Suppression of Thermally Induced Fullerene Aggregation in

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Suppression of Thermally Induced Fullerene Aggregation in Poly(fullerene) Based Multi-Acceptor Organic Solar Cells Simon A. Dowland, Michael Salvador, Jose Dario Perea, Nicola Gasparini, Stefan Langner, Sambatra Rajoelson, Hasina Harimino Ramanitra, Benjamin D Lindner, Andres Osvet, Christoph J. Brabec, Roger C. Hiorns, and Hans-Joachim Egelhaaf ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00401 • Publication Date (Web): 06 Mar 2017 Downloaded from http://pubs.acs.org on March 8, 2017

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ACS Applied Materials & Interfaces

Suppression of Thermally Induced Fullerene Aggregation in Poly(fullerene) Based MultiAcceptor Organic Solar Cells Simon A. Dowland*αβ, Michael Salvadorγδ, J. Darío Pereaγ, Nicola Gaspariniγ, Stefan Langnerγ, Sambatra Rajoelsonβ, Hasina H. Ramanitraε, Benjamin D. Lindnerζ, Andres Osvetγ, Christoph J. Brabecγ, Roger C. Hiornsε, Hans-Joachim Egelhaafα α

ZAE Bayern, Auf AEG, Bau 16, 1. OG Fürther Str. 250, 90429 Nürnberg, Germany. β

Belectric OPV GmbH, Landgrabenstr. 94, 90443 Nürnberg, Germany.

γ

Institute of Materials for Electronics and Energy Technology (i-MEET), Friedrich-AlexanderUniversity of Erlangen-Nürnberg, Martensstrasse 7, 91058, Erlangen, Germany. δ

Instituto de Telecomunicações, Instituto Superior Técnico, Av. Rovisco Pais, P-1049-001 Lisboa, Portugal ε

Université de Pau et des Pays de l’Adour (UPPA), IPREM (EPCP, CNRS-UMR 5254), 2 Avenue Président Angot, 64053 Pau, France.

ζ

Department of Chemistry and Pharmacy & Interdisciplinary Center for Molecular Materials

(ICMM), Friedrich-Alexander-University Erlangen-Nürnberg (FAU), 91054 Erlangen, Germany.

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Keywords: (organic photovoltaics, main-chain polyfullerenes, thermal stability, multi acceptor composite blend, fullerene aggregation)

ABSTRACT

A novel main-chain poly(fullerene) poly{(fullerene)-alt-[2,5-bis(octyloxy)terephthalaldehyde]} (PPC4) is investigated for its hypothesized superior morphological stability as an electron accepting material in organic photovoltaics relative to the widely used fullerene phenyl-C61butyric acid methyl ester (PCBM). When mixed with poly(3-hexylthiophene-2,5-diyl) (P3HT) PPC4 affords low charge generation yields due to poor intermixing within the blend. The adoption of a multi-acceptor system, by introducing PCBM into the P3HT:poly(fullerene) blend, was found to lead to a threefold enhancement in charge generation, affording power conversion efficiencies very close to the prototypical P3HT:PCBM binary control. Upon thermal stressing, and in contrast to the P3HT:PCBM binary, photovoltaic devices based on the multi-acceptor system demonstrated significantly improved stability, outperforming the control due to the suppression of the PCBM migration and aggregation processes responsible for rapid device failure. We rationalize the influence of fullerene miscibility and its implications on device performance in terms of a thermodynamic model based on Flory–Huggins solution theory. Finally, the potential universal applicability of this approach for thermal stabilisation of organic solar cells is demonstrated utilizing an alternative low bandgap polymer donor system.


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INTRODUCTION Organic photovoltaics (OPV) continue to attract significant interest as a promising, commercially viable technology due to their potential for large scale, low cost production of low weight and flexible modules.1,2 While perovskite-based cells might be seen as formidable competitors,3 there is an expectation of complementarity between these two systems, given that OPV can be more acceptably installed on buildings and in appliances due to their non-toxic nature. In the commercial sphere, OPV has witnessed a dramatic increase in its employment as building integrated photovoltaics around the world, underscoring its potential beyond niche markets.4 As the technology emerges from adolescence, it becomes even more important to address the current hindrances to its further progress. Factors, such as device and module efficiency as well as processibility have developed substantially in recent years with many systems achieving power conversion efficiencies (PCEs) of over 10% and 5%, respectively.5–8 However, as OPV becomes more competitive with respect to the more established technologies such as CIGS and a-Si (21.0% and 10.2% lab scale efficiencies respectively),7 it is important for the research community to continue building an understanding of and resolving the fundamental challenges with respect to long term device stability, addressing reagents and mechanisms responsible for device failure. Much work has been conducted in this regard identifying and elucidating the influence of the key drivers of device degradation, namely, oxygen, water, heat and light.9–14 Further advances are expected to ensure very long, i.e. >20 year, module operations, a milestone for building integrated applications. It is important to note that the challenges posed by oxygen and water ingress on solar cell lifetime may be overcome entirely by the use of effective encapsulation materials and strategies, if the device itself cannot be made


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more resistant to the processes responsible for its degradation. Furthermore, the incorporation of UV blocking materials into the encapsulation architecture may also afford additional protection against detrimental light induced chemical reactions.15,16 However, heat driven processes as imposed by light irradiation and/or essential annealing steps, leading to detrimental morphological rearrangement of the active layer, for instance, cannot be effectively countered with the use of encapsulation. Moreover, thermal instability in otherwise promising systems precludes them from further development as potential systems for industrial application in spite of their initial favourable optoelectronic properties.17,18 Thus, the suppression of adverse morphological rearrangement will be key to improving the stability of OPV and allow wider use of otherwise thermally unstable systems.19,20 In most OPV systems, fullerene small molecules are utilized as the acceptor component. The bulk heterojunction (BHJ) is typically induced into a metastable state and kinetically locked upon removal of the solvent. Under thermal stress, the morphology of the BHJ is subject to change towards suboptimal mixing facilitated through the migration and aggregation of the fullerene and crystallisation of the polymer phase. This demixing may evolve well beyond the limits imposed by the exciton diffusion length, leading to a reduction in photovoltaic performance.13,21,22 The extent to which degradation occurs through this mechanism is dependent on a number of factors, including: the surface energy of the substrate,23 the choice of processing conditions,24,25 the level of photoinduced fullerene dimerisation in the active layer26 as well as the diffusivity and miscibility of the fullerene within the donor phase.17 Evidently, the suppression of fullerene aggregate nucleation and growth mechanisms should lead to vastly improved thermal stability.


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Extensive work already exists in the literature proposing a range of approaches for improving the thermal stability of OPV.27–33 However, many are not universally applicable approaches or add additional complexity to the manufacturing process. A simple yet effective approach has been to tether fullerenes together into dimers or oligomers and introduce these into the BHJ.18,26,34 Previously, we reported on the incorporation of fullerene oligomers into polymer fullerene blends and achieved some success in retarding photovoltaic performance loss correlating with a noticeable effect on the nucleation and growth of fullerene crystallites under thermal stress.34 However, no clear mechanism could be proposed due to significant impurities in the starting oligomeric material. We recently expanded on the concept of fullerene oligomers to demonstrate the synthesis of a novel class of polymers containing fullerene in the backbone.35–37 These polymers have the advantages of being easily purified, maintaining the flexibility afforded by fullerene chemistry with regards to tuning frontier orbital energy levels and solubility while avoiding a tendency to aggregate as frequently observed in polymers utilising the more typical approach of incorporating fullerenes as a pendent moiety.38,39 These combined benefits position poly(fullerene)s as attractive candidates for inhibiting morphologically driven changes within the BHJ of efficient organic solar cells and for elucidating the mechanisms by which the BHJ is stabilised. Herein, we report the incorporation of the fullerene polymer poly{(fullerene)-alt-[2,5bis(octyloxy)terephthalaldehyde]} (PPC4) into the active layer of a series of OPV blend systems. It is demonstrated that systems containing the poly(fullerene) acceptor demonstrate remarkable thermal stability although they suffer from inferior photovoltaic performance. It is further demonstrated, however, that through the combination of mono- and poly(fullerene) acceptors the photovoltaic performance can be mostly recovered while the improved thermal stability imparted


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by the poly(fullerene) is maintained. Finally, a mechanism by which stabilisation is achieved is proposed and discussed.

RESULTS AND DISCUSSION PPC4 was synthesized by a sterically controlled azomethine ylide bis-addition to buckminsterfullerene (C60) as described elsewhere, yielding a polymer with a molar mass of 15200 gmol-1 and a polydispersity of 2.2.35 The electronic energy levels of the resulting polymer, as determined from cyclic voltammetry (Figure S1a), are displayed along with its’ molecular structure in Figure 1. In comparison to the commonly used fullerene acceptor phenyl-C61butyric acid methyl ester (PCBM), a shift of the LUMO energy of ca. 100 mV was observed, consistent with bis-adduct functionalisation of the fullerene cage.40,41 The HOMO was determined from estimation of the optical band gap (Figure S1b) resulting in HOMO/LUMO values of -5.90/-4.16 eV respectively.

Figure 1. Structure of PPC4 (left) and PCBM (right) and their respective energy levels as determined by cyclic voltammetry (for LUMO levels) and absorption spectroscopy (for optical bandgaps and HOMO levels) (Figure S1). Note that for visual clarity the majority PPC4


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regioisomer, based on trans-3 addition is shown; the polymer is regioregular with a minor presence of trans-2, trans-4 and equatorial isomers. A more detailed characterization can be found in reference 35.

Initial experiments were conducted using PPC4 as the acceptor in combination with the prototypical polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) as the system electron donor. Samples were processed from a o-xylene:1-methylnaphthaline (19:1) mixed solvent system instead of more commonly used chlorinated compounds. It was considered important that such studies be conducted using benign, industrially applicable solvents so that the role that they will inevitably play in influencing solar cell performance and stability may be better appreciated. Indeed, to the best of the authors’ knowledge, there are very few studies to be found in the literature, which explore the stability of systems processed from any benign solvent. Absorption and photoluminescence measurements were conducted in order to study the influence of PPC4 weight fraction on blend optical properties. To make an informative comparison with PPC4, PCBM was employed here as a reference acceptor. Figure 2a and b show the effect of increasing PCBM and PPC4 loadings on the absorption spectra of blends with P3HT, respectively.


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Figure 2. Normalized absorption spectra of (a) P3HT:PCBM and (b) P3HT:PPC4 blend systems at varying donor acceptor ratios. Normalization was carried out with respect to the highest energy P3HT absorption feature. Normalization of the absorption spectra of the pure fullerenederivative films was carried out by bringing the value at 400 nm to 1. (c) PL quenching efficiencies of the corresponding P3HT:PCBM and P3HT:PPC4 blends with respect to the plain P3HT photoluminescence as a function of fullerene weight fraction in the blend. The lines in (c) are intended as guides to the eye.

As the loading of PCBM in P3HT is increased gradually a corresponding weakening of absorption features correlating with structural disorder in P3HT in the solid state can be observed. A reduction in the peak at ~600 nm, attributed to coupled π – π* transitions of adjacent P3HT chains,42 relative to the other P3HT absorption features is clearly apparent and is accompanied by a P3HT blue shift as expected from reduced P3HT order and consistent with observations in comparison between regioregular and regiorandom P3HT.43 In contrast, as the loading of PPC4 is likewise increased, only minimal change in these features is observed until very high loadings of PPC4 (>0.67) are reached. Such differences in blend ratio and their effect on absorption spectra are telling of the relative intermixing between components, or more accurately, the ability of the acceptor material to infiltrate and disrupt the order of the donor phase, which in the present case is more pronounced when using PCBM. In order to achieve higher sensitivity when probing the intermixing of PPC4 with P3HT, photoluminescence (PL) spectra were taken of films with increasing PPC4 loading and the integrated spectra compared against

a

pristine

P3HT

control,

giving

rise

to

the

PL

quenching

efficiency

(1-PL(P3HT:acceptor)/PL(P3HT)). The photoluminescence quenching of these systems was then


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corrected for sample absorption and a comparison between PPC4 and PCBM, as a function of weight fraction, was made (Figure 2c). As expected, already at low loadings of PCBM, a sharp rise in P3HT emission quenching to as high as ~80% (excitation at 525 nm) is observed implying that the PCBM is highly miscible and relatively well dispersed throughout the P3HT.44 The further addition of PCBM thus appears to have no significant observable effect on quenching levels allowing the conclusion to be drawn that these predominantly collect around existing quenching centres forming aggregates and percolating pathways. In contrast, the gradual addition of PPC4 leads to only a modest level of quenching, plateauing at ~40% as the acceptor loading is increased further. Such low quenching levels are suggestive of the presence of large pure emissive P3HT regions. That such behaviour is observed regardless of PPC4 loading also strongly supports the idea that PPC4 facilitates only a limited degree of intermixing. It may thus be inferred that the dispersion of this particular poly(fullerene), throughout the donor matrix, is markedly worse than in the case of PCBM with negative implications for charge generation and therefore device efficiency. Indeed devices produced using the poly(fullerene) as an acceptor achieved only modest efficiencies (Figure S2 and Table 1).


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Figure 3. (a) PL quenching of films (black circles, left y-axis), device short circuit currents (grey circles, right y-axis) and (b) device power conversion efficiencies of the multi-acceptor systems as a function of PCBM weight fraction. Increasing amounts of PCBM were added to the optimum P3HT:PPC4 ratio (0.7:0.3, see also Table 1). Dashed lines are intended as a guide to the eye. Table 1. Blend compositions of the multi-acceptor systems and their corresponding photovoltaic device parameters. Data represent the average of eight devices.


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The apparently significant difference in acceptor dispersion suggests that in the case of the optimum PPC4 system, large domains of relatively pure P3HT phase are likely to exist. The addition of PCBM to these regions in a multi-acceptor system could offer an opportunity to enhance charge generation from these domains while maintaining a strong interconnected acceptor network throughout the matrix as afforded by the poly(fullerene). To test this idea, increasing amounts of PCBM were added to the optimum P3HT:PPC4 ratio (0.7:0.3) in films and devices in order to study their influence on charge generation and overall device performance. The results of photoluminescence quenching measurements conducted on these films and a comparison with the observed device short circuit current densities (JSC’s) of equivalent devices are shown in Figure 3a. The P3HT:PPC4 (0.0 PCBM weight fraction) binary blend exhibited a PL quenching efficiency of only ~40%, however the addition of merely a 0.1 weight fraction of PCBM (with respect to the total system mass) to the system leads to a sharp increase in PL quenching to over 65%. The addition of 0.3 and 0.5 weight fractions of PCBM to the system further increases the quenching up to over 80% and 90%, respectively; more comparable to that of the P3HT:PCBM binary (>80%) and suggestive of an incremental improvement in potential charge generation yields with increasing PCBM content in the blend. This rise in quenching is also accompanied by an increase in observed JSC’s from 2.56 mAcm-2 to as high as 8.08 mAcm-2 in the multi-acceptor system, almost on par with the 9.30 mAcm-2 exhibited by the P3HT:PCBM binary blend. Interestingly, a mismatch between the degree of increased PL quenching and extracted JSC’s is evident, particularly at both low and very high PCBM loadings and implicative of a barrier to the extraction of some of the additional charge generated in these systems. The corresponding device power conversion efficiencies (PCE’s) for the blends exhibit a trend closely following that of the JSC (Figure 3b). A low PCE of 0.95% was


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exhibited by the P3HT:PPC4 binary devices. The introduction of PCBM into the system leads to a substantial increase in device PCE’s up to almost 2.9%, with a peak in PCE existing between PCBM weight fractions 0.3 and 0.4, and almost achieving comparable efficiency with the 3.39% reached by the P3HT:PCBM binary control. Furthermore, higher observed VOC’s for samples containing large amounts of PPC4 can be rationalized in terms of the higher-lying LUMO of PPC4 as compared to PCBM and at PCBM weight fractions greater than 0.2, a clear contribution to improved device performance from an improved fill factor (FF) is also apparent (Table 1). Recalling that the P3HT:PPC4 blend appears to have many regions of relatively pure P3HT, it should be expected that PCBM would disperse relatively well through these regions due to its demonstrably high miscibility and result in the formation of mixed P3HT:PCBM regions. At low PCBM loadings, it is highly likely that electrons generated within these mixed regions do not have an effective percolation pathway out of the active layer. As the PCBM loading is increased beyond a required threshold it begins to form percolation pathways allowing the extra generated charge to exit the active layer more effectively. However, at very high PCBM loadings, it is likely that a high level of intermixing between P3HT and PCBM causes massive disruption of the P3HT order, as evidenced by the notable blue shift in P3HT absorption relative to all other samples in the series (Figure S3), leading to the breakdown of effective P3HT percolation. The P3HT:PPC4:PCBM ratio of 0.42:0.18:0.4 (T 0.4) thus reflects an optimum scenario for charge generation and extraction in this mixed acceptor blend. We now turn to the effects of thermal treatment on the opto-electronic properties of P3HT blends.

Thermal degradation of OPV has often been associated with the aggregation and

breakdown in percolation of fullerene in the active layer, leading to a reduction in effective charge generation and extraction.13,21,45 In order to monitor the potential growth of acceptor


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aggregates, optical and atomic force micrographs were taken of the various multi-acceptor blends, periodically, during annealing (Figure 4 and S4).

Figure 4. Optical micrographs of the multi-acceptor system active layers with varying composition (cf. Table 1 for details), sandwiched between ITO/ZnO and PEDOT:PSS layers, taken as a function of annealing time at 120 oC. The scale bars each represent 50 µm.

In the case of the P3HT:PCBM control, after 5 h of annealing at 120 oC, micron-scale aggregates are already visible, which remain until the end of the experiment. Worthy of note is that neither the number of crystallites nor average size appears to be changing at longer timescales. This would suggest that P3HT:PCBM blends very quickly achieve a steady state. After 25 h and 100 h large aggregates become visible in the case of the T 0.5 and T 0.4 samples, respectively. In all other samples no aggregates are observed even after 200 h of annealing.


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Importantly, the total lack of aggregation in the PPC4 control supports the hypothesis that the polymeric fullerene acts to inhibit its aggregation and thus suppress related degradation mechanisms. Conversely, the almost immediate onset of aggregation in the PCBM control sample followed shortly by that in the T 0.5 sample suggests that the rapid migration of PCBM readily takes place and is consistent with the observation of initial rapid degradation in photovoltaic performance. The later onset or rather gradual suppression of this phenomenon correlates with the decrease in fullerene monomer content and increase in poly(fullerene) content in the films. Indeed, at PCBM weight fractions of 0.3 and below, there appears to be no evidence of PCBM aggregation at all which should lead to noticeably improved stability in devices. In order to elucidate the potential benefit of incorporating poly(fullerene)s into the BHJ we subjected P3HT-based devices with varying fullerene composition to an elevated temperature of 120 °C in an otherwise inert environment (no light and no environmental contaminants). Figure 5 shows the change in device parameters for devices of both binary systems as well as the most promising multi-acceptor mixture (T 0.3) investigated as a function of time (representative current-voltage curves can be found in Figure S5). At very short time scales (i.e. in the first six hours) slight improvements in performance were observed among all samples due to favourable morphological rearrangement of the active layer associated with crystallisation of the donor and acceptor phases.46,47 After this process has run its course, deterioration of the BHJs begins to take place. A rapid and extensive degradation in device PCE is observed within the first 20 h in the P3HT:PCBM control upon annealing; driven by a loss in current density output and on a time scale that is consistent with the appearance of fullerene crystallites in the optical micrographs. In contrast, only a negligible performance decay is observed in the P3HT:PPC4 control and T 0.3 sample over the same timescale. A slower but


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significant degradation phase lasting until 100 h is observed in all samples followed by an effective plateauing of device PCE up to 200 h and driven by a gradual reduction in VOC. Interestingly, the lack of an accompanying change in JSC strongly suggests that an additional and independent degradation mechanism is at play, which could be related to widening of the density of states due to increasing disorder within the active layer or gradual deterioration of the interfaces.48–50 It should also be noted that the observed drop in VOC is less pronounced in the multi-acceptor system than in the PCBM binary. Further investigation was, however, deemed beyond scope of this study.


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Figure 5. Photovoltaic device parameter (a) PCE, (b) JSC, (c) VOC and (d) FF as a function of annealing time at 120 oC for P3HT:PCBM (black circles) and P3HT:PPC4 (light grey circles) binaries and the P3HT:PPC4:PCBM (T 0.3) (grey circles) multi-acceptor system. The data represents an average of 8 devices.

In order to better understand the source of early performance decay, PL quenching measurements were conducted on films of the active layer as a function of annealing time


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(Figure 6a) under otherwise equivalent conditions. As evidenced by the microscopy in Figure 4 and the short-circuit current in Figure 5, macroscopic evolution of the morphology was not apparent after 25 h of annealing and it was considered sufficient therefore to observe PL changes within this timeframe. In the case of both binary systems quenching of P3HT photoluminescence is reduced rapidly with most of the loss occurring within the first 5 h of annealing. This observation is suggestive of rapid changes in morphological properties of the films and the growth of relatively purer and increasingly emissive P3HT domains. In the case of the multiacceptor system this rapid loss of quenching is greatly suppressed, and instead a much slower rate of loss is observed. Much work in the literature has been conducted demonstrating the tendency for P3HT towards rapid crystallisation upon being subjected to annealing processes. It thus follows that the enhanced crystallisation process correlates with a reduction in donor acceptor mixing and the forced segregation of the acceptor phase. In the case of PCBM this is expressed as the formation of large aggregates that grow to the micron scale within the first few hours of thermal stressing as clearly apparent from AFM topography and phase images as well as a strong increase in film roughness (Figure S4). On the other hand, AFM micrographs reveal that both the PPC4 binary and T 0.3 sample appear to show no such growth even on the nanoscale. The reduction in quenching in the case of PPC4 is therefore likely due to a slight enhancement in crystallinity of the P3HT domains accompanied by a relatively minor increase in phase segregation. In the case of the T 0.3 sample, the less pronounced decrease in quenching is implicative of the retardation of the initial P3HT crystallisation process as well as the maintenance of a higher level of intermixing between the P3HT and PCBM at higher temperatures, as will be shown further below. It may thus be suggested that effective exciton


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dissociation and charge generation are also preserved, playing a key role in the observed photovoltaic stabilisation.

Figure 6. (a) Photoluminescence quenching efficiency and (b) charge carrier mobility measurements (Photo-CELIV) of the PPC4 binary, PCBM binary and T 0.3 system with P3HT as donor. The lines represent guides to the eye.

Although changing morphology can have a significant impact on the photogeneration of charges, functioning solar cells also require that their charges be able to effectively exit the device. Thus, to better understand the influence of changing morphology, the impact of thermal degradation on charge transport in devices was monitored using the photoinduced charge carrier extraction by linearly increasing voltage (photo-CELIV) technique.51–53 Photo-CELIV measurements were performed directly on the solar cells, allowing calculation of the charge carrier mobilities as a function of the evolution of thermal degradation in the devices. PhotoCELIV traces of PCBM, PPC4 and T 0.3 devices were recorded upon laser excitation of 405 nm


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at a 50 µs delay time by applying a 2 V linearly increasing reverse bias over the course of 60 µs (Figure S6). Figure 6b shows the calculated mobility as a function of the annealing time for the corresponding devices (see experimental for details). A significant drop in mobility for P3HT:PCBM cells (~95% reduction) is observed, whereas the T 0.3 and PPC4 devices exhibit a relatively smaller decrease of ~55% and ~45% compared to fresh devices over the course of 100 h, respectively. It is known that the number of percolation pathways for electron transport in a fullerene containing BHJ is reduced upon thermal stressing mainly due to a decreasing of PCBM concentration in the mixed phase.45 These results suggest that the vacation of PCBM from the mixed phase is notably suppressed in the PPC4 containing devices (T 0.3 devices), in agreement with observations from both the PL quenching and microscopy measurements presented above and suffering only a slightly higher drop in mobility than that observed in PPC4 only devices. It is suggested that the change in mobility in the PPC4 only devices is likely to be influenced by nanoscopic changes at the active layer / electrode interface, which are not necessarily linked to bulk effects.54 This observation also corroborates the hypothesis that PPC4 (and likely other similar fullerene polymers) has the capability to inhibit severe detrimental changes in the charge transport behaviour of BHJ solar cells driven by changes within the active layer over long hours of thermal stress. As a means to elucidating the structural benefit of a multi-acceptor blend, a well-established thermodynamic model based on Flory–Huggins solution theory was employed to determine the relative miscibilities of the active layer components. Following Flory–Huggins, the general phase behaviour or thermodynamic stability of a mixture consisting of components A and B, whose sizes are significantly dissimilar, can be assessed according to the following equation55:

=

(

+ )

(1)


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Where is the molar volume of the lattice site in the Flory-Huggins model, , and , are the densities and molecular weights of the two components, respectively, and

is the volume

fraction of compound B. The spinodal line(χspinodal) predicts the condition for which the mixture becomes unstable (the spinodal line defines the transition from energetically favoured ∂2∆Gmix/∂ϕ2 > 0 to energetically unfavoured ∂2∆Gmix/∂ϕ2 < 0 mixing). At the same time, the Flory–Huggins interaction parameter (: ) for a given binary blend can be estimated from the change in melting enthalpy as a function of the composition (Figure S11).56 Typically, a value of χ towards 0 is indicative of good miscibility while higher values represent a reduced tendency for intermixing. Figure 7a shows the variation in melting point depression observed by differential scanning calorimetry (DSC) as a function of PCBM volume fraction in blends with PPC4 and P3HT. From these, the Flory-Huggins interaction parameter (χ) for each blend was calculated, yielding χPPC4:PCBM and χP3HT:PCBM of 0.99 and 0.78 respectively (see Experimental for details).

Figure 7. (a) Melting point depression for P3HT:PCBM (black circles – left axis) and PPC4:PCBM (grey circles – right axis) binary mixtures, as a function of PCBM volume fraction. The Flory-Huggins interaction parameter can be calculated by fitting this data to equation 3 (see


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Experimental). (b) Spinodal demixing lines for P3HT:PCBM (black line) and PPC4:PCBM (grey line) systems. Horizontal lines represent the experimentally determined Flory-Huggins interaction Parameters for the P3HT:PCBM (black line) and PPC4:PCBM (grey line) systems.

Figure 7b shows the calculated spinodal line plots as a function of PCBM volume fraction in blends with PPC4 and P3HT using equation (1). Importantly, it should be noted that a far larger two-phase region for the PPC4:PCBM blend is observed making a thermodynamically stable mixed phase unlikely at middling volume fractions. Overlaying the calculated Flory-Huggins interaction parameters (χP3HT:PCBM and χPPC4:PCBM - horizontal lines) for each mixture with these plots, allows determination of the compositions at which the blends tend towards phase separation or homogeneity. The parameter χP3HT:PCBM intersects the corresponding spinodal line close to the PCBM volume fraction utilized in this study (ϕ = 0.48), implying that some small degree of spinodal demixing is to be expected. In contrast, the χPPC4:PCBM overlays broadly with the two phase region at PCBM volume fractions between 0.1 and 0.9 indicating that separation of the PCBM phase from that of PPC4 occurs readily and to a far greater extent than in the case with P3HT. There is thus little driving force for PCBM to remain in the PPC4 phase, suggesting that PCBM is substantially more effectively dispersed in the P3HT matrix, consistent with the PL measurements. Conversely, under the effect of thermal degradation it is likely that PCBM migration would require diffusion through regions in the BHJ that are rich in PPC4. The shape of the spinodal lines, however, strongly implies that such a migration out of the P3HT would be inhibited by the low PCBM miscibility in PPC4, forcing the majority of the acceptor to remain in the P3HT phase.


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So far, it has been demonstrated that the introduction of the poly(fullerene), PPC4, into BHJ blends of P3HT and PCBM has the ability to suppress the deterioration of key solar cell parameters responsible for device failure under thermal stress. In order to explore the universal applicability of poly(fullerene)s as effective agents for the thermal stabilisation of OPV, PPC4 was incorporated into an alternative BHJ system. The donor polymer PBTZT-stat-BDTT-8 (Figure S7) was chosen for its good initial photovoltaic performance and its demonstrated “green” solvent and large area compatibility.4

Figure 8. (a) Change in power conversion efficiencies for PBTZT-stat-BDTT-8:PCBM (1:1.5), PBTZT-stat-BDTT-8:PPC4 (1:0.43) and PBTZT-stat-BDTT-8:PPC4:PCBM (1:0.25:1.5) based devices as a function of annealing time at 120 oC. (b) Optical micrographs taken of films of the corresponding active layers after 0 h, 1h and 4h of annealing under N2 at 120 oC

Figure 8a shows the change in device PCE’s upon thermal annealing at 120 °C under nitrogen for PCBM and PPC4 binary systems and a mixed system incorporating both acceptors. Initial PCE’s achieved for the PCBM control gave an average of ~6%, in line with previous reports


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Page 24 of 38

using a similar device layout. In contrast, again, the poly(fullerene) exhibited an inferior performance yielding devices with an average of only ~0.6% due to low device current outputs (Figure S8). In light of the initial P3HT experiments above, this difference is believed to be caused by the poor intermixing of PPC4 with the donor polymer in the active layer (for the solvent system utilized). In analogy to the P3HT case, attempts to restore this performance by the building of a multi-acceptor system succeeded in recovering the majority of the lost PCE reaching ~4.2% for a BHJ containing a PBTZT-stat-BDTT-8:PPC4:PCBM ratio of 1:0.25:1.5. It should be noted that fine-tuning of the multi-acceptor composition is expected to yield efficiencies even closer to the binary system. We further emphasize that the PPC4 binary achieved a very promising VOC of 1 V, significantly higher than the PCBM containing blends. Addressing the poor intermixing of the poly(fullerene), (e.g. by modification of sidechains, processing conditions, etc) could therefore lead to greatly improved performance. Upon thermal annealing, an immediate drop in the PCE of the PCBM sample is observed, reaching 2% within a single hour. In contrast, both the poly(fullerene) control and the mixed system showed almost unchanged power output over the course of 5 hours. It should be noted that in the work of others, devices made using a PBTZT-stat-BDTT-8:PCBM active layer have been shown to be reasonably thermally stable over a 120 min timeframe at 120 °C,4 however, the results presented here were performed using different processing conditions, active layer materials as well as supporting architecture, unoptimized with respect to enhancing long term stability. Thus, these results serve as an accelerated test in comparison to a more optimized system, demonstrating the eventual tendency for PCBM aggregation and it’s hindrance with the incorporation of PPC4. This confirms the general benefit of incorporating poly(fullerene)s into otherwise efficient but thermally unstable systems to substantially enhance the lifetime of


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fullerene containing thin-film devices by suppressing the mechanisms responsible for rapid degradation. Indeed, the fact that there is a strong correlation between changes in device PCE and changes in JSC as well as FF (Figure S8 and S9) is certainly implicative of a reduction in effective charge generation and/or extraction induced by fullerene migration out of the mixed regions of the BHJ. A rise in the VOC under thermal stressing also is consistent with the behavior of other systems utilizing amorphous donor polymers.57 Optical micrographs taken of films of the active layer at various stages during annealing are shown in Figure 8b. Initial images show no significant fullerene aggregates in all cases. After a single hour of annealing needle like formations become visible in the film containing PCBM as the only acceptor. After 3 more hours of annealing these formations appear to have grown substantially, covering the entire surface of the film. In contrast, films containing only PPC4 or a combination of both acceptors exhibit no such behaviour. While similarly promising results have been obtained before (using cross linkers, etc), the development of poly(fullerene)s alleviates the use of additional components, which could lead to charge transport limitations. Furthermore poly(fullerene)s represent an interesting test bed for the study of morphological (polymer-single molecule vs polymer-polymer interactions) and electronic compatibility, envisioning a wide range of applications that require a high level of thermal stability including, photodetectors, solar cells and sensors.

CONCLUSION In conclusion, an approach for the stabilization of fullerene containing BHJ solar cells, by the incorporation of the poly(fullerene) PPC4, is presented. Solar cells utilising the PPC4 exhibit high thermal stability. However, the use PPC4 as electron acceptor causes suboptimal intermixing in the blend and thus low performance due to low current generation as a result of


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Page 26 of 38

poor exciton quenching. Although the potential for this class of poly(fullerene)s as a favourable n-type semiconductor is apparent, further work is required to elucidate the structure-function relationship dictating the physical properties exhibited, particularly in relation to miscibility. It is expected that the use of alternative solubilizing groups and solvent combinations will have a strong impact on the blend morphology and device performance and may pave the way for highly efficient and stable binary systems incorporating poly(fullerene)s. It is also demonstrated that, by combining PPC4 and PCBM into a single blend, it is possible to achieve high power conversion efficiencies while maintaining the thermal stability of the system. Given the intrinsic morphological instability of many bulk heterojunctions this methodology may be applied to a variety of donor acceptor blends, offering a facile general approach to maintaining a favourable level of component intermixing over the long term, which should translate into significantly improved device lifetimes.

EXPERIMENTAL SECTION Cyclic Voltammetry (CV): CV studies were performed using a standard three-electrode cell. A platinum disk electrode was used as working electrode, a platinum wire as counter-electrode and silver/silver

chloride

(Ag/AgCl)

as

reference

electrode.

Tetrabutylammonium

hexafluorophosphate (NBu4PF6; ≥ 98%) was used as electrolyte. Measurements were recorded using a BAS CV-50W Voltammetric Analyzer with a BAS C3 Cell Stand, which was connected to a personal computer running BAS CV-50W software version 2.31. In a typical experiment, 25 mg of the material was diluted in in a mixture of o-dichlorobenzene/acetonitrile (4/1 by volume) in the presence of 0.1 M NBu4PF6. The cyclic voltammetry graphs were recorded at a potential scan rate of 100 mV s-1 under nitrogen atmosphere. The reduction potentials were


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calibrated against an internal ferrocene/ferrocenium (Fc/Fc+) redox couple. The oxidation potentials of the fullerenes were mostly obscured under these conditions. Sample Preparation: Glass and Glass/ITO substrates were prepared by sonication in acetone followed by isopropanol. The active layer solutions were prepared by dissolving P3HT, PBTZTstat-BDTT-8, PCBM and PPC4 in varying proportions in a xylene:1-methylnaphthaline (19:1) solution and stirring overnight in an inert atmosphere at 80/100 °C for P3HT/PBTZT-statBDTT-8 containing solutions, respectively. The PEDOT:PSS (Heraeus) solution was prepared bys dilution of the stock with 3 parts isopropanol and left under continuous stirring at room temperature for over an hour before use. Devices were constructed with the following architecture:

ITO/ZnO/photoactive layer/PEDOT:PSS/Ag.

A 5 wt%

ZnO nanoparticle

suspension was deposited directly onto the ITO and thermally annealed at 140 °C in air for 5 minutes. The active layer and PEDOT:PSS (in the case of complete devices) were then sequentially deposited over the substrates. All aforementioned depositions were carried out with a doctor blade. P3HT:PCBM, P3HT:PPC4 and T 0.3 active layers were coated from 30 mgml-1 solutions at 65 °C and 40, 20 and 40 mms-1 respectively. After coating all samples were annealed for 5 minutes in an inert atmosphere at 120 °C to drive off residual solvents from the PEDOT layer. Electrode deposition was carried out by thermal evaporation of silver under vacuum to produce complete solar cells with active areas of 27 mm². Optoelectronic Characterisation: Current voltage characteristics were measured under nitrogen using a Keithley 2400 source meter and a Xenon arc lamp normalized against a cailbrated Si-photodiode to an intensity of approximately 100 mWcm-2. Spectroscopic Characterisation: Absorption spectra were taken with a Perkin Elmer Lambda 950 UV/Vis Spectrometer. Photoluminescence spectra were taken by excitation of samples at


27


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525 nm using a Jasco FP-8500 Spectrofluorometer.

Page 28 of 38

Photoluminescence quenching was

calculated from the ratios of the emission spectra of the samples and a neat P3HT film, integrated over the space 630-850 nm and corrected for absorption at the excitation wavelength. Optical Microscopy: P3HT blend images were taken through transmission using a Nikon Eclipse

L200

optical

microscope

on

device

stacks

comprised

of

ITO/ZnO/active

layer/PEDOT:PSS. PBTZT-stat-BDTT-8 blend images were taken through transmission using a Keyence VH-Z500R on active layer films on glass substrates. Atomic Force Microscopy: Intermittent contact atomic force microscopy was performed in air on a Solver Nano from NT-MDT using gold-coated silicon cantilevers with a resonance frequency of ~300 kHz (NSG30, NT-MDT). Photo-CELIV measurements: The devices were illuminated with a 405 nm laser-diode. Current transients were recorded across an internal 50 Ω resistor of an oscilloscope (Agilent Technologies DSO-X 2024A). We used a fast electrical switch to isolate the cell and prevent charge extraction or sweep out during the laser pulse and the delay time. After a variable delay time, a linear extraction ramp is applied via a function generator. The ramp, which was 60 µs long and 2 V in amplitude, was set to start with an offset matching the VOC of the cell for each delay time. Differential Scanning Calorimetry (DSC): The Flory-Huggins interaction parameter for P3HT:PCBM and PPC4:PCBM was determined by DSC. Therefore chlorobenzene solutions with various donor:acceptor ratios were prepared. After stirring over night at 60 °C, each solution was coated on glass via drop-casting. The drop-casting solution was annealed for 10 min at 70 °C to remove the solvent. The remaining solid films were then removed using a scalpel and filled into a crucible. The DSC process was initiated at a temperature of -50 °C followed by a


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ramp of 5 °C/min until a final temperature of 300 °C was reached. After a holding time of 5 min, the sample was then cooled down back to -50 °C. Because of the fact that a small amount of chlorobenzene may remain in the samples, two heating cycles were run. The investigations of the melting point depression and the calculations of the interaction parameter are based on the second heat curve. Flory-Huggins Parameter Determination: The melting point depression of a polymer in a polymer-diluent system can be expressed as:

"# $

−"

'(#

#$ &

'(#

= ∆* ( $ Φ. − ∆* ( $ χ0. Φ. + #,

+ #,

(2)

Here Tm is the melting point of the polymer in the blend, Tm0 - the melting point of the pristine polymer, Vm - the molar volume, R – the ideal gas constant, ∆Hf – the enthalpy of fusion, χ0. – the Flory-Huggins interaction parameter and ΦB – the volume fraction of PC61BM. The data can be fitted by the following function: y = aΦ. − b Φ.

(3)

The interaction parameter is then simply the ratio of b to a: χ0. =

4 5

(4)

In Figure S11a and S11b the specific heat flow as a function of temperature is depicted for blends of P3HT:PCBM and PPC4:PCBM, respectively. For P3HT a strong shift of the melting point from 237 °C to 217 °C can be observed. Compared to P3HT, PPC4 is more amorphous. Thus, here only a slight melting point depression caused by PCBM is identifiable. The melting point depression as a function of the PCBM volume fraction is depicted in Figure 7a. The data was fitted by equation (3). Calculation of the Spinodal Demixing graphs: The DFT quantum chemical model COSMORS58 was employed to predict the densities of P3HT, PCBM and PPC4. The densities employed


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in equation (1) were as follows: (P3HT) 1.17 gcm-3, (PCBM) 1.66 gcm-3, (PPC4) 1.60 gcm-3. The molar volume of the solvent o-xylene, calculated as 94 cm3mol-1, was taken as the lowest lattice molar volume of the system.

ASSOCIATED CONTENT Supporting Information available containing Cyclic voltammetry and absorption spectra for the neat polyfullerene, device optimisation and lifetime studies, optical and atomic force microscopy, Photo-CELIV and differential scanning calorimetry. This material is accessible free of charge via the internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author Simon A. Dowland, Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENTS The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2011) under grant agreement ESTABLIS no. 290022. MS acknowledges primary support from a fellowship by the Portuguese Fundação para a Ciência e a


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Tecnologia (SFRH/BPD/71816/2010). J.D.P. is funded by a doctoral fellowship grant of the Colombian Agency COLCIENCIAS. S.L is financed by the Deutsche Forschungsgemienschaft (DFG) in the framework of SFB 953 (Synthetic Carbon Allotropes) and Cluster of Excellence “Engineering of Advanced Materials”, Solar Technologies go Hybrid (SolTech), and Umweltfreundliche Hocheffiziente Organische Solarzellen (UOS).

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Suppression of Thermally Induced Fullerene Aggregation in

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