High-Resolution Kelvin Probe Force Microscopy Imaging of Interface

High-Resolution Kelvin Probe Force Microscopy Imaging of Interface Dipoles and Photogenerated Charges in Organic Donor− Acceptor Photovoltaic Blends Franz Fuchs,†,‡,§,∥ Florent Caffy,†,‡,§ Renaud Demadrille,†,‡,§ Thierry Mélin,⊥ and Benjamin Grévin*,†,‡,§ †

Univ. Grenoble Alpes, ‡CNRS Alpes, and §CEA, INAC-SPrAM, F-38000 Grenoble, France ⊥ Institut d’Electronique, Microélectronique et Nanotechnologie (IEMN), CNRS UMR 8520, CS 60069, Avenue Poincaré, 59652 Villeneuve d’Ascq, France S Supporting Information *

ABSTRACT: We present noncontact atomic force microscopy and Kelvin probe force microscopy studies of nanophase segregated photovoltaic blends based on an oligothiophene−fluorenone oligomer and [6,6]-phenyl C70 butyric acid methyl ester. We carried out a complete analysis of the influence of the tip−surface interaction regime on the topographic, in-dark contact potential and surface photovoltage contrasts. It is demonstrated that an optimal lateral resolution is achieved for all channels below the onset of a contrast in the damping images. With the support of electrostatic simulations, it is shown that in-dark contact potential difference contrasts above subsurface acceptor clusters are consistent with an uneven distribution of permanent charges at the donor−acceptor interfaces. A remarkable dependence of the surface photovoltage magnitude with respect to the tip−surface distance is evidenced and attributed to a local enhancement of the electromagnetic field at the tip apex. KEYWORDS: organic photovoltaics, donor−acceptor interfaces, NC-AFM, KPFM, surface photovoltage

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These permanent interface dipoles can in some cases assist the geminate pair dissociation,7 and theoretical investigations3,6 have shown that the magnitude of the local electrostatic fields at the interfaces intimately depends of the local molecular conformation. As a consequence, one expects significant fluctuations of the interfacial dipoles on the molecular scale in BHJs. In that frame, recent photoluminescence experiments outlined the impact of the relative orientation between D and A molecules on the exciton dissociation process.8 So far, however, a direct visualization of the electrostatic landscape at the D−A interfaces has not been achieved even if using scanning probe microscopy techniques. More generally, there is still a lack of a complete understanding of the blends’ micro- and nanostructure. In principle, percolating interpenetrated D−A networks at the scale of the exciton diffusion length (i.e., 10 nm) are highly desirable. However, recent studies have shown that pure donor and acceptor domains may often coexist with intermixed or cocrystallized phases where the donor and acceptor are

owadays, with power conversion efficiency records of over 10% for a single junction1 and 11% for multiple junctions,2 organic solar cells are regarded as a promising alternative to conventional silicon-based photovoltaic devices. However, a comprehensive and clear understanding of all physical processes at work within organic solar cells is still missing. Bulk heterojunction (BHJ) organic devices rely on nanophase-segregated donor (D) and acceptor (A) materials, arranged to efficiently separate excitons into free charges at the D−A interfaces. The electronic structure at these organic/organic interfaces plays a key role in the efficiency of the active device. A universal feature of the D−A heterojunctions is that the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbital (LUMO) lie energetically higher in one material than in the other. However, interfacial electronic interactions can dramatically impact the energy level alignment at D−A interfaces and result in the appearance of significant electrostatic dipoles. Interfacial dipoles may originate from charge transfer mechanisms, from mutual polarization effects between the interacting molecules, or even from charge redistribution caused by a wave function overlap at the D−A interface.3−6 © XXXX American Chemical Society

Received: September 15, 2015 Accepted: January 11, 2016

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DOI: 10.1021/acsnano.5b05810 ACS Nano XXXX, XXX, XXX−XXX

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Figure 1. (a,b) Chemical structures of the electron donor (FG1, a) and acceptor ([70]PCBM, b) molecules. (c) Single FG1 molecule in its most stable conformation, as determined from molecular mechanics calculations.26 The main molecular dipole D at the fluorenone core is highlighted by an arrow. (d) Single FG1 molecular stack26 (top view).

Figure 2. (a) Model proposed for the FG1:[70]PCBM blend morphology. FG1 stacks and [70]PCBM aggregates are represented by blue lines and red circles, respectively. Upon annealing, a demixing process occurs, resulting in the formation of mesoscopic [70]PCBM clusters, highlighted by red-dotted contours in (a) and (c). A nanometric [70]PCBM cluster buried in the vicinity of the surface is indicated by the black-dotted circle. (b,c) Large-scale (5000 × 5000 nm2) topographic (b) and KPFM potential (c) images of the blend after in situ annealing (Δf = −10 Hz, AVib = 9 nm). (d−h) High-resolution (400 × 400 nm2) topographic (d), damping (e,g), and KPFM potential (f,h) images recorded above a “FG1-rich” area (Δf = −24 Hz, AVib = 9 nm) in the dark (d−f) and under illumination (1 mW) at 515 nm (g,h). (i) Surface photovoltage (SPV) image (Gaussian smooth filtered) calculated as the difference between images (h) and (f) (the raw image displays the same features but with a slightly higher noise level). Black-dotted circles highlight subsurface [70]PCBM aggregates, and the two black lines delimitate FG1 stacks with a higher level of positive charging. The white line indicates a path used for profile analysis in Figure 4.

photovoltage (SPV) of photoactive thin films and devices11−18 can be mapped by analyzing the contact potential difference (CPD) shift upon illumination. Besides, in-dark CPD images may also, in principle, be used to investigate the permanent charges and electrostatic dipoles at the D−A interfaces. However, resolving structural and electrostatic contrasts on BHJ blends at the sub-10 nm scale19 remains a challenge even in the case of investigations carried out in ultrahigh vacuum (UHV) by noncontact AFM (nc-AFM). It becomes mandatory to minimize the tip−surface distance and operate the AFM in

intimately mixed.9,10 These last works point out the role of intermixed phases in the exciton dissociation and carriers dynamics. There is therefore a crucial need for experimental techniques able to probe simultaneously the morphology and the (opto)electronic properties at the nanometer scale. In combination with atomic force microscopy (AFM), Kelvin probe force microscopy (KPFM) is a well-established technique in the field of organic, hybrid, and inorganic photovoltaics. Several reports have already demonstrated that the local surface B


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ACS Nano the regime where the fast decaying short-range forces are dominant.20 In such a regime, it has been shown on other kinds of surfaces21−23 and on single molecules24 that nonconventional mechanisms may significantly contribute to the local CPD (LCPD) contrast formation, raising the question of the physical interpretation of LCPD and local SPV contrasts recorded on D−A blends. Actually, it is not clear how the chosen interaction regime may influence the SPV images obtained by KPFM. In this article, we address these issues by analyzing nc-AFM and KPFM images (i.e., compensation bias images; see the polarity convention in the Methods section) acquired on model donor−acceptor organic BHJ thin films (ca. 350 nm thick). These blends are based on an oligothiophene−fluorenone oligomer25 (FG1, see Figure 1a) and [6,6]-phenyl C70 butyric acid methyl ester ([70]PCBM, Figure 1b). Due to the liquidcrystalline properties of the electron donor25 FG1, the sample morphology can be tuned via in situ (under UHV) sample annealing (see Supporting Information, Figure S1), resulting in samples for which nanometer-sized [70]PCBM clusters are buried in the subsurface of a matrix of self-assembled π-stacked FG1 molecular wires26 (Figure 1d). These extremely flat samples constitute model systems for high-resolution KPFM investigations, thanks to the highly homogeneous nature of the surface layer which reduces the risk of cross-talk effects between the topographic and CPD channels. To discuss the physical origin of the KPFM contrasts, it is essential to understand the nature of the sample morphology, for which a model is given in Figure 2a. In the case of FG1− [70]PCBM blends, the [70]PCBM clusters aggregate upon annealing into “acceptor-rich” mesoscopic domains which can be easily identified in large-scale topographic (Figure 2b) and tip compensation bias (Figure 2c) images (in the following, the tip compensation bias will be called the KPFM potential for simplicity). The remaining parts of the sample consist in “donor-rich” domains, where FG1 molecules form selfassembled π-stacked lamellar structures (Figure 2d). This organization in well-ordered lamellae appears to be promoted by the annealing process. Molecular mechanics and dynamics simulations have shown26 that FG1 can form two-dimensional monolayers of polar or nonpolar π-stacked self-assemblies, for which the orientation of the molecular dipole alternates or is identical from one π-stacked wire to the next. In the D−A blends, however, the exact nature of the three-dimensional supramolecular assembly remains unknown, and both kind of structures (alternate and nonalternate) may coexist in the donor matrix. In these “FG1-rich” domains, neither topographic (Figure 2d) nor damping images (Figure 2e) display features which may be attributed to [70]PCBM clusters. On the other hand, KPFM potential images (Figure 2f) display bright spots with diameters in the range of a few tens of nanometers, revealing the existence of small [70]PCBM aggregates buried in the vicinity of the sample surface. In fact, even the mesoscopic [70]PCBM aggregates seem to be covered by a uniform capping layer of FG1 stacks, as shown by high-magnification images (Figure S1). Actually, most of the fullerene derivatives have to be buried deeper in the bulk with a concentration increase toward the ITO/PEDOT:PSS electrode. The mean compensation bias directly reflects the existence of a bulk electrostatic dipole (labeled D in Figure 3a) at the recessed [70]PCBM−FG1 interface. Such a model for the bulk morphology is well

Figure 3. (a) In the dark state, the mean compensation bias directly reflects the existence of a bulk electrostatic dipole D at the recessed [70]PCBM−FG1 interface ([70]PCBM, red circles; FG1, blue lines), which shifts the local vacuum level downward (short-dotted line) with respect to its reference value (symbolized by the largedotted line). Local KPFM contrasts reflect the existence of a nonuniform charge distribution at the interface between the FG1 matrix and subsurface [70]PCBM clusters. Local Vtip increases (toward more positive values) are consistent with local effective dipoles δ pointing upward. (b) Surface is uniformly covered by a capping layer of FG1, and most of the [70]PCBM is recessed at the ITO/PEDOT:PSS interface: as a consequence, the mean surface photovoltage is positive, and subsurface [70]PCBM clusters appear as local minima in SPV images.

supported by the comparison with the KPFM potential values recorded on pristine films of FG1 and [70]PCBM deposited on the same substrate as for the blends (Figure S2). The relative potential values are fully consistent with the energy level alignments expected in the frame of the integer charge transfer (ICT) model27,28 at the FG1−PEDOT:PSS, [70]PCBM− PEDOT:PSS, and [70]PCBM−FG1 interfaces (Figure S3). In the following, we will mainly focus the discussion on the “FG1-rich” domains as a benchmark system for investigations of local KPFM contrasts. First, we discuss the in-dark contrasts, which originate from the permanent charge distribution at the D−A interfaces (Figure 3a). More precisely, the potential increase detected above the recessed [70]PCBM clusters reveals the existence of local positive charges or dipoles oriented upward (Figure 3a). To determine whether the charges are uniformly or unevenly distributed at the D−A interfaces, the experimental potential profiles (Figure 4 and Figure S4) have been compared to simulated cross sections of the KFPM signal. Three-dimensional (3D) electrostatic potentials and KPFM profiles have been computed using the Comsol Poisson solver (see the Methods section and Figure

Figure 4. Experimental KPFM potential profiles in the dark, under illumination at 515 nm and SPV profile, extracted above a subsurface [70]PCBM aggregate (profile path highlighted by white lines in Figure 2f,h,i). C


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ACS Nano S5) for spherical inclusions of variable radius RPCBM (ranging from 1 to 8 nm) buried at different distances zPCBM below the sample surface. First, a homogeneous dipole distribution at the D/A interface has been tentatively modeled for a [70]PCBM sphere with RPCBM = 3 nm, with an outer layer thickness of 1 nm carrying a positive charge +|e| distributed in its volume and an inner layer of thickness of 1 nm carrying a negative charge −|e|. This situation leads to zero potential signals (not shown) within the calculation accuracy, which can be attributed to the closed geometry of the sphere surface dipole distribution, only leading to a negative electrostatic potential inside the sphere with no influence on the outside potential. In other words, a [70]PCBM sphere covered with a uniform dipole layer shall be “electrostatically transparent” for the KPFM regulation. Consequently, the system in consideration must display a certain electrostatic anisotropy, which may originate either from geometrical effects or from uneven charge transfer or polarization effects across different sections of the D−A interface. The former hypothesis implies the existence of [70]PCBM aggregates displaying an highly anisotropic aspect ratio and a proper orientation with respect to the surface. This is not likely to be the case for all [70]PCBM clusters buried just below the surface. In fact, the experimental data can be fairly well reproduced with [70]PCBM spheres carrying a positive charge +|e| uniformly distributed at their upper half surface and a negative charge −|e| at their lower half, for which the computed 3D potential is shown as an inset in Figure 5a. The cross section of the KFPM signal has been simulated as a function of the burial depth for a sphere with a radius of 3 nm (Figure 5a). The best fit in terms of peak intensity (experimental data in the 150−200 mV range; see Figures 4 and S4) is achieved for zPCBM = 3 nm, which is remarkably close to the height expected for a single molecular layer of the donor FG1. In a second step, the potential profiles have been computed as a function of the sphere radius, keeping the burial depth fixed at 3 nm. The simulated cross sections agree well with the experimental data in terms of peak intensity and full width at half-maximum (fwhm) for a sphere radius between 3 nm (peak fwhm = 12 nm) and 8 nm (peak fwhm = 20 nm). It is obvious that the real morphology of nanometer-sized [70]PCBM clusters may differ from the perfectly spherical geometry adopted for our calculations. At this stage, our simulations show anyway that the local KPFM contrasts can be reasonably accounted for by “effective” dipoles, which reflect most probably the existence of a more complex charge distribution around the D/A interface. Recent theoretical investigations of pentacene−fullerene heterojunctions3,6 provide a plausible explanation for our observations. These calculations demonstrated that the local vacuum level shift critically depends on the orientation of the pentacene π-system relative to the adjacent C60. Such a model may as well be applied to the case of FG1/[70]PCBM. As a consequence of the high degree of ordering within the matrix formed by the πstacked FG1 wires, the donor π-orbitals display a strongly anisotropic orientation with respect to the [70]PCBM clusters. Moreover, it is stressed that FG1 molecules carry a net molecular dipole at their fluorenone core (Figure 1c). It can be easily shown that a spherical object embedded in a polar FG1 matrix is equivalent to a recessed effective dipole (Figure S6). Furthermore, the charge distribution used for the simulations is realistic compared to the polarization per volume unit of the FG1 matrix (see the Supporting Information).

Figure 5. (a) Calculated cross sections of the KPFM signal (in the dark) associated with a PCBM spherical inclusion with a radius of 3 nm, buried below the surface at various depths. Inset: illustration of the 3D electrostatic potential calculated using a grounded substrate (not shown), a grounded tip, and the PCBM cluster with negative (−|e|) and positive (+|e|) charge distributions at its lower half and upper half, respectively. (b) Calculated KPFM cross sections for PCBM spheres with various radius buried 3 nm below the surface.

We stress that the polarity of the observed KPFM contrasts puts some constraints on the models that could be proposed for the supramolecular organization of FG1−[70]PCBM blends near the surface. In particular, it is highly likely that FG1 molecular dipoles are oriented downward (Figure S6), resulting in effective dipoles at the recessed [70]PCBM clusters pointing toward the surface. Further investigations combining molecular mechanics and dynamics simulations with quantum chemical methods and microelectrostatic simulations will be needed to establish a more comprehensive model for the charge distribution at the FG1−[70]PCBM interface. In a next step, we now focus on the analysis of the surface photovoltage (SPV) images (Figure 2i; see also Figures S8 and S9). A positive SPV (mean value of more than 100 mV) is systematically recorded above FG1-rich domains, which confirms the existence of a uniform capping layer of donor molecules (Figure 3b) and of a recessed layer of [70]PCBM at the PEDOT:PSS interface. Less positive (or even slightly negative) shifts can be eventually observed above the mesoscopic [70]PCBM-rich clusters (Figure S9). Remarkably, the potential shift is completely reversible (Figures S8), revealing the absence of permanent charge trapping effects in these blends. One can take advantage of this perfect reversibility to carry out a quantitative comparison of the SPV levels recorded for different tip−surface interaction regimes. D


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Figure 6. (a,e,i) Topographic, (b,f,j) damping, (c,g,k) in-dark KPFM potential, and (d,h,l) calculated SPV images (800 × 800 nm2) recorded with a vibration amplitude of 9 nm and a frequency shift of −4 Hz (a−d), −14 Hz (e−h), and −24 Hz (i−l). The black-dotted circles and the black arrows in (d,h,l) pinpoint a subsurface [70]PCBM cluster and FG1 stacks, respectively, with the latter displaying a more positive photocharging.

Figure 7. (a) KPFM potential in the dark (black symbols) and under illumination (1 mW, 515 nm, green symbols) recorded for different Δf set points at the same location (data from the same measurement series as presented in Figure 6). (b) Mean values of the KPFM potentials as a function of Δf; the data have been plotted by employing symbols according to the ones used for the histograms in (a). (c) Illustration explaining the dependence of the surface photovoltage on the tip−surface separation, z, by an optical field exaltation effect at the AFM tip apex. A typical Δf(z) curve has been represented in red. In the long-range regime, z decreases quickly when Δf is decreased (to more negative values, from 1 to 2), resulting in an enhanced SPV. In the short-range regime, the tip−surface separation almost no longer evolves, resulting in a SPV saturation for decreasing (i.e., more negative) Δf set points. NB: the dotted lines in the Δf(z), Δf(Vtip), and SPV(z) curves highlight the interaction regimes that have not been probed in this experiment. Especially, for very large tip−surface gaps (from 0 to 1), there may be no field-effect enhancement of the SPV. Probing this regime is impractical in the case of single-pass KPFM experiments.

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range of materials.32,33 For apertureless near-field optical microscopy, several groups have successfully used monolithic silicon-based AFM cantilevers.33−36 In that context, our results are consistent with the optical near-field distribution and enhancement factors reported in the literature for silicon tips. Even without taking into account the confinement effects due to optical tip−sample coupling, simulations for free-standing tips32 have shown that the electromagnetic field can be amplified by a factor of 10 near the apex. The field intensity decreases rapidly as a function of the distance (relative to the tip apex), with a typical decay length of a few nanometers. Consequently, in our case, the saturation of the SPV increase in the SR regime can be simply accounted for by the typical dependence of the tip−surface separation distance z as a function of the frequency shift (Figure 7c). In the long-range regime, z decreases quickly when lowering Δf (to more negative values), resulting in an enhanced SPV due to the increase of the effective optical field intensity at the surface. On the other hand, the tip−surface separation almost no longer changes in the short-range regime, resulting in a SPV saturation for decreasing (i.e., more negative) Δf set points. Besides, we also note that the SPV displays a logarithmic dependence as a function of the illumination intensity (Figure S11). This effect, related to the electron−hole recombination kinetics,37 can also contribute to the SPV saturation at minimal tip−surface distances. Last, we underline that this optical field enhancement effect may affect local measurements of the surface photovoltage decay time by intensity-modulated KPFM.41−43 Recent investigations have demonstrated the capability of intensitymodulated KPFM to probe local recombination rates in organic photovoltaic samples.43 In the presence of an optical field enhancement at the tip apex, the nongeminate charge recombination should be faster at minimal tip−surface distances (due to the higher carrier density under amplified optical excitation). It is likely that forthcoming experiments will reveal a tip−surface distance dependency of the recombination time measured by intensity-modulated KPFM. In conclusion, the present work demonstrates that permanent and photogenerated charge carriers can be probed with an increased resolution by operating KPFM at minimal tip−surface distances. The results are in line with recent theoretical investigation, which predict that electrostatic dipoles display significant nonuniformities at the D−A interfaces in organic blends. High-resolution SPV imaging paves the way for local investigations of complex systems where pure donor and acceptor domains coexist with intermixed phases. Finally, it was shown that the magnitude of the local surface photovoltage is increased by the optical field enhancement at the tip apex. Hence, advanced numerical modeling of the tip−surface nanogap will be necessary for a quantitative analysis of the SPV.

Besides the global positive shift, the SPV images display heterogeneities at the nanometer scale. Local SPV minima are nearly always observed above the [70]PCBM clusters buried in the vicinity of the surface (see Figure 2i and the SPV profile in Figure 4). In addition, stripe-like features are revealed by the SPV images, revealing photocharging variations within FG1 supramolecular stacks on the nanometer scale. Here, it is crucial to establish that all these features have a physical origin and do not result from a misalignment of the set of source images used for the SPV calculation (i.e., KPFM potential under illumination and in the dark). In the case of the high-resolution image displayed in Figure 2i, the analysis of damping profiles recorded simultaneously with the KPFM data (Figure S10) shows that the lateral misalignment is at a maximum of 1.5 nm. Thus, the local SPV contrasts (as highlighted in Figure 2i) reflect the negative photocharging of the subsurface [70]PCBM aggregates and the existence of nonuniformly positively charged FG1 supramolecular stacks. Therefore, the influence of the tip−surface interaction regime on the SPV contrasts can be studieda crucial dependency not yet investigated to the best of our knowledge. For that purpose, several series of nc-AFM/KPFM images have been acquired in the dark and under illumination on the same area for decreasing (i.e., more negative) frequency shift set points (while keeping the vibration amplitude constant). Figure 6 shows the topography, damping, KPFM potential (in the dark), and SPV for three different frequency shift set points in the long-range (LR) regime (Δf = −4 Hz), at the onset of the short-range (SR) regime (Δf = −14 Hz), and finally in the SR regime (Δf = −24 Hz). Here, the occurrence of fast decaying short-range forces is characterized in a phenomenological manner by the appearance of a contrast in the damping images.20 For all channels, a significant increase in lateral resolution and contrast is achieved in the SR regime. We stress that the general features in the KPFM potential and SPV images stay the same whatever the interaction regime. This confirms that the local contrasts achieved at minimal tip− surface distances originate from the electrostatic contributions of permanent dipoles and photogenerated charges. Furthermore, the increase in resolution is attributed to a reduction of averaging effects at reduced tip−sample distances and eventually by additional contributions of SR electrostatic forces to the KPFM contrasts. Strikingly, the mean value of the “illuminated” KPFM potential (and consequently of the SPV) increases strongly when reducing the tip−surface separation in the LR regime and saturates in the SR regime (Figure 7a,b). This behavior is unambiguously related to the physics of photogenerated charge carriers, as the average “in-dark” KPFM potential is independent of the frequency shift set point. This effect is tentatively attributed to an optical field enhancement effect in the nanogap between the AFM tip and the surface sample, which creates locally higher effective illumination intensities for decreasing tip−surface distances. This effect is well-known and becomes useful in apertureless near-field optical microscopy or in surface-enhanced Raman scattering. In general, geometrical effects (i.e., the so-called lightning rod) and plasmon resonances can both contribute to the amplification of the electromagnetic field at the scanning probe tip apex.29 Therefore, the most advantageous materials for local field enhancement are, in principle, metals like silver30 or gold31 due to their strong surface plasmon resonances. However, it has been shown that significant field exaltation factors can be obtained with a wide

METHODS Experimental Measurements. The nc-AFM experiments were performed with an Omicron VT-AFM/STM setup in UHV and at room temperature. The silicon supersharp AFM cantilever (resonance frequency around 275 kHz) was argon-sputtered in vacuum to remove the oxide layer and possible contaminants. KPFM measurements were performed in frequency modulation mode with the compensation voltage VDC applied to the cantilever (tip bias Vtip = VDC). In that configuration, the contact potential difference was the opposite of VDC, CPD = Φtip − Φsample (Φ being the work function). In this work, an “electrostatically friendly” convention was employed, based on the analysis of compensation bias (Vtip = − CPD) images (here also called F


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ACS Nano KPFM potential images). This facilitates the interpretation of KPFM data, as a local positive charging of the surface (or an electrostatic dipole pointing upward) corresponds to a positive Vtip change. FG1: [70]PCBM (in a 1:2.5 ratio) bulk heterojunction thin films (approximately 350 nm thick) were deposited via spin-coating on indium−tin−oxide (ITO) functionalized with PEDOT:PSS. In situ sample annealing (150 °C for 15 min) was performed at pressures of 10−10 mbar and under temperature control with an optical pyrometer. A dual laser source with 515 and 685 nm wavelengths and variable light intensities (OmicronLaser, Germany) was used for sample illumination (through an optical viewport of the UHV chamber) in backside geometry due to the use of specifically designed sample holders. Surface photovoltage images were calculated as the difference between the compensation bias images recorded under selective illumination and in the dark. Last, prior to the investigations on the D−A blends, reference measurements were carried out with pristine films of pure FG1 deposited on ITO/PEDOT:PSS (Figure S7) to confirm the absence of any photovoltage related to the silicon cantilever itself. Electrostatic Simulations. KPFM signals have been computed from static force fields acting on an AFM tip38 using Comsol Poisson solver with ac/dc module in a three-dimensional geometry. The tip was modeled in a sphere−cone geometry with a tip radius Rtip = 5 nm, a tip-cone half-angle α = 15°, an oscillation amplitude A = ±9 nm (accordingly to the experimental parameters), and a minimum tip− substrate distance ztip = 0.5 nm. Since we deal with frequencymodulated KPFM, only side capacitances associated with the tip apex and tip cone contribute to the KPFM averaging effects. As a consequence, the AFM cantilever was not modeled. The tip height was limited by including a flat “cover” above the tip cone, and we checked that capacitance second derivatives associated with this cover do not play any role in the KPFM calculations. A PCBM inclusion was defined as a sphere with radius RPCBM and a relative dielectric constant39 εPCBM = 3.9, buried by a distance zPCBM in a 350 nm thick dielectric FG1 layer with a dielectric constant εFG1 = 2.5 (which is a reasonable value for such π-conjugated architecture40). The spherical inclusion can also be displaced in the sample plane by a distance xPCBM. This was used to calculate the KPFM cross sections by varying the lateral position xPCBM of the inclusion with respect to the AFM tip (see the sketch of the tip−sample geometry and the Comsol meshing geometry in Figure S5). Last, the tip oscillation was obtained via a variable tip−surface distance z = ztip + A × (1 − cos θ), in which θ varies in the [0, π] range (with typically n = 50 steps) to compute the force integrals.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Present Address ∥

Franz Fuch: Lithium Energy and Power GmbH & Co. KG, Engineering Systems and Electronics, P.O. Box 300 220, 70442 Stuttgart, Germany. Notes

The authors declare no competing financial interest.

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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/acsnano.5b05810. Topographic and KPFM potential images of the blend as a function of the annealing temperature. Topographic and KPFM potential images of pristine FG1, pristine [70]PCBM, FG1/[70]PCBM blend as-cast, and FG1/ [70]PCBM blend after annealing at 150 °C. Model for the energy level alignment at the organic/substrate and organic/organic interfaces. Series of experimental profiles extracted from the KPFM image recorded in the dark. Sketch of the tip−substrate geometry and meshing geometry in Comsol. Effective dipoles of [70]PCBM clusters embedded in a polar FG1 matrix. Reference SPV measurements on pristine FG1 films. KPFM potential shift reversibility on the FG1/[70]PCBM blend. SPV images recorded above a mesoscopic [70]PCBM cluster. Estimation of the SPV image lateral resolution via the analysis and comparison of damping profiles in the dark and under illumination. SPV as a function of the laser source intensity (PDF) G


Article

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High-Resolution Kelvin Probe Force Microscopy Imaging of Interface

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