Chemical Analysis of the Interface in Bulk ... - ACS Publications

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Chemical Analysis of the Interface in Bulk-Heterojunction Solar Cells by X‑ray Photoelectron Spectroscopy Depth Profiling Yan Busby,*,† Emil J. W. List-Kratochvil,‡ and Jean-Jacques Pireaux† †

Research Center in the Physics of Matter and Radiation (PMR), Laboratoire Interdisciplinaire de Spectroscopie Electronique (LISE), University of Namur, B-5000 Namur, Belgium ‡ Institut für Physik, Institut für Chemie & IRIS Adlershof, Humboldt-Universität zu Berlin, Brook-Taylor-Straße 6, 12489 Berlin, Germany S Supporting Information *

ABSTRACT: Despite the wide use of blends combining an organic p-type polymer and molecular fullerene-based electron acceptor, the proper characterization of such bulk heterojunction materials is still challenging. To highlight structure-to-function relations and improve the device performance, advanced tools and strategies need to be developed to characterize composition and interfaces with sufficient accuracy. In this work, high-resolution X-ray photoelectron spectroscopy (XPS) is combined with very low energy argon ion beam sputtering to perform a nondestructive depth profile chemical analysis on full Al/P3HT:PCBM/PEDOT:PSS/ITO (P3HT, poly(3hexylthiophene); PCBM, [6,6]-phenyl-C61-butyric acid methyl ester; PEDOT, poly(3,4-ethylenedioxythiophene; PSS, polystyrenesulfonate; ITO, indium tin oxide) bulk-heterojunction solar cell device stacks. Key information, such as P3HT and PCBM composition profiles and Al-PCBM chemical bonding, are deduced in this basic device structure. The interface chemical analysis allows us to evidence, with unprecedented accuracy, the inhomogeneous distribution of PCBM, characterized by a strong segregation toward the top metal electrode. The chemical analysis highresolution spectra allows us to reconstruct P3HT/PCBM ratio through the active layer depth and correlate with the device deposition protocol and performance. Results evidence an inhomogeneous P3HT/PCBM ratio and poorly controllable PCBM migration, which possibly explains the limited light-to-power conversion efficiency in this basic device structure. The work illustrates the high potential of XPS depth profile analysis for studying such organic/inorganic device stacks. KEYWORDS: bulk heterojunction, solar cells, XPS, P3HT:PCBM, depth profile, chemical analysis, low-energy ion beams

1. INTRODUCTION Organic bulk-heterojunction solar cells based on a conjugated polymer acting as an electron donor, and a fullerene derivative acting as an electron acceptor, have been intensively investigated for many device applications, particularly in photovoltaics.1 After the original discovery, scientific interests stretched from the fundamental aspects related to charge transfer and charge separation processes in the bulk heterojunction to the technological development of costeffective large area deposition processes based on roll-to-roll printing or coating.2 Within polymer-based solar cells, devices based on blends of poly(3-hexylthiophene) (P3HT) and fullerene derivatives, such as [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), are by far the most studied. Simple device structures based on P3HT:PCBM have shown power conversion efficiencies up to 4%; however, the device typically suffers from poor reproducibility and limited lifetime. In such a donor−acceptor system, the possibility to achieve fast and efficient charge transfer by exciton splitting is known to critically depend on the morphology of the two interpenetrating networks. Indeed, a uniform distribution of the heterojunction components would ensure an efficient exciton splitting and limit the internal exciton recombination. The © 2017 American Chemical Society

performance of P3HT:PCBM solar cells has shown a clear correlation with the P3HT crystallinity, molecular weight (MW),3−5 regioregularity,6 and the P3HT:PCBM mass ratio.7,8 Postdeposition, thermal, or solvent annealing is generally performed to increase the P3HT crystallinity and optimize the heterojunction morphology, resulting in enhanced device stability and lifetime.9 Efficiency of P3HT:PCBM devices has also been frequently related to the vertical distribution of PCBM within the active layer;10,11 in this context, it is highly desirable to develop strategies allowing for a direct and easy evaluation of the distribution of P3HT:PCBM in a device stack. Since its invention almost 50 years ago, X-ray photoelectron spectroscopy (XPS) or Electron Spectroscopy for Chemical Analysis (ESCA) has developed as a key and unique technique to analyze materials: its capability to deliver quantitative chemical composition, with utmost surface sensitivity when probing solid materials, resulted in a continuous instrumental development and at the same time a broadening and a focusing of its extremely diverse fields of applications. For the “nano” Received: November 17, 2016 Accepted: January 10, 2017 Published: January 10, 2017 3842

DOI: 10.1021/acsami.6b14758 ACS Appl. Mater. Interfaces 2017, 9, 3842−3848

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ACS Applied Materials & Interfaces

layers (without top electrode) with Ar cluster ion beam.22 Both surface energy considerations and charge-transfer mechanisms have been evoked to play a role in promoting the diffusion of PCBM toward metal electrodes. This is why, in an inverted solar cell structure,23 the hole transport layer (poly(3,4ethylenedioxythiophene):polystyrenesulfonate, PEDOT:PSS) is inserted between the P3HT:PCBM layer and the top electrode to reduce the PCBM segregation and increase the power conversion efficiency.24 In this work, we have performed the in-depth chemical analysis on full solar cell stacks; the P3HT vertical distribution is evaluated from the S signal after subtracting the contribution from the PEDOT:PSS, while the PCBM distribution is evaluated from the O signal after subtracting the contribution from Al2O3 and PEDOT:PSS. Our strategy resides in the fact that, if two atomic elements, A and B, are bound to form a compound ABγ, high-resolution XPS spectra of both A and B core levels will include a component (a1 and b1) relative to this specific chemical state. Now, if the peak a1 overlaps with signals from other chemical components (a2, a3...), its atomic percentage (%a1) can still be evaluated from the peak analysis on the other atomic element (%b1), when knowing the proportionality factor (γ) which relates the two atomic percentages (i.e., %a1= γ·%b1). In the proposed methodology, the proportionality factor γ is experimentally determined so that the overall procedure has the advantage of not being critically dependent on the XPS peak-fitting parameters. In the data analysis discussed in the next sections, the challenge will be to prove that XPS coupled with low-energy monatomic ion beam is capable of offering a truly valuable analysis at the nanoscale of such multilayered device stack containing only C, O, and S atoms.

era, stand-alone XPS equipment does not provide a sufficient lateral resolution capability: instruments are struggling to reach the micrometer lateral resolution, clearly forbidding interesting information at the nanoscale in chemical images. This is not true for the in-depth study of multilayered materials or stacks containing different superimposed films with typical thicknesses in the nanometer range. Because of its high surface sensitivity, or short probing depth (of a few nanometers), XPS has been teamed up with ion sputtering to get unprecedented quantitative in-depth information; for comparison, the XPS detection limit is not as good as with secondary ion mass spectrometry (dynamic SIMS and ToF-SIMS) techniques which are, however, only semiquantitative and XPS analysis is not as fast and as laterally resolved as with Auger spectroscopy which, however, cannot be applied to study organic or polymer materials. In depth profiles on organic materials, the use of Ar+ ion beam is usually not recommended as it may heavily damage or induce chemical modifications in the layer, eventually resulting in charring which impedes even a simple elemental indepth analysis. Argon cluster (Arn+) ion beams are ideal for profiling organic films;12 however, hybrid interfaced materials are not easily profiled since only very high-energy Arn+ clusters (above 20 keV) are able to sputter metal or oxide layers.13 Ar possibility for profiling hybrid and biological films, developed with ToF-SIMS, is by using a very low energy (250−500 eV) Cs+ beam.14−17 In this work, a very low energy (500 eV) monatomic Ar+ beam is used to preserve chemical information and extract the in-depth distribution of P3HT and PCBM compounds in a full bulk-heterojunction solar cell which is shown, together with the molecular structure of its main constituents, in Figure 1. The

2. EXPERIMENTAL SECTION Samples were prepared with two P3HT molecular weights (MW), namely, samples C-1 and C-2, which correspond to a low MW (25 000−45 000 g·mol−1), while C-3 corresponds to a high MW (45 000−65 000 g·mol−1). Solar cells with structure Al/P3HT:PCBM/ PEDOT:PSS/ITO/glass (ITO, indium tin oxide) have been deposited, tested, and analyzed in the same conditions. Regioregular P3HT and PCBM (Sigma Aldrich, St. Louis) have been mixed in equal masses (R = 1:1) without further purification, dissolved in chlorobenzene (18 mg/mL), heated up to 50 °C for 3 h, and continuously stirred for 12 h at room temperature to promote the complete dissolution of the components. Glass/ITO substrates (Delta Technologies Limited, Loveland, CO) have been cleaned in acetone and isopropanol ultrasonic baths for 10 min. Poly(3,4ethylenedioxythiophene):poly(styrenesulfonate), PEDOT:PSS Clevios VP Al 4083 (Heraeus), was filtered through a polyvinylidene fluoride filter (0.45 μm) and spin-coated at 2500 rpm for 40 s. To prevent any further water and oxygen contamination during the fabrication, samples were transferred into a glove box and further processed under inert Ar atmosphere. Residual water within the PEDOT:PSS layer was removed by thermal annealing at T = 120 °C for 20 min under vacuum. The P3HT:PCBM polymer blend is then spin-coated at 1200 rpm for 120 s. The thickness of the PEDOT:PSS layer (∼50 nm) and the P3HT:PCBM blend (∼80 nm) were measured by tapping-mode atomic force microscopy on a scratched film (Dimension V, Veeco Digital Instruments, Plainview, NY). The Al top electrode was finally deposited via thermal evaporation of Al at a pressure below 10−6 mbar through a shadow mask, yielding a final thickness of 200 nm and an electrode area of 0.1 cm2. The whole device stack was successively exposed to thermal annealing at T = 150 °C for 15 min to trigger the crystallization and the diffusion of P3HT and PCBM to form the final nanomorphology of interpenetrating electron donor and electron acceptor material phases. Sample were subsequently transferred to an Ar-filled glove box for device testing

Figure 1. Conventional P3HT:PCBM organic bulk-heterojunction solar cell device stack and chemical structure of its main constituents.

PCBM segregation toward the top metal electrode has been previously reported in P3HT:PCBM blends by different techniques:18 XPS surface analysis was applied af ter delaminating the top electrode to measure the intensity of the S 2p peak, from thiophene, at the surface of the active layer; thermal annealing was shown to be associated with a considerable lowering of the sulfur signal, that is, a top segregation of PCBM.19 More recently, XPS depth profiling of the Al/ P3HT:PCBM interface revealed the presence of three chemical components (doublets) corresponding to Al2O3, Al−O−C, and Al−Al chemical environments.20,21 Furthermore, the effect of thermal annealing on the vertical distribution of P3HT and PCBM was recently studied by depth profiling spin-coated 3843


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ACS Applied Materials & Interfaces under the solar simulator illumination (SolarTest 1200 Atlas Material Testing Technology, USA) providing an AM 1.5G spectrum with an intensity of 100 mW/cm2. Current density−voltage (J/V) measurements were carried out with a Keithley 2400 source unit driven by a customized software routine written in LabVIEW (National Instruments, Austin, TX). XPS analysis (K-Alpha, by Thermo Scientific, East Grinstead, England) was performed with a monochromatic (Al Kα) X-ray beam, on a 300 × 300 μm spot area in a spectrometer equipped with a flood gun for charge compensation. Solar cells were profiled to determine the in-depth distribution of atomic species; a low-energy (500 eV) monatomic Ar+ beam was used to reduce as much as possible ion beam damages and preserve chemical information. To generate the depth profile, survey scans and high-resolution spectra of aluminum (Al 2p), carbon (C 1s), oxygen (O 1s), sulfur (S 2p), and indium (In 3d) were acquired at each profile step. As it will be described in details in the following section, the quantity of P3HT is successively evaluated from the spectral analysis of the S 2p region, while the PCBM one is deduced from the total measured oxygen atomic percent after subtraction of the atomic percentages of oxygen to be accounted for alumina, ITO, and PEDOT:PSS. Because of the strong overlap between O 1s spectrum components, the chemical analysis has been quantitatively deduced from XPS spectral analysis from complementary elements, namely, Al 2p, S 2p, and In 3d. Once the respective components are accurately fitted, the proportionality factors with oxygen are experimentally determined at convenient profile depths. More precisely, fitting parameters of each component such as the binding energy, full width at half-maximum (fwhm), and line shape (Lorentz/Gauss, L/G ratio) are determined at a convenient profile depth, far from interfaces, where no overlap occurs; fit conditions are then propagated to all profile levels, allowing for small constraints variations (typically 0.1 eV on energies and few tens of electronvolts on fwhms) to have a good peak fit. Shirley-type background was always used except for Al spectrum for which a linear background was preferred for achieving more stable and reliable fits along the depth profile. The choice of fit parameters (background selection, FMHM, L/G···) can sensibly affect the quantification in XPS; however, as will be clear from the following discussion, by experimentally determining proportionality factors (later referred as γ), the final P3HT and PCBM distributions result remarkably in being less affected by the choices made during the peaks fitting.

Figure 2. J/V curves of solar cells made with low-molecular-weight (C1, C-2) and high-molecular-weight (C-3) P3HT. Full lines correspond to devices under illumination and dotted curves to dark conditions.

Table 1. Survey of Solar-Cell-Related Figures of Merits Extracted from the J/V Characterizationa solar cell

VOC (±10) [mV]

JSC (±0.1) [mA/cm2]

FF (±0.02)

PCE (±0.1) [%]

C-1 C-2 C-3

605 593 516

7.12 7.20 9.04

0.45 0.44 0.44

1.93 1.87 2.05

a

C-1 and C-2 solar cells are prepared with low MW P3HT, while C-3 is prepared with a higher MW P3HT.

500 eV energy Ar+ beam; this allows one to roughly linearly correlate the sputtering time with the depth. Indeed, this is confirmed in this experiment, where a sputtering rate of about 0.13 nm/s is obtained on an Al electrode and 0.08 nm/s in P3HT:PCBM layer. Interestingly, very similar composition profiles are obtained in our cells, motivating the need for a more detailed chemical analysis to extract valuable information. In Figure 4 we report the detailed peak-fitting procedure on Al 2p spectra acquired at different sputtering depths during the profile of C-3. In agreement with ref 1, at the Al/P3HT:PCBM interface, three chemical states are clearly identified in the Al 2p spectrum, namely, Al0 and Al2+, corresponding to Al−O−C, that is, Al bound to PCBM and Al3+ corresponding to Al2O3. As shown in Figure 5a,b, profiles of C-1 and C-2 samples, deposited in exactly the same conditions, show a remarkably variable Al2+ signal intensity. This component, testifying to the chemical bonding of Al-PCBM, results in being very intense at the Al/P3HT:PCBM interface in samples C-1 and C-3, while it is almost absent in the profile of sample C-2. This is thus a parameter which is clearly poorly reproducible. Chemical Analysis (P3HT and PCBM Profiles). To extract quantitative chemical information from depth profiles, it is mandatory to exclude ion-beam-induced chemical modifications. Here, a low-energy Ar+ ion beam was used to limit the degradation of organic molecules; to ensure this, the ratio between peak components in high-resolution spectra was checked to remain constant as long as the profile is in the same layer. For example, the ratio between two sulfur components relative to thiophene (PEDOT) and sulfonate (PSS) was checked to be constant within the PEDOT:PSS layer, that is, far

3. RESULTS AND DISCUSSION Solar Cells Characterization. Solar cells have been characterized in an inert atmosphere. Resulting J/V characteristics are shown in Figure 2. From this data, the most important figures of merit are extracted (see Table 1). As a general result, extracted from results from a few tens of cells, samples made with a high MW P3HT exhibit a higher short-circuit current (JSC), about 15% lower open-circuit voltage (VOC), resulting in a higher total power conversion efficiency (%PCE) as compared to devices based on low MW P3HT. Besides the reported absolute values, these solar cells also exhibit a lack in reproducibility and stability; excluding the low performance of two shortened cells, within the eight cells simultaneously deposited on each glass/ITO substrate, we observed a 17% and 6% variation in the PCE of high and low MW cells, respectively. In the following section, interface analysis will be performed to highlight the possible origin of such device-to-device variations and reliability issues. Interface Analysis. Solar cells interfaces are easily identified in XPS depth profiles by following the atomic percentage (at. %) evolution of C, Al, O, S, and In elements as a function of the sputtering time (see Figure 3, and example of raw data are reported in Figure S2, Supporting Information). According to our experience, similar sputtering rates are generally obtained on organic and inorganic materials with a 3844


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Figure 3. Atomic composition profiles (LOG scale) from C-1 (a) and C-3 (b) solar cells performed by 500 eV Ar+ beam sputtering; interfaces are identified from at. % of oxygen, carbon, aluminum, sulfur, and indium evaluated from the respective high-resolution spectra (O 1s, C 1s, Al 2p, S 2p, and In 3d).

Figure 4. High-resolution Al 2p spectra acquired at different sputtering time (depth) of device C-3 close the Al/P3HT:PCBM interface for a sputtering time of 1200 s (a), 1290s (b) and 1380s (c). Three oxidation states are clearly identified, namely metallic Al (Al0), Al bond to PCBM (Al2+) and Al from Al2O3 (Al3+). The total Al at% are indicated in the top left corner of each panel.

Figure 5. XPS chemical profiles showing the at. % evolutions (in LOG scale) of aluminum (Al0, Al2+, and Al3+), carbon (C), sulfur, (S), and oxygen (O) at the Al/P3HT:PCBM interface for the solar cells C-1 (a), C-2 (b), and C-3 (c). A strong variability is observed in the Al2+ intensity.

from interfaces (see Figure 6). Moreover, the effects of lowenergy Ar+ sputtering on a pure P3HT layer have been investigated and show that after 250 s of sputtering the C 1s and S 2p peaks shape is not modified and C/S composition is constant within 10% (see Figure S1 in the Supporting Information). A consistent peak-fitting methodology was developed to quantitatively determine the in-depth distribution of P3HT and

PCBM from their specific atomic element (S and O, respectively). The P3HT distribution is thus evaluated from the spectral analysis of S 2p, by subtracting from the total S signal (STOT), contributions from the underlying PEDOT:PSS hole extraction layer and from Al2S3 due to the atmosphere contamination of the Al electrode (see the S profile in Figure 3). At the bottom interface, sulfur is present in P3HT (SP3HT), PEDOT (SPEDOT), and PSS (SPSS), that is, %STOT = %SP3HT + 3845


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Figure 6. Typical XPS high-resolution S 2p spectra measured at different profile depths, namely, at the Al/P3HT interface (a) in the P3HT:PCBM layer (b) and deep in the PEDOT:PSS layer (c).

Figure 7. Depth profile chemical analysis of Al/P3HT:PCBM/PEDOT:PSS layers from solar cells C-1 (a), C-2 (b), and C-3 (c). The XPS at. % distribution of %SP3HT (black), %OPCBM (red), Al bound to PCBM (Al2+, green), and the smoothed SP3HT/OPCBM ratio (S/O, blue) within the P3HT:PCBM layer are displayed. Interfaces with Al and PEDOT:PSS are indicated by the gray-shaded regions.

evaluation of γ1 since it allows counterbalancing possible errors related to the peak fitting, in particular, the determination of the exact peak full width at half-maximum (fwhm). The second proportionality factor γ2 = OTOT/In is determined in the same way deep in the ITO layer (note that no fit is needed in the In peak because the proportionality factor is calculated with respect to the total In%). Finally, γ3 = (%OTOT − γ2·%InITO)/ SPSS is evaluated in the PEDOT:PSS/ITO region. Resulting at. % profiles of %SP3HT and %OPCBM, %SP3HT/%OPCBM ratio (S/ O), and %Al2+ (∼Al−PCBM) are reported in Figure 7. The % OPCBM profile indicates that PCBM tends to segregate toward the Al electrode in each cell; the top segregation of PCBM does not appear as a limiting factor for the PV performance as far as a sufficient PCBM concentration is present in the active layer. From the comparison of the profiles in C-1, C-2, and C-3 solar cells, a higher S/O ratio is observed in C-2 close to the PEDOT:PSS interface. This inhomogeneous heterojunction composition is thought to explain the slightly lower efficiency compared to that of the C-1 cell. P3HT and PCBM profiles in C-3 suggests that such a strong surface segregation of PCBM could favor the electron collection at the Al electrode and allow for a higher JSC value with respect to samples prepared with low-MW P3HT. The relation between the PCBM segregation, the solar cell efficiency, and the P3HT MW is not straightforward; however, a higher segregation toward the top electrode and a more symmetrical (triangular-shaped) S/O distribution is clearly obtained in the high MW cell C-3 (Figure 7c), while the S/O is more than 2.5 times higher in sample C-2

%SPEDOT + %SPSS. Despite the obvious superposition of thiophene-related S 2p components (SPEDOT and SP3HT), in PEDOT:PSS, SPEDOT is proportional to the well-separated sulfonate component (SPEDOT = α SPSS) and this proportionality factor (α) was experimentally determined deep in the PEDOT:PSS layer. SPEDOT and SPSS contribution could be thus isolated and then subtracted from the total sulfur to get the P3HT distribution, %SP3HT = %STOT − γB %SPSS (with γB = 1 + α). Indeed, the at. % profile of %SP3HT through the whole profile is derived from the obvious relation %SP3HT = %STOT − γA(%AlAl2S3) − γB(%SPSS). A further check on γA and γB values was made by verifying that the SP3HT signal vanishes at the top and bottom interfaces. Similarly, oxygen from PCBM (% OPCBM) is evaluated from the total oxygen signal (OTOT) after subtracting the contributions from (a) Al2O3, evaluated on the Al2O3 component, (b) ITO, evaluated on the total In peak (since the In2+ component is proportional to the total indium content, no peak fit was necessary), and (c) PEDOT:PSS, evaluated from the sulfonate component (again, the oxygen contribution from PEDOT is proportional to that of PSS, so no interference occurs with P3HT), through the equation %OPCBM = %OTOT − γ1·%Al3+ − γ2·%In − γ3·%SPSS. The low at. % of OPCBM requires an accurate evaluation of proportionality factors γ1−3. For Al2O3, the theoretical ratio γ1 = %O/%Al3+ is obviously 1.5; however, its experimental evaluation (resulting in values typically differing up to 10% from the theoretical value), made at the top of the Al2O3 layer, that is, where oxygen signal comes exclusively from alumina, results in more accurate 3846



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close to the PEDOT:PSS interface (Figure 7b). The effect of metal bonding of PCBM is not evident from our results and would need further investigations. Results suggest that a more extended PCBM diffusion occurs in low-MW P3HT cells, resulting in a more irregular PCBM distribution which correlates well with the lower %PCE and JSC values. To summarize, surface segregation of PCBM appears to mainly influence the PV performance by modifying the P3HT:PCBM heterojunction composition and the charge collection at upper and lower interfaces. Depth profile XPS analysis highlights poor control of the P3HT and PCBM distributions in such a device structure which correlates well with the poor reproducibility of the device-to-device performance.

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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/acsami.6b14758. Results from the profile with 500 eV Ar+ beam on a P3HT/Si sample; raw data of the XPS profile of sample C-1 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.B.). ORCID

Yan Busby: 0000-0002-6826-6142 4. CONCLUSION XPS depth profile analysis was successfully applied to full conventional Al/P3HT:PCBM/PEDOT:PSS/ITO bulk-heterojunction solar cells to derive, with high accuracy, the in-depth distribution of P3HT and PCBM. Low-energy Ar+ sputtering beam was used to profile such hybrid interfaces without sensibly degrading organic layers. Contributions from Al2S3 and PEDOT:PSS have been subtracted from the total S signal to determine the profile of sulfur from P3HT (%SP3HT), while oxygen from Al2O3, ITO, and PEDOT:PSS have been subtracted from the total oxygen signal to isolate the contribution from PCBM (%OPCBM). Results evidence a marked PCBM segregation toward the top Al metal electrode in each device and chemical analysis evidenced the formation direct bonds between PCBM and Al. The PCBM-Al bonding results in being a poorly controllable effect with no evident consequences on the device performance. The correlation between the P3HT and PCBM distribution and the solar cell parameters extracted from J−V curves is not straightforward: the segregation of PCBM seems to be associated with two competing effects: on one hand, a high JSC is obtained where a rather strong PCBM segregation occurs toward the top electrode, suggesting that a more efficient charge collection occurs; on the other hand, if the PCBM content left at a certain depth in the active layer is “too low”, then the cell efficiency lowers. The SP3HT/OPCBM ratio has been identified as the most relevant parameter for evidencing such a “too low” PCBM content. In particular, in In low-MW cells, the lower efficiency was associated with a less-symmetrical SP3HT/OPCBM distribution with values exceeding 10, which correspond to regions with a particularly low PCBM content. Further dedicated experiments would be helpful in elucidating more details on the effects of a nonoptimized heterojunction composition on the cell efficiency. In this sense, our strategy could be easily applied to a larger family of devices based on more complex bulk heterojunctions and thus illustrate the high potential of highresolution XPS depth profile analysis combined with lowenergy ion beams for performing quantitative chemical interface analysis on such material stacks. This characterization has the potential to elucidate structure-to-function properties, guide, and inspire the design of more performing and reliable bulk heterojunction devices. For this, more controlled systematic studies are needed to associate the molecular components distribution with the device performance and identify the effects of thermal treatments, aging, or the addition of functional buffer layers, such as LiF, Ca, and PFN, which are commonly inserted between the P3HT:PCBM layer and the metal electrode.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Authors contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge F. Kolb for the preparation and J−V characterization of solar cells. REFERENCES

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