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The local breakdown behavior may be harmful to solar cells and could possibly permanently damage the cell. Therefore, understanding the breakdown mechanisms in commercially competitive photovoltaic devices such as monocrystalline silicon (Si) solar cells is of great importance. Here, by using the reverse-biased electroluminescence (ReBEL) imaging technique, we observed three types of breakdown phenomena in monocrystalline Si solar cells: defect-induced breakdown, avalanche breakdown, and early breakdown. We have applied a variety of methods to diagnose each breakdown mechanism. The positions of defect-induced breakdown were first determined by combining EL and ReBEL imaging. An innovation method, the distributed circuit modeling was further introduced to trace the formation of different defect-induced breakdown sites. It is firstly applied this approach in the analysis of the breakdown mechanism. Then, avalanche breakdown was demonstrated by the temperature coefficient. The origin of its emission spectra was analyzed by the Si energy band structure combined with Baraff theory. Moreover, the characteristic of early breakdown was found to be consistent with the Zener effect, which may be caused by the metal stains such as aluminum (Al) during the manufacturing process.

In this paper, Sb2S3 thin-film solar cells are fabricated by the vapor transport deposition (VTD) method. The effect of the source-substrate proximity on the performance of Sb2S3 thin-film solar cells has been investigated and comparative studies of different source-substrate proximity are carried out. The device efficiency is improved from 0.83 to 3.02% by optimizing the source-substrate proximity with the augment of open-circuit voltage, short-circuit current density and fill factor. X-ray diffraction and scanning electron microscopy studies indicate that the deposited Sb2S3 films can achieve optimal grain orientation, high crystallinity, and compact morphology. Moreover, the current transport mechanism is analyzed in detail from dark current density–voltage (J-V) measurements and shows the optimal sample to be least affected by Shockley-Read-Hall recombination and space-charge-limited current (SCLC). Meanwhile, temperature and light intensity-dependent open-circuit voltage measurements reveal the carrier recombination rates are lowest for the optimal cell in all regions, including the CdS/Sb2S3 interface, the space-charge region (SCR), and the quasi-neutral region (QNR). These can account for the efficiency enhancement of the optimal cell and can be used to facilitate the further development of Sb2S3 thin-film solar cells.

We demonstrate that the magneto-optical Kerr effect at normal incidence in the terahertz (THz) frequency range is useful for evaluating carrier transport properties of particular layers of a p-n junction. Since a single p-type thin film only exhibits a small magneto-optical Kerr effect, magneto-optical Kerr spectroscopy cannot be used to determine the carrier densities of such a film with high sensitivity. However, because the electric field is enhanced at the p-layer in a p-n junction due to the interference between the THz waves that are reflected at the highly doped substrate and the p-layer at the surface, it is possible to conduct magneto-optical Kerr spectroscopy with a higher sensitivity. We numerically calculate and experimentally determine the spectra of the ellipticity and polarization rotation angles for single n- and p-GaAs epitaxial layers and GaAs-based photovoltaic devices with a p-i-n structure and evaluate the carrier densities of the n- and p-layers. At normal incidence, this method has a high spatial resolution, which is beneficial for imaging of large-area devices.

Despite many successful realizations of laser operations in various micro/nanostructured lead halide perovskites (LHPs), the electron–hole plasma (EHP) lasing dynamics has only rarely been reported...

In this paper, we demonstrate numerical evidence that interfacial passivation in the Sb2Se3 solar cell forming the configuration of indium tin oxide (ITO)/SnO2/CdS/Sb2Se3/Au is beneficial for suppr...

We report a quantum-dot single-photon source (QD SPS) hybrid integrated on a silicon waveguide embedding a photonic crystal mirror, which reflects photons and enables efficient unidirectional output from the waveguide. The silicon waveguide is constituted of a subwavelength grating so as to maintain the high efficiency even under the presence of stacking misalignment accompanied by hybrid integration processes. Experimentally, we assembled the hybrid photonic structure by transfer printing, and demonstrated single-photon generation from a QD and its unidirectional output from the waveguide. These results point out a promising approach toward scalable integration of SPSs on silicon quantum photonics platforms., 17 pages, 5 figures

The reduction of the non-radiative recombination rate by modifying the solar cell structure without improving the material quality is discussed. Our calculations show that adopting a heterojunction design instead of a homojunction design can lead to a reduction of the non-radiative recombination rate in the depletion region due to formation of a depletion region in the wide–gap emitter material, which has a lower intrinsic carrier density. We experimentally confirm the relation between the non-radiative recombination rate in the depletion region and the carrier concentration in the wide-gap emitter layer.

Electron tunneling dynamics in asymmetric coupled triple InGaN/GaN quantum wells (ACQWs) with different well thicknesses of 3.0 nm (QW1), 2.5 nm (QW2), and 2.0 nm (QW3) were quantitatively investigated based on the time- and spectrally-resolved photoluminescence (PL) measurements and the rate-equation theory. Under weak excitation, only the emission peak of the widest well was observed at room temperature due to the effective electron tunneling from a wide to a narrow well, while all three emission peaks of the distinct wells were obtained at a high excitation level. The PL-intensity ratios of the wells in the initial transient spectra differed from those in the time-integrated spectra. With a set of rate equations and the experimental results of PL ratios and decay times, a 2 ns tunneling time from QW2 to QW1 was extracted and was decreased to 0.5 ns with increasing excitation, while the one from QW3 to QW2 was extracted to be ∼ 170 p s . The extracted tunneling times are in good qualitative agreement with the data from the exponential fitting of the PL decay traces, which can be interpreted by the energy mismatches between relevant energy levels in the ACQWs. These results provide not only a better understanding of the carrier recombination and tunneling processes in the ACQW systems but also a useful guidance for high-performance ACQW-based optoelectronic and functional devices.

Heat-recovery (HERC) solar cell is a concept of solar cell that converges heat into electricity and enables silicon solar cell to have high efficiency exceeding the Shockley-Queisser limit without requiring fast carrier extraction. In this paper, we show by numerical simulation its robustness against strong non-radiative recombination of the carriers and a positive effect arising from the bandgap reduction in the energy filtering layers.

We report efficient quantum-dot single-photon sources with unidirectional output integrated on Si by transfer printing. The device was designed to be robust against misalignment accompanied by the hybrid integration. Efficient single-photon generation was experimentally confirmed.

Bioluminescence imaging (BLI) demonstrates cellular events as a light signal at the single-cell level using a highly sensitive, cooled CCD camera. However, BLI signals are relative values and thus, images taken on different days or using different equipment cannot be compared directly. We established a reference LED light source that was characteristic of the total flux and light distribution and calibrated the BLI system as an absolute light signal. This calibrated BLI system revealed that the average light signal of beetle luciferase was at an attowatt level per sec at the single cell level.

Perovskite nanocrystals (PNCs) have emerged as highly promising optical gain materials for laser applications. Despite the recent surge of reports on their lasing performance, it remains a challenge to achieve PNCs-based single-mode vertical cavity surface emitting lasers (VCSELs) because of their inhomogeneous gain saturation. Here, we successfully demonstrate high performance single-mode VCSELs using CsPbBr3 PNCs that were synthesized via a facile ligand-assisted reprecipitation strategy. The optical gain medium with wide crystal size range was fabricated, and the dense assembly of each individual PNCs was expected to offer a homogeneous optical continuum to reduce gain saturation inhomogeneity. The fabricated VCSELs exhibit excellent single-mode lasing at ~541 nm without mode hopping and high spontaneous emission coupling factor of ~0.24, which is one order of magnitude higher than those of nitride-based VCSELs. Through streak camera direct observations, the stimulated emission of the laser device exhibits an ultrafast decay of ~10 ps, and the electron hole plasma (EHP) mechanism is considered responsible for stimulated emission in the VCSELs. This work paves the way toward next-generation high performance PNC-based single-mode laser devices with reduced fabrication cost for practical application.

The combination of lasers and solar cells has potential applications in many areas. Therefore, investigating the laser induced damage and defect formation in such cells is crucial. In this study, a Cu(In, Ga)Se2 solar cell was irradiated with a 532 nm continuous wave laser beam. The damage threshold power density with irradiation for 10 s was found to be approximately 5000 W/cm2 by analyzing the light current density–voltage curves and the variation of the solar cell efficiency after laser exposure. Further, absolute electroluminescence (EL) imaging with distributed circuit modeling was used to determine the laser induced defects. By simulating the measured injection current dependent EL intensities of the defect spots, the defects created by exposure to laser power densities less than 5000 W/cm2 could be mainly attributed to increased series resistance or increased transparent conductive oxide layer resistance, whereas those created by exposure to laser power densities greater than 5000 W/cm2 could be mainly attributed to decreased shunt resistance.

Current defect inspection methods for photovoltaic (PV) devices based on electroluminescence (EL) imaging technology lack juggling both labor-saving and in-depth understanding of defects, restricting the progress towards yield improvement and higher efficiency. Herein, we propose an adaptive approach for automatic solar cell defect detection and classification based on absolute EL imaging. Specifically, we first develop an unsupervised algorithm to automatically detect defects referring to the defect features in EL images. Then a diagnosis approach is proposed, which statistically classifies the detected defects based on the electrical origin. To the best of our knowledge, the proposed method is the first effort to integrate automatic defect detection with fine-grained classification. Experimental results on multiple types of solar cells show that the proposed method can achieve the average uncertainty of 5.15% at the minimum, with by up to 98.90% optimization ratio compared with two conventional methods. The proposed method is expected to provide more guiding feedback in both practical design and reliability diagnosis of the PV industry.

A spatially resolved absolute electroluminescence (EL) imaging method was utilized to analyze the photovoltaic properties and resistive loss properties of a GaAs thin-film solar cell. The I–V relation was extrapolated from the absolute EL efficiency measurements in conjunction with the external-quantum-efficiency (EQE) measurements; the EL extrapolated I–V relation has a merit over the conventional I–V relation measured with a solar simulator that it could eliminate the series resistance effect caused by external probe contact. Then, the mapping of the internal voltage of the solar cell and the sheet resistance of the window layer of the solar cell were obtained from the calibrated absolute EL imaging method. Finally, optic electroconversion losses of the solar cell including radiative loss, nonradiative loss, thermalization loss, transmission loss, and junction loss were quantified given by the EL and EQE measurements.

High power pulsed lasers with tunable pulse widths are highly favored in many applications. When combined with power amplification, gain-switched semiconductor lasers driven by broadband tunable electric pulsers can meet such requirements. For this reason, we designed and produced a low-cost integrated CMOS pulse generator with a minimum pulse width of 80 ps and a wide tuning range of up to 270 ns using a 40-nm microelectronic process technique. We used this pulser to drive a 1.3-µm semiconductor laser diode directly, and thereafter investigated the gain-switching properties of the laser system. The optical pulses consist of a spike followed by a steady state region. Tuning the width of the electrical pulse down to approximately 1.5 ns produces optical pulses consisting only of the spike, which has a minimum pulse-width of 100 ps. Moreover, the duration of the steady state can be tuned continuously by tuning the electrical pulse width, with a peak power of approximately 5 mW. The output voltage of the electric pulser has a tuning range of 0.8–1.5 V that can be used to directly drive semiconductor laser diodes with wavelengths in the near-infrared spectrum, which are suitable for power amplification with rare-earth doped fiber amplifiers.

By using absolute electroluminescence measurement, we characterized current-dependent external radiative efficiency of GaInAs/GaAsP multiple quantum well (MQW) single-junction solar cells with and without a back distributed Bragg reflector (DBR). Internal radiative efficiency ( η int) under illuminated condition was quantified for analyzing their difference in photovoltaic performances. It was revealed that MQWs showed an advantage of improved η int compared with a bulk layer, and that a back DBR indeed improved conversion efficiency via double-pass absorption of sun light, but improvement via reduction of rear radiative emission loss toward substrate was small. Efficiency add-on via improved material quality, or η int, in the same structures was predicted.

Using heat that otherwise would be lost is important for efficient use of solar energy, but continues to be held back for lack of an ultrathin photovoltaic absorber that permits both sufficient absorption of light and ultrafast extraction of ``hot'' carriers before they lose their kinetic energy. This article presents a concept to enable heat recovery in a solar cell $w\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}h\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}u\phantom{\rule{0}{0ex}}t$ the need for ultrafast carrier extraction. This thermoelectric approach should impact engineering solutions for further improvement in silicon solar cells, beyond the Shockley-Queisser limit.

We report local wavelength tuning of quantum-dot single-photon sources integrated on a CMOS silicon chip as well as in-situ spectral matching between two dissimilar sources. All the necessary heterogeneous integration processes were performed in a simple pick-and-place manner based on transfer printing, largely facilitating the fabrication of complicated device structures.

Two organic-inorganic halide CH3NH3PbI3 perovskite solar cells prepared in the same experimental batch but having different power conversion efficiencies −18.46% and 17.15%—were investigated based on the absolute electroluminescence (EL) efficiency measurements and traditional I-V measurements. The possible factors that affect the power conversion efficiency were also investigated. Comparing the experimental I-V curves of the two solar cells, it was found that the short-circuit currents (Jsc) were nearly the same; however, the open-circuit voltages (Voc) were markedly different. Moreover, the deduced I-V curves from the absolute EL efficiencies of the two solar cells, which were mainly affected by the material quality, were almost the same, proving that the device processing technology has a vital effect on Voc.

An attempt to apply quantum-dot (QD) intermediate band solar cell (IBSC) as bottom cell in 4-terminal tandem solar cell instead of a conventional bulk bottom cell is presented. Considering the inferior lattice qualities in QD cells, we introduce a parameter η IC = 1 ~ 0.0001 to represent slow or fast conduction-band-to-dots non-radiative relaxation rate respectively corresponding to strong or weak phonon-bottleneck effects, aiming to develop a more general IBSC model. Our results exhibit a well-designed QD solar cell can more sufficiently utilize the below-1.41eV photons passing through the GaAs top cell, which would be a better candidate as bottom cell. Compared with the highest efficiency (11.15% absolute) achieved by the best bulk bottom cell (E g = 0.93eV), QD bottom cell with E g = 1.74eV and E 1 = 1.04eV will raise an over 3% absolute of efficiency boost, when phonon bottleneck effects are running well (η IC ≥0.1). If use a GaAs-host QD bottom cell with E 1 = 0.98eV, 1.5% absolute of efficiency add-on can still be achieved, which attributes to a significant voltage boost by the separation of quasi-Fermi levels between IB and CB. Even for those QD bottom cells with inferior lattice qualities (η IC 1 .

We have directly generated optical pulses having a duration of 0.56 ps with a peak power of 25 W by gain switching of multi-section semiconductor lasers in which the optimized lengths of the absorption and gain regions were 50 and 200 µm, respectively. Even though the experiment was conducted via impulsive optical pumping at a low temperature, we observed that the multi-section gain switching suppresses the low-energy tail and chirping inherent to conventional gain switching in single-section lasers and is useful in direct short-pulse generation.

Via absolute electroluminescence measurements, we characterized degradations of internal luminescence quantum yield or internal radiative efficiency ( $\eta _{{\rm int}i}$ ) in respective subcells in GaInP/GaAs/Ge triple-junction and GaInP/GaAs double-junction solar cells after irradiations of protons and electrons with different energy and fluence ( $\phi$ ). Compared with typical open-circuit-voltage characterizations, $\eta _{{\rm int}i}$ turned out to be a sensitive, high-dynamic-range, quantitative, and fair indicator of radiation damage, since it purely represents material-quality change due to radiation damage, independently from small differences in bandgap energy due to alloy composition fluctuations and in other cell structures. A detailed fluence-dependence study has shown that the data of $\eta _{{\rm int}i}$ versus $\phi$ in moderate and high $\phi$ regions are very similar and almost independent of subcell materials, while the difference in beginning-of-life qualities of GaInP and GaAs materials causes dominant difference in subcell sensitivity to the low-radiation damages.

All-inorganic perovskite micro/nanolasers are emerging as a class of miniaturized coherent photonic sources for many potential applications, such as optical communication, computing, and imaging, owing to their ultracompact sizes, highly localized coherent output, and broadband wavelength tunability. However, to achieve single-mode laser emission in the microscale perovskite cavity is still challenging. Herein, we report unprecedented single-mode laser operations at room temperature in self-assembly CsPbX 3 microcavities over an ultrawide pumping wavelength range of 400–2300 nm, covering one- to five-photon absorption processes. The superior frequency down- and upconversion single-mode lasing manifests high multiphoton absorption efficiency and excellent optical gain from the electron–hole plasma state in the perovskite microcavities. Through direct compositional modulation, the wavelength of a single-mode CsPbX 3 microlaser can be continuously tuned from blue-violet to green (427–543 nm). The laser emission remains stable and robust after long-term high-intensity excitation for over 12 h (up to 4.3 × 10 7 excitation cycles) in the ambient atmosphere. Moreover, the pump-wavelength dependence of the threshold, as well as the detailed lasing dynamics such as the gain-switching and electron–hole plasma mechanisms, are systematically investigated to shed insight into the more fundamental issues of the lasing processes in CsPbX 3 perovskite microcavities.

Obtaining whispering gallery mode (WGM) lasing with prominent stability and high quality optical cavities is extremely desirable but still a great challenge. Despite the recent surge of reports on microspheres, nanowires and nanoplatelets metal halide crystals, there has few reports on the morphology of Hemispheres (HSs) in the form of uniform, component-tunable microcrystals. Here, CsPbX3 (X = Cl/Br) HSs with smooth surface and regular geometry structure were synthesized on silicon substrates by chemical vapor deposition (CVD) method, and the wavelength of laser can be continuously tuned from blue to green (450–540 nm) at room temperature by varying the halide composition. WGM laser with polarization ratio of 0.70 and cavity quality factor Q of 2485 were realized in the CsPbBr3 HS cavity. Besides, the crystal cavity suggested a high photonic and thermal stability for the superb capability of continuous stable lasing output under ambient conditions. All these results illustrate that the synthesis of cesium lead halide perovskite HSs may provide an alternative solution for fundamental researches and applications in optical devices.

Silicon quantum photonics provides a promising pathway to realize large-scale quantum photonic integrated circuits (QPICs) by exploiting the power of complementary-metal-oxide-semiconductor (CMOS) technology. Toward scalable operation of such silicon-based QPICs, a straightforward approach is to integrate deterministic single-photon sources (SPSs). To this end, hybrid integration of deterministic solid-state SPSs, such as those based on InAs/GaAs quantum dots (QDs), is highly promising. However, the spectral and spatial randomness inherent in the QDs pose a serious challenge for scalable implementation of multiple identical SPSs on a silicon CMOS chip. To overcome this challenge, we have been investigating a new hybrid integration technique called transfer printing, which is based on a pick-and-place operation and allows for the integration of desired QD SPSs on any locations on the silicon CMOS chips at will. Nevertheless, even in this scenario, in-situ fine tuning for perfect wavelength matching among the integrated QD SPSs will be required for interfering photons from the dissimilar sources. Here, we demonstrate in-situ wavelength tuning of QD SPSs integrated on a CMOS silicon chip. To thermally tune the emission wavelengths of the integrated QDs, we augmented the QD SPSs with optically driven heating pads. The integration of all the necessary elements was performed using transfer printing, which largely simplified the fabrication of the three-dimensional stack of micro/nanophotonic structures. We further demonstrate in-situ wavelength matching between two dissimilar QD sources integrated on the same silicon chip. Our transfer-printing-based approach will open the possibility for realizing large-scale QPICs that leverage CMOS technology.

Silicon photonics is a powerful platform for implementing large-scale photonic integrated circuits (PICs), because of its compatibility with mature complementary-metal-oxide-semiconductor (CMOS) technology. Exploiting silicon-based PICs for quantum photonic information processing (or the so-called silicon quantum photonics) provides a promising pathway for large-scale quantum applications. For the development of scalable silicon quantum PICs, a major challenge is integrating on-silicon quantum light sources that deterministically emit single photons. In this regard, the use of epitaxial InAs/GaAs quantum dots (QDs) is a very promising approach, because of their capability of deterministic single-photon emission with high purity and indistinguishability. However, the required hybrid integration is inherently difficult and often lacks the compatibility with CMOS processes. Here, we demonstrate a QD single-photon source (SPS) integrated on a glass-clad silicon photonic waveguide processed by a CMOS foundry. Hybrid integration is performed using transfer printing, which enables us to integrate heterogeneous optical components in a simple pick-and-place manner and thus assemble them after the entire CMOS process is completed. We observe single-photon emission from the integrated QD and its efficient coupling into the silicon waveguide. Our transfer-printing-based approach is fully compatible with CMOS back-end processes, and thus will open the possibility for realizing large-scale quantum PICs that leverage CMOS technology., 15 pages, 5 figures

Femtosecond semiconductor lasers are ideal devices to provide the ultrashort pulses for industrial and biomedical use because of their robustness, stability, compactness and potential low cost. In particular, gain-switched semiconductor lasers have significant advantages of flexible pulse shaping and repetition rate with the robustness. Here we first demonstrate our laser, which is initiated by very strong pumping of 100 times the lasing threshold density, can surpass the photon lifetime limit that has restricted the pulse width to picoseconds for the past four decades and produce an unprecedented ultrashort pulse of 670 fs with a peak power of 7.5 W on autocorrelation measurement. The measured phenomena are reproduced effectively by our numerical calculation based on rate equations including the non-equilibrium intraband carrier distribution, which reveal that the pulse width is limited by the carrier–carrier scattering time, instead of the photon lifetime. Femtosecond lasers are used for a vast variety of applications where super resolution is required. The authors present gain-switched semiconductor-laser operations using an extreme optical pump allowing them to generate ultrashort, high power pulses.

We systematically analyzed the detailed-balance-limit-conversion efficiency of solar cells with quantum dots (QDs) embedded in host materials. We calculated their open-circuit voltage, short-circuit current, and conversion efficiency within single-photon absorption conditions, both in the radiative limit and in other cases with non-radiative recombination loss, using modeled absorption band with various absorptivities and energy widths formed below that of the host material. Our results quantitatively revealed the existence of intrinsic and significant drops in the open-circuit voltage and conversion efficiency of QD solar cells, in addition to extrinsic drops due to degraded material quality.

Owing to the improvement of device design and material properties, the conversion efficiencies of state-of-the-art solar cells are approaching the theoretical limit. However, identification of the remaining losses is still difficult. Therefore, the quantitative analysis of losses is important for further optimization of the conversion efficiencies. In this study, we propose a method to quantitatively analyze the losses in open-circuit voltage of multi-junction solar cells. This method employs absolute electroluminescence and external quantum efficiency measurements including uncertainties. We analyze the voltage output losses for three triple-junction solar cells and discuss possible improvements of multi-junction solar cells.

A contactless measurement of the picosecond dynamics of photovoltage in two GaAs solar cells, one with n-layer on the top and the other with p-layer, was performed using time-resolved photoemission spectroscopy. We found that the shift of photoelectron spectra occurs in opposite directions for the two samples. This agrees with the interpretation that the spectral shift is caused by photovoltage at the p-n junction. We also summarize the limiting factors on the present technique including the degradation of the temporal resolution caused by propagation effect, and the deformation of spectra originating from the work-function cutoff, and discuss the possible solutions.

Via absolute electroluminescence imaging measurement, we systematically examine the drops in open-circuit voltage (V OC ) and conversion efficiency for an InAs quantum dot (QD) solar cell, in contrast to a similar geometric GaAs bulk solar cell. Our results quantitatively reveal that apart from an extrinsic (V OC ) drop of 137mV stemming from the inferior lattice quality, an intrinsic drop of 115mV also co-exist in QD cell caused by the practical band edge extending towards lower energy. Moreover, the extrinsic and intrinsic drops in conversion efficiency are respectively evaluated as 5.6% and 1.6%, indicating even without lattice deterioration, such QD cell still cannot exceed the conversion efficiency of the bulk-host-material solar cell, if the boost in photocurrent cannot compensate the large intrinsic voltage drop induced by the narrow-gap QD material. This study could contribute to clarify the empirical characteristics of realistic QD solar cells, and also to verify the validity of the present theoretical QD-cell models.

We propose a new concept of a solar cell that enables heat recovery to improve its conversion efficiency, namely “heat recovery (HERC) solar cell”. The two key elements are an absorber hotter than electrodes and carrier-energy selecting potential barriers placed between the absorber and electrodes, whose bandgap is larger than that of the absorber. HERC solar cell operating out of equilibrium can have high efficiency exceeding the detailed-balance limit. Unlike hot carrier solar cells, it does not require fast carrier extraction within the thermalization time, which largely improves the feasibility. HERC solar cell can be realized with a silicon absorber.

Magnetic field effect on enzymatic reactions is under intensive study in the past decades. Recently, it was reported that firefly bioluminescence was suppressed and red-shifted significantly when exposed to external magnetic field. However in this work, by means of selective excitation, we confirmed that emission properties of firefly light emitter “oxyluciferin” are completely immune to external magnetic field of up to 53 T. These findings pose strong contrast to existing relevant results. Potential reasons for the discrepancies found and the underlying physics towards the understanding of firefly bioluminescence were discussed.

We investigated the gain-switching properties of GaN-based ridge-waveguide lasers on free-standing GaN substrates with low-cost nanosecond current injection. It was observed that the output pulses with intense injection consisted of an isolated short pulse with a duration of around 50 ps at the high-energy side and a long steady-state component at the lower energy side independent of the electric pulse duration. The energy separation between the short pulse and steady-state component can be over 30 meV, favoring short-pulse generation with the spectral filtering technique. The duration of the steady-state component can be tuned freely by controlling the duration and voltage of the electric pulse, which is very useful for generating pulse-width-tunable optical pulses for various applications.

Absolute electroluminescence (EL) measurements have been becoming essential methods not only to evaluate internal properties of individual subcells in multi-junction solar cells but also to characterize performance of series-connected solar-cell arrays. A photodetector-contact method., in which a spectral-sensitivity-calibrated photodetector is proximately placed on a sample surface in face-to-face geometry, is a simple and useful quantitative method for characterizing the absolute EL intensities of the solar cells. In this work, we investigated collection efficiencies of emission for this method by numerical calculations based on a ray-tracing calculation model and evaluated accuracy in the absolute EL measurements of the solar-cell devices using this method.

Aiming at the realization of high efficiency and low cost simultaneously, we modeled light absorption by subcells for a substrate-free thin-film multi-junction cell with a rear texture. The model evaluated subcell photocurrents and the requisite thicknesses at current matching. For InGaP/GaAs/lnGaAs metamorphic 3-junction solar cells, the rear texture significantly enhanced light absorption in the InGaAs bottom cell and GaAs middle subcells, and the requisite thickness of InGaAs is almost less than 10% of that with flat rear reflector, even GaAs is also less than half of that without rear texture, if InGaP-subcell is not greater than 600 nm. The reduced lower subcell thickness by the texture rear surface will mitigate the difficulty of metamorphic growth and contribute to the cost reduction of multi-junction solar cells.

Absolute electroluminescence (EL) measurements have been becoming essential methods to evaluate internal properties of individual subcells in multi-junction (MJ) solar cells. To establish methodology of absolute EL for standardization of MJ solar-cell characterizations, we developed independently calibrated absolute EL methods and verified accuracy and uncertainties via round-robin studies measuring EL emission rates for InGaP/GaAs/Ge triple-junction and single-junction solar cells. The results showed good quantitative agreements within 10% discrepancy among the individually calibrated methods, which corresponds to less-3-meV uncertainties in the estimation of the open-circuit voltages. This enables accurate comparison of the deviations between the open-circuit voltages via the transport experiment and via the absolute EL methods and detailed characterization of the internal properties of MJ solar cells.

Low threshold lasing at room temperature was achieved in optically pumped GaN-based vertical-cavity surface-emitting lasers (VCSELs) with sub-nanometer roughness polishing. The cavity region sandwiched by two dielectric distributed Bragg reflectors, incorporating InGaN/GaN multiquantum wells, a p-type AlGaN layer, and n- and p-type GaN layers, is a typical structure for electrically driven VCSELs. We observed lasing at a wavelength of 431.0 nm with a low threshold pumping energy density of ~ 3.2 mJ/cm2 and a high spontaneous emission coupling factor of ~ 0.09. These results were attributed to the significant reduction of the internal cavity loss by the removal of the high-dislocation GaN region, the reduction of cavity length, and the achievement of sub-nanometer level surface roughness (root mean square roughness of 0.3 nm) via inductively coupled plasma etching and chemical mechanical polishing. The loss mechanism is discussed and loss is quantitatively calculated in this letter.

We have investigated the electrical and optical properties of p–i–n junction structures in which un-doped GaNAs/GaAs multiple quantum wells (MQWs) were sandwiched by p- and n-doped GaAs layers. The samples were formed on the GaAs (001) substrates by plasma assisted molecular beam epitaxy (RF-MBE) using the modulated N radical beam method. We have prepared several samples for various GaNAs MQW structures. The electroluminescence (EL) measurements showed slightly different spectra to those of photoluminescence (PL) and the EL intensities were almost proportional to the applied currents. The room temperature EL measurement revealed strong electron confinement of GaNAs/GaAs MQW.

In-situ characterization is one of the most powerful techniques to improve material quality and device performance. Especially in view of highly efficient tandem solar cells this is an important issue for improving the cost-performance ratio. Optical techniques are suitable characterization methods, since they are non-destructing and contactless. In this work, we measured the power dependence of photoluminescence (PL) from the InGaP and GaAs subcells of an industry-standard triple-junction solar cell. High luminescence yields enhance the luminescence coupling, which was directly verified by time-resolved PL measurements. We present a new method to determine the internal luminescence efficiencies of InGaP and GaAs subcells with the aid of luminescence coupling. High luminescence efficiencies of 90% for GaAs and more than 20% for InGaP were found, which suggest that the material quality of the grown GaAs layer is excellent while the intrinsic luminescence limit of InGaP is still not reached even for high excitation conditions. The PL method is useful for probing the intrinsic material properties of the subcells in flat band condition, without influence of transport. Since no calibration of absolute PL is required, a fast screening of the material quality is possible, which should be extremely helpful for the solar cell industry.

We performed time-resolved terahertz reflection spectroscopy in a GaAs p-i-n device. While p- and n-doped layers are not transparent in low frequency region, we observed the characteristic spectral modulation under the photoexcitation. From the spectral analysis, we quantitatively evaluate the carrier density in the active layer with low concentration. We propose this scheme for a new tool for evaluating devices.