Understanding Physico-Chemical Aspects in the Depth Profiling of

Article pubs.acs.org/JPCC

Understanding Physico-Chemical Aspects in the Depth Profiling of Polymer:Fullerene Layers Supriya Surana,†,‡ Thierry Conard,*,† Claudia Fleischmann,† Jeffrey G. Tait,†,§ Joaõ P. Bastos,†,§ Eszter Voroshazi,† Rasmus Havelund,∥ Mathieu Turbiez,⊥ Pierre Louette,# Alexandre Felten,# Claude Poleunis,∇ Arnaud Delcorte,∇ and Wilfried Vandervorst†,‡ †

IMEC, Kapeldreef 75, B-3001, Leuven, Belgium KU Leuven Instituut voor Kern- en Stralingsfysica (IKS), Celestijnenlaan 200D, B-3001, Leuven, Belgium § KU Leuven, ESAT, Kasteelpark Arenberg 10, B-3001, Leuven, Belgium ∥ National Physical Laboratory, Hampton Road, Teddington, Middlesex, TW11 0LW, U.K. ⊥ BASF Schweiz AG, Schwarzwaldallee 215, CH-4002 Basel, Switzerland # Research Center in Physics of Matter and Radiation (PMR), University of Namur, 61 rue de Bruxelles, B-5000, Namur, Belgium ∇ Université catholique de Louvain, IMCN/BSMA, Croix du Sud 1, L7.04.01, B-1348, Louvain-la-Neuve, Belgium ‡

S Supporting Information *

ABSTRACT: Layers composed of binary blends of a polymer and a fullerene derivative are at the heart of bulk heterojunction organic photovoltaic cells. The efficiency and stability of these devices critically depend on the distribution of the polymer and fullerene materials within the blend layer. Whereas time-of-flight secondary ion mass spectrometry, particularly in combination with cluster ion beams, has frequently been used to probe similar organic materials, the quantification of the results is often missing due to a lack of understanding of all the parameters influencing the measurement results. This study contributes to improved quantification by exploring the role of the bulk composition and interfacial material on parameters such as sputtering yield, mass fragmentation, and ionization. We show that the argon cluster sputtering yields of these materials may be described by two widely acknowledged sputtering yield relationships. Their fitting parameters depend on the layer composition in a manner that is consistent with a lower energy required for sputtering of layers with higher fullerene derivative content. Similarly, we show that changes in composition impact the ion yields nonlinearly, which is an important source of quantification uncertainty. We provide evidence that, for the case of the fullerene derivative mixed with different donor materials, the matrix effect (i.e., the deviation from a linear response) correlates linearly with the electronegativity of the species in the donor material. Finally, with respect to the quantification of the composition at the interface with the substrate, we present a charge transfer mechanism that describes observed enhancements of the secondary ion intensities.

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(SPM),10,11 and X-ray photoelectron spectroscopy (XPS)7,12 can provide depth-related quantitative elemental or electrical information; specific molecular informationwhich is of great importance for unambiguous interpretation of the link to the compositionis absent. To obtain comprehensive chemical information about the photoactive layer as a function of its depth, one of the most suited and explored analysis techniques is time-of-flight secondary ion mass spectrometry (ToF-SIMS).13−22 Notwithstanding its successes, detailed studies of the impact of the analysis conditions on the results are required. Particularly on polymer:fullerene blend layers, Smentkowski et al.23 demonstrated the superiority of the molecular depth profiles when

INTRODUCTION Polymer−fullerene blends have received a great amount of attention, largely due to their application as the photoactive layer in bulk heterojunction organic photovoltaic cells.1 The depth distribution of the polymer and the fullerene materials in such devices has an enormous influence on their efficiency.2 Over the past decade, various techniques3 have been employed to characterize photoactive blends. Although X-ray techniques such as grazing incidence wide angle X-ray scattering (GIWAXS),4,5 grazing incidence small angle X-ray scattering (GISAXS),5,6 and near-edge X-ray absorption fine structure (NEXAFS)7 have provided detailed information about the morphology and composition of the photoactive layer, the cost of access and complex data analysis impair the accessibility of these techniques for routine applications. On the other hand, more routine (laboratory) techniques such as transmission electron microscopy (TEM),8,9 scanning probe microscopy © XXXX American Chemical Society

Received: September 30, 2016 Revised: November 14, 2016 Published: November 18, 2016 A

DOI: 10.1021/acs.jpcc.6b09911 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT - supplied by Saint Jean Photo Chemicals), and poly(diketopyrrolopyrrole−quinquethiophene)34 (PDPP5T - supplied by BASF). A C60 fullerene derivative, phenyl-C61-butyric acid methyl ester (PC60BM - supplied by Nano-C), was used as the acceptor material for the case of P3HT and MDMO-PPV, while, for the case of PCDTBT and DPP5T, a phenyl-C71butyric acid methyl ester (PC70BM - supplied by Nano-C) was used as the acceptor material. In each case, layers with varying concentrations of the fullerene in the donor material were fabricated. The blend layers were spin-coated onto suitable charge transport layers.35 In order to calculate the sputtering yields, layer thicknesses were extracted by measuring the step height from surface topography profiles obtained on scratched films by using a Dektak 150 profiler. The charge transport layers employed were thermally evaporated MoO3 for the P3HT, PCDTBT, and MDMOPPV samples while a spin-coated TiO2 layer was used for the PDPP5T samples. The charge transport layer was deposited onto ultrasonically cleaned indium tin oxide (ITO) substrates, and the photoactive layer was spin-coated inside the glovebox. The sample architecture and the chemical structures of the materials used are shown in Figure 1.

using Ar-cluster sputter beams compared to the monatomic Ar sputter beam. Along the same lines, Mouhib et al.24 evaluated the effect of three sputter beams, Ar1700+, Cs+, and C603+, on such layers and found that Ar1700+ generates relatively artifactfree molecular depth profiles provided its impact energy, E, per atom in the cluster, n, is less than a critical value of 6 eV/atom. At higher E/n, sample degradation sets in and the molecular information is lost. Presumably, under the influence of the energy deposition by the sputter beam, cross-linking occurs, precluding steady-state molecular depth profiling. Molecular dynamics25 have predicted that this process becomes dominant at beam energies exceeding ∼5 eV/atom. Fleischmann et al.26 complemented this work by demonstrating that the quality of the profiles depends not only on E/n but also equally on the total kinetic energy of the sputter beam. This led to the conclusion that, for the analysis with high depth resolution of such soft organic materials, one needs to adhere to a low E/n sputter beam with a low total kinetic energy.27−31 As these conditions lead to reduced sputtering yields, one has to consider carefully the additional constraint imposed by the dual beam ToF-SIMS configuration whereby one alternates between the sputter beam and the analysis beam. Imperative for analysis with this configuration is that the amount of material removed by the sputtering beam should remain considerably larger than the removal and damage rates of the analysis beam such that any accumulation of damage by the analysis beam is prevented.32 Moving toward the low E/n regime for the sputter beam leads to (very) reduced sputtering yields and thus slow removal rates such that maintaining a proper removal ratio between the sputter beam and the analysis beam becomes harder to realize. Equally important for the analysis of blend materials is the need to extract quantitative information concerning their composition. In many cases, this is challenging as signal intensities of mass peaks, representative for the respective constituents, do not scale linearly with their concentration. The latter is often referred to as “the matrix effect” and can imply an increase or a decrease of the ionization efficiency with increasing concentration.33 Fleischmann et al.26 demonstrated for instance that, in the case of two polymeric donor matrixes, P3HT and PCDTBT, the negative fullerene ion does not show a monotonic increase with concentration. The objective of this work is to study the matrix effects, both in the bulk and at the interface, that hinder quantification of the composition in such layers. We first investigate the applicability of two sputter yield relationships on these layers by interpreting the fitting parameters relative to the layer composition. Subsequently, we study the evolution of the ion yields of the characteristic negative fragment of the acceptor material, as a function of both sample composition as well as the donor matrix used. This enables us to derive a quantitative assessment of the matrix effect and provide a framework for its understanding. Finally, we establish a relationship between the band structure of the materials involved and the enhancement in the ion intensities observed at the interface between the blend layer and the underlying inorganic layer.

Figure 1. Schematic of the sample architecture and chemical structures of the materials used.

To perform a detailed study of the interface phenomena, the PDPP5T:PC70BM layer was also deposited on a Si substrate, without a charge transport interlayer. Depth Profiling. ToF-SIMS depth profiles were obtained on a TOF-SIMS5 instrument from IONTOF GmbH, used in the dual beam configuration. To expose subsurface layers and progressively form a crater, an Arn+ beam was rastered at 45° to the surface, while a pulsed Bi3+ beam with a cycle time of 200 us in the high current bunched mode was employed to generate the mass spectrum from a central 1/9th area of the crater floor, carefully avoiding any contributions from the crater walls. The sputter-to-analysis dose ratios were maintained above 1000, which has been shown (T. Conard, AVS 59- 2012, unpublished) to be high enough to avoid degradation of the material by the analysis beam for such materials. The characteristic ionic fragments from PCBM were the intact small molecule, PCBM−, in the negative polarity and the fullerene ion (C60 − m/z ∼ 720 or C70 − m/z ∼ 840) in both the negative and the positive polarities. For PDPP5T, PCDTBT, and P3HT, the negative thiophene ion, C4HS− (m/z ∼ 81) was used as the characteristic fragment. The secondary ion yields were computed by normalizing the number of secondary ions detected with the Bi3+ dose values extracted by measuring the Bi3+ beam current. During the studies on the impact of the E/n, the total kinetic energy of the Arn+ cluster beam was kept constant at 5 keV, whereas the cluster size was varied from ∼1250 to 3400 (Δn/n ∼ 20−30%) to achieve E/n values of 4, 3, 2.5, 2, and 1.5 eV/

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EXPERIMENTAL METHODS Materials and Sample Structure. The donor polymers used in this study are poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV - supplied by Merck), poly(3-hexylthiophene) (P3HT - supplied by Rieke Chemicals), poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5B


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The Journal of Physical Chemistry C atom. In order to compute the sputtering yield, the dose required to reach the interface of the layer was used as well as the layer thickness determined with the surface profiler. To estimate the number of molecules in the analysis volume,36 an assumption of the same densities for all of these systems has been made, using the average volume of a C or O atom as (0.27 nm)3 as proposed by Seah.37 XPS analysis was performed into the depth of the layers on a Thermo Escalab 250Xi instrument, using monochromated Al Kα photons (1486.6 eV). In order to expose subsurface layers, a MAGCIS cluster ion source from Thermo Instruments was used at an angle of 40° with the normal in the large Arn mode (n ∼ 2000 atoms) at 4 keV, with a current of ∼7 nA and raster size 2 μm.

in sputter yield, arising from differences in composition, will cause development of surface topography. The trend is fairly independent of the energy of the beam per atom, whereas the actual sputter yield value at fixed composition increases with E/ n. This experimentally observed energy dependence of the sputtering yield can be evaluated within the formalisms proposed by Seah37 and Cumpson.38 A universal equation for sputtering yields (Y) and their energy dependence on the cluster size (n) has been proposed by Seah37 as

RESULTS Sputtering Yields. The variation of the sputter yield, as shown in Figure 2 in units of nm3/Arn+ ion, with the sample

It uses three parameters. B allows one to define the yield in nm3 instead of atoms, which is a more appropriate measure for organic materials that contain large molecules, A is related to the mean energy required to release a fragment per atom (excluding hydrogen) in the fragment, and q describes the energy dependence of the sputter beam in the low E/n regime (based on a power law). As an example (see Figure 3a), for a 50 wt % PC70BM sample, the values of the parameters are A = 1.87 eV, B = 0.004 nm3, and q = 3.83. These are comparable to the values reported for other organic materials.37 An alternative model describing the relationship between the sputter yield (Y) of materials and E/n of the sputter beam is proposed by Cumpson et al.38 as

⎛ E ⎞q − 1⎤ ⎛ E ⎞q ⎡ Y (nm 3) ⎟ / ⎢1 + ⎜ ⎟ ⎥ = B∗⎜ ⎝ A∗n ⎠ ⎦ ⎝ A∗n ⎠ ⎣ n

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⎡ ⎛ ε − U ⎞⎤ ⎟⎥ Y = nε0A⎢1 + erf⎜ 0 ⎝ s ⎠⎦ ⎣

which assumes a Gaussian distribution of the energies of individual Ar atoms (ε0) at the moment of impact. An effective sputter threshold U is defined, below which the Ar atom would not cause any sputtering. The two additional parameters are s, which describes the distribution of the individual energies of the Ar atoms released upon the impact, and A, which is a linear scaling coefficient. Again, (as seen in Figure 3a), for the 50 wt % PC70BM sample, a good fit to the experimental data can be obtained, using U = 1.80 eV, A = 0.001 nm3/ion-eV and s =

Figure 2. Sputtering yields (in nm3/ion) of the PDPP5T:PC70BM layers analyzed by sputter beams of different E/n.

composition (increasing fraction of PC70BM in the PDPP5T: PC70BM layers) has a fairly weak dependence up to ∼50 wt % PC70BM; the sputter yield dramatically increases. The latter is an important observation, impacting the quantification accuracy and depth resolution of heterogeneous films, as local differences

Figure 3. (a) The universal Ar-cluster sputtering equation fit and the sputter threshold model on the data obtained from layers with 50 wt % PC70BM in a PDPP5T matrix. (b) Fitting parameters obtained from each model on the data obtained as a function of the PC70BM fraction in the layer. C


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monotonously increasing trend. A similar bell-shaped trend is seen for the negative fullerene ions in each of the four systems studied, as shown in Figure 4b. It is clear that, with such a nonlinear behavior, simply taking ratios to determine the composition will not lead to unique quantification results. Hence, a detailed study of the matrix effect will be presented in the subsequent Discussion section. Interface Phenomena. Whereas, in the previous section, we focused on quantifying the bulk composition, it is equally important to achieve a correct quantification at the interface as in such devices, the interfaces are optimized for enhancing the device efficiencies. One can expect that, at the interface, the matrix effects might be present as well. For instance, out of the four systems studied, the PCBM related negative ion depth profiles in the PDPP5T:PC70BM system, shown in Figure 5a,

1.50 eV. For the polymeric materials studied, Cumpson et al. report U ≈ s,38 which is consistent with our data. Since these models provide a reasonable fit to the data obtained, of specific interest here is how the fitting parameters evolve with the PCBM fraction in the sample. This is explored for the PDPP5T:PC70BM system in Figure 3b and will be further discussed in the Discussion section. We observe that both parameters, A from the universal sputtering equation and U from the sputter threshold model, are very close to each other, independently of the composition of the materials. Matrix Effect (Quantification of the Bulk Composition). The prime objective of the ToF-SIMS depth profiles would be to provide a quantitative estimate of the composition of the films. For this purpose, we first explore the response of the characteristic ion intensities relative to the blend composition. In order to avoid convolution with transients and other effects at the surface and the buried interface, the intensity of the characteristic fragments was taken in the steady state part of the depth profile. As shown for the PDPP5T: PC70BM system in Figure 4a, the negative fullerene-related ion yields (C70− and PCBM−) vary highly nonmonotonically with concentration and display a bell-shaped trend as a function of the PCBM fraction in the layer. On the contrary, the steady state ion intensities of the positive fullerene ion (C70+) show a

Figure 5. (a) Depth profiles of the characteristic fragments in the negative polarity obtained on various substrates. (b) Positive fullerene depth profiles as obtained from pure PC70BM on TiO2 as well as the 75 wt % PC70BM on Si. All the depth profiles have been obtained by a 2.5 eV/atom Ar-cluster sputter beam.

display an order of magnitude increase in intensity at the interface with the TiO2. This enhancement cannot be linked to the increase in the ionization efficiency to positive ions in the presence of oxygen39 at the interface. Although such an enrichment could be attributed to a phase segregation and even has been predicted theoretically,40,41 the validity of this interpretation becomes questionable due to two observations: no complementary depletion in the intensity of the thiophene fragment in the blend layer and the presence of a similar intensity enhancement for a pure PC70BM layer. Furthermore, the positive fullerene depth profile in Figure 5b does not exhibit

Figure 4. (a) Secondary ion yields in the steady state of the characteristic fragments from both the polymer and the fullerene materials, as obtained on the PDPP5T:PC70BM system using a 2.5 eV/ atom Ar-cluster sputter beam. (b) The secondary ion yields, in the steady state region, of the negative fullerene ion as a function of the layer composition in each of the polymer matrix employed. The fall in the intensities beyond a certain PCBM loading of the sample is attributed to the matrix effect. D


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The Journal of Physical Chemistry C an enhancement of a comparable order. When the PDPP5T: PC70BM layer is deposited on a Si substrate, no such increase in the fullerene related fragments can be seen for either of the polarities (Figure 5b). This indicates that the enhancement of the negative PC70BM-related ion intensity is most likely a matrix effect linked to the underlying TiO2 layer and not indicative of any segregation. To avoid the matrix effects and assess the existence of any enrichment at the interface, XPS depth profiles for a pure PCBM and a 75 wt % PCBM (25 wt % PDPP5T) layer deposited on a TiO2/ITO substrate, as well as the mixed layer on a Si substrate, were measured. A direct comparison between the XPS and TOF-SIMS depth profiles might be obscured by the broader cluster size distribution of MAGCIS as opposed to the ION-TOF GCIB. However, our findings demonstrate that the interface enhancement manifests itself in every depth profile obtained within the 1.5−4.5 eV/atom range, which is likely to accommodate the broad range of cluster sizes from MAGCIS (see the Supporting Information). This indicates that the interface enhancement is not very sensitive to the sputter conditions and provides support to the validity of a comparison between the ToF-SIMS and XPS results. Quantitative analysis into the depth of the layers was performed using an Ar-cluster source by XPS whereby the C 1s peak was deconvoluted using a linear combination of reference spectra from the PCBM and PDPP5T compounds. As shown in Figure 6a, for the pure PCBM layer on TiO2, it is necessary to include an additional component in the C 1s spectra at the interface, suggesting an interaction between the PCBM and the TiO2 layer. On the other hand, for the PDPP5T:PCBM layer, some PCBM/ PDPP5T segregation is observed at the interface when deposited on TiO2 and not on Si as shown in Figure 6b.

Figure 6. XPS depth profile analysis performed using Ar-cluster sputtering. (a) C 1s peak as a function of layer depth of a pure PC70BM layer on a TiO2/ITO substrate. Inset: the C 1s peaks in the XPS spectrum. (b) C 1s peaks as a function of layer depth from a 75 wt % PC70BM layer on Si and TiO2/ITO substrates. The C 1s is deconvoluted using a linear combination of PCBM and PDPP5T reference spectra. On the TiO2/ITO substrate, an enhancement of the C 1s from the PC70BM and a depletion of the C 1s peak from the PDPP5T indicate a PC70BM enhancement at the interface. However, no enhancement or depletion of either component is seen on the layer with the Si substrate.

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DISCUSSION Sputtering Yields. The sputtering yields of bulk heterojunction layers have been shown by Fleischman et al.26 to be nonlinear with respect to the PCBM concentration. The nonlinearity of sputter yield has been explained by associative interactions between the two components of the blend in a model proposed by Seah et al.42 The data obtained on both the PDPP5T:PC70BM, (Figure 2) and the MDMO-PPV:PC60BM material systems show a similar nonlinear dependence. On the basis of fitting the data with the universal equation for sputtering yields with Ar-cluster beams, cf. Figure 3b, we find that the parameter A decreases linearly when the PC70BM fraction in the layer is increased. Since this parameter is associated with the removal energy per atom for an average fragment,37 this implies that the mean sputtered fragment size increases with the PC70BM fraction in the layer. This is corroborated by the fact that the characteristic ionic fragments from the fullerene derivatives are the gigantic C70 and intact PC70BM molecules, while those from the polymer materials are tiny thiophene units. Hence, the increasing dominance of these large fragments in the sputtered flux will increase the average sputter yield. This is also in agreement with reports where the effects of molecular weights of polymeric materials on the sputter yield have been studied.43,44 In the sputter threshold model, we find that the effective sputter threshold decreases as the PC70BM fraction in the layer is increased, cf. Figure 3b. This trend is also intuitively expected, since polymeric materials are held together by strong covalent bonds as opposed to the weak van der Waals forces that hold the fullerene derivatives together. Therefore, the

sputter threshold would be expected to decrease as the PC70BM fraction in the sample was increased and average binding forces would decrease. The sputtering yields of polymer:fullerene layers can be approximated by both models. The residuals seen in Figure 3a show that the sputter threshold model has a slightly closer fit to the data obtained. Although the trends of the parameters obtained strengthen the postulate that the parameters have a dependence on material properties, the existence of a sputter threshold, which is the fundamental tenet of contention between these two relationships, cannot be established by the data from polymer:fullerene layers as the measurements are primarily in the linear E/n regime where the two models effectively agree. Matrix Effect. Fleischmann et al.26 demonstrated the bellshaped trend of the steady state negative fullerene ion intensities as a function of the PCBM concentration in the film. The initial increase in the fullerene ion yields, as seen in Figure 4a, can be correlated with the increased fraction of the parent species in the layer. The subsequent fall of the negative fullerene ion intensities, seen in layers with a higher PCBM loading, needs to be explained. Although one could attempt to E


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The Journal of Physical Chemistry C justify this by cross-linking between the fullerenes at high PCBM loadings, leading to reduced ion yields, the data obtained on the PDPP5T:PC70BM in the positive polarity, as shown in Figure 4a, suggest that, beyond a certain PCBM loading, positive ionization of PCBM is favored as opposed to negative ionization. This rules out the possibility of crosslinking between the fullerene derivatives as the main matrix mechanism in films with higher PCBM loading and indicates that the matrix effect is induced by the interaction between the polymer donor material and the fullerene acceptor. It was also speculated by Fleischmann et al.26 that the matrix effect of the PCDTBT and P3HT on the PCBM fragments could be due to the electronic properties of the thiophene unit which is common to both materials. To test this hypothesis, we employed a non-thiophene donor, MDMO-PPV. The trend of the negative fullerene ion from a matrix of MDMO-PPV as a function of the PC60BM loading is presented in Figure 4b. Since MDMO-PPV does not contain the thiophene unit, and the matrix effect on the negative fullerene ion still exists, other effects of the donor matrix need to be considered. Since we strive toward a generalized model, it is important to define a quantity that is independent of the analysis conditions. For this purpose, we look at the secondary ion yield of the daughter as a fraction of the number of parent molecules in the analyzed volume, which we will refer to as the fractional secondary ion yield. fractional secondary ion yield fullerene secondary ion yield = PCBM molecules in the analysis volume

In a system without a matrix effect, the fractional secondary ion yield would remain the same regardless of the composition of the layer. However, the fact that, in the systems studied here, the fractional secondary ion yield varies with the layer composition is evidence of a strong matrix effect in polymer−fullerene binary blends. The fractional secondary ion yield shows, in all cases, an exponential decrease with increasing PCBM composition, cf. Figure 7a, which can be quantified by a characteristic decay parameter. The exponential gives an excellent description of the data for a PCBM concentration equal to or higher than 20%, but it is clear that this cannot be extrapolated to the dilute regime, below 1% PCBM, where the fractional ion yield may be expected to be constant. A possible explanation for the decline in the fractional secondary ion yield of the negative fullerene ion would be that the polymer material acts as the source of electrons for the negative ionization of PCBM and fullerene molecules. An increase of the PCBM fraction in the layer would decrease the interfacial area between the polymer and the PCBM material (per PCBM molecule), thereby, greatly reducing the interfacial area where an electron can be transferred from the donor material to PCBM. Consequently, the negative ionization probability of PCBM is reduced and, with this, the fractional secondary ion yield of the PCBM related species decreases. Since the decay parameter of the fractional secondary ion yield is a function of the donor material, we speculate that this trend is induced by a variation in the ionization probability resulting from different charge distributions between the fullerene-containing acceptor molecules and the donor species depending on the material properties of the donor. To approximate the electron density of the donor matrix, we consider that the highly electronegative

Figure 7. (a) The fullerene ion yield per PCBM molecule in the analyzed volume as obtained from various PCBM loadings on the 4 different systems studied. Also depicted by lines are the first-order exponentials approximating the data obtained. (b) The decay parameter of the exponentials shown in (a) as a function of the sum of the O-N-S electronegativity in the donor material divided by the monomer mass.

nonconjugated elements in the donor material affect the transfer of electrons from the polymer matrix to the fullerene. To exemplify this, we introduce an ONS electronegativity factor (ONS EN) defined as the sum of the electronegativitieson the Pauling electronegativity scaleof O, N, and S in the donor material normalized by the mass of the monomer. This is fundamentally equivalent to normalizing toward the volume of the donor material as only small variations of mass densities are observed. One would thus expect that, as the electronegativity per volume of the donor material increases, the fractional secondary ion yield decreases and, thus, the decay parameter would decrease. Figure 7b shows clearly this expected relation between the ONS EN factor and the decay parameter of the exponential fit. Moreover, this relation is linear. Interface Phenomenon. The intensity enhancement of PCBM-related fragments at the interface can be explained by the formation of interface dipoles as a result of the energy-level alignment. Comparing the energy diagrams of the materials used in this study, as depicted by Figure 8a, it is clear that the lowest unoccupied molecular orbital (LUMO) of TiO245,46is offset by only a small amount, 0.1 eV, below the LUMO of PC70BM.47 A transfer of electrons from the LUMO of TiO2 to F


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negative fullerene ion due to the presence of the donor material. Furthermore, we have also quantified the matrix effect by means of an exponential decay of the fractional secondary ion yield. The decay constant shows a linear relationship with the electronegativity of the O, N, and S species per volume of the polymer. This relationship could serve as a predictive model for the matrix effect on the negative fullerene ion in a given donor matrix. We provide a model for the anomalous interfacial enhancement of the negative fullerene based on the electron transfer from the TiO2 substrate to the PCBM.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b09911. Depth profiles obtained on a 75 wt % PCBM layer by Arn+ sputter beams ranging from 1.5 to 4.5 eV/atom (PDF)

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Figure 8. (a) Energy band diagram of the PC70BM:DPP5T/TiO2 layers. The negative charge transfer state of PC70BM (ICT-) is depicted by a dotted line. (b) Energy band diagram of PC70BM: DPP5T layers on Si. Also shown for reference is the work function of MoO3.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Supriya Surana: 0000-0001-7495-0672 Notes

The authors declare no competing financial interest.

the LUMO of PCBM is forbidden. However, it has been demonstrated that the negative charge transfer state in PCBM is ∼4.2 eV48 and is, thus, lower than the LUMO of TiO2. In this case, a transfer of an electron from the LUMO of TiO2 to the negative charge transfer state of PC70BM is energetically favorable.49 The integer charge transfer model postulates that electrons flow spontaneously from TiO2 to the PCBM.49 This flow of electrons causes the PC70BM in the proximity of the TiO2 interface to become enhanced in negative charge, while the TiO2 becomes positively charged until an equilibrium is reached. Potentially, this enhanced negative charge on the PC70BM increases the negative ionization probability of the PC70BM molecules at the interface, leading to the increased intensity of these negative ions at the interface Vice versa, in the case of the PDPP5T:PC70BM layer on the Si substrate, the work function of Si is sufficiently lower than the LUMO of PC70BM, cf. Figure 8b, such that the transfer of electrons from the Si substrate to PCBM is energetically unfavorable, explaining the absence of the intensity enhancement at the interface.

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ACKNOWLEDGMENTS We gratefully acknowledge the funding for this work through the European Metrology Programme for Innovation and Research (EMPIR) Project 3DMetChemIT. The EMPIR initiative is cofunded by the European Union’s Horizon 2020 research and innovation programme and the EMPIR Participating States.

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REFERENCES

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CONCLUSIONS The sputtering yields of polymer:fullerene layers, and their dependence on argon cluster ion energy and cluster size, can be estimated by both the universal Ar-sputtering equation and the sputter threshold model proposed by Seah37 and Cumpson,38 respectively. The fitting parameters obtained in either case show an unambiguous dependence on the layer composition. For both models, the critical energy parameter decreases with increasing PCBM content. Quantification of the composition of such layers simply using ion intensities is impeded by the matrix effect. The latter has been quantified in terms of the fractional secondary ion yield, i.e., fullerene ion yield per parent PCBM molecule. Using this quantity, we have provided an understanding of the matrix effect in terms of a change in the ionization probability of the G


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