Hybrid organic-inorganic perovskites are highly promising candidates for next generation single- and multi-junction solar cells. Despite their extraordinarily good semiconducting properties, there is a need to increase the intrinsic material stability against heat, moisture and light exposure. Understanding how variations in synthesis affect the bulk and surface stability is therefore of paramount importance to achieve a rapid commercialization on large scales. In this work, I show for the case of methylammonium lead iodide that a thorough control of the methylammonium iodide (MAI) partial pressure during co-evaporation is essential to limit photostriction and phase purity, which dictate the absorber stability. Kelvin probe force microscopy (KPFM) measurements in ultra-high vacuum combined with X-Ray diffraction measurements corroborate that off-stoichiometric absorbers prepared with an excess of MAI partial pressure exhibit trances of stacked perovskite sheets that have adverse effects on the intrinsic material stability. Furthermore, I discuss the impact of processing conditions on grain boundary band bending. I compare the results from UHV KPFM to measurements carried out in air and highlight the importance of FM-KPFM in order to achieve a reliable estimate of the workfunctions. In addition I discuss measurements carried out on absorbers grown on different substrates and I show that surface photovoltage measurements are likely to be falsified by band bending at the rear interface. Finally, I will present preliminary work on halide perovskites grown via van-der-Waals epitaxy on graphene.

Over the last decade, the 3D characterization of samples has become of increasing importance, as modern devices (e.g. state-of-the-art energy devices and electronic devices) integrate a number of advanced materials, very often in a complex 3D manner at nanometre spatial scales. The development of innovative characterization tools providing high spatial resolution in 3D combined with excellent chemical or elemental sensitivity is hence of paramount importance to advance the frontiers of science and technology in numerous areas of research.

In order to overcome the limitations of individual techniques, correlative microscopy has been recognized as a powerful approach to obtain complementary information about the investigated materials [1]. In this context, we have combined Secondary Ion Mass Spectrometry (SIMS), which is an extremely powerful technique for analysing surfaces owing in particular to its excellent sensitivity, high dynamic range, high mass resolution and ability to differentiate between isotopes, with Helium Ion Microscopy (HIM) and Atomic Force Microscopy (AFM), respectively. We have developed integrated HIM-SIMS [2,3] and AFM-SIMS [4,5] instruments, which allow the chemical or elemental information obtained by SIMS to be correlated with nano-scale 3D mapping of the investigated samples obtained by a photogrammetric approach on the HIM or AFM scanning, respectively.

In this presentation, we will first introduce the HIM-SIMS and AFM-SIMS instruments and discuss their performance characteristics. We will then present a number of examples taken from various fields of materials science and life science to show the powerful correlative microscopy possibilities enabled by these new in-situ methods.

Transport across the interfaces in complex oxides attracts a lot of attention because it allows creating novel functionalities useful for device applications, e.g. for electric energy storage or non-volatile memories. It has been observed that movable domain walls (DWs)  in epitaxial BiFeO₃ films possess enhanced conductivity that can be used for ferroelectric-based memories [1,2]. In this case, the apparent memristive effect is based on the low resistance state induced by writing the DWs and high resistance by their erasure. In this work, the relation between the polarization, strain, and conductivity was investigated in sol-gel BiFeO₃ (BFO) films with special emphasis on grain boundaries (GBs) as natural interfaces in polycrystalline ferroelectrics. To explore the interplay between the transport properties of the GBs and polarization we studied domain structure and local conductivity by vector Piezoresponse Force Microscopy (PFM) and conductive Atomic Force Microscopy (c-AFM). We found that the individual grains were all single domain and did not contain any domain walls inside the grains. Surprisingly, we found a self-assembled arrangement of the domain structure confined in “clusters” with the correlated orientation of the spontaneous polarization. In addition, grain boundaries at the cluster circumference were highly conductive, while the electrical conductivity of GBs inside the clusters was similar to that in the bulk. Thus, the conductivity of GBs was dependent on the polarization state in adjacent grains being maximized for the opposite polarization directions. The enhanced conductivity was attributed to the strain concentration at GBs coupled with the local band bending. In order to put the proposed qualitative explanation of GBs effect on a theoretical basis, we used well-known Landau-Ginsburg-Devonshire (LGD) formalism to model the interface charge and polarization distribution in BFO films. PFM tip was modeled as a quasi-planar electrode separated from the film surface by a gap. In accordance with the proposed model, the experimentally observed conductivity enhancement at the GBs was explained by the electric potential and elastic strain variation at the interfaces. Finite element modeling (FEM) was used to calculate the polarization, electric field, potential, and elastic stress distributions across the GBs for the analysis of c-AFM contrast in BFO films. The observed phenomena provide further insight into the physics of interfaces in polycrystalline ferroelectrics and may strongly affect their technological applications, such as capacitors for energy storage or non-volatile memory cells.

The carbon dioxide transformation into useful fuels by means of just sunlight aims to solve a crucial problem of nowadays society. Lamentably, just an insufficient 1% of the generated CO₂ is reused today due, primarily, to the lack of efficient conversion technologies. However, the scientific accomplishments achieved during the last decade [1] have renewed the interest in this field.

            A fundamental part in a photocatalytic process is the electron-hole pair formation, due to the absorption of photons. One of the main limitations to obtain good conversion efficiency is the fact that electron-hole pair recombination speed is between 2 and 3 orders of magnitude faster than the speed of separation and charge transport. A recent proposal consists in the deposition of metal nanoparticles (NPs) on the TiO₂ surface. NPs trap the semiconductors excited electrons, thus reducing the photo generated carriers recombination speed and, therefore, enhancing the photocatalytic activity [2].

            Despite the fact that TiO₂ is the most studied photocatalyzer, the mechanisms that operate in the process of charge separation and transfer are still not completely understood [3]. Particularly, the metal nanoparticle trapping electron phenomenon is still an unexplored field but relevant in the redesigning of a more efficient photocatalyzer. In this work, we have studied the surface charge changes of individual ligand-free Au nanoparticles deposited on n-doped TiO₂ (110), under different catalytic conditions. Among other results, we will report the increased rate of hole trapping by the catalyzer and the decrease of contact potential difference experienced by the Au nanoparticles, as a function of light irradiance. 

Photoelectrochemical (PEC) water splitting is a promising route for efficient conversion of solar energy into chemical fuels. The chemical transformation of water into oxygen and hydrogen takes place at the photoelectrode surface. Consequently, the activity, efficiency, and reaction pathway are critically controlled by the material surface properties. Under operating conditions, surface properties depend on the surrounding environment, and may be altered in the course of the reaction. Thereby, absorption of molecules can modify the chemistry at the surface, for example by influencing the kinetics of reactants, products, or reaction intermediates, but they can also directly impact the electronic transport properties by acting as surface trap states. In this context, improved understanding of these complex surface interactions will aid the development of highly efficient light absorbers as well as the integration of effective passivation and catalyst layers for these materials.

Among different photoelectrode materials, bismuth vanadate (BiVO₄) is one of the most actively investigated oxide semiconductors. Here, we employ photoconductive AFM under controlled in-situ conditions to gain insight into the relationship between surface interactions and interfacial charge transport characteristics in polycrystalline BiVO₄ thin films. We demonstrate that the low intrinsic bulk conductivity of BiVO₄ limits charge transport through the film, and that the transport mechanism can be attributed to space charge limited current in the presence of trap states.[1] By analyzing the space charge limited current in selective gas environments, we are able to quantify the impact of surface adsorbates on bulk transport properties. We find that surface adsorbed oxygen acts as a shallow trap state and accounts for 40% of the effective trap density in BiVO₄ thin films.[2] For humid environments, our results are consistent with the adsorption of water as an oriented dipole layer, which does not induce a surface charge transfer but partially inhibits the adsorption of oxygen at the surface. Disentangling the individual effects of oxygen and water on charge carrier trapping underpins the importance of trap state passivation for efficient transport of photogenerated charge carriers in BiVO₄.

For Cu(In,Ga)Se2 (CIGSe) solar cells an alkali-fluoride (AlkF) post-deposition treatment (PDT) has recently led to a significant increase in the efficiencies. It has been observed by Kelvin probe force microscopy that the AlkF-PDT passivates the grain boundaries (GBs) in CIGSe by modifying their electronic properties in a beneficial way [1]. However, these results rely on surface potential information collected at the surface of the AlkF-PDT CIGSe.

In the present work, we explore the depth dependent electrical properties of a rubidium-fluoride (RbF)-PDT CIGSe, a treatment which has led to efficiencies as high as 22.6%. We apply conductive atomic force microscopy (C-AFM) tomography to study the depth dependent conductive properties of the CIGSe absorber, aiming at getting electronic information about the grains and grain boundaries inside the material. In order to achieve the depth resolution, we use highly-doped diamond-coated tips for the experiments with an applied tip force of several µN, leading to the removal of a thin layer of material with every scan frame. This combination of tip-induced material erosion with the sensing capability of C-AFM, enables a slice-and-view conductive tomography technique. Thus, three-dimensional (3D) quantitative current measurements are obtained, providing deeper insight into the conductive paths in relation to grains and grain boundaries. In order to understand different current signals obtained on different grains, we performed electron backscatter diffraction (EBSD) on the same areas, providing information about the crystallographic orientation. Based on these correlative c-AFM and EBSD data, we attribute the differences in currents to different surface dipoles related to the different crystallographic orientations and atomic arrangements of the different grains

The combination of functional scanning probe microscopy with advanced signal processing techniques in recent years has enabled new discoveries in a wide range of energy materials. These “big data” methods merge the availability of affordable data storage with the advances of the broader data science community to extract hidden information from gigabytes of raw time-dependent cantilever response data. In this talk, we will discuss our work using data-driven scanning probe methods, in particular time-resolved electrostatic force microscopy, for analyzing mixed organic-inorganic halide perovskites in situ in response to illumination. Through signal processing of the raw cantilever deflection signal during photoinduced charging, it is possible to extract the photoresponse of materials at microsecond timescales via analysis of the instantaneous frequency or the reconstructed electrostatic force. Importantly, we show that in layered (n=1) perovskites it is possible to observe photovoltage dynamics with timescales comparable to ion motion or trap-mediated carrier motion, in contrast to device-level studies. Furthermore, these timescales exhibit strong spatial dependence, with grain centers showing faster response compared to grain boundaries. This result is confirmed by general mode scanning Kelvin probe microscopy as well as by unsupervised clustering methods like k-means. These data indicate that layered perovskite materials may be more defect-prone than previously thought. Lastly, we discuss our work on analysis of hyperspectral photoinduced force microscopy for studying the spatial distribution of components in layered perovskites.

We study ion migration in 2D lead halide perovskites of varying dimensionality using scanning-Kelvin probe microscopy.  We perform potentiometry on micron-scale lateral junctions in the absence of injected charge and we compare how ion motion varies between prototypical two-dimensional n-butylammonium lead iodide perovskites (BA₂PbI₄, n=1), and methylammonium-incorporated quasi-2D perovskites (BA₂MA₃Pb₄I₁₃, ~<n>=4) both in the dark and under illumination. For pure 2D BA₂PbI₄ films (n=1) under applied bias, we observe symmetric potential profiles with charges migrating towards the anode and cathode (the charging process), and then away from the anode and cathode when the electric field is removed (the discharging process), both in the dark and under illumination.  In contrast, we observe asymmetric charging and discharging potential profiles for quasi-2D BA₂MA₃Pb₄I₁₃ films in the dark, which become symmetric under illumination. We attribute such a difference to the n=1 film being intrinsic and the n=4 film being self p-doped, on which the electric field is then screened by photogenerated carriers. We also measure the relaxation of the bias-induced ionic charge distributions at different temperatures to extract the activation energies associated with the ionic motion in each case. The relaxation dynamics during the discharging of both positive and negative potentials are similar for the n=1 film, but vary significantly for the n=4 film. Finally, we propose an explanation for these phenomena by hypothesizing that ion motion in purely 2D BA₂PbI₄ perovskite films is dominated by paired halide and halide vacancy motion, whereas for quasi-2D BA₂MA₃Pb₄I₁₃ films, the ion motion is a combination of both halide and methylammonium (vacancy) migration. These data show that dimensionality in these systems plays a critical role in the ion dynamics.