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National Science Foundation (U.S.) (CA No. EEC-1041895)

High-temperature annealing, known as Tabula Rasa (TR), proves to be an effective method for dissolving oxygen precipitate nuclei in n-Cz silicon and makes this material resistant to temperature-induced and process-induced lifetime degradation. Tabula Rasa is especially effective in n-Cz wafers with oxygen concentration >15 ppma. Vacancies, self-interstitials, and their aggregates result from TR as a metastable side effect. Temperature-dependent lifetime spectroscopy reveals that these metastable defects have shallow energy levels ~0.12 eV. Their concentrations strongly depend on the ambient gases during TR because of an offset of the thermal equilibrium between vacancies and self-interstitials. However, these metastable defects anneal out at typical cell processing temperatures ≥850°C and have little effect on the bulk lifetime of the processed cell structures. Without dissolving built-in oxygen precipitate nuclei, high-temperature solar cell processing severely degrades the minority carrier lifetimes to below 0.1 millisecond, while TR-treated n-Cz wafers after the cell processing steps exhibit carrier lifetimes above 2.2 milliseconds.

In this work, we applied internal quantum efficiency mapping to study the recombination activity of grain boundaries in High Performance Multicrystalline Silicon under different processing conditions. Wafers were divided into groups and underwent different thermal processing, consisting of phosphorus diffusion gettering and surface passivation with hydrogen rich layers. After these thermal treatments, wafers were processed into heterojunction with intrinsic thin layer solar cells. Light Beam Induced Current and Electron Backscatter Diffraction were applied to analyse the influence of thermal treatment during standard solar cell processing on different types of grain boundaries. The results show that after cell processing, most random-angle grain boundaries in the material are well passivated, but small-angle grain boundaries are not well passivated. Special cases of coincidence site lattice grain boundaries with high recombination activity are also found. Based on micro-X-ray fluorescence measurements, a change in the contamination level is suggested as the reason behind their increased activity.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2018., Cataloged from PDF version of thesis., Includes bibliographical references (pages 135-145)., To meet climate targets by 2030, manufacturing capacity for photovoltaic (PV) modules must be scaled at 22-25% annual growth rate while maintaining high performance and low selling price. The most suitable material substrate to enable this scale-up is cast multicrystalline silicon (mc-Si) due to its low operating cost and capital requirements compared to other technologies. However, a new form of light-induced degradation was discovered when transitioning mc-Si to the latest high efficiency device architecture. Light- and elevated temperature-induced degradation (LeTID) causes performance to decrease by about 10% (relative) under field-relevant conditions within only four months. In this work, the root cause of LeTID is investigated in three parts: (1) Candidate hypotheses are developed for LeTID; (2) Targeted experiments are carried out toward developing a defect-based description of LeTID; and (3) The basis for a predictive model of LeTID is proposed. Techniques including minority carrier lifetime spectroscopy, synchrotron-based X-ray fluorescence, intentional contamination, and process simulation are employed to probe the defect causing LeTID. The results indicate that LeTID is caused by at least two reactants-hydrogen and one or more reactants that can be modified by high-temperature processing-and that the defect at the point of maximum degradation has recombination characteristics similar to a deep-level donor in silicon. By providing the basis for a predictive model, this work enables both identification of the root cause of LeTID and de-risking of novel solar cell architectures based on mc-Si, allowing assessment of the impact of LeTID on the future of the PV industry. This work also enables development of mitigating strategies for LeTID., Funding from the National Science Foundation Graduate Research Fellowship Program and grants from the National Science Foundation and the U.S. Department of Energy, by Mallory Ann Jensen., Ph. D.

We investigate the microscopic distributions of sub-band-gap luminescence emission (the so-called D-lines D1/D2/D3/D4) and the band-to-band luminescence intensity, near recombination-active subgrain boundaries in multicrystalline silicon wafers for solar cells. We find that the sub-band-gap luminescence from decorating defects/impurities (D1/D2) and from intrinsic dislocations (D3/D4) has distinctly different spatial distributions, and is asymmetric across the subgrain boundaries. The presence of D1/D2 is correlated with a strong reduction in the band-to-band luminescence, indicating a higher recombination activity. In contrast, D3/D4 emissions are not strongly correlated with the band-to-band intensity. Based on spatially resolved, synchrotron-based micro-X-ray fluorescence measurements of metal impurities, we confirm that high densities of metal impurities are present at locations with strong D1/D2 emission but low D3/D4 emission. Finally, we show that the observed asymmetry of the sub-band-gap luminescence across the subgrain boundaries is due to its inclination below the wafer surface. Based on the luminescence asymmetries, the subgrain boundaries are shown to share a common inclination locally, rather than being orientated randomly., Australian Research Council, Australian Renewable Energy Agency (gramt RND009), National Science Foundation (U.S.). Graduate Research Fellowship (Grant No.1122374), United States. Department of Energy. Office of Science (Contract No. DE-AC02-06CH11357)

The presence of metal impurities and their interactions with structural defects (e.g., dislocations) are deleterious to the performance of Si-based solar cell devices. To achieve higher minority carrier lifetimes that translate into higher solar cell efficiencies, novel growth methods with low dislocation densities and reduced metal impurity concentrations have recently been developed. These methods simultaneously aim to achieve low capital expense (capex), necessary to ensure rapid industry scaling. Monocrystalline Si grown by the non-contact crucible method (NOC-Si) has the potential to achieve high bulk minority carrier lifetimes and high efficiencies at low cost given its low structural defect density. Growth in large-diameter crucibles ensures high throughput consistent with low capex. However, high temperatures, coupled with conditions during Si growth (e.g., crucible and ambient gas) can lead to the in-diffusion of impurities, compromising the potential to achieve high efficiency solar cell devices. Herein, we report high minority-carrier lifetimes exceeding 3 milliseconds (ms) in n-type NOC-Si material, achieved through a strict impurity-control procedure at the growth stage that prevents in-diffusion of impurities to the melt, coupled with a tailored defect-engineering process via optimized phosphorus gettering. Keywords: defects; impurities; minority-carrier lifetime; non-contact crucible; top-seeded solution growth; silicon; solar cells, National Science Foundation (U.S.) (Grant 1122374), National Science Foundation (U.S.) (Contract EEC-1041895), United States. Department of Energy (Contract EEC-1041895)

In recent years, high-performance multicrystalline silicon (HPMC-Si) has emerged as an attractive alternative to traditional ingot-based multicrystalline silicon (mc-Si), with a similar cost structure but improved cell performance. Herein, we evaluate the gettering response of traditional mc-Si and HPMC-Si. Microanalytical techniques demonstrate that HPMC-Si and mc-Si share similar lifetime-limiting defect types but have different relative concentrations and distributions. HPMC-Si shows a substantial lifetime improvement after P-gettering compared with mc-Si, chiefly because of lower area fraction of dislocation-rich clusters. In both materials, the dislocation clusters and grain boundaries were associated with relatively higher interstitial iron point-defect concentrations after diffusion, which is suggestive of dissolving metal-impurity precipitates. The relatively fewer dislocation clusters in HPMC-Si are shown to exhibit similar characteristics to those found in mc-Si. Given similar governing principles, a proxy to determine relative recombination activity of dislocation clusters developed for mc-Si is successfully transferred to HPMC-Si. The lifetime in the remainder of HPMC-Si material is found to be limited by grain-boundary recombination. To reduce the recombination activity of grain boundaries in HPMC-Si, coordinated impurity control during growth, gettering, and passivation must be developed. Keywords: Defects; dislocation recombination activity; dislocations; eccentricity variation; high-performance multicrystalline silicon (HPMC-Si); minority-carrier lifetime; photovoltaics; recombination; synchrotron, United States. Department of Energy (Contract EEC-1041895), National Science Foundation (U.S.) (Contract EEC-1041895)

The bulk minority-carrier lifetime in p-And n-type kerfless epitaxial (epi) crystalline silicon wafers is shown to increase >500× during phosphorus gettering. We employ kinetic defect simulations and microstructural characterization techniques to elucidate the root cause of this exceptional gettering response. Simulations and deep-level transient spectroscopy (DLTS) indicate that a high concentration of point defects (likely Pt) is "locked in" during fast (60 °C/min) cooling during epi wafer growth. The fine dispersion of moderately fast-diffusing recombination-active point defects limits as-grown lifetime but can also be removed during gettering, confirmed by DLTS measurements. Synchrotron-based X-ray fluorescence microscopy indicates metal agglomerates at structural defects, yet the structural defect density is sufficiently low to enable high lifetimes. Consequently, after phosphorus diffusion gettering, epi silicon exhibits a higher lifetime than materials with similar bulk impurity contents but higher densities of structural defects, including multicrystalline ingot and ribbon silicon materials. Device simulations suggest a solar-cell efficiency potential of this material >23%., United States. Department of Energy (Contract DE-EE0005314), United States. Department of Energy (Contract EEC-1041895), National Science Foundation (U.S.) (Contract EEC-1041895)

Chromium (Cr) can degrade silicon wafer-based solar cell efficiencies at concentrations as low as 10¹⁰cm⁻³. In this contribution, we employ synchrotron-based X-ray fluorescence microscopy to study chromium distributions in multicrystalline silicon in as-grown material and after phosphorous diffusion. We complement quantified precipitate size and spatial distribution with interstitial Cr concentration and minority carrier lifetime measurements to provide insight into chromium gettering kinetics and offer suggestions for minimizing the device impacts of chromium. We observe that Cr-rich precipitates in as-grown material are generally smaller than iron-rich precipitates and that Cr[subscript i] point defects account for only one-half of the total Cr in the as-grown material. This observation is consistent with previous hypotheses that Cr transport and CrSi₂ growth are more strongly diffusion-limited during ingot cooling. We apply two phosphorous diffusion gettering profiles that both increase minority carrier lifetime by two orders of magnitude and reduce [Cr[subscript i]] by three orders of magnitude to 10¹⁰cm⁻³. Some Cr-rich precipitates persist after both processes, and locally high [Cr[subscript i]] after the high-temperature process indicates that further optimization of the chromium gettering profile is possible., United States. Department of Energy (Contract DE-EE0005314), National Science Foundation (U.S.) (Contract EEC-1041895), United States. Department of Energy (Contract EEC-1041895), National Science Foundation (U.S.) (Grant 1122374)

Impurity contamination in thin-film solar cells remains an uncertain risk due to the little-known impact of impurities on recombination. Building upon previous work, in which we intentionally contaminated lead halide perovskite (LHP) solar cells with iron, we further examine the distribution and charge state of iron-induced defects in LHP films using synchrotron-based X-ray techniques. X-ray absorption measurements suggest that iron-rich regions, which form among iron feedstock concentrations that exceed 100 ppm, most closely resemble the chemistry of Fe2O3. Iron distributed within the bulk may form a mix of Fe2+and Fe3+, the latter of which is not expected to be recombination active, potentially allowing LHPs to incorporate more iron than traditional semiconductors. X-ray beam induced current measurements show little correlation between the presence of iron-rich regions and charge collection, which further suggests low recombination activity at these sites. These results further elucidate the recombination behavior caused by iron incorporation in LHP films, revealing insight into how inhomogeneous incorporation of impurities may mitigate photovoltaic performance degradation., National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (award number DMR-1419807), United States. Department of Energy. Office of Science User Facility (Contract No. DE-AC02-06CH11357), National Science Foundation (U.S.) (NSF EECS Award No. 1541959), Martin Family Society of Fellows for Sustainability, National Science Foundation (U.S.). Graduate Research Fellowship (Grant No. 1122374), National Science Foundation (U.S.). (CA No. EEC-1041895)

In recent years, high-performance multicrystalline silicon (HPMC-Si) has emerged as an attractive alternative to traditional ingot-based multicrystalline silicon (mc-Si), with a similar cost structure but improved cell performance. Herein, we evaluate the gettering response of traditional mc-Si and HPMC-Si. Microanalytical techniques demonstrate that HPMC-Si and mc-Si share similar lifetime-limiting defect types but have different relative concentrations and distributions. HPMC-Si shows a substantial lifetime improvement after P-gettering compared with mc-Si, chiefly because of lower area fraction of dislocation-rich clusters. In both materials, the dislocation clusters and grain boundaries were associated with relatively higher interstitial iron point-defect concentrations after diffusion, which is suggestive of dissolving metal-impurity precipitates. The relatively fewer dislocation clusters in HPMC-Si are shown to exhibit similar characteristics to those found in mc-Si. Given similar governing principles, a proxy to determine relative recombination activity of dislocation clusters developed for mc-Si is successfully transferred to HPMC-Si. The lifetime in the remainder of HPMC-Si material is found to be limited by grain-boundary recombination. To reduce the recombination activity of grain boundaries in HPMC-Si, coordinated impurity control during growth, gettering, and passivation must be developed.