The Role of C60 Barrier Layer in Improving the Performances of

The Role of C60 Barrier Layer in Improving the Performances of Efficient ... and EDXR techniques, may provide a structural interpretation of the perfo...
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J. Phys. Chem. C 2009, 113, 19740–19747

The Role of C60 Barrier Layer in Improving the Performances of Efficient Polymer-Based Photovoltaic Devices: An AFM/EDXR Time-Resolved Study B. Paci,*,† A. Generosi,† V. Rossi Albertini,† P. Perfetti,† and R. de Bettignies‡ Istituto di Struttura della Materia, C.N.R., Via Fosso del CaValiere 100, 00133 Roma, Italy, and Laboratoire Composants Solaires CEA INES-RDI, SaVoie Technolac, BP 332, 50 aVenue du lac Le´man, 73377 LE BOURGET DU LAC, France ReceiVed: August 07, 2009; ReVised Manuscript ReceiVed: September 25, 2009

Here, we report an in situ study of the effect of morphological/structural aging processes on polymerbased photovoltaic cell performances. The devices were provided with a fullerene film as a barrier layer between the active element and the metallic cathode. The experimental method adopted consists of the joint use of atomic force microscopy (AFM) and energy dispersive X-ray reflectivity (EDXR), an original coupling particularly effective in the study of stratified media. These techniques were applied first to the intermediate stages of the device construction and, finally, to a complete cell. The problems related with the surface/interface modifications of the devices elements in operating conditions were investigated in depth, with particular concern on the role of the C60 barrier layer. The C60 film surface topography was monitored by AFM experiments during illumination, which evidenced a surface reorganization of the C60 layer molecules over time. Conversely, the C60 film bulk and its interface with the active layer, investigated by EDXR analysis in the same conditions, turned out to remain unchanged. Then the cathode buried interface of a complete cell was studied, by EDXR measurements in working conditions, thus demonstrating that the C60 layer guarantees a good structural stability of the cell. In addition, the in situ AFM/EDXR characterization established that the observed reorganization process of the C60 layer molecules does not affect the film physical barrier role. This finding was confirmed by power conversion efficiency measurements, showing that the C60/LiF/Al cathode cell efficiency is preserved over time. This work also demonstrates how the morphological properties of organic device layered components, investigated in situ by the two noninvasive and independent AFM and EDXR techniques, may provide a structural interpretation of the performance preservation or fading. 1. Introduction Plastic photovoltaic (PV) devices, although still much less efficient in comparison to silicon ones, have attracted much attention due to interesting characteristics such as low cost, flexibility, and the possibility of covering large surfaces. In particular, in polymer-based photovoltaics, a breakthrough in the efficiency of PV conversion was obtained when photoinduced electron transfer from a conjugated polymer (donor) to buckminsterfullerene C60 (acceptor) was demonstrated.1 It follows that the buckminsterfullerene and its soluble derivates play a key role in the development of organic solar cells. Indeed, by sublimating C60 films over polymer single layers, heterojunction devices with an improved efficiency in comparison with single layer cells5,6 were obtained.2-4 More recently, much attention has been devoted to bulk heterojunction solar cells, based on blends of a conjugated polymer and a soluble fullerene derivative.7,8 In this case, the donor-acceptor material interfacial area is largely increased, and therefore, both charge generation and transport are enhanced. In particular, devices based on polythiophene and its derivatives are characterized by good PV performances9-12 and efficiencies above 6%13-16 have been * To whom correspondence should be addressed. E-mail: [email protected]. Telephone: +39 0649934174. Fax: +39 0649934153. † Istituto di Struttura della Materia, C.N.R.. ‡ Laboratoire Composants Solaires CEA INES-RDI.

reached using P3HT (poly(3-hexyl thiophene)) blended with methano-fullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM). Nevertheless, a deeper inspection of the various mechanisms involved in the conversion of solar energy and of the possible causes of the device aging is still required. In particular, the role of buffer layers in enhancing the properties of organic devices is under intensive debate. In photovoltaics, it was demonstrated that, by adding a poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) layer between the ITO electrode and the active layer, a raise in both the short circuit current and the fill factor can be obtained. Moreover, in order to fulfill the stability requirement, the possible use of a barrier layer, between the active film and the metal electrode, is under investigation.2,17,18 In this context, LiF films have been studied, together with MgO19 or Al2O3,20 as potentially interesting barrier layers, as reported in organic light-emitting devices19,21,22 studies. Concerning PV devices, a LiF barrier film, permeable to electrons, may be used to avoid the direct contact of the active layer with the reactive electrode metal, thus minimizing the effect of possible chemical reactions. Indeed, evidence that the modification of the interface between the electrode and the polymer, due to photochemical reaction between the organic material and aluminum, may be responsible for a fast reduction of the device’s lifetime has been reported.23,17 The interposition of a LiF layer was shown to improve the PV performance,

10.1021/jp907636f CCC: $40.75  2009 American Chemical Society Published on Web 10/19/2009

Role of C60 Barrier Layer inhibiting, for example, photoinduced oxidation of the electrode buried interface.24,25 Nevertheless, since LiF films exhibit insulator properties, they must be extremely thin (not to inhibit the electron transport to the electrode), which may jeopardize its barrier role. In this respect, C60 has been indicated as an interesting alternative, thanks to its superior electron conducting properties.26 It follows that it is important to clarify the role of the C60 barrier layer in the device lifetime and the effect of its possible structural and morphological changes. Indeed, such changes may well induce a degradation of both the C60 film/ active layer and the C60 film/electrode interfaces, as discussed in the next section. It is, therefore, crucial to detect possible modifications occurring in the C60 film in real working conditions of the devices. Heating due to illumination (temperatures up to 70 °C may be reached inside the cells) may result in thermal induced modifications of the active layer and consequent phase separations, PCBM clustering,27,28 and interfacial diffusion.29 In this framework, the development and application of specific characterization techniques, allowing in situ studies of the cells in working conditions and elucidating the various aging mechanisms, is of utmost importance to gain technological advances. 2. Experimental Section 2.1. Materials and Devices. The active element of the bulk heterojunction solar cells was made from a blend of methanofullerene [6,6]-phenyl C61-butyric acid methyl ester and poly(3hexyl thiophene), denoted as PCBM and P3HT, respectively. The complete cell consisted of an indium tin oxide (ITO) substrate cleaned in an ultrasonic bath of acetone and isopropanol, rinsed in deionized water, dried in an oven, and, finally, treated with UV-ozone. The active layer of P3HT:PCBM was deposited by spin-casting from an anhydrous chlorobenzene solution. The C60 layers were thermally sublimed through the electrode shadow mask on top of the active layer in a vacuum chamber. The devices being based on thin films, very small amounts of C60 and PCBM material are used, preserving the low cost requirement for organic devices. The aluminum electrodes were prepared by evaporation in a vacuum chamber through the same shadow mask. Aluminum, LiF, and C60 were evaporated at a pressure of 10-6 mbar. The top contact was a 100 nm thick Al layer. The cells had an active surface of 32 mm2. The first samples (sample #1 and sample #2) represent an intermediate step in the organic device construction missing the top layer cathode, in order to have the possibility of studying the C60 film surface by AFM. Subsequently, complete cells were prepared by depositing the metallic cathode. Energy DispersiWe X-ray ReflectiWity (EDXR) Setup. The experimental apparatus consisted of a noncommercial reflectometer30 characterized by a very simple setup geometry, since neither monochromator nor goniometer is required in the energy dispersive mode, with no movement being needed during the measurements. The Bremsstrahlung of the X-ray tube (3 kW power, tungsten anode) is used as a probe, and an EG&G high purity germanium solid-state detector, whose energy resolution is about 1.5-2% in the 20-50 keV energy range, accomplishes the energy scan. The measurements were performed with the samples placed inside an X-ray transparent chamber. The experiments on the cell were performed in short circuit conditions. Atomic Force Microscopy (AFM) Setup. The AFM measurements were performed in noncontact mode using a noncom-

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Figure 1. Sketch of the PV device: top view (A) and side view (B).

mercial air-operating atomic force microscope.31 Both the EDXR and the AFM measurements were carried out during illumination with a white light lamp (10 mW/cm2). 3. Results and Discussion Here, a synergic use of AFM and X-ray reflectivity techniques has been made to address the critical issue in the development of organic devices of the structural stability. In the bulk heterojunction devices investigated, based on an active layer of P3HT:PCBM, a fullerene film is used as a barrier layer between the active element and the metallic cathode. The approach we propose allows one to simultaneously monitor the morphology of the device layers, detecting possible surface and interface effects that may occur in working conditions. The joint use of AFM and EDXR is particularly effective in the present case, since the former technique is a measurement of the film surface in the direct space, while the latter is a probe (in the reciprocal space) of both the surface and interface morphology, at the angstrom resolution. Moreover, as will be demonstrated in the following, the combined use of the two techniques allows one to quantify the extent to which the modifications observed by AFM, that is, local probe, affect the overall morphology (provided by EDXR, which provides an average value of the sample morphological parameters) of the material under study. The EDXR technique is based on the optical properties of X-rays: in the small angle approximation, the Snell rule applied to this energy range implies32 that the reflected intensity is a function of the momentum q transferred from the X-rays to the sample electrons.33 The latter, in turn, depends on both the reflection angle and the energy of the X-rays. In the energy dispersive mode,30 the reflectivity patterns are collected using a polychromatic beam and carrying out an energy scan by means of an energy sensitive detector (unlike the conventional angular dispersive mode, which makes use of a fixed X-ray energy and performs an angular scan). This allows one to keep the experimental geometry unchanged during data collection, which is a fundamental advantage for in situ studies of the type presented here. Indeed, in these grazing angle conditions, mechanical movements may produce misalignments of the sample, and, in any case, the variable geometry implies changes in the scattering volume as well as in the X-ray footprint on the sample surface (difficult to be taken into account in real devices). Conversely, a systematic study of the materials behavior during the subsequent steps of the device construction can be easily carried out by collecting sequences of AFM images and EDXR patterns, thus elucidating their roles prior to the final investigation of the complete cell. Figure 1 shows a sketch of the cell structure. 3.1. Investigating the C60 Layer: Time-Resolved AFM and ex Situ EDXR Experiments. In order to elucidate the role of the fullerene layer morphology on the stability of the device, first a C60 film (sample #1; structure, glass/ITO/ PEDOT:PSS/

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Figure 2. AFM measurements of C60 film deposited on glass/ITO/ PEDOT:PSS: (a) AFM 3 × 3 µm topographic image representative of the C60 film surface and (b) 3D 1 × 1 µm zoom of the image in (a).

C60) was studied by in situ AFM and ex situ EDXR. A preliminary AFM analysis was performed to characterize the morphological quality of the film, by monitoring various regions of the as-deposited sample. Figure 2 shows a typical 3 × 3 µm atomic force microscopy image of the C60 film and a three-dimensional (3D) 1 × 1 µm zoom. The film exhibits a granular morphology, characterized by several spherical aggregates composed of C60 clusters with a peak-to-valley amplitude in the range of 7 nm and an average root-mean-square roughness, σAFM, as low as 0.5 nm. The average in-plane size of the buckminsterfullerene grains is around 5 × 5 nm. A successive time-resolved analysis aimed to monitor the effect of light and of the consequent heating of the C60 layer. This was done by sampling a fixed portion of the film surface over time by AFM measurements, carried out during illumination with a white light lamp (10 mW/cm2) for 24 h. A selection of the topographic Figures obtained as the result of the time-resolved AFM measurements is reported in Figures 3 and 4. Each image may be considered as a sort of moVie photogram of the surface real-time evolution, so that a rich visual description of the phenomenon is gained. The first nine images (Figure 3) show the modification of the sample topography during illumination (5-13 h). All the AFM images were acquired for 1 h. The selection was performed considering that no significant modification is observed for the first 4 h. The line profiles taken in correspondence of point A are reported under each image. Only a minimal drift in the sampled zone is present, despite heating induced by light and the long lasting images sequence. The first image, collected after 4 h of illumination, shows that the sample is still quite uniform, although characterized by several aggregates. The film is still very smooth, with the mean roughness being below 1 nm (see Figure 3). The size of the aggregate has increased since the beginning of the irradiation. Following the sample evolution, image by image, a rapid growth of the pre-existing structures, giving rise to the formation of large petal-like aggregates, is well detectable, even at a first visual analysis of the sequence in Figure 3. Both size and shape are in agreement with literature reports on aggregates of fullerene clusters.34-37 After about 10 h, no further increase of the cluster height is detectable. Nevertheless, focusing only on the region of the three largest clusters, it is evident how the clusters go on expanding in the plane of the film surface. This is shown in Figure 4, which reports the topographic images measured during illumination (16-22 h). In order to understand the dynamics of the process, a quantitative analysis was performed, image by image, giving

Paci et al. the mean surface roughness σAFM and the grain size of two main topographic structures (labeled 1 and 2 in Figures 3 and 4), considered as representative of the overall cluster ensemble. The variation over time of σAFM is reported in Figure 5a. For each image, in Figures 3 and 4, line profiles were taken both horizontally in correspondence of point A and vertically in correspondence of point B to evaluate the grain size. The evolution of the grain height (h) is reported in Figure 5b. The lateral dimensions in the surface plane (x and y dimensions, corresponding to the horizontal line profile taken in correspondence of point A and vertically in correspondence of point B, respectively) are reported separately in Figure 5c. The effect of illumination is visibly a single step increase for the cluster growth in height, with a characteristic time of about 7 h (see Figure 5b). Conversely, a two-step increase is observed both in σAFM and in the grain size in the plane (x,y). The fit of the ζ(t) curves, with ζ being σAFM, x and y, respectively, was carried out using two subsequent Boltzmann curves: the first curve, ζ(t) ) ζ0 + (ζ1 - ζ0)(1 - exp-(t/τ1)), describes the progressive increase of the film σAFM and of the cluster dimensions in the plane, from an initial value ζ0 up to a first asymptotical value ζ1 in a characteristic time τ1 of about 7 h, exactly as for h. The second one, ζ(t) ) ζ1 + (ζ2 - ζ1)(1 - exp-(t/τ2)), describes a further increase, beginning when the first one has reached the intermediate value ζ1, coinciding with the saturation value of the first process, until a final asymptotical value ζ2, in a characteristic time τ2 of about 3 h. The above description provides further qualitative information on the nature of such processes, which can be summarized as follows. The clustering of the structures monitored (labeled 1 and 2) is a two-step process: the first is characterized by a 3D growth of the pre-existing aggregates of C60 molecules, and the second is a subsequent 2D in-plane enlargement. It can be noticed that the first process is almost concluded at the onset of the second. Therefore, the AFM in situ analysis evidences a local reorganization of the film organic molecules during illumination, resulting in a substantial increase of the size of the small preexisting clusters. This is likely due to the enhanced molecule mobility, induced by sample heating consequent to irradiation. The dynamics of the process suggests that this aggregation may be due both to the diffusion of the molecules on the surface and to those in the layer bulk. Once the process was concluded, various zones of the sample surface were monitored by AFM, as reported in the inset of Figure 6. The results show that, although several large aggregates are now present, the regions amidst the clusters (that is, the largest part of the sample) exhibit a homogeneous topography very similar to that of the as-deposited sample (in Figure 2). Finally, EDXR was performed on the sample to evaluate the film thickness and overall surface roughness. The technique enables one to determine the morphological parameters (thickness and roughness) of the layers of stratified systems, such as organic photovoltaic cells.38,39 From a qualitative point of view, an EDXR pattern can be visually interpreted by a simplified model, prior to any quantitative analysis: the period of the oscillations is related to the film thickness, d, while the oscillation damping is related to the film roughness, σ.40 The quantitative analysis of the data is performed by using the Parratt model40 describing the reflection of an X-ray beam by a film of well-defined thickness, separated from its substrate by a sharp interface. When the surface and/or the interface are not sharp, the

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Figure 3. Time-resolved AFM measurements of C60 film deposited on glass/ITO/PEDOT:PSS. The same region was monitored by 3 × 3 µm images acquired over time during illumination. (a-i) C60 film topography during light exposure (5-13 h, respectively). The line profiles taken in correspondence of point A are reported under each image.

Figure 4. Time-resolved AFM measurements of C60 film deposited on glass/ITO/PEDOT:PSS. The same region was monitored by AFM images acquired over time during illumination. (a-f) C60 sample topography during light exposure (16, 17, 18, 20, 21, 22 h, respectively). The line profiles taken in correspondence of point A are reported under each image.

reflected intensity is modified by a roughness factor that plays a role similar to the Debye-Waller factor in diffraction patterns.

Considering only the dominant term of the film surface roughness, the general expression for the reflected intensity is

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) {|Rl |exp(-2k0kfσ2)|+|Rr - 2Re[RlRr exp(-

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2k0kfσ2)exp(2ikfd)]/1 + |Rl |Rr |exp(-2k0kfσ2)|2Re[RlRr exp(-2k0kfσ2)exp(2ikfd)]} (1) where k0 ) q/2 is the radiation wavenumber in the air, Rl ) (k0 - kf)/(k0 + kf) is the Fresnel film material reflectivity, Rr ) (kf - ks)/(kf + ks) is the Fresnel substrate material reflectivity, d is

Figure 5. Time evolution of C60 film morphology under illumination. The analysis of the time-resolved AFM measurements reported in Figures 3 and 4 allowed the major characteristics of the film topography to be obtained. (a) Mean surface roughness, σAFM, versus time. (b) Grain size for the larger structure in Figure 3 and 4 (structure labeled 1). The evolution of the grain height (h), full dot line, and of the dimensions in the plane (x, y), empty dot line and empty triangle line, are reported separately. (c) Grain size for the smaller structure in Figure 3 and 4 (structure labeled 2). The evolution of the grain height (h), full dot line, and of the dimensions in the plane (x,y), empty dot line and empty triangle line, are reported separately. The sample has the following multilayered structure: glass/ITO/PEDOT:PSS/C60.

Figure 6. EDXR pattern collected on C60 film, after 24 h of illumination and data fit (red line). Inset: AFM 10 × 10 µm image representative of the C60 film surface and 2 × 2 µm zoom. The sample has the following multilayered structure: glass/ITO/PEDOT:PSS/C60.

Paci et al. the average thickness of the film, and σ is the surface roughness. The quantities kf and ks are the components perpendicular to the surface of the radiation wavenumbers in the film and in the substrate, respectively, which both depend on k0: kf ) (k02 4πFf)1/2, ks ) (k02 - 4πFs)1/2. The results of the EDXR experiments are reported in Figure 6. The measured C60 film thickness and roughness are about 61 and 0.14 nm, respectively. It must be noted that the latter result, although numerically much below that provided by AFM, is consistent with it. Indeed, while the EDXR value represents the average roughness of the sample, the AFM one is a local measurement. Comparing the results, we can state that the average roughness, obtained by EDXR, is comparable with the one obtained by the AFM “local” analysis restricted to the smoother (aggregate-free) regions (σAFM ) 0.6 nm, comparable with the as-deposited sample). Indeed, the aggregates are too big and irregular to specularly reflect X-rays and provide just a small contribution to the diffuse component of the reflection pattern. 3.2. Investigating the P3HT:PCBM/C60 System: TimeResolved EDXR and ex Situ AFM. At this point, it appears essential to be able to verify the effect of illumination on the structural stability of the C60 layer bulk and of its interface with the P3HT:PCBM layer, namely, the active component of the device. Moreover, of particular concern is the effect of the increase in temperature, due to illumination, on the morphology of both the C60 barrier layer and the bulk heterojunction. Indeed, in working conditions, temperatures above the glass transition temperature (Tg) of the polymeric component of the blend are reached,41 reducing the P3HT:PCBM organic layer morphological stability.42 It follows that heating may be responsible for the enhancement of diffusion, both due to a softening of the polymeric matrix above Tg, which decreases the viscosity, and to an augmented mobility of the diffusing molecules. In turn, diffusion processes (for example, of active layer PCBM molecules or of electrode decomposition products) may further contribute to the cell degradation, so that the role of barrier layers is crucial. For this reason, sample #2 (glass/ITO/ PEDOT:PSS/ P3HT: PCBM/C60) was prepared and studied by EDXR and AFM. In the present case, we performed the EDXR in situ and the AFM ex situ, since our attention was mostly devoted to the interface of the C60 layer with the active film, and EDXR represents a unique nondestructive tool for interfaces studies. On the other hand, ex situ AFM allows one to detect possible local effects and to correlate the results obtained on this sample with the results obtained on the previous one. The C60 film has a nominal thickness of less than 10 nm, as required by its role of buffer layer in the PV device. Preliminarily, the surface topography was studied by AFM. It appears stable, if the sample is stored in the dark (see Figure 7, reporting the AFM measurements repeated at a distance of several months). By comparison with the images of the previous sample reported in Figure 2, it can be noticed that the topographical characteristics are unchanged. This means that the deposition of a thin C60 film on the organic blend has the same morphological quality of a thicker film deposited directly on the glass/ITO/PEDOT:PSS substrate. Then an in situ EDXR experiment was performed. It consisted of collecting a sequence of X-ray reflection patterns, each acquired for 3 h, under illumination with a white light lamp. The reflection patterns are reported in Figure 8. They show a double modulation, due to the contributions of both the C60 and the P3HT:PCBM films.

Role of C60 Barrier Layer

Figure 7. AFM images collected on a C60 film. Comparison of 1 × 1 µm images, representative of the film surface, repeated at a distance of several months. The sample has the following multilayered structure: glass/ITO/PEDOT:PSS/P3HT:PCBM/C60.

Figure 8. Time-resolved EDXR patterns collected on glass/ITO/ PEDOT:PSS/P3HT:PCBM/C60 multilayered sample under illumination.

In general, to perform an accurate data fit, several oscillations are required. Instead, the C60 layer is far too thin, so that only one complete oscillation produced by it is visible. Therefore, the thickness of such a layer cannot be precisely determined. However, an estimation of its value could be obtained by an approximate model, that is, a Gaussian fit of the broad oscillations present in the reflection pattern, to which a simplified formula valid for large q values was applied, D ) ∆q/2π. The C60 film thickness turned out to be about 10 nm. The same formula was used, once the C60 film contribution to the signal was subtracted, to perform the analysis of the oscillations frequency of the P3HT:PCBM film, giving a thickness of about 33 nm. Although these calculations are not very sophisticated, the relative error on the morphological parameters is rather low, so that the following conclusions are substantially reliable, not being invalidated by the attribution of imprecise values to the morphological parameters. As can be noticed, the patterns of this sequence overlap well. Consequently, since no change in the oscillation period occurs for both contributions, the morphological parameters of the two films must have remained unchanged upon working conditions

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Figure 9. AFM images collected on glass/ITO/PEDOT:PSS/P3HT: PCBM/C60 multilayered sample, after 36 h of illumination (during EDXR experiment): (a) 10 × 10 µm topographic image of the C60 film, (b) 2 × 2 µm zoom, corresponding to the evidenced portion in (a), showing the cluster-free region, (c) new 10 × 10 µm topographic image of the C60 film, and (d) 2 × 2 µm zoom, corresponding to the evidenced portion in (c), focused on an aggregate.

of the device. In particular, neither the surface of the C60 layer nor the interface of the latter with the P3HT:PCBM film show any change. Nevertheless, as for the previous film, also in the present case, the ex situ AFM analysis reveals the formation of large spherical-like aggregates composed by petal-like clusters of C60 molecules on the film surface, as reported in Figure 9. However, in the aggregate-free regions, the film topography is similar to that observed before illumination (see Figure 9b versus Figure 7). At this point a consideration of the additional information that may be gained by the joint use of the AFM/EXDR realtime characterizations can be made. Indeed, in order to account for the formation of the large C60 clusters detected by the AFM experiments, a “multiple contributions” aggregation process may be hypothesized: in addition to the thermally enhanced molecule mobility in the C60 layer itself, diffusion and consequent clustering of the PCBM molecules coming from the P3HT: PCBM blend may occur.43 In this case, the quantification of the latter phenomenon is of utmost importance, since an uncontrolled migration of PCBM molecules from the active layer may result in a demixing of polymer and PCBM that would be detrimental for the exciton dissociation and, therefore, for the PV device efficiency. On the other hand, the time-resolved EDXR experiments, showing no modification of the morphological parameters of the P3HT:PCMB film upon illumination, demonstrate that, if present, the PCMB diffusion process does not affect the morphology of the active layer. Strictly speaking, a diffusion process of the PCBM molecules should be accompanied by a change in the blend film density. The latter, in principle, would be revealed as a change in the position of the relative total reflection edge in the EDXR patterns. However, in the present case, it would produce effects well below the detection threshold of the EDXR technique and no relevant change in the patterns profile is expected. Therefore, although it is not possible to exclude a migration of the PCBM molecules from the blend to the C60 film, induced by the polymer softening that may occur upon illumination (temperature above Tg may be reached), the experimental evidence indicates that, if present, the process should involve a negligible number of molecules (likely localized close to the interface), not modifying the blend bulk structure. It is worth reaffirming that this is an

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Figure 10. Time-resolved EDXR patterns collected on glass/ITO/ PEDOT:PSS/P3HT:PCBM/C60/LiF/Al PV device under illumination.

encouraging result, since an uncontrolled migration of PCBM molecules from the blend to the C60 film would be detrimental for the bulk heterojunction. To summarize the results obtained up to this point, joint timeresolved AFM and EDXR techniques were applied to study the subsequent stages of PV device fabrication, allowing a description of the aging effects of the device components. What was found can be schematized as follows: (i) a surface reorganization of the C60 film molecules under illumination was detected, whose dynamics was clarified and (ii) the substantial stability of the morphology of the C60 film bulk, of the P3HT:PCBM active layer, and of their mutual interface. At this point, it appears essential to verify the structural stability of the complete cell under working conditions and, in particular, of the cathode buried interface, which represents one of the major critical points in the improvement of organic PV devices. Therefore, a further in situ EDXR analysis aiming to detect possible aging effects was performed. 3.3. Investigation of the Complete PV Device in Working Conditions: Time-Resolved EDXR. In order to obtain the information on the interface between the C60 film and the LiF/ Al electrode, EDXR was applied in situ to a cell of the following structure upon illumination: glass/ITO/PEDOT:PSS/P3HT: PCBM/C60/LiF/Al. The results are shown in Figure 10. In the reflectivity profiles (shifted in height for clarity), the Kiessig fringes are due to the Al film. The patterns are perfectly overlapping, which demonstrates that the buried interface of the Al electrode is stable (see the inset reporting the thickness and roughness versus time curves). Such a result, confirming that the observed reorganization process of the C60 layer molecules is a local effect, restricted to limited portions of the film, is of extreme importance. Indeed, as a consequence of this finding, one may conclude that the C60 film maintains its role of mechanical barrier layer, fundamental for the preservation of the chemical-physical properties of the metal electrode, and for the integrity of its buried interface, during the time the cell is working. 3.4. Investigation of the Effect of the Morphology on the Device Efficiency over Time. The previous studies aimed to elucidate the role played by possible morphological changes of the cell layers in the maintenance or fading of the cell efficiency upon working. For this reason, the processes discussed above

Paci et al.

Figure 11. Normalized PCE measurements under simulated AM1.5 100 mW/cm2 illuminations as a function of time. Comparison of the PCE curve for three cells of the following structures: glass/ITO/PEDOT: PSS/P3HT:PCBM/Al (red squares line), glass/ITO/PEDOT:PSS/P3HT: PCBM/LiF/Al (gray triangles line), and glass/ITO/PEDOT:PSS/P3HT: PCBMC60/LiF/Al electrode (blue dots line). J/V characteristics are reported in the inset.

were finally correlated to the cell performances, by means of real-time power conversion efficiency (PCE) measurements. In Figure 11, normalized PCE under simulated AM1.5 100 mW/cm2 illumination is reported as a function of time for three cells, one with an Al electrode, one with a LiF/Al electrode, and one with a C60/LiF/Al electrode. The PCE values are normalized to unity for better comparison, since our major aim was to address the role of the C60 layer in limiting the aging over time of the working device. The J/V characteristics are reported in the inset. The PV parameters are: Voc ) 0.39 V, Jsc ) 9.4 mA/cm2, FF ) 53.5%, and η ) 2.0% for the Al cell; Voc ) 0.60 V, Jsc ) 9.9 mA/ cm2, FF ) 63.2%, and η ) 3.4% for the LiF/Al cell; and Voc ) 0.56 V, Jsc ) 9.5 mA/cm2, FF ) 60.3%, and η ) 3.2% for the C60/LiF/Al cell. The comparison of the J/V characteristics of the studied PV cells further demonstrates the good efficiency of the C60 barrierlayer-based devices. Therefore, the C60 layer, while acting as a mechanical barrier, still maintains the contact between the organic layer and the metal electrode. Moreover, from the comparison of the PCE curves, it is evident that for the C60based cells, the efficiency is preserved over time. This confirms what was observed in the AFM/EDXR study reported above, namely that the C60 active layer and the C60 electrode interface are stable under working conditions and, consequently, guarantee the durability of the device performance. 4. Conclusions We report on an in situ AFM/EDXR study of organic bulk heterojunction PV devices making use of C60 barrier layers. The results obtained upon the cell working allowed us to correlate, for the first time, the modifications of the film surface topography (observed by time-resolved AFM) with the film bulk morphology (detected by time-resolved EDXR). In particular, the AFM measurements allowed us to follow in real time the occurrence of local rearrangements of the C60 molecules upon illumination. Importantly, although the modifications experienced by the fullerene layer were revealed to be more complex than expected, the joint use of the time-resolved EDXR and AFM techniques permitted us to observe, with extreme accuracy, the dynamics of the process. The substantial stability of the C60 film bulk, of the P3HT:PCBM active layer, and of their mutual interface was ascertained. In addition, the study of a complete

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