The effect of post-processing treatments on inflection points in current–voltage curves of roll-to-roll processed polymer photovoltaics

Solar Energy Materials & Solar Cells 94 (2010) 2018–2031

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The effect of post-processing treatments on inflection points in current–voltage curves of roll-to-roll processed polymer photovoltaics Mathilde R. Lilliedal, Andrew J. Medford, Morten V. Madsen, Kion Norrman, Frederik C. Krebs n Risø National Laboratory for Sustainable Energy, Technical University of Denmark, Frederiksborgvej 399, DK-4000 Roskilde, Denmark

a r t i c l e in f o

a b s t r a c t

Article history: Received 6 February 2010 Received in revised form 12 April 2010 Accepted 9 June 2010

Inflection point behaviour is often observed in the current–voltage (IV) curve of polymer solar cells. This phenomenon is examined in the context of flexible roll-to-roll (R2R) processed polymer solar cells in a large series of devices with a layer structure of: PET–ITO–ZnO–P3HT:PCBM–PEDOT:PSS–Ag. The devices were manufactured using a combination of slot-die coating and screen printing; they were then encapsulated by lamination using a polymer based barrier material. All manufacturing steps were carried out in ambient air. The freshly prepared devices showed a consistent inflection point in the IV curve and a corresponding poor performance or lack of photovoltaic behaviour. Upon exposure to 1000 Wm  2 illumination at ca. 85 1C and repeated IV scans (photo-annealing) the inflection point gradually disappeared, and performance drastically increased over time. The characteristics and stability of this ‘‘photo-annealing’’ behaviour was further investigated by studying the effects of several key factors: temperature, illumination and atmosphere. The results consistently showed that the inflection point is a dynamic phenomenon which can be removed under specific conditions. Subsequently, chemical characterization of device interfaces was carried out in order to identify possible chemical processes that are related to photo-annealing. A possible mechanism based on ZnO photoconductivity, photooxidation and redistribution of oxygen inside the cell is proposed, and it is anticipated that the findings are applicable to various other device structures based on semi-conducting oxides. The findings may have influences on the possibilities and scale-up of polymer solar technologies. & 2010 Elsevier B.V. All rights reserved.

Keywords: Scale-up Zinc oxide IV-curve S-curve Inverted polymer solar cells Roll-to-roll processing

1. Introduction 1.1. Polymer solar cell device structures Polymer solar cells [1,2] are characterised by simple construction and an exceptional ease of processing [3]. Currently, active areas of research include studies of morphology, power conversion efficiency, device stability, processing, manufacture and demonstration of the technology. The current state of the art devices rely on the bulk heterojunction [2] and are typically prepared by simple solution processing of the active layers followed by evaporation of metallic back electrodes resulting in the ‘‘normal’’ device geometry (photo-generated electrons under working conditions flow from transparent to back electrode). This normal geometry allows easy processing on the lab scale; yet the requirements for manufacture and scale-up are not taken into consideration. The capital investment in the equipment required for vacuum processing is not warranted by the current performance of the technology which is in the order of 5% efficiency (although a few reports of efficiencies approaching higher values in the 6–8% range have appeared recently) [4–6].

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More recently, ‘‘inverted’’ device geometries (photo-generated electrons under working conditions flow from back electrode to transparent electrode) were reported [7]. These devices yield similar efficiencies, yet they allow the use of a solution processed back electrode; with an active stack structure of ITO–ZnO– P3HT:PCBM–PEDOT:PSS–Ag the PEDOT:PSS layer provides a protective barrier from the solvents in the Ag ink [8]. With inverted device structures it becomes possible to avoid vacuum processing; polymer solar cells can be printed and coated with existing equipment, thus drastically lowering the capital investment necessary for scale-up. The possibility to prepare polymer solar cells on a relatively large scale under humble conditions has enabled several demonstration projects of the technology. These preliminary demonstrations show feasibility and confirm the advantages of inverted devices [9]. However, considerable work must be done to further optimize and understand the inverted structure such that the large-scale vision for the polymer solar cell technology can be realized. 1.2. Roll-to-roll coated polymer cells Recently accurate protocols for full roll-to-roll coating of polymer solar cell modules were reported which enable manufacture of polymer solar cells under ambient conditions without

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the use of vacuum coating [7,9,10]. One of the developed methods (termed ProcessOne [7]) works with most active materials and gives semi-transparent modules. In the photovoltaic industry it is commonly reported that manufactured devices typically reach 60–70% of the performance of laboratory cells [9b]. Devices manufactured according to ProcessOne show average active area efficiencies in the range 1–2% for modules with an active area of 120 cm2 when using P3HT:PCBM as the active material (record efficiencies on smaller devices with an active area of 4.8 cm2 was up to 2.3%). This corresponds to 30–40% of laboratory scale devices, which demonstrates considerable room for improvement. One major drawback of the current process is the fact that the devices perform extremely poorly, or not at all, when freshly prepared due to an inflection point phenomenon. This can be overcome by ‘‘photo-annealing’’ under illumination and elevated temperatures in order to reach optimum performance; however, this photo-annealing requires considerable energy input and is hence a major obstacle in the scale-up of the technology. For this reason it is of great interest to study and understand the effects of various parameters on the inflection point behaviour. 1.3. Inflection point behaviour Under normal circumstances the current–voltage (IV) curve of solar cells is ‘‘J-shaped’’ and can be rationalized through the diode equation; however, for some devices an inflection point appears and causes the curve to be ‘‘S-shaped’’. The difference between these is depicted in Fig. 1. This behaviour has been studied in detail by some authors, [11–14] has been observed by others [15–41] and can be seen in some published manuscripts with little explanation [42–45]. The inflection point behaviour is not observed in the dark curve and only appears under illumination [13]. Furthermore, the behaviour is observed for a wide range of devices including normal and inverted geometries [12,13] standard and polymer– polymer heterojunctions [11], tandem cells [19] and devices containing various oxide materials [12,19,40,46,47]. Several theoretical models and explanations of this behaviour have been proposed in the literature. The inflection is often attributed to an energy barrier, caused by a poor carrier transport in one of the layers or interfaces which prevents charge extraction [11,13,19,47]. The fact that this phenomenon is observed for such a large range of devices suggests that it is general and warrants further investigation. In this report the inflection point behaviour in a large number of inverted polymer solar cells prepared by R2R coating is examined. The consistency of this behaviour is established by reporting its appearance in a vast number of cells and under

Fig. 1. Illustration of IV-curves which under illumination exhibit ‘‘diode’’ or ‘‘J-shaped’’ behaviour (green), ‘‘inflection point’’ or ‘‘S-shaped’’ behaviour (red) and a typical dark curve for the device (black). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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multiple post-production treatments. Subsequently, the characteristics of the inflection point over time were carefully examined at various temperatures, atmospheres, and illuminations. Chemical characterization with X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) of device interfaces were carried out in order to identify possible chemical processes related to photo-annealing. The disappearance of the inflection point is attributed to photoconductivity and degradation of impurities in the ZnO and a favorable redistribution of oxygen within the encapsulated cell.

2. Experimental 2.1. General materials Flexible polyethyleneterphthalate (PET, 130 mm thickness) foil with a sputtered layer of indium tin oxide (ITO, 80 nm thickness) having a nominal sheet resistance of 60 O square  1 was patterned by R2R screen printing a UV-curing etch resist followed by etching, stripping and washing. This material was cleaned with isopropanol prior to use. The ZnO nanoparticles were prepared by a method similar to the one described earlier and the ink prepared in acetone instead of o-xylene/WS-1 [7,48,49] The ZnO particles were stabilized with 10% methoxyethoxyacetic acid and filtered (0.45 mm) prior to use. The final concentration of the ZnO solution was 42.5 mg mL  1. The P3HT:PCBM ink was prepared by dissolving P3HT and PCBM in half the required volume of 1,2-dichlorobenzene at 110 1C for 2 h followed by the addition of one volume of chloroform. The final concentrations were 24 mg mL  1 for P3HT and 22 mg mL  1 for PCBM. The PEDOT:PSS was purchased from Agfa (EL-P 5010) and diluted with approximately one volume of isopropanol to give a final viscosity of 270 mPa s. The silver ink was from Dupont (PV410) and used without modification. The barrier foil was provided by Alcan Packaging with barrier performance that was 0.01 cm3 m  2 bar  1 day  1 with respect to oxygen (measured according to ASTM D 3985-81) and 0.04 g m  2 day  1 with respect to water vapour (measured according to ASTM F 372-78). The adhesive was from 467 MPF from 3 M. 2.2. Roll-to-roll production The R2R processing of these modules has been described in detail earlier [9]. The specific details of this production run are provided here. The PET/ITO substrate was cleaned immediately prior to coating on both sides (backside first) using Corona treatment (1075 W) and washing with isopropanol. Each module was labeled for later identification. The ZnO nanoparticle solution had a concentration of 42.5 mg mL  1 and was slot-die coated at 2 m min  1. The dry ZnO layer thickness was 60 nm. The active layer was slot-die coated at 1.4 m min  1 giving a dry layer thickness of 300 nm. PEDOT:PSS was slot-die coated at 0.3 m min  1 using wetting of the film with isopropanol immediately before the coating meniscus to enable good adhesion and avoid dewetting during drying of the PEDOT:PSS. The dry thickness of the PEDOT:PSS was estimated to be  20 mm. Finally a silver grid electrode was screen printed as described earlier [50]. All drying temperatures were 140 1C. Postproduction, the devices were encapsulated by R2R lamination using the barrier foil at 21 m min  1. The device side was laminated first followed by lamination of the uncoated (front) side. A total of 100 m was prepared (364 modules). There was some loss of modules at the beginning and end of the roll limiting the technical yield of functional modules to the middle 270 modules. Of these 220 modules were passed on to the experiments. The encapsulation

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procedure has been described in the literature [51]. P3HT was obtained from BASF (Sepiolid P200), [60]PCBM was obtained from Solenne B.V., PEDOT:PSS was obtained from AGFA (EL-P 5010) and silver was obtained from Dupont (PV410). 2.3. Outline of experiments A vast amount of experiments were conducted under a variety of conditions, and for this reason a schematic of the production route for each sample is presented in Fig. 2. The approximate time passed between cell production and experiment is also given for each set of experiments. 2.4. Characterization and treatments in ambient conditions A large number of R2R processed cells were subjected to photo and thermal post-treatments (Fig. 2a) and subsequently characterized in ambient conditions (Fig. 2b). These cells were taken directly from the coating roll, and consisted of 8 independent 0.6  25 cm2 cells connected in series with a nominal active area of 120 cm2. 2.4.1. Post-production treatments The following provides details of the treatments listed in Fig. 2a. Illumination: Prior to photo-annealing, some cells were illuminated under high-intensity and low-intensity conditions. The high-intensity treatment took place under a KHS solar constant 575 solar simulator with an illumination of ca. 1000 W m  2. The temperature was not actively controlled and was measured at 70 75 1C using a Fluke 52 II digital thermometer and a bimetallic thermocouple placed on the back of a cell during annealing. The low intensity treatment was conducted under a KHS solar constant 1200 solar simulator with an illumination of ca. 280 W m  2 and temperature of 45 75 1C. During both

treatments the cells were cycled from  1.25 to 1.25 V/cell with a Keithley 2400 series SourceMeter. The IV-behaviour was recorded in voltage steps of 100 mV and scans were repeated every 4 min. The photo-annealing was conducted for 90 min in ambient atmosphere and humidity (4575% relative humidity). Thermal annealing: Thermal annealing was carried out in a P-selecta oven at temperatures of 65, 85 and 105 1C. The cells were annealed in an air atmosphere for 90 min in the dark. 2.4.2. Electrical characterization (photo-annealing) The electrical properties of the solar cells were measured using Keithley Series 2400 SourceMeters, controlled by custom software. The scan range was 10 to 10 V (  1.25 to 1.25 V/cell) with voltage steps of 100 mV. In most cases, the characterization was carried out over a period of 30–90 min, where the cell was repeatedly scanned every minute until an approximately steadystate performance was achieved. The illumination was provided by a KHS solar constant 575 at an illumination intensity of ca. 1000 W m  2. Measurements were made in an ambient atmosphere and humidity; the temperature was elevated to 85 75 1C due to radiation from the illumination. The temperature was measured using a Fluke 52 II digital thermometer and a bimetallic thermocouple placed on the back of the cell, and was not actively controlled. This applies to experiments outlined in Fig. 2b. 2.5. Characterization in controlled conditions 2.5.1. Sample preparation Cells tested under controlled conditions had an active area of 0.66 cm2. These smaller samples were obtained by cutting one of the 8 independent cells from a module as shown in Fig. 2c/d, and masking the active area to 0.6  1.1 cm2 using black tape adhered to the front of the cell. The cell was removed from its module such that there was no other cell in parallel. Un-encapsulated samples were obtained from the same coating roll, but on a special section

Fig. 2. Schematic outlining the sample preparation for each set of experiments, along with approximate time from production. Details for (a) are given in Section 2.4.1. Details for (b) are given in Section 2.4.2, and results shown in Figs. 3–5 and 10. Details for (c) are given in Section 2.5, and results in Figs. 6,7 and 9. Details for (d) are also in Section 2.5, and results in Figs. 11 and 12.

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which was never laminated (Fig. 2d). It is important to note that these un-encapsulated cells were stored in air with relative darkness for approximately 10 weeks prior to testing. The cells were contacted by puncturing the barrier layers with metallic button contacts at the electrodes. 2.5.2. Atmosphere and temperature control The temperature and atmosphere were controlled using a custom designed airtight steel vessel as described in Ref. [52] with minor modifications. The atmosphere was set by pumping the vessel to vacuum, purging with 99.9% pure N2 or 95% pure O2, and repeating the process. Temperature was raised by placing a hot plate under the vessel, or lowered by pumping cold water through a heat exchanger. Fine tuning of the temperature could be achieved by modulating the fan speed. The temperature was recorded using a bimetallic thermocouple, and actively adjusted to 73 1C of the desired operating temperature. 2.5.3. Illumination and UV exposure The effects of illumination and UV exposure were conducted on encapsulated R2R single cells (Fig. 2c). The illumination intensity was provided by a KHS solar constant 575 and calibrated to various illumination intensities using a Hamamatsu S1133 KG5. The spectrum approaches AM1.5G and is expected to pass through the quartz window without significant changes [52]. In order to remove the UV light, a ThorLabs FGL400S filter was placed over the light portal and restricted the spectrum to wavelengths of longer than 400 nm. 2.5.4. Electrical characterization During the controlled condition experiments the electrical properties of the cells were measured in-situ using a Keithley Series 2400 SourceMeter controlled by custom software. The scan range was 1.25 to 1.25 V/cell in voltage steps of 20 mV and scans were repeated every minute. This applies to experiments outlined in Fig. 2c/d. 2.6. TOF-SIMS, XPS and UV–vis 2.6.1. Time-of-flight secondary ion mass spectrometry TOF-SIMS analysis was performed using a TOF-SIMS IV (ION¨ TOF GmbH, Munster, Germany). 15-ns pulses of 25-keV Bi + (primary ions) were bunched to form ion packets with a nominal temporal extent of o0.9 ns at a repetition rate of 10 kHz yielding a target current of 0.9 pA. These primary ion conditions were used for obtaining mass spectra and depth profiles. Mass spectra were acquired on 100  100 mm2 surface areas for 60 s. Depth profiling was performed using an analysis area of 300  300 mm2 and a sputter area of 500  500 mm2. 20 nA of 3-keV Xe + was used as sputter ions. Electron bombardment (20 eV) was in each case used to minimize charge built-up at the surface. Desorbed secondary ions were accelerated to 2 keV, mass analyzed in the flight tube, and post-accelerated to 10 keV before detection. 2.6.2. X-ray photoelectron spectroscopy XPS analyses were performed on a K-alpha (Thermo Electron Limited, Winsford, UK) using a monochromated Al–Ka X-ray source and a take-off angle of 901 from the surface plane. Atomic concentrations were determined from surface spectra (0–1350, 200 eV detector pass energy) and were calculated by determining the relevant integral peak intensities using a Shirley type background. All XPS and TOF-SIMS analyses were repeated five times on different surface locations.

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2.6.3. UV–visible spectroscopy UV–vis was performed on a Shimadzu UV-1700 PharmaSpec spectrophotometer. ZnO nanoparticles were spin-coated onto glass at 500, 1000 and 2000 rpm from a 45 gm/ml solution in acetone and dried at 140 1C for 1 min. The background spectrum of glass was then subtracted from the measured absorption. The transmission of the PET barrier and 400 nm filter were measured directly. Scans were performed from 200 to 800 nm with a resolution of 1 nm and a medium scan speed. Data below 300 nm were discarded due to poor signal-noise ratio. The direct band-gap of ZnO particles was estimated by plotting (ahn)2 vs. hn (a ¼absorption coefficient, h¼Planck’s constant and n ¼frequency) and taking the intercept of the linear region (hn ¼3.45–3.60 eV). Three repetitions were used for each spin coating speed, and the variation due to spinning speed was found to be negligible. The mean value of nine measurements 71.96 standard deviations (95% confidence) is reported.

3. Results and discussion 3.1. Dynamic nature of the inflection point The inflection point behaviour can be overcome by exposing the cells to a combination of illumination and temperature. Although the transition from the S-shape to J-shaped curves is observed consistently for all cells, the time it takes for the device to reach optimum performance varies. In Fig. 3 the power conversion efficiencies (ZPCE) after two different post-production treatments are presented. In one treatment, 40 cells were exposed to a ‘low intensity’ treatment at 280 Wm  2 and 45 75 1C for a fixed time of 90 min. It is clear that these cells showed little to no photovoltaic effect after the treatment (Fig. 3, open red circles). Another 45 cells were exposed to a ‘high intensity’ treatment at 1000 Wm  2 and 7575 1C for 90 min. A large variation in efficiency is observed in the performance, yet almost all cells show photovoltaic behaviour (Fig. 3, open blue diamonds). The same cells were subsequently photo-annealed to optimal performance (no increase in ZPCE after 5 scans) at 1000 Wm  2. As seen in Table 1 the times necessary to reach maximum performance varied considerably between cells and treatments. Despite these significant variations, Fig. 3 and Table 1 show that the final ZPCE was fairly consistent across the roll and between treatments. The statistical variation in the final ZPCE is likely to be a result of many factors, and it has been independently reported that

Fig. 3. Power conversion efficiencies after post-treatments for 90 min, cell 1–40 low-intensity (open red circles), cell 40–80 high-intensity (open blue diamonds) and after individual photo-annealing to optimum performance cell 0  40 low intensity (red dots), cell 40  80 high-intensity (blue diamonds). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Table 1 Initial and final power conversion efficiencies and photo-annealing times for low-intensity and high-intensity post-production treatments. Values reported as mean 7 standard deviation. Treatment

ZPCE after treatment (%)

ZPCE after photo-annealing (%)

Photo-annealing time (min)

Low intensity High intensity All cells

0.009 70.024 0.325 7 0.264 0.167 7 0.245

1.502 7 0.118 1.488 7 0.180 1.485 7 0.151

34.27 15.6 24.37 10.0 29.27 13.9

Fig. 4. (a) Evolution of IV-curves over time during standard photo-annealing of an encapsulated R2R cell at 1000 W m  2, 857 5 1C. The initial curve is red, the blue curve is after 10 min and the green is after 40 min (b) development of VOC, JSC, fill factor, and ZPCE with time during photo-annealing. Colors in (a) correspond to time marked in (b). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

quantitatively repeatable results are difficult to obtain with devices employing ZnO [46]. Fig. 4 depicts a typical ‘photo-annealing’ experiment. It is clear that the majority of the improvement in performance is a result of removing the inflection point in the IV-curve. This occurs fairly quickly, and corresponds to a sharp increase in the open circuit voltage (VOC) and fill factor. The performance then improves with a slow increase of the short circuit current density (JSC). This time dependence of the device performance adds another dimension to the characterization of cells, and requires that performance as a function of time is measured to fully characterize a cell. If the cells are left in ambient conditions for a period of time on the order of days, it is observed that the inflection point to some extent will re-appear in the IV-curve, as can be seen in Fig. 5. This re-appearance of the inflection is less severe than the original one, yet it still leads to an appreciable decrease in performance. This instability implies that the photo-annealing is not simply a tendency towards thermal equilibrium. The inflection can be removed again by extended exposure to heat and illumination (Fig. 5). It is likely that the same mechanism contributes to both the initial inflection point and the re-appearance of the inflection point. Several features of the IV-curves in the ‘‘re-annealing’’ experiment are worth noting (Fig. 5c and d). The initial few IV-scans reflect a decreasing short circuit current density but an

increase in the extracted photocurrent at 1.25 V. This actually leads to a more prominent inflection point after around 3 min. The next regime of the re-annealing is analogous to the original photo-annealing in that a relatively fast increase in performance is observed as the inflection is removed; however this increase is somewhat slower than the initial photo-annealing. The final regime is also similar to the original case of a slowly increasing performance with a relatively constant diode-shaped curve. Some hysteresis is observed between the original optimal performance and the final after re-annealing. This is attributed to oxidation of P3HT and other typically reported degradation mechanisms [41]. In order to further understand the evolution of performance over time the phenomenon was investigated under various illumination, temperature and atmospheric conditions.

3.2. Influence of illumination and UV exposure The effect of illumination intensity on the inflection point in the IV curve was examined by testing small area cells (0.66 cm2) in a carefully controlled environment. The illumination intensities used were 250, 500 and 1000 W m  2 and the temperature was set to 85 73 1C. The results are shown in Fig. 6. From Fig. 6 it is clear that the cell performance will rise much faster as the illumination intensity increases. This provides

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Fig. 5. Evolution of IV-curves for the same cell during (a) photo-annealing, (b) storage in dark and (c) re-annealing. (d) The change of JSC with time for each interval. Colors in (a), (b) and (c) correspond to time marked in (d).

Fig. 6. IV-curve development under photo-annealing at different illumination (a) 250 W m  2, (b) 500 W m  2 and (c) 1000 W m  2. (d) The change of JSC with time for illumination of 250 W m  2 (dotted), 500 W m  2 (dashed) and 1000 W m  2 (solid). Colors in (a), (b) and (c) correspond to time marked in (d).

evidence that the process is not merely thermal, and implies that a photoactive process is taking place. Obviously the P3HT is photoactive, and it is also well documented that ZnO exhibits photoactive and photocatalytic behaviour. The absorbance of ZnO occurs around 360–395 nm [53], depending on the particle size [46], while P3HT absorbs mostly in the visible spectrum [46]. Thus it is possible to probe the contributions from P3HT and ZnO separately by filtering light below wavelengths of 400 nm. Fig. 7 shows the results of two cells simultaneously photoannealed for 16.5 h in identical environments. The temperature was actively cooled to 5575 1C in order to minimize thermal contributions. In the case of UV exposure, the device reaches a very stable performance after 16.5 h, and thus the experiment was terminated. The curve after 16.5 h shows no inflection, as seen in Fig. 7a. Conversely, the cell not exposed to UV showed a remarkably slow increase in performance, and still showed significant inflection behaviour after 16.5 h (Fig. 7b, orange

curve). At 16.5 h the UV filter was removed, upon which an almost instantaneous increase is seen in the performance. This increase corresponds to a very rapid disappearance of the inflection point (Fig. 7b, blue curve). This is in agreement with other observations for a similar system, and is attributed to the photoconductivity of ZnO [46,47,54]. It is expected that some of this increase is accounted for by the additional UV photons; however, this should not contribute more than 10% increase. The observed increase is around 400%, and also shows a removal of the inflection, which should not occur simply due to additional photons. The performance of the cell under full illumination continued to increase slowly with kinetics similar to the case of the cell exposed to UV from the onset of the experiment. The filter was again placed over the cell after 2 h. This resulted in an immediate decrease in short-circuit current (possibly due to the decrease of incident photons), followed by a slow decay of performance.

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Fig. 7. Influence of UV on photo-annealing at 55 1C. (a) IV-curve development with UV illumination (b) IV-curves after 0, 16.5 h (no UV), 16.5 h (with UV), 18.5 and 47 h. (c) The change of JSC with time at constant UV illumination (dashed line) and with a short period of UV exposure (solid line—UV filter was removed during shaded area). Colors in (a) and (b) correspond to time marked in (c).

The device was monitored for another 30 h, after which the performance had decayed to a value only slightly above the value prior to UV exposure. The final IV curve after 50 h does not show the obvious inflection behaviour of the initial IV-curves. The improved cell performance and lack of inflection after exposure to UV light can be explained by the photoconductivity of ZnO [46,47,55]. It is well known that the conductivity of ZnO increases upon exposure to UV light [56], yet the process is not fully understood. The electrical properties of ZnO are complex and depend strongly upon processing conditions which can lead to a range of dark-conductivities and an array of conductivity responses upon exposure to visible and UV radiation [55–58]. The band gap of the particles used in the study was estimated at 3.418 70.004 eV using UV–visible spectroscopy and a Tauc plot. Light with wavelengths of o363 nm contains sufficient energy to photogenerate electrons and holes in the ZnO, and in principle either of these charges could induce a metastable conductivity increase. In these devices the ZnO layer is in direct contact with P3HT which is photoactive in the visible range and also acts as a hole conductor. In the case of illumination under a full spectrum (UV included) electron–hole pairs will be photo-generated in both the ZnO and P3HT. However, illumination without UV will result in exciton formation only in P3HT:PCBM; the excitons dissociate at the ZnO–P3HT:PCBM interface and in the bulk of P3HT:PCBM with electrons being transferred to ZnO while holes are transported away from the ZnO by the hole-conducting P3HT [46]. Thus mobile electrons are present in ZnO in both cases, but holes present only in the case of exposure to UV. Since the rapid disappearance of the inflection point is only observed with UV radiation, it can be concluded that the ZnO conductivity increases primarily due to the presence of holes. This experiment is analogous to previous work by Verbakel et al. [14,55] and the results are consistent. The enhanced

conductivity is hypothesized to occur due to the combination of mobile holes with adsorbed oxygen. The adsorbed oxygen acts as a surface trap state, and upon desorption the concentration of mobile electrons increases [55,56]. Upon removal of UV light oxygen begins to re-adsorb, leading to the observed decrease in conductivity. The re-adsorption process is kinetically very slow [56], which explains the long time needed for cell performance to decay to near-initial levels. Verbakel et al. also reported [14,55] the increase in conductivity upon positive charge injection via strong electrical potentials; similar behaviour was observed in R2R coated cells processed analogous to the ones presented in this report. The situation is further complicated by the outer barrier layer. The transmission of wavelengths shorter than 390 nm is attenuated by the PET barrier, as shown in Fig. 8. Free charge carriers will be generated in ZnO at wavelengths below 363 nm; however, the transmission of the PET barrier is only around 2% at this value. It is hypothesized that at such a low transmission the photogenerated charges will all contribute to oxygen desorption rather than transient photoconductivity. Experiments on glass substrates without the barrier layer between the cell and illumination do not show the same inflection behaviour. This is attributed to a transient photoconductivity induced by an excess of free charge carriers generated by photons with energies greater than 3.42 eV. The inflection will return if the barrier is placed over the glass cell within the first  5 min; however, after a few minutes this photoconductivity becomes stable even when the barrier is permanently placed between the glass cell and the light. This supports a transient photoconductivity due to population of the conduction band by excited electrons, as well as a metastable photoconductivity caused by desorption of oxygen. The former is expected to be observed devices made without any UV barrier, while the latter is hypothesized to be the dominant mechanism in the photo-annealing reported here. It is also worthwhile to note that there was some increase in cell performance even prior to UV exposure. Theoretical calculations indicate that some increase in ZnO conductivity may also occur due to the formation of oxygen vacancies which are stable under illumination of light with energies of ca. 2.3 eV [58]. It is also possible that the ZnO photo-catalyzes the decomposition of residual organics, leading to an improved morphology. [59,60]. A first-order linear extrapolation of the curve prior to UV exposure (Fig. 7, dashed line) indicates that after sufficient time the inflection may be removed even in the absence of UV. It is also possible that this is simply the result of imperfect filtering of the UV light, as shown in Fig. 8.

3.3. Influence of temperature The dynamics of the inflection point are also examined as a function of various thermal treatments. A controlled condition study was conducted in which the temperature was held at 65, 85, and 105 1C and the illumination was kept constant at 1000 W m  2. The results, shown in Fig. 9, indicate that the inflection point can be removed much faster at elevated temperatures. This is an expected result, as increased temperature will enhance the rate of most chemical and transport processes and can also lead to improved interlayer contact. It is also worthwhile to note that the onset of device improvement varies considerably between the temperatures. This may be the result of simple heat transfer; cold cells (20 1C) were placed into the preheated chamber and the experiment was started immediately. Although this heat transport limitation may explain the differences in onset, the rate of cell improvement is also drastically increased at higher temperatures, which indicates the presence of a thermal barrier.

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Fig. 8. UV–vis transmission of PET barrier (dashed), PET barrier +UV filter (dotted) and absorbance of ZnO nanoparticles (solid). Absorbance of ZnO with PET barrier (blue) and with PET barrier + UV filter (blue) is given in the inset. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 9. Evolution of IV-curves with time during photo-annealing at (a) 65 1C (b) 85 1C and (c) 105 1C. (d) The change of JSC with time at 65 1C (dotted), 85 1C (dashed) and 105 1C (solid). Colors in (a), (b) and (c) correspond to time marked in (d).

The final currents are expected to be slightly higher when measured at higher temperatures, but measurements on cold cells after annealing indicate that this effect contributes to o10% of the final current. In an attempt to de-convolute the effects of temperature and illumination several cells were thermally annealed in the absence of light at 65, 85 and 105 1C. The cells were then photo-annealed, and their performance monitored over time as shown in Fig. 10. After reaching maximum performance the cells were stored in ambient conditions for 35 days, after which they were re-annealed (Fig. 10d). The results of cells dark-annealed at 65 and 85 1C are quite similar during the primary photo-annealing; both show a distinct inflection which is removed on the order of 20 min and end at

a similar current density of 6 mA cm  2. The device darkannealed at 105 1C exhibited a significantly different behaviour in which the initial curve did not exhibit a prominent inflection point. This resulted in an initial current density of close to  4 mA cm  2 which improved quickly to  7 mA cm  2 after 10 min. This result is surprising in the context of the ZnO photoconductivity process previously proposed, as the inflection behaviour can apparently be removed by thermal treatment alone. It is important to note that the hypothesized ZnO photoconductivity mechanism is not typical transient photoconductivity due to population of the conduction band by photogenerated charges, but rather a metastable conductivity increase as a result of oxygen desorption. It is unclear whether oxygen

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Fig. 10. Cells were thermally pre-annealed for 90 min at different temperatures followed by photo-annealing, storage for 35 days in the dark and finally re-annealed. Evolution of IV-curves after dark thermal annealing at (a) 65 1C (b) 85 1C and (c) 105 1C. (d) The change of JSC with time under photo-annealing and re-annealing at 65 1C (dotted), 85 1C (dashed) and 105 1C (solid). Colors in (a), (b) and (c) correspond to time marked in (d).

desorption from the ZnO surface could occur thermally at 105 1C, but some other reports have shown that the electrical properties of ZnO and ZnO nano-composites change after annealing at temperatures of ca. 105 1C due to changes in morphology and/or defect mobility [61,62]. The thermally annealed devices were stored in ambient conditions for 35 days, and occasional IV scans showed that the performance decreased relatively linearly. After 35 days the cells were re-annealed with illumination of 1000 W m  2, 8575 1C. Interestingly, the re-annealing experiment and final current density obtained was nearly identical for each cell. Thermal treatment may enhance the decomposition of organic impurities in ZnO, improve the morphology of the P3HT:PCBM heterojunction, or affect the concentration of oxygen in various layers of the device; however, after a sufficiently long time the cell performance will still decrease back to a common point.

3.4. Influence of atmosphere The role of atmosphere on the inflection point phenomenon was also examined. Encapsulated cells were first tested in air, oxygen and nitrogen environments at 8573 1C and 1000 Wm  2 illumination. The results (Fig. 11d, e and f) indicate quite similar behaviour regardless of atmosphere. The curve begins with a clear inflection which is removed over the course of 10 min; slight deviations between cells are expected to be the result of statistical variation. These experiments were complemented by analogous ones with cells which were never encapsulated after processing. The results, also shown in Fig. 11 (a, b and c), indicate distinctly different behaviour in various atmospheres. The device does not show photovoltaic response in air or oxygen regardless of time; yet the photocurrent extracted at 1.25 V decreases quickly in air and even more rapidly in oxygen. The deteriorating photocurrent is attributed to an oxidative degradation of P3HT which is accelerated by temperature and illumination, and such behaviour is commonly reported for organic solar cells [63]. In a nitrogen atmosphere the behaviour is considerably different. The cell still displays an inflection on the first scan, but the S-shape has almost completely disappeared within 5 min.

This increased rate of cell improvement further supports the oxygen desorption hypothesis presented previously. In a nitrogen environment the external partial pressure of oxygen will be extremely low, and is expected to favor a quick desorption upon exposure of the cell to UV radiation. The effects of atmosphere on inflection point behaviour have been reported previously. The most similar experiments were performed on un-encapsulated glass based devices with a TiO2 layer [12]. The results from these experiments show a conflicting dependence on atmosphere in which photo-annealing is slower in nitrogen. Other reports also indicate ambiguous effects of atmosphere on the inflection point for devices employing both TiO2 and ZnO [40,46,64]; however, the exact nature of the devices and the pre-processing conditions vary extensively across these experiments. In the present work the un-encapsulated devices were stored in air for 10 weeks prior to testing. This would likely lead to a saturation of oxygen at a concentration approaching the solubility limits of oxygen in the polymers at room temperature, which leads to a significantly different initial condition than in any previously presented reports. The profound effect of the barrier layer is also important to mention. Fig. 11 provides strong evidence that the barrier layer protects the active material from the atmosphere. It also implies that the encapsulated cells on which all the other experiments are performed act as closed systems with respect to the atmosphere on the timescale of a single experiment. It is thus unlikely that a significant amount of oxygen is able to enter or leave the cell during photo-annealing experiments. In order to further explore the affects of oxygen on the device, the encapsulated cell tested in oxygen was left in an oxygen-rich atmosphere for 14 days. The cell was subsequently photo-annealed under standard conditions (air, 85 1C, AM1.5G, 1000 W m  2) and the results are shown in Fig. 12. The first section of the plot shows the performance change with time upon the initial photo-annealing (IV-curves are given in Fig. 11e). The performance shows a rapid increase and the inflection point is no longer apparent after ca. 10 min. Conversely, the re-annealing occurs extremely slowly. During the first 25 min the performance decreases slightly, and the inflection becomes more pronounced. The short circuit current density then begins to

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Fig. 11. Change of IV-curves under photo-annealing of un-encapsulated and encapsulated cells in different atmospheres (a) un-encapsulated in air, (b) un-encapsulated in O2, (c) un-encapsulated in N2, (d) encapsulated in air, (e) encapsulated in O2 and (f) encapsulated in N2. Colors of curves correspond to time (red) 1 min, (brown) 5 min, (orange) 10 min, (blue) 15 min, (pink) 20 min and (green) 25 min. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

kinetics. The hysteresis is attributed to the oxidative degradation of P3HT performance. The storage of the device in an oxygen rich environment for longer time will lead to higher oxygen concentration in the cell, which is expected to result in increased oxidation of P3HT. The reduced kinetics are also in agreement with the oxygen desorption induced photoconductivity of ZnO; Melnick et al. [56] report reduced kinetics of photoconductivity at higher oxygen pressures. Nevertheless, the source of the decreasing initial performance during the secondary photo-annealing is not clear. The fact that it is again present and more pronounced in the case of increased oxygen concentration indicates that it is not an experimental anomaly, and also suggests that oxygen plays an important role; further work is necessary to gain a complete understanding.

3.5. Interface and surface chemistry

Fig. 12. Change in JSC under photo-annealing in oxygen atmosphere (before x-axis break) and re-annealing after storage in a 95% oxygen atmosphere for 14 days (after x-axis break). Inset shows evolution of IV-curves at times (red) 0 min, (brown) 25 min, (orange) 50 min, (blue) 75 min, (pink) 100 min and (green) 250 min. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

increase slowly until around 50 min, where an inflection is still apparent. The change in curve shape begins around 50 min, and a diode shaped curve is obtained after 100 min. The short circuit current density then increases slowly with time, but reaches a relatively steady state at approximately 25% of the value which was obtained for the same cell after the primary photo-annealing. The secondary photo-annealing follows a trend analogous to the re-annealing behaviour of the cell stored in ambient conditions (Fig. 5) but with significantly more hysteresis and much slower

It is reasonable to assume that the dynamic inflection phenomenon is related to processes that are chemical in nature. Chemical characterization of the devices will identify possible processes that are related to photo-annealing, but are not able to directly relate chemical changes to the appearance of an inflection point in the IV-curve. The chemical surface characterization techniques XPS and TOF-SIMS were employed to map possible chemical differences between an as-produced and a photoannealed device. The limited probe depths of XPS (  10 nm) and TOF-SIMS (  1 nm) make it necessary to delaminate the devices such that the interior surfaces become exposed and analysable. The as-produced devices delaminate very easily at the P3HT:PCBM– PEDOT:PSS interface, and after considerable mechanical deformation can be delaminated at the ITO–ZnO interface. The photo-annealed devices also delaminate at the P3HT:PCBM– PEDOT:PSS interface; however, despite considerable efforts the ITO–ZnO or the ZnO–P3HT:PCBM interfaces could not be accessed on photo-annealed devices. These observations suggest that processes occur at interfaces within the device as a result of photo-annealing. The phenomenon that takes place under photoannealing at the ITO–ZnO interface increases the adhesion between the two layers to an extent that makes it impossible to access by mechanical delamination.

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The P3HT:PCBM–PEDOT:PSS interfaces for the as-prepared and photo-annealed devices were analyzed on both exposed surfaces (P3HT:PCBM surface and PEDOT:PSS surface). XPS and TOF-SIMS analyses were performed on five different surface locations. Both XPS and TOF-SIMS could not detect any statistically significant chemical differences between the as-prepared and the photoannealed device at the P3HT:PCBM–PEDOT:PSS interface. In summary, it is likely that no major chemical processes are taking place at the P3HT:PCBM–PEDOT:PSS interface during photoannealing. In an attempt to reach the ZnO interfaces by other means than mechanical delamination TOF-SIMS depth profiling was employed. The advantage is that chemical information is obtained through all layers starting from the exposed P3HT:PCBM surface and ending in the PET film. The disadvantage of TOF-SIMS depth profiling is the depth resolution, which is on a scale that makes it difficult to detect interface phenomena. Fig. 13 displays depth profiles through the photo-annealed and the as-prepared device starting from the exposed P3HT:PCBM surfaces. Two differences were found between the photo-annealed and the as-prepared device. The sputter time used for penetrating the P3HT:PCBM layer was 20 min shorter for the photo-annealed device, which is attributed to shrinkage during the annealing process. The change in density likely occurs due to improved packing of polymer chains induced by the increased temperatures during photo-annealing. The profiles in Fig. 12 were corrected for this time difference in order to clarify the data. The second difference is observed in the ITO profile for the photo-annealed device, which has two intensity maxima instead of the expected one. The hump on the corresponding ZnO profile is most likely a matrix effect caused by the ITO phenomenon. The most likely explanation for the additional ITO intensity maximum is the occurrence of a different phase in the ITO (in the half of the ITO layer facing PET) that has different physical properties, e.g. a different crystallographic phase. The origins of this phenomenon are unclear; however it is deemed to be of little practical relevance since the changes are on the PET side of the ITO layer and should thus have a negligible effect on the photovoltaic properties. In summary, photo-annealing shrinks the P3HT:PCBM layer and induces an unknown phenomenon in the second half of the ITO layer adjacent to PET, while no change could be detected at the ZnO–P3HT:PCBM interface. The depth profiling analysis (Fig. 13) did not detect any chemical differences in the ZnO–P3HT:PCBM interface, however,

100

Normalized intensity (%)

ZnO annealed ITO annealed

80

ZnO 60

ITO

40

this could be due to poor depth resolution. An alternative experiment was thus chosen at this point. An equivalent ZnO solution was spin-coated on a glass slide and subsequently exposed to thermal annealing analogous to the heat treatments used during cell production (110 1C). The ZnO film was then analyzed with both XPS and TOF-SIMS and finally photo-annealed for 22 h at 1000 W m–2. The photo-annealed ZnO film was then once again analyzed with XPS and TOF-SIMS. Table 2 summarizes the result of the XPS analysis. Only carbon, oxygen and zinc were detected on the ZnO surface before and after photo-annealing (except for trace amounts of chlorine on the photo-annealed surface). As is evident from Table 2 photo-annealing reduces the carbon content by almost 50%. Since the oxygen content is unchanged it must mean that oxygen was also removed from the material during photoannealing (i.e. zinc cannot be added). One possible explanation could be that the thermal annealing process does not remove all of the organic components that are grafted to the ZnO nanoparticles in order to make them soluble. The TOF-SIMS analysis provides information on the identity of the organic components which are present. Fig. 14 shows the TOF-SIMS mass spectra for the ZnO surface before and after photo-annealing. The peak labeled ‘‘1’’ in Fig. 14 corresponds to methoxyethoxyacetate which was used to make the ZnO nanoparticles soluble. Characteristic fragment ions from the ionization process are observed, so there is no doubt regarding the identity. Photoannealing completely removes the remains of the methoxyethoxyacetate under the given conditions. As expected (based on the XPS analysis) most of the carbon signals are absent for the photo-annealed surface. It is also worth noting the presence of the O2H– peak (Fig. 14) that increases in intensity after photoannealing. This is an indicator ion for the superoxide ion which is known to form during photo-excitation of ZnO in the presence of oxygen, and contributes to the degradation of organics including P3HT [41,65]. A chlorine signal is observed on the photo-annealed surface consistent with the XPS analysis where trace amounts of chlorine were detected, but its origin is unknown. The only point during manufacture where the materials are subjected to chlorine is during coating of the active layer from dichlorobenzene. Only trace amounts of chlorine was observed but it should be stressed that the presence of chlorine could adversely affect performance and a process free from chlorinated solvents and organic solvents would be desirable while currently not available. In the case of the model ZnO surfaces no chlorine containing materials are employed at any stage so we assume that it is trace amounts. The CHO2 signal does not disappear after photo-annealing. An XPS signal corresponding to a carbonyl carbon is also present after annealing (not shown). These observations suggest that some of the zinc is bound as a formiate, acetate or carbonate after photoannealing under the conditions in question. In summary, thermal annealing does not remove all the methoxyethoxyacetate. During photo-annealing the remaining methoxyethoxyacetate is completely removed if the ZnO surface is exposed directly in ambient air for 22 h at 1000 W m–2 (AM1.5G, 85 1C). Knowledge on kinetics of photo-decomposition methoxyethoxyacetate would be useful, but is outside the scope of this work.

20 0 0

1

2 3 Sputter time (hours)

4

5

Fig. 13. TOF-SIMS depth profiles through P3HT:PCBM–ZnO–ITO–PET for a photoannealed and a non-photo-annealed device after delamination. The sputter time scale for the photo-annealed device was corrected by 20 min due to a smaller P3HT:PCBM thickness caused by shrinkage (due to the annealing process).

Table 2 Element compositions of a ZnO surface before and after photo-annealing measured using XPS. Values reported as mean 7standard deviation. Sample

Carbon (atom%)

Oxygen (atom%)

Zinc (atom%)

Before photo-annealing After photo-annealing

22.5 70.8 12.3 71.0

43.1 7 0.3 43.3 7 0.5

34.47 0.6 44.47 0.7

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Fig. 14. TOF-SIMS mass spectra (negative ions) of a ZnO surface before and after 22 h of photo-annealing at 1000 W m–2 in ambient air.

Fig. 15. Schematic of proposed oxygen redistribution upon illumination and heating. Layers are approximately to scale.

3.6. Effects of oxygen redistribution The dynamic inflection phenomenon could be explained by redistribution of oxygen within the encapsulated cell resulting from photochemical and thermal processes. The initial oxygen concentration in a cell is determined by the oxygen solubility of each layer, and is assumed to be at equilibrium for cells after production. Upon illumination the photogenerated holes in ZnO will cause desorption of oxygen from the surface of ZnO [55,56], presumably leading to an increased oxygen concentration in the ZnO layer, as shown schematically in Fig. 15. This mobile oxygen would disturb the equilibrium of oxygen concentrations within the device layers. Furthermore, the oxygen solubility and transport in barrier and adhesive layers in the cell can affect the oxygen concentration

distribution in the photovoltaic layers. Solubility and permeability data are not readily available for the commercial adhesives and substrates used, but the solubility of oxygen in the polymer materials involved (PET and polyacrylate) is known and is in the order of 1  10  6 cm3 cm  3 bar  1 at STP [66]. The solubility of oxygen in these polymers is expected to decrease by 50% when the temperature increases from 20 to 100 1C [67]. Examination of the to-scale layer schematic in Fig. 15 reveals that the barrier and adhesive layers are orders of magnitude thicker than the photovoltaic layers which indicate that as solubility decreases a substantial amount of mobile oxygen would be available for transport to the photovoltaic layers via diffusion (schematically depicted as arrows in Fig. 15). The rate of mobile oxygen generation and transport would depend upon illumination, temperature and atmosphere/encapsulation. In the studied cells the ZnO is in direct contact with P3HT, and likely contains some residual organics from the ZnO synthesis. The results of XPS analysis showed that the carbon content is reduced by almost 50% after photo-annealing. It is expected that the ZnO will catalyze the oxidation of residual organics under illumination; ZnO has previously shown photocatalytic behaviour towards the oxidation of organics and polymers in the presence of UV light via the formation of hydroxyl, hydroperoxyl and superoxide ions [65,68,69]. This is supported by the presence of O2H  ions in the TOF-SIMS analysis and suggests that degradation of organics also depends on mobile oxygen availability. The removal of organics from within the ZnO layer is expected to positively affect the morphology and conductive properties of the ZnO. In addition, ZnO may also catalyze the oxidation of P3HT which will lead to some degradation of cell performance due to a decrease in the amount of photoactive P3HT; however, oxygen can also act as a dopant for P3HT. It has previously been demonstrated that inverted cells based on ZnO and other semi-conducting metal oxides show better performance in air than in inert atmospheres [12,40, 70–72]. In addition, the kinetics of ZnO photoconductivity will vary depending on the availability of oxygen [56]. Although it is clear that cell performance is a function of oxygen availability, the exact details of how mobile oxygen will re-distribute upon illumination and heating are difficult to predict. Results of the presented experiments do, however, allow the formulation of several reasonable hypotheses. The consistent appearance of the inflection point after production and extended storage at ambient conditions implies that the oxygen equilibrium reached in the production results in a low conductivity state of ZnO. The results of un-encapsulated and encapsulated cells in different atmospheres Fig. 11 clearly show that the barrier

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layer is effective in isolating the cell from atmospheric conditions on the time scale of a photo-annealing experiment (30–60 min). The observation that the inflection point re-appears in the order of days suggests that the return to the initial oxygen concentration profile is extremely slow, and that atmospheric oxygen can permeate the cell over a long period time. The significantly slower kinetics and degraded ultimate performance after storage in an oxygen-rich atmosphere Fig. 12 support the latter. Photodesorbed oxygen from ZnO followed by redistribution of oxygen is capable of explaining most of the observed behaviour in these experiments, and could also account for variations in inflection point behaviour between other reported devices. Based on the experimental evidence presented here, further work should necessarily include systematic chemical doping of ZnO in order to alleviate the conductivity hysteresis. The practical implications of our work are that devices employing ZnO will likely require some pre-treatments (or chemical doping in the bulk or on the surface) in order to optimize performance, and that the encapsulation of inverted devices is not trivial.

4. Conclusions The experiments in this report clearly show the consistent appearance of an inflection point in the IV-curves of R2R processed inverted polymer solar cells after production. The inflection point can be effectively removed by photo-annealing which results in an increased efficiency of devices. The appearance of the inflection point is dynamic as it partially re-appears if the cells are stored at ambient conditions for a number of days, and can again be removed via photo-annealing. Results show that the photo-annealing process is accelerated by increased illumination intensity and temperature; the exposure to UV light with wavelengths between 360 and 400 nm is also a critical factor in photo-annealing. Atmospheric experiments indicate that the barrier layer is effective in isolating the cell from atmospheric conditions at short time scales, and that increased oxygen content in the cell will degrade performance and slow the photo-annealing process. TOF-SIMS and XPS analyses of interfaces and surfaces verify that ZnO can photocatalyze the degradation of organic species and suggest that the major chemical processes are taking place in the bulk of the nanoporous ZnO and at the ZnO–P3HT:PCBM interface. A mechanism is proposed based on redistribution of oxygen within the cell. The oxygen is present as a result of photo-desorption from ZnO and/or decreased oxygen solubility in encapsulation layers. The hypothesis is capable of explaining observed behaviour in these experiments. It can be concluded that devices employing ZnO will likely require some pre-treatments and/or chemical doping in order to optimize performance, and that the encapsulation of inverted devices can significantly change their behaviour by changing the exposure to oxygen and/or UV light. Acknowledgements Andrew J. Medford is grateful for funding provided by the Danish-American Fulbright Commission. This work was supported by the Danish Strategic Research Council (DSF 2104-050052 and 2104-07-0022), EUDP (j. nr. 64009-0050) and PV-ERA-NET (project acronym POLYSTAR). References [1] P.W.M. Blom, V.D. Mihailetchi, L.J.A. Koster, D.E. Markov, Device physics of polymer: fullerene bulk heterojunction solar cells, Adv. Mater. 19 (2007) 1551–1566.

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