Luminescent Coupling Research Articles

The Role of Luminescent Coupling in Monolithic Perovskite/Silicon Tandem Solar Cells.

Luminescent coupling (LC) is a key phenomenon in monolithic tandem solar cells. This study presents a nondestructive technique to quantitatively evaluate the LC effect, addressing a gap in the existing predictions made by optical modeling. The method involves measuring the ratio of photons emitted from the high bandgap top cell that escape through the rear, contributing additional current to the bottom cell, and to those escaping from the front side of top cell. The findings indicate that in the analyzed monolithic perovskite/silicon tandem solar cells, more than 85% of the emitted photons escaping from the perovskite top cell are used to generate additional current in the bottom cell. This process notably reduces the mismatch in the generated current between each subcell, particularly when the current is limited by the low bandgap subcell. The presented method is applicable to a variety of monolithic tandem structures, providing vital information for subcell characterization, providing vital information for predicting energy output and optimization for outdoor applications.

Measuring the device‐level EQE of multi‐junction photonic power converters

AbstractMulti‐junction photonic power converters (PPCs) are photovoltaic cells used in photonic power transmission systems that convert monochromatic light to electricity at enhanced output voltages. The junctions of a multi‐junction PPC have overlapping spectral responsivity, which poses a unique challenge for spectrally resolved external quantum efficiency (EQE) measurements. In this work, we present a novel EQE measurement technique based on a wavelength‐tunable laser system and characterize the differential multi‐junction device‐level EQE (dEQEMJ) as a function of the monochromatic irradiance over seven orders of magnitude. The irradiance‐dependent measurements reveal three distinct irradiance regimes with different dEQEMJ. For the experimentally studied 2‐junction GaAs‐based device, at medium irradiance with photocurrent densities between 0.3 and 90 mA/cm2, dEQEMJ is independent of irradiance and follows the expected EQE of the current‐limiting subcell across all wavelengths. At higher irradiance, nonlinear device response is observed and attributed to luminescent coupling between the subcells. At lower irradiances, namely, in the range of conventional EQE measurement systems, nonlinear effects appear, which mimic luminescent coupling behavior but are instead attributed to finite shunt resistance artifacts that artificially inflate dEQEMJ. The results demonstrate the importance of measuring the device‐level dEQEMJ in the relevant irradiance regime. We propose that device‐level measurements in the finite shunt artifact regime at low monochromatic irradiance should be avoided.

Effects of Photon Recycling and Luminescent Coupling in All‐Perovskite Tandem Solar Cells Assessed by Full Opto‐electronic Simulation

The impact of photon recycling (PR) and of luminescent coupling (LC) on the photovoltaic performance of all‐perovskite tandem solar cells is analyzed by the means of optical and full opto‐electronic device simulation. Optical processes are assessed using a comprehensive Green function formalism that considers wave optical effects also in emission. Starting from a consistent fit of experimental sub‐cell and tandem characteristics, the effects of re‐absorption are propagated from the optical limit to a situation with consideration of realistic charge transport across the entire tandem device. This also provides insight into the origin of performance losses due to sub‐cell and interconnection quality.

Luminescence coupling in 28.4%-efficient tandem solar cell utilizing 20.8%-efficient perovskite minimodule stacked on silicon cell

The total power conversion efficiency (PCE) of 28.4% with an aperture area of 64 cm2 on a four-terminal (4T) tandem solar cell with a 20.8% efficiency-semitransparent perovskite module (SPM) with a series interconnection structure and a conventional passivated emitter and rear cell type crystalline silicon solar cell is independently confirmed. This is the independently confirmed record PCE of a 4T perovskite and crystalline silicon tandem solar cell with a structure having the same active area for both top and bottom cells. The impact of the luminescence coupling monitored by the silicon solar cell in the 4T tandem solar cell was investigated based on electrical properties of the top SPM analyzed by a simple one-diode equivalent circuit model.

A simulation study on the bias and wavelength dependence of luminescent coupling in top-limited multi-junction solar cells

Abstract Luminescent coupling in a top-limited multi-junction solar cell was investigated by employing a 3D circuit model and semiconductor physics. The calculation methods for computing both electro-luminescent and photo-luminescent coupling efficiencies were introduced. Since the calculation processes were circuit independent, they can be applied directly to any form of an equivalent circuit model of a solar cell. Moreover, an external approach to counteract the luminescent coupling effect was provided and the luminescent coupling process in a top-limited solar cell was illustrated. Finally, the bias and wavelength dependence of luminescent coupling efficiency was applied to the 3D circuit model and the simulation results were consistent with the measured data.

A modeling framework to quantify the intermediate layer impact in III–V//Si multijunction solar cells

Multijunction solar cells (MJSCs) are capable of converting sunlight to electricity more efficiently than single-junction solar cells. The intermediate scattering layers between the individual junctions contribute to high efficiency by impacting the generated currents, photon recycling (PR), as well as luminescent coupling (LC) in the device. The MJSC efficiency can be simulated using expressions that involve a simplified and idealized intermediate layer structure but cannot accurately reflect its actual performance. This work, however, aims to establish a systematic optical model for MJSCs with complicated intermediate layers. It begins with incorporating the LC and PR effects into the developed model, emphasizing requirements for the cut-off wavelength and long-wavelength transmission of the intermediate layer. Furthermore, a three-dimensional metallic nanocylinder array is designed as the intermediate layer to improve device performance. With the model, high-performance MJSCs can be designed and optimised by quantifying the impact of PR and LC on device parameters.

Origins of the short circuit current of a current mismatched multijunction photovoltaic cell considering subcell reverse breakdown.

In the photovoltaic community, short circuit current (Isc) of a current mismatched multijunction photovoltaic (MJPV) cell was usually thought to be limited by the lowest subcell photocurrent (Imin). However, under certain conditions for multijunction solar cells, Isc≠Imin was observed by researchers, while this effect has not been studied in multijunction laser power converters (MJLPCs). In this work, we provide an in-depth analysis of the formation mechanisms for the Isc of the MJPV cell by measuring I-V curves of the GaAs and InGaAs LPCs with different number of subcells and simulating the I-V curves with the reverse breakdown of each subcell considered. It is found that Isc of an N-junction PV cell can be theoretically equal to any current value within a range from a current lower than Imin to the maximum subcell photocurrent, which is up to the number of subcell current steps in the forward biased I-V curve. An MJPV cell with a constant Imin will demonstrate a higher Isc if it has more subcells, smaller subcell reverse breakdown voltage and smaller series resistance. As a result, Isc tends to be limited by the photocurrent of a subcell closer to the middle cell and is less sensitive to the optical wavelength than Imin. This should be another possible reason why the measured EQE of a multijunction LPC exhibits a wider spectrum width than the calculated Imin-based EQE, whereas this was usually attributed to the luminescent coupling effect merely.

Effects of proton radiation on the InGaAs component cells of inverted metamorphic four-junction solar cells

In this study, 5-MeV proton radiation was incident on the InGaAs (1.0 eV) and InGaAs (0.7 eV) component cells of inverted metamorphic four-junction (IMM4J) solar cells. Light current–voltage (LIV), external quantum efficiency (EQE), and deep level transient spectroscopy (DLTS) were used to examine the performance degradation and the radiation defects. For the two InGaAs component cells, in case of LIV, open circuit voltage (Voc), short-circuit current (Isc), and the maximum output power (Pmax) decreased linearly with the function of logarithmic change of the proton fluence. Compared to InGaAs (1.0 eV) component cells, for the InGaAs (0.7 eV) component cells, Voc and Pmax decreased considerably and Isc decreased slightly. In EQE, an evident measurement artifact related to the luminescence coupling effect was detected in InGaAs (1.0 eV) before proton radiation, and it disappeared after radiation. This phenomenon was not observed in InGaAs (0.7 eV). In DLTS, a hole trap, H0, was detected at Ev + 0.09 eV in InGaAs (1.0 eV), and three hole traps H1, H2, and H3, were detected at Ev + 0.07 eV, Ev + 0.30 eV, and Ev + 0.45 eV, respectively, in InGaAs (0.7 eV). H0 and H1 were attributed to shallow traps introduced during radiation, and H2 and H3 were the initial defects due to the growth process; moreover, H2 and H3 were determined to be VGa (0/-1) and VGa(-1/-2) or probably VAs(+3/+1).

Electrical passivation of III-V multijunction solar cells with luminescent coupling effect

Perimeter recombination is one of the causes identified for having a nonuniform luminescent coupling effect in III-V multijunction solar cells. A potential solution could be the electrical passivation of the multijunction solar cell perimeter. To test this hypothesis, electrical passivation of InGaP/GaAs/Ge 3-junction solar cells was done by the atomic layer deposition of thin Al 2 O 3 films. Perimeter passivation of a 3-junction solar cell relatively increased current collection measured at luminescent coupling and direct subcell excitation by as much as 11.4% and 29.8%, respectively. Meanwhile, the current homogeneity improved relatively by as much as 7.9% after electrical passivation. At 1-sun global standard illumination (AM 1.5G), 0.2% absolute conversion efficiency increase was achieved. Therefore, electrical passivation of fully working III-V multijunction solar cells is a non-invasive, post-treatment process that can recover losses due to surface defects caused by oxidation. • Post-treatment passivation for fully working multijunction solar cell. • Larger conversion efficiencies in multijunction solar cells with smaller active areas. • Spatial current uniformity improvement after perimeter passivation. • Reduction of shunt leakage current in a subcell made of indirect bandgap material. • Alleviate lateral resistance effects by electrical passivation.

Theoretical modeling and ultra-thin design for multi-junction solar cells with a light-trapping front surface and its application to InGaP/GaAs/InGaAs 3-junction.

Light-trapping design is a good strategy to obtain ultra-thin solar cells without sacrificing conversion efficiency. If applied to III-V compound multi-junction solar cells (MJSCs), it not only can greatly reduce the cell cost and weight, but also improve its radiation tolerance when operating in space. This paper formulates all subcell absorptance in an arbitrary N-junction solar cell with an ideal front textured surface and perfect rear mirror, including the effects of complex absorption and luminescence coupling in the stack. Taking the well-known InGaP/GaAs/InGaAs triple-junction solar cell (3J) for instance, the ultra-thin design and the conversion efficiency both in radiative limit and that with subcell internal radiative efficiency below-unity are predicted. Our results show that such front-textured 3J with top-subcell thickness varying from 200 to 500 nm can enhance light absorption so significantly that more than 28% of top-subcell, 56% of middle-subcell, and 90% of bottom-subcell thickness will be cut down when compared with the smooth-surfaced 3J. Typically, (350 nm, 315 nm, 28 nm) is recommended as the optimal design for the front-textured 3J with an experimental efficiency of over 38%. For the same benchmarks on photocurrent of 15.1 mA/cm2 or detailed balance limit of 44%, the minimum total thickness (all subcells only) in the front-textured 3J is only 1453 nm, that is even 71% of that in the rear-textured 3J, quantitatively revealing front texturization has a greater potential for material cut-down than rear texturization. Finally, the impacts of non-ideal scattering texturization on cell performance and ultra-thin design are also discussed. This work provides theoretical guidance for experimental studies on ultra-thin and high-efficient MJSCs with various light-trapping strategies.

Luminescent coupling and efficiency of bifacial GaAs/Si tandem solar cells

The limiting efficiencies of monofacial and bifacial GaAs/Si tandem solar cells are calculated from the physical properties of GaAs and Si, with light trapping by Lambertian scattering. The highest efficiency GaAs/Si tandem cells are obtained when there is an air gap between the GaAs and Si layers, which maximizes the voltage of the GaAs cell. A 4-terminal tandem air gap cell with GaAs layer 4 μm thick and Si layer 200 μm thick was found to have an efficiency of 40.5% under AM1.5G illumination. The highest efficiency 2-terminal air gap tandem (37.3%) has a 0.37 μm GaAs layer and a 200 μm thick Si layer for which the photocurrents in the cells are equal. For thicker GaAs layers the photocurrent in the GaAs exceeds the photocurrent in the Si and in this case the most efficient 2-terminal cells (36.1%) for GaAs 4 μm thick and Si 200 μm thick, are obtained when the two cells are in physical contact which maximizes the luminescent coupling. In this regime the efficiency is insensitive to current mismatch because of compensation from luminescent coupling. The efficiency of these cells is also insensitive to changes in rear illumination intensity in bifacial operation.

A Smart Molecule Showing Spin Crossover Responsive Aggregation-Induced Emission

A Smart Molecule Showing Spin Crossover Responsive Aggregation-Induced Emission

A self-consistent interactive model for the study of luminescence coupling in multijunction solar cells

In this work, we propose a self-consistent interactive model based on the detailed balance approach to investigate the impact of luminescence coupling (LC) in multijunction solar cells (MJSCs). The proposed model is innovative in not using any empiric parameter input in considering the interactive nature of LC within the detailed balance framework to correlate the emissive and electrical properties of the junctions under interaction. We apply the model to radiative-limited series-connected MJSCs under different illumination conditions to demonstrate the influence of LC on the current matching condition and the impact on the power conversion efficiency (PCE) limits of such devices. As a result, we show that LC does not change the optimum bandgap energy combination leading to the highest PCE for a given operation condition but widens the span of configurations reaching high PCE, information that aids in the design of high PCE MJSC. Additionally, we analyzed some selected MJSC configurations with two to six junctions, well known from the literature to reach high PCE under different illumination conditions showing that even better performance can be achieved without either the need for changing the existent active materials or using optical thinning but using more efficient photon management concepts. Finally, we indicate MJSC configurations that can achieve high PCE for terrestrial applications under high coupling conditions, including some promises for low-cost high-efficiency photovoltaics, especially the ones involving stacks with silicon, perovskites, chalcogenides, and/or III–V materials.

Characterization of multiterminal tandem photovoltaic devices and their subcell coupling

Three-terminal (3T) and four-terminal (4T) tandem photovoltaic (PV) devices using various materials have been increasingly reported in the literature, but measurement standards are lacking. Here, multiterminal devices measured as functions of two load variables are characterized unambiguously as functions of three device voltages or currents on hexagonal plots. We demonstrate these measurement techniques using two GaInP/GaAs tandem solar cells, with a middle contact between the two subcells, as example 3T devices with both series-connected and reverse-connected subcells. Coupling mechanisms between the subcells are quantified within the context of a simple equivalent optoelectronic circuit. Electrical and optical coupling mechanisms are most clearly revealed using coupled dark measurements. These measurements are sensitive enough to observe very small luminescent coupling from the bottom subcell to the top subcell in the prototype 3T device. Quick simplified measurement techniques are also discussed within the context of the complete characterization. Two types of 3T GaInP/GaAs tandems are fabricated and characterized Hexagonal plots unambiguously characterize multiterminal tandem solar cells Five zero-power points of 3T tandems are analogous to the V OC and J SC of 2T devices Tiny luminescent coupling from the bottom to the top subcell of 3T tandem is quantified Three- and four-terminal tandem photovoltaic devices are becoming increasingly relevant. Geisz et al. demonstrate meaningful measurement techniques for unambiguously characterizing these devices using 3T GaInP/GaAs tandem solar cells as examples. Subcell coupling is sensitively quantified with coupled dark measurements that are fit to a simple equivalent optoelectronic circuit.

Multijunction solar cell spectral response determination at radiation damage study

This paper discusses multijunction solar cells with optically coupled p-n junctions under radiation exposure photosensitivity spectral response study. It is shown that if the measurement technique does not consider the luminescent coupling and does not track the optical coupling degradation, then instead of a decrease (which is a natural response of a photoconverter to radiation damage), an abnormal increase in the narrow-bandgap photoresponse (receiving luminescent radiation) subcell due to radiation damage can be observed. Accordingly, with an increase in the irradiation dose, an increase in the subcell photocurrent forming a “negative” degradation dependence of MJ SC being used in space applications is recorded.

Directional luminescence of the diamond NV center via Bloch surface waves in one-dimensional photonic crystals

In this work, we numerically study the luminescence of nanodiamonds with NV centres embedded in a polymer layer on the surface of one-dimensional photonic crystal. The interaction of NV center spontaneous emission with the Bloch surface wave (BSW) is demonstrated. The presence of a photonic crystal leads to a change in the angular distribution of the emitter radiation due to the coupling of luminescence to BSW. We show that the best coupling efficiency of 71% is observed when NV centres are located in the close proximity to the BSW field maximum.

A review of modeling of luminescent coupling effect in multi-junction solar cell based on diode equation

Abstract The luminescent coupling effect in multi-junction solar cell is a phenomenon where extra photocurrent in one sub-cell is driven by radiative recombination of electron–hole pairs in another sub-cell. This paper focuses on the analysis of the modeling of luminescent coupling effect in multi-junction solar cell. These modelings are based on the two-diode equivalent circuit. Finally, it concludes that the study of the luminescent coupling effect requires not only a suitable modeling but also a good experimental method for detecting the characteristics of sub-cells in multi-junction solar cell.

Study of luminescent coupling effect in modeling a concentrator photovoltaic system with III-V triple junction solar cell

Abstract A theoretical model is proposed for a concentrator photovoltaic (CPV) system. This model is based on the two-diode equation and considers the luminescent coupling (LC) effect in III-V triple junction solar cell. By using the model, the ‘I-V’ curve of the system is fitted. The fitting result is in good agreement with the experimental data. The system’s open circuit voltage is also predicted in a certain temperature range. Based on the analysis of the simulation results and the two-diode equation, it is concluded that the LC effect should be a non-negligible mechanism for analyzing the operation performance of CPV system.

MOVPE Growth and EQE Artifact Analysis of Upright Metamorphic Quadruple Junction Solar Cell

The performance of metal-organic vapor phase epitaxy (MOVPE) grown upright metamorphic (UMM) AlGaInP/AlGaInAs/GaInAs/Ge quadruple junction (QJ) solar cells have been investigated. Metamorphic (MM) epitaxy is achieved through compositionally graded buffer (CGB) layer varying in lattice constants between Ge and Ga0.8In0.2As. High-resolution X-ray diffraction (HRXRD) was used to study the relaxation of strain which is about 98%. Threading dislocation density (TDD) at about 3 × 105 cm−2 was estimated from cathodoluminescence (CL) images. Incorporation of oxygen is effectively suppressed by elevated growth temperature and phosphine flow, which can decrease the nonradiative recombination rate in active layers. Systematic spectra response measurements were used to analyze the comprehensive effect of reverse breakdown (RBD) and luminescence coupling (LC) in multijunction solar cells, including bias light- and voltage-modulated analysis and corresponding analytical models.

Perovskite/Silicon Tandem Solar Cells: Effect of Luminescent Coupling and Bifaciality

The power conversion efficiency of the market‐dominating silicon photovoltaics approaches its theoretical limit. Bifacial solar operation with harvesting additional light impinging on the module back and the perovskite/silicon tandem device architecture are among the most promising approaches for further increasing the energy yield from a limited area. Herein, the energy output of perovskite/silicon tandem solar cells in monofacial and bifacial operation is calculated, for the first time considering luminescent coupling (LC) between two sub‐cells. For energy yield calculations, idealized solar cells are studied at both standard testing as well as realistic weather conditions in combination with a detailed illumination model for periodic solar panel arrays. Typical experimental photoluminescent quantum yield values reveal that more than 50% of excess electron–hole pairs in the perovskite top cell can be utilized by the silicon bottom cell by means of LC. As a result, LC strongly relaxes the constraints on the top‐cell bandgap in monolithic tandem devices. In combination with bifacial operation, the optimum perovskite bandgap shifts from 1.71 eV to the range 1.60–1.65 eV, where already high‐quality perovskite materials exist. The results are very important for developing optimal perovskite materials for tandem solar cells.