Photoluminescence Imaging Research Articles

Probing the dipole–dipole interaction in single 0D/2D heterostructures by tightly focused cylindrical vector beams

Energy transfer from zero-dimensional (0D) quantum dots (QDs) to two-dimensional (2D) materials has attracted much attention for both the manipulation of fundamental material properties and their potential device applications. An understanding of the effect of dipole interactions on energy transfer rate in the hybrid dimensional system is essential for improving optoelectronic device performance. Here, we report the dipole-orientation-dependent energy transfer from individual core–shell CdSe/ZnS QDs to bilayer molybdenum disulfide (MoS2) by utilizing tightly focused azimuthally and radially polarized cylindrical vector beams. With second-order photon correlation measurements [g2(τ)], we show the single-photon emission behavior from QDs in 0D/2D heterostructures, indicating that the investigated heterostructure is constructed from single QDs. By polarization resolved photoluminescence (PL) imaging and PL lifetime measurements, we observe a fast energy transfer rate of the system excited with azimuthally polarized beams and interpret it based on dipole–dipole interactions with Förster energy transfer theory. Our work provides an in-depth understanding of the dipole-orientation-dependent energy transfer mechanism in 0D/2D systems, which could offer guidance for designing the related optoelectronic device applications.

Integration of a superconducting nanowire single-photon detector into a confocal microscope for time-resolved photoluminescence (TRPL)-mapping: Sensitivity and time resolution.

This report highlights the combination of the MicroTime 100 upright confocal fluorescence lifetime microscope with a Single Quantum Eos Superconducting Nanowire Single-Photon Detector (SNSPD) system as a powerful tool for photophysical research and applications. We focus on an application in materials science, photoluminescence imaging, and lifetime characterization of Cu(InGa)Se2 (CIGS) devices intended for solar cells. We demonstrate improved sensitivity, signal-to-noise ratio, and time-resolution in combination with confocal spatial resolution in the near-infrared (NIR) range, specifically in the 1000-1300nm range. The MicroTime 100-Single Quantum Eos system shows two orders of magnitude higher signal-to-noise ratio for CIGS devices' photoluminescence imaging compared to a standard NIR-photomultiplier tube (NIR-PMT) and a three-fold improvement in time resolution, which is now limited by the laser pulse width. Our results demonstrate the advantages in terms of image quality and time resolution of SNSPDs technology for imaging in materials science.

Nature of contaminants introduced in silicon wafers during molecular beam epitaxy chamber annealing

Epitaxial monolithic III–V/Si tandem solar cells are one of the most promising technologies to be adopted by the industry after the efficiency of the current market dominating single junction silicon solar cell saturates at its fundamental limit. One of the key limitations of this technology is the degradation of silicon wafers during in situ annealing in the molecular beam epitaxy chamber. Determining the nature of contaminants in this process is key to improve the efficiency of epitaxial tandem solar cells. However, to date, the nature of contaminants from molecular beam epitaxy chambers remains unknown. In this work, we use photoluminescence imaging, lifetime spectroscopy, and deep level transient spectroscopy to measure the electronic properties of extrinsic impurities incorporated during annealing in the molecular beam epitaxy chamber. Photoluminescence images reveal that at least two impurities diffuse into silicon wafers during molecular beam epitaxy annealing. One is highly localized, while the other one is distributed uniformly across the whole wafer. Phosphorus diffusion is found to confine the localized impurity within the diffused layer but is ineffective at preventing the indiffusion of other impurities. Lifetime spectroscopy shows that metastable impurities with characteristic similar to Cr and CrB in our molecular beam epitaxy annealed silicon wafers. No evidence of Fe or FeB was observed. The emission rates and concentrations of the electrically active defects were measured with deep-level transient spectroscopy: The emission rates of detected defects do not match that of known Cr-related defects.

Identifying and Investigating Spatial Features in InGaAs Solar Cells by Hyperspectral Luminescence Imaging.

Hyperspectral luminescence imaging adds high-resolution spectral data to electroluminescence and photoluminescence images of photovoltaic materials and devices. This enables absolute calibration across a range of spectra, and subsequently enhances the information that can be gained from such measurements. We present a temperature-dependent luminescence hyperspectral imaging study of dilute InGaAs solar cells. We are able to identify the cause of dark spots on the device as local areas with increased defect-related recombination and identify a likely candidate for the type of defect. Hyperspectral images also reveal a device-wide pattern in low-energy-tail luminescence and In alloy fraction, which corresponds with increased nonradiative recombination. This pattern would not be identifiable with conventional imaging methods. Detailed information on such features is useful as, paired with knowledge of fabrication processes and device design features, it can help identify ways to reduce associated non-radiative recombination and improve device performance.

Interfacial-Water-Modulated Photoluminescence of Single-Layer WS2 on Mica.

Because of their bandgap tunability and strong light-matter interactions, two-dimensional (2D) semiconductors are considered promising candidates for next-generation optoelectronic devices. However, their photophysical properties are greatly affected by their surrounding environment because of their 2D nature. In this work, we report that the photoluminescence (PL) of single-layer WS2 is substantially affected by interfacial water that is inevitably present between it and the supporting mica substrates. Using PL spectroscopy and wide-field imaging, we show that the emission signals from A excitons and their negative trions decreased at distinctively different rates with increasing excitation power, which could be attributed to the more efficient annihilation between excitons than between trions. By gas-controlled PL imaging, we also prove that the interfacial water converted the trions into excitons by depleting native negative charges through an oxygen reduction reaction, which rendered the excited WS2 more susceptible to nonradiative decay via exciton-exciton annihilation. Understanding the role of nanoscopic water in complex low-dimensional materials will eventually contribute to devising their novel functions and related devices.

Integration of a superconducting nanowire detector into a confocal microscope for TRPL-mapping: Sensitivity and time resolution.

Superconducting Nanowire Single Photon Detectors (SNSPDs) were introduced to the market in 2003 and are used in multiple quantum optics applications, due to their outstanding properties such as fast instrument response and high quantum efficiency. The high quantum efficiency is important for materials science applications with emission in IR-range >1000 nm, where other available single photon detectors have a low sensitivity, high dark noise, and slow time response. We have integrated a SNSPD (Single Quantum) into a confocal photoluminescence lifetime microscope MicroTime 100 (PicoQuant) in order to compare the performance of different SNSPD designs with a standard IR-PMT for time-resolved photoluminescence (TRPL) measurements and imaging of IR sample e.g., CIGS device. One used SNSPD channel was a classical single mode fiber coupling to guide the light onto the sensor, the other channel used an internal multimode fiber instead. We achieved a significant increase in photoluminescence sensitivity in both designs compared to a standard IR-PMT, as well as a several times higher sensitivity of the multimode-fiber coupled nanowire compared to the singe-mode fiber one, despite comparable photon quantum efficiencies in this wavelength range. The increased sensitivity combined with the lower dark count rate resulted in an increase of signal-to-noise ratio by more than 2 orders of magnitude compared to the IR-PMT. The high sensitivity of SNSPDs combined with high temporal resolution (instrument response function of this setup was below 100 ps) allows to identify and investigate highly quenched signals of samples even at low illumination levels. The integration of the SNSPD detector to a scanning confocal microscope will also enhance the capabilities in field of single oxygen sensing and cell imaging in NIR by using NIR emitting dyes or nanoparticles without autofluorescence.

Integrating Photoluminescence and Ferromagnetism in Carbon Quantum Dot/ZnO by Interfacial Orbital Hybridization for Multifunctional Bioprobes.

Integrating ferromagnetism (FM) and photoluminescence (PL) into one particular nanostructure as biological probe plays an irreplaceable role in accurate clinical diagnosis combining magnetic resonance and photoluminescence imaging technology. However, magnetic emergence generally needs a spin polarization at Fermi level to display a half-metallic electronic feature, which is not beneficial for preserving radiation recombination ability of photo-excited electron-hole carriers. To overcome this intrinsic difficulty, we propose a feasible atomic-hybridization strategy to anchor carbon quantum dots (CQDs) onto ZnO microsphere surface via breakage of C=O bonds at CQDs and subsequent Zn-3d and C-2p orbital hybridization, which not only ensures the carrier recombination but also leads to a room-temperature magnetism. Herein, the photoluminescence and magnetism coexist in this multifunctional heterojunction with outstanding biocompatibility. This work suggests that integration of magnetism and photoluminescence could be accomplished by particular interfacial orbital hybridization.

Anisotropic exciton drift-diffusion in a monolayer WS2x Se(2–2x) alloy with a gradually changing composition

We demonstrate anisotropic exciton drift-diffusion in a WS2x Se(2–2x) alloy monolayer with a gradually changing exciton energy caused by the spatial variation in the chalcogen composition ratio. The photoluminescence (PL) images under isotropic laser excitation are extended toward the lower exciton energy side. Moreover, the degree of asymmetry in the PL image increases at the positions with a steep exciton energy gradient. The anisotropy in the PL images and its correlation with the exciton energy gradient are reproduced considering the exciton kinetics obeying a drift-diffusion equation. The findings may support the development of excitonic devices using transition metal dichalcogenides.

Investigations on photovoltaic material absorptivity using hyperspectral photoluminescence excitation imaging

Photoluminescence imaging has become a standard method to characterize solar cells. However, performing some quantitative analysis of this technique requires the assumption of uniform local absorptivity, which cannot be directly measured using traditional methods. This study presents a novel approach to measure the local relative absorptivity over a broad spectral range for a perovskite absorber deposited on a charge extraction layer and an electrode. By analyzing the photoluminescence intensity as a function of the incident photon energy, we were able to determine the relative absorptivity of the incident light above the bandgap energy. Additionally, luminescence spectra allow us to accurately assess the absorptivity near the bandgap energy from the reciprocity between absorption and emission. Reflectivity measurements were also performed to further understand the possible limitations of our experiment and to discuss our results. Finally, this method was able to distinguish between variations in the photoluminescence response caused by slight differences in the local film thickness and changes in the local carrier lifetime.

Impact of organic particles from wafer handling equipment on silicon heterojunction pseudo-efficiency

Within this paper a systematic analysis of particle transfer onto SHJ Solar cell precursors by handling with suction cups and the impact on the pseudo efficiency is presented. The study establishes a correlation between particle area coverage and a resulting loss of pseudo solar cell parameters. The analysis was carried out on one hand by means of SEM measurements at the contact points between suction cup and wafer to quantify particle transfer and on the other hand by means of suns photoluminescence imaging measurements to evaluate the resulting losses. It is shown that the choice of contact material and the wafer temperature have a significant influence on the transferred particle number, their size and the resulting particle area coverage. A local electrical defect was observed at these particle-rich spots, which also affected a larger area around this insufficiently passivated region. This had a significant negative effect on the pseudo efficiency, which is more pronounced for increasing particle area coverage. If the particle density is increased by 0.1% within an area of 800 mm2, the pseudo efficiency in this area decreases by almost 1.2%Relative. The correlation found can be used to predict an efficiency loss using standard photoluminescence images.

Impact of acetic acid exposure on metal contact degradation of different crystalline silicon solar cell technologies

Degradation due to acetic acid in photovoltaic (PV) modules has been a commonly observed phenomenon for both damp-heat exposure and outdoor operations. Acetic acid is a degradation byproduct of ethylene-vinyl acetate (EVA), a common module encapsulant. To address this issue, robust metallization pastes and cell technologies are being developed. However, it is important to assess how these technologies perform in an acetic acid environment and withstand degradation before they are implemented in the solar market. In this work, we investigate the impact of acetic acid exposure on four different cell groups: monofacial passivated emitter and rear contact (PERC) cells with advanced telluride-based front contact pastes, bifacial PERC cells with novel aluminum rear contact pastes, bifacial tunnel oxide passivated contacts (TOPCon) cells, and silicon heterojunction (SHJ) cells. These cells were exposed to acetic acid for different time increments. The recombination losses were characterized by Suns- V OC , and multi-variate regression analysis of intensity-dependent photoluminescence (PL) images with Griddler AI. Resistive losses were tracked with the transmission line method (TLM). Samples showing severe performance degradation were selected for further materials characterization to understand the root cause. Top-down and cross-sectional scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) were performed to investigate the change in materials properties. Our study shows that the front contacts of the bifacial TOPCon cells and monofacial PERC cells were significantly affected by acetic acid exposure. The SHJ cells were found to be the most stable. • Effect of acetic acid on metallization pastes and cell technologies was investigated. • Front Ag-Al contacts for Bifacial TOPCon cells degrade the most due to acid exposure. • Monofacial PERC cells with Pb-free Te-O paste is the second group that is affected. • Bifacial PERC cells were stable with Al back contacts showing no signs degradation. • SHJ cells were the most stable group in this experiment.

A SWIN TRANSFORMER IN THE CLASSIFICATION OF IR IMAGES OF PHOTOVOLTAIC MODULE

The faults occurring in the photo voltaic module as to be detected in order to increase its efficiency. The infrared images, electroluminescent images, and photo luminescent images of the photo voltaic modules have been used to detect and classify the faults. The infrared data set is used for classification and it is highly imbalanced. To make it balanced, generative adversarial networks are used to generate images for each fault category. If the fault classification is done using a convolution neural network, the feature maps are generated by convolution operation with filters on images. The convolution neural network model is to be trained for 1000 epochs and the time required for training is greater than 24 hours. The computational cost of a convolution neural network (CNN) is reduced in transformers through the attention mechanism. The Swin (shifted window) transformers which are used for classification as to be trained for 40 epochs and the maximum training time is less than 6 hours. The computation time, categorical classification accuracy, and top-5-accuracy obtained using Swin classifier are compared with the existing methods. It is found that the computational cost is very much reduced by the Swin transformer and top-5-accuracy of 99.04% is obtained by it while classifying 11 faults of IR images.

Synergetic Passivation of Metal‐Halide Perovskite with Fluorinated Phenmethylammonium toward Efficient Solar Cells and Modules

AbstractSurface passivation with organic halide salts is a powerful strategy to enhance the performance of perovskite solar cells. However, the inevitable formed in‐plane favored two‐dimensional perovskite layers with low carrier mobility and high binding energy inhibit the interfacial charge transfer within the device. Herein, a bulky fluorinated phenmethylammonium salt is designed and synthesized to passivate the perovskite film without forming 2D perovskites. A strong interaction which is induced by an electron donation from passivation agent to perovskite not only reduces the defects at the top surface of the perovskite, but also suppresses the recombination reaction at the buried surface due to a permeation of the organic halide salt. Moreover, the results of time resolved photoluminescence and confocal microscopy images suggest that the interfacial charge transfer speed and uniformity are enhanced. As a result, the efficiency of a small‐area device increases from 20.7 ± 0.9% to 22.8 ± 0.4% (aperture: 0.16 cm2). Moreover, a stabilized efficiency of 18.0% (aperture: 10.0 cm2) is achieved for larger‐area modules with 6‐series connected sub‐cells. Equally important, the non‐encapsulated modules show significantly improved stability at ambient conditions (ISOS‐D‐1). These significant improvements provided by a simple and reproducible procedure can be readily adopted in other types of devices.

Partial dislocation structures at expansion terminating areas of bar-shaped single Shockley-type stacking faults and basal plane dislocations at the origin in 4H-SiC

Partial dislocation (PD) combinations near the substrate/epilayer interface and the epilayer surface of 4H-SiC are analyzed for bar-shaped single Shockley-type stacking faults (1SSFs) by plan-view transmission electron microscopy (TEM) with the aid of photoluminescence imaging. Although the PDs are found to have a zigzag structure similar to that found in triangular 1SSF by TEM observation, the combination is thought to be different, consisting of a 30° Si-core + 90° Si-core for the triangular 1SSF and 30° Si-core + 30° C-core for the bar-shaped 1SSF. The features of the basal plane dislocation at the origin are speculated by also identifying the converted threading edge dislocation by additional Burgers vector determination of the PD loop by TEM.

In Situ Process Monitoring and Multichannel Imaging for Vacuum‐Assisted Growth Control of Inkjet‐Printed and Blade‐Coated Perovskite Thin‐Films

AbstractVacuum‐assisted growth (VAG) control is one of the most promising methods for controlling nucleation and crystallization of printed and coated large area lead halide perovskite‐based layers for optoelectronics. To coat or print homogeneous high‐quality perovskite thin‐films at high fabrication yield, real‐time process monitoring of the VAG is pivotal. In response, a 2.1‐megapixel multichannel photoluminescence (PL) and reflection imaging system is developed and employed for the simultaneous spatial in situ analysis of drying, nucleation, and crystal growth during VAG and subsequent thermal annealing of inkjet‐printed and blade‐coated perovskite thin‐films. It is shown that the VAG process, for example, evacuation rate and time, affects the film formation and provide detailed insight into traced PL and reflection transients extracted from sub‐second videos of each channel. Based on correlative analysis between the transients and, for example, perovskite ink composition, wet‐film thickness, or evacuation time, key regions which influence crystal quality, film morphology, and are base for prediction of solar cell performance are identified.

Structural and Photophysical Properties of Guanidinium–Iodide‐Treated Perovskite Solar Cells

Use of the guanidinium iodide (GAI) cation is widely recognized as an interface engineering technique for perovskite solar cells that deliver stability improvements via defect passivation on surfaces and grain boundaries. However, a comprehensive understanding of the relationship between the structural and photophysical properties is lacking. Herein, GAI‐induced perovskite structural modifications, including derivative phases and underlying transitions, are detected in GAI surface‐treated Cs0.07MA0.14FA0.79Pb(I0.83Br0.17)3 through an analysis of X‐ray and electron diffraction and microscopy data. An optimum GAI solution concentration at 10 mg mL−1 can eliminate excess PbI2, improve crystallinity, and increase grain size of the as‐prepared perovskite films. However, a further increase to 20–40 mg mL−1 induces new (FAPbI3) x (GA2PbI4) x phases and a reduction in crystallinity and grain size. In addition, from confocal photoluminescence imaging, it is observed that 10 mg mL−1 GAI also helps to remove the microscale spatial heterogeneities, demonstrating optimum device performance. These results show that understanding the impact on structure and microstructure of the selection and concentration of surface treatment agents is critical for the homogenization of perovskite optoelectronic properties and achieving efficient device.

Exciton spectroscopy and unidirectional transport in MoSe2-WSe2 lateral heterostructures encapsulated in hexagonal boron nitride

Chemical vapor deposition (CVD) allows lateral edge epitaxy of transition metal dichalcogenide heterostructures. Critical for carrier and exciton transport is the material quality and the nature of the lateral heterojunction. Important details of the optical properties were inaccessible in as-grown heterostructure samples due to large inhomogeneous broadening of the optical transitions. Here we perform optical spectroscopy of CVD grown MoSe2-WSe2 lateral heterostructures, encapsulated in hBN. Photoluminescence (PL), reflectance contrast and Raman spectroscopy reveal optical transition linewidths similar to high quality exfoliated monolayers, while PL imaging experiments uncover the effective excitonic diffusion length of both materials. The typical extent of the covalently bonded MoSe2-WSe2 heterojunctions is 3 nm measured by scanning transmission electron microscopy (STEM). Tip-enhanced, sub-wavelength optical spectroscopy mapping shows the high quality of the heterojunction which acts as an excitonic diode resulting in unidirectional exciton transfer from WSe2 to MoSe2.

Printed Hybrid Upconversion/Perovskite Nanoparticle Cluster and Its Individual Photoluminescence Imaging

AbstractA versatile strategy to print hybrid upconversion/perovskite nanoparticle clusters based on nanoxerography has been developed. The individual hybrid cluster can be tuned by different upconversion nanoparticles (UCNPs), perovskite quantum dots (QDs), and their combinations to visualize multicolor. Based on the precise positioning of the array system, the structural and optical properties of each cluster in the array can be in situ and simultaneously characterized. This approach provides a straightforward and robust platform for high‐throughput preparation and characterization of the hybrid clusters, and the ability to position functional nanoparticles at a desired area with nanoscale accuracy exhibits great potential in fabricating advanced nanodevices.

Epitaxially Grown p‐type Silicon Wafers Ready for Cell Efficiencies Exceeding 25%

Combining the advantages of a high‐efficiency solar cell concept and a low carbon footprint base material is a promising approach for highly efficient, sustainable, and cost‐effective solar cells. In this work, we investigate the suitability of epitaxially grown p‐type silicon wafers for solar cells with tunnel oxide passivating contact rear emitter. As a first proof of principle, an efficiency limiting bulk recombination analysis of epitaxially grown p‐type silicon wafers deposited on high quality substrates (EpiRef) unveils promising cell efficiency potentials exceeding 25% for three different base resistivities of 3, 14, and 100 Ω cm. To understand the remaining limitations in detail, concentrations of metastable defects Fei, CrB and BO are assessed by lifetime‐calibrated photoluminescence imaging and their impact on the overall recombination is evaluated. The EpiRef wafers’ efficiency potential is tracked along the solar cell fabrication process to quantify the impact of high temperature treatments on the material quality. We observe large areas with few structural defects on the wafer featuring lifetimes exceeding 10 ms and an efficiency potential of 25.8% even after exposing the wafer to a thermal oxidation at 1050 °C.

Up/Down-Scaling Photoluminescent Micromarks Written in Diamond by Ultrashort Laser Pulses: Optical Photoluminescent and Structural Raman Imaging.

Elongated photoluminescent micromarks were inscribed inside a IaAB-type natural diamond in laser filamentation regime by multiple 515 nm, 0.3 ps laser pulses tightly focused by a 0.25 NA micro-objective. The micromark length, diameter and photoluminescence contrast scaled as a function of laser pulse energy and exposure, coming to a saturation. Our Raman/photoluminescence confocal microscopy studies indicate no structural diamond damage in the micromarks, shown as the absent Raman intensity variation versus laser energy and exposition along the distance from the surface to the deep mark edge. In contrast, sTable 3NV (N3)-centers demonstrate the pronounced increase (up to 40%) in their 415 nm zero-phonon line photoluminescence yield within the micromarks, and an even higher-ten-fold-increase in NV0-center photoluminescence yield. Photogeneration of carbon Frenkel "interstitial-vacancy" (I-V) pairs and partial photolytic dissociation of the predominating 2N (A)-centers were suggested to explain the enhanced appearance of 3NV- and NV-centers, apparently via vacancy aggregation with the resulting N (C)-centers or, consequently, with 2N- and N-centers.