Yohan Yoon

Nanophotonic Atomic Force Microscope Transducers Enable Chemical Composition and Thermal Conductivity Measurements at the Nanoscale

The atomic force microscope (AFM) offers a rich observation window on the nanoscale, yet many dynamic phenomena are too fast and too weak for direct AFM detection. Integrated cavity-optomechanics is revolutionizing micromechanical sensing; however, it has not yet impacted AFM. Here, we make a groundbreaking advance by fabricating picogram-scale probes integrated with photonic resonators to realize functional AFM detection that achieve high temporal resolution (<10 ns) and picometer vertical displacement uncertainty simultaneously. The ability to capture fast events with high precision is leveraged to measure the thermal conductivity (η), for the first time, concurrently with chemical composition at the nanoscale in photothermal induced resonance experiments. The intrinsic η of metal-organic-framework individual microcrystals, not measurable by macroscale techniques, is obtained with a small measurement uncertainty (8%). The improved sensitivity (50×) increases the measurement throughput 2500-fold and enables chemical composition measurement of molecular monolayer-thin samples. Our paradigm-shifting photonic readout for small probes breaks the common trade-off between AFM measurement precision and ability to capture transient events, thus transforming the ability to observe nanoscale dynamics in materials.

Nanoscale imaging of photocurrent enhancement by resonator array photovoltaic coatings

Nanoscale surface patterning commonly used to increase absorption of solar cells can adversely impact the open-circuit voltage due to increased surface area and recombination. Here, we demonstrate absorptivity and photocurrent enhancement using silicon dioxide (SiO2) nanosphere arrays on a gallium arsenide (GaAs) solar cell that do not require direct surface patterning. Due to the combined effects of thin-film interference and whispering gallery-like resonances within nanosphere arrays, there is more than 20% enhancement in both absorptivity and photocurrent. To determine the effect of the resonance coupling between nanospheres, we perform a scanning photocurrent microscopy based on a near-field scanning optical microscopy measurement and find a substantial local photocurrent enhancement. The nanosphere-based antireflection coating (ARC), made by the Meyer rod rolling technique, is a scalable and a room-temperature process; and, can replace the conventional thin-film-based ARCs requiring expensive high-temperature vacuum deposition.

Effects of Focused-Ion-Beam Processing on Local Electrical Measurements of Inorganic Solar Cells

Quantitative determination of electronic properties at high spatial resolution is crucial for the development of high-efficiency solar cells. Electron beam induced current (EBIC) is a powerful technique in which electron-hole pairs are created in proximity to an exposed surface, and the carrier collection efficiency is measured as a function of excitation position [1]. Cross-sections of device are often created by focused ion beams (FIB) due to the flexibility of the patterning and milling processes. However, the irradiating Ga ions of the FIB fabrication may introduce unintended artifacts, affecting local electronic properties. In this study, we investigate the impact of the FIB process observed in EBIC measurements and two-dimensional finite element simulations.

Nanoimaging of local photocurrent in hybrid perovskite solar cells via near-field scanning photocurrent microscopy

Photocurrent generation of methylammonium lead iodide (CH3NH3PbI3) hybrid perovskite solar cells is observed at the nanoscale using near-field scanning photocurrent microscopy (NSPM). We examine how the spatial map of photocurrent at individual grains or grain boundaries is affected either by sample post-annealing temperature or by extended light illumination. For NSPM measurements, we use a tapered fiber with an output opening of 200 nm in the Cr/Au cladded metal coating attached to a tuning fork-based atomic force microscopy (AFM) probe. Increased photocurrent is observed at grain boundaries of perovskite solar cells annealed at moderate temperature (100 °C); however, the opposite spatial pattern (i.e., increased photocurrent generation at grain interiors) is observed in samples annealed at higher temperature (130 °C). Combining NSPM results with other macro-/microscale characterization techniques including electron microscopy, x-ray diffraction, and other electrical property measurements, we suggest that such spatial patterns are caused by material inhomogeneity, dynamics of lead iodide segregation, and defect passivation. Finally, we discuss the degradation mechanism of perovskite solar cells under extended light illumination, which is related to further segregation of lead iodide.

Cathodoluminescence Measurements of CdTe in Transmission Electron Microscope

Cathodoluminescence (CL) is an important spectroscopic method for characterizing photovoltaic materials in electron microscopes. When electron-generated free carriers recombine, CL signals are emitted from the luminescent material and provide spectroscopic information that can be used to reveal features of the electronic structure, such as the band gap and defect states near the band edge. These characteristics can be correlated with the microstructure and microstructural defects that limit the solar cell efficiency. Ultimately, this knowledge can be used to develop processing schemes that optimize material structure and performance. For example, grain boundaries create interfaces in the bulk of thinfilm solar cells, leading to interfacial states may have an impact on the bulk recombination events and rates[1,2]. When investigating the influence of grain boundaries in scanning electron microscopes, the spatial resolution of CL is limited by the lateral size of the interaction volume (at least 250 nm). Much higher resolutions are needed to extract quantitative information from individual grain boundaries. Therefore, it is necessary to use high-spatial-resolution CL so that the change in electronic structure due to the interfacial states can be characterized.

Local photocarrier dynamics in CdTe solar cells under optical and electron beam excitations

We compare local carrier dynamics in n-CdS / p-CdTe solar cells, where the electron-hole pairs are generated by either near-field optical illumination or highly focused electron beam excitation. An ion beam milling process was used to prepare a smooth surface of cross-sectional devices. The spatially resolved photocurrent images confirm high carrier collection efficiency at grain boundaries. An analytical model was used to extract material parameters at the level of single grains. We find that the minority carrier diffusion lengths extracted from both local measurements are in excellent agreement, but are smaller than the values determined from macro-scale measurements.

Nanoscale Photocurrent Microscopy for Thin Film Solar Cells Using Focused Electron Beam and Near-Field Optical Excitations

Characterization techniques based on scanning probe microscopy are increasingly used for investigating microstructure, compositions, and optoelectronic properties of photovoltaic devices. In photocurrent microscopy, excess carriers are generated by an injected beam of photons, which are collected by a Schottky contact or a p-n junction. The value of measured photocurrent corresponds to the local efficiency of carrier collection, which is determined by the local built-in potential and applied electric field, as well as the carrier recombination rate. This direct imaging technique has been frequently used for characterizing bulk properties of semiconductor devices (e.g., defects, diffusion lengths).