Light-modulated scanning tunneling spectroscopy (LM-STS) was applied to evaluating spatial variation of the local generation efficiency in an organic solar cell (OSC) with a bulk heterojunction (BHJ) structure. The acquired surface morphology and microscopic current-voltage (J-V) characteristics under dark and illuminated conditions allowed us to map not only the potential, i.e. the open-circuit voltage (VOC) but also the short-circuit current (JSC), fill-factor (FF) and internal resistance of the OSC on a scanning tunnel microscopy (STM) surface morphology. Understanding the device level macroscopic J-V characteristics of OSC's from the individual nanoscale J-V characteristics will be the key issue to develop new OSC devices with superior performances.
A system for time-resolved photoemission electron microscopy (TR-PEEM) conducted with femtosecond laser pulses has been developed to explore the photogenerated electron dynamics on semiconductor surfaces. Attained space and time resolutions were 100 nm and 100 fs, respectively. The present manuscript introduces the TR-PEEM system, and also reports the observation of different photogenerated electron lifetimes in different nanoscale structural defects randomly distributed on a semiconductor surface. The results were explained based on Schockley-Read-Hall (SRH) model relating the carrier recombination time and the defect state density. The defect state density in each defect was successfully estimated.
Kelvin-probe force microscopy (KPFM) is a surface potential mapping technique based on dynamic-mode atomic force microscopy (AFM). It is useful to visualize carrier injection barriers and trapped charges in operating organic thin-film transistors (OTFTs). Since it is desirable to perform KPFM experiments in vacuum conditions, frequency modulation (FM) technique is often used to operate AFM/KPFM. We review two operating modes of KPFM using FM-AFM in vacuum and demonstrate visualization of the carrier injection barriers and trapped charges in OTFTs. We also introduce a method to visualize the transient distribution of the trapped charges being evacuated from the channel of the operating OTFT.
High-sensitive visualization of the local electric and magnetic field is possible using a low energy electron beam combining with a simple grid detector configuration. The beam deflection goes along with the clear principle that the larger deflection can be induced by lower energy electron based on the Rutherford scattering scheme. The field distribution around two-dimensional materials allows quantitative analysis of the local field, showing good agreements with FEM simulation. Well defined beam scanning control established in the recent scanning electron microscope (SEM) can project the detector grid image superimposed on the specimen image. And thus the localized field distribution was easily visualized through a simple E-filed vector translation based on the deflection configuration. Detailed techniques and the analysis were described from the viewpoint of practical applications.