Photoconductive Atomic Force Microscopy Research Articles

Optical Tuning of Resistance Switching in Polycrystalline Gallium Phosphide Thin Films.

The nanoscale resistive switching characteristics of gallium phosphide (GaP) thin films directly grown on Si are investigated as a function of incident light. The formation of conductive channels along the grain boundaries is attributed to the presence of point defects and structural disorder, which provide the ideal environment to enable the filamentary switching process. Both first-principles calculations and UV-vis and photoluminescence spectroscopy strongly point to the possibility of mid-gap electronic states in the polycrystalline GaP film due to such defects. To study the photonic excitation, photoconductive atomic force microscopy (phAFM) measurement is conducted. We observe photocurrents even for incident photon energies lower than the band gap, consistent with the presence of mid-gap electronic states; the photocurrents increase in direct proportion to the incident photon energy with a concomitant decrease in the filament resistance. This demonstrates GaP directly integrated on Si can be a promising photonic resistive switching materials system.

Photoelectrical Properties Investigated on Individual Si Nanowires and Their Size Dependence

Periodically ordered arrays of vertically aligned Si nanowires (Si NWs) are successfully fabricated with controllable diameters and lengths. Their photoconductive properties are investigated by photoconductive atomic force microscopy (PCAFM) on individual nanowires. The results show that the photocurrent of Si NWs increases significantly with the laser intensity, indicating that Si NWs have good photoconductance and photoresponse capability. This photoenhanced conductance can be attributed to the photoinduced Schottky barrier change, confirmed by I–V curve analyses. On the other hand, electrostatic force microscopy (EFM) results indicate that a large number of photogenerated charges are trapped in Si NWs under laser irradiation, leading to the lowering of barrier height. Moreover, the size dependence of photoconductive properties is studied on Si NWs with different diameters and lengths. It is found that the increasing magnitude of photocurrent with laser intensity is greatly relevant to the nanowires’ diameter and length. Si NWs with smaller diameters and shorter lengths display better photoconductive properties, which agrees well with the size-dependent barrier height variation induced by photogenerated charges. With optimized diameter and length, great photoelectrical properties are achieved on Si NWs. Overall, in this study the photoelectrical properties of individual Si NWs are systematically investigated by PCAFM and EFM, providing important information for the optimization of nanostructures for practical applications.

In Situ Visualization of Localized Surface Plasmon Resonance-Driven Hot Hole Flux.

Nonradiative surface plasmon decay produces highly energetic electron–hole pairs with desirable characteristics, but the measurement and harvesting of nonequilibrium hot holes remain challenging due to ultrashort lifetime and diffusion length. Here, the direct observation of LSPR‐driven hot holes created in a Au nanoprism/p‐GaN platform using photoconductive atomic force microscopy (pc‐AFM) is demonstrated. Significant enhancement of photocurrent in the plasmonic platforms under light irradiation is revealed, providing direct evidence of plasmonic hot hole generation. Experimental and numerical analysis verify that a confined |E|‐field surrounding a single Au nanoprism spurs resonant coupling between localized surface plasmon resonance (LSPR) and surface charges, thus boosting hot hole generation. Furthermore, geometrical and size dependence on the extraction of LSPR‐driven hot holes suggests an optimized pathway for their efficient utilization. The direct visualization of hot hole flow at the nanoscale provides significant opportunities for harnessing the underlying nature and potential of plasmonic hot holes.

Scanning Probe Microscopy Analysis of Nonfullerene Organic Solar Cells.

In this work, scanning probe microscopies (SPMs) are used for the analysis of PBDB-T, ITIC, and PBDB-T:ITIC layers of solar cells (OSCs). Scanning tunneling microscopy (STM) images of PBDB-T reveal that thin films (<1 nm) tend to form worm-like pattern (amorphous type) domains with an average chain-to-chain distance of 950 pm; likewise, STM images of ITIC show that side arms form chain-like patterns. STM images of PBDB-T:ITIC blend suggest why PBDB-T domains could facilitate charge dissociation. Further, a strong interchain π-π interaction of the ITIC molecules could promote self-organization, and under the mutual interaction with the PBDB-T polymer, it could influence the pathway formation for electron transport. Moreover, when correlating electrostatic force microscopy (EFM) and photoconductive atomic force microscopy (pc-AFM), the blend morphology and its electrical/electronic properties are determined; the ideal domain size of PBDB-T:ITIC blend phases for maximizing the generated photocurrent is 15-35 nm. Furthermore, phase contrast and surface electric potential characteristics with Kelvin probe force microscopy (KPFM) are measured to examine additional details about the surface and potential changes due to the domain differences in the active layer. OSCs based on the nonfullerene PBDB-T:ITIC active layer reach an average power conversion efficiency (PCE) of 9.1% (best 9.2%).

Mechanistic Understanding of Material Transformations and Charge Transport at the Nanoscale in Solar Water Splitting Via Advanced Characterization Techniques

Stable, cost effective, and efficient solar water splitting is the “Holy Grail” for sustainable clean fuels from two most abundant resources, sunlight and water. This approach requires the combination of light absorbers and surface coating to perform catalytic reactions and enhance the stability and charge transport. Thus, accurate characterization of the material transformations and charge transport mechanism is critical to enable design and development of new functional systems. In our approach, photoconductive atomic force microscopy and Kelvin probe measurements combined with photoelectrochemistry were developed to correlate morphological and functional heterogeneity in metal oxides. Furthermore, the pc-AFM combined with other spectroscopies, e.g., X-ray photoemission spectroscopy, X-ray absorption spectroscopy, and electron energy loss spectroscopy were carried out on nitrides materials to fundamentally understand stability during operating conditions.

Visible‐light‐stimulated Alkalis‐triggered Platinum Cocatalyst with Electron Deficient Interface for Hydrogen Evolution

AbstractSolid–liquid interface engineering has recently emerged as a promising technique to optimize the activity. Here, the cation identity and induced interfacial electronic structure of Pt have been shown to have a particularly significant impact on the activity of desired photocatalytic hydrogen evolution. The behavior of Pt aggregation synergistically induced by photo‐electron accumulation on Pt surface and the ionic potential of alkalis was logically elucidated. X‐ray photoelectron spectroscopy (XPS) and photoconductive atomic force microscopy (pcAFM) results revealed that the surface of alkali‐triggered Pt nanoparticles (Pt NPs) aggregates are electron‐deficient, which favors photo‐induced charge separation, leading to improved reaction kinetics. Based on the density functional theory (DFT) calculation, alkalis‐triggered Pt NPs has lower Gibbs free energy and potential energy surface of H2 formation for hydrogen evolution in thermodynamics. As a result, K+ triggered Pt cocatalyst displays the highest activity (3306.6 μmol h−1 g−1) on pure CN, almost 4‐fold increase compared to the reference without the addition of alkalis, showing high utilization efficiency of Pt.

Self-Powered Perovskite/CdS Heterostructure Photodetectors.

Methylammonium lead halide perovskites have gained a lot of attention because of their remarkable physical properties and potential for numerous (opto)electronic applications. Here, high-performance photodetectors based on CH3NH3PbI3 (MAPbI3)/CdS heterostructures are demonstrated. The resulting self-powered MAPbI3/CdS photodetectors show excellent operating characteristics including a maximum detectivity of 2.3 × 1011 Jones with a responsivity of 0.43 A/W measured at 730 nm. A temporal response time of less than 14 ms was achieved. The mechanisms of charge separation and transport at the interface of the MAPbI3/CdS junction were investigated via conductive atomic force microscopy (AFM) and photoconductive AFM. Obtained results show that grain boundaries exhibit higher photocurrent than flat regions of the top perovskite layer, which indicates that excitons preferentially separate at the grain boundaries of the perovskite thin film, that is, at the edges of the MAPbI3 crystals. The study of the photoelectric mechanism at the nanoscale suggests the device performance could potentially be fine-tuned through grain boundary engineering, which provides essential insights for the fabrication of the high-performance photodetector. The demonstrated self-powered photodetector is promising for numerous applications in low-energy consumption optoelectronic devices.

Understanding the High Performance of over 15% Efficiency in Single-Junction Bulk Heterojunction Organic Solar Cells.

The highly efficient single-junction bulk-heterojunction (BHJ) PM6:Y6 system can achieve high open-circuit voltages (VOC ) while maintaining exceptional fill-factor (FF) and short-circuit current (JSC ) values. With a low energetic offset, the blend system is found to exhibit radiative and non-radiative recombination losses that are among the lower reported values in the literature. Recombination and extraction dynamic studies reveal that the device shows moderate non-geminate recombination coupled with exceptional extraction throughout the relevant operating conditions. Several surface and bulk characterization techniques are employed to understand the phase separation, long-range ordering, as well as donor:acceptor (D:A) inter- and intramolecular interactions at an atomic-level resolution. This is achieved using photo-conductive atomic force microscopy, grazing-incidence wide-angle X-ray scattering, and solid-state 19 F magic-angle-spinning NMR spectroscopy. The synergy of multifaceted characterization and device physics is used to uncover key insights, for the first time, on the structure-property relationships of this high-performing BHJ blend. Detailed information about atomically resolved D:A interactions and packing reveals that the high performance of over 15% efficiency in this blend can be correlated to a beneficial morphology that allows high JSC and FF to be retained despite the low energetic offset.

High-Performance Photoresistors Based on Perovskite Thin Film with a High PbI₂ Doping Level.

We prepared high-performance photoresistors based on CH3NH3PbI3 films with a high PbI2 doping level. The role of PbI2 in CH3NH3PbI3 perovskite thin film was systematically investigated using scanning electron microscopy, X-ray diffraction, time-resolved photoluminescence spectroscopy, and photoconductive atomic force microscope. Laterally-structured photodetectors have been fabricated based on CH3NH3PbI3 perovskite thin films deposited using precursor solution with various CH3NH3I:PbI2 ratios. Remarkably, the introduction of a suitable amount of PbI2 can significantly improve the performance and stability of perovskite-based photoresistors, optoelectronic devices with ultrahigh photo-sensitivity, high current on/off ratio, fast photo response speed, and retarded decay. Specifically, a highest responsivity of 7.8 A/W and a specific detectivity of 2.1 × 1013 Jones with a rise time of 0.86 ms and a decay time of 1.5 ms have been achieved. In addition, the local dependence of photocurrent generation in perovskite thin films was revealed by photoconductive atomic force microscopy, which provides direct evidence that the presence of PbI2 can effectively passivate the grain boundaries of CH3NH3PbI3 and assist the photocurrent transport more effectively.

Direct Imaging of Surface Plasmon-Driven Hot Electron Flux on the Au Nanoprism/TiO2.

Direct measurement of hot electron flux from a plasmonic Schottky nanodiode is important for obtaining fundamental insights explaining the mechanism for electronic excitation on a surface. Here, we report the measurement of photoinduced hot electrons on a triangular Au nanoprism on TiO2 under incident light with photoconductive atomic force microscopy (pc-AFM), which is direct proof of the intrinsic relation between hot electrons and localized surface plasmon resonance. We find that the local photocurrent measured on the boundary of the Au nanoprism is higher than that inside the Au nanoprism, indicating that field confinement at the boundary of the Au nanoprism acts as a hot spot, leading to the enhancement of hot electron flow at the boundary. Under incident illumination with a wavelength near the absorption peak (645 nm) of a single Au nanoprism, localized surface plasmon resonance resulted in the generation of a higher photoinduced hot electron flow for the Au nanoprism/TiO2, compared with that at a wavelength of 532 nm. We show that the application of a reverse bias results in a higher photocurrent for the Au nanoprism/TiO2, which is associated with a lowering of the Schottky barrier height caused by the image force. These nanoscale measurements of hot electron flux with pc-AFM indicate efficient photon energy transfer mediated by surface plasmons in hot electron-based energy conversion.

Semiconducting Polymer Thin Films Used in Organic Solar Cells: A Scanning Tunneling Microscopy Study

AbstractThe widely used semiconducting polymers poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) (PH1000), poly(3‐hexylthiophene‐2,5‐diyl) (P3HT), and Poly[[4,8‐bis[(2‐ethylhexyl)oxy]benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl][3‐fluoro‐2‐[(2‐ethylhexyl)carbonyl]thieno[3,4‐b]thiophenediyl]] (PTB7) for organic photovoltaic (OPV) cells, are deposited on highly ordered pyrolytic graphite substrates and investigated by scanning tunneling microscopy (STM) under the liquid/solid interface technique. PEDOT:PSS film morphology shows an ellipsoidal shape of PEDOT surrounded by PSS with 4 nm size. STM images of P3HT reveal that films tend to form well‐defined crystalline domains with an average interchain distance around 1.41 nm and chain length ≈41 nm. PTB7 chains are self‐aggregated by using chlorobenzene as solvent; however, when using 1‐phenyloctane with 3% of 1,8‐diiodooctane, these chains show a worm‐like pattern with a distance of ≈2 nm between backbone chains and a chain length of ≈90 nm. For P3HT and PTB7 based OPVs, local J–V plots are determined from photoconductive atomic force microscopy (pc‐AFM) measurements and fitted with the modified Mott−Gurney equation, the reached parameters are correlated with those achieved from the macroscopic J–V graphs. Active films of OPVs cells based on PTB7 are also analyzed by using AFM phase imaging showing domains size of 80–120 nm. This current STM analysis can elucidate some new important insights on polymer film formation and morphology and therefore, their influence on the OPV performance.

Characterization of Light Effect on Photovoltaic Property of Poly-Si Solar Cell by Using Photoconductive Atomic Force Microscopy

We investigate the effect of light intensity and wavelength of a solar cell device using photoconductive atomic force microscopy(PC-AFM). A POCl3 diffusion doping process is used to produce a p-n junction solar cell device based on a poly-Si wafer, and the electrical properties of prepared solar cells are measured using a solar cell simulator system. The measured open circuit voltage(Voc) is 0.59 V and the short circuit current(Isc) is 48.5 mA. Moreover, the values of the fill factors and efficiencies of the devices are 0.7 and approximately 13.6%, respectively. In addition, PC-AFM, a recent notable method for nano-scale characterization of photovoltaic elements, is used for direct measurements of photoelectric characteristics in limited areas instead of large areas. The effects of changes in the intensity and wavelength of light shining on the element on the photoelectric characteristics are observed. Results obtained through PC-AFM are compared with the electric/optical characteristics data obtained through a solar simulator. The voltage(VPC-AFM) at which the current is 0 A in the I-V characteristic curves increases sharply up to 18 W/m2, peaking and slowly falling as light intensity increases. Here, VPC-AFM at 18 W/m2 is 0.29 V, which corresponds to 59 % of the average Voc value, as measured with the solar simulator. Furthermore, while the light wavelength increases from 300 nm to 1,100 nm, the external quantum efficiency(EQE) and results from PC-AFM show similar trends at the macro scale but reveal different results in several sections, indicating the need for detailed analysis and improvement in the future.

Direct Observation of Perovskite Photodetector Performance Enhancement by Atomically Thin Interface Engineering.

Lead trihalide perovskites have been integrated with atomically thin WS2 and served as absorption layers to improve photoresponsivity in photodetectors. The combination of perovskites and two-dimensional (2D) transition-metal dichalcogenide (TMDC) materials provides the platform to study light-matter interactions and charge transfer mechanisms in optoelectronic devices. Herein, conductive and photoconductive atomic force microscopy were used to image the dark current and photocurrent generated in WS2/CH3NH3PbI3 (MAPbI3) heterostructures. Dark current measurement in the applied voltage range displays characteristic diode behavior, which can be well described by thermionic emission theory. Under laser illumination at 532 nm, the spatially resolved photocurrent images exhibit location-dependent photoresponse, where the photocurrent increases remarkably for the WS2/MAPbI3 heterostructures compared with the bare MAPbI3 regions. Furthermore, comparative surface roughness and 2D Fourier analysis of the topographic and current maps reveal that the interfacial conditions of the WS2/MAPbI3 heterojunctions play an important role in the charge separation process. In addition, WS2/MAPbI3-based photodetectors have been fabricated. Our study provides direct evidence that atomically thin TMDC monolayers can effectively assist the charge separation process and improve the light-to-electric energy conversion, which aids in the design principles and understanding of 2D heterostructured optoelectronic devices.

Characterisation of InGaN by Photoconductive Atomic Force Microscopy.

Nanoscale structure has a large effect on the optoelectronic properties of InGaN, a material vital for energy saving technologies such as light emitting diodes. Photoconductive atomic force microscopy (PC-AFM) provides a new way to investigate this effect. In this study, PC-AFM was used to characterise four thick (∼130 nm) InGaN films with x = 5%, 9%, 12%, and 15%. Lower photocurrent was observed on elevated ridges around defects (such as V-pits) in the films with %. Current-voltage curve analysis using the PC-AFM setup showed that this was due to a higher turn-on voltage on these ridges compared to surrounding material. To further understand this phenomenon, V-pit cross sections from the 9% and 15% films were characterised using transmission electron microscopy in combination with energy dispersive X-ray spectroscopy. This identified a subsurface indium-deficient region surrounding the V-pit in the lower indium content film, which was not present in the 15% sample. Although this cannot directly explain the impact of ridges on turn-on voltage, it is likely to be related. Overall, the data presented here demonstrate the potential of PC-AFM in the field of III-nitride semiconductors.

Disentangling the Role of Surface Chemical Interactions on Interfacial Charge Transport at BiVO4 Photoanodes.

Chemical transformations that occur on photoactive materials, such as photoelectrochemical water splitting, are strongly influenced by the surface properties as well as by the surrounding environment. Herein, we elucidate the effects of oxygen and water surface adsorption on band alignment, interfacial charge transfer, and charge-carrier transport by using complementary Kelvin probe measurements and photoconductive atomic force microscopy on bismuth vanadate. By observing variations in surface potential, we show that adsorbed oxygen acts as an electron-trap state at the surface of bismuth vanadate, whereas adsorbed water results in formation of a dipole layer without inducing interfacial charge transfer. The apparent change of trap state density under dry or humid nitrogen, as well as under oxygen-rich atmosphere, proves that surface adsorbates influence charge-carrier transport properties in the material. The finding that oxygen introduces electronically active states on the surface of bismuth vanadate may have important implications for understanding functional characteristics of water splitting photoanodes, devising strategies to passivate interfacial trap states, and elucidating important couplings between energetics and charge transport in reaction environments.

Nanoscale imaging of charge carrier transport in water splitting photoanodes

The performance of energy materials hinges on the presence of structural defects and heterogeneity over different length scales. Here we map the correlation between morphological and functional heterogeneity in bismuth vanadate, a promising metal oxide photoanode for photoelectrochemical water splitting, by photoconductive atomic force microscopy. We demonstrate that contrast in mapping electrical conductance depends on charge transport limitations, and on the contact at the sample/probe interface. Using temperature and illumination intensity-dependent current–voltage spectroscopy, we find that the transport mechanism in bismuth vanadate can be attributed to space charge-limited current in the presence of trap states. We observe no additional recombination sites at grain boundaries, which indicates high defect tolerance in bismuth vanadate. These findings support the fabrication of highly efficient bismuth vanadate nanostructures and provide insights into how local functionality affects the macroscopic performance.

Effective charge collection area during conductive and photoconductive atomic force microscopy

Conductive atomic force microscopy (C-AFM) has been widely used to map the nanoscale electrical properties of conducting polymers, nanomaterials, and organic electronic devices. While these measurements provide valuable insight into the spatial dependence of electrical performance, reported current densities and electrical conductivities measured by C-AFM are consistently much higher than those measured at the macroscopic scale. Here, we demonstrate that these anomalously high current densities and conductivities arise from ignoring current spreading and hence underestimating the current-carrying area. We present a simple experimental means of estimating the effective charge collection area during C-AFM measurements. Using semiconducting polymer poly(3-hexylthiophene) films as a test case, we find that the effective charge collection area can be as much as three orders of magnitude larger than the mechanical contact area between the probe and the film. Calibrated conductivity maps are obtained, with a quantitative correspondence with accepted values, and C-AFM photocurrent measurements of a nanostructured hybrid organic-inorganic solar cell active layer yield short-circuit current densities that match those reported for macroscopic devices. Finally, we address the observation that current spreading increases the effective charge collection area beyond the size of the probe-sample contact but does not preclude an imaging resolution below 10 nm.

Local Optoelectronic Characterization of Solvent-Annealed, Lead-Free, Bismuth-Based Perovskite Films.

Traditional organolead-halide perovskite-based devices have shown rapid improvements in their power conversion efficiency in less than a decade, yet challenges remain for improving stability and film uniformity, as well as the elimination of lead to address toxicity issues. We fabricated lead-free methylammonium bismuth iodide (MBI) perovskite films and studied the effect of solvent annealing with dimethylformamide (DMF) on both (1) the crystallinity and structure of the films with X-ray diffraction and scanning electron microscopy and (2) the local optoelectronic properties of the films as measured via (photo)conductive atomic force microscopy. We found that solvent annealing leads to improved crystallinity and increased grain size in the MBI films as compared to the thermally annealed films. Furthermore, solvent-annealed MBI films show significantly increased electrical conductivity in the out-of-plane direction. Photoconductivity in both solvent-annealed and thermally annealed MBI films was increased in the grain interiors versus the grain boundaries. It was observed that DMF-induced solvent annealing impacts charge transport through the film, which can be a unique design parameter for optimizing local optoelectronic properties. By studying how solvent annealing affects the MBI film structure and changes the ways in which charges are transported through the film, we have developed a better understanding of how local optoelectronic properties are affected by DMF annealing.

(Invited) Impacts of Nanostructures and Interfaces on Perovskite Solar Cell Performance

Organometallic halide perovskite materials have a variety of remarkable optoelectronic properties, such as suitable band gap (1.5eV), high absorption coefficient, long carrier diffusion lengths and lifetimes. All these unique characters pave the way for perovskite-based devices, especially the photovoltaic cells. This presentation will start with the variation of the surface potential, photo-generated voltage, and photocurrent networks of perovskite solar cells (PSCs) with different film topography has been studied by coupling scanning Kelvin probe force microscopy (SKPM) and photoconductive atomic force microscopy (pcAFM). Then a series of TiO2 compact films was prepared by sol-gel method with different sol aging time resulting in change of, thickness, surface roughness and compactness. High roughness of the films benefits the light absorption of perovskite layer and the injection efficiency at interface. The films with better compactness perform better hole blocking effect. The Ag@TiO2 nanoparticles made by polyol solution method are embedded in perovskite films, resulting in an enhanced short-circuit current density due better light absorption and reduced charge recombination. Finally, both excess CH3NH3Cl and thiourea were introduced into the pristine CH3NH3PbI3 precursor for antisolvent precipitation at low temperature and fabricated high quality perovskite films with desired morphology, crystallinity and optical properties. CH3NH3Cl exerted significant impacts on the perovskite crystallization process by controlling the delivery speed of PbI2 from the intermediate phase CH3NH3I2Cl. Further tuning the chemical composition with controlled film growth condition resulted in an average power conversion efficiency above 21% under the standard light illumination condition.

Crystalline orientation dependent photoresponse and heterogeneous behaviors of grain boundaries in perovskite solar cells

Using photoconductive atomic force microscopy and Kelvin probe force microscopy, we characterize the local electrical properties of grains and grain boundaries of organic-inorganic hybrid perovskite (CH3NH3PbI3) thin films on top of a poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT:PSS)/ITO substrate. Three discrete photoconductivity levels are identified among perovskite grains, likely corresponding to the crystal orientation of each grain. Local J-V curves recorded on these grains further suggest an anti-correlation behavior between the short circuit current (JSC) and open circuit voltage (VOC). This phenomenon can be attributed to diffusion-limited surface recombination at the non-selective perovskite-tip contact, where a higher carrier mobility established in the perovskite grain results in an enhanced surface recombination and thus a lower VOC. In addition, the photoresponse of perovskite films displays a pronounced heterogeneity across the grain boundaries, with the boundaries formed between grains of the same photoconductivity level displaying even enhanced photocurrent and open circuit voltage compared to those of the adjacent grain interiors. These observations highlight the significance of controlling the microstructure of perovskite thin films, which will be a necessary route for further improving the efficiency of perovskite solar cells.