Efficient charge extraction within solar cells explicitly depends on the optimization of the internal interfaces. Potential barriers, unbalanced charge extraction, and interfacial trap states can prevent cells from reaching high power conversion efficiencies. In the case of perovskite solar cells, slow processes happening on time scales of seconds cause hysteresis in the current–voltage characteristics. In this work, we localized and investigated these slow processes using frequency-modulation Kelvin probe force microscopy (FM-KPFM) on cross sections of planar methylammonium lead iodide (MAPI) perovskite solar cells. FM-KPFM can map the charge density distribution and its dynamics at internal interfaces. Upon illumination, space charge layers formed at the interfaces of the selective contacts with the MAPI layer within several seconds. We observed distinct differences in the charging dynamics at the interfaces of MAPI with adjacent layers. Our results indicate that more than one process is involved in hysteresis. This finding is in agreement with recent simulation studies claiming that a combination of ion migration and interfacial trap states causes the hysteresis in perovskite solar cells. Such differences in the charging rates at different interfaces cannot be separated by conventional device measurements.Keywords: charge trapping; ion migration; Kelvin probe force microscopy; perovskite solar cells; scanning probe microscopy; space charge layer

Here we investigated the stability of an aptamer, which is formed by two RNA strands and binds the antibiotic streptomycin. Molecular dynamics simulations in aqueous solution confirmed the geometry and the pattern of hydrogen bond interactions that was derived from the crystal structure (1NTB). The result of umbrella sampling simulations indicated a favored streptomycin binding with a free energy of ΔGbind° = −101.7 kJ mol–1. Experimentally, the increase in oligonucleotide stability upon binding of streptomycin was probed by single-molecule force spectroscopy. Rate dependent force spectroscopy measurements revealed a decrease in the natural off-rate (koff-COMPLEX = 0.22 ± 0.16 s–1) for the aptamer–streptomycin complex compared to the aptamer having an empty binding pocket (koff-APTAMER = 0.49 ± 0.11 s–1). This decrease in the natural off-rate corresponds to a decrease in the Gibbs free energy of ΔΔGsheer ≈ −3.4 kJ mol–1. The simulated binding pattern and the experimental results led to the conclusion that hydrogen bonds between both RNA strands mainly contribute to the decrease in natural off-rate of the aptamer system studied.

Methylammonium lead halide perovskites (MAPbI3) are very sensitive to humid environments. We performed in situ scanning force microscopy and in situ X-ray diffraction measurements on MAPbI3 films to track changes in the film morphology and crystal structure upon repeated exposure to a high relative humidity environment (80%). We found that the appearance of monohydrate (MAPbI3·H2O) Bragg reflections coincided with the appearance of additional grain boundaries. Prolonging the exposure time to humidity induced more grain boundaries and steps in the MAPbI3 films, and the peak intensities of the monohydrate MAPbI3·H2O increased. The monohydrate was not stable under dry atmosphere and could be reversed to MAPbI3. However, the humidity-induced grain boundaries persisted. The presence of these additional grain boundaries was most likely the reason for an increase in hysteresis in JV behavior upon humidity exposure. Morphological changes were not observed for exposure to humidity ≤50% for a duration of 144 h.

We report measurements of structure, mechanical properties, glass transition temperature, and contact angle of a novel nanocomposite material consisting of swellable silsesquioxane nanoparticles with grafted poly(ethyl methacrylate) (PEMA) brushes and PEMA matrices with varying molecular weight. We measured the interparticle distance at the surface of the composites using scanning probe microscopy (SPM) and in the bulk of ∼0.5 μm thick films by grazing incidence small angle X-ray scattering (GISAXS). For a given molecular weight of the brush unstable dispersions at high molecular weight of the matrix indicate an intrinsic incompatibility between polymer-grafted-nanoparticles and homopolymer matrices. This incompatibility is affirmed by a high contact angle between the polymer-grafted-nanoparticles and the high molecular weight matrix as measured by SPM. For unstable dispersions, we measured a decreased glass transition temperature along with a decreased plateau modulus by dynamic mechanical thermal analysis (DMTA) which indicates the formation of a liquid-like layer at the brush–matrix interface. This proves the ability to decouple the structural and mechanical properties from the potential to be swollen with small molecules. It opens a new area of use of these soft nanocomposites as slow release materials with tailored mechanical properties.Keywords: GISAXS; glass transition temperature; mechanical properties; nanocomposite; polymer brushes; scanning force microscopy;

Measurements with nanomechanical cantilever (NMC) sensors often reveal only qualitative results. Here we overcome this issue by inkjet printing well-defined polyelectrolyte multilayers (PEMs). We present a method that allows fabricating a 40 bilayer (BL) thick and 5 mm long line made of poly(allylamine hydrochloride) (PAH) and polystyrene sulfonate (PSS). NMC sensors were used to quantify the uptake of water in thin PEMs. We measured and analyzed the mass loading and the swelling response of the PEMs upon exposure to relative humidity between 5% and 80%. For a film made of 5 BLs we determined a Young’s module of ∼390 MPa for low humidity (<5%). Thicker PEM films made by 10 BLs exhibited a higher Young’s module of ∼560 MPa. The Young’s module decreased in both cases to 2–3 MPa at 80% relative humidity. Furthermore, the NMC measurements of mass and swelling upon exposure to humidity indicated a thickness-dependent swelling of the PEMs.

•Scanning force microscopy methods allow to investigate electrical properties of organic solar cells.•Electrical modes are unique for the correlation of structural and electric information on a nanometer scale.•Integration of an additional light source into the scanning force microscope setup allows to study photo-induced processes.The application of electrical modes in scanning probe microscopy helps to understand the electrical function of materials that are structured on the nanometer scale. Scanning force microscopes are routinely used for the investigation of surface topography. Here we accentuate the use of electrical modes that are unique for the correlation of structural and electric information on a nanometer scale. This is particularly important for analyzing organic solar cell materials. A special focus is given to experiments aiming at the investigation of light-induced processes which requires the integration of an additional light source into the scanning force microscope setup. Furthermore, we address future challenges for scanning force microscopy investigation of electrical properties of soft matter materials.Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slide

•Self-assembly at the air/water interface into mono-, bi- and multilayers.•Preparation of highly functional thin films with DNA facing the air interface for subsequent surface modification via molecular recognition.•Locally tunable molecular orientation.The self-assembly of amphiphilic hybrid materials containing an oligonucleotide sequence at the air/water interface was investigated by means of pressure–molecular area (Π–A) isotherms. In addition, films were transferred onto solid substrates and imaged using scanning force microscopy. We used oligonucleotide molecules with lipid tails, which consisted of a single stranded oligonucleotide 11mer containing two hydrophobically modified 5-(dodec-1-ynyl)uracil nucleobases (dU11) at the 5′-end of the oligonucleotide sequence. The air/water interface was used as confinement for the self-assembling process of dU11. Scanning force microscopy of films transferred via Langmuir–Blodgett technique revealed mono-, bi- (Π ≥ 2 mN/m) and multilayer formation (Π ≥ 30 mN/m). The first layer was 1.6 ± 0.1 nm thick. It was oriented with the hydrophilic oligonucleotide moiety facing the hydrophilic substrate while the hydrophobic alkyl chains faced air. In the second layer the oligonucleotide moiety was found to face the air. The second layer was found to cover up to 95% of the sample area. Our measurements indicated that the rearrangement of the molecules into bi- and multiple bilayers happened already at the air/water interface. Similar results were obtained with a second type of oligonucleotide amphiphile, an oligonucleotide block copolymer, which was composed of an oligonucleotide 11mer covalently attached at the terminus to polypropyleneoxide (PPO).

We explore the effect of an ultrathin elastic coating to optimize the mechanical stability of an underlying polymer film for nanoscale applications. The coating consists of a several nanometer thin plasma-polymerized norbornene layer. Scanning probes are used to characterize the system in terms of shear-force-induced wear and thermally assisted indentation. The layer transforms a weakly performing polystyrene film into a highly wear-resistive system, ideal for high-density and low-power data storage applications. The result can be understood from the indentation characteristics with a hot and sharp indenter tip. The latter gives rise to a deformation mode in the fully plastic regime, enabling a simple interpretation of the results. The softening transition and the yield stress of the system on a microsecond time scale and a nanometer size scale were obtained. We show that the plastic deformation is governed by yielding in the polystyrene sublayer, which renders the overall system soft for plastic deformation. The ultrathin protection layer contributes as an elastic skin, which shields part of the temperature and pressure and enables the high wear resistance against lateral forces. Moreover, the method of probing polymers at microsecond and nanometer size scales opens up new opportunities for studying polymer physics in a largely unexplored regime. Thus, we find softening temperatures of more than 100 °C above the polystyrene glass transition, which implies that for the short interaction time scales the glassy state of the polymer is preserved up to this temperature.Keywords: data storage; indentation; plasma polymerization; protective coating; scanning force microscopy; thin polymer films; time−temperature superposition

A combination of energy filtered transmission electron microscopy (EFTEM) and thermally stimulated current (TSC) was used in order to investigate the effect of thermal annealing on the performance of an organic solar cell based on P3HT and PCBM as a well-studied reference system. By probing specific elements, EFTEM allowed spectroscopic imaging with enhanced resolution compared to standard TEM techniques. Here, we applied EFTEM to cross-sections of pristine and thermally annealed organic solar cells to probe the sulfur concentration as a measure for the P3HT distribution within the photoactive layer. Thermal annealing for 10 min at 130 °C resulted in a reordering of P3HT and PCBM into better defined domains. The effect of the morphological changes on the presence of trap states was studied by TSC measurements. The TSC spectra recorded for the pristine and the thermally annealed solar cells showed three contributions, respectively, that could be assigned to the neat materials P3HT and PCBM as well as the blend. The pristine solar cell revealed a significantly lower density of trap states in the P3HT phase compared to the annealed solar cell. In combination with our EFTEM measurements, we were able to attribute this finding to the increased number of P3HT rich domains present in the annealed device. Annealing of P3HT:PCBM solar cells had a beneficial impact not only on the local molecular order, but in particular on providing percolation paths for both charge carrier types.

Heated micromechanical cantilevers were used to probe the thermal properties of nanogram samples in two ways: (i) as a micromechanical thermogravimetric balance and (ii) as a local nano-heater for heating individual objects. For micromechanical thermogravimetry, 6 ng of polyurethane nanocapsules containing silverazide were freeze dried and attached to the micromechanical cantilever apex. The resonant frequency of the micromechanical cantilever upon heating was recorded. After heating to 250 °C, 25% mass was found remaining on the micromechanical cantilever. In order to image structural changes, the tip of the heatable micromechanical cantilever was positioned on individual nanocapsules weighing less than 6 fg, and the capsule was imaged using a scanning force microscope. Then the cantilever was heated and temperature–deflection curves were recorded. Both methods revealed a decomposition temperature of 180 °C for silverazide encapsulated in a polyurethane nanocapsule. Topographic images of individual capsules after decomposition showed a rupture of the shell at one position.Graphical abstractHighlights► Analysis of thermal properties of polymeric nanocapsules containing gas generators. ► Comparison of two different SPM based techniques using heatable cantilevers. ► Cantilevers as ultrasensitive balance for nano-thermal gravimetric analysis. ► Analysis and imaging of single capsules. ► Analysis of decomposition temperature and shell rupture.

Nanomechanical cantilevers (NMC) were used for the characterization of the film formation process and the mechanical properties of colloidal monolayers made from polystyrene (PS). Closely packed hexagonal monolayers of colloids with diameters ranging from 400 to 800 nm were prepared at the air–water interface and then transferred in a controlled way on the surface of NMC. The film formation process upon annealing of the monolayer was investigated by measuring the resonance frequency of the NMC (≈12 kHz). Upon heating of non-cross-linked PS colloids, we could identify two transition temperatures. The first transition resulted from the merging of polymer colloids into a film. This transition temperature at 147 ± 3 °C as measured at ≈12 kHz remained constant for subsequent heating cycles. We attributed this transition temperature to the glass transition temperature Tg of PS which was confirmed by dynamic mechanical thermal analysis (DMTA) and using the time temperature superposition principle. The second transition temperature (175 ± 3 °C) was associated with the end of the film formation process and was measured only for the first heating cycle. Furthermore, the transition of the colloidal monolayer into a homogeneous film preserved the mass loading on the NMC which allowed determination of the Young’s modulus of PS (≈3 GPa) elegantly.

Work function changes of Au were measured by Kelvin probe force microscopy (KPFM) in the nonpolar liquid decane. As a proof of principle for the measurement in liquids, we investigated the work function change of an Au substrate upon hexadecanethiol chemisorption. To relate the measured contact potential difference (CPD) during the chemisorption of alkanethiols to a change of the work function, the influence of physisorbed decane must be taken into account. It is crucial that either the work function of the scanning probe microscope (SPM) tip or the sample surface remains constant throughout the reaction, since both contribute to the CPD. We describe two routes for determining the work function shift of Au coated with a monolayer of alkanethiols: In the first route, the SPM tips were taken as reference surfaces (constant tip work function). For this approach, we used Au(111) surfaces and kept the SPM tip ex situ during the adsorption process. In the second route, structured surfaces with reactive and inert parts were studied by KPFM (constant reference work function). For this route, we prepared nanometer sized Au structures by nanosphere lithography on SiOx substrates. Now, the SiOx served as the inert reference surface. The shifts in the work function after exposure to the hexadecanethiol (HDT) solution were determined to be ΔΦAu+HDT,decane-Au,air = −1.33 eV ± 0.07 eV (route I) and ΔΦAu+HDT,decane-Au,air = −1.46 eV ± 0.04 eV (route II). Both values are in excellent agreement with the work function shifts determined by ultraviolet photoemission spectroscopy (UPS) reported in literature. The presented procedures of measuring work function changes in decane open new ways to study local reactions at solid–liquid interfaces.

The rupture force of a split (bipartite) aptamer that forms binding pockets for adenosine monophosphate (AMP) was measured by atomic force spectroscopy. Changes in the rupture force were observed in the presence of AMP, while this effect was absent when mutant aptamers or inosine were used. Thus, changes in the rupture force were a direct consequence of specific binding of AMP to the split aptamer. The split aptamer concept allowed the detection of nonlabeled AMP and enabled us to determine the dissociation constant on a single-molecule level.

Redox active polymers with phenothiazine moieties have been synthesized by Atomic Transfer Radical Polymerization (ATRP). These novel polymers reveal bistable behaviour upon application of a bias potential above the oxidation threshold value. Using conductive Scanning Force Microscopy, two distinguishable conductivity levels were induced on a nanoscale level. These levels were related to a high conducting “On” and a low conducting “Off” state. The “On” state is generated by the oxidation of the phenothiazine side chains to form stable phenothiazine radical cations. The formation and stability of the radical sites was examined by cyclic voltammetry, electron spin resonance and optical spectroscopy. Polymers with phenothiazine moieties show the ability to retain their redox state for several hours and can therefore be used for nonvolatile organic memory devices. Furthermore, thin films made from the phenothiazine containing polymers show high mechanical nanowear stability.

In this paper we report on the unprecedented deformation behavior of stratified ultrathin polymer films. The mechanical behavior of layered nanoscale films composed of 8−12 nm thin plasma polymerized hexamethyldisiloxane (ppHMDSO) films on a 70 nm thick film of polystyrene was unveiled by atomic force microscopy nanoindentation. In particular, we observed transitions from the deformation of a thin plate under point load to an elastic contact of a paraboloid of revolution, followed by an elastic−plastic contact for polystyrene and finally an elastic contact for silicon. The different deformation modes were identified on the basis of force−penetration data and atomic force microscopy images of residual indents. A clear threshold was observed for the onset of plastic deformation of the films at loads larger than 2 μN. The measured force curves are in agreement with an elastic and elastic−plastic contact mechanics model, taking the amount of deformation and the geometry of the layer that presumably contributed more to the overall deformation into account. This study shows that the complex deformation behavior of advanced soft matter systems with nanoscale dimensions can be successfully unraveled.

We investigated the photoinduced changes in the surface potential and conductivity for locally degraded active layers of organic solar cell materials using electrical modes of scanning force microscopy. Samples were degraded under different partial pressures of oxygen and humidity in the presence of light. Degraded and nondegraded areas were investigated by Kelvin Probe Force Microscopy (KPFM) and conductive scanning force microscopy (cSFM). The analysis allowed us to quantify the extent of degradation and compensate the contribution of the probe tip. Two typical blends used for organic solar cell, i.e., P3HT:PCBM and PCPDTBT:PCBM, were investigated. We observed that P3HT:PCBM photodegraded significantly more than PCPDTBT:PCBM for an environment containing oxygen. For short photodegradation times (1 h), we verified that changes in the surface potential and conductivity of P3HT:PCBM films were fully reversible after annealing. For individual layers of P3HT and PCBM, we found that only P3HT degrades. However, the blend material of P3HT and PCBM leads to an accelerated degradation supporting the interpretation that PCBM undergoes a series of oxidations in the blend.

We investigated the reinforcement of blends made from poly(ethyl methacrylate) (PEMA) and PEMA-grafted nanoparticles at a nanometer length scale. The reinforcement was probed by nanowear experiments based on scanning force microscopy (SFM). In addition to imaging, the SFM is applied to wear the surface of samples at the length scale of nanoparticles. Blends with different miscibility of nanoparticles were prepared by varying the molecular weights of polymer grafted from the nanoparticles (N) and polymer forming the matrix (P). We were able to associate a critical force for the onset of nanowear by analyzing the nanowear patterns resulting from different normal loads during the nanowear experiment. The definition of this critical force allows quantitative comparison of nanoparticle-polymer systems of different composition. Nanowear tests indicated that only mixtures where N/P > 1 reinforced the composite material compared to the pure homopolymer. Under these conditions the grafted polymers were swollen and the nanoparticles acted as additional anchor sites. As reference experiments we used a blend made from PEMA homopolymer and unmodified particles. Non-grafted nanoparticles clearly did not account for any reinforcement.Graphical abstractSFM topography image after a nanowear test on a nanocomposite sample composed of polymer grafted nanoparticles and hompolymers.Highlights► The wear of nanocomposites was studied on a nanometer scale. ► The onset of nanowear is characterized by a critical normal force. ► We investigate nanowear of nanocomposites made of modified nanoparticles and polymers. ► Polymer grafted nanoparticles are reinforcing low Mw polymers. ► Unmodified nanoparticles clearly did not account for any reinforcement.

A gentle method that combines torsion mode topography imaging with conductive scanning force microscopy is presented. By applying an electrical bias voltage between tip and sample surface, changes in the local sample conductivity can be mapped. The topography and local conductivity variations on fragile free-standing nanopillar arrays were investigated. These samples were fabricated by an anodized aluminum oxide template process using a thermally cross-linked triphenylamine-derivate semicondcutor. The nanoscale characterization method is shown to be nondestructive. Individual nanopillars were clearly resolved in topography and current images that were recorded simultaneously. Local current−voltage characteristics suggest a space-charge limited conduction in the semiconducting nanopillars.

The interface roughness of adjacent films which were made by plasma polymerization of hexamethyldisiloxane were investigated. Multilayered structures were made by using different plasma conditions in alteration resulting in different mechanical properties within each layer. Scanning force microscopy on the face side of fractured pieces of the multilayer structures revealed a significant phase contrast between the layers. The direct visualization of the interface using the mechanical contrast between layers allowed the estimation of the interfacial roughness. We found that the interfaces between hexamethyldisiloxane films deposited at a radio frequency (RF) input power of 90 W in the presence of oxygen on top of films made by 48 W without oxygen resulted in an interface roughness of ≈10 nm. In the reverse case, a significantly lower interface roughness of ≈3 nm was determined. We attribute the increase of the interfacial roughness compared to the surface roughness being <1 nm to partial etching of the films by the subsequent deposition process. A key role in the appearance of higher interface roughness plays the RF-input power that determines the cross linking density and the hydrocarbon content in layers.Graphical abstractScanning force microscopy revealed an asymmetric interface roughness in films which were made by plasma polymerization of hexamethyldisiloxane.Research highlights► Scanning force microscopy revealed an asymmetric interface roughness of stacked layers of plasma polymerized films made from HMDSO. ► A key role in interface roughness plays the RF-input power and the presence of oxygen. ► Film defects have been imaged using the phase contrast mode.

Composite materials made of metallic nanoparticles embedded in an organic matrix are highly promising candidates as coatings for novel chemical sensors. Micromechanical cantilever measurements revealed that the Au-nanoparticle terphenyldithiol composite material swells upon dosing with toluene vapor. The mass increase was found to be linear with the toluene vapor concentration, ≈ 40 fg/ppm. Furthermore, significant differences in the mechanical transduction of ≈100 nm thick Au-nanoparticle terphenyldithiol composite material that was prepared on a 3-aminopropyldimethylmonoethoxysilane (APDMES) surface and on a Au surface were observed. The transduction of swelling of the composite film into a mechanical deflection was found to be more efficient for the composite film prepared on the Au surface attributed to covalent binding of the terphenyldithiol molecules with the Au surface. In contrast, the interface of the APDMES layer and the Au−terphenyldithiol composite material is based on electrostatic interaction between the Au nanoparticles and the amino interface. The analysis of the micromechanical cantilever sensor measurements lead to the conclusion that the composite film at the APDMES interface is more mobile compared to a similar film that was prepared on Au.

We have studied self-assembled monolayers (SAMs) of asymmetric dialkyldisulfide derivatives of the form CH3–(CH2)11+m–S–S–(CH2)11–OH with m = −4, −3, 0, +2 and +4 on gold. Sub-nanoscale changes in the length of the CH3-terminated alkylchain have been used to selectively protrude one particular end group in the resulting film. The alteration of the chain length in only two methylene units already results in changes of surface properties, which have been detected with local (chemical force microscopy) and macroscopic (contact angle) techniques. In particular, advancing contact angles can be adjusted between 40° and 80°. The adhesion between a hydrophobic tip and these SAMs in water is determined by the chemical nature of the protruding end group. Chemical force microscopy, X-ray photoelectron spectroscopy and infrared reflection absorption spectroscopy have shown that these SAMs are composed of mixed, well-packed CH3– and OH–alkylthiolate branches. The surface composition ratio is close to 1:1 for all investigated SAMs.

Swelling of plasma-polymerized allylamine (PPAA) films has been investigated by using a combination of micromechanical cantilever sensor (MCS) and surface plasmon resonance (SPR) spectroscopy. The bending responses of the polymer-coated MCS were compared with simultaneously measured reflectivity changes recorded by SPR for both, highly and low crosslinked plasma-polymerized films in N2 atmospheres with different humidity. Signals attributed to reversible swelling for both plasma polymers were obtained. With increasing the humidity, the thickness increase of a highly crosslinked film was lower than that of a low crosslinked film. In contrast, the MCS coated with a highly crosslinked film exhibited a larger deflection. This demonstrates that a plasma-polymerized allylamine film of higher crosslink density can transduce more efficiently the swelling to the MCS bending than a film of lower crosslink density under identical environmental conditions.