Quantum Efficiency Measurements Research Articles

Dy3+-doped lithium aluminoborate glass for luminescent light guides with high luminance

Lithium aluminoborate glass optically activated with the lanthanide ion dysprosium is investigated for its potential as luminescent light guide. For this, ray-tracing simulations are performed on the basis of transmission, photoluminescence, and quantum efficiency measurements. The luminous flux at the end of the light guide depends significantly on its length as well as on the roughness of the output face. The best results are obtained for a light guide length of 70–80 mm with the side faces of the light guide coated with a 100 % reflecting mirror and a rough output face with Lambertian scattering characteristic. An additional coating of the input face with a half-transmitting mirror is investigated. The mirror is transmissive for the excitation wavelength of 388 nm but reflective for the emission bands in the visible spectral range. For this light guide, a luminance of approximately 20 cd/mm 2 is achieved for an excitation power density of 1 W/mm 2 . The geometry of the light guide (cuboid/cylinder) has only a slight effect on the maximum luminance value. • Analytical estimate of luminescence output of dysprosium-doped light guides. • Ray-tracing simulations for different lengths, surface properties, and geometries. • Maximum luminance of approximately 20 cd/mm 2 for 388-nm excitation.

Thin c-Si Solar Cells Based on VOx Heterojunctions With Texturized Rear Surface

Solar cells based on thin crystalline silicon (c-Si) substrates have the potential to provide relevant efficiencies with less material. In this article, we report on the fabrication of thin solar cells starting from a 20 μ m-thick c-Si wafer. Special attention is paid to optical performance of the rear surface where we need very high internal reflection and light scattering to improve light trapping properties of the device. In this sense, we introduce a texturization of the rear surface, which is excellently passivated by aluminium oxide films. In addition, for the rear contacts we use metal-covered wide-bandgap materials to improve back reflectance with a Vanadium Oxide/c-Si heterojunction for the emitter regions while the base contacts are defined by laser processing phosphorus doped amorphous silicon carbide films. Best solar cell has a reasonable efficiency of 8.7% and, more importantly, external quantum efficiency measurements demonstrate a better performance for photons with λ > 900 nm than in the case of a flat rear surface.

Optical Quality of InAs/InP Quantum Dots on Distributed Bragg Reflector Emitting at 3rd Telecom Window Grown by Molecular Beam Epitaxy.

We present an experimental study on the optical quality of InAs/InP quantum dots (QDs). Investigated structures have application relevance due to emission in the 3rd telecommunication window. The nanostructures are grown by ripening-assisted molecular beam epitaxy. This leads to their unique properties, i.e., low spatial density and in-plane shape symmetry. These are advantageous for non-classical light generation for quantum technologies applications. As a measure of the internal quantum efficiency, the discrepancy between calculated and experimentally determined photon extraction efficiency is used. The investigated nanostructures exhibit close to ideal emission efficiency proving their high structural quality. The thermal stability of emission is investigated by means of microphotoluminescence. This allows to determine the maximal operation temperature of the device and reveal the main emission quenching channels. Emission quenching is predominantly caused by the transition of holes and electrons to higher QD’s levels. Additionally, these carriers could further leave the confinement potential via the dense ladder of QD states. Single QD emission is observed up to temperatures of about 100 K, comparable to the best results obtained for epitaxial QDs in this spectral range. The fundamental limit for the emission rate is the excitation radiative lifetime, which spreads from below 0.5 to almost 1.9 ns (GHz operation) without any clear spectral dispersion. Furthermore, carrier dynamics is also determined using time-correlated single-photon counting.

Photoelctrochemically Fabricated and Heated Cu2O/CuO Bilayers with Enhanced Photovoltaic Characteristics.

Cu2O/CuO bilayers were fabricated by electrodeposition of the CuO layer in a copper(II)–ammonia complex aqueous solution, followed by photoelectrochemical deposition of the Cu2O layer at potentials ranging from −0.3 to −1.0 V referenced to a Ag/AgCl electrode in a copper(II)–lactate complex aqueous solution under light irradiation, and the effects of varied potentials of the photoelectrochemical Cu2O depositions and post-heating conditions on their structural, optical, and photovoltaic characteristics were investigated with X-ray diffraction, field emission-scanning electron microscopy, optical absorption measurements, and external quantum efficiency (EQE) measurements with and without applied bias voltage. The Cu2O layers with a characteristic 2.1 eV band gap energy were adhesively stacked on the thorn-like grains of the CuO layers possessing a characteristic 1.5 eV band gap energy, and dense and defect-free Cu2O/CuO bilayers could be fabricated at the potentials of −0.4 and −0.5 V, but the grain size of Cu2O decreased at −0.5 V. In addition, the metallic Cu was deposited simultaneously at potentials less than −0.7 V. The Cu2O/CuO bilayer fabricated at −0.4 V revealed photovoltaic features at wavelengths ranging from 350 nm to approximately 900 nm, and a maximum EQE value of 56.8% was achieved at 510 nm in wavelength with a bias voltage of −0.1 V. The maximum EQE value, however, decreased to 1.2% accompanied with the peak wavelength shift to 580 nm, and no photovoltaic feature was observed at potentials of −0.3, −0.7, and −1.0 V. The photovoltaic performance for the Cu2O/CuO bilayer fabricated at −0.4 V was ameliorated by heating at 423 K, and the maximum EQE values were enhanced to 87.7% at 550 nm and 89.8% at 530 nm in an ambient atmosphere and vacuum. Both the Cu2O and CuO layers acted as photovoltaic layers in the Cu2O/CuO bilayer fabricated at −0.4 V and heated at 423 K, and the electrical characteristic including the carrier mobility affected the photovoltaic performance. The photovoltaic feature, however, disappeared by heating above 523 K due to the formation of nanopores inside the CuO layer and near the CuO heterointerface to the Cu2O and fluorine-doped tin oxide substrate.

Microstructured antireflective encapsulant on concentrator solar cells

AbstractMicrostructured antireflective coatings (ARCs) can reduce reflection losses over a wide range of incidence angles when applied to the surface of a high‐efficiency III‐V photovoltaic cell in a concentrator photovoltaic (CPV) system. In this article, we present a microstructured ARC consisting of a monolayer of close‐packed silica microbeads partially submerged within a polydimethylsiloxane (PDMS) cell encapsulant for use within a reference 500× CPV submodule. Comparing a commercialized SiOx encapsulant to this microstructured coating with 25% submerged 1,000 nm‐diameter beads, angle‐dependent external quantum efficiency measurements yield a 2.6% current gain for the microstructured coating. Simulations demonstrate good agreement with measurements, predicting a 2.4% current gain for the same configuration. Extrapolating with our validated model, we estimate a maximum and achievable (within a large manufacturing tolerance) current gain of 3.4% and 2.9 ± 0.4% using 60% submerged and 10%–32% submerged 760 nm‐diameter beads, respectively.

Quantum efficiency, intrinsic emittance, and response time measurements of a titanium nitride photocathode

The photoemission properties of photocathodes affect the ultimate performance of electron accelerator based facilities and the cost. Several major properties of titanium nitride (TiN) irradiated by ultraviolet femtosecond laser pulses were studied. Specifically, the quantum efficiency and intrinsic emittance were measured, and the response time was estimated indirectly. For photon energies higher than the work function, quantum efficiency values of $7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}--5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ were obtained. The measured intrinsic emittance was $0.54\text{ }\text{ }\ensuremath{\mu}\mathrm{m}/\mathrm{mm}$ for the typical wavelength of 266 nm (4.66 eV). When the photon energy was lower than the work function, two-photon photoemission appeared, and the intrinsic emittance increased with the laser intensity. At photon energies of higher than and slightly lower than the work function, the intrinsic emittance ranged from 0.28 to $0.60\text{ }\text{ }\ensuremath{\mu}\mathrm{m}/\mathrm{mm}$ and tended toward a minimum constant of $0.28\text{ }\text{ }\ensuremath{\mu}\mathrm{m}/\mathrm{mm}$ if only single-photon photoemission was considered. The response time of TiN was proven to be close to that of gold by comparing the transverse expansion of electron beams under space charge force for the irradiated photon energy higher than the work function. The autocorrelation measurement of the photoemission of TiN and gold were performed, and a comparison of these results indicated that the two-photon photoemission of TiN excited by a photon energy slightly lower than the work function was prompt.

Effect of Additives and Annealing on the Performance of Nonfullerene‐Based Binary and Ternary Organic Photovoltaics

Fine tuning of blend morphology is a key factor that limits the performance of the bulk‐heterojunction organic photovoltaics (BHJ‐OPVs). Herein, the morphological control of the binary (PM6:Y7) and ternary (PM6:Y7:PC70BM) blends is conducted through 1‐chloronaphthalene (CN) solvent additive and thermal annealing (TA) treatment with respect to their influence on the photovoltaic performance. Moreover, a distinct study is accomplished on the optical and electronic properties of the treated and nontreated binary and ternary devices by external quantum efficiency measurements and impedance spectroscopy. The results indicate that these treatments affect the performance of the binary and ternary OPVs differently. Regarding the 2% CN addition, the current density of the binary devices is improved by ≈27%, whereas the fill factor of the ternary devices shows a pronounced increment of ≈22%. A contradictory behavior is exhibited by TA for the binary and ternary OPVs. The PCEs for binary devices (with/without CN) and 2% CN‐treated ternary ones are improved, while diminishing the PCEs of the ternary ones with 0% CN. Accordingly, the highest efficiencies of the binary and ternary OPVs are obtained due to the dual effect of 2% CN solvent additive along with the TA treatments.

Effect of air mass on carrier losses in bifacial silicon heterojunction solar cells

We investigate the effect of incident spectra on current loss as a function of depth and voltage into high efficiency textured bifacial silicon heterojunction solar cells. We integrate thin-film ellipsometry measurements with a 3D optical model and a 2D electronic model and validate our model with measurements of external quantum efficiency and Suns-Voc. For front illumination at normal incidence, an increasing air mass of AM1.5 to 10 reduces current density loss due to parasitic absorption in ITO and a-Si:H from 8.1% to 4.0%, and increases recombination loss at maximum power from 4.2% to 4.7%, resulting in an overall increase in collected current (88.2% to 90.5%). Cell performance metrics are summarized as a function of air mass, with efficiency peaking at AM5.0 for front illuminated and rear illuminated cells with an albedo of unity. We further demonstrate the impact of spectra on bifacial efficiency by calculating rear-side performance with the spectral albedo of dry grass. Overall, current-collection and efficiency trends emphasize the importance of considering spectral effects in energy yield models. These results are of particular importance for cell structures with high bifaciality and significant spectral albedo contributions, locations with large proportions of diffuse light, and high air mass locations as in mid-to-high latitudes. • Optoelectronic model is validated with measurements of EQE and Suns-Voc. • Collected photogenerated current increases from 88% to 91% between AM1-10. • Maximum cell efficiency occurs at AM5 but is heavily influenced by spectral albedo. • Encapsulation increases parasitic absorption, decreasing efficiency by 1.4% abs. • Most impactful for modules with high bifaciality, high albedo, mid to high latitude.

Predicting color and short-circuit current of colored BIPV modules

Photovoltaic modules for façade integration should have a widely modifiable appearance to adjust to the architect’s requirements. However, architects today usually have only a limited number of already manufactured samples to choose from. Changing the color will also change the photovoltaic yield. Therefore, it would be helpful to have a procedure that allows us to determine the appearance and expected yield in advance of module fabrication. We present such a method for creating a digital prototype of a colored building integrated photovoltaic module. Using reflectance and external quantum efficiency measurements of eight colored modules, we simulate the appearance and respective energy yield for arbitrary module colors. We validate our predictions for 29 different colored modules. We use textiles that have been colored by printing and laminate them onto the modules to change the appearance of the modules. However, our digital prototyping model is also applicable to other coloring techniques. We achieve an average color difference of ΔE00 = 1.34 between predicted and measured colors, which is barely perceptible to the human eye. The predicted short-circuit current density of the digital prototype deviates on average less than 1% from the measured one.

Temperature and intensity dependence of the open-circuit voltage of InGaN/GaN multi-quantum well solar cells

We have analyzed the temperature and intensity dependence of the open-circuit voltage of InGaN/GaN multi-quantum well solar cells up to 725 K and more than 1000 suns. We show that the simple ABC model routinely used to analyze the measured quantum efficiency data of InGaN/GaN LEDs can accurately reproduce the temperature and intensity dependence of the measured open-circuit voltage if a temperature-dependent Shockley-Read-Hall lifetime is used and device heating is taken into account.

Absolute Electrochemiluminescence Efficiency Quantification Strategy Exemplified with Ru(bpy)32+ in the Annihilation Pathway.

This work presents a thorough guide to procedures for absolute electrochemiluminescence (ECL) quantum efficiency (ΦECL) measurements, which if employed effectively should raise the research impact of ECL studies for any luminophore. Absolute measurements are not currently employed in ECL research. Instead, ECL efficiencies have been determined relative to Ru(bpy)32+ under similar conditions, regardless of whether the conditions are favorable for Ru(bpy)32+ emissions or not. In fact, the most cited Ru(bpy)32+ ΦECL is from the pioneering work by the Bard research group in 1973 by means of a rotating ring-disk electrode revolving at 52 rotations per second measured with a silicon photodiode. Our presented technique uses a common disk electrode, spectrometer, and photomultiplier tube to measure the ΦECL. The more common light detection hardware and electrodes combined with an in-depth calculation walkthrough will provide ECL researchers the necessary tools to implement ΦECL measurement procedures in their own laboratories. Following a facile instrument setup and calculation, a systematic study of Ru(bpy)32+ ΦECL finds comparable results to those performed by Bard and co-workers.

High current of GaInNAs solar cell integrating distributed Bragg reflectors grown by metalorganic vapor phase epitaxy

GaInNAs junctions can be applied to the lattice-matched five-junction (5J) or six-junction (6J) solar cells to achieve ultra-high efficiency. In this work, a GaInNAs solar cell integrating distributed Bragg reflectors (DBR) is grown by metalorganic vapor phase epitaxy. According to external quantum efficiency measurements, the photocurrent can be improved by ∼20% due to DBR. Meanwhile, a short-circuit current (J sc ) of GaInNAs DBR-cell is obtained high as 13.78 mA cm−2 under the air mass (AM0) spectrum, which is completely satisfied the demand of 36%-efficiency 5J solar cell (∼ 12 mA cm−2). Finally, Dark-IV characteristics reveal that the introduction of DBR does not deteriorate GaInNAs quality.

Two local built-in potentials of H2S processed CZTSSe by complex impedance spectroscopy

We have fabricated CZTSSe solar cells with H2S sulfo-selenization processes and investigated the electronic structure at the PN hetero- and back contact junctions by impedance spectroscopy. By decoupling each junction’s impedance spectroscopic responses, we systematically characterized the built-in potential of two local junction interfaces. A developed equivalent circuit model has been optimized for decoupling each junction’s property at different frequency bands. Modeling and numerical simulations were conducted with the in-house MATLAB modeling suites connected to external simulators of Sentaurus TCAD and LEVM/LEVMW software to estimate the impact of each junction component to impedance spectra. The optimized model is comprised of a parallel circuit combination with resistance and capacitor-like elements (constant phase elements), connected to one inductive element. The conversion efficiency of CZTSSe devices is 6.2% with bandgap energy 1.13 eV based on external quantum efficiency measurements. From the equivalent circuit model, the built-in potential of the hetero-junction is characterized as 956 meV, which is ~3% smaller than an ideal case from TCAD, 987 meV. Conversely, the built-in potential of the back contact junction is 476 meV. The apparent built-in potential is estimated as 480 meV from the Mott-Schottky equation.

A universal Urbach rule for disordered organic semiconductors

In crystalline semiconductors, absorption onset sharpness is characterized by temperature-dependent Urbach energies. These energies quantify the static, structural disorder causing localized exponential-tail states, and dynamic disorder from electron-phonon scattering. Applicability of this exponential-tail model to disordered solids has been long debated. Nonetheless, exponential fittings are routinely applied to sub-gap absorption analysis of organic semiconductors. Herein, we elucidate the sub-gap spectral line-shapes of organic semiconductors and their blends by temperature-dependent quantum efficiency measurements. We find that sub-gap absorption due to singlet excitons is universally dominated by thermal broadening at low photon energies and the associated Urbach energy equals the thermal energy, regardless of static disorder. This is consistent with absorptions obtained from a convolution of Gaussian density of excitonic states weighted by Boltzmann-like thermally activated optical transitions. A simple model is presented that explains absorption line-shapes of disordered systems, and we also provide a strategy to determine the excitonic disorder energy. Our findings elaborate the meaning of the Urbach energy in molecular solids and relate the photo-physics to static disorder, crucial for optimizing organic solar cells for which we present a revisited radiative open-circuit voltage limit.

Novel non-rare-earth red-emitting phosphor Li4AlSbO6:Mn4+ for plant growth: Crystal structure, luminescence properties and its LED device

Bright red emission of Mn4+ ions was obtained in a novel non-rare-earth host of Li4AlSbO6. Luminescence properties as well as crystal structure and particle morphology of Li4AlSbO6:Mn4+ phosphors were studied. The phosphors can effectively produce intensive red emission at 673 nm with the excitation of UV or blue light. The optimum doping concentration of Mn4+ is obtained at 0.15 mol%. The internal quantum efficiency measurement shows that Li4AlSbO6:Mn4+ is an efficient phosphor with the best quantum yield of 54.8%. The electroluminescence of phosphor-converted LED device based on Li4AlSbO6:Mn4+ is well coupled with the absorption of several important plant pigments, which suggests that the phosphor is a promising candidate for indoor plant lighting.

The Bottlenecks of Cs2AgBiBr6 Solar Cells: How Contacts and Slow Transients Limit the Performance

AbstractCs2AgBiBr6 has attracted much interest as a potential lead‐free alternative for perovskite solar cells. Although this material offers encouraging optoelectronic features, severe bottlenecks limit the performance of the resulting solar cells to a power conversion efficiency of below 3%. Here, the performance‐limiting factors of this material are investigated in full solar cells featuring various architectures. It is found that the photovoltaic parameters of Cs2AgBiBr6‐based solar cells strongly depend on the scan speed of the J/V measurements, suggesting a strong impact of ionic conductivity in the material. Moreover, a sign change of the photocurrent for bias voltages above 0.9 V during the measurement of the external quantum efficiency (EQE) is revealed, which can be explained by non‐selective contacts. The radiative loss of the VOC from sensitive subgap‐EQE measurements is calculated and it is revealed that the loss is caused by a low external luminescence yield and therefore a high non‐radiative recombination, supported by the first report of a strongly red shifted electroluminescence signal between 800 and 1000 nm. Altogether, these results point to a poor selectivity of the contacts and charge transport layers, caused by poor energy level alignment that can be overcome by optimizing the architecture of the solar cell.

Measurement of the relative quantum efficiency of Hamamatsu model R5912-20MOD photomultiplier tubes at liquid argon temperature

The model R5912-20MOD photomultiplier tube(PMT) is made for cryogenic application by Hamamatsu. In this paper, we report on the measurement of relative quantum efficiency (QE) of this model PMT at liquid argon(LAr) temperature. Furthermore, a specially designed setup and relevant test method are introduced. The relative QE is measured in visible wavelengths with the PMT emerged in high purity nitrogen atmosphere. The results show that the change of QE at LAr temperature is within about 5% compared with room temperature around 420 nm. However, the QE increases about 10% in the shorter wavelength range and decreases significantly after 550 nm.

Photovoltaic module failures after 10 years of operation in the tropics

This paper presents a case study of photovoltaic (PV) module failures after over 10 years of operation in the tropical climate of Singapore. Three types of modules (two samples from each type) were analysed: multi-crystalline silicon (multi-Si), mono-crystalline silicon (mono-Si), and copper indium selenide (CIS). Visual inspection revealed several problems, including encapsulant discoloration (to different extents), backsheet yellowing, and soiling among others. Different degradation behaviour of the samples was observed from current-voltage (IV) and electroluminescence characterization. The maximum power of the multi-Si samples degraded by more than 9% (on average), likely due to corrosion around the cell edges. The mono-Si modules suffered a catastrophic power reduction (>40%) that could be ascribed to a combination of encapsulant discoloration, potential-induced degradation (PID), and corrosion. While one CIS module was primarily affected by encapsulant discoloration and possibly corrosion, the other sample also exhibited signatures of PID and experienced about 45% power drop. Furthermore, external quantum efficiency measurements of the multi-Si modules identified cell mismatch and changes to the additives in the encapsulant. Overall, PID, corrosion and encapsulant degradation are found to be the most detrimental degradation processes for PV modules in the tropical climate of Singapore.

Energy Down-Conversion Cs3Cu2Cl5 Nanocrystals for Boosting the Efficiency of UV Photodetector

Zero-dimension (0-D) lead halide perovskite nanocrystals (NCs) have attracted a sight of interest in the field of optoelectronic devices due to their outstanding properties, such as high photoluminescence quantum yield (PLQY) and size- and composition-controlled tunable emission wavelengths. However, the toxicity of lead (Pb) element in the lead perovskite NCs is the bottleneck for the commercial application of perovskite NCs. Herein, we report a facile ligand-assisted synthesis to achieve lead-free Cs3Cu2Cl5 NCs with a high PLQY of ∼70% and good stability against environmental oxygen/moisture as a promising down-conversion material. It has good merits of high PLQY and large Stokes shift (∼300 nm) originated from the effect of Jahn–Teller distortion and self-trapped excitons (STEs). Furthermore, the Cs3Cu2Cl5 NCs embedded composite films (NCCFs) were utilized to enhance the ultraviolet (UV) response of silicon (Si) photodetectors. External quantum efficiency (EQE) measurements show that the UV response can be greatly improved from 3.3 to 19.9% @ 295 nm based on NCCFs combined with Si photodiodes. Our work offers an effective approach to develop highly efficient and stable lead-free Cs3Cu2Cl5 NCs for the application in the solar-blind UV photodetector.

On Quantum Efficiency Measurements and Plasmonic Antennas

Quantum efficiency is a key quantity that describes the performance of light-emitting materials and is, thus, an important metric for assessing novel nanophotonic systems. This Perspective provides a concise discussion of the difficulties encountered in the characterization of quantum efficiencies, especially for studies that involve single emitters. In particular, we review various approaches that have been recently used for determining quantum efficiencies of emitters coupled to plasmonic antennas and highlight the subtleties and challenges that hinder precise measurements.