Luminescent Coupling Research Articles

Temperature dependence of luminescence coupling effect in InGaP/GaAs/Ge triple junction solar cells

Optimizing the luminescence coupling (LC) effect can reduce the current mismatch in multijunction solar cells (MJSCs), thereby enhancing their performance. In a previous work, the LC current collection was measured at room temperature. The temperature dependence of LC effect in InGaP/GaAs/Ge triple junction solar cells was observed by light J–V characteristics measurements and by laser beam-induced current mapping at operating temperatures between 18°C and 72°C. Results were analyzed using quasi-two-dimensional electro-optical model. At low operating temperatures, the photocurrent collection by LC effect has been found to be more dominant than the current collection due to bandgap reduction and thermal excitation of carriers. At 18°C and 72°C, when Ge bottom cell was made current limiting, the conversion efficiencies calculated were 6.68% and 0.01%, respectively. These findings show that the LC effect can contribute best to the MJSC performance when operated at the lowest temperature possible.

Extracting Nonradiative Parameters in III–V Semiconductors Using Double Heterostructures on Active p-n Junctions

A novel method of extracting the nonradiative lifetime of Ga0.51In0.49P lattice matched to GaAs by exploiting luminescence coupling is discussed. The method requires a GaInP double heterostructure monolithically grown on a GaAs photodetector for quantum efficiency measurements. The method then extracts the nonradiative lifetime by modeling the luminescence coupling between the GaInP and the GaAs active region. The bulk nonradiative lifetime of disordered GaInP doped to 1017 cm−3 under low-level injection (∼4 × 1012 cm−3) is determined to be 47 ns for GaInP layer thicknesses ranging from 200 to 1500 nm, with a surface recombination velocity of 660 cm/s. The results are in agreement with nonradiative lifetimes extracted using power-dependent relative photoluminescence measurements, whereby the lifetimes increase as a function of injection level to 0.5 μ s. Surface recombination velocities are also reported as a function of injection level using this technique, decreasing from 1800 cm/s under low injection to 50 cm/s under high injection (∼1017 cm−3). Lastly, the effective radiative lifetimes (accounting for photon recycling) are also reported for the studied GaInP samples.

Luminescence dating of qanat technology: prospects for further development

With few exceptions in which dating is implied by indirect association with adjacent settlements or incorporation of diagnostic artefacts in upcast sediment, individual qanats have proven very difficult to date. This absence of a chronological framework hampers both our understanding of technology transfer, as well as the study of local settlement and landscape evolution and the temporal correlation of land use with climatic and palaeoenvironmental data. However, surface shaft mounds potentially contain a sequence of upcast deposits collected periodically from the tunnel, starting with initial construction and persisting until the last maintenance episode, less any material lost by surface erosion. The sedimentary nature of the upcast lends itself to the application of luminescence dating to determine the burial age, in particular, using the techniques based on optically stimulated luminescence. We examine the results produced by two recent dating studies where luminescence techniques were applied to two qanat systems with the aim of building a chronostratigraphy for the deposits within their upcast mounds. These studies show that the extent to which a complete record of the deposition since initial construction survives may differ between qanat systems, and even shaft mounds within the same system. Providing there is a close coupling of luminescence and sedimentological analysis in the testing of qanat mounds, these formative studies suggest that there are good prospects for introducing a valuable tool in the study of various types of hydraulic feature where upcast has been preserved and guidance regarding further fieldwork is provided.

Redox-induced switch between luminescence and magnetism in a trinuclear cyanide-bridged compound.

A trinuclear cyanide-bridged luminescent compound, trans-RuII(DMAP)4(CN)2[(PY5Me2)Mn]2[PF6]4 (1-R), and its oxidation product, ferromagnetic trans-RuIII(DMAP)4(CN)2[(PY5Me2)Mn]2-[PF6]5 (1-O), were synthesized and fully characterized. This is the first example of a molecular material showing a redox-controlled transformation between fluorescence and ferromagnetism.

About significance of absolute photocurrent values determination during the solar cell external quantum efficiency measurements

In the work, the link of registered relative and absolute current signals of different amplitude and shape in recording spectral characteristics of the sunlight PV converter external quantum efficiency is investigated, and the need to use absolute current in spectral response studies of multijunction solar cells with clearly pronounced luminescent coupling is established.

Temporal and Remote Tuning of Piezophotonic-Effect-Induced Luminescence and Color Gamut via Modulating Magnetic Field.

Light-emitting materials have been extensively investigated because of their widespread applications in solid-state lighting, displays, sensors, and bioimaging. In these applications, it is highly desirable to achieve tunable luminescence in terms of luminescent intensity and wavelength. Here, a convenient physical approach of temporal and remote tuning of light-emitting wavelength and color is demonstrated, which is greatly different from conventional methods. It is shown that by modulating the frequency of magnetic-field excitation at room temperature, luminescence from the flexible composites of ZnS:Al, Cu phosphors induced by the piezophotonic effect can be tuned in real time and in situ. The mechanistic investigation suggests that the observed tunable piezophotonic emission is ascribed to the tilting band structure of the ZnS phosphor induced by magnetostrictive strain under a high frequency of magnetic-field excitation. Furthermore, some proof-of concept devices, including red-green-blue full-color displays and tunable white-light sources are demonstrated simply by frequency modulation. A new understanding of the fundamentals of both luminescence and magnetic-optics coupling is thus provided, while offering opportunities in magnetic-optical sensing, piezophotonics, energy harvesting, novel light sources, and displays.

Up-conversion of sunlight by GaInP/GaAs/Ge cell stacks: Limiting efficiency, practical limitation and comparison with tandem cells

Recent work proposed an alternative approach for photon up-conversion in that two or more low-band-gap solar cells drive a large-band-gap light-emitting diode (LED), emitting high-energy photons to be absorbed by a large-band-gap solar cell on the top. Compared with the old tandem structure, this multijunction up-conversion system has superior spectral robustness valuable under varying spectral conditions in real world. An example of interest is the use of a GaAs/Ge solar cell stack to drive a GaInP LED, behind a GaInP bifacial solar cell. This work investigates its one-sun limiting efficiency as well as possible practical efficiency if it were fabricated using state-of-the-art technologies for a world-record GaInP/GaAs/Ge tandem cell, taking luminescent coupling between subcells into account. It is shown that, although the limiting efficiency for such an up-conversion system is close to the record efficiency of a tandem device, its practical performance may be much lower due to carrier recombination in low-band-gap cells reducing the LED’s emissions significantly. To obtain more insight into practical designs, the theoretical efficiency is explored further for the lattice-mismatched case where the band-gap of GaInP cells is reduced, both when there are luminescent couplings between cells in the rear up-converter and there are not. With a maximum theoretical efficiency of 44%, it is shown that the top cell’s band-gap is a trade-off and luminescent coupling between rear cells reduces the efficiency.

Impact of optically nonuniform luminescence coupling effect to the limiting cell conversion efficiency in InGaP/GaAs/Ge triple junction solar cell

The impact of nonuniform spatial distribution of the luminescence coupling (LC) effect to the limiting cell conversion efficiency of multijunction solar cells (MJSCs) has been investigated. For this purpose, the laser beam induced current distribution maps of the limiting bottom cell have been acquired experimentally under varying middle-to-bottom cell LC efficiencies. The minimum and the maximum LC efficiencies demonstrated were 8.5% and 69%, respectively. To further analyze the measurement results, a quasi-two-dimensional simulation model considering the spatially nonuniform nature of the LC effect has been developed. A good agreement between the simulation and the measurement results suggests that the nonuniform LC current distribution is induced by optical phenomena such as photon escape and internal reflection. This nonuniformity then causes the absolute conversion efficiency of the limiting cell to be reduced by 1.35% at maximum LC efficiency. This reduction, when suppressed, can yield higher limiting cell conversion efficiency, which in turn may improve the overall MJSC conversion efficiency.

Optical leakage versus luminescent coupling in a multijunction solar cell: how to recognize a source of additional photocurrent in subcells

A procedure for estimating an optical leakage of incoming radiation through a wide-bandgap subcell in a GaInP/GaInAs/Ge multijunction solar cell to photoactive layers of narrow-bandgap ones is proposed. A sequence for experimental determination of currents induced in a Ge subcell owing to an optical leakage or via a luminescent coupling is described.

Subcell Characterization in Multijunction Solar Cells Using Pulsed Light

The photocurrent versus open-circuit voltage characteristic of individual subcells in a multijunction solar cell can be measured by illuminating the subcell with a pulse sequence of spatially homogeneous laser light. In this paper, we demonstrate the applicability of this method for four junction solar cells. Furthermore, the degradation of triple junction solar cells after electron irradiation is analyzed on the subcell level. Subcell performance parameters are derived, and results are compared with electroluminescence-based results. The influence of luminescent coupling and semitransparency effects to lower subcells are quantified by simulations.

Cross-Sectional Transport Imaging in a Multijunction Solar Cell

We combine a highly localized electron-beam point source excitation to generate excess free carriers with the spatial resolution of optical near-field imaging to map recombination in a cross-sectioned multijunction ( ${\rm{Ga}}_{{\rm{0.5}}}{\rm{In}}_{{\rm{0.5}}}{\rm{P/ GaIn}}_{{\rm{0.01}}}{\rm{As/ Ge}}$ ) solar cell. By mapping the spatial variations in emission of light for fixed generation (as opposed to traditional cathodoluminescence (CL), which maps integrated emission as a function of position of generation), it is possible to directly monitor the motion of carriers and photons. We observe carrier diffusion throughout the full width of the middle (GaInAs) cell, as well as luminescent coupling from point source excitation in the top cell GaInP to the middle cell. Supporting CL and near-field photoluminescence (PL) measurements demonstrate the excitation-dependent Fermi level splitting effects that influence cross-sectioned spectroscopy results, as well as transport limitations on the spatial resolution of conventional cross-sectional far-field measurements.

Internal luminescence efficiencies in InGaP/GaAs/Ge triple-junction solar cells evaluated from photoluminescence through optical coupling between subcells.

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.

On the temperature dependence of dual‐junction laser power converters

AbstractDual‐junction GaAs laser power converters optimized for one monochromatic wavelength are presented, and their temperature dependence is experimentally evaluated. External quantum efficiency and irradiance‐dependent current–voltage measurements (10 to 104 W/cm2) under monochromatic laser light (809 nm) have been undertaken to quantify temperature‐ and irradiance‐dependent effects on the performance. The temperature dependence of the current matching of the two subcells, caused by the temperature‐dependent absorbance, is quantified. Losses in performance due to variations in operating temperature for different power‐by‐light applications are calculated to be between 16.2% and 21.0%. Future potential enhancements in cell performance are discussed. For elevated temperatures, super‐linear behavior of the spectral response with increasing irradiance is observed, which is attributed to effective luminescent coupling from the top to the bottom subcell as the device becomes more radiative limited. For low temperatures, where the bottom cell is overproducing, no dependence on irradiance is found, which shows the influence of photon transport losses to the substrate. © 2016 The Authors. Progress in Photovoltaics: Research and Applications published by John Wiley & Sons Ltd.

Current relaxation in MJ SCs and its influence on IV curve formation in presence of light coupling

In the work, a procedure for simulating I-V characteristics of multijunction solar cells (MJ SCs) with accounting for the processes of current relaxation determined by the luminescent coupling between separate p-n junctions has been considered. The effect of the mentioned above processes on the I-V characteristic shape was investigated. It has been shown that, in the case of well pronounced reemission processes in high- efficient MJ SCs, in recording their I-V characteristics, variation of fill factor and rise of the open circuit voltage values being registered take place.

Quantitative Determination of Luminescent Coupling in Multijunction Solar Cells from Spectral Photovoltage Measurements

We present a simple method to quantify the magnitude of luminescent coupling (LC) between stacked subcells in multijunction photovoltaic devices. The effect of luminescence produced at high-band-gap subcells on underlying low-gap units within the same device can be directly accessed as a measurable open-circuit voltage difference by comparing two photovoltage spectra. Additionally, our study unambiguously identifies LC as the modulation mechanism across multijunction solar cells generating a response from buried subcells in photoreflectance measurements.

Realization of GaInP/Si Dual-Junction Solar Cells With 29.8% 1-Sun Efficiency

Combining a Si solar cell with a high-bandgap top cell reduces the thermalization losses in the short wavelength and enables theoretical 1-sun efficiencies far over 30%. We have investigated the fabrication and optimization of Si-based tandem solar cells with 1.8-eV rear-heterojunction GaInP top cells. The III–V and Si heterojunction subcells were fabricated separately and joined by mechanical stacking using electrically insulating optically transparent interlayers. Our GaInP/Si dual-junction solar cells have achieved a certified cumulative 1-sun efficiency of 29.8% ± 0.6% (AM1.5g) in four-terminal operation conditions, which exceeds the record 1-sun efficiencies achieved with both III–V and Si single-junction solar cells. The effect of luminescent coupling between the subcells has been investigated, and optical losses in the solar cell structure have been addressed.

Optimum band gap combinations to make best use of new photovoltaic materials

The detailed balance approach has been used to analyze the optimum use of band gaps in a multi-junction device of up to 6 sub-cells. Results for the AM1.5G spectrum suggest that as the number of sub-cells increases the importance of the bottom sub-cell band gap becomes less critical, assuming the optimum band gap combination for that value can be obtained. Given this greater freedom in choice, the potential for the use of silicon as an active substrate is investigated along with a cell thinning ‘current sharing’ approach to improve current mismatch in the device. Results show a more robust design space of multi-junctions with active silicon substrates when the current sharing approach is used, with performances close to the optimum for a completely free choice of band gaps. The use of the AM1.5D spectrum for a concentration ratio of 100, shows similar results for the substrate and a slight increase in band gap sensitivity for the upper band gaps in the stack. Inclusion of optical coupling between the sub-cells lowers limiting efficiency, with luminescent coupling mitigating the band gap sensitivity. The results and approach outlined are useful for determining how best to deploy new photovoltaic materials in multi-junction solar cells.

Determination of subcell open circuit voltages and Iph–Voc curves in multijunction solar cells by sequentially pulsed, monochromatic illumination

The open circuit voltages Voc of individual subcells in a multijunction solar cell are measured by illuminating a given subcell with a pulse of spatially homogeneous, nearly monochromatic light with a rising edge in the μs regime. The influence of luminescent coupling and semi-transparency on Voc is eliminated by over-illuminating all subcells below this subcell with a preceding light pulse. By using a suns-Voc approach, the two-diode model dark saturation currents of each subcell are extracted. The proposed method is verified experimentally as well as through simulations on three and four-junction solar cells.

Analysis of luminescent coupling effects innseries-connected multijunction solar cells

In this work, the luminescent coupling (LC) effects on photocurrent-matched and photocurrent-mismatched multijunction solar cells are investigated from fundamental physical theories and modeled by spice circuit simulations. It is demonstrated that the voltage increase of a photocurrent-matched cell is constant in the voltage range from maximum power point to open-circuit point, and this increase depends on the number of junctions and LC efficiency. Through the LC effects, it is shown that at the operation point of a photocurrent-matched double-junction (2J) cell, the photons from radiative recombination (which is the dominated recombination mechanism) of the top junction, is not wasted, but instead couples downward to the bottom junction, doubles the second junction's recombination current density, and leads to a significant voltage increase of = 17.8 mV. The same physics extended to a photocurrent-matched triple-junction (3J) cell with an increase of = 46 mV and n-junction cell with an increase of . Furthermore, it is shown that the theoretical prediction matches the circuit simulations exactly, and this tens of millivolts enhancement in voltage increases with increasing the number of junctions, consequently leads to a greater improvement in cell performance.

Simulation study of GaAsP/Si tandem solar cells

A model, adapted from the Shockley–Queisser detailed balance model to tandem solar cells with a monolithically grown GaAsxP1−x top junction on a Si bottom junction, has been developed. Updated data have been used for the absorption spectrums. Two surface geometries, flat and ideally textured, have been investigated. As an important improvement over existing models, the effects of threading-dislocations-related Shockley–Read–Hall recombinations in the GaAsxP1−x cell, due to the lattice mismatch between the GaAsxP1−x epilayers and the Si substrate, have been taken into consideration. Auger recombinations in the Si bottom cell and luminescent coupling between the cells have also been considered. For a dislocation-free 2-μm-thick top cell, maximal theoretical efficiencies of 41.6% and 39.1% have been calculated for a textured and a flat surface, respectively. For threading dislocation (TD) densities below 104cm−2, the impact of TDs in the GaAsxP1−x layers on the solar cell performances is very limited. With TD densities over 105cm−2, the top cell open-circuit voltage is reduced, hence the overall efficiency. For TD densities over 4×106cm−2, as the diffusion length of minority carriers in the base gets smaller than the base thickness, the short-circuit current in the top GaAsxP1−x cell is also reduced, resulting in a decrease in the optimal top cell bandgap. Using non-ideal EQEs and surface recombination rates from published experimental data, the long-term efficiency potential of the investigated technology has been estimated to be ~35.1% for an ideally textured GaAsxP1−x/Si tandem cell with a TD density of 105cm−2 (~33.0% with a flat surface).