Laser beam induced current microscopy and photocurrent mapping for junction characterization of infrared photodetectors

For non-destructive optical characterization, laser beam induced current(LBIC) microscopy has been developed into as a quantitative tool to examine individual photodiodes within a large pixel array. Two-dimensional LBIC microscopy, also generally called photocurrent mapping(PC mapping), can provide spatially resolved information about local electrical properties and p-n junction formation in photovoltaic infrared(including visible light) photodetectors from which it is possible to extract material and device parameters such as junction area, junction depth, diffusion length, leakage current position and minority carrier diffusion length etc. This paper presents a comprehensive review of research background, operating principle, fundamental issues, and applications of LBIC or PC mapping.

A positive correlation between local photocurrent and grain size in a perovskite solar cell

Developing an interplay between the local morphological character and its local photovoltaic(PV)parameters in a perovskite thin film is essential for guiding the construction of highly-efficient perovskite solar cells(PSCs). To achieve a higher PSC performance, great efforts have been devoted to the growth of larger perovskite grains;however, how the gain size can influence the PSC performance in a working device remains unclear. Herein, using laser-scanned confocal microscopy coupled with a photocurrent detection module, we realize local photocurrent, photoluminescence(PL) intensity and PL lifetime mappings directly in a working PSC. For perovskite grains of various sizes(from ~500 nm to a few micrometers), their local photocurrent exhibit a statically positive correlation with the grain size, but anti-correlated with the grain’s local PL intensity. This result suggests that a larger perovskite grain likely has fewer defects and more importantly better interfacial contact with the charge collection layers and thus leads to higher charge collection efficiency, and the optimum grain size is found to be ≥2 μm.Our result provides important guidance to the growth and control of perovskite microstructures toward the further improvement of PSC performance.

Nanoscale visualization of hot carrier generation and transfer at non-noble metal and oxide interface

The conversion efficiency of energy-harvesting devices can be increased by utilizing hot-carriers(HCs).However,due to ultrafast carrier-carrier scattering and the lack of carrier injection dynamics,HC-based devices have low efficiencies.In the present work,we report the effective utilization of HCs at the nanoscale and their transfer dynamics from a non-noble metal to a metal oxide interface by means of real-space photocurrent mapping by using local probe techniques and conducting femtosecond transient absorption(TA)measurements.The photocurrent maps obtained under white light unambiguously show that the HCs are injected into the metal oxide layer from the TiN layer,as also confirmed by conductive atomic force microscopy.In addition,the increased photocurrent in the bilayer structure indicates the injection of HCs from both layers due to the broadband absorption efficiency of TiN layer,passivation of the surface states by the top TiN layer,and smaller barrier height of the interfaces.Furthermore,electrostatic force microscopy and Kelvin probe force microscopy provide direct evidence of charge injection from TiN to the MoO_(x)film at the nanoscale.The TA absorption spectra show a strong photo-bleaching signal over wide spectral range and ultrafast decaying behavior at the picosecond time scale,which indicate efficient electron transfer from TiN to MoO_(x).Thus,our simple and effective approach can facilitate HC collection under white light,thereby achieving high conversion efficiency for optoelectronic devices.