Luminescent Coupling Effects Research Articles

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.

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.

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.

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.

Application of perovskite quantum dots in carrier redistribution in III-V multijunction solar cells with luminescent coupling effect

The luminescent coupling effect in a multijunction solar cell is known to help achieve current matching among subcells through carrier redistribution. We demonstrate the carrier redistribution in III-V multijunction solar cell devices using a moisture-resistant, all-inorganic perovskite quantum dot (PQD) film. This hydrophobic PQD film was applied on a full III-V multijunction solar cell device. This successfully demonstrated current redistribution vertically, shown by the increased current collection in the lower bandgap subcells, and laterally, as observed from improved current collection homogeneity in the lower bandgap subcell adjacent to the higher bandgap subcell where the luminescence originated.

Efficiency increase in multijunction monochromatic photovoltaic devices due to luminescent coupling

We present a multijunction detailed-balance model that includes the effects of luminescent coupling, light trapping, and nonradiative recombination, suitable for the treatment of multijunction solar cells and photonic power converters—photovoltaic devices designed to convert the narrow-band light. The model includes both specular and Lambertian reflections using a ray-optic formalism and treats nonradiative processes using an internal radiative efficiency. Using this model, we calculate and optimize the efficiency of multijunction photonic power converters for a range of material qualities and light-trapping schemes. Multijunction devices allow increased voltage with lower current, decreasing series resistance losses. We show that efficiency increases significantly with an increased number of junctions, even without series resistance, when the device has an absorbing substrate. Such an increase does not occur when the device has a back reflector. We explain this effect using a simplified model, which illustrates the origin of the decreased radiative losses in multijunction devices on substrates.

A new model for thin-film solar cells using photon cycling

Photovoltaics has emerged as one of the most promising and reliable renewable energy resources. By maximizing the light confinement in solar junctions, the absorption increases, and consequently enhances the generation rate. Photon cycling plays a role model in thin-film solar cells, hence a new analytical model for evaluating the generation rate is developed. This photon cycling effect records an improvement of 144% on the total generation rate in 50 μm Si junctions. Photon recycling (PR) and luminescent coupling (LC) effects are also examined in the presence of photon recycling in Si/GaAs tandem solar cells. The PR effect enhances the GaAs generation rate by 65%, and by 1.5% for the LC in the Si junction.

Spectral response measurements of each subcell in monolithic triple-junction GaAs photovoltaic devices

Multijunction photovoltaic devices have gained attention for both solar cells and wireless power transmission. Herein, we present the spectral response of each subcell in a monolithic triple-junction photovoltaic device composed of only GaAs subcells with different thicknesses, which clarifies current matching among the GaAs subcells under simulated solar illumination conditions. In addition, the spectral response curves of each subcell show that the power-dependent spectral responses under laser illumination with wavelengths of 405, 660, and 785 nm are explained by the top GaAs subcell, which has a slightly lower shunt resistance and experiences the effects of luminescent coupling.

Luminescent coupling effect in wafer-bonded III-V on silicon multijunction solar cells

Strong luminescent coupling (LC) effect reduces current mismatch in III-V-based multijunction solar cells and consequently improves their power conversion efficiency. The LC current production in the Si bottom cell of InGaP/AlGaAs//Si triple junction solar cells having different sizes was investigated to determine the cell area dependence of LC effect. This was probed through laser beam-induced current (LBIC) mapping. The areal mapping was then assessed by obtaining the absolute difference between the quantum efficiencies calculated from the fitted LBIC map and simulated LC current considering uniform distribution using a quasi-two-dimensional electro-optical prediction model. At 9.17 suns concentration difference between the AlGaAs middle cell and the current-limiting Si bottom cell, the absolute LC quantum efficiency differences ranged between 0.16% and 1.68% in various cell sizes, which indicate potential current production increase if the LC current is made uniform.

Temperature dependence of wafer-bonded III-V on silicon multijunction solar cells with luminescent coupling effect

Strong luminescent coupling (LC) effect in a multijunction solar cell (MJSC) allows better current balance among its subcells. The temperature dependence of wafer-bonded III-V on silicon (Si) MJSCs with LC effect was investigated. Experimentally, this was explored through light current density–voltage (J – V) characteristic measurements, external quantum efficiency measurements, and laser beam-induced current mapping of InGaP/AlGaAs//Si triple junction solar cells at different cell temperatures. Measurement results were analyzed using a quasi-two-dimensional electro-optical prediction model for LC current production and bandgap temperature dependence models of various semiconductors. It was revealed that at lower temperatures, LC current production in the limiting Si bottom cell resulted in a larger absolute LC quantum efficiency difference. At 15°C, the absolute LC quantum efficiency difference calculated was 0.17%, which indicates potential current production improvement if vertical LC current is made uniform.

Analysis of luminescence coupling effect in three-terminal tandem solar cells

The properties of luminescence coupling (LC) in multijunction solar cells were investigated in three-terminal InGaP//InGaAsP and InGaP/GaAs//InGaAsP tandem solar cells fabricated using the bonding technique through metal nanoparticle (MNP) arrays, where // denotes the mechanical stacking using MNP arrays. Power extraction in the three-terminal tandem solar cells is evaluated, showing that the power extraction in the devices is equivalent to a sum of the power extraction of the top and bottom subcells. In addition, the properties of the LC effect are investigated in the three-terminal tandem devices. The LC current increases with the illuminated light intensity and shows a bias-voltage dependency of the subcell that is emitting luminescence. We also discuss the impact of LC on the power extraction of the three-terminal tandem solar cells.

Transient analysis of luminescent coupling effects in multi-junction solar cells

We investigate the luminescent coupling (LC) effects in a four-junction GaInP/GaAs//GaInAsP/GaInAs concentrator solar cell based on transient open-circuit voltage (Voc) measurements under monochromatic illumination. Photocurrent generation in the non-absorbing GaInAs bottom subcell due to LC from upper subcells shows superlinear behavior with increasing light intensity. Along with this, a Voc enhancement is observed and quantified for illumination intensities that span almost six orders of magnitude. The Voc increase is explained and studied using a series-connected diode model including subcell shunt resistances, capacitances, and LC effects. The impact of unilluminated subcells on the subcell Voc determination is discussed for multi-junction solar cells. Finally, in the analysis of the LC generated photocurrent, namely, the coupling factor from the GaInAsP to the non-absorbing GaInAs subcell, a characteristic dependency on bias voltage is shown and explained by a result of competing photo- and electroluminescence mechanisms.

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.

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.

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.

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.

Luminescent coupling in planar opto-electronic devices

Effects of luminescent coupling are observed in monolithic 5 V, five-junction GaAs phototransducers. Power conversion efficiency was measured at 61.6% ± 3% under the continuous, monochromatic illumination for which they were designed. Modeling shows that photon recycling can account for up to 350 mV of photovoltage in these devices. Drift-diffusion based simulations including a luminescent coupling term in the continuity equation show a broadening of the internal quantum efficiency curve which agrees well with experimental measurements. Luminescent coupling is shown to expand the spectral bandwidth of the phototransducer by a factor of at least 3.5 for devices with three or more junctions, even in cases where multiple absorption/emission events are required to transfer excess carriers into the limiting junction. We present a detailed description of the novel luminescent coupling modeling technique used to predict these performance enhancements.