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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.

Under adverse conditions such as shade or elevated temperatures, cotyledon expansion is reduced and hypocotyl growth is promoted to optimize plant architecture. The mechanisms underlying the repression of cotyledon cell expansion remain unknown. Here, we report that the nuclear abundance of the BES1 transcription factor decreased in the cotyledons and increased in the hypocotyl in Arabidopsis thaliana under shade or warmth. Brassinosteroid levels did not follow the same trend. PIF4 and COP1 increased their nuclear abundance in both organs under shade or warmth. PIF4 directly bound the BES1 promoter to enhance its activity but indirectly reduced BES1 expression. COP1 physically interacted with the BES1 protein, promoting its proteasome degradation in the cotyledons. COP1 had the opposite effect in the hypocotyl, demonstrating organ-specific regulatory networks. Our work indicates that shade or warmth reduces BES1 activity by transcriptional and post-translational regulation to inhibit cotyledon cell expansion.

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.

SummaryThe intracellular colonization of plant roots by the beneficial fungal endophyteSerendipita indicafollows a biphasic strategy. After an early biotrophic phase, the interaction transitions to a host cell death phase restricted to the epidermal and cortex layers of the root. Host cell death contributes to the successful accommodation of the fungus during the beneficial interaction inArabidopsis thaliana. How host cell death is initiated and controlled is largely unknown. Here we show that two fungal enzymes, the ecto-5’-nucleotidaseSiE5NT and the nucleaseSiNucA, act synergistically in the plant apoplast at the onset of cell death to produce deoxyadenosine (dAdo), a potent cell death inducer in animal systems. The uptake of extracellular dAdo, but not the structurally related adenosine (Ado), activates a previously undescribed cell death mechanism inA. thaliana. Mutation of the equilibrative nucleoside transporter ENT3 inA. thalianaresults in resistance to cell death triggered by extracellular dAdo and reduced fungal-mediated cell death during root colonization. A library screen ofA. thalianaT-DNA insertion lines identified a toll/interleukin-1 receptor nucleotide-binding leucine-rich repeat (TIR-NLR) protein as an additional intracellular component in dAdo-triggered cell death. Mutation of this previously uncharacterised TIR-NLR, which we have named ISI (induced byS.indica), affects host cell death, fungal colonization and growth promotion, suggesting a key role in the regulation of root cell death and plant-microbe interaction. Our data show that the combined activity of two fungal apoplastic enzymes leads to the production of a metabolite that, upon uptake, triggers TIR-NLR-modulated plant cell death, providing a link to immunometabolism in plants.Short summaryEfficient intraradical colonization by the beneficial fungal endophyteSerendipita indicarequires restricted host cell death. How this symbiotic host cell death is initiated and controlled is largely unknown. Here we show that two fungal enzymes, the ecto-5’-nucleotidaseSiE5NT and the nucleaseSiNucA, act synergistically in the apoplast at the onset of cell death to produce deoxyadenosine (dAdo), a potent cell death inducer in animal systems. Uptake of extracellular dAdo activates a previously undescribed cell death mechanism in plants. Mutation of theA. thalianaequilibrative nucleoside transporterENT3leads to resistance to cell death triggered by uptake of extracellular dAdo and to reduced fungal-mediated cell death during colonization. A library screen ofA. thalianaT-DNA insertion lines identified a TIR-NLR protein as an additional intracellular component in dAdo-triggered cell death, providing a link to immunometabolism in plants.In a nutshellRegulated host cell death is part of the plant defense strategy against pathogens, but it is also involved in the accommodation of certain beneficial microbes in the roots. We have identified extracellular metabolites and intracellular metabolic signals that contribute to colonization by beneficial root fungal endophytes and uncovered a conserved cell death mechanism likely co-opted for establishing plant-endophyte symbiosis.

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.

In this work we report on a procedure to increase the reproducibility of multi-junction solar cell growth. This is achieved by applying a systematic offset to room-temperature photoluminescence measurement to match the required band gap for obtaining current match as simulated by optical modelling. We found that the difference in band gap extracted from photoluminescence and external quantum efficiency respectively is systematic and depends on the material system under investigation. With the help of this procedure, we modelled and built a 4-junction solar cell that reaches a power conversion efficiency of 43.6 % under the AM1.5d solar spectrum that was 710-fold concentrated.

This work addresses general aspects of the front side TCO and grid electrode for application in a Perovskite-Silicon tandem device by optical and electrical modelling. We highlight the need to optimize the front electrode for the tandem case, where in contrast to silicon single junction devices, there is (i) about half the current, which significantly reduces resistive losses in the front electrode, (ii) a lower refractive index of the absorber (most Perovskites at 600 nm:~2.4 vs. Si: ~4.2), (iii) almost no lateral transport in the perovskite top cell, and (iv) a lower thermal device stability (~100-130°C vs. 200°C), which results in less efficient sintering of the Ag pastes and hence higher metal grid resistivity. This study concludes that compared to silicon heterojunction cells (SHJ), the thickness of the front side TCO electrode should be reduced from 75 nm to 20 nm. For the grid electrode, the low-temperature Ag pastes results in higher line resistivities and curing durations. Furthermore, we showcase that for the multi-wire interconnection concept, the number of wires can be significantly reduced from 18 wires to less than 9 depending on the front metallization.

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.

We present recent results achieved in the field of photonic power conversion, i.e. monochromatic light to electricity conversion, using photovoltaic cells. Based on a thin film processing approach we leverage photon recycling and optical resonance effects with a GaAs/AlGaAs rear-heterojunction photovoltaic cell. A back reflector yields increased effective minority carrier lifetime and, as a consequence, an increase in voltage. Optical resonance in the microcavity yields high absorptance close to bandgap despite weak absorptivity. Hence, high current is reached while thermalization losses are minimized. Maximal spectral response SR=0.653 A/W is measured at 858 nm. At this wavelength, thermalization losses diminish to only 21 meV per photon or 1.5% rel . Based on the calibrated spectral response combined with light I-V measurements under broad band and monochromatic light, we determine a maximum equivalent monochromatic power conversion efficiency at 858 nm of 68.9%±2.8%.

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

As an answer to increased demand for more radiation tolerant solar cells for space applications, we investigate the dependence of radiation hardness of III-V compounds on composition of InGaAsP. Four solar cells with different composition were grown on InP and metamorphically on GaAs and irradiated with 1 MeV electrons. A simulation-based analysis was used to determine the material specific irradiation damage. In addition, the regeneration ability is investigated under typical operating conditions (60 °C and AM0 illumination) and under the ECSS standard. While a clear decrease in radiation damage compared to GaAs is observed in all samples, the effect is stronger with increasing InP-fraction. The irradiation induced defect recombination coefficient can be described with a linear function of InP-fraction.

One drawback of passivating contacts in crystalline silicon solar cells is the current loss due to parasitic absorption within the involved material layers. When employed on an illuminated side of the cell, the full spectrum of the incident light will be partly absorbed before reaching the silicon bulk. Additionally, near-infrared (NIR) absorption can substantially reduce the cell's NIR spectral response (SR) also when employed on the non-illuminated side. As those losses are hard to measure directly, optical modeling is crucial for their quantification. This paper presents an extension to an analytical light-trapping model to account for parasitic absorption in the near-surface region via a new parameter Appp (absorbed fraction per perpendicular pass). We test the model by analyzing i) reflectance measurements on samples with varying doping profiles, ii) SR measurements on TOPCon solar cells with varying back-side poly-silicon layers, and iii) SR measurements of a bifacial silicon heterojunction cell. We show that the model can well be calibrated by fitting a single value for Appp to reflectance measurements. This enables a quantification of the parasitic absorption loss without requiring knowledge of all layer's optical properties. The model also is able to predict parasitic absorption in TOPCon cells when knowing the thickness and doping density of the poly-Si layer. Having proven the usefulness of Appp to represent parasitic absorption as a single-valued quantity, we suggest Appp as a third figure of merit for the quality of a passivating contact, next to the recombination parameter J0c and the contact resistivity rc.

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.

Copper is a common impurity in photovoltaic silicon. While reported to precipitate instantly in n-type Si, copper causes light-induced degradation (Cu-LID) in p-type Si. Recently, partial recovery of Cu-LID was observed after only few minutes of dark annealing at 200°C. In this contribution, we investigate the effects of the dark anneal on Cu-LID-limited minority carrier lifetime both experimentally and by simulations. Surprisingly, after initial recovery, the dark anneal results in further degradation corresponding to a many-fold increase in recombination activity compared to the degraded state after illumination. This anneal-induced degradation can potentially cause additional losses in accidentally Cu-contaminated devices when exposed to elevated temperatures, for example during recovery and regeneration treatments of solar cells. Transient ion drift measurements confirmed that the anneal-induced degradation cannot be attributed to residual interstitial Cu after illumination. After hundreds of hours of annealing, the samples showed another recovery. To analyze these experimental results, a comparison to simulations is performed at the end of the paper.

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.

Electric orbit raising increases the radiation dose for space solar arrays significantly. This leads to the need for a more radiation resistant, highly efficient space solar cell. We propose a new wafer-bonded 4-junction structure which allows reaching begin-of-life efficiencies up to 34.7% (AMO) and efficiencies up to 30.8% (AMO) after 1*1015 cm−2 1-MeV electron irradiation. The high radiation hardness is a result of specific material properties of InP-rich compounds which benefit from significant defect annealing under typical operating conditions in space. A new four-j unction space solar cell, based on high InP fractions containing GaInAsP and Ge is currently under development in the EU project RadHard and first devices achieve an efficiency of 21.4% (AM0) before irradiation. After irradiation, as expected, a strong annealing effect after 3 days at AM0 & 60°C is found for this device. Already at this early development stage 78% of the end of life open circuit target voltage of 3.09 V under AM0 has been reached.

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.

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.

In this paper, we study the effect of an enlarged surface area of nanotextured crystalline silicon wafers on the formation of n-type emitters using a tube diffusion process applying POCl 3 as P dopant source. A fast, single-step and industrially viable F 2 -based dry texturing process is used to perform nanotexturing of Si wafers. This process is presented as an alternative route of nanotexturing in comparison to the two-step nanotexturing approach of creating black silicon and then modifying it with an alkaline or acidic solution. Predictive simulations of phosphorous in-diffusion aided by microscopical characterization of the selected emitters are used to understand the formation of emitter in nanotextured surfaces. Based on these investigations, we show that the optimized emitter leads to a significant improvement in short circuit current density ( J sc ≥0.7 mA/cm 2 ) of nanotextured mc-Si solar cells in comparison to the industrial standard acidic textured solar cells.

In this contribution, we provide an insight into the light-induced degradation of multicrystalline (mc-) silicon caused by copper contamination. Particularly we analyze the degradation kinetics of intentionally contaminated B- and Ga-doped mc-Si through spatially resolved photoluminescence (PL) imaging. Our results show that even small copper concentrations are capable of causing a strong LID effect in both B- and Ga-doped samples. Furthermore, the light intensity, the dopant and the grain quality were found to strongly impact the degradation kinetics, since faster LID was observed with stronger illumination intensity, B-doping and in the grains featuring low initial lifetime. Interestingly after degradation we also observe the formation of bright denuded zones near the edges of the B-doped grains, which might indicate the possible accumulation of copper impurities at the grain boundaries.

Recent progress in the electronic quality of high-performance (HP) multicrystalline silicon material is reported with measurements and modeling performed at various institutions and research groups. It is shown that recent progress has been made in the fabrication at Trina Solar mainly by improving the high excess carrier lifetimes τ due to a considerable reduction of mid-gap states. However, the high lifetimes in the wafers are still reduced by interstitial iron by a factor of about 10 at maximum power point (mpp) conditions compared to mono-crystalline Cz wafers of equivalent resistivity. The low lifetime areas of the wafers seem to be limited by precipitates, most likely Cu. Through simulations, it appears that dislocations reduce cell efficiency by about 0.25% absolute. The best predictors for PERC cell efficiency from ingot metrology are a combination of mean lifetime and dislocation density because dislocations cannot be improved considerably by gettering during cell processing, while lifetime-limiting impurities are gettered well. In future, the material may limit cell efficiency above about 22.5% if the concentrations of Fe and Cu remain above 1010 and 1013 cm-3, respectively, and if dislocations are not reduced further.

For future 4-junction Ge based multi-junction solar cells, the current generated in the Ge sub-cell gets very important. We developed an efficient rear-side passivation stack, which results in minority carrier lifetimes $(\tau _{eff}) \approx 200\mu s$ and investigated its performance in an accelerated aging experiment (1MeV electron irradiation). The aging caused a strong lifetime decrease down to $\tau _{eff}=4\mu s$, whereas the carrier mobility stayed constant. These experimental values provide the basis for Beginning-of-Life and End-of-Life solar cell simulations, which show that the potential of the rear-side passivation for 3-junction solar-cell performance is limited, but very promising for 4-junction solar cells.

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.

In general, the charge carrier lifetime in n -type silicon is less sensitive to common dissolved metals compared to p -type silicon. However, in experiments it was observed that metal impurities limit the lifetime in n -type multicrystalline (mc) silicon even if high purity feedstock was used. By evaluating Neutron Activation Analysis (NAA) and Inducitvely Coupled Plasma Mass Spectrometry (ICP-MS) at blocks grown with high purity feedstock, we identified the main currently unavoidable metal impurities present in n -type mc silicon. In addition, we measured the charge carrier lifetime on surface-passivated wafers after different solar cell processes. The measurements were compared to simulations in order to identify the limitations by different impurities and to evaluate the influence of metal precipitates. We found that Cr i is an important defect in good grains of as-grown wafers. Dissolved Co can have a severe impact on lifetime after process steps with fast cooling. The lower lifetime in the edge region of the blocks is attributed to FeSi 2 precipitates, which explains the poor gettering response of the edge region in contrast to grains in the block center. From the results obtained for FeSi 2 -precipitates, we concluded that Cu 3 Si- and NiSi 2 -precipitates located at crystal defects may be responsible for significant recombination in the block center.

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.

We propose a fast and easy way to calculate the precipitate-related carrier recombination / lifetime in n- and p-type silicon as a function of the minority carrier density, the doping level and the precipitate size distribution. The calculation is based on a parameterization of numerical simulation results modeling the thermionic emission currents (recombination currents) at the internal Schottky junction between metallic precipitates and the semiconductor. The carrier lifetimes calculated with the parameterization are in good accordance with the numerically simulated values for a wide range of material properties relevant for silicon photovoltaics, the calculation needing only two sets of parameters (one for each p- and n-type Si). While the numerical simulation of precipitate-related recombination may take several hours, the calculation time using the parameterization is negligible.

An important aspect of Process Simulators for photovoltaics is prediction of defect evolution during device fabrication. Over the last twenty years, these tools have accelerated process optimization, and several Process Simulators for iron, a ubiquitous and deleterious impurity in silicon, have been developed. The diversity of these tools can make it difficult to build intuition about the physics governing iron behavior during processing. Thus, in one unified software environment and using self-consistent terminology, we combine and describe three of these Simulators. We vary structural defect distribution and iron precipitation equations to create eight distinct Models, which we then use to simulate different stages of processing. We find that the structural defect distribution influences the final interstitial iron concentration ([Fe[subscript i]]) more strongly than the iron precipitation equations. We identify two regimes of iron behavior: (1) diffusivity-limited, in which iron evolution is kinetically limited and bulk ([Fe[subscript i]]) predictions can vary by an order of magnitude or more, and (2) solubility-limited, in which iron evolution is near thermodynamic equilibrium and the Models yield similar results. This rigorous analysis provides new intuition that can inform Process Simulation, material, and process development, and it enables scientists and engineers to choose an appropriate level of Model complexity based on wafer type and quality, processing conditions, and available computation time., National Science Foundation (U.S.), United States. Dept. of Energy (NSF CA No. EEC-1041895), Tekes (Agency) (project ‘‘PASSI’’ (project No. 2196/31/ 2011)), Academy of Finland (project ‘‘Low- Cost Photovoltaics.’’), German Federal Ministry for the Environment, Nature Conservation and Nuclear (research cluster ‘‘SolarWinS’’ (contract No. 0325270A-H)), Alexander von Humboldt-Stiftung (Feodor Lynen Postdoctoral Fellowship), Massachusetts Institute of Technology. Department of Mechanical Engineering (Peabody Visiting Professorship), Harvard University (Real Colegio Complutense, RCC Fellowship), Finnish Cultural Foundation (grant No. 00150504)

Copper contamination causes minority carrier lifetime degradation in p-type silicon bulk under illumination, leading to considerable efficiency losses in affected solar cells. Although the existence of this phenomenon has been known for almost two decades, ambiguity prevails about the underlying defect mechanism. In Paper I of this two-part contribution, we propose the first comprehensive mathematical model for Cu-related light-induced degradation in p-type silicon (Cu-LID). The model is based on the precipitation of interstitial Cu ions, which is assumed to be kinetically limited by electrostatic repulsion from the growing Cu precipitates. Hence, growth and dissolution rates of individual Cu precipitates are derived from the drift-diffusion equation of interstitial Cu and used in a kinetic precipitation model that is based on chemical rate equations. The kinetic model is interlinked to a Schottky junction model of metallic precipitates in silicon, enabling accurate calculation of the injection-dependent electric field enclosing the precipitates, as well as the precipitate-limited minority carrier lifetime. It is found that a transition from darkness to illuminated conditions can cause an increase in the kinetics of precipitation by five orders of magnitude. Since our approach enables a direct connection between the time evolution of precipitate size-density distribution and minority carrier lifetime degradation under illumination, a procedure for calculating the Cu-LID-related lifetime as a function of illumination time is included at the end of this article. The model verification with experiments is carried out in Paper II of this contribution along with a discussion of the kinetic and energetic aspects of Cu-LID.

In a series of crystallization experiments, the directional solidification of silicon was investigated as a low cost path for the production of silicon wafers for solar cells. Instrumental neutron activation analysis was employed to measure the influence of different crystallization parameters on the distribution of 3d-metal impurities of the produced ingots. A theoretical model describing the involved diffusion and segregation processes during the solidification and cooling of the ingots could be verified by the experimental results. By successive etching of the samples after the irradiation, it could be shown that a layer of at least 60 μm of the samples has to be removed to get real bulk concentrations.

Within (crystalline silicon) solar cell modeling, a skin means the thin region from the quasi-neutral bulk to the actual surface or metal contact, including e.g. doping profiles, induced band-bending or top-cells of a tandem configuration. A typical highly doped skin is commonly characterized by its main lumped properties: effective recombination via J 0 , skin and lateral conductance via R sheet . When applied as a boundary condition to bulk carrier transport modeling, it is known as the conductive boundary model. However, the detailed resolution of physics inside the skin is then lacking but required in many cases, and possible complexities, like injection dependence of the lumped parameters, are commonly neglected. This work introduces a general parameterization of skins, which accounts fully for injection dependence and a vertical resistance, and is thus able to accurately describe arbitrarily complex skins by lumped parameters. A “skin solver” is implemented in the solar cell simulation software Quokka3 to solve a detailed skin in 1D and to perform the general parameterization. Additionally, the performance of the multidimensional quasi-neutral bulk (qn-bulk) solver is largely improved compared to Quokka2, enabling, for the first time, the 3D discretization and solution of up to an entire 156 mm × 156 mm solar cell in manageable computing times. Quokka3 can then consistently couple the skin solver with the qn-bulk solver. With this multiscale modeling approach, the user can define and solve a solar cell device including the details of the skins orders of magnitude faster compared to generic device simulation software, without loss of accuracy for the majority of conditions in wafer-based silicon solar cells. The new capabilities are demonstrated by showing how the front phosphorus diffusion of a PERC solar cell can be optimized with unprecedented completeness and accuracy. Besides accurately modeling 3D current transport in the bulk, the single solution domain intrinsically accounts for the busbar influence (both recombination and shading), the distributed resistance of the fingers, and the limited current collection independent of the surface area enlargement by texturing.

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)