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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...

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.

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.

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.

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.

The possibility of evaluating minority-carrier mobility and lifetime by measuring fast photovoltage dynamics of completed cells via time-resolved photoemission spectroscopy is investigated. A numerical simulation based on drift-diffusion model is performed to investigate how mobility and lifetime can be extracted. A simulation assuming a constant lifetime shows that photovoltage decay reflects carrier lifetime over two orders of magnitude of the photo-injection level while photoluminescence decay does not. The possibility of a detailed analysis on recombination mechanisms is discussed. The discrepancy in the magnitude of photovoltage observed in the previous measurement was investigated. The contribution from surface band-bending is discussed.

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 .

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.

In this study, Sb2(S,Se)3 thin films are fabricated using the vapor transport deposition (VTD) method, with Sb2Se3 and Sb2S3 mixed powders as the evaporation source. The performance of the corresponding glass/ITO/CdS/Sb2(S,Se)3/Au solar cells are found to be correlated to the mass ratio between Sb2S3 and the overall powder mixture. The properties of the Sb2(S,Se)3 thin films and cell devices adopting four different Sb2S3 mass ratios (x = 0.1, 0.25, 0.5 and 0.75) are compared. Further, the electrical properties – from dark and light J-V measurements; structural properties – from X-ray diffraction and scanning electron microscope measurements; and carrier-recombination rates at the buffer/absorber interface in the space-charge region (SCR) and in the quasi-neutral region – from temperature-illumination-dependent open-circuit voltage (VOC) measurements – are compared. It is found that a Sb2(S,Se)3 solar cell with a Sb2S3 mass ratio of 0.25 had optimal crystallinity, the lowest density of deep traps and the smallest carrier-recombination rates at the interface, leading to a high efficiency of 7.31%.

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.

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.

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.

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.

The ideal solar cell conversion efficiency limit known as the Shockley-Queisser (SQ) limit, which is based on a detailed balance between absorption and radiation, has long been a target for solar cell researchers. While the theory for this limit uses several assumptions, the requirements in real devices have not been discussed fully. Given the current situation in which research-level cell efficiencies are approaching the SQ limit, a quantitative argument with regard to these requirements is worthwhile in terms of understanding of the remaining loss mechanisms in current devices and the device characteristics of solar cells that are operating outside the detailed balance conditions. Here we examine two basic assumptions: (1) that the photo-generated carriers lose their kinetic energy via phonon emission in a moment (fast thermalization), and (2) that the photo-generated carriers are extracted into carrier reservoirs in a moment (fast extraction). Using a model that accounts for the carrier relaxation and extraction dynamics, we reformulate the nonequilibrium theory for solar cells in a manner that covers both the equilibrium and nonequilibrium regimes. Using a simple planar solar cell as an example, we address the parameter regime in terms of the carrier extraction time and then consider where the conventional SQ theory applies and what could happen outside the applicable range., Comment: 25 pages, 15 figures

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.

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.

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 measured gain spectra of highly uniform current-injection T-shaped quantum wire lasers and investigated the physics of gain-spectral-shape modulation due to strong Coulomb interactions by comparing current-injection and optical-pumping measurements and additional calculations including screened Coulomb interaction. We found remarkable features for current injection: redshifts and long high- and low-energy tails of the gain peak. These characteristic current-injection features were ascribed to band-gap renormalization, state-filling, and carrier-carrier scattering inherent to charge-imbalanced electron-hole systems (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We measured the time profiles of terahertz (THz) radiation emitted from a GaAs-based solar cell under weak excitation with ultrashort optical pulses. The time-domain THz waveform directly reflects the characteristic saturation and overshooting of the drift current under high fields in the thin depletion layer of the solar cell, from which we can evaluate the internal electric field quantitatively. We also measured the time profiles of THz radiation for simultaneous excitation with ultrashort pulses and continuous light and observed the reduction of the internal field in the depletion layer of the solar cell during operation. Since nonlinear responses of the drift current in the depletion layer can be observed, we conclude that THz measurements provide a key technology for characterizing the solar cell performance that can be expected under practical operating conditions.We measured the time profiles of terahertz (THz) radiation emitted from a GaAs-based solar cell under weak excitation with ultrashort optical pulses. The time-domain THz waveform directly reflects the characteristic saturation and overshooting of the drift current under high fields in the thin depletion layer of the solar cell, from which we can evaluate the internal electric field quantitatively. We also measured the time profiles of THz radiation for simultaneous excitation with ultrashort pulses and continuous light and observed the reduction of the internal field in the depletion layer of the solar cell during operation. Since nonlinear responses of the drift current in the depletion layer can be observed, we conclude that THz measurements provide a key technology for characterizing the solar cell performance that can be expected under practical operating conditions.

A high-power beam can be achieved in spectrally beam combined systems due to the interaction between internal and external cavities. In order to investigate effects of the coupled cavity on the combined beam, we started with the derivation of the equivalent reflectance and transmittance of the external cavity based on a multilayer dielectric grating (MDG). Then, photons in active regions of diode lasers with/without feedback were calculated by utilizing Lang-Kobayashi rate equations. The results indicate that the lens and the MDG have an obvious impact on both the reflectance and transmittance of the external cavity, which determine whether the photons of a laser with feedback are larger or smaller than those without feedback. The photons of the laser with feedback and the transmittance of the external cavity codetermine the combining efficiency. Impacts of the confinement factor and spontaneous emission factor of diode laser emitters on the coupled cavity are similar to those on a common laser internal cavity. Spectral compositions of the combined beam have different brightness from each other.

Gain spectra of 1500-nm-wavelength GaInAsP/InP multiple-quantum-well (MQW) laser diodes were obtained over wide wavelength and injection current ranges. To cover wide spectral ranges, we combined gain spectra derived from amplified spontaneous emission (ASE) spectra and white-light transmittance spectra. In addition, gain (absorption) spectra for very low injection currents were obtained through ASE measurements using a liquid-nitrogen-cooled InGaAs photodiode array detector with high sensitivity. From these gain spectra, accurate estimations of the material gain and carrier-density-dependent internal loss as well as the material-transparency current were obtained.

We developed methodologies and calibration standards for absolute electroluminescence (EL) measurements for CONTACT-LESS evaluation of various internal properties of multi-junction and arrayed solar cells, such as open-circuit voltages, external and internal radiative efficiencies, and luminescence-coupling efficiency. Several independent calibration methods were compared that used: 1) a calibrated EL imaging system, 2) proximity measurement with a large-area photodiode, 3) an integrating-sphere system, and 4) planar light-emitting diodes with a circular aperture. The comparison clarified the advantages and disadvantages of each method, and showed consistency within 30% uncertainty, resulting in a 7-meV uncertainty in open-circuit voltage measurements.

We have used in situ micro-Raman spectroscopy to clarify the mechanism of the luminescence efficiency improvement of GaAsN alloys by laser irradiation. Raman peak intensity of GaAs-like longitudinal optical (LO) mode phonon was observed to increase with the laser irradiation time. With increasing laser power density, the Raman intensity of GaAs-like LO mode phonon was found to increase more rapidly and to be saturated in a shorter time. Changes in the Raman intensity can be expressed by the same equation used for the luminescence efficiency improvement. Time constants of changes in the Raman intensity were almost in agreement with those of the luminescence efficiency improvement. This indicates that the increase in Raman intensity of GaAs-like LO phonon is closely related to the luminescence efficiency improvement.

We developed absolute electroluminescence (EL) calibration standards to evaluate absolute radiative-emission rates from subcells in multi-junction (MJ) solar cells. The absolute-EL-measurement system consists of an EL imaging setup and an emission-intensity-calibrated planar light-emitting diode with a circular open aperture as an emission-intensity standard. We applied this system to the measurements of the absolute EL imaging of a monolithic satellite-use InGaP/GaAs/Ge MJ solar cell. From the observed absolute EL images, we characterized external EL quantum efficiencies and internal open-circuit voltages of InGaP and GaAs subcells.

Cavity-length dependence of the property of optically pumped GaN-based vertical-cavity surface-emitting lasers (VCSELs) with two dielectric distributed Bragg reflectors was investigated. The cavity lengths were well controlled by employing etching with inductively coupled plasma and chemical mechanical polishing. It was found that the lasing characteristics including threshold, slope efficiency and spontaneous emission coupling factor were substantially improved with reducing the cavity length. In comparison with the device pumped by a 400 nm pulsed laser, the lasing spectrum was featured by a red shift and simultaneous broadening with increasing the pumping energy of a 355 nm pulsed laser. Moreover, the lasing threshold was much higher when pumped by a 355 nm pulsed laser. These were explained by taking into account of the significant heating effect under 355 nm pumping. Our results demonstrate that a short cavity length and good heat-dissipation are essential to GaN-based VCSELs.

We fabricated ITO/biomolecule/Al junctions using chrolophyll a, cytochrome c, myoglobin, hemin, Vitamin B12. All the junctions worked as organic light-emitting diode (OLED). The quantum efficiencies of the fabricated OLED were of the order of 1 × 10−7 around V = 10 V, and did not seriously depend on whether or not a compound exhibits photoluminescence. Based on the energy diagram of the ITO/cytochrome c/Al junction, we discuss the difference of the EL spectrum from the photoluminescence or absorption spectrum.

We fabricated biomolecular light-emitting diodes (BIODE) using hemin, and measured the electroluminescence (EL) spectra, current–voltage and EL intensity–voltage characteristics. The current–voltag...

We have fabricated ITO-insulator-Al junctions using cytochrome c and myoglobin as insulating layers. The junctions act as light-emitting diodes. The electro-luminescence (EL) spectra of these diodes exhibit broad peaks around 530 and 700 nm, and a weak shoulder around 410 nm. These features are consistent with the absorption spectra. Although the reproducibility of the data is quite poor for the current and the EL intensity, the external quantum efficiency obtained from the two quantities are well expressed by a function of the applied voltage. The quantum efficiency almost linearly increases above the applied voltage of 7 V and reaches to ∼1 × 10 −7 at V = 14 V for both cytochrome c and myoglobin diodes.

We fabricated organic light-emitting diodes (OLED) from horse-heart cytochrome c, and measured the electroluminescence (EL) spectra, current–voltage and EL intensity–voltage characteristics. The ex...

A T-shaped single quantum-wire laser with high spatial uniformity has been fabricated by a cleaved-edge overgrowth method with molecular beam epitaxy and a growth-interrupt annealing technique. Using micro-photoluminescence imaging and spectroscopy, we confirmed the formation of a spatially uniform quantum wire over 20 μm long without hetero-interface roughness. By optical pumping, we achieved single-mode lasing from the ground state of the single quantum wire at a threshold excitation power as low as 5 mW at 5 K .

High-quality T-shaped quantum wire lasers are fabricated by cleaved-edge overgrowth with the molecular beam epitaxy on the interface improved by a growth-interrupt high-temperature anneal. Micro-photoluminescence (PL) and PL excitation spectroscopy reveals unprecedented high quality of the wires, and structures of one-dimensional (1D) free excitons and 1D continuum states. At high pumping levels, PL evolves from a sharp free exciton peak via a biexciton peak to a red-shifted broad band. Lasing has been achieved with low lasing threshold. The lasing energy is on the red-shifted broad band and is about 5 meV below the free exciton. The observed shift excludes free excitons in lasing, and suggests contribution of highly Coulomb-correlated electron–hole plasma.

We developed a straightforward method based on detailed balance relations to analyze individual subcells in multi-junction solar cells via measuring absolute electroluminescence quantum yields. This method was applied to characterization of a InGaP/GaAs/Ge 3-junction solar cell for satellite use. In addition to subcell I-V characteristics and internal luminescence yields, we derived balance sheets of energy and carriers, which revealed respective subcell contributions of radiative and nonradiative recombination losses, junction loss, and luminescence coupling. These results provide important diagnosis and feedback to fabrications. We calculated conversion-efficiency limit and optimized bandgap energy in 2-, 3-, and 4-junction tandem solar cells, including finite values of sub-cell internal luminescence quantum yields to account for realistic material qualities in sub-cells. With reference to the measured internal luminescence quantum yields, the theoretical results provide realistic targets of efficiency limits and improved design principles of practical tandem solar cells.

World-wide studies on multi-junction (tandem) solar cells have led to record-breaking improvements in conversion efficiencies year after year. To obtain detailed and proper feedback for solar-cell design and fabrication, it is necessary to establish standard methods for diagnosing subcells in fabricated tandem devices. Here, we propose a potential standard method to quantify the detailed subcell properties of multi-junction solar cells based on absolute measurements of electroluminescence (EL) external quantum efficiency in addition to the conventional solar-cell external-quantum-efficiency measurements. We demonstrate that the absolute-EL-quantum-efficiency measurements provide I-V relations of individual subcells without the need for referencing measured I-V data, which is in stark contrast to previous works. Moreover, our measurements quantify the absolute rates of junction loss, non-radiative loss, radiative loss, and luminescence coupling in the subcells, which constitute the "balance sheets" of tandem solar cells.

Planar-type Lambertian light-emitting diodes (LEDs) with a circular aperture of several tens of μm to a few mm in diameter were developed for use as radiant-flux standard light sources, which have been in strong demand for applications such as quantitative or absolute intensity measurements of weak luminescence from solid-state materials and devices. Via pulse-width modulation, time-averaged emission intensity of the LED devices was controlled linearly to cover a wide dynamic range of about nine orders of magnitude, from 10 μW down to 10 fW. The developed planar LED devices were applied as the radiant-flux standards to quantitative measurements and analyses of photoluminescence (PL) intensity and PL quantum efficiency of a GaAs quantum-well sample. The results demonstrated the utility and applicability of the LED standards in quantitative luminescence-intensity measurements in Lambertian-type low radiant-flux level sources.

GaN-based vertical-cavity surface-emitting lasers (VCSELs) with high optical gain and short cavity lifetime are favorable for the generation of ultra-short pulses in the blue and green regions. In our previous works, 6 and 2 picosecond short-pulses have been generated from gain-switched InGaN VCSELs with 3- and 10-period InGaN/GaN quantum wells (QWs) in the active layers by using an up-conversion measurement system. To further increase the gain of the VCSEL for the generation of even shorter pulses, 20-period InGaN/GaN QWs samples were fabricated. The emission characteristics of these high-gain VCSELs were investigated and analyzed under the optical pumping at room temperature.

We developed a straightforward and potentially standard method of accessing the balance sheets of energy and carriers as well as respective subcell photovoltaic properties in multi-junction tandem solar cells by measuring absolute electroluminescence quantum yields. This method was indeed applied to a GaInP/GaAs/Ge 3-junction solar cell for satellite use. The 28.7% efficiency derived as a sum of 15.2% (top), 10.7% (mid) and 2.8% (bottom) of 3 subcells via this method shows good agreement with the measured 27.4% efficiency via an I–V curve under 1-sun illumination at AM0. Moreover, the derived balance sheets of energy and carriers revealed respective subcell contributions of radiative and nonradiative recombination losses, junction loss, thermalization loss, and luminescent coupling, to provide detailed diagnosis and valuable feedback to fabrications.

Ultra-short pulses in blue region generated from compact and low-cost semiconductor lasers have attracted much attention for a wide variety of applications. Nitride-based vertical-cavity surface-emitting lasers (VCSELs), having intrinsic high material gain and short cavities, favor the generation of ultra-short blue pulses via a simple gain-switching technique. In this study, we fabricated a single-mode InGaN VCSEL consisting of 10-period InGaN/GaN quantum wells (QWs). The output pulses were evaluated accurately with an up-conversion measurement system having time resolution of 0.12 ps. We demonstrated that ultra-short blue pulses, as short as 2.2 ps at 3.4 K and 4.0 ps at room temperature, were generated from the gain-switched InGaN VCSEL via impulsive optical pumping, without any post-processing. The gain-switched pulses we obtained should greatly promote the development of ultra-short blue pulse generation. In addition, this successful assessment demonstrates the up-conversion technique's usefulness for characterizing ultra-short blue pulses from semiconductor lasers.