A Unified Parameterization of the Formation of Boron Oxygen Defects and their Electrical Activity

AbstractThe 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

Hierarchical time-series approaches for photovoltaic systems performance forecasting with sparse datasets

Solar-based power generation presents challenges for system and grid operators due to the intermittent nature of power supply. Predicting the performance of photovoltaic (PV) power plants and rooftop systems can often be challenging due to difficulties in data collection and incoherencies in interconnected systems. Following the hierarchical aggregation structure from geographical and temporal similarities between PV systems, we suggest a simplified approach to predicting the performance of individual installations and evaluating the impact of these hypothetical installations on the overall grid. We use the hierarchical nature of power generation and ascertain weather datasets to predict the performance of new or existing systems for locations with unmeasured input data. We demonstrate an approach that could improve grid stability by using a hierarchical model on publicly available datasets on utility and rooftop installations. Ensemble Machine Learning algorithms are trained with 16 weeks of known hourly input training features to form a baseline model for known locations. The prediction accuracy is then directly compared for locations with known and unknown input features, both on a granular and subregion level. We observe a reduction in prediction accuracy by 6-8 % using the hierarchical approach. The accuracy of the hierarchical model can be further enhanced beyond our work by increasing the training dataset temporally as well as by augmenting nested layers of the hierarchy

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

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Gallium doped silicon for high efficiency commercial PERC solar cells

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

Electrically tunable Si-based THz photomodulator using dielectric/polymer surface gating

Silicon-based terahertz (THz) photomodulators suffer from a modulation speed limited by the lifetime of the charge carriers photoexcited in the silicon. We report a silicon-based THz photomodulator scheme offering real-time reconfiguration of the switching behavior by manipulation of effective charge carrier lifetime. Atomic layer deposition was used to coat silicon samples with dielectric layers to passivate the surfaces with a conductive polymer (PEDOT:PSS) subsequently deposited to enable electrical gating over the whole surface. The resulting gated photomodulators are characterized using photoconductance decay and photoluminescence imaging. A gated photomodulator with HfO2 passivation is then implemented into a THz time domain spectroscopy setup to demonstrate the potential for live photomodulation optimization during a single-pixel imaging experiment. We use the device to achieve a real-time improvement of the signal-to-noise ratio of the images by a factor of 8

Epitaxially grown p‐type silicon wafers ready for cell efficiencies exceeding 25%

Combining the advantages of a high‐efficiency solar cell concept and a low carbon footprint base material is a promising approach for highly efficient, sustainable, and cost‐effective solar cells. In this work, we investigate the suitability of epitaxially grown p‐type silicon wafers for solar cells with tunnel oxide passivating contact rear emitter. As a first proof of principle, an efficiency limiting bulk recombination analysis of epitaxially grown p‐type silicon wafers deposited on high quality substrates (EpiRef) unveils promising cell efficiency potentials exceeding 25% for three different base resistivities of 3, 14, and 100 Ω cm. To understand the remaining limitations in detail, concentrations of metastable defects Fe i , CrB and BO are assessed by lifetime‐calibrated photoluminescence imaging and their impact on the overall recombination is evaluated. The EpiRef wafers’ efficiency potential is tracked along the solar cell fabrication process to quantify the impact of high temperature treatments on the material quality. We observe large areas with few structural defects on the wafer featuring lifetimes exceeding 10 ms and an efficiency potential of 25.8% even after exposing the wafer to a thermal oxidation at 1050 °C

Lifetime-limiting defects in monocrystalline Silicon

This thesis discusses studies performed by the author at the Fraunhofer Institute for Solar Energy Systems, ISE in cooperation with the Freiburg Materials Research Center, FMF. The main achievements are: A new measurement technique was developed that allows the investigation of the distribution of interstitial oxygen atoms in silicon wafers. The method is based on photoluminescence imaging and the measurement of resistivity changes upon annealing at 450 °C. This results in a high lateral resolution on full samples with reasonable effort. It thereby opens up pathways to improve the understanding of the distribution of impurities during crystal growth. Furthermore, the method can be applied to very thin wafers without loosing precision. It is therefore feasible for typical sample thicknesses in photovoltaic research, where application of infrared absorption spectroscopy becomes problematic. An extensive literature review of the broad field of studies of the light-induced degradation caused by boron-oxygen defects is given. The review provides an overview over aspects that were subject of vivid discussions in literature. It reduces the pronounced fragmentation of the scientific discourse in the field by identifying established and controversial findings in literature. Detailed experiments to investigate the activation kinetics of boron-oxygen defects were performed on compensated n-type silicon. The results provide unambiguous confirmation of the strong dependence of the activation rates on the concentration of holes during illumination. This influence was demonstrated to apply to both, the fast and the slow activation processes. This finding indicates the involvement of two holes in both defect state transitions. The recombination activity of boron-oxygen defects was investigated independence of sample doping and injection conditions. The experiments provide strong evidence that boron-oxygen defects introduce at least two energetic levels in the silicon band gap that interact during recombination. A light-induced degradation of the charge carrier lifetime in p-type float-zone silicon was observed at elevated temperature and studied in detail. The investigations indicate that the effect arises from bulk defects involving hydrogen introduced from dielectric surface passivation layers. Several similarities to a light-induced degradation at elevated temperature in multicrystalline silicon are observed. A long-term stability test of several passivation schemes involving aluminium oxide was performed. The experiment demonstrates that the good passivation quality of such layers is stable for illumination durations above 1000 hours at elevated temperature

Superacid-treated silicon surfaces : extending the limit of carrier lifetime for photovoltaic applications

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