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This article describes the successful integration of a passivated, highly reflecting Ge rear side into a III-V multijunction solar cell. The use of lowly doped Ge and the new rear side leads to the aimed increase in Ge cell current up to 1.6 mA.cm (exp -2), demonstrated by external quantum efficiency and I-V measurements. For the contact formation, two different types of laser processes were conducted and evaluated-the laser fired contact route and the PassDop route. In both cases, the formation of a local back surface field preserves the passivation of the contact points. A laser pitch and laser power variation leads to a good performing back contact. The passivation effect is proven experimentally and is qualitatively accessed with cell simulations.

Passivating contacts created via a thin interfacial oxide and a highly doped polysilicon layer, e.g., the TOPCon technology, are on the verge of being implemented in solar cell mass production. Investment decisions rely on R&D to identify the most promising implementation option, meaning a trustworthy roadmap based on predicted performance gains. This article shows how to thoroughly quantify the performance potential via numerical simulation, focusing on an evolutionary upgrade of a busbarless p-type bifacial passivated emitter rear cell (PERC) technology. We specifically highlight the need to consider not only the electrical gains of passivating contacts, but also the associated optical losses due to parasitic absorption in the polysilicon layers for front and rear illumination. The influence of free-carrier absorption in polysilicon on the solar cell optics is characterized on experimental test structures in order to verify our optical simulation model. Introducing TOPCon fully at the rear and also locally aligned to the front fingers can boost the PERC efficiency by approximately 1% abs . The final device is strongly limited by losses in the p-type c-Si bulk and phosphorus-doped front emitter. Consequently, the presented evolutionary TOPCon upgrades may well be of increased relevance for future improved p-PERC cells, as an alternative to the current focus on n-type TOPCon cells with boron emitter.

Minimizing transport losses in novel solar cell concepts is often linked to improvements at the transparent conductive oxide (TCO)/doped silicon contact. A detailed understanding of the determining factors for an efficient transport at this heterojunction is essential, such as work function matching and efficient tunneling transport. In this article, we analyze the different TCO contact parameters experimentally and by numerical device simulations. We show that work function matching by using a proper interlayer [e.g., tungsten oxide (WO x )] can be an effective means to improve the fill factor of silicon heterojunction solar cells. However, we showcase that an improved work function matching achieved by changing the doping of a TCO interlayer can be superimposed by a less efficient tunneling transport, for e.g., due to an interfacial oxide. Furthermore, we show that for n-tunnel oxide passivating contacts, an unintentionally grown oxide at the TCO/poly-Si contact could be a possible explanation for recently observed transport losses.

In this work an inverted metamorphic four junction (IMM4J) solar cell with 30.9% conversion efficiency in beginning of life conditions under the AM0 (1367 W/m2) spectrum is presented. Additionally, our newest improved IMM3J cell, consisting of Ga0.51In0.49P/GaAs/Ga0.73In0.27As subcells, with 30.6% efficiency is also shown. The IMM4J solar cells consist of Al0.05Ga0.46In0.49P/Al0.14Ga0.86As/Ga0.89In0.11As/Ga0.73In0.27As subcells and are epitaxially grown by metal organic vapor phase epitaxy (MOVPE) on a GaAs substrate. These IMM solar cells achieve power-to-mass ratios of 3 W/g or more, which is more than three times higher than standard germanium based triple or four junction space solar cells. The losses in comparison to the simulated near-term potential efficiency of 33.8% for the IMM4J are analyzed in detail. Furthermore, the irradiation behavior for 1 MeV electron fluences of 1 × 1014 e−/cm2 and 2.5 × 1014 e−/cm2 for the IMM4J cells was investigated. A roadmap to further develop this concept towards an IMM5J with a realistic begin of life (BOL) efficiency potential of 35.9% under AM0 is presented.

A thin, lightweight, flexible solar cell is developed that maximizes the power-to-mass ratio under AM0 illumination and has a competitive efficiency after typical high energy electron irradiation. The inverted metamorphic triple junction (IMM3J) solar cells with Ga0.51In0.49P/GaAs/Ga0.73In0.27As subcells are grown on GaAs substrates and have a total epitaxy thickness of about 10 μm. After epitaxial growth, the inverted layer stack is metallized, with the metal serving as back-contact, back reflector and support layer for the ultra-thin solar cells before the GaAs substrate is separated by an epitaxial lift-off (ELO) process. The nondestructive nature of the ELO process makes multiple reuses of the GaAs substrate possible. The solar cell structure is optimized for maximum EOL efficiency, that is, after 1-MeV electron irradiation with a fluence of 1 × 1015 cm-2, by means of simulations that include the irradiation induced defects in the various semiconductor alloys. Assuming realistic charge carrier lifetime in the materials, we predict a near-term efficiency potential for the IMM3J ELO of 30.9% under AM0 illumination before and 26.7% after irradiation. Several IMM3J ELO solar cells with an area of approximately 20 cm2 from different development stages were tested under AM0 illumination. The newest solar cells (generation III) with a mass density of only 13.2 mg/cm2 reach conversion efficiencies of 30.2% at 25°C. The resulting power-to-mass ratio of 3.0 W/g for the bare solar cell is one of the highest published ratios. After irradiation, a conversion efficiency of 25.4% was measured for “generation II” devices under AM0 illumination, which corresponds to a power-to-mass ratio of 2.6 W/g. IMM3J ELO solar cells from “generation I” were also tested for mechanical stability as “de-risking” test of this new cell technology. No degradation of the cell performance was found after dipping the cell in liquid N2 and then heating up to 25°C for five times, despite of strong deformation of the flexible cell during the temperature cycle.

For solar cells operating under the broad-band solar spectrum, the photovoltaic conversion efficiency is fundamentally limited by transmission and thermalization losses. For monochromatic light, these losses can be minimized by matching the photon energy and the absorber material's bandgap energy. Furthermore, for high-crystal-quality direct semiconductors, radiative recombination dominates the minority carrier recombination. Light-trapping schemes can leverage reabsorption of thereby internally generated photons. Such photon recycling increases the effective excess carrier concentration, which, in turn, increases photovoltage and consequently conversion efficiency. Herein, a back surface reflector underneath a GaAs/AlGaAs rear-heterojunction structure leverages photon recycling to effectively reduce radiative recombination losses and therefore boost the photovoltage. At the same time, resonance in the created optical cavity is tailored to enhance near-bandgap absorption and, thus, minimize thermalization loss. With a thin film process and a combined dielectric-metal reflector, an unprecedented photovoltaic conversion efficiency of 68.9 ± 2.8% under 858 nm monochromatic light at an irradiance of 11.4 W cm−2 is demonstrated.

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

A new approach to model edge recombination in silicon solar cells is presented. The model accounts for recombination both at the edge of the quasi-neutral bulk as well as at an exposed space-charge-region (SCR), the latter via an edge-length-specific diode property with an ideality factor of 2: a localized J02, edge. The model is implemented in Quokka3, where the $J_{02,edge}$ is applied locally to the edges of the three-dimensional geometry, imposing less simplifying assumptions compared with the common way of applying it as an external diode. A “worst-case” value for $J_{02,{\rm{edge}}}$ , assuming very high surface recombination, is determined by fitting to full detailed device simulations which resolve the SCR recombination. A value of $\sim \text{19 nA/cm}$ is found, which is shown to be largely independent of device properties. The new approach is applied to model the impact of edge recombination on full cell performance for a substantial variety of device properties. It is found that recombination at the quasi-neutral bulk edge does not increase the $J_{02}$ of the dark J–V curve, but still shows a nonideal impact on the light J–V curve similar to the SCR recombination. This needs to be considered in the experimental evaluation of edge losses, which is commonly performed via fitting $J_{02}$ to dark J–V curves.

The applicability of different high (low) work function contact materials for the formation of alternative passivating and hole (electron) selective contacts is currently re-explored for silicon solar cells. To assist the engineering of those contacts, which is still in its infancy, numerical device simulations are used to improve knowledge regarding relevant heterojunction and thin film properties with the focus on metal oxide based hole contacts. The importance of 1) a high metal oxide work function for the induced c-Si pn-junction is shown. It is elucidated that for an efficient hole transport from this induced c-Si junction into the external electrode, via the buffer and the metal oxide, 2) the metal oxide's conduction band must be below the valence band of the buffer (or c-Si absorber) for direct band-to-band tunneling, or 3) bulk traps near the valence band edge of the buffer (or c-Si absorber) are needed for trap-assisted tunneling.

33rd European Photovoltaic Solar Energy Conference and Exhibition; 218-225, This paper discusses degradation phenomena in crystalline silicon. We present new investigations of the light- and elevated temperature induced degradation (LeTID) of multicrystalline silicon. The investigations provide insights into the defect parameters as well as the diffusivity and solubility of impurity species contributing to the defect. We discuss possible defect precursor species and can rule out several metallic impurities. We find that an involvement of hydrogen in the defect could explain the characteristic observations for LeTID. Furthermore, we demonstrate analogies to the light-induced degradation mechanisms at elevated temperatures observed in floatzone silicon, where several experimental results also indicate an involvement of hydrogen in the defect. Based on the similarities between multicrystalline and floatzone silicon we suggest that both degradation phenomena might be caused by the same or similar defects. As we do not expect large concentrations of cobalt in floatzone silicon, we suggest that complexes of intrinsic lattice defects and hydrogen might cause both degradation phenomena.

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.

In this paper, we study the impact of change in emitter diffusion profiles on the electrical characteristics of nanotextured surfaces formed by an inline plasma-less dry-chemical etching process. Our experimental results and process simulations suggest that a deeper highly doped region and a significantly higher inactive P concentration in the emitter plays a determining role in defining recombination, as well as the resistive losses in nanotextured surfaces. Low emitter saturation current densities on phosphorous-diffused surfaces are achievable after passivation with either SiN x ( $j_{{\rm{0e,min}}}\,\approx \,{\text{81 fA/ cm}}^{2}$ ) or AlO x /SiN x ( $j_{{\rm{0e,min}}}\,{\rm{\approx}} \,\text{31 fA/ cm}^{2}$ ) if the emitter recombination channels are suppressed. Based upon macroscopic measurement of contact resistivity and microscopic analysis of the contact areas, we propose that the formation of numerous metal–semiconductor direct contact points on the peak and the plateaus of the nanostructures are mainly responsible for a low specific contact resistivity ( $\rho _{{\rm{c,min}}}\,{\rm{\approx}} \,\text{1.2}\; \text{m}\Omega \cdot \text{cm}^{2}$ ) achievable in these surfaces.

Correlations between defect-related luminescence (DRL) and recombination mechanisms of multicrystalline silicon wafers are investigated by hyperspectral photoluminescence (PL) imaging at cryogenic temperatures (∼80 K) and by PL-based techniques for charge carrier lifetime at room temperature. This unique combination of measurement techniques is used to spectrally compare the DRL in n-type and p-type wafers and to investigate the DRL as a function of block height in a p-type block. Further, the dependence of DRL on interstitial and precipitated metallic impurities has been investigated by comparison of simulated concentration profiles of interstitial and precipitated iron with the spatial distribution of DRL. Our results indicate that the origins of the dislocation-related emission lines (D-lines) are independent of the doping type and suggest that the spectral shape, rather, is determined by the dominating recombination mechanism in the material. In regions with high structural defect density, we observe increased intensities of the D-lines D1-D4 in the DRL spectrum. In regions with a high concentration of either iron or other metallic precipitates, we observe reduced emission intensities of D3 and D4. It is, thus, likely that precipitates of either iron or other impurities partly supress the D3 and D4 emission intensities.

III–V devices outperform all other solar cells in terms of efficiency. However, the manufacturing of these cells is expensive and prevents their use in a number of applications, which would benefit from the high efficiency. A major contribution to the cost is the metal–organic vapor phase epitaxy process for the III–V compounds. Increasing growth rates and, hence, machine throughput, as well as the growth efficiency, are important steps toward reducing the cost of III–V solar cells. We demonstrate the growth of GaAs solar cells at extremely high growth rates of 100 μm/h and achieve a V OC of 1.028 V, a base diffusion length of 6.5 μm, and an efficiency of 23.6% under AM1.5g conditions. Furthermore, we show reactor adjustments leading to growth rates up to 140 μm/h and reach conditions where more than half of the Ga from the precursor is incorporated into the solar cell layers. The results are encouraging and demonstrate a pathway toward lower cost III–V solar cell manufacturing.

In this study, we demonstrate the potential of multicrystalline (mc) n-type silicon for the fabrication of highly efficient mc-Si solar cells. High-quality mc n-type silicon wafers are obtained from a research ingot crystallized in a high-purity crucible, using high-purity granular silicon as seed layer in the crucible bottom and high-purity polysilicon feedstock for the block. An mc p-type silicon block crystallized under identical conditions (same seed and feedstock, crucible system, and temperature profiles) serves as reference and enables measurements of the interstitial iron and chromium concentrations by metastable defect imaging. In combination with 2-D simulations for in-diffusion and precipitation of chromium, the limitation of n-type high-performance mc silicon by these metals is assessed after different solar cell processing steps. Material-related efficiency losses are assessed by an “efficiency limiting bulk recombination analysis,” which combines injection-dependent photoluminescence imaging of minority charge carrier diffusion length with PC1D cell simulations. Finally, based on this material, boron-diffused front-junction mc n-type silicon solar cells with a full-area passivated rear contact (TOPCon) are fabricated. The record cell features an efficiency of 19.6%, which is the highest efficiency reported for an mc n-type silicon solar cell.

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.

A parameterized model is proposed in order to analytically calculate the metallic precipitate-related carrier recombination/lifetime that depends on excess carrier and doping concentration, as well as precipitate size and density in both p- and n-type silicon. The parameterization is based on numerical simulations of recombination at the precipitate–silicon interface, assuming that the dominant physical mechanism is thermionic emission currents due to internal Schottky contacts between metal (silicide) and semiconductor. Application examples and the range of validity of the proposed analytical calculation are discussed.

This work presents recent advances in the characterisation of carrier recombination and impurities at Fraunhofer ISE. The role of iron contamination during crystallisation is analysed in more detail. Numerical simulations and comparisons to experimental data are presented which demonstrate the impact of iron from the crucible and crucible coating and show the in-diffusion of iron into the silicon melt as well as into the solid silicon during crystal cooling. Measurements of spatially resolved carrier lifetime and interstitial iron concentration on wafers after phosphorus diffusion gettering are used as input for cell efficiency modelling which reveals the specific and quantitative role of iron on cell parameters in multicrystalline silicon. A new photoluminescence based method is presented which quantitatively determines the interstitial iron concentration in finished solar cells. We finally present advances in defect characterisation with sub-micrometre resolution: We show recent progress in micro photoluminescence spectroscopy for the quantitative measurement of interstitial chromium with high spatial resolution. A further development of this setup will be discussed: By combining the principle of Light Beam Induced Current (LBIC) or voltage (LBIV) and the highly localized illumination, images of carrier recombination at local defects are presented which feature a, compared to EBIC, higher signal-to-noise ratio.

The aim of this paper is to analyze the limiting role of crucible and coating impurities on material quality of multicrystalline silicon. Both solid body diffusion and diffusion into the silicon melt are considered in this study. Two ingots of size G1 have been analyzed. One of them was crystallized in a standard crucible, whereas the other was crystallized in a quartz crucible of very high purity. Focus is put on iron and cobalt as examples of typical impurity species. Iron was found in large concentrations in standard crucibles, and cobalt was proven to be a suitable marker impurity that is mainly found in the coating. Inductively coupled plasma mass spectroscopy data are exploited for the determination of impurity concentrations in crucible, coating, and within the crystal. With higher sensitivity for low concentration, PL imaging is applied for carrier lifetime and interstitial iron concentration measurements. The different findings are compared with modeling results of iron and cobalt in-diffusion by Sentaurus Process. The analysis of silicon wafers before and after gettering steps enable a quantification of impurity-limiting cell efficiency potential. Conclusions about the role of impurities from coated crucibles in large-scale crystallization are deduced.

The nanostructured surface – also called black silicon (b-Si) – is a promising texture for solar cells because of its extremely low reflectance combined with low surface recombination obtained with atomic layer deposited (ALD) thin films. However, the challenges in keeping the excellent optical properties and passivation in further processing have not been addressed before. Here we study especially the applicability of the ALD passivation on highly boron doped emitters that is present in crystalline silicon solar cells. The results show that the nanostructured boron emitters can be passivated efficiently using ALD Al2O3 reaching emitter saturation current densities as low as 51 fA/cm2. Furthermore, reflectance values less than 0.5% after processing show that the different process steps are not detrimental for the low reflectance of b-Si. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this study, we investigate the influence of different diffusion temperature profiles on the interstitial iron concentration before and after contact firing. It is shown that the positive influence of a low-temperature anneal after phosphorus diffusion is strongly reduced after contact firing as the formed precipitates dissolve again. On the other hand, a dissolution peak before the diffusion aims on a decrease in precipitate density and, therefore, leads to a reduced interstitial iron concentration especially after firing. The physical mechanisms during diffusion and firing are investigated with Sentaurus Process simulations. Furthermore, lifetime improvements in standard multicrystalline material could be achieved applying the dissolution peak before diffusion.

We simulate the precipitation of iron during the multicrystalline ingot crystallization process and the redistribution of iron during subsequent phosphorus diffusion gettering with a 2-D model. We compare the simulated size distribution of the precipitates with the X-ray fluorescence microscopy measurements of iron precipitates along a grain boundary. We find that the simulated and measured densities of precipitates larger than the experimental detection limit are in good agreement after the crystallization process. Additionally, we demonstrate that the measured decrease of the line density and the increase of the mean size of the iron precipitates after phosphorus diffusion gettering can be reproduced with the simulations. The size and spatial distribution of iron precipitates affect the kinetics of iron redistribution during the solar cell process and, ultimately, the recombination activity of the precipitated iron. Variations of the cooling rate after solidification and short temperature peaks before phosphorus diffusion strongly influence the precipitate size distribution. The lowest overall density of iron precipitates after phosphorus diffusion is obtained in the simulations with a temperature peak before phosphorus diffusion, followed by moderate cooling rates.

We present a method for the purification of silicon ingots during the crystallization process that reduces significantly the width of the low charge carrier lifetime region at the ingot top. The back-diffusion of impurities from the ingot top is suppressed by adding a small amount of pure aluminum into the silicon melt right at the end of the solidification. We study the aluminum gettering effect by instrumental neutron activation analysis (INAA) and Fei imaging. Furthermore, we present a model for aluminum gettering of Fe in the silicon ingot that is in agreement with literature data for aluminum gettering at lower temperature. The distribution of iron in the ingots with and without aluminum is fairly well predicted by a combination of this model with a model for Fe contamination from the crucible system. A simulation with varying Al content exhibits further potential for an increased yield of silicon wafers with high charge carrier lifetime. (© 2016 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

Interstitial iron (Fei) and iron-boron pairs influence or even limit the recombination lifetime in industrial block cast multicrystalline (mc) silicon, though the proportions in the total iron concentration are generally small. Most of the iron in mc silicon is precipitated and less recombination active. This work aims for a better understanding of the distribution of iron in its different states (precipitated or dissolved) over the block height, as well as in regions of different crystal quality. In experimental studies several features of iron in mc silicon were observed, which occur due to the high extended defect density. In our 2-dimensional model for mc silicon, trapping of interstitial Fe at extended defects and precipitation at the extended defects are taken into account. The results are compared with NAA-data and spatial resolved measurements of the Fei concentration.

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)

This study aims for a quantitative investigation of the material limitations and the efficiency potential of an entire multicrystalline (mc) n-type silicon block in comparison with an mc p-type block of the same purity level in order to predict the potential of mc n-type silicon for the industrial production of solar cells. Therefore, two standard mc silicon blocks were crystallized under identical conditions (same high purity feedstock, crucible system, and temperature profiles), only differing in their type of doping. The material quality of wafers along the whole block height is analyzed after different solar cell process steps by photoluminescence imaging of the diffusion length. The bulk recombination related efficiency losses are assessed by an ""efficiency limiting bulk recombination analysis (ELBA),"" combining injection dependent lifetime images with PC1D cell simulations. The influence of the base resistivity variation along the block is considered in the PC1D cell simulations and backed up by Sentaurus Device simulations. This analysis predicts a significantly higher material-related efficiency potential after typical solar cell processes along the whole block height for mc n-type silicon compared with mc p-type silicon. In addition, the efficiency potential for mc n-type silicon depends less on block position.

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.

The presence of copper contamination is known to severely degrade the minority carrier lifetime of p‐type silicon upon exposure to illumination. In this contribution, we have analyzed the recombination activity of light‐activated copper defects in deliberately Cu‐contaminated p‐type silicon by means of a recombination model that quantitatively defines the effect of metallic precipitates on minority carrier lifetime. The excellent agreement between the model and the experimental data indicates that (i) the formation of Cu precipitates is the probable root‐cause behind Cu‐LID and (ii) in the samples examined in this work, the precipitate radius varies between few to several tens of nm with corresponding densities estimated to be in the range of 108-1010 cm−3. Further evidence of these results was obtained from the analysis of temperature‐dependent lifetime data. While applied here to light‐activated copper defects, the procedure described in this article can be applied for characterizing lifetime‐limiting precipitates originated by other transition metals (e.g., Fe or Ni).

The impact of iron point defects on the measured injection-dependent minority carrier lifetime in silicon after different processing steps (described by the Shockley-Read-Hall equation) is well known. However, in some parts of multicrystalline silicon (mc-Si) (used for solar cells), a large share of the iron atoms is precipitated. In this study, we simulate realistic iron precipitate distributions in mc-Si after crystallization, as well as after phosphorus diffusion gettering within grains by employing the Fokker-Planck equations. Taking the Schottky contact between metallic precipitates and semiconductor into account, in a second step, we analyze the effect of recombination at iron precipitates on carrier lifetime by means of finite-element simulations. The results are compared with experimental injection-dependent lifetime measurements on a p-type mc-Si wafer before and after phosphorus diffusion. Our simulations show that in the low-lifetime edge region close to the crucible wall, a considerable share of the carrier recombination can be attributed to iron precipitates in both the as-grown and in the phosphorus-diffused state. In addition, the simulated injection dependences of iron precipitates and iron interstitials differ significantly, with the precipitates influencing the carrier lifetime especially in the mid- to high-injection range, which is supported by carrier lifetime measurements.

The interstitial iron concentration measured directly on the side face of a silicon brick after crystallization and brick squaring can give important early and fast feedback regarding its material quality. Interstitial iron is an important defect in crystalline silicon, particularly in directionally solidified ingots. Spectral photoluminescence intensity ratio imaging has recently been demonstrated to independently provide high-resolution bulk lifetime images and is therefore ideally suited to assess spatially variable multicrystalline silicon bricks. Here, we demonstrate this technique to enable imaging of the interstitial iron concentration on boron-doped silicon bricks and thick silicon slabs. We present iron concentration studies for two directionally solidified silicon bricks of which one is a standard multicrystalline and the other is a seeded-growth ingot. This lifetime-based measurement technique is highly sensitive to interstitial iron with detection limits down to concentrations of about 1 × 10 10 cm -3 . Its accuracy is enhanced, as the injection level remains below 2 × 10 12 cm -3 during the measurement and, hence, avoids the influence of injection level dependences on the conversion factor, although it remains dependent on the knowledge of the electron capture cross section of interstitial iron in silicon. Access to both bulk lifetime and dissolved iron concentration provides a valuable parameter set of as-grown crystal quality and the relative recombination fraction of interstitial iron via Shockley-Read-Hall (SRH) analysis. Simulated interstitial iron concentration profiles support the presented experimental data.

The electrical material quality of multicrystalline (mc) silicon for photovoltaic applications suffers from crystal defects as well as from impurities that originate from the feedstock, the quartz crucible, and its coating. In this study, we investigate the influence of impurities from the crucible on efficiency losses in mc silicon solar cells, focusing on the limitation due to iron. The applicability of p-type mc silicon, crystallized in G1 sized crucibles of industrial material quality and very pure electrically fused silica, for a high-efficiency solar cell process is examined by measuring lifetime and interstitial iron concentration in the wafers after different processing steps and by estimating the cell efficiency potential from injection-dependent bulk lifetime measurements. Interstitial iron concentrations extracted from 2-D simulations of iron precipitation at crystal defects and gettering during processing agree well with Fei measurements at different process stages and explain the observations. Efficiency losses are quantified to losses due to segregated impurities diffused into the silicon melt, losses due to decorated crystal defects and losses due to solid-state diffusion into the crystal. By using a high-purity crucible, losses are reduced significantly and an efficiency gain of 0.5% absolute is estimated to be attainable on wafers with edge region.

An effective passivation on the front side boron emitter is essential to utilize the full potential of solar cells fabricated on n-type silicon. However, recent investigations have shown that it is more difficult to achieve a low surface recombination velocity on highly doped p-type silicon than on n-type silicon. Thus, the approach presented in this paper is to overcompensate the surface of the deep boron emitter locally by a shallow phosphorus diffusion. This inversion from p-type to n-type surface allows the use of standard technologies which are used for passivation of highly doped n-type surfaces. Emitter saturation current densities (J0e) of 49 fA/cm2 have been reached with this approach on SiO2 passivated lifetime samples. On solar cells a certified conversion efficiency of 21.7% with an open-circuit voltage (Voc) of 676 mV was achieved. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We studied the precipitation of chromium in multicrystalline silicon during the crystallization process and temperature treatments typical for solar cell processing. A model which was already successfully used for simulating heterogeneous precipitation of iron is transferred to chromium, allowing two-dimensional simulations of dissolved chromium and precipitate density. The observed accordance with spatially resolved measurements demonstrates the similarity of chromium to iron precipitation and the ability of our model to predict the dissolved chromium concentration in multicrystalline silicon. After the crystallization process, a high impact of chromium on the carrier lifetime of wafers originating from an ingot intentionally contaminated with 20 ppma chromium in the melt was observed. The concentration of dissolved chromium was significantly reduced by phosphorus diffusion gettering or oxidation at 815°C. Copyright © 2012 John Wiley & Sons, Ltd.

Metals are detrimental to silicon solar cells in two ways: (i) they typically introduce defect levels in the bandgap, leading to enhanced carrier recombination and thus to lower voltage in solar cells; and (ii) they may, in the form of precipitates, contribute to the formation of shunts and reverse breakdown sites. This paper provides a review on techniques to access the spatial distribution of recombination sites for multicrystalline silicon. Methods to detect metal precipitates as well as, in the case of iron, dissolved point defects are presented. These methods are applied to clarify the distribution of iron after high-temperature processes and the identification of breakdown sites.

A novel method of extracting the nonradiative lifetime of Ga 0.51 In 0.49 P lattice matched to GaAs by exploiting luminescence coupling is discussed. The method requires a GaInP double heterostructure monolithically grown on a GaAs photodetector for quantum efficiency measurements. The method then extracts the nonradiative lifetime by modeling the luminescence coupling between the GaInP and the GaAs active region. The bulk nonradiative lifetime of disordered GaInP doped to 10 17 cm -3 under low-level injection (~4 × 10 12 cm -3 ) is determined to be 47 ns for GaInP layer thicknesses ranging from 200 to 1500 nm, with a surface recombination velocity of 660 cm/s. The results are in agreement with nonradiative lifetimes extracted using power-dependent relative photoluminescence measurements, whereby the lifetimes increase as a function of injection level to 0.5 μs. Surface recombination velocities are also reported as a function of injection level using this technique, decreasing from 1800 cm/s under low injection to 50 cm/s under high injection (~10 17 cm -3 ). Lastly, the effective radiative lifetimes (accounting for photon recycling) are also reported for the studied GaInP samples.