Search

The effect of light- and elevated temperature-induced degradation (LeTID) can be nonpermanently reversed by charge carrier injection below the degradation temperature (commonly used degradation temperatures are above ~70 °C). In this study, we show that the rate of temporary recovery depends strongly on the excess carrier density. We observe that the order of the reaction changes from pseudo-zero to first with increasing injection. The rate decreases slightly with increasing temperature. Since the samples can go through multiple degradation/recovery cycles without distinct changes in the degradation kinetics, the experimentally accessible recovered and degraded states are interpreted as manifestations of the equilibrium concentrations of the defect responsible for LeTID at different temperatures. Based on our observations, we argue that the process underlying LeTID degradation is the dissociation of a precursor rather than an association of two or more components. In light of the relation between LeTID susceptibility and bulk hydrogen concentration, we hypothesize that the LeTID precursor dissociates into the LeTID defect and monatomic hydrogen. Numerical simulations of the coupled rate equations including hydrogen interactions well reproduce the experimental observations; according to these results, the presence of a sink for the atomic hydrogen such as dopant atoms is paramount for the LeTID degradation.

Gallium-doped silicon material has been rapidly gaining importance in the photovoltaic industry as a boron-oxygen defect-free material with promising minority carrier lifetime. We investigate the influence of different cell process flows [passivated emitter and rear cell tunneling-oxide-passivating contact, and a 'hot oxidation' process] on the bulk material quality of an industrial Ga-doped Cz-grown silicon material, as well as its light-and elevated temperature induced degradation degradation behavior under light at elevated temperature. We measure a generally high carrier lifetime level, which remains limited by an unknown recombination-Active defect after most processes. Hydrogenation seems to passivate this unknown defect. In addition, we demonstrate that such high-quality p-Type material can suffer noticeably by iron even for extremely low concentrations below 109 at/cm3.

Characterisation and optimization of next-generation silicon solar cell concepts rely on an accurate knowledge of intrinsic charge carrier recombination in crystalline silicon. Reports of measured lifetimes exceeding the previous accepted parameterisation of intrinsic recombination indicate an overestimation of this recombination in certain injection regimes and hence the need for revision. In this work, twelve high-quality silicon sample sets covering a wide doping range are fabricated using state-of-the-art processing routes in order to permit an accurate assessment of intrinsic recombination based on wafer thickness variation. Special care is taken to mitigate extrinsic recombination due to bulk contamination or at the wafer surfaces. The combination of the high-quality samples with refined sample characterisation and lifetime measurements enables a much higher level of accuracy to be achieved compared to previous studies. We observe that reabsorption of luminescence photons inside the sample must be accounted for to achieve a precise description of radiative recombination. With this effect taken into account, we extract the lifetime limitation due to Auger recombination. We find that the extracted Auger recombination rate can accurately be parameterized using a physically motivated equation based on Coulomb-enhanced Auger recombination for all doping and injection conditions relevant for silicon-based photovoltaics. The improved accuracy of data description obtained with the model suggests that our new parameterisation is more consistent with the actual recombination process than previous models. Due to notable changes in Auger recombination predicted for moderate injection, we further revise the fundamental limiting power conversion efficiency for a single-junction crystalline silicon solar cell to 29.4%, which is within 0.1%abs compared to other recent assessments.

It is well-known that the radiative recombination coefficient B rad of silicon decreases with increasing charge carrier densities, usually modelled via the scaling factor B rel parameterized by Altermatt et al. Additionally, photon recycling (PR) is effectively lowering the radiative recombination rate. This work aims to comprehensively reassess those influences on radiative recombination. Firstly, it is clarified that Altermatt’s parameterization of B rel is largely dominated by the effect of band-gap narrowing (BGN) on the intrinsic charge carrier density, and that a change of the band-to-band absorption coefficient is not observable. Next, a photon-reabsorption model is suggested, which accounts for free-carrier absorption (FCA) and can predict PR and luminescence intensity. The model is shown to be useful in improving the interpretation of very high lifetime measurements, in particular towards an improved quantification of Auger recombination. Furthermore, it is found that FCA may have a significant influence on photon reabsorption beyond charge carrier densities of 1016 cm-3 for relatively thick samples, in particular affecting luminescence. This means that the assumption of direct proportionality between luminescence intensity and radiative recombination, which is fundamental to most luminescence-based measurement techniques, may fail for such conditions, but can be corrected using this work’s model.

Commercial silicon is prone to form silicon oxide precipitates during high-temperature treatments typical for solar cell production. Oxide precipitates can cause severe efficiency degradation in solar cells. We have developed a model describing the nucleation and growth of oxide precipitates that considers silicon self-interstitial defects and surface effects influencing the precipitate growth in ∼150 μm thick wafers during the solar cell processing. This kinetic model is calibrated with experiments that cause a well-defined and strong precipitate growth to give a prediction of the carrier lifetime limitation because of the oxide precipitates. We test the oxide precipitate model with scanning Fourier-transform infrared spectroscopy, selective etching, and lifetime measurements on typical Cz solar cell wafers before and after solar cell processes. Despite the relatively rough saw damaged etched surfaces and the thin wafers, we observe recurring ring patterns in the measurements of interstitial oxygen reductions, oxide precipitate etch pit density, and recombination activity by photoluminescence imaging. The concentration of precipitated oxygen correlates with the recombination activity and with the initial interstitial oxygen concentration. However, we found lifetime measurements to be a more sensitive technique to study oxide precipitates and using these we find smaller precipitates not detected by selective etching are very recombination active too. The measured concentrations of precipitated oxygen and lifetime agree fairly well with the predictions of the model.\ud

In order to push silicon solar cell efficiencies further towards their limit, as well as to ensure accuracy of luminescence based characterization techniques, an accurate modeling of radiative recombination is important. It is well-known that the radiative recombination coefficient Brad of silicon shows a substantial charge carrier density dependence (c-dependence), often modelled via the scaling factor Brel quantified by Altermatt et al. Another effect lowering the total radiative recombination is photon recycling (PR), which depends mainly on the width and optical properties of the sample. PR also depends on free-carrier absorption (FCA), which introduces a further c-dependence. This work comprehensively reassesses and quantifies those influences on radiative recombination in silicon for photovoltaic (PV) applications. Firstly, it is evidenced that Altermatt's Brel model is dominated by the effect of band-gap narrowing (BGN). This clarifies that it should not be combined with a different BGN model to avoid modeling inconsistencies. It is thereby further confirmed that the band-to-band absorption coefficient of silicon does not show a relevant c-dependence for PV conditions. Next, a PR model is suggested which calculates a scaling factor Brel,PR. The model is proven useful in improving the interpretation of very high lifetime measurements. Finally, it is found that the c-dependence introduced by FCA affects the direct proportionality between luminescence signal and radiative recombination beyond charge carrier densities of 1016 cm−3 for samples thicker than typical wafer widths, and should then be considered in characterization techniques like e.g. calibrated lifetime measurements via photoluminescence.

Hydrogenation of poly-Si based passivating contacts (TOPCon) is an essential process to achieve a very high level of surface passivation, especially on textured surface. This contribution is dedicated to improve the understanding of the hydrogenation mechanism. We compare different hydrogen sources with regard to their ability to chemically passivate defects at the Si/SiOx interface and presumably in the doped poly-Si layer as well as their thermal stability in a low and high temperature range. To this end, hotplate annealing series were performed on textured n-type TOPCon structures and Al2O3/SiNx multi-layer stacks were exposed to fast-firing processes. Very distinct activation characteristics were detected. It was also observed that the Al2O3 capping layers enable a higher level of surface passivation and higher thermal stability compared to SiNx. When implemented in multi-layer stacks, Al2O3 acts as a hydrogen diffusion barrier und prevents effusion from the TOPCon structure that deteriorates the passivation quality irreversibly.

The degradation kinetics of multicrystalline silicon solar cells and wafers at elevated temperature (often termed “LeTID”) depend on the specific temperature and injection conditions. We apply different forward biases in the dark at a constant temperature of ~75 °C to industrial passivated emitter rear contacted (PERC) solar cells fabricated on p-type multicrystalline wafers from a variety of material producers and determine the degradation rate constant in dependence of the excess carrier density at the p-n junction. We find that whereas the specific material properties influence the degradation extent, the degradation rate constant is comparable for all materials but depends on the excess carrier concentration. This implies involvement of one electron in the rate-limiting step of LeTID defect formation. The result not only is an important contribution to elucidate the physical mechanism underlying LeTID, but can also be used as a guideline for devising degradation tests of multicrystalline silicon wafers and solar cells.

Minimizing carrier recombination at interfaces is of extreme importance in the development of high-efficiency photovoltaic devices and for bulk material characterization. Here, we investigate a temporary room temperature superacid-based passivation scheme, which provides surface recombination velocities below 1 cm/s, thus placing our passivation scheme amongst state-of-the-art dielectric films. Application of the technique to high-quality float-zone silicon allows the currently accepted intrinsic carrier lifetime limit to be reached and calls its current parameterization into doubt for 1 Ω·cm n-type wafers. The passivation also enables lifetimes up to 65 ms to be measured in high-resistivity Czochralski silicon, which, to our knowledge, is the highest ever measured in Czochralski-grown material. The passivation strategies developed in this work will help diagnose bulk lifetime degradation under solar cell processing conditions and also help quantify the electronic quality of new passivation schemes.

For solar cell application, the stability of interface passivation quality to in-field conditions is crucial. We have performed an experiment to test the resilience of different aluminium oxide based passivation schemes to illumination at 75 °C. Different thermal treatments to activate the passivation and/or simulate contact firing were performed before light soaking. The experiment was performed on 1 Ωcm float-zone silicon of both p- and n-type doping. The study demonstrates that good passivation quality can be achieved both by atomic layer deposition and by PECVD and that addition of silicon nitride capping layers greatly enhances thermal stability. On p-type wafers a severe but temporary degradation of the electrical quality of the wafer bulk was observed during the first hours upon application of such capping layers. Besides this effect, reasonable temporal stability of the effective lifetime was observed for p-type samples while n-type samples featured excellent long-term stability.

This paper gives an overview on the current understanding of a technologically relevant defect group in crystalline silicon related to the presence of boron and oxygen. It is commonly addressed as boron–oxygen defects and has been found to affect silicon devices, whose performance depends on minority charge carrier diffusion lengths—such as solar cells. The defects are a common limitation in Czochralski-grown p-type silicon, and their recombination activity develops under charge carrier injection and is, thus, commonly referred to as light-induced degradation. A multitude of studies investigating the effect have been published and introduced various trends and interpretations. This review intends to summarize established trends and provide a consistent nomenclature for the defect transitions in order to simplify discussion.

To maximize the annual energy output of silicon based photovoltaic modules beyond standard testing conditions (STC), it is important to assess temperature dependent device properties of silicon solar cells. In this study we characterize the temperature dependence of the dark saturation current density J 0 of diffused regions (e.g. emitter layers). In particular, we test whether the theoretical first-order approximation of J 0 /n i 2 being temperature independent holds. For this, the lifetime of symmetrically diffused silicon samples with varying dopant types and profiles is measured by temperature dependent modulated photoluminescence to extract J 0 as a function of injection and temperature. We find that for passivated diffused regions J 0 /n i 2 shows a temperature coefficient in the range of -0.5 to -0.7 %/K, while even lower coefficients and more scattering is observed for the unpassivated samples. The presented data and approach can be used to improve the beyond-STC predictive power of device simulations which use J 0 inputs.

Czochralski‐grown gallium‐doped silicon wafers are now a mainstream substrate for commercial passivated emitter and rear cell (PERC) devices and allow retention of established processes while offering enhanced cell stability. We have assessed the carrier lifetime potential of such Czochralski‐grown wafers in dependence of resistivity, finding effective lifetimes well into the millisecond region without any gettering or hydrogenation processing, thus demonstrating one advantage over boron‐doped silicon. Second, the stability of gallium‐doped PERC cells are monitored under illumination (>3000 h in some cases) and anomalous behavior is detected. While some cells are stable, others exhibit a degradation then recovery, reminiscent of light and elevated temperature‐induced degradation (LeTID) observed in other silicon materials. Surprisingly, cells from one ingot exhibit LeTID‐like behavior when annealed at 300 °C but near stability when not annealed, but, for another ingot, the opposite is observed. Moreover, a stabilization process typically used to mitigate boron–oxygen degradation does not influence any cells that are studied. Secondary‐ion mass spectrometry of the PERC cells reveals significant concentrations of unintentionally incorporated boron in some cases. Nevertheless, even in the absence of mitigating light‐induced degradation, Ga‐doped silicon is still more stable than unstabilized B‐doped silicon under illumination.

The magnitude of light-induced degradation of solar cells based on Czochralski grown silicon strongly depends on material properties. We have performed experiments to describe the activation and recombination activity of boron oxygen defects in boron compensated n -type silicon. Compensated n -type material enables flexible assessment of charge carrier influences on the defect that cannot be distinguished on p -type material. The results can be generalized to p -type material and thus provide valuable insights to the defect. Our measurements demonstrate the two-level defect nature of the slow-formed boron oxygen defect component and allow the study of the dopant dependency of the defect concentrations. Our findings strongly support a revision of the existing model of the defect composition. Based on the experimental results and literature data we have created a parameterization of the lifetime limitation in silicon due to BO defects. Established findings from literature for uncompensated p -type silicon are taken into account and ensure general validity. The parameterization is useful to discuss BO defect influences and can serve to predict material properties after LID.

Silicon is the most studied semiconductor, having almost every aspect of it being investigated. All this information is spread over a large set of publications, review articles and textbooks and cannot be found in a single location. Furthermore, the available data is not always consistent and depends on the techniques and samples used. This problem even exists for more specialised areas such as the study of defects in silicon photovoltaics, which is the focus of this paper. Currently, if a signature of a defect is experimentally determined a literature search must then be performed through texts going back decades in the hope to find a defect with similar properties. This paper addresses this time consuming activity by introducing an open source text based repository, which anyone can access or contribute to, and that provides clearly arranged information about defects in silicon.

We present new experimental data on light-induced degradation due to the boron oxygen defect in compensatedn-type silicon. We are the first to show that both defect components known fromp-type silicon are formed in compensatedn-type silicon. A parameterization of the injection dependent recombination activity of the slower formed defect component is established. The formation kinetics of both defect components are studied and modeled under different conditions. It is found that the same rate factors as inp-type can describe the degradation, if the actual hole concentration under illumination is taken into account. The regeneration process known to permanently deactivate boron oxygen defects inp-type is successfully applied ton-type material and the illumination stability of the regenerated state is tested and proven.

The stability of passivation layers under the conditions of field application of solar modules is a crucial parameter. We have performed an experiment to test the stability of aluminum oxide based passivation schemes under halogen lamp illumination of 1 sun equivalent intensity at 75 °C. We compare aluminum oxide layers with atomic layer deposition and plasma-enhanced chemical vapor deposition with and without an amorphous silicon nitride capping layer. Different thermal treatments are performed to activate the passivation or simulate solar cell contact firing. All passivation schemes are tested on full four inch 1 Ω·cm p- and n-type float-zone silicon wafers. The quality and stability of the surface passivation are tested by means of repeated charge carrier lifetime measurements. No significant surface passivation degradation due to light-soaking is evident on capped samples for several hundred hours. N-type samples are demonstrated to maintain excellent passivation quality for at least 3700 h and only slight degradation of p-type samples is found.

We present a fast method to create interstitial oxygen concentration maps from resistivity calibrated photoluminescence images prior to and after a heat treatment at 450 °C. The method utilizes the influence of thermal donors on the effective doping concentration of a sample. Although the determination of thermal donor concentrations from conductivity measurements is customary in literature, we found that implementation of a mobility model is necessary to determine accurate concentrations of thermal donors. Therefore an iterative correction algorithm is presented, which allows precise determination of thermal donor concentrations from resistivity measurements. The determination of interstitial oxygen concentrations from thermal donor concentrations is based on an updated parameterization based on a model of Wada et al. (Phys. Rev. B: Condens. Matter 30 (1984) 5884–5895) that is also presented in this paper. The method is demonstrated on a 1.3 Ω cm p-type Czochralski grown silicon sample with an interstitial oxygen concentration in the range of 7.5×10 17 cm −3 and yields good agreement with FTIR measurements.

We investigated the light-induced degradation of compensated Czochralski grown n-type silicon and found a fast-forming and a slow-forming component similar to p-type silicon. A study by means of extended lifetime spectroscopy shows that the ”slow” defect introduces two recombination-active energy levels in the silicon band gap. One level resembles the literature data from p-type silicon of a donor-like level at Et1 = ECB – (0.41 ± 0.02 eV). The second level is found at Et2 = EVB + (0.26 ± 0.02 eV) and exhibits a strong acceptor-like capture asymmetry. The two-level parameterization constitutes a unified model for the description of the injection dependent lifetime on both p- and n-type silicon and is physically more plausible than previous ones featuring multiple independent centers. A comparison to literature data demonstrates the improved description quality achieved with the new parameterization. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

Photoluminescence-based impurity imaging methods have been shown to be able to quantify impurities with excellent detection limits of approximately 1010 cm−3. They are, however, limited to metastable defects in p-type silicon only. In this paper, we present an approach that overcomes this limitation by evaluating temperature- and injection-dependent photoluminescence imaging. In contrast with temperature- and injection-dependent lifetime spectroscopy, we are not aiming for determining precise impurity parameters of known contaminants, but rather for identifying lifetime-limiting metal impurities using established impurity parameters from the literature. Our approach is to measure spatially resolved injection-dependent lifetimes by photoluminescence imaging and fit them with respect to established defect parameters. Additional measurements at higher temperature enhance the information content of the analysis. The two-defect approach first identifies the two lifetime-limiting defects and derives a candidate for a third defect. Subsequently, we can determine their concentrations. The presented method is not limited to doping type nor metastable defects and is, therefore, a promising method to characterize the spatially resolved distribution of a large variety of impurities.

Up to now the existence of trap levels − defect levels in the forbidden band gap which temporary trap minority charge carriers − in Cz silicon was controversially discussed. We directly monitor the transient dynamics of the free electron density in the conduction band by the means of a time correlated single photon counting of photoluminescence. A variation of the experimental conditions reveals both a decrease of the electron density on a timescale of microseconds, which is not governed by recombination and an apparent generation of electrons on a scale of up to multiple seconds. We discuss that the transient dynamics may be excellently described by trap levels, providing strong evidence for their existence.

Identification of the lifetime limiting defects in silicon plays a key role in systematically optimizing the efficiency potential of material for solar cells. We present a technique based on temperature and injection dependent photoluminescence imaging to determine the energy levels and capture cross section ratios of Shockley-Read-Hall defects. This allows us to identify homogeneously and inhomogeneously distributed defects limiting the charge carrier lifetime in any silicon wafer. The technique is demonstrated on an n-type wafer grown with the non-contact crucible (NOC) method and an industrial Czochralski (Cz) wafer prone to defect formation during high temperature processing. We find that the energy levels for the circular distributed defects in the Cz wafer are in good agreement with literature data for homogeneously grown oxide precipitates. In contrast, the circular distributed defects found in NOC Si have significantly deeper trap levels, despite their similar appearance.

Light-induced degradation due to BO defects in silicon consists of a fast initial decay within a few seconds followed by a slower decay within hours to days. Determination of injection dependent charge carrier lifetime curves during the initial decay is challenging due to this short timeframe. We have developed a suitable measurement technique based on in situ photoluminescence measurements and present results of our studies of the fast degradation component. The temporal evolution of the recombination activity is studied and assessed by means of a two-level Shockley-Read-Hall statistics. A quadratic dependence of the fast defect activation on the hole concentration during illumination is demonstrated. We suggest a new parameterization of the recombination activity introduced by fast-formed BO defects featuring energy levels 0.34 eV below the conduction band and 0.31 eV above the valence band. The capture asymmetry ratio determined for the donor level of 18.1 is significantly smaller than previous paramet...

Broad range injection-dependent carrier lifetime measurements in crystalline silicon are most relevant for both defect level spectroscopy, and for the investigation of recombination properties of novel solar cell technologies. The approach presented in this paper combines a determination of the effective carrier lifetime via time-dependent (quasi-steady-state) photoluminescence with a steady-state photoluminescence lifetime scan over a broad range of excess carrier densities. Thereby, the power of a virtually artifact-free time-domain approach is combined with the high sensitivity of steady-state photoluminescence at extremely low injection conditions. Time-dependent fluctuations of dark photocurrent measurements are identified as the essential source of uncertainty under such low injection. A measurement design which eliminates this source of uncertainty is presented and tested. Lifetime measurements at excess carrier densities as low as 10 8 cm −3 are shown in detail.

The passivation mechanisms and qualities of Al2O3, SiNx, SiO2 and a-Si:H( i ) on p - and n -type silicon are investigated by quasi-steady-state photoluminescence measurements. This technique allows effective lifetime measurements in an extremely large injection range between 1010 cm-3 and 1017 cm-3. The measurements are discussed focusing on injections below 1012 cm-3 in order to determine the most effective passivation layer for solar cells arranged for indoor applications. Fixed negative charges in the passivation layer cause field-effect passivation due to band bending leading to either accumulation or inversion at the passivation layer/silicon interface. Accumulation causes a stable passivation quality at low level injection. Inversion leads to effective lifetime losses similar to the losses in the space charge region. On p -type silicon the most effective surface passivation at low injections is provided by Al2O3 or a-Si:H( i ). The n -type silicon samples passivated with a-Si:H( i ) show the best effective lifetimes. SiNx and SiO2 show lifetimes one order of magnitude below a-Si:H( i ). Al2O3 on n -type is the most effective passivation at high injections around 1015 cm-3. Due to inversion losses at low level injections the passivation quality decreases more than two orders of magnitude for injections around 1010 cm-3.

A study of the activation of the light-induced degradation in compensated n-type Czochralski grown silicon is presented. A kinetic model is established that verifies the existence of both the fast and the slow components known from p-type and proves the quadratic dependence of the defect generation rates of both defects on the hole concentration. The model allows for the description of lifetime degradation kinetics in compensated n-type silicon under various intensities and is in accordance with the findings for p-type silicon. We found that the final concentrations of the slow defect component in compensated n-type silicon only depend on the interstitial oxygen concentration and on neither the boron concentration nor the equilibrium electron concentration n0. The final concentrations of the fast defect component slightly increase with increasing boron concentration. The results on n-type silicon give new insight to the origin of the BO defect and question the existing models for the defect composition.