The functionality of electronic devices to a large extent is governed by the interfaces of semiconductors with contact materials. In solar cells, thermodynamic equilibration of the electrochemical potential of electrons in the absorber and in the contact phases leads to built in electric fields, which under solar excitation give rise to the photo-potential. In this talk it is demonstrated, how photoelectron spectroscopy can be applied to visualize potential variations in space charge regions of thin film solar cells and how these change under illumination.

As photoelectron spectroscopy is a surface analytical method the band structure of buried interfaces classically is determined by step by step experiments using vacuum integrated deposition and PES systems. In order to analyze the electronic structure of ex-situ solution processed devices, we developed tapered cross section photoelectron spectroscopy (TCS-PES) [1]. Here we use a shallow angle wedge to project the nm scale of the normal cross section to the mm scale on the TCS. Line scan PES with local resolution in the 10µm range now gives access to band bending in space charge regions in the dark and operating under illumination. In addition we use in vacuo prepared interfaces and ex situ prepared device stacks of classical and inverted solar cell structures to corroborate the TCS-PES findings [2]. Perovskite absorbers of different compositions are demonstrated to be doped n-type.  Simulations of the potential distribution on TCS shows the importance of strong p-doping of the hole extraction layer to obtain large photo­potentials. 

In contrast to the current believe, we demonstrate that the photoactive interface of efficient MAPI₃ and (FAPbI₃)_0.85(MAPbBr₃)_0.15 solar cells is a n-p heterojunction i.e. the working principle is n-n-p and not n-i-p.

Many nanostructured materials when observed in a fluorescence microscope demonstrate fluctuations of local photoluminescence intensity. The fluctuations vary from drastic ON/OFF blinking of individual semiconductor nanocrystals to rather mild PL intensity flickering of micrometre-sized grains in metal halide perovskite films. The following necessary conditions for this phenomenon to occur can be formulated:

1) the material must contain some very strong metastable PL quenching sites (or efficient non-radiative (NR) recombination centres sometimes called super traps[1]) which switch from their active to passive state at the time scale of the experiment,

2) just one such NR centre should be able to substantially decrease PL quantum yield of the whole region over which photogenerated excitations (electrons, holes, excitons) can diffuse,[2,3]

3) these regions must be isolated from each other in such a way that luminescence microscope resolves them.

Luminescence micro-spectroscopy is an ideal tool study local PL intensity fluctuations and correlate them with local PL spectroscopic properties with spatial resolution limited by light diffraction. Moreover, naturally fluctuating PL allows for application of super-resolution methods based on PL blinking to increase the spatial resolution of imaging and local spectroscopy.[1] In addition activation/de-activation of a super trap in a nanocrystal drastically changes the local energy relaxation pathways and can reveal local energy inhomogeneities via PL spectral fluctuations couples with fluctuations of PL intensity.[4]

In my lecture I will illustrate the general ideas described above on MAPbI₃ perovskite semiconductor where a very pronounced PL blinking effect allows for studying of local photophysics and photochemistry of this semiconductor.[5]

Halide perovskites have emerged as high-performance semiconductors for efficient optoelectronic devices, not least because of their bandgap tunability using mixtures of different halide ions. In this talk, I will give an overview of our recent work on understanding why these materials show such unexpectedly high photoluminescence (PL) yields.

We show that spatially varying energetic disorder in the electronic states of such mixed-halide films causes local charge accumulation, creating photodoped regions, which unearths a strategy for efficient light emission at low charge-injection in solar cells and light-emitting diodes [1].

Combining temperature-dependent PL microscopy with computational modelling we quantify the impact of local bandgap variations from disordered halide distributions on the global PL yield in mixed-halide perovskite films [2]. We find that fabrication temperature, surface energy, and charge recombination constants are key for describing local bandgap variations and charge carrier funneling processes that control the PL quantum efficiency. Finally, we show that further luminescence efficiency gains are possible through tailored bandgap modulation in the future, even for materials that have already demonstrated high luminescence yields.

[1] Feldmann et al., Nature Photonics 14, 123 (2020).

[2] Feldmann et al., Advanced Optical Materials 18, 2100635 (2021).

Long-range propagation of energy and information is highly desirable for solar energy harvesting and quantum information applications. However, there currently lacks experimental tools to investigate transport with high temporal and spatial resolutions to directly elucidate coherent and incoherent regimes. To address this challenge, my research group has developed ultrafast microscopy tools to image energy transport in molecular and nanostructured materials with simultaneously high spatial and temporal resolutions.

In my talk, I will focus on our recent progress on the visualization of exciton and charge transport in the nonequilibrium and coherent regimes. One example is the quasi-ballistic transport of hot carriers in hybrid perovskite materials, which leads to 230 nanometers transport distance in 300 fs. These results suggest potential applications of hot carrier devices based on hybrid perovskites. Another example is the transport of delocalized excitons in molecular aggregates. Our measurements demonstrated that delocalization can greatly enhance exciton diffusion, even when the excitons are only weakly delocalized (< 10 molecules). Finally, I will discuss nonequilibrium transport resulting from many-body exciton interactions. We have shown that the migration of interlayer excitons in WS₂-WSe₂ heterobilayers is controlled by the interplay between the moiré potentials and strong many-body interactions, leading to exciton-density- and twist-angle-dependent transport length that deviates significantly from normal diffusion.

Nano- and meso- scale materials properties are crucial to the macroscopic performance of a wide range of functional and photovoltaic devices. Photoconductivity, ferroelectricity, and coupled effects have been microscopically investigated for decades, especially in 2-dimensions using continuously evolving variations of Atomic Force Microscopy. Our work, along with many others, reveals how these properties are frequently mediated by strain, orientation, grain boundaries, and other microstructural defects or heterogeneities. However, practical devices are often sensitive to, or even controlled by, otherwise inaccessible sub-surface effects or thickness dependencies related to microstructure and concentration, polarization, and/or field gradients. Therefore, we are advancing Tomographic AFM for volumetric materials property mapping, with voxels of properties on the order of ~10 nm³. With polycrystalline photovoltaics such as MAPbI₃ and CdTe, tomographic photoconductive AFM literally uncovers new pathways to improve carrier separation via inter- and intra- granular defects. For BiFeO₃, Tomographic piezo-force microscopy confirms Kay-Dunn thickness scaling, LGD behavior with a minimum switchable thickness of <5 nm, and even co-located domain and current maps which together directly reveal sub-surface topological defects. In multiferroic CoFe₂O₄-BiFeO₃ vertically aligned nanocomposites, TAFM directly visualizes lateral strain coupling, while superlattices demonstrate depth resolution on the few nanometer scale. Such volumetric insight is increasingly important for engineering optimal performance and reliability of real-world, 3-Dimensional materials devices for energy production and/or efficiency.

The functional properties of halide perovskite semiconductors at the mirco and nanoscales are heterogeneous,¹ and there is a need to understand how these properties (electrical, chemical, etc.)² affect overall photovoltaic (PV) performance. In this presentation, we begin by showing a variety of nanoscale microscopy methods that have been applied to perovskite solar cells to investigate the local properties of single-junction devices. These results reveal that the charge carrier dynamics in perovskites depend on the incident photon energy ³ and passivated perovskite films can exhibit chemical heterogeneities due to the inclusion of additives that subsequently affect the electrical response in three dimensions.²

Not only are perovskites complex on their own (as a thin film or single-junction device), but when embedded within a textured multi-junction solar cell, the interplay between device architecture and material performance introduces new challenges. It is not well understood how the local light-matter interactions of the perovskite material behave when incorporated into this type multijunction configuration. Therefore, we present optoelectronic microscopy measurements correlated with optical simulations of perovskite on textured c-Si solar cells. We measure the photon out-coupling of the perovskite material and find both a spectral and spatial dependence on the geometrical patterning which dominates any grain-to-grain variation.⁴ Such heterogeneity reveals the importance of the underlying c-Si texture design, suggesting that tuning the surface morphology could lead to a homogenization of the perovskite’s PL emission.

Kelvin-probe force microscopy (KPFM) has been widely used as a nanoscale surface potential measurement technique for investigating electrical properties of various materials and devices. However, this method cannot be used in an electrolyte since the application of a dc bias voltage between a tip and a sample often induces uncontrolled electrochemical reactions and/or redistribution of ions. To overcome this limitation, we have developed open-loop electric potential microscopy (OL-EPM) [1, 2], where we apply only a high-frequency ac bias voltage and detect the first and second harmonic vibration amplitude and phase of the cantilever to calculate the local potential near the sample surface. So far, we demonstrated that this method allows us to visualize nanoscale distribution of corrosion reaction activities at the surface of duplex stainless steel and cupper fine wires in electrolyte [3]. However, it has not been applied to the investigations on a specific important question in corrosion science.

  In this study, we apply OL-EPM to the studies on the nanoscale corrosion mechanisms of aluminum alloys around Al-Fe intermetallic particles (IMPs) or grain boundaries in electrolyte. Aluminum alloys have attracted growing attention due to the increasing demands for reducing the weight and fuel consumption of cars, aircraft, and robots. To this end, tremendous efforts have been made for understanding their corrosion mechanisms and improving their corrosion resistance. However, this has been impeded by the difficulties in visualizing nanoscale distribution of local corrosion cells during the corrosion process. Here we use OL-EPM to visualize the dynamic changes in the surface potential around Al-Fe IMPs and grain boundaries and present the model to explain the corrosion mechanisms. These results provide important guidelines for the future improvements in the corrosion resistance of aluminum alloys, and also demonstrate the applicability of OL-EPM to the nanoscale studies on the corrosion mechanisms.

 Rechargeable battery technology, for example Li ion batteries (LIBs), has attracted much attention as demand for their use in portable electronics, electric vehicles, and so on has increased. To further improve the device performance, a deeper understanding of their working mechanisms is essential to gain valuable insights into design guidelines. In general, evaluations of a battery are performed by electrochemical measurements, including charge/discharge tests and cyclic voltammetry (CV). These macroscopic characterizations are the first necessary steps for grasping the battery performance but for investigating device working principles from fundamental aspects, additional direct microscopic analyses are desired, which clarifies how local electrochemical reactions proceed during the electrochemical measurements.
 In this work, we have developed a method based on Kelvin probe force microscopy (KPFM) that enables dynamically imaging the change of potential distribution in an operating electrochemical device and used it for characterizing an all-solid-state Li ion battery (ASS-LIB)[1,2]. Experiments were conducted on the cross-section of the ASS-LIB with the KPFM setup equipped with an electrochemical measurement system. We continuously obtained CPD images at the cathode composite region during a CV operation of the ASS-LIB.
 Our results clarified that electrochemical reactions proceeded non-uniformly from the outer electrode side in the charging process while those in the discharging process progressed uniformly over the composite electrode. Furthermore, from a direct comparison of the variation in the internal potential distribution with the CV characteristic, we demonstrated that the difference in the local electrochemical reactions can be explained by the change in the condition of the electronic conductive path network in the composite electrode, arising from the change of the electronic conductivity of the cathode active material due to charging/discharging reactions (Li-ion extraction and insertion).

Perovkites have been shown as potential candidates for next generation optoelectronic applications. Therefore, their basic crystallographic features need to be understood deeply. Ferroelastic twin domains are crystallography related structures that affect the charge carrier dynamics in methylammonium lead iodide (MAPI) crystals. They form in MAPI crystals due to its tetragonal crystal structure with low symmetry. In this presentation, the methods such as piezoresponse force microscopy (PFM), polarized optical microscopy (POM), and x-ray diffraction (XRD) to monitor the ferroelastic twin domains in MAPI and strain engineering procedures to change the domain will be discussed. In addition, characterization techniques like time-resolved photoluminescence (TRPL) and PL microscopy on twin domain effect on charge carrier diffusion will be presented.

Metal-halide perovskites are a versatile material platform for light-harvesting and light-emitting applications as their variable chemical composition allows the optoelectronic properties to be tailored to specific applications. Halide mixing is one of the most powerful techniques to tune the optical bandgap of metal-halide perovskites across wide spectral ranges. However, halide mixing has commonly been observed to result in phase segregation, which reduces excited-state transport and limits device performance. While the current emphasis lies on the development of strategies to prevent phase segregation, it remains unclear how halide mixing may affect excited-state transport even if phase purity is maintained.

In this work, we use transient photoluminescence microscopy to study excitonic excited-state transport in phase pure mixed-halide 2D perovskites. We show that, despite phase purity, halide mixing inhibits exciton transport in these materials. We find a significant reduction even for relatively low alloying concentrations, with bromide-rich perovskites being particularly sensitive to the introduction of iodide ions. Performing Brownian dynamics simulations, we are able to reproduce our experimental results and attribute the decrease in diffusivity to the energetically disordered potential landscape that arises due to the intrinsic random distribution of alloying sites. Our results suggest that even in the absence of phase segregation, halide mixing may still impact carrier transport due to the local intrinsic inhomogeneities in the energy landscape.

Employing 2D/3D heterostructures has become a prominent passivation strategy to enhance the open-circuit voltage (V_OC) and fill factor (FF) of perovskite solar cells (PSC). The most widely employed 2D/3D interfaces are based on Ruddlesden Popper (RP) perovskites via treating the perovskite film surface with long chain alkylammonium salts. However, the detailed microscopic and electronic structure at the surface of such passivated films and the mechanisms governing the observed enhancements in device performance are still poorly understood. Here, we analyze methylammonium-free Cs_0.18FA_0.82PbI₃ perovskite films that are passivated employing a recently developed passivation strategy using phenethylammonium chloride, that was shown to form 2D RP (PEA)₂(Cs_1−xFA_x)_n−1Pb_n(I_1−yCl_y)3_n+1 phase with n ~1-2 at the GBs and film surface. [1] We perform cathodoluminescence (CL) in conjunction with Kelvin probe force microscopy (KPFM) on unpassivated reference films, grain boundary passivated films and grain boundary & surface passivated films with the aim to correlate the observations from these complementary techniques. We discuss the challenges and limitations of the measurements. Our results thereby improve the understanding of perovskite films passivated employing 2D RP perovskite phases.

Plasmonic resonances can concentrate light into exceptionally small volumes, approaching the molecular scale. The extreme light confinement provides an advantageous pathway to probe molecules at the surface of plasmonic nanostructures with highly sensitive spectroscopies, such as surface-enhanced Raman scattering. Unavoidable energy losses associated with metals, which are usually seen as a nuisance, carry invaluable information on energy transfer to the adsorbed molecules through the resonance linewidth. We measured a thousand single nanocavities with sharp gap plasmon resonances that spanned the red to near-infrared spectral range and used changes in their linewidth, peak energy and surface-enhanced Raman scattering spectra to monitor the energy transfer and plasmon-driven chemical reactions at their surface. Using methylene blue as a model system, we measured shifts in the absorption spectrum of molecules on surface adsorption and revealed a rich plasmon-driven reactivity landscape that consists of distinct reaction pathways that occur in separate resonance energy windows.

The family of layered thio- and seleno-phosphates has gained attention as possible control dielectrics for the rapidly growing family of 2D and quasi-2D electronic materials. Ferrielectric CuInP₂S₆ has been discovered in the 80’s but only recently it was revealed that this material exhibits a broad spectrum of unusual and even anomalous dielectric properties including negative electrostriction. Here we report a combination of density-functional-theory (DFT) calculations, DFT-based molecular-dynamics (MD) simulations, and variable-temperature, -pressure, and -bias piezoresponse force microscopy (PFM) data to predict and verify the existence of an unusual ferroelectric property – a uniaxial quadruple potential well for Cu displacements – enabled by the van-der-Waals (vdW) gap in ferrielectric CuInP₂S₆. Dependent on the position of Cu atoms, the polarization is either ±5 µC/cm2 when Cu resides within the layers or ±11 µC/cm2 when Cu displaces into the vdW gaps and forms ionic bonds with adjacent layers. Theoretical calculations provide values for the longitudinal piezoelectric coefficient of each of those four polarization states. Properly calibrated PFM amplitude and phase response can be related to the piezoelectric coefficient, which allows to identify these states. Consequently, we are able to track phase transitions as well as polarization switching in response to external stimuli. The calculated potential-energy landscape for Cu displacements is strongly influenced by strain, accounting for the origin of the giant negative piezoelectric coefficient. The combined theory-experiment approach also allows to map strain and stress in the material on the length scales of 10’s of nm using PFM. These phenomena offer new opportunities for both fundamental studies and applications in data storage and electronics.

Halide perovskite semiconductors are making waves for their exceptional performance as photovoltaic materials, but since they are such profound optoelectronic materials, discovery is underway to elucidate additional breakthrough applications. In this talk, we will show how forming heterojunctions containing perovskites enable unconventional control over spin, charge and light. 
In traditional optoelectronic approaches, control over spin, charge, and light requires the use of both electrical and magnetic fields. In a spin-polarized light-emitting diode (spin-LED), charges are injected, and circularly polarized light is emitted from spin-polarized carrier pairs. Typically, the injection of carriers occurs with the application of an electric field, whereas spin polarization can be achieved using an applied magnetic field or polarized ferromagnetic contacts. Here, we used chiral-induced spin selectivity (CISS) to produce spin-polarized carriers and demonstrate a spin-LED that operates at room temperature without magnetic fields or ferromagnetic contacts. The CISS layer consists of oriented, self-assembled small chiral molecules within a layered organic-inorganic metal-halide hybrid semiconductor framework. The spin-LED achieves ±2.6% circularly polarized electroluminescence at room temperature.[1]
Long-lived photon-stimulated conductance changes in solid-state materials can enable optical memory and brain-inspired neuromorphic information processing. It remains challenging to realize optical switching with low-energy consumption, and new mechanisms and design principles giving rise to persistent photoconductivity (PPC) can help overcome an important technological hurdle. Here, we demonstrate versatile heterojunctions between metal-halide perovskite nanocrystals and semiconducting single-walled carbon nanotubes that enable room-temperature, long-lived (thousands of seconds), writable, and erasable PPC. Optical switching and basic neuromorphic functions can be stimulated at low operating voltages with femto- to pico-joule energies per spiking event, and detailed analysis demonstrates that PPC in this nanoscale interface arises from field-assisted control of ion migration within the nanocrystal array. Contactless optical measurements also suggest these systems as potential candidates for photonic synapses that are stimulated and read in the optical domain. The tunability of PPC shown here holds promise for neuromorphic computing and other technologies that use optical memory.[2]

[1] Kim et al., Science 371, 1129–1133 (2021) 10.1126/science.abf5291

[2] Hao et al., Sci Adv. 2021 Apr; 7(18): eabf1959.  10.1126/sciadv.abf1959