Light And Elevated Temperature Induced Degradation Research Articles

Passivation with sputtered silicon nitride and modified heat treatment for lifetime improvement

It is widely inferred that Light and Elevated Temperature Induced Degradation (LeTID) is attributed to the diffusion of hydrogen into bulk silicon from the rear PECVD SiNx:H layer of PERC during the high-temperature firing process. We propose not only a simplified heat treatment flow for the typical PERC structure but also a new degradation-reduced structure with sputtered SiNx. The common N2 annealing after Al2O3 formation can be skipped to reduce process complexity and cost because we found that the lifetime can be even improved by omitting the moderate-temperature annealing while retaining the unavoidable high-temperature firing process for metallization. In the light soaking measurement of lifetime, the sputtered-SiNx passivated wafer displayed a higher lifetime than the PECVD-SiNx:H passivated wafer. The absence of excess hydrogen in the sputtered-SiNx film contributes to the suppression of LeTID. We further demonstrate that the sputtered-SiNx cells own higher VOC compared to PECVD-SiNx:H cells under light soaking.

Enhancing Reliability and Regeneration of Single Passivated Emitter Rear Contact Solar Cell Modules through Alternating Current Power Application to Mitigate Light and Elevated Temperature‐Induced Degradation

The study explores a novel method to combat the Light and Elevated Temperature‐Induced Degradation (LeTID) in solar cell modules, which significantly reduces their efficiency and lifespan. This method involves applying alternating current (AC) of various waveforms (triangular, sinusoidal, and square) and frequencies (5 and 100 kHz) to boron‐doped p‐type passivated emitter rear contact (p‐PERC) solar cell modules. This approach effectively lowers the series resistance at the critical junction between the silver (Ag) contact and the silicon emitter layer of the PERC solar cell, thereby reducing charge recombination hindered by high resistance, especially at elevated temperatures. As a result, there is an improved flow of electrical charges, leading to decreased energy loss and increased solar cell efficiency. The study's findings indicate that a slow, smooth sinusoidal AC waveform at 100 kHz is particularly effective, restoring about 100% of the original performance of the panel. Moreover, oscillations at 5 kHz also show considerable efficacy, recovering more than 96% of the performance. The sinusoidal waveform is noted to surpass both triangular and square waveforms in recovery efficiency. This research highlights the use of high‐frequency AC electricity as a viable strategy to extend the lifespan and enhance the performance of solar panels.

Deciphering the Role of Hydrogen in the Degradation of Silicon Solar Cells under Light and Elevated Temperature

In recent years, significant attention has been paid to the research of light‐ and elevated‐temperature‐induced degradation (LeTID) in silicon solar cells due to the substantial power loss and instability it causes. It has been discovered that the presence of hydrogen is closely linked to the occurrence of LeTID. In this study, a thorough review and re‐assessment of previously published results is conducted and connected with newly obtained data. The findings indicate a complex interaction between different hydrogen complexes and the LeTID defect states. The precursor of LeTID is connected to molecular hydrogen (H2), while the LeTID degradation and regeneration are related to the binding of atomic hydrogen to the precursor and defect, respectively. A detailed description of the various reactions that occur under illumination and in the dark is provided. Additionally, explanation is given on how pre‐annealing can significantly affect the kinetics of LeTID during subsequent light soaking. Furthermore, a comprehensive hydrogen model that incorporates these various reactions and demonstrates an agreement between simulation and experimental results is developed. Finally, the implications of the findings on strategies for mitigating LeTID are discussed.

Why is gallium-doped silicon (sometimes) stable? Kinetics of light and elevated temperature induced degradation

Light and elevated temperature induced degradation (LeTID) can induce high power losses in photovoltaic modules built from various types of silicon wafers. After the industry's rapid transition from Boron- to Gallium-doping, it is still unclear how the new dopant atom affects the degradation process and why it entails an apparently higher resistance to LeTID. We treat identically processed Gallium-doped Czochralski silicon wafers at 16 different conditions by screening temperature and minority charge carrier density (in the following “injection”). The illumination source is regulated to keep the injection constant at each condition. This method allows for an improved quantitative analysis of the LeTID kinetics based on effective lifetime measurements. Our results show that Gallium-doping shifts the equilibrium between the formation of LeTID defects and their temporary recovery (TR) to the latter, leading to a reduced degradation extent at temperatures up to 80 °C. The efficacy of this TR-induced LeTID suppression depends delicately on both temperature and injection which explains why Gallium-doped silicon appears to be LeTID-immune at high illumination intensities. By accounting for the influence of TR, we extract activation energies and injection exponents that relate to the dominant defect transitions separately, revealing a large discrepancy to effective values reported in literature. The increased accuracy of our kinetic parameters enhances the reliability of existing efficiency models. Finally, we find that a degradation as expected under field conditions can be probed at a 100-fold accelerated rate in the lab. By shining light on LeTID kinetics in Gallium-doped silicon, we explain the dopant atoms' influence on the degradation behavior, establish a basis for precise yield modelling and formulate guidelines for accelerated test protocols.

Method and Equipment for Reducing the Efficiency Degradation of Monocrystalline Passivated Emitter and Rear Cells

Infrared soldering as a step in module encapsulation, which would cause light-induced degradation (LID) and light- and elevated-temperature-induced degradation (LeTID) effects on solar cells, may cause efficiency mixing among solar cells that were originally in the same grade within the module after soldering. Furthermore, the problem of bright and dark regions would appear, which would result in a decrease in the CTM value. Current injection is considered to be one of the effective methods to solve the above problem. However, after the current injection treatment, there is still a 10% probability of the appearance of bright and dark regions in modules. In this work, we first adopted the conventional current injection process in monocrystalline passivated emitter and rear cells (PERCs). The effects of injected currents, temperature and time were systematically optimized, and cells with or without the current injection under the optimal parameters were illuminated with 1 sun at 85 °C for 25 h. Secondly, a piece of equipment was developed to further stabilize the performance of solar cells and improve the CTM value. The results showed that the best current injection parameters were a temperature of 185 °C, an injected current of 11 A and an injection time of 770 s. Compared with the cells without any pretreatment, the relative changes in the η, Voc, Isc and FF of the cells pretreated with the optimal conditions mentioned above were 0.23%, 0.08%, 0.02% and 0.08% larger, respectively, after 25 h of degradation. Then, solar cells processed by current injection were processed with our equipment, and the probability of a problem occurring was reduced from 10% to 2%. Meanwhile, the CTM value increased by 0.4%. Finally, a balance mechanism between H0 and H0-X has been proposed to explain the mechanism of the equipment.

Influence of AlOx Interlayers on LeTID Kinetics in Ga-Doped Cz-Si

Light and elevated temperature-induced degradation (LeTID) is causing a reduction in efficiency especially in p-type silicon based solar cells. It is assumed to be strongly influenced by the hydrogen content in the bulk material. The presented work focuses on the impact of differently thick (5-25 nm) atomic layer-deposited aluminum oxide (AlOx) interlayers underneath the hydrogen-rich silicon nitride (SiNy:H) capping layer. The interlayer acts as a diffusion barrier for H during the firing step. It is demonstrated that the AlOx interlayer has a comparable effect on the LeTID kinetics in Ga-doped Cz-Si (Cz-Si:Ga) as it is observed in B-doped Cz-Si (Cz-Si:B). Additionally, it substantially minimizes lifetime degradation in the Cz-Si:Ga sample. With a determined ratio of electron to hole capture cross sections k=26(3), the degradation phenomena are attributed to the LeTID kinetics. Deposition of AlOx barrier layers exceeding 10 nm in thickness does not yield additional positive effects. Resistivity measurements revealed that the change in hole concentration correlates with the defect density for varying AlOx layer thicknesses. The doping concentration seems to influence the change in maximum defect density for varying AlOx layer thicknesses.

Reducing Time and Costs of FT-IR Studies of the Effect of SiNx, Dopants, and Emitter on Hydrogen Species in Si Wafers and Solar Cell Structures

Accurately measuring the hydrogen content in silicon (Si) solar cells is essential due to its connection to surface degradation and light and elevated temperature induced degradation (LeTID). Fourier Transform-Infrared (FT-IR) spectroscopy provides a quantitative technique for determining the content of various hydrogen species in Si wafers that have undergone various process steps. In this study, we examine both the effect of a silicon nitride (SiNx:H) layer during FT-IR spectroscopic measurements on hydrogen species, as well as the impact of an emitter present during firing on the amount of hydrogen introduced into Si wafers. We find that the presence of SiNx:H during measurements has negligible effects on the measured hydrogen species, potentially simplifying the preparation steps for FT-IR. For the emitter investigation we analyze boron (B)- and gallium (Ga)-doped p-type wafers to detect H-B, H-Ga, Oi-H2, and H2. We observe that hydrogen species initially present in B- and Ga-doped Si wafers differ significantly. Only H-Ga is detected in Ga-doped wafers, while H-B, Oi-H2, and H2 signals are measured in B-doped wafers. Moreover, we cannot confirm an increased release of H through the emitter into the bulk during the firing process. Finally, we conduct measurements at different temperatures and confirm that cryogenic temperatures are more effective for detecting H-B and H2 with concentrations in the 1014 cm-3 range. Nevertheless, useful spectra can still be obtained at liquid nitrogen (N2) temperatures.

Stability of industrial gallium-doped Czochralski silicon PERC cells and wafers

The carrier lifetime stability of gallium-doped silicon wafers and performance stability of industrial PERC solar cells produced from sister wafers were investigated under four different illumination conditions and temperatures. The seven investigated materials feature a resistivity variation of 0.4–1.0 Ωcm and lifetime samples were processed to create high hydrogen content (with PECVD SiNx) or low hydrogen content (with ALD Al2O3 or HfO2). Our results confirm that the material itself is prone to light and elevated temperature induced degradation (LeTID), however experiments on PERC cells produced utilising the same silicon material indicate that the production process can successfully suppress LeTID. In contrast to earlier studies, we observe only small levels of degradation at the cell level, with some showing an improvement in cell parameters under LeTID testing conditions. Our results indicate that LeTID is not necessarily a major issue for the performance of modern passivated emitter and rear cells made from gallium-doped silicon substrates.

Impact of Fast-Firing Conditions on Light- and Elevated-Temperature-Induced Degradation (LeTID) in Ga-Doped Cz–Si

Impact of Fast-Firing Conditions on Light- and Elevated-Temperature-Induced Degradation (LeTID) in Ga-Doped Cz–Si

Impact of Hydrogen in Ga‐Doped Silicon on Maximum LeTID Defect Density

Many studies suggest that hydrogen is an important factor for light and elevated temperature‐induced degradation (LeTID) in p‐type c‐Si solar cells. The exact mechanism of this defect is still unknown. Here, Ga‐doped Si wafers fired with an SiN x :H layer present were used to establish a correlation between the initial concentration of GaH pairs and H2 dimers on one and the maximum defect density evolving during degradation on the other hand. Degradation of all samples is performed at constant excess charge carrier injection. The correlation to LeTID defect density is found to be linear in the case of [H2], hence, a direct involvement of H2 in the defect formation is expected. In contrast, the correlation between GaH pairs and defects is found to scale with the fraction of GaH on total hydrogen concentration. This fraction is not constant but rather decreases with an increase in total hydrogen concentration. In addition, changes in [GaH] and lifetime are examined under different degradation conditions with either fixed injection up to and temperatures up to 180 °C. Under these conditions, LeTID evolves but no dissociation of [GaH] takes place. The effective activation energy of LeTID defect formation is determined to be 0.76(17) eV.

LeTID mitigation via an adapted firing process in p-type PERC cells from gallium-doped Czochralski silicon

In this work, we analyse the light and elevated temperature induced degradation (LeTID) behaviour of gallium (Ga)-doped Czochralski (Cz) passivated emitter and rear cells (PERC). We show that these cells, based on industrial precursors, are prone to LeTID, degrading on average 8.3%rel in efficiency. The degradation and regeneration is slower than in boron (B)-doped PERC cells leading to a greater loss of energy over a 20 year lifespan of a solar module on a German rooftop, since, according to our experiments, regeneration does not start during that time frame. We show that the method of mitigating LeTID by slightly altering the temperature profile during fast firing, previously introduced on B-doped PERC cells, also works on Ga-doped PERC cells, reducing the maximum degradation to between 1%rel and 4%rel. This process does only slightly reduce the initial efficiency of the cell by up to 0.8%rel. Lastly, we show that increasing illumination and temperature from 0.15 suns eq. to 2 suns eq. and 75 °C to 140 °C, respectively, is an adequate way to accelerate LeTID testing on Ga-doped samples by more than two orders of magnitude while achieving qualitatively and quantitatively similar results to those obtained in a standard test.

Understanding the impact of the cooling ramp of the fast-firing process on light- and elevated-temperature-induced degradation

In current silicon solar cell technologies, hydrogen is incorporated into the solar cell during the fast-firing process. It passivates defects at the surface and in the bulk, but also leads to light- and elevated-temperature-induced degradation (LeTID). Although it is known that the hydrogen content and the LeTID extent can be reduced by employing a slower cooling ramp during the fast-firing process, the exact mechanism behind this phenomenon remains unclear. This study aims at closing this gap by investigating the impact of cooling ramps with different temperature plateaus on hydrogen (complexes) in B-doped FZ-Si wafers. The fired wafers are analyzed with FT-IR spectroscopy, four-point-probe resistivity measurements, and LeTID tests via effective lifetime measurements. Our findings provide evidence that hydrogen not only diffuses into the silicon bulk but can also effuse out of it during the cooling ramp. A one-dimensional hydrogen model is built in Sentaurus TCAD to simulate the in- and out-diffusion of hydrogen and to compare it with the experimental results. The experimentally determined hydrogen concentrations align with our simulation, with the diffusion being dominated by the fast-diffusing neutral hydrogen H0. Moreover, we find stronger out-diffusion at higher temperatures, resulting in a lower total hydrogen concentration for slower cooling ramps. The extent of LeTID and surface-related degradation (SRD) are found to scale with the final total hydrogen concentration in the bulk. Therefore, modifying the cooling ramp can be an effective tool to optimize the hydrogen content and minimize the impact of degradation phenomena on silicon solar cells.

Interactions of hydrogen atoms with boron and gallium in silicon crystals co-doped with phosphorus and acceptors

Reports showing that hydrogen and group-III acceptors play an important role in Light- and elevated Temperature-induced Degradation (LeTID) of Si-based solar cells highlight the need for a better understanding of interactions between these two species. In this contribution, a combination of junction spectroscopy techniques and first principles modelling has been used to study hydrogen-induced changes in electrical properties of either boron or gallium Czochralski-grown silicon co-doped with phosphorus in order to produce n-type material facilitating novel techniques to assess recombination active defects. The interactions of hydrogen with acceptor atoms have been induced via annealing of these co-doped hydrogenated samples with the application of reverse bias (RBA). These treatments have resulted in a significant increase in the net shallow donor concentration in depletion regions of both materials and in the appearance of a strong electron emission signal due to a trap with an energy level at about Ec −0.18 eV in the DLTS spectra of Si:P + B material. It is argued that this trap is related to the donor level of a BH2 complex. Calculations using density functional theory have shown that the BH2 defect has a charge-state dependent geometry, which turns out to be crucial for the proposed non-radiative recombination mechanism. The BH2 defect is therefore suggested to be the root cause of LeTID in boron-doped Si. In contrast, modelling results predict that GaH2 is a defect with shallow energy levels, without the characteristic features of a recombination centre. This is corroborated by the results of electrical measurements on hydrogenated Si:P + Ga subjected to RBA. Conventional annealing treatments were subsequently used to assess the thermal stability of acceptor-H related defects. Based on the obtained results, the peculiarities of hydrogen interactions with boron and gallium acceptors are discussed.

Assessing the stability of p+ and n+ polysilicon passivating contacts with various capping layers on p-type wafers

Polysilicon (poly-Si)-on-oxide passivating contact structures (POLO/TOPCon) enable high-efficiency solar cells as they simultaneously provide a very high level of surface passivation and a high conductance for either electrons or holes. The ease of incorporation with existing manufacturing lines and their tolerance for high-temperature processing has increased the wide acceptance of this structure in the PV industry. In this report, we explore the effects of short high-temperature annealing required for effective hydrogenation and formation of ohmic screen-printed contacts across a wide temperature range (636 °C–846 °C) on the stability of passivating contact structures. We study this on p-type c-Si substrates with phosphorus-doped (n-type) or boron-doped (p-type) polysilicon contacts capped with either an AlOx or SiNx coating. Our experimental results show that irrespective of the poly-Si doping type, AlOx-capped samples suffer a loss in surface passivation across the investigated temperature range, while SiNx-capped samples show an improvement at lower annealing temperatures. Above 744 °C, severely ruptured blisters occur for the samples coated with a SiNx layer, leading to lift-off of the poly layer in extreme cases, and in all cases, significant surface passivation losses, up to 99%. A study of the long-term stability of these fired samples under 1-sun illumination @ 140 °C shows that they suffer from both bulk and surface-like instabilities. Two degradation cycles were observed: the first, a boron-oxygen light-induced degradation (BO-LID) observed after 5 min, with capture cross-section ratios of 15.8–19.2, and a slower secondary degradation, similar to light and elevated temperature-induced degradation (LeTID), with maximum degradation reached after ∼ 14 days. The presence of a silicon nitride layer does not appear to influence the kinetics of post-degradation recovery. Our results suggest that the effect of firing may be influenced by the polarity of the bulk c-Si or perhaps the chemistry of the SiNx film and highlight that passivating contact structures based on p-type c-Si may offer better long-term stability than those based on n-type c-Si.

The Mechanism of Hot Spots Caused by Avalanche Breakdown in Gallium-Doped PERC Solar Cells

Gallium-doped p-type passivated emitter and rear contact (PERC) solar cells, which eliminate light-induced degradation (LID) and reduce the impact of light- and elevated-temperature-induced degradation (LeTID), have completely replaced boron-doped p-type PERC cells. However, in previous experiments, we found hot spots in the center of gallium-doped PERC solar cells. In this study, it was found that gallium-doped PERC cells had uneven resistivity, which caused hot spots brought about by the avalanche breakdown of PN junctions. There were significant hot spots in the center of the tested cells, with an average resistivity of 0.4–0.5 Ωcm and nonuniformity greater than 30%, or at an average resistivity of 0.5–0.6 Ωcm with nonuniformity greater than 40%. In this paper we describe and study in detail hot spots triggered by the uneven resistivity of gallium-doped cells and analyze the causes and related influencing factors, thereby providing guidance and a reference for the improvement of the performance and reliability of gallium-doped PERC solar cells.

The Impact of Different Hydrogen Configurations on Light- and Elevated-Temperature- Induced Degradation

In this article, the impact of different hydrogen configurations and their evolution on the extent and kinetics of light- and elevated-temperature-induced degradation (LeTID) is investigated in float-zone silicon via charge carrier lifetime measurements, low-temperature Fourier-transform infrared spectroscopy, and four-point-probe resistance measurements. Degradation conditions were light soaking at 77 °C and 1 sun-equivalent illumination intensity and dark anneal at 175 °C. The initial configuration of hydrogen is manipulated by varying the wafer thickness, the cooling ramp of the fast-firing process, and the dopant type (B- or P-doped). We find lower hydrogen concentrations in thinner samples and samples with a slower cooling ramp. This suggests that hydrogen diffuses out of the sample during the cool-down, which strongly affects the final concentration of hydrogen molecules H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , and to a smaller degree the concentration of boron-hydrogen (BH) pairs. A regeneration of potential LeTID defects and a presumed LeTID degradation during dark annealing is found in n-type Si. In p-type Si, the LeTID extent was found to scale with H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , suggesting a direct link between both. The temporal evolution of BH pairs, LeTID degradation/regeneration, and surface degradation depends on wafer thickness and the cooling ramp of the fast-firing process. Based upon these findings, we formulate a theory of the hydrogen-related mechanism behind LeTID: Hydrogen originating from H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> moves between different temporary traps. First, hydrogen binds to LeTID precursors and acceptor atoms in the silicon bulk, later moving toward the surface. This leads first to the LeTID degradation and regeneration and then to the degradation of surface passivation.

Analysis of variation in recombination characteristics due to light and heat in industrial silicon solar cells

This work presents a comprehensive analysis of the variation in performance parameters of monocrystalline and multicrystalline silicon solar cells when subjected to illumination at elevated temperature. Higher degradation and better regeneration in performance parameters namely open circuit voltage (VOC), short circuit current density (JSC) and fill factor (FF) are observed for monocrystalline solar cells. Suns-VOC measurement clearly indicate that the recombination current densities within the space charge region (J02) and rest of the solar cells (J01) together play an important role in light and elevated temperature induced degradation (LeTID) and subsequent regeneration characteristics. During the degradation phase, both J01 and J02 changed significantly for monocrystalline and multicrystalline solar cells. During regeneration, even though J01 improved for both the types of solar cells, notable reduction in J02 was observed only for monocrystalline solar cells. FF loss analysis shows that the recombination in the space charge region is majorly responsible for degradation and regeneration in FF. In contrast to most of the previous studies, we report similar reduction in external quantum efficiency (EQE) at 984 nm, 877 nm and 658 nm and propose that the formation of defects related to LeTID are not only in the deep bulk, but are distributed throughout the bulk. Absence of any improvement in EQE after prolonged exposure to light and heat suggests that defect transformation and gettering are not sufficient enough to regenerate JSC in multicrystalline solar cells. EQE maps at 407 nm suggest that LeTID behavior of emitter is different from that of bulk.

Atomic structure of defect responsible for light-induced efficiency loss in silicon solar cells in warmer climates

Light-induced degradation of Si solar cells when deployed in warmer climates can cause up to a ∼10% relative degradation in efficiency, but the atomic structure of the defect responsible for this degradation remains elusive. Herein, using electron paramagnetic resonance, we show that the defect responsible for light- and elevated-temperature-induced degradation (LeTID) is likely an Si dangling bond within an extended defect such as a vacancy agglomerate, with H atoms in its vicinity and likely O nearby. Our atomistic-level insights suggest that the defect responsible for LeTID can be mitigated by targeted engineering of the intrinsic defect populations by optimizing annealing routines prior to or during device fabrication and by controlling the amount of H injected in the Si bulk during cell processing. Mitigating LeTID through detailed knowledge of its atomic structure can help preserve the long-term efficiency of gigawatts of future worldwide installations of solar based on crystalline Si.

Influence and mitigation of interference by LID and LETID in damp heat and thermal cycling tests on PV modules

Accelerated aging tests as defined in testing standards such as IEC 61215 are important to assure the quality and safety of photovoltaic (PV) modules. The test conditions often contain high temperatures and sometimes carrier injection, which can cause light induced degradation (LID) effects, such as boron-oxygen LID (BO LID) or light and elevated temperature induced degradation (LETID). These effects can interfere with the interpretation of results or produce false fails or passes in certification tests. To address the most severe cases, an option for a regeneration procedure for BO LID after damp heat was recently included in IEC 61215:2021. However, positive performance deviations due to BO LID, as well as the general influence of LETID, are still not excluded. Variations of damp heat and thermal cycling tests on mini-modules built from the monocrystalline passivated emitter and rear cells (PERC) are performed and combined with latest approaches for BO LID regeneration, BO degradation, and LETID temporary recovery. The results show that LETID can superimpose procedures applied for BO LID regeneration but can be easily temporary recovered by one additional step. A combined stabilization procedure, which can exclude influences from both BO LID and LETID on accelerated aging test results, is proposed.

Electronic Properties of Light- and Elevated Temperature-Induced Degradation in Float-Zone Silicon

Light- and elevated temperature-induced degradation (LeTID) causes long-term instabilities, especially in passivated emitter and rear cells, leading to severe performance loss of commercial modules. Despite the hundreds of LeTID reports available in the literature, there is little consensus regarding the underlying defects and defect formation mechanism responsible for this degradation and its exact electronic properties. Recently, it has been shown that a form of carrier-induced degradation similar to that of LeTID is also observed in some high-purity silicon crystals grown by the float-zone method. In this work, using deep level transient spectroscopy and lifetime spectroscopy, we study the role of nitrogen on the degradation. Intentional contaminated samples with different and known level of nitrogen have been specially grown and characterized for this study. We detect the appearance of a set of majority carrier traps in the degraded state of the samples with activation energies 0.1 (H85), 0.43 (H270A), 0.39 (H270B), and 0.46 eV (H200) with respect to the valence band, from which H270A appears to correlate with the degradation. The results show that the extent of degradation in the nitrogen-rich samples is at least double that of the nitrogen-lean samples.