Time-resolved Electrostatic Force Microscopy Research Articles

Nanoscale visualization of fast carrier dynamics in organic thin-film transistors by time-resolved electrostatic force microscopy

Fast carrier dynamics in organic thin-film transistors (OTFTs) was investigated by time-resolved electrostatic force microscopy (tr-EFM). We found that the carrier diffusion in the OTFTs proceeded in two stages: fast diffusion and slow diffusion. By applying the instantaneous frequency method to EFM, the temporal evolution of the spatial distribution of fast carriers in the channel region of the OTFTs, which took place on the timescale of several hundreds of nanoseconds, was evaluated. The inhomogeneous distribution of the local decay time constant showed that the carrier diffusion of the OTFTs was limited by the grain boundaries between each crystalline region. The quantitative capability of the method was verified by comparing the values of the carrier mobility estimated by the tr-EFM measurement and a numerical simulation. The mobility estimated from the experiment and the simulation showed good agreement, showing the possibility of the tr-EFM to evaluate the time evolution of dynamic phenomena in semiconductor devices.

Time-resolved electrostatic force microscopy under base-bias-level control

We propose a base-bias-level control method, in which the contact potential difference is always compensated in a similar way to Kelvin probe force microscopy, applicable to time-resolved electrostatic force microscopy using the pump–probe method. We experimentally acquired temporal waveforms of the electrostatic force signal on two SiO2/n-type Si samples, one of which was as-grown and the other was treated by forming gas annealing. Consequently, the effectiveness of the proposed method was confirmed.

Understanding the evolution of lithium dendrites at Li6.25Al0.25La3Zr2O12 grain boundaries via operando microscopy techniques

The growth of lithium dendrites in inorganic solid electrolytes is an essential drawback that hinders the development of reliable all-solid-state lithium metal batteries. Generally, ex situ post mortem measurements of battery components show the presence of lithium dendrites at the grain boundaries of the solid electrolyte. However, the role of grain boundaries in the nucleation and dendritic growth of metallic lithium is not yet fully understood. Here, to shed light on these crucial aspects, we report the use of operando Kelvin probe force microscopy measurements to map locally time-dependent electric potential changes in the Li6.25Al0.25La3Zr2O12 garnet-type solid electrolyte. We find that the Galvani potential drops at grain boundaries near the lithium metal electrode during plating as a response to the preferential accumulation of electrons. Time-resolved electrostatic force microscopy measurements and quantitative analyses of lithium metal formed at the grain boundaries under electron beam irradiation support this finding. Based on these results, we propose a mechanistic model to explain the preferential growth of lithium dendrites at grain boundaries and their penetration in inorganic solid electrolytes.

A Robust Neural Network for Extracting Dynamics from Electrostatic Force Microscopy Data.

Advances in scanning probe microscopy (SPM) methods such as time-resolved electrostatic force microscopy (trEFM) now permit the mapping of fast local dynamic processes with high resolution in both space and time, but such methods can be time-consuming to analyze and calibrate. Here, we design and train a regression neural network (NN) that accelerates and simplifies the extraction of local dynamics from SPM data directly in a cantilever-independent manner, allowing the network to process data taken with different cantilevers. We validate the NN's ability to recover local dynamics with a fidelity equal to or surpassing conventional, more time-consuming, calibrations using both simulated and real microscopy data. We apply this method to extract accurate photoinduced carrier dynamics on n = 1 butylammonium lead iodide, a halide perovskite semiconductor film that is of interest for applications in both solar photovoltaics and quantum light sources. Finally, we use SHapley Additive exPlanations to evaluate the robustness of the trained model, confirm its cantilever-independence, and explore which parts of the trEFM signal are important to the network.

Visualization of Charge Migration in Conductive Polymers via Time-Resolved Electrostatic Force Microscopy.

Charge dynamics play an important role in numerous natural phenomena and artificial devices, and tracking charge migration and recombination is crucial for understanding the mechanism and function of systems involving charge transfer. Tip-synchronized pump-probe electrostatic force microscopy simultaneously permits highly sensitive detection, microsecond time resolution, and nanoscale spatial resolution, where the spatial distribution in static measurement (usual EFM) reflects differences in the carrier density and the time evolution reveals the surface carrier mobility. By using this method, carrier injection and ejection in sulfonated polyaniline (SPAN) thin films were visualized. Comparison of tr-EFM results of SPAN thin films with different doping levels revealed the individual differences in carrier density and mobility.

Surface Charge Migration Tracked by Tip-Synchronized Time-Resolved Electrostatic Force Microscopy

Nanoscale observation of charge migration is crucial for understanding and controlling functional materials and devices. We developed tip-synchronized time-resolved electrostatic force microscopy. The analysis of cantilever motion provides the temporal resolution with the timescale of cantilever vibration cycle. The observation of sub-microsecond charge migration is achieved for photovoltaic bilayer and conductive polymer thin films by a movie with 0.3 microsecond frame step time resolution.

Time-Resolved Electrical Scanning Probe Microscopy of Layered Perovskites Reveals Spatial Variations in Photoinduced Ionic and Electronic Carrier Motion.

We study light-induced dynamics in thin films comprising Ruddlesden-Popper phases of the layered 2D perovskite (C4H9NH3)2PbI4. We probe ionic and electronic carrier dynamics using two complementary scanning probe methods, time-resolved G-mode Kelvin probe force microscopy and fast free time-resolved electrostatic force microscopy, as a function of position, time, and illumination. We show that the average surface photovoltage sign is dominated by the band bending at the buried perovskite-substrate interface. However, the film exhibits substantial variations in the spatial and temporal response of the photovoltage. Under illumination, the photovoltage equilibrates over hundreds of microseconds, a time scale associated with ionic motion and trapped electronic carriers. Surprisingly, we observe that the surface photovoltage of the 2D grain centers evolves more rapidly in time than at the grain boundaries. We propose that the slower evolution at grain boundaries is due to a combination of ion migration occurring between PbI4 planes, as well as electronic carriers traversing grain boundary traps, thereby changing the time-dependent band unbending at grain boundaries. These results provide a model for the photoinduced dynamics in 2D perovskites and are a useful basis for interpreting photovoltage dynamics on hybrid 2D/3D structures.

Time-resolved electrostatic force microscopy using tip-synchronized charge generation with pulsed laser excitation

Nanoscale observation of charge distribution and electric polarization is crucial for understanding and controlling functional materials and devices. In particular, the importance of charge dynamics is well recognized, and direct methods to observe charge generation, transfer, and recombination processes are required. Here, we describe tip-synchronized time-resolved electrostatic force microscopy. Numerical modeling clarifies that the tip-synchronized method provides temporal resolution with the timescale of the cantilever oscillation cycle. This method enables us to resolve sub-microsecond charge migration on the surface. The recombination of photo-excited carriers in a bilayer organic photovoltaic thin film is observed as a movie with a 0.3 µs frame step time resolution. Analysis of the images shows that the carrier lifetime is 2.3 µs near the donor/acceptor interface. The tip-synchronized method increases the range of time-resolved electrostatic force microscopy, paving the way for studies of nanoscale charge dynamics.

Noncontact Imaging of Ion Dynamics in Polymer Electrolytes with Time-Resolved Electrostatic Force Microscopy.

Ionic-transport processes govern performance in many classic and emerging devices, ranging from battery storage to modern mixed-conduction organic electrochemical transistors (OECT). Here, we study local ion-transport dynamics in polymer films using time-resolved electrostatic force microscopy (trEFM). We establish a correspondence between local and macroscopic measurements using local trEFM and macroscopic electrical impedance spectroscopy (EIS). We use polymer films doped with lithium bis(trifluoromethane)sulfonimide (LiTFSI) as a model system where the polymer backbone has oxanorbornenedicarboximide repeat units with an oligomeric ethylene oxide side chain of length n. Our results show that the local polymer response measured in the time domain with trEFM follows stretched-exponential relaxation kinetics, consistent with the Havriliak-Negami relaxation we measure in the frequency-domain EIS data for macroscopic samples of the same polymers. Furthermore, we show that the trEFM results capture the same trends as the EIS results-changes in ion dynamics with increasing temperature, increasing salt concentration, and increasing volume fraction of ethylene oxide side chains in the polymer matrix evolve with the same trends in both measurement techniques. We conclude from this correlation that trEFM data reflect, at the nanoscale, the same ionic processes probed in conventional EIS at the device level. Finally, as an example application for emerging materials syntheses, we use trEFM and infrared photoinduced force microscopy (PiFM) to image a diblock copolymer electrolyte for next-generation solid-state energy storage applications.

Functional Scanning Probe Imaging of Nanostructured Solar Energy Materials.

From hybrid perovskites to semiconducting polymer/fullerene blends for organic photovoltaics, many new materials being explored for energy harvesting and storage exhibit performance characteristics that depend sensitively on their nanoscale morphology. At the same time, rapid advances in the capability and accessibility of scanning probe microscopy methods over the past decade have made it possible to study processing/structure/function relationships ranging from photocurrent collection to photocarrier lifetimes with resolutions on the scale of tens of nanometers or better. Importantly, such scanning probe methods offer the potential to combine measurements of local structure with local function, and they can be implemented to study materials in situ or devices in operando to better understand how materials evolve in time in response to an external stimulus or environmental perturbation. This Account highlights recent advances in the development and application of scanning probe microscopy methods that can help address such questions while filling key gaps between the capabilities of conventional electron microscopy and newer super-resolution optical methods. Focusing on semiconductor materials for solar energy applications, we highlight a range of electrical and optoelectronic scanning probe microscopy methods that exploit the local dynamics of an atomic force microscope tip to probe key properties of the solar cell material or device structure. We discuss how it is possible to extract relevant device properties using noncontact scanning probe methods as well as how these properties guide materials development. Specifically, we discuss intensity-modulated scanning Kelvin probe microscopy (IM-SKPM), time-resolved electrostatic force microscopy (trEFM), frequency-modulated electrostatic force microscopy (FM-EFM), and cantilever ringdown imaging. We explain these developments in the context of classic atomic force microscopy (AFM) methods that exploit the physics of cantilever motion and photocarrier generation to provide robust, nanoscale measurements of materials physics that are correlated with device operation. We predict that the multidimensional data sets made possible by these types of methods will become increasingly important as advances in data science expand capabilities and opportunities for image correlation and discovery.

Fast time-resolved electrostatic force microscopy: Achieving sub-cycle time resolution.

The ability to measure microsecond- and nanosecond-scale local dynamics below the diffraction limit with widely available atomic force microscopy hardware would enable new scientific studies in fields ranging from biology to semiconductor physics. However, commercially available scanning-probe instruments typically offer the ability to measure dynamics only on time scales of milliseconds to seconds. Here, we describe in detail the implementation of fast time-resolved electrostatic force microscopy using an oscillating cantilever as a means to measure fast local dynamics following a perturbation to a sample. We show how the phase of the oscillating cantilever relative to the perturbation event is critical to achieving reliable sub-cycle time resolution. We explore how noise affects the achievable time resolution and present empirical guidelines for reducing noise and optimizing experimental parameters. Specifically, we show that reducing the noise on the cantilever by using photothermal excitation instead of piezoacoustic excitation further improves time resolution. We demonstrate the discrimination of signal rise times with time constants as fast as 10 ns, and simultaneous data acquisition and analysis for dramatically improved image acquisition times.

Faster Time-Resolved Electrostatic Force Microscopy

Faster Time-Resolved Electrostatic Force Microscopy

Imaging Charge Transfer State Excitations in Polymer/Fullerene Solar Cells with Time-Resolved Electrostatic Force Microscopy.

We demonstrate nanoscale imaging of charge transfer state photoexcitations in polymer/fullerene bulk heterojunction solar cells using time-resolved electrostatic force microscopy (trEFM). We compare local trEFM charging rates and external quantum efficiencies (EQE) for both above-gap and below-gap excitation of the model system poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM). We show that the local trEFM charging rate correlates with device EQE for both above-gap and below-gap photoexcitation, demonstrating that EFM methods have sufficient sensitivity to detect the low EQEs associated with CT state formation, a result that could be useful for probing weak subgap excitations in nanostructured materials such as quantum dot and organometal halide perovskite solar cells. Further, we use trEFM to map spatial variations in EQE arising from subgap CT excitation in organic photovoltaics (OPVs) and find that the local distribution of photocurrent arising from these states is nearly identical to the spatial variation in EQE from above-gap singlet excitation. These results are consistent with recent work showing that both above-gap and below-gap excitation have similar internal quantum efficiency.

Morphology-Dependent Trap Formation in Bulk Heterojunction Photodiodes

We show that local structural variation affects the rate of aging in nanostructured polymer solar cells by comparing time-resolved electrostatic force microscopy (trEFM) and conventional device measurements on model polymer blends. Specifically, we study photovoltaic devices made from 1:1 blends of the polyfluorene copolymers poly(9,9'-dioctylfluorene-co-bis-N,N'-(4-butylphenyl)-bis-N,N'-phenyl-1,4-phenylene-diamine) (PFB) and poly(9,9'-dioctylfluorene-co-benzothiadiazole) (F8BT). We photooxidize these films in situ using 365, 405, and 455 nm illumination under ambient conditions, with the wavelengths chosen to preferentially excite the different components. During photooxidation, we observe a faster loss of photocurrent generation from F8BT-rich domains, leaving the PFB-rich phases to show higher photoresponse even at wavelengths absorbed predominantly by F8BT. We propose that this effect is due to the more rapid degradation of PFB hole-transport pathways in the F8BT-rich regions, resulting in a loss of percolation pathways for hole transport in the F8BT-rich phase.

Imaging Local Trap Formation in Conjugated Polymer Solar Cells: A Comparison of Time-Resolved Electrostatic Force Microscopy and Scanning Kelvin Probe Imaging

We study local photooxidation and trap formation in all-polymer bulk-heterojunction organic photovoltaics (OPVs) using both time-resolved electrostatic force microscopy (trEFM) and conventional scanning Kelvin probe microscopy (SKPM). We create electron-trapping defects at known locations by locally photooxidizing blends of poly[(9,9′-dioctylfluorene-alt-(bis(N,N′-(4-butylphenyl))-bis(N,N′-phenyl-1,4-phenylenediamine)] and poly[9,9′-dioctylfluorene-alt-1,4-benzothiadiazole]. We then compare the local surface photovoltage shifts measured via SKPM and the changes in local photoinduced charging rates measured via trEFM with changes in the performance of macroscopic photodiodes that have been exposed to similar photooxidation. We find that the trEFM charging rate images can identify local photooxidation and trap formation with much better sensitivity than conventional SKPM images. In addition, the changes in the trEFM charging rates correlate well with the external quantum efficiencies of the macroscopic photodiodes. In contrast, the SKPM images not only are less sensitive to trap formation but also show a more complicated response. We conclude that trEFM is well suited to studying local trap formation in organic solar cells and caution that SKPM data by itself can be difficult to interpret on OPV films, especially when materials have been exposed to photooxidation.

New SPM techniques for analyzing OPV materials

Organic solar cells hold promise as an economical means of harvesting solar energy due to their ease of production and processing. However, the efficiency of such organic photovoltaic (OPV) devices is currently below that required for widespread adoption. The efficiency of an OPV is inextricably linked to its nanoscale morphology. High-resolution metrology can play a key role in the discovery and optimization of new organic semiconductors in the lab, as well as assist the transition of OPVs from the lab to mass production. We review the instrumental issues associated with the application of scanning probe microscopy (SPM) techniques such as photoconductive atomic force microscopy and time-resolved electrostatic force microscopy that have been shown to be useful in the study of nanostructured organic solar cells. These techniques offer unique insight into the underlying heterogeneity of OPV devices and provide a nanoscale basis for understanding how morphology directly affects OPV operation. Finally, we discuss opportunities for further improvements in scanning probe microscopy to contribute to OPV development.

Time-resolved electrostatic force microscopy of polymer solar cells

Blends of conjugated polymers with fullerenes, polymers, or nanocrystals make promising materials for low-cost photovoltaic applications. Different processing conditions affect the efficiencies of these solar cells by creating a variety of nanostructured morphologies, however, the relationship between film structure and device efficiency is not fully understood. We introduce time-resolved electrostatic force microscopy (EFM) as a means to measure photoexcited charge in polymer films with a resolution of 100 nm and 100 micros. These EFM measurements correlate well with the external quantum efficiencies measured for a series of polymer photodiodes, providing a direct link between local morphology, local optoelectronic properties and device performance. The data show that the domain centres account for the majority of the photoinduced charge collected in polyfluorene blend devices. These results underscore the importance of controlling not only the length scale of phase separation, but also the composition of the domains when optimizing nanostructured solar cells.