Internal Radiative Efficiency Research Articles

Understanding and Weakening Photon Recycling in Solar Cells to Approach the Radiative Limit.

Photon recycling plays an important role in the light outcoupling of state-of-the-art solar cells and is considered a necessary condition to achieve the radiative limit of open-circuit voltage (VOC) and efficiency. However, due to the impact of photon recycling on bulk and surface radiation of solar cells being overlooked, experimental demonstrations on the accuracy of existing photon recycling models are scarce and some contrary theoretical results also emerge. Here, the relationship between photon recycling and radiation processes, as well as the corresponding VOC losses of solar cells based on the principle of detailed balance is clarified. It is shown that weakening photon recycling is more effective to boost the device performance than enhancing it, promoting the theoretical efficiencies of solar cells, such as perovskite, Si, and GaAs, to 98.5%, 94.9%, and almost 100% of their radiative limit, respectively. Moreover, weakening photon recycling also helps to maintain higher efficiency when the internal radiative efficiency decreases, which benefits higher device stability. This work provides an in-depth understanding of the role of photon recycling in solar cells and helps to push efficiency to a new limit.

Diagnosis and Breakeven Analysis of GaInNAs Subcell Incorporated in Monolithic Lattice-Matched Five-Junction Solar Cell

We focus on the diagnosis and breakeven analysis of the critical dilute nitride subcell incorporated in a monolithic AlGaInP/AlGaInAs/InGaAs/GaInNAs/Ge five-junction (5J) solar cell grown by metalorganic vapor phase epitaxy. Via absolute electroluminescent measurement and with the help of the control cells, some key internal performance of our latest 5J with AM0 efficiency of 28.8% are disclosed, where the subcell <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oc</sub> from top to bottom are assessed as 1576±1 mV, 1253±4 mV, 1001±1 mV, 372 mV, and 196 mV, respectively. The yield contributed by the top three subcells is assessed as 26.8%, while that by the GaInNAs- and Ge-subcell is only 2.0% in total. The evaluated internal radiative efficiency of the top three subcells reveal that GaInNAs-subcell growth leads to obvious degradation of the lattice quality in the closest GaInAs-subcell and slight drops in the AlGaInP- and AlGaInAs-subcells. Finally, a semiempirical approach is used to predict the essential parameters on the 1-eV subcell used for such 5J architecture so that it can exceed the efficiency benchmark of 30%–34%.

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.

Limiting mechanisms for photon recycling in thin‐film GaAs solar cells

AbstractPhoton recycling mechanisms in single junction thin‐film GaAs solar cells are evaluated in this study. Modelling supported by experimentally obtained results is used in order to correlate the reflectance of the cell's rear layers, the photon recycling probability, and the solar cell performance. Solar cells with different top and bottom metallization configurations are produced, and their performance is analyzed from the optical and electrical point of view. It is shown that the photon recycling probability increases with the rear mirror reflectance and solar cell thickness, which results in the increase of the devices open circuit voltage. However, the front grid coverage, usually disregarded in rear mirror focused studies, strongly reduces the photon recycling probability. Furthermore, perimeter and interface recombination hinder the internal radiative efficiency of the solar cells, preventing further increase of the devices' open circuit voltage as a result of improvements of the rear mirror reflectivity. In order to exploit the significant benefit of increased photon recycling probability to the solar cell performance, these limiting mechanisms need to be properly addressed.

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.

Modeling and design for low‐cost multijunction solar cell via light‐trapping rear texture technique: Applied in InGaP/GaAs/InGaAs triple junction

AbstractTo realize high efficiency in parallel with low cost, a light‐trapping rear texture was proposed to be implemented in substrate‐free thin‐film multijunction (MJ) cells. A detailed‐balance theory was formulated taking account of the finite light absorption in thin subcells. Such presented absorption model is general and useful to optimize the subcell thickness for MJ solar cells with light‐trapping design. It is applied for InGaP/GaAs/InGaAs triple‐junction solar cells to simulate subcell photocurrents and to obtain the current‐matching (minimum requisite) subcell thicknesses combinations. Furthermore, the detailed‐balance conversion efficiency was estimated for both radiative limit and the cases with below‐unity internal radiative efficiency. For InGaP/GaAs/InGaAs MJ cells with InGaP subcell thickness less than 600 nm, adding a random‐textured rear reflector can enhance light absorption so significantly that over 90% of InGaAs‐cell thickness and even 50% of GaAs‐cell thickness would be cut without any penalty in conversion efficiency, compared with the subcell thicknesses in traditional MJ cells with flat rear reflectors. Additionally, the thickness combination, (InGaP, GaAs, and InGaAs) = (450 nm, 333 nm, and 26 nm), is recommended to achieve both high conversion efficiency and low material cost. This work provides a very important theoretical guidance for the development on low‐cost and high‐efficiency MJ devices.

Exploring the Internal Radiative Efficiency of Selective Area Nanowires

Nanowires are ideal building blocks for next-generation solar cell applications. Nanowires grown with the selective area (SA) approach, in particular, have demonstrated very high material quality, thanks to high growth temperature, defect-free crystalline structure, and absence of external catalysts, especially in the InP material system. A comprehensive study on the influence of growth conditions and device processing on optical emission is still necessary though. This article presents an investigation of the nanowire optical properties, performed in order to optimize the internal radiative efficiency. In an initial preamble, the motivation for this study is discussed, as well as the morphology and crystallinity of the nanowires. The effect on the nanowire photoluminescence of several intrinsic and extrinsic parameters and factors are then presented in three sections: first, the influence of basic growth conditions such as the temperature and the precursor ratio is studied. Subsequently, the effects of varying dopant molar flows are explored, keeping in mind the intended solar cell application. Third, the manner in which the processing and the passivation affect the nanowire optical emission is discussed. Precise control of the growth conditions allows maximizing the nanowire internal radiative efficiency and thus their performance in solar cells and other optoelectronic devices.

Optimization of the defects and the nonradiative lifetime of GaAs/AlGaAs double heterostructures

We used Raman scattering and time-resolved photoluminescence spectroscopy to investigate the molecular-beam-epitaxy (MBE) growth parameters that optimize the structural defects and therefore the internal radiative quantum efficiency of MBE-grown GaAs/AlGaAs double heterostructures (DH). The DH structures were grown at two different temperatures and three different As/Ga flux ratios to determine the conditions for an optimized structure with the longest nonradiative minority carrier lifetime. Raman scattering measurements show an improvement in the lattice disorder in the AlGaAs and GaAs layers as the As/Ga flux ratio is reduced from 40 to 15 and as the growth temperature is increased from 550 to 595 °C. The optimized structure is obtained with the As/Ga flux ratio equal to 15 and the substrate temperature 595 °C. This is consistent with the fact that the optimized structure has the longest minority carrier lifetime. Moreover, our Raman studies reveal that incorporation of a distributed Bragg reflector layer between the substrate and DH structures significantly reduces the defect density in the subsequent epitaxial layers.

Diagnosis of GaInAs/GaAsP Multiple Quantum Well Solar Cells With Bragg Reflectors via Absolute Electroluminescence

By using absolute electroluminescence measurement, we characterized current-dependent external radiative efficiency of GaInAs/GaAsP multiple quantum well (MQW) single-junction solar cells with and without a back distributed Bragg reflector (DBR). Internal radiative efficiency ( η int) under illuminated condition was quantified for analyzing their difference in photovoltaic performances. It was revealed that MQWs showed an advantage of improved η int compared with a bulk layer, and that a back DBR indeed improved conversion efficiency via double-pass absorption of sun light, but improvement via reduction of rear radiative emission loss toward substrate was small. Efficiency add-on via improved material quality, or η int, in the same structures was predicted.

Characterizations of Radiation Damage in Multijunction Solar Cells Focused on Subcell Internal Luminescence Quantum Yields via Absolute Electroluminescence Measurements

Via absolute electroluminescence measurements, we characterized degradations of internal luminescence quantum yield or internal radiative efficiency ( $\eta _{{\rm int}i}$ ) in respective subcells in GaInP/GaAs/Ge triple-junction and GaInP/GaAs double-junction solar cells after irradiations of protons and electrons with different energy and fluence ( $\phi$ ). Compared with typical open-circuit-voltage characterizations, $\eta _{{\rm int}i}$ turned out to be a sensitive, high-dynamic-range, quantitative, and fair indicator of radiation damage, since it purely represents material-quality change due to radiation damage, independently from small differences in bandgap energy due to alloy composition fluctuations and in other cell structures. A detailed fluence-dependence study has shown that the data of $\eta _{{\rm int}i}$ versus $\phi$ in moderate and high $\phi$ regions are very similar and almost independent of subcell materials, while the difference in beginning-of-life qualities of GaInP and GaAs materials causes dominant difference in subcell sensitivity to the low-radiation damages.

Nonradiative lifetime extraction using power-dependent relative photoluminescence of III-V semiconductor double-heterostructures

A power-dependent relative photoluminescence measurement method is developed for double-heterostructures composed of III-V semiconductors. Analyzing the data yields insight into the radiative efficiency of the absorbing layer as a function of laser intensity. Four GaAs samples of different thicknesses are characterized, and the measured data are corrected for dependencies of carrier concentration and photon recycling. This correction procedure is described and discussed in detail in order to determine the material's Shockley-Read-Hall lifetime as a function of excitation intensity. The procedure assumes 100% internal radiative efficiency under the highest injection conditions, and we show this leads to less than 0.5% uncertainty. The resulting GaAs material demonstrates a 5.7 ± 0.5 ns nonradiative lifetime across all samples of similar doping (2–3 × 1017 cm−3) for an injected excess carrier concentration below 4 × 1012 cm−3. This increases considerably up to longer than 1 μs under high injection levels due to a trap saturation effect. The method is also shown to give insight into bulk and interface recombination.

Conversion efficiency limits and bandgap designs for multi-junction solar cells with internal radiative efficiencies below unity.

We calculated the conversion-efficiency limit ηsc and the optimized subcell bandgap energies of 1 to 5 junction solar cells without and with intermediate reflectors under 1-sun AM1.5G and 1000-sun AM1.5D irradiations, particularly including the impact of internal radiative efficiency (ηint) below unity for realistic subcell materials on the basis of an extended detailed-balance theory. We found that the conversion-efficiency limit ηsc significantly drops when the geometric mean ηint* of all subcell ηint in the stack reduces from 1 to 0.1, and that ηsc degrades linearly to logηint* for ηint* below 0.1. For ηint*<0.1 differences in ηsc due to additional intermediate reflectors became very small if all subcells are optically thick for sun light. We obtained characteristic optimized bandgap energies, which reflect both ηint* decrease and AM1.5 spectral gaps. These results provide realistic efficiency targets and design principles.

Implications of Redesigned, High-Radiative-Efficiency GaInP Junctions on III-V Multijunction Concentrator Solar Cells

Nonradiative recombination in inverted GaInP junctions is dramatically reduced using a rear-heterojunction design rather than the more traditional thin-emitter homojunction design. When this GaInP junction design is included in inverted multijunction solar cells, the high radiative efficiency translates into both higher subcell voltage and high luminescence coupling to underlying subcells, both of which contribute to improved performance. Subcell voltages within two and four junction devices are measured by electroluminescence and the internal radiative efficiency is quantified as a function of recombination current using optical modeling. The performance of these concentrator multijunction devices is compared with the Shockley–Queisser detailed-balance radiative limit, as well as an internal radiative limit, which considers the effects of the actual optical environment in which a perfect junction may exist.

Deterministic coupling of a single silicon-vacancy color center to a photonic crystal cavity in diamond.

Deterministic coupling of single solid-state emitters to nanocavities is the key for integrated quantum information devices. We here fabricate a photonic crystal cavity around a preselected single silicon-vacancy color center in diamond and demonstrate modification of the emitters internal population dynamics and radiative quantum efficiency. The controlled, room-temperature cavity coupling gives rise to a resonant Purcell enhancement of the zero-phonon transition by a factor of 19, coming along with a 2.5-fold reduction of the emitter's lifetime.

Effects of Internal Luminescence and Internal Optics on $V_{\bf oc}$ and $J_{\bf sc}$ of III--V Solar Cells

For solar cells dominated by radiative recombination, the performance can be significantly enhanced by improving the internal optics. We demonstrate a detailed model for solar cells that calculates the external luminescent efficiency and discuss the relationship between the external and internal luminescence. The model accounts for wavelength-dependent optical properties in each layer, parasitic optical and electrical losses, multiple reflections within the cell, and assumes isotropic internal emission. For single-junction cells, the calculation leads to Voc, and for multijunction cells, the calculation leads to the Voc of each junction as well as the luminescent coupling constant. In both cases, the effects of the optics are most prominent in cells with high internal radiative efficiency. Exploiting good material quality and high luminescent coupling, we demonstrate a two-junction nonconcentrator cell with a conversion efficiency of (31.1 ± 0.9)% under the global spectrum.

Enhanced external radiative efficiency for 20.8% efficient single-junction GaInP solar cells

We demonstrate 1.81 eV GaInP solar cells approaching the Shockley-Queisser limit with 20.8% solar conversion efficiency, 8% external radiative efficiency, and 80–90% internal radiative efficiency at one-sun AM1.5 global conditions. Optically enhanced voltage through photon recycling that improves light extraction was achieved using a back metal reflector. This optical enhancement was realized at one-sun currents when the non-radiative Sah-Noyce-Shockley junction recombination current was reduced by placing the junction at the back of the cell in a higher band gap AlGaInP layer. Electroluminescence and dark current-voltage measurements show the separate effects of optical management and non-radiative dark current reduction.

An improved carrier rate model to evaluate internal quantum efficiency and analyze efficiency droop origin of InGaN based light-emitting diodes

A carrier rate model taking carrier delocalization into account is presented to analyze current dependent internal quantum efficiency of InGaN based light-emitting diodes (LEDs). By fitting normalized experimental internal quantum efficiency-current curves, both injection efficiency and radiative recombination efficiency depending on current can be obtained. Based on the fitting results from two LED samples with and without the InGaN interlayer beneath the active regions of 5 InGaN quantum wells (QWs), carrier delocalization and carrier leakage are believed to lead to the efficiency droop effect under considerable and even larger injection, respectively. By investigating two LED samples with 8 and 10 QWs, it is found that the 8-QWs LED has the highest radiative recombination efficiency over 80% and the 10-QWs one has the highest injection efficiency over 50% under 120 A/cm2. This means that increasing QW number is an effective method to suppress droop effect.

Spontaneous Radiative Efficiency and Gain Characteristics of Strained-Layer InGaAs–GaAs Quantum-Well Lasers

The optical gain spectra, unamplified spontaneous emission spectra, and spontaneous radiative efficiency are extracted from the measurement of amplified spontaneous emission (ASE) on a single pass, segmented contact 0.98-mum-emitting aluminum-free InGaAs-InGaAsP-GaAs quantum-well (QW) laser diode. These measurements provide a baseline for which to compare higher strain InGaAs QW lasers emitting near 1.2 mum. The peak gain-current relationship is extracted from gain spectra and the peak gain parameter go is found to agree within 25% of the value extracted using conventional cavity length analysis for 0.98-mum-emitting devices. The spontaneous radiative current is extracted using the fundamental connection between gain and unamplified spontaneous emission, which in turn gives an estimate of the amount of nonradiative recombination in this material system. The spontaneous radiative efficiency, the ratio of spontaneous radiative current to total current, at room temperature of 0.98-mum-emitting InGaAs QW laser material is found to be in the range of 40%-54%, which is 2.5-3.5 times larger than that of highly strained InGaAs QW laser emitting near lambda = 1.2 mum. Whereas the gain parameter, g <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> = dg/d(ln j), was measured to be 1130 and 1585 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> for the 0.98-mum- and 1.2-mum-emitting materials, respectively. From the calculated below threshold current injection efficiency of 75%-85%, we deduce that the internal radiative efficiency of the QW material is ~ 20% higher than the ratio of internal radiative current to external injected current extracted directly from ASE measurements.

Rigorous carrier dynamic model of electroluminescent metal-oxide-semiconductor silicon tunneling diodes

The carrier dynamics of electroluminescent metal-oxide-semiconductor silicon tunneling diodes is rigorously modeled in this study. Various tunneling and recombination current densities are formulated without using the Maxwell-Boltzmann approximation for the carrier concentrations. This model satisfactorily explains the current-voltage characteristics in strong accumulation using a self-consistent formulation. It also relates light emission efficiency to the interface-state density and the bulk-trap density in a straightforward manner. The internal radiative recombination efficiency in the bulk Si substrate is estimated to be in tens of percents. The model also explains clearly the strongly influenced small-signal light-current response time with respect to the injection current density. It enables the small-signal method to be useful in extracting the parameters of the interface-state density and the bulk-trap density.

Efficient Infrared Electroluminescent Devices Using Solution‐Processed Colloidal Quantum Dots

AbstractWe report efficient electroluminescence in the near‐infrared from PbS–MEH‐PPV (poly(2‐methoxy‐5‐(2′‐ethylhexyloxy)‐1,4‐phenylenevinylene)) large‐area, solution‐cast nanocomposite devices. We employ multivariate optimization of the structural and materials components that govern the radiative, energy‐transfer, and bipolar‐injection efficiencies into the devices. As a result, we report an external electroluminescence quantum efficiency of 0.27 %, which corresponds to an internal electroluminescence quantum efficiency of 1.9 %. The very best devices exhibit internal‐radiative‐efficiency‐limited performance and not transport‐ or capture‐limited performance, indicating that further gains in efficiency may be achieved if the internal radiative efficiency of the nanocrystal–polymer composite can be further increased without compromising transfer and device bipolar‐injection efficiency.