Toward Megapixel Resolution Compressed Sensing Current Mapping of Photovoltaic Devices Using Digital Light Processing

Abstract

Photocurrent response mapping is a powerful imaging technique for assessing defects and losses in photovoltaic devices. However, it has not enjoyed widespread application because high‐resolution measurements of large samples can last several hours, while weak signals from micrometer‐sized laser spots require lock‐in amplification. An alternative approach presented recently is the use of digital micromirror devices combined with compressed sensing theory. There are significant benefits when using such methods, such as signal amplification, undersampling options, and simplified measurement systems. Nevertheless, high computational requirements have limited the experimental application of this method to low‐resolution outputs. Herein, the mathematical background and the experimental approach toward megapixel resolution, ultrafast compressed sensing current mapping are presented, overcoming previous computational and experimental barriers. A high‐power digital light processing projection system is developed for the experimental application. Solutions to computational issues, sampling optimization, and measurement strategies are presented and the flexibility of the system regarding the sizes of photovoltaic devices that can be measured is demonstrated.

Compressed sensing for optical metrology of semiconductor materials and devices

“…Only contributions from pixels that coincide with unmasked (illuminated) areas of the pattern are taken into account, and their sum is calculated for each wavelength, for the whole area under consideration (area of projected pattern). The process is the equivalent of the experimental process analytically described elsewhere 16 .…”

“…In previous work, CS photocurrent mapping was introduced for optoelectronic devices 14 . The advantages of the methodology such as increased SNR, speed and flexibility of measurements were investigated 15 , and megapixel resolutions have been achieved 16 . Similar principles can also be applied for CS in the spectral domain, with patterns applied on a spectral profile and the reconstruction process yielding a spectrum instead of an image 17 .…”

Development of time-resolved photoluminescence microscopy of semiconductor materials and devices using a compressed sensing approach

“…Two types of patterns were considered in these simulations-one based on pseudo-random number generation and the other on a Walsh-Hadamard transform (WHT) [23]. Examples of a few Hadamard patterns are shown in figure 2.…”

“…algorithms have been created to compute Fast Fourier Transform quickly, without needing to first calculate the whole transform matrix. Hadamard patterns have been already demonstrated for sampling in recent CS imaging applications [23].…”

“…spatial patterns to be projected onto the sample or detector so that the compressed measurement can be acquired. CS and DLP have been successfully applied in various measurement applications such as light-beam induced current imaging for PV [23] and fluorescence microscopy for biology applications [24]. Recent work has also been carried out towards achieving temporally or spectrally resolved PL imaging using liquid crystal spatial modulators [25].…”