Phosphorimetric Characterization of Solution-Processed Polymeric

Article pubs.acs.org/JPCC

Phosphorimetric Characterization of Solution-Processed Polymeric Oxygen Barriers for the Encapsulation of Organic Electronics Eduardo Aluicio-Sarduy,‡ Aliaksandr Baidak,‡,† Georgios C. Vougioukalakis,§ and Panagiotis E. Keivanidis‡,* ‡

Centre for Nanoscience and Technology@PoliMi, Fondazione Istituto Italiano di Tecnologia, Via Giovanni Pascoli 70/3, 20133 Milano, Italy § Laboratory of Organic Chemistry, Department of Chemistry, University of Athens, Panepistimiopolis Zografou, 15771 Athens, Greece S Supporting Information *

ABSTRACT: We describe the use of a modified Stern−Volmer photokinetic model for the determination of the oxygen-permeation coefficient (PO2) of materials that can be used as barriers against oxygen permeation in organic electronic applications. The model is applied on photophysical data collected based on the use of the optical technique of phosphorimetry for the oxygen-sensing organometallic complex (2,3,7,8,12,13,17,18octaethylporphyrinato)platinum(II) (PtOEP). PtOEP is used as a phosphorescent probe encapsulated by a set of model solution-processed transparent oxygen-barrier layers made by the polymers of poly(norbornene), poly(methyl methacrylate), poly(styrene), and Zeonex. For each barrier system the oxygen-induced quenching of the PtOEP phosphorescence is monitored with the study of the time-integrated and time-resolved PtOEP phosphorescence intensity, as a function of the partial pressure of oxygen. The advantage of utilizing the presented photokinetic model is based on the consideration of the fractional accessibility of the excited triplet states to the permeant oxygen. The extracted values of PO2 are in excellent agreement with the previous literature, confirming the validity of the modified Stern−Volmer model employed in the analysis of the photophysical data. The results suggest that phosphorimetric characterization is a simple and inexpensive methodology for the fast screening of next-generation barrier materials for organic electronic devices. The high sensitivity of the phosphorimetric technique is shown in the successful characterization of a commonly used glass/epoxy barrier system for which PO2 = 39 × 10−16 cm3 (STP) ·cm·cm−2·s−1·Pa−1 is found. The findings of the phosphorimetric characterization are in qualitative agreement with a preliminary shelf lifetime stability test of organic solar cell devices that were encapsulated with some of the barrier materials of the study.

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devices from both oxygen and water.18−21 These methodologies rely on the use of transparent films that practically surround the photoactive OPV layer and act as oxygen/water permeation barriers.22,23 In this context, next-generation transparent barrier materials are currently sought that can impede the permeation of oxygen and hence prolong the operational stability of organic solar cells and other organic electronic devices. Ideally, these oxygen barriers must be solution-processed so that they can be incorporated in the rollto-roll lines that are used for the device fabrication.24 The effectiveness of these new materials in oxygen-barrier applications is addressed with the analytical technique of coulometry, based on isostatic, steady-state conditions.25 In this case, the established figure of merit is the oxygen transmission rate (OTR), which corresponds to the rate at which the permeant oxygen goes through a material of a specific area. It depends on the thickness of the barrier material and the driving

INTRODUCTION The emerging field of organic electronics offers the advantage of low manufacturing costs for the fabrication of highperformance semiconducting circuits and optoelectronic modules such as light-emitting diodes,1 organic solar cells,2,3 field-effect transistors,4,5 and photodetectors.6,7 Recent reports suggest that the gamut of applications for organic materials extends to spintronic devices.8 Despite the rapid development of elaborate organic device architectures, the fundamental problem of material instability for all of the ingredients of the organics active layers has not yet been fully resolved.9−12 Particularly in the case of power-generating applications, organic photovoltaic (OPV) devices13 still suffer from issues of operational stability originating due to the tendency of organic materials to undergo photoinduced reactions with the ambient oxygen and moisture.9,14−16 The issue of device stability is gaining major attention and is becoming a high priority now that the OPVs have a realistic potential to enter the market.17 To circumvent the stability problem, several encapsulation methods are currently being developed for protecting the OPV © XXXX American Chemical Society

Received: December 4, 2013 Revised: January 14, 2014

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dx.doi.org/10.1021/jp4118908 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Figure 1. Time-integrated PtOEP phosphorescence spectra of (a1) a 370 nm thick PNB:PtOEP layer, (b1) a 520 nm thick PNB:PtOEP layer, and (c1) a 5000 nm thick PS:PtOEP/PNB bilayer, recorded as a function of partial pressure of oxygen. Stern−Volmer plot for the oxygen-induced quenching of the PtOEP phosphorescence intensity of (a2) a 370 nm thick PNB:PtOEP, (b2) a 520 nm thick PNB:PtOEP layer, and (c2) a 5000 nm thick PS:PtOEP/PNB bilayer. Modified Stern−Volmer plot for the oxygen-induced quenching of the PtOEP phosphorescence intensity of (a2) a 370 nm thick PNB:PtOEP layer, (b2) a 520 nm thick PNB:PtOEP layer, and (c2) a 5000 nm thick PS:PtOEP/PNB bilayer. The solid lines in the modified Stern−Volmer plots are fits on the data according to eq 4. The chemical structure of PtOEP is shown in the inset of b2.

force or concentration gradient of the permeant.26 To compare the barrier properties of different materials measured in different conditions, the oxygen-permeation coefficient (PO2) can be used instead. It reflects the ability of oxygen to diffuse through the free volume present in the microstructure of a barrier film on which it is adsorbed26,27 when the film is between a different oxygen pressure. The value of PO2 is not sample-specific, and it reflects a material property due to its independence from film thickness and pressure gradient. Moreover, PO2 is proportional to the OTR. If optical spectroscopy is employed for the study of oxygen permeation, the photochemical process of triplet-fusioninduced delayed luminescence can be utilized.28 Alternatively, one straightforward and low-cost quantitative characterization approach is the use of phosphorimetry.29−31 The effective barrier property of oxygen barriers can be validated with the use of organic triplet emitters that are phosphorescent when excited with visible light.27,32 According to this method, thin solid films of oxygen molecular sensors are enclosed by the encapsulation barrier under investigation. The interaction of molecular oxygen with the triplet excited states of the molecular sensor

result in the decrease of their phosphorescence intensity and lifetime. Therefore, the measured phosphorescence intensity is inversely proportional to the oxygen concentration. Having the probe in a controlled environment of oxygen and monitoring the phosphorescence intensity at increasing oxygen levels allow for the determination of oxygen permeability through the encapsulation barrier in question. The interpretation of phosphorimetric data is normally based on the utilization of the Stern−Volmer plot formalism.33 Equation 1 quantifies the observed quenching of the phosphorescence intensity and provides information on the Stern−Volmer quenching constant, KSV. I0 = 1 + KSV[O2 ] I

(1)

In eq 1, I0 and I correspond to the phosphorescence intensity in the absence and in the presence of the quenching agent, respectively, whereas [O2] corresponds to the molar concentration of the oxygen quenchers. When the partial pressure of molecular oxygen, pO2, is used instead, eq 1 becomes27 B



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Table 1. Fitting Results of the Modified Stern−Volmer Photokinetic Model That Was Used in This Study As Applied on the Phosphorimetric Data of the Characterized Barriersa system PNB:PtOEP PNB:PtOEP PS:PtOEP/PNB PMMA120k:PtOEP PMMA350k:PtOEP PMMA996k:PtOEP Zeonex:PtOEP PS:PtOEP PS:PtOEP/Zeonex PS:PtOEP/glass+epoxy

fα 0.91 0.92 0.80 0.16 0.25 0.23 0.65 0.72 0.73 0.30

KSVSO2 (Pa−1) 21.70 24.40 6.39 1.79 2.27 3.99 5.58 7.08 4.62 0.21

× × × × × × × × × ×

PO2 (cm3(STP)·cm·cm−2·s−1·Pa−1)

−4

2.827 3.083 0.683 0.220 0.282 0.545 0.731 0.916 0.607 0.039

10 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4

× × × × × × × × × ×

10−13 10−13 10−13 10−13 10−13 10−13 10−13 10−13 10−13 10−13

⟨τ0⟩ (μs)

in vacuo , pO2 (Pa)

thickness (nm)

91 94 111 97 96 87 91 92 91 64

8.47 × 10−5 1.60 × 10−5 1.55 × 10−5 1.32 × 10−5 2.1 × 10−5 1.32 × 10−5 2.94 × 10−5 3.36 × 10−5 4.20 × 10−5 1.05 × 10−4

370 520 5000 1100 1900 2400 2300 800 6800 not measured

a

The deduced permeation coefficient PO2 and the layer thickness for each barrier system are reported together with the corresponding average lifetime value τ0 as measured in vacuo conditions.

I0 = 1 + KSVSO2pO 2 I

(PtOEP)35 as an oxygen sensor;36 see the inset in Figure 1b2. PtOEP is dispersed in a set of solution-processable barrier materials. Thin films of the polymer:PtOEP blends are fabricated onto quartz substrates, either by spin-coating or dip-coating deposition. The polymeric barriers studied are poly(norbornene) (PNB), poly(methyl methacrylate) (PMMA), poly(styrene) (PS) (Mw = 192k, Aldrich 430102), and Zeonex (Zeon Europe GmbH, ZEONEX 480R). For PMMA, three different molecular weights are tested; Mw = 120k (Aldrich, 182230), Mw = 350k (Aldrich, 445746), and Mw = 996k (Aldrich, 182265). All polymers except PNB were purchased and used as received, whereas PNB was synthesized according to a previously reported synthetic protocol.37 In all cases, a PtOEP content of 1 wt % is used. The exact fabrication protocol for each studied system is described in the Supporting Information. Parts a1 and b1 of Figure 1 show the time-integrated photoluminescence (PL) spectra of two PNB:PtOEP films with different film thicknesses made by this blend (d1 = 370 nm and d2 = 520 nm), after photoexcitation at 532 nm. In both cases a progressive reduction of the PtOEP luminescence intensity can be seen as the partial oxygen pressure is increased. A similar effect was observed in the PL spectra of a PS:PtOEP blend film (see Figure 1b2) that was covered with a solution-processed PNB overlayer by dip-coating. On the basis of the timeintegrated PL spectra obtained from the PNB:PtOEP blends and the PS:PtOEP/PNB bilayer, the Stern−Volmer formalism was employed to deduce the KSVSO2 product of the PtOEP luminescence quenching in each system. Parts a2, b2, and c2 of Figure 1 present the Stern−Volmer plots for the two PNB:PtOEP blend films and the PS:PtOEP/PNB bilayer. For all three systems, a clear deviation from linearity and a downward curvature toward the x-axis is observed. This is a typical situation where nonlinear Stern−Volmer curves are obtained due to the different accessibilities of the O2 quenchers to the triplet excited state population of PtOEP. In order to deduce the fraction of PtOEP sites that are accessible to O2, modified Stern−Volmer plots were constructed for these systems according to eq 4.38

(2)

where SO2 corresponds to the solubility coefficient of oxygen in the medium within which the phosphorescent probe is embedded. The corresponding PO2 value of the material under study can then be deduced, based on the extracted product of the KSV and SO2 parameters, KSVSO2, according to eq 3.32 P O2 =

1000KSVSO2 4πNAαστ0

(3)

Here, α corresponds to the probability for the oxygen quenchers to collide with the triplet excited states and result in luminescence quenching, σ corresponds to the size of the encounter complex formed between molecular oxygen and triplet excited PtOEP during the collisional quenching process, τ0 corresponds to the lifetime of the PtOEP triplet excited state in vacuo, and NA corresponds to the Avogadro number. Despite its simplicity, the Stern−Volmer formalism in most cases fails to adequately describe the process of phosphorescence quenching by oxygen in polymeric blend films. The microstructural heterogeneities of a barrier:triplet emitter composite system allows for the permeant oxygen to probe only a fraction of the overall triplet excited state population. Consequently, phosphorescence emission is quenched by the interaction of oxygen with the accessible excited triplet state population. In this case, the obtained phosphorimetric quenching data exhibit nonlinear Stern−Volmer plots,31,34 and eq 2 cannot be utilized effectively in the determination of KSVSO2. In this work, we present the use of a modified Stern−Volmer photokinetic model that can be successfully employed in the determination of PO2of barrier materials, by taking into account the fractional accessibility of the excited triplet states to the permeant oxygen. Our approach is applied in a set of phosphorimetric data collected for a variety of different barrier materials. The obtained results are coherent with the literature. They are also in qualitative agreement with the results of a preliminary shelf lifetime stability study of OPV devices encapsulated with some of the studied barrier materials.

I0 1 1 = + ΔI fa fa KSVSO2pO

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2

RESULTS AND DISCUSSION For our study we use the organometallic complex (2,3,7,8,12,13,17,18-octaethyl-porphyrinato)platinum(II)

(4)

In eq 4, ΔI = I0 − I, where I0 and I correspond to the phosphorescence intensities of the triplet emitter PtOEP in the absence and in the presence of oxygen, respectively. The C



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parameter fα corresponds to the fraction of the PtOEP triplet excited states that are accessible to oxygen. It can be of great use in future studies aiming to address the microstructural properties of the barrier materials.39 Equation 4 has been successfully applied in PL quenching studies of emissive amino acids in enzyme systems.38,40 The modified Stern−Volmer plots for the systems studied herein and their linear fits are presented in Figure 1a3,b3,c3. Table 1 summarizes the fitting results of the modified Stern− Volmer plots. In the case of the PNB:PtOEP blend films, it is found that 90% of the PtOEP excited state population is accessible to the O2 quenchers. Thus, the use of the PNB barrier as an overlayer for the protection of the PS:PtOEP layer is found to be more effective, allowing less accessibility of the O2 quenchers toward the PtOEP sites. Based on the deduced KSVSO2 values for the three systems and taking into account that (i) the quenching probability of a triplet excited state is α = 1/ 931 and (ii) the size σ of the encounter complex formed between O2 and triplet excited PtOEP is 1 nm,32 the permeability coefficient PO2 was deduced according to eq 3. The time-resolved PL characterization of the PNB:PtOEP blend films and the PS:PtOEP/PNB bilayer confirmed that the lifetime of the PtOEP phosporescence was also reduced as the partial pressure of oxygen was increased (see the Supporting Information). At low oxygen partial pressures, the PtOEP phosphorescence is found to decay monoexponentially but a biexponential fit is necessary to better describe the phosphorescence decay at higher partial pressures. For all phosphorescence decay curves the average lifetime of PtOEP, ⟨τ⟩, is deduced based on eq 5.38,41 2

⟨τ ⟩ =

A1τ1 + A 2 τ2 A1τ1 + A 2 τ2

Figure 2. Stern−Volmer plots for (a) the oxygen-induced quenching of PtOEP phosphorescence intensity and (b) the oxygen-induced quenching of PtOEP phosphorescence lifetime of a PS:PtOEP layer (squares), a Zeonex:PtOEP layer (circles), a PMMA996k:PtOEP layer (down-triangles), and a PS:PtOEP/Zeonex bilayer (up-triangles). The solid lines are guides for the eye.

2

(5)

In eq 5, A1 and A2 correspond to the amplitude values and τ1 and τ2 correspond to the respective lifetime values of the two components of the biexponential fit. In light of the timeresolved phosphorimetric data, we can rule out the occurrence of static quenching effects35 and we can safely assign the reduction of the PtOEP emission intensity to the dynamic quenching of the emissive PtOEP triplet states by molecular oxygen. We have extended our phosphorimetric studies to the rest of the solution-processable polymeric layers used for the fabrication of the blend films in order to validate the general applicability of the modified Stern−Volmer photokinetic model. Time-integrated and time-resolved PL spectra were recorded for the PS:PtOEP, PMMA:PtOEP, Zeonex:PtOEP, and PS:PtOEP/Zeonex systems in a range of partial pressures of oxygen. Figure 2 presents the Stern−Volmer plots for these systems when either the spectral integrals or the average lifetimes of the PtOEP phosphorescence were used. Similar to the case of the PNB:PtOEP blend films (Figure 1), both types of the obtained Stern−Volmer plots are nonlinear and they exhibit a clear indication of downward curvature toward the xaxis. In contrast, the application of the nonlinear Stern−Volmer photokinetic model for the fractional accessibility of molecular oxygen to the PtOEP triplet excited states successfully reproduces the time-integrated phosphorimetric data of Figure 2a. In Figure 3, the data of Figure 2a are plotted according to eq 4 and the linear fits provide information on the corresponding KSVSO2 and fα parameters. For each studied system, the deduced value of the PO2 parameter is reported in Table 1. According to

Figure 3. Modified Stern−Volmer plot for the oxygen-induced quenching of the PtOEP phosphorescence intensity of a PS:PtOEP layer (open squares), a Zeonex:PtOEP layer (open circles), a PS:PtOEP/Zeonex bilayer (open up-triangles), a PMMA996k:PtOEP layer (open down-triangles), a PMMA350k:PtOEP layer (open righttilted triangles), a PMMA120k:PtOEP layer (open left-tilted triangles), and a PS:PtOEP/glass+epoxy layer (solid circles). The solid lines are fits on the data according to eq 4.

the results, the fraction of the PtOEP triplet excited state population available for phosphorescence quenching by oxygen is the lowest for the PMMA:PtOEP blend films and these systems exhibit the lowest values for the PO2 parameter. Note that the deduced values of the PO2 parameters are in excellent D



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Figure 4. Evolution of the main device figures of merit of an organic solar cell of glass/ITO/PEDOT:PSS/P3HT:PCBM[70]/Al geometry without encapsulation (squares), encapsulated with a solution-processed Zeonex barrier (triangles), and encapsulated with glass/epoxy barrier (circles).

agreement with previous reports; i.e., for PS a value of POPS2 = 0.982 × 10 −13 cm3 (STP)·cm·cm −2 ·s −1 ·Pa −1 has been reported.27 Moreover, the PMMA-based films exhibit single exponential decay dynamics of the PtOEP phosphorescence in the whole range of partial oxygen pressures that were monitored. We have furthermore characterized an oxygen-barrier system based on the use of epoxy resin and glass, which is commonly employed for the encapsulation of organic photovoltaic devices.42 In this case, a PS:PtOEP blend film was deposited on a glass substrate and was then enclosed in a glass cavity. The edges of the glass cavity on the glass substrate of the PS:PtOEP film were sealed with the specific epoxy glue under study, without the need for additional thermal annealing for the curing of the epoxy. The corresponding KSVSO2 and the fα and PO2 parameters were deduced on the basis of the phosphorimetric characterization results of this epoxy-sealed PS:PtOEP system. Figure 3 presents the time-integrated phosphorimetric data for this sample, plotted according to eq 4. The corresponding PO2 parameter has a value of 39 × 10−16 cm3(STP)·cm·cm−2·s−1· Pa−1. This finding highlights the high sensitivity of the phosphorimetric technique owning to the high quantum efficiency of phosphorescence and the long phosphorescence lifetime of the PtOEP triplet emitter. Therefore, the suggested photokinetic model is an appropriate method for the phosphorimetric characterization of high-efficiency barrier materials such as poly(ethylene terephthalate) (PET) with PO2PET = 30 × 10−16 cm3(STP)·cm·cm−2·s−1·Pa−1, commonly used in food packaging applications, and the water-soluble O2 = 0.3 × 10 −16 poly(vinyl alcohol) (PVA)43 with PPVA 3 −2. −1 −1 cm (STP)·cm·cm ·s ·Pa that could be an appropriate barrier for OPV devices. The permeation coefficient values in Table 1 are reported without the corresponding standard deviation parameter, since no multiple measurements were performed for averaging the time-integrated and time-resolved phosphorimetric data of the studied systems. The alternative approach of using the uncertainty values of the determined fα−1 and ( fαKSVSO2)−1 parameters, as derived by fitting the data of Figure 3, is not an appropriate method for determining the uncertainty of PO2. To

perform the propagation of the error in the calculation of the PO2 value, the parameters fα‑1 (intercept) and (fαKSVSO2)−1 (slope) should be uncorrelated; however this is not the case because both parameters originate from the same least-squares fit. The determined PO2 values are reaching the sensitivity limit of the phosphorimetric technique when the PtOEP oxygen sensor is used.32 In principle, the longer the lifetime of the phosphorescent probe used, the higher the sensitivity of phosphorimetry is, for detecting the amounts of oxygen that can infinitesimally quench the phosphorescence emission and reduce the triplet-state lifetime of the probe. In order to compare the results of the phosphorimetric characterization, as deduced by the use of the modified Stern− Volmer photokinetic model, with the barrier performance of the studied materials, we also fabricated a set of bulk heterojunction solar cells utilizing these materials as barriers. We then monitored the evolution of the basic device parameters by leaving the cells in ambient conditions over a period of 7 days. Three types of OPV devices were fabricated by using the binary composite system of poly(3-hexyl thiophene):[6,6]phenyl-C 71 -butyric methyl acid ester ((P3HT):PCBM[70]) as the device photoactive layer on glass/indium tin oxide (ITO) substrates.44,45 Poly(styrene sulfonate)-doped poly(ethylene dioxythiophene) (PEDOT:PSS) and aluminum (Al) were used as the hole-collecting and electron-collecting electrodes, respectively. One of the three types remained without encapsulation, the second was encapsulated with the solution-processed barrier material Zeonex, and the third was encapsulated with glass/epoxy. All devices were electrically characterized every 2 days under similar experimental conditions, by illuminating them with simulated solar light (AM1.5G, 0.79 Suns). During the waiting time the devices were stored in the dark in a laboratory environment (average laboratory temperature, 23 °C; average laboratory relative humidity, 43.2%). Figure 4 presents the evolution of the basic device parameters in this period of time, namely, the open-circuit voltage (VOC), fill factor (FF), shortcircuit current density (JSC), and power conversion efficiency (PCE). For each device type at least four different devices were characterized to ensure the reproducibility of the obtained E



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CONCLUSION In conclusion, we have presented phosphorimetric studies for oxygen permeation in a set of solution-processed barriers in combination with shelf stability studies of organic photovoltaic devices, encapsulated with these barrier materials. The results presented herein show that the spectroscopic technique of phosphorimetry is a versatile, straightforward diagnostic tool of high sensitivity that can be routinely employed in the fast and low-cost screening evaluation of future oxygen-barrier materials for organic electronics. When organic triplet emitters are mixed with or covered by the solid-state barrier materials under study, the process of dynamic phosphorescence quenching can be simply described by a modified Stern−Volmer plot that takes into account the fractional accessibility of the excited triplet states to oxygen. The presented photokinetic model was successfully tested in a set of different polymeric barrier materials. The use of other molecular probes with longer excited-state lifetimes and with higher PL quantum yields47,48 is expected to increase further the sensitivity of the phosphorimetric technique.

results. Figure 4 clearly shows that the epoxy/glass encapsulation prevents the degradation of the P3HT:PCBM[70] device whereas the other two types of devices are completely degraded after the period of 2 days. The evolution of the corresponding external quantum efficiency (EQE) spectra of the studied devices is presented Figure 5.

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EXPERIMENTAL DETAILS Solution Preparation and Film Fabrication. For all PS:PtOEP, Zeonex:PtOEP, and PMMA:PtOEP blend films, toluene is used as the solvent. In all cases a PtOEP content of 1 wt % was used. Bilayer films of PS:PtOEP/PNB and PS:PtOEP/Zeonex are also included in the study, after using the dip-coating technique for depositing a PNB or a Zeonex overlayer on top of a PS:PtOEP blend film. Both overlayer materials were dissolved in cyclohexane for minimizing the dissolution of the PS:PtOEP layer during the dip-coating process. UV−vis characterization was performed for evaluating the impact of the dip-coating process on the thickness of the PS:PtOEP layer of the bilayers (see Figure S1 of the Supporting Information). All blend films were prepared by spin-coating (Laurell Technologies Corp., WS-650SX-6NPP/LITE). A dipcoating unit (Holmarc Opto-Mechatronics Pvt Ltd., HO-TH01) was used for the fabrication of the bilayer systems in a N2filled glovebox. Table S1 of the Supporting Information summarizes the exact fabrication protocol for each system. For all samples the spin-coating and dip-coating steps were performed inside the glovebox. Toluene was degassed by purging N2 for 15 min in ambient condition during sonication at 25 °C while purging the gas. After degassing, the solvent was transferred inside the glovebox in a capped bottle and all subsequent operations were performed in the N2 atmosphere of the glovebox. Cyclohexane was not degassed since it was manufactured and supplied in oxygen- and moisture-free grade. The required quantities of the solvent were taken with a needled syringe without destroying the manufacturer’s septum crown cap. Film-Thickness and UV−Vis Absorption Characterization. Film-thickness determination of the films was performed with a surface profiler measuring system (Bruker, D150). UV−vis absorption spectra of the produced films were recorded with a UV-2700 Shimadzu spectrophotometer. Phosphorimetric Characterization. Time-integrated and time-resolved phosphorescence spectra of all systems were recorded in a range of partial oxygen pressures. The samples were photoexcited, with the output of an optical parametric oscillator (Spectra-Physics VersaScan Midband 120) pumped by the third harmonic of a Nd:YAG Laser (Spectra-Physics

Figure 5. The evolution of the EQE spectra in a time span of 7 days after the fabrication of the P3HT:PCBM[70] devices: (a) without encapsulation, (b) encapsulated with a solution-processed Zeonex barrier, and (c) encapsulated with a glass/epoxy barrier.

The performed shelf stability study provides an indication that the low PO2 value of the specific glass/epoxy combination, as determined by the phosphorimetric characterization, is in line with the relative efficiency of the material to act as an efficient oxygen barrier. The value determined for the PO2 parameter of the epoxy/glass combination is almost 2 orders of magnitude lower than the corresponding value of the Zeonex-based barrier. Nonetheless, this device degradation experiment cannot deconvolve the detrimental effects of oxygen and moisture permeation through the barrier materials on the electrical properties of the studied cells. More accurate shelf lifetime studies are required that should be performed in environmental chambers where the relative humidity and temperature parameters can be tuned and stabilized on demand.46 F



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than the device active area. To achieve that, the monochromatic output light was directed through a circular iris onto the active area of the characterized device during the measurement. A calibrated Si photodiode (818-UV Newport) was used as a reference in order to determine the intensity of the light incident on the device, allowing the deduction of the EQE spectrum. For each system characterized, the reproducibility of the results was checked by measuring at least four to six devices. Photovoltaic Characterization under AM1.5G Simulated Solar Light Illumination. Photocurrent−voltage characteristics of the fabricated solar cells were recorded with a 2440 Keithley source measure and a Sol3A Oriel solar simulator (AM1.5G) with an irradiance of 79.35 mW/cm2. For each system characterized the reproducibility of the results was checked by measuring at least four to six devices.

INDI-40-10-HG), at 532 nm by a train of 10 ns pulses at a repetition rate of 10 Hz, with average pulse energy of 10 μJ/ pulse. The emitted light was dispersed in a spectrograph (Andor Shamrock Spectrograph, SR303i) with a 150 lines/mm grating and detected with a gated intensified charge-coupled device camera (Andor iCCD, iStar DH320T-25U-73). Timeintegrated PL spectra were recorded by using an exposure time of 500 ms. During the phosphorimetric characterization the samples were kept at laboratory temperature in a Janis cryostat (VPF-100) that was evacuated by a turbomolecular pump (Pfeiffer TSH 071E economy dry vacuum pumping station). In order to achieve the lowest vacuum values, either the cryostat chamber was evacuated for 2−3 h prior to the first measurement or the pump was left running overnight. All consecutive measurements were performed after introducing the desirable pressure value of atmospheric air and allowing the system to equilibrate at these conditions for 15 min. Solar Cell Device Fabrication. P3HT was purchased from Sigma-Aldrich (698997), and PCBM[70] was purchased from Solenne BV. Both materials were used as received without any further purification. Surface polished glass/(ITO) substrates were purchased from Xin Yan Technology Ltd. (15 Ω/square, XY15S). Organic solar cell devices were prepared of the type glass/ITO/poly(styrene sulfonate-doped) poly(ethylene dioxythiophene) (PEDOT:PSS)/P3HT:PCBM[70]/aluminum (Al). The glass/ITO substrates were precleaned with acetone, isopropanol, and detergent Hellmanex III (Hellma) and dried under a flow of dry nitrogen. Before the deposition of the organic layers the ITO substrates were cleaned in O2 plasma for 10 min. A PEDOT:PSS (Baytron P VP AI 4083, H.C. Stark) solution was then spin-coated (Laurell Technologies Corp., WS-400B-6NPP/LITE) at 4000 rpm onto the cleaned ITO substrates resulting in a thickness of approximately 40 nm. The PEDOT:PSS layer was annealed for 30 min at 140 °C in air. After that the devices were transferred into a N2-filled glovebox (GP-concept-II-P glovebox, P[SYS]-II-P gas purification module, Jacomex). For the preparation of the active layer, P3HT and PCBM[70] were dissolved in chlorobenzene at a concentration of 23.5 mg mL−1. The active layer was then spincoated on top of the PEDOT:PSS film at 1500 rpm resulting in a film thickness of ∼135 nm. Finally, aluminum counter electrodes were deposited by sputter-coating (Emitech, KL75R005) through a shadow mask on top of the active layer with a thickness of 60 nm. The films were postannealed at 140 °C for 15 min on a hot plate in the glovebox. For all devices, the active area of the pixels as defined by the overlap of anode and cathode areas was 0.0525 cm2. The device pixels were then clipped to lead frames (Tyco Electronics, 1544169-4) to allow electrical connection and encapsulated in glass using an epoxy resin and hardener (Robnor Resins Ltd.), without the need for additional thermal annealing. Curing of the epoxy was completed 24 h after the fabrication of the devices that during this period were left in the glovebox. External Quantum Efficiency Characterization. A 2401 Keithley source-measure unit was used for recording the shortcircuit current whereas monochromatic light was provided by a quartz tungsten halogen lamp dispersed through a Newport Oriel Apex monochromator illuminator. The light output from the quartz tungsten halogen lamp was monochromated by a Newport Oriel Cornerstone 130 1/8 monochromator, and the short-circuit device photocurrent was monitored by the electrometer as the monochromator was scanned. Extreme care was taken so that the illumination spot size was smaller

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ASSOCIATED CONTENT

S Supporting Information *

Tables listing protocols for solution preparation and phosphorescence intensity decay transients and figures showing layer deposition, UV−vis spectra, time-integrated phosphorescence spectra, and phosphorescence intensity decay transients. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address

† Dalton Cumbrian Facility, Dalton Nuclear Institute, The University of Manchester, Westlakes Science & Technology Park, Moor Row, Cumbria CA24 3HA, U.K.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

All authors acknowledge SAES Getters S.p.A. for financial support and for providing access in their premises where the encapsulation process of the PS:PtOEP layer with the glass/ epoxy barrier took place. P.E.K. acknowledges the financial support of an Intra European Marie Curie Fellowship (Project DELUMOPV) within the 7th Framework Programme of the European Commission (Grant FP7-PEOPLE-2011-IEF). P.E.K. and G.C.V. acknowledge the financial support of the John S. Latsis Public Benefit Foundation (Project “Development of next-generation oxygen-barrier materials for organic electronic and dye-sensitized solar cell applications”). Zeon Europe GmbH is acknowledged for providing a generous gift of Zeonex.

(1) Mesta, M.; Carvelli, M.; de Vries, R. J.; van Eersel, H.; van der Holst, J. J. M.; Schober, M.; Furno, M.; Lüssem, B.; Leo, K.; Loebl, P.; et al. Molecular-Scale Simulation of Electroluminescence in a Multilayer White Organic Light-Emitting Diode. Nat. Mater. 2013, 12, 652−658. (2) Janssen, R. A.; Nelson, J. Factors Limiting Device Efficiency in Organic Photovoltaics. Adv. Mater. 2013, 25, 1847−1858. (3) Dou, D.; You, J.; Hong, Z.; Xu, Z.; Li, G.; Street, R. A.; Yang, Y. 25th Anniversary Article: A Decade of Organic/Polymeric Photovoltaic Research. Adv. Mater. 2013, 25, 6642−6671.

G



Article

(24) Krebs, F. C. Fabrication and Processing of Polymer Solar Cells: A Review of Printing and Coating Techniques. Sol. Energy Mater. Sol. Cells 2009, 93, 394−412. (25) Abdellatief, A.; Welt, B. A. Comparison of New Dynamic Accumulation Method for Measuring Oxygen Transmission Rate of Packaging against the Steady-State Method Described by ASTM D3985. Packag. Technol. Sci. 2013, 26, 281−288. (26) Siracusa, V. Food Packaging Permeability Behaviour: A Report. Int. J. Polym. Sci. 2012, 302029. (27) Hess, S.; Demir, M. M.; Yakutkin, V.; Baluschev, S.; Wegner, G. Investigation of Oxygen Permeation Through Composites of PMMA and Surface-Modified ZnO Nanoparticles. Macromol. Rapid Commun. 2009, 30, 394−401. (28) Borisov, S. M.; Larndorfe, C.; Klimant, I. Triplet−Triplet Annihilation-Based Anti-Stokes Oxygen Sensing Materials with a Very Broad Dynamic Range. Adv. Funct. Mater. 2012, 22, 4360−4368. (29) Papkovsky, D. B.; Olah, J.; Troyanovsky, I. V.; Sadovski, N. A.; Rumyantseva, V. D.; Mironov, A. F.; Yaropolov, A. I.; Savitsky, A. P. Phosphorescent Polymer Films for Optical Oxygen Sensors. Biosens. Bioelectron. 1992, 7, 199−206. (30) Papkovsky, D. B. New oxygen sensors and their application to biosensing. Sens. Actuators, B 1995, 29, 213−218. (31) Lu, X.; Winnik, M. A. Luminescence Quenching in Polymer/ Filler Nanocomposite Films Used in Oxygen Sensors. Chem. Mater. 2001, 13, 3449−3463. (32) Hess, S.; Becker, A.; Baluschev, S.; Yakutkin, V.; Wegner, G. A Comparative Study of Oxygen Permeabilities of Film-Forming Polymers by Quenching of Platinum Porphyrin Phosphorescence. Macromol. Chem. Phys. 2007, 208, 2173−2188. (33) Stern, O.; Volmer, M. Ü ber die Abklingzeit der Fluoreszenz. Phys. Z. 1919, 20, 183−188. (34) Demas, J. N.; Deora, B. A.; Xut, W. Modeling of Luminescence Quenching-Based Sensors: Comparison of Multisite and Nonlinear Gas Solubility Models. Anal. Chem. 1995, 67, 1377−1380. (35) Keivanidis, P. E.; Baluschev, S.; Lieser, G.; Wegner, G. Inherent Photon Energy Recycling Effects in the Up-Converted Delayed Luminescence Dynamics of Poly(fluorene)-Pt(II)octaethyl Porphyrin Blends. ChemPhysChem 2009, 10, 2316−2326. (36) Mills, A. Optical Oxygen Sensors Utilising the Luminescence of Platinum Metal Complexes. Platinum Met. Rev. 1997, 41, 115−127. (37) Vougioukalakis, G. C.; Stamatopoulos, I.; Petzetakis, N.; Raptopoulou, C. P.; Psycharis, V.; Terzis, A.; Kyritsis, P.; Pitsikalis, M.; Hadjichristidis, N. Controlled Vinyl-Type Polymerization of Norbornene with a Nickel(II) Diphosphinoamine Methylaluminoxane Catalytic System. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 5241− 5250. (38) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Springer: Berlin, 2010. (39) Jayarajah, C. N.; Yekta, A.; Manners, I.; Winnik, M. A. Oxygen Diffusion and Permeability in Alkylaminothionylphosphazene Films Intended for Phosphorescence Barometry Applications. Macromolecules 2000, 33, 5693−5701. (40) Xing, D.; Dorr, R.; Cunningham, R. P.; Scholes, C. P. Endonuclease III Interactions With DNA Substrates, 2: The DNA Repair Enzyme Endonuclease III Binds Differently to Intact DNA and to Apyrimidinic/ Apurinic DNA Substrates as Shown by Tryptophan Fluorescence Quenching. Biochemistry 1995, 34, 2537−2544. (41) Masoumi, Z.; Stoeva, V.; Yekta, A.; Pang, Z.; Manners, I.; Winnik, M. A. Luminescence Quenching Method for Probing the Diffusivity of Molecular Oxygen in Highly Permeable Media. Chem. Phys. Lett. 1996, 261, 551−557. (42) Foster, S.; Finlayson, C. E.; Keivanidis, P. E.; Huang, Y. S.; Hwang, I.; Friend, R. H.; Otten, M. B. J.; Lu, L. P.; Schwartz, E.; Nolte, R. J. M.; et al. Improved Performance of Perylene-Based Photovoltaic Cells Using Polyisocyanopeptide Arrays. Macromolecules 2009, 42, 2023−2030. (43) Gaume, J.; Wong-Wah-Chung, P.; Rivaton, A.; Therias, S.; Gardette, J.-L. Photochemical Behavior of PVA as an Oxygen-Barrier

(4) Gelinck, G.; Heremans, P.; Nomoto, K.; Anthopoulos, T. D. Organic Transistors in Optical Displays and Microelectronic Applications. Adv. Mater. 2010, 22, 3778−3798. (5) Baeg, K.-J.; Caironi, M.; Noh, Y.-Y. Toward Printed Integrated Circuits based on Unipolar or Ambipolar Polymer Semiconductors. Adv. Mater. 2013, 25, 4210−4244. (6) Gelinck, G. H.; Kumar, A.; Moet, D.; van der Steen, J.-L.; Shafique, U.; Malinowski, P. E.; Myny, K.; Rand, B. P.; Simon, M.; Rütten, W.; et al. X-ray Imager Using Solution Processed Organic Transistor Arrays and Bulk Heterojunction Photodiodes on Thin, Flexible Olastic substrate. Org. Electron. 2013, 14, 2602−2609. (7) Baeg, K.-J.; Binda, M.; Natali, D.; Caironi, M.; Noh, Y.-Y. Organic Light Detectors: Photodiodes and Phototransistors. Adv. Mater. 2013, 25, 4267−4295. (8) Ando, K.; Watanabe, S.; Mooser, S.; Saitoh, E.; Sirringhaus, H. Solution-Processed Organic Spin−Charge Converter. Nat. Mater. 2013, 12, 622−627. (9) Yan, Y.; Rothberg, L. J.; Papadimitrakopoulos, F.; Galvin, M. E.; Miller, T. M. Defect Quenching of Conjugated Polymer Luminescence. Phys. Rev. Lett. 1994, 73, 744−747. (10) Jørgensen, M.; Norrman, K.; Krebs, F. C. Stability/Degradation of Polymer Solar Cells. Sol. Energy Mater. Sol. Cells 2008, 92, 686−714. (11) Sirringhaus, H. Reliability of Organic Field-Effect Transistors. Adv. Mater. 2009, 21, 3859−3873. (12) Di Pietro, R.; Fazzi, D.; Kehoe, T. B.; Sirringhaus, H. Spectroscopic Investigation of Oxygen- and Water-Induced Electron Trapping and Charge Transport Instabilities in n-type Polymer Semiconductors. J. Am. Chem. Soc. 2012, 134, 14877−14889. (13) Jørgensen, M.; Norrman, K.; Gevorgyan, S. A.; Tromholt, T.; Andreasen, B.; Krebs, F. C. Stability of Polymer Solar Cells. Adv. Mater. 2012, 24, 580−612. (14) Anthopoulos, T. D.; Anyfantis, G. C.; Papavassiliou, G. C.; de Leeuw, D. M. Air-Stable Ambipolar Organic Transistors. Appl. Phys. Lett. 2007, 90, 122105. (15) Tournebize, A.; Bussière, P.-O.; Rivaton, A.; Gardette, J.-L.; Medlej, H.; Hiorns, R. C.; Dagron-Lartigau, C.; Krebs, F. C.; Norrman, K. New Insights Into the Mechanisms of Photodegradation/ Stabilization of P3HT:PCBM Active Layers Using Poly(3-hexyl-d13Thiophene). Chem. Mater. 2013, 25, 4522−4528. (16) Girtan, M. On the Stability of the Electrical and Photoelectrical Properties of P3HT and P3HT:PCBM blends thin films. Org. Electron. 2013, 14, 200−205. (17) Nelson, J.; Emmott, C. J. M. Can Solar power Deliver? Philos. Trans. R. Soc., A 2013, 371 (no. 1996), 20120372. (18) Dennler, G.; Lungenschmied, C.; Neugebauer, H.; Sariciftci, N. S.; Latreche, M.; Czeremuszkin, G.; Wertheimer, M. R. A New Encapsulation Solution for Flexible Organic Solar Cells. Thin Solid Films 2006, 511−512, 349−353. (19) Cros, S.; de Bettignies, R.; Berson, S.; Bailly, S.; Maisse, P.; Lemaitre, N.; Guillerez, S. Definition of Encapsulation Barrier Requirements: A Method Applied to Organic Solar Cells. Sol. Energy Mater. Sol. Cells 2011, 95, S65−S69. (20) Hermenau, M.; Schubert, S.; Klumbies, H.; Fahlteich, J.; MüllerMeskamp, L.; Leo, K.; Riede, M. The Effect of Barrier Performance on the Lifetime of Small-Molecule Organic Solar Cells. Sol. Energy Mater. Sol. Cells 2012, 97, 102−108. (21) Sondergaard, R. R.; Makris, T.; Lianos, P.; Manor, A.; Katz, E. A.; Gong, W.; Tuladhar, S. M.; Nelson, J.; Toumi, R.; Sommeling, P.; et al. The Use of Polyurethane as Encapsulating Method for Polymer Solar Cells An Inter Laboratory Study on Outdoor Stability in 8 Countries. Sol. Energy Mater. Sol. Cells 2012, 99, 292−300. (22) Kim, N.; Potscavage, W. J., Jr.; Sundaramoothi, A.; Henderson, C.; Kippelen, B.; Graham, G. A Correlation Study Between Barrier Film Performance and Shelf Lifetime of Encapsulated Organic Solar Cells. Sol. Energy Mater. Sol. Cells 2012, 101, 140−146. (23) Ahmad, A.; Bazaka, K.; Anderson, L. J.; White, R. D.; Jacob, M. V. Materials and Methods for Encapsulation of OPV: A Review. Renewable Sustainable Energy Rev. 2013, 27, 104−117. H



Article

Polymer for Solar Cell Encapsulation. RSC Adv. 2011, 1, 11471− 11481. (44) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Plastic Solar Cells. Adv. Funct. Mater. 2001, 11, 15−26. (45) Wienk, M. M.; Kroon, J. M.; Verhees, W. J. H.; Knol, J.; Hummelen, J. C.; van Hal, P. A.; Janssen, R. A. J. Efficient Methano[70]fullerene/MDMO-PPV Bulk Heterojunction Photovoltaic Cells. Angew. Chem., Int. Ed. 2003, 42, 3371−3375. (46) Gevorgyan, S. A.; Medford, A. J.; Bundgaard, E.; Sapkota, S. B.; Schleiermacher, H.-F.; Zimmermann, B.; Wurfel, U.; Chafiq, A.; LiraCantu, M.; Swonke, T.; et al. An Iinter-Laboratory Stability Study of Roll-To-Roll Coated Flexible Polymer Solar Modules. Sol. Energy Mater. Sol. Cells 2011, 95, 1398−1416. (47) Douglas, A.; Eaton, K. Response Characteristics of Thin Film Oxygen Sensors, Pt and Pd Octaethylporphyrins in Polymer Films. Sens. Actuators B 2002, 82, 200−208. (48) Huynh, L.; Wang, Z.; Yang, J.; Stoeva, V.; Lough, A.; Manners, I.; Winnik, M. A. Evaluation of Phosphorescent Rhenium and Iridium Complexes in Polythionylphosphazene Films for Oxygen Sensor Applications. Chem. Mater. 2005, 5, 4765−4773.

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