good applicability of this flexible device approach. Not only roll-to-roll processing, but even completely rollable products can be realized with TMEs, AgNWs, and the ALD barriers used here. Enabling a low cost and large area ALD process is possible by using spatial ALD. 37 This approach is compatible with roll-to-roll technology and strongly reduces precursor waste. Moreover, Emmott et al. reported that AgNW electrodes are cheaper and less energy-consumptive than ITO in mass-produced OSC modules. 38

2.2

Flexible Organic Solar Cells

Organic solar cells are fabricated on flexible transparent electrodes (TME, NW90, NW35) with pPEN/AlOx as barrier substrate. ITO on glass substrates serves as reference electrode. An additional pPEN/AlOx sheet is laminated onto the flexible OSC using UV-curable barrier glue (tesa SE, Germany) with latent getter against lateral moisture diffusion. This serves as top encapsulation and protection against mechanical stress. Custom encapsulation glasses with a getter-filled cavity serve as rigid top encapsulation for ITO-based devices. A schematical layer stack including the barrier substrate is shown in Figure 1. DCV5T-Me and C60 are used as donor and acceptor molecules in a bulk heterojunction with n-i-p architecture. This system stands out as highly efficient with PCEs up to 8.3 % and 9.7 % in a single- and multijunction OSC, respectively. 39 8


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Figure 4: jV curves under AM1.5G illumination (100 mW cm−2 ) of organic solar cells on ITO, TME, NW35 and NW90. Figure 4 displays the current density vs. voltage (jV ) characteristics of the organic devices and Table 2 summarizes relevant photovoltaic performance parameters. ITO-based devices exhibit a PCE of 7.2 % with highest open circuit voltage (VOC ), short circuit current density (jSC ) and fill factor (F F ) of 961 mV, 11.9 mA cm−2 , and 62.6 %, respectively. A PCE of 7.0 % is reached with a TME electrode, exhibiting VOC , jSC and F F values of 953 mV, 11.6 mA cm−2 , and 64 %, respectively. On NOA/AgNW electrodes, PCEs of 5.7 % (NW35) and 5.6 % (NW90) are reached with a similar VOC of 955 mV and jSC values of 9.5 mA cm−2 (NW35) and 9.8 mA cm−2 (NW90). Highest F F of 64.1 % is achieved with NW35 whereas NW90 shows only 59.8 %. The PCE of TME-based devices is close to the reference value. F F values are 2 % higher as compared to the reference due to a lower RS of the electrode. However, the jSC is remarkably close to the reference jSC although TME transmission is comparably low. This seems to arise from a high specular TME reflection (Rvis ) of (27 ± 7) % (averaged over the visible part of the spectrum) leading to a microcavity effect and enabling an increased jSC . 40,41 This is supported by measurement and optical simulations of the external quantum efficiency (EQE) (see SI S3). The experimentally obtained difference in jSC as compared to ITO (6 to 7 %) matches well with the optical simulation. 9



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Table 2: Photovoltaic performance parameters of DCV5T-Me:C60 solar cells on different transparent bottom electrodes. As encapsulation, glass or 20 nm AlOx are used. For every solar cell type, 2 samples containing 4 devices each are prepared under identical conditions. The yield states the amount of working devices. Maximum values are written in brackets. Type

jSC (mA cm−2 ) 11.7 ± 0.3 (11.9)

VOC (mV) 960 ± 1 (961)

FF (%) 61 ± 2 (62.6)

PCE (%) 6.8 ± 0.3 (7.2)

Yield (x/8) 6

pPEN/AlOx | TME | pPEN/AlOx

11.0 ± 0.3 (11.6)

952 ± 1 (953)

63.6 ± 0.3 (64.0)

6.7 ± 0.2 (7.0)

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pPEN/AlOx | NOA63/NW35 | pPEN/AlOx

9.3 ± 0.4 (9.5)

955 ± 1 (955)

63.5 ± 0.3 (64.1)

5.6 ± 0.2 (5.7)

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pPEN/AlOx | NOA63/NW90 | pPEN/AlOx

9.7 ± 0.1 (9.8)

955 ± 1 (957)

59 ± 1 (59.8)

5.4 ± 0.1 (5.6)

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Glass | ITO | Glass

The PCE of AgNW-based devices is approximately (15 to 20) % lower as compared to ITO. Main reason for this decrease is a loss in jSC on NW35 and NW90 and additionally a reduced F F only visible on NW90. Two reasons are considered for the jSC loss. pPEN blocks light below 400 nm. Charges are not generated in this spectral region although the absorbing system has the ability of charge carrier generation down to 300 nm (cf. SI S3). Further, AgNW electrodes exhibit a low specular reflection Rvis of only (5.5 ± 0.5) %. Large voids are present within the network where only the highly transmissive NOA is located. The low reflection does not support a microcavity effect as discussed in the context of TME leading to a lower jSC despite higher electrode transmission. A F F of only 59.8 % on NW90 is explainable with the interplay of microscopic AgNW network structure and overlying organic layers. The NW90 electrode exhibits large voids between the nanowires in the square micrometer range (see SI S1) which are much larger than on NW35. If the conductivity of the adjacent charge transport material is too low, charges are inefficiently transported to the highly conductive AgNW network. N-doped bisHFl-NTCDI exhibits a conductivity of 2 × 10−4 S cm−1 . This is the crucial conductivity at which losses in F F and jSC emerge, as previously reported. 19 Summarizing the solar cell characterization, we show fully-flexible, encapsulated solar cells 10


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with an efficiency larger than 5.5 % on AgNW electrodes and reaching 7.0 % with TME, being comparable to rigid ITO reference devices with maximum 7.2 %.

2.3

Degradation in 38 ◦C/50 % Climate under Illumination

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Figure 5: PCE over time in illuminated 38 ◦C/50 % climate. Values are normalized to the starting value. Left: ITO and TME. Right: ITO and AgNW electrode. We further study degradation of fully-flexible OSCs on TME, NW35 and NW90 electrodes with pPEN/AlOx top and bottom encapsulation and compare it to glass-glass encapsulated ITO-based counterparts. Devices are aged in 38 ◦C, 50 % RH climate and illuminated under AM1.5G conditions with 100 mW cm−2 intensity. In Figure 5, the loss in efficiency over time is plotted for all investigated OSCs within this study. ITO-based devices exhibit a distinct burn-in such that 80 % of its initial efficiency (T80 ) are reached after only 160 h. Responsible is mainly a decrease in F F , the exact cause is not clear. Subsequently, the devices enter a regime of slow degradation with linear shape. Degradation in this case is mostly driven by intrinsic factors. An exponential decay is visible for TME electrodes until approximately 1000 h where it reaches T80 . After 1000 h, a breakdown occurs leading to an accelerated degradation toward 11



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50 % of the initial efficiency at 1600 h. Until 1000 h, the degradation is similar as compared to ITO which indicates only intrinsic degradation mechanisms. The reason for the sudden change in degradation behavior is most likely a pPEN substrate degradation leading to a decrease of barrier performance. PEN is known to yellow, stiffen, and change morphology under UV illumination and moisture ingress. 42–44 It is likely that the moisture barrier becomes locally permeable due to morphological changes after approximately 1000 h. The water ingress mainly leads to a top electrode delamination and degradation 45 resulting in additional losses in jSC and F F . An increased jSC loss and F F degradation is in fact visible in the IV curves at later aging stages (see SI S4). The AgNW-based OSCs (Figure 5 right) degrade within (25 ± 5) h to 50 % of the initial PCE. A strongly s-shaped IV curve emerges (see SI S4). The s-shape is a consequence of the formation of energetic barriers within the device. 46 As the organic layer stack is intrinsically stable (cf. Figure 5), we attribute this fast breakdown to electrode failure. This happens specifically on NOA/AgNW electrodes, as IV curves on TME show none of these features. Possible reasons of energetic barriers on NOA/AgNW are the formation of silver oxide or silver sulfide with a work function greather than 5 eV, 47,48 strongly increasing the energetic difference with respect to the electron transport layer LUMO (4 eV). Moreover, the interfacial n-dopant W2 (hpp)4 is prone to oxidation, 49 possibly resulting in a Schottky instead of an Ohmic contact between NOA/AgNW electrode and electron transport layer. SEM images of bare NOA/AgNW electrodes are taken before and after degradation for 7 d in a climate identical to solar cell aging. These are shown in Figure 6 for NOA/NW35 electrodes before and after degradation with and without light incidence. Nanowires in the freshly prepared sample exhibit intact junctions and nanowires. In contrast, the degraded sample with light incidence shows many spots with broken nanowires. Over the entire area, nanowires exhibit a grainy structure. If degraded without light incidence, a grainy structure is also visible but the nanowires themselves are still intact. The SEM images substantiate the assumption of an electrode failure. The electrode decomposes and is destroyed under

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prevent direct contact between NOA and silver nanowires without affecting AgNW contact to subsequent organic layers. Secondly, usage of polymers (e.g. polyimide 24,56,57 ) which cure under annealing or have a different chemical composition might prevent reactions of the polymer with AgNWs.

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Conclusion

We demonstrate lifetime studies of DCV5T-Me:C60 solar cells in 38 ◦C, 50 % RH climate. Utilizing ITO, a transparent metal electrode, and silver nanowires as transparent electrode, efficiencies of 7.2 %, 7.0 %, and 5.7 % are reached, respectively. With the latter two electrode technologies, we are able to fabricate fully-flexible encapsulated solar cells employing a highly transparent AlOx thin-film with W V T Rs down to 3 × 10−5 g m−2 d−1 . Long device lifetimes T80 of 1000 h are reached on TME which is comparable to glass-glass-encapsulated ITO-based references. On silver nanowires, devices degrade fast within one day of aging. Reason is the failure of silver nanowires due to photo-oxidization and electromigration or chemical reaction of silver with NOA63. As a first step towards competitive OSCs, a combination of AlOx barrier, TME electrode and DCV5T-Me:C60 organic device is found which is flexible, highly efficient, and reasonably stable. This makes it suitable for scaling up, enabling large area roll-to-processing and low production cost.

4

Experimental

Experiments are performed on pre-cleaned and pre-structured (2.5 × 2.5) cm2 BK7 glass- and ITO substrates (Schott, Germany). Silver nanowires with d = 35 nm, l = (12 ± 3) µm (NW35) and d = 90 nm, l = 30 µm length (BlueNano, USA) are diluted with ethanol to a concentration of 0.2 mg ml−1 and spray-coated on glass substrates at 80 ◦C substrate temperature. NW90 films are heated for 90 min at 210 ◦C to obtain low RS . The polymer "Norland Optical Adhesive 63" (Norland Products, USA) is used as delivered. After spin coating the polymer onto AgNW layers (1200 min−1 , 30 s), a no-name nailcuring device with 4 UV energy saving lamps (9 W each) is used to cure the layers within 30 min. Layers are peeled off with tweezers from the glass substrate

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and laminated with barrier glue (tesa SE, Germany) on pPEN/AlOx substrates. 20 nm of AlOx are atomic layer deposited at 100 ◦C electrode temperature using trimethylaluminum and ozone as precursors in a Sentech SI ALD LL plasma-enhanced deposition system (Sentech Instruments GmbH, Germany). Details are described elsewhere. 27 The ALD deposition takes place onto planarized Teonex PEN PQA1M (Dupont Teijin Films Ltd., UK). Transparent metal electrodes are vacuum-deposited onto pPEN/AlOx substrates prior to solar cell fabrication in the same evaporation system. NOA63/AgNW electrodes are treated with argon plasma for 5 min at 4 × 10−1 mbar to increase wettability and contact area for subsequent solar cell deposition. C60 (Creaphys, Germany), 2,2-((3,4-dimethyl-[2,2:5,2:5,2:5,2-quinquethiophene]-5,5-diyl)bis(methanylylidene))dimalon onitrile (DCV5T-Me), n,n’-((diphenyl-n,n’-bis)9,9,-dimethyl-fluoren-2-yl)-benzidine (BF-DPB) (Synthon, Germany), n,n-bis(fluoren-2-yl)-naphtalenetetracarboxylic diimide (Bis-HFlNTCDI) (synthesized in-house), tetrakis(1,3,4,6,7,8-hexahydro-2h-pyrimidol[1,2-a]pyrimidinato)ditungsten (W2 (hpp)4 ), "Novaled P Dopant 9" (NDP9), 2,2’-(perfluoronaphthalene-2,6-diylidene)dimalononitrile (F6-TCNNQ) (Novaled AG, Germany), 9,9-bis[4-(N,N-bis-biphenyl-4-yl-amino)phenyl]-9H-fluorene (BPAPF) (Lumtec, Taiwan), molybdenum oxide (Sigma Aldrich, Germany), gold (Allgemeine Goldund Silberscheideanstalt AG, Germany), silver and aluminum (Lesker, UK) are deposited using a single-chamber thermal evaporation system at a base pressure of 1 × 10−8 mbar (Lesker, UK). All organic materials are purified at least twice by gradient sublimation. The layer sequence is depicted in Figure 4. For top encapsulation purposes, getter-containing cavity glasses are glued on top of the devices using epoxy XNR 5592 (Nagase, Japan) under N2 atmosphere. Alternatively, pPEN/AlOx sheets are laminated with barrier glue onto the devices. EQE measurements for spectral mismatch correction are carried out using a lock-in amplifier under monochromatic illumination. Initial IV curves are recorded spectrally mismatch corrected using a source measurement unit (Keithly, USA) under AM1.5G irradiation at 1000 W m−2 with a sun simulator "16 S-003–300" (SolarLight Company Inc., USA). A four-point-probe setup (Lucas Labs, USA) is used for measuring sheet resistance. Transmission and reflection measurements are carried out using an UV-VIS-NIR photospectrometer with integrating sphere unit (Shimadzu, Japan). All transmission values are stated including the substrate. Atomic force microscopy images are taken in tapping mode with a CombiScope (AIST, USA) and TAP-Al-G tips (BudgetSensors, Bulgaria) at a resonance frequency of 300 kHz. Aging of solar cells and electrodes is done in a climate chamber "PL-3 J" (Espec, Japan) with mounted solar simulator "SOL2000" (Hönle AG, Germany) under AM1.5G illumination at 1000 W m−2 . A custom-built circuit board in the chamber electrically connects the samples to a source measurement unit. A measurement PC records IV curves every hour. During measurement down-times, the samples are in open-circuit condition. Bending tests are performed in a nitrogen-filled glovebox using several cylindrical objects with different radii. A multimeter is used for measuring the resistance. WVTRs of bent barriers are measured using electrical calcium corrosion tests. Here, the following layer stack is evaporated directly onto the flexible barrier film: 20 nm of C60 for mechanical decoupling, 58 60 nm Ca sensor, and 100 nm Al electrodes in four-point-probe conductivity measurement geometry. 31 After deposition, encapsulation glasses are glued onto the sample backsides with UV-curing XNR-5592 epoxy in inert environment. Samples are then introduced to a custom measurement setup and measurements are taken every 5 min. A current is sent through the outer electrode fingers until a voltage of 20 mV is measured between the inner sensing electrodes. During measurement down-times, no voltage is applied. A constant RH of 90 % is applied using a saturated, aqueous Na2 SO4 solution. This setup is described in detail elsewhere. 59

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Acknowledgement This work was funded by the European Community’s Seventh Framework Program (FP7/2007e2013) under grant agreement No. 314068 ("Treasores") and within the DFG Cluster of Excellence "Center for Advancing Electronics Dresden". Financial support from the Bundesministerium für Bildung und Forschung (BMBF) within the Innoprofile Transfer project 03IPT602A is gratefully acknowledged. The authors thank Tobias Günther and Andreas Wendel for sample preparation, Sven Kunze and Andreas Büst for maintenance of the measurement systems, and Susanne Goldberg for acquiring the SEM images.

Supporting Information This article entails the following supporting information: Atomic force microscopy images of NW90 and NW35 films with and without NOA embedment. Resistance measurement of pPEN/AlOx /TME and pPEN/AlOx /TME/pPEN layers in dependence on bending radius. Optical simulation of EQE for DCV5T-Me:C60 OSCs with ITO or TME as bottom electrode. IV curves of flexible DCV5TMe:C60 OSCs on TME or NW35 in dependence of degradation time. This material is available free of charge at http://pubs.acs.org.

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Degradation of Flexible, ITO-Free Oligothiophene Organic Solar Cells

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