The role of the alkali metal cations in halide perovskite solar cells is not well understood. Using synchrotron-based nano-x-ray fluorescence and complementary measurements, we found that the halide ...distribution becomes homogenized upon addition of cesium iodide, either alone or with rubidium iodide, for substoichiometric, stoichiometric, and overstoichiometric preparations, where the lead halide is varied with respect to organic halide precursors. Halide homogenization coincides with long-lived charge carrier decays, spatially homogeneous carrier dynamics (as visualized by ultrafast microscopy), and improved photovoltaic device performance. We found that rubidium and potassium phase-segregate in highly concentrated clusters. Alkali metals are beneficial at low concentrations, where they homogenize the halide distribution, but at higher concentrations, they form recombination-active second-phase clusters.
We identify two engineering solutions to mitigate light-induced degradation (LID) in p-type multicrystalline silicon passivated emitter and rear cells, including modification of metallization firing ...temperature and wafer quality. Lifetime measurements on etched-back samples confirm that LID has a strong bulk component. Spatially resolved lifetime maps indicate that the defects responsible for LID are dispersed ubiquitously across the wafer. Reversibility of LID upon low-temperature annealing suggests a low-activation-energy barrier inconsistent with precipitated impurity dissolution. Lifetime spectroscopy of the LID-affected state reveals an asymmetry of electron and hole capture cross sections of ~28.5, consistent with a deep-level donor point defect (e.g., interstitial Ti, interstitial Mo, substitutional W), charged nanoprecipitate, or charged structural defect, such as a dislocation. Finally, we explain two possible root causes of this LID, including 1) a point-defect complex involving a hydrogen atom and a deep-level donor and 2) configurational change of a point-defect complex involving fast-diffusing impurities.
When untreated, light-induced degradation (LID) of p-type multicrystalline silicon (mc-Si)-based passivated emitter and rear cell (PERC) modules can reduce power output by up to 10% relative during ...sun-soaking under open-circuit conditions. Identifying the root cause of this form of LID has been the subject of several recent investigations. Lifetime spectroscopy analysis, including both injection and temperature dependencies (IDLS and TIDLS), may offer insight into the root-cause defect(s). In this paper, to illustrate the root-case defect identification method, we apply room-temperature IDLS to intentionally Cr-contaminated mc-Si. Then, we apply this technique to the p-type mc-Si that exhibits LID in PERC devices, and we provide further insights by analyzing qualitatively the injection-dependent lifetime as a function of temperature. We quantify the sensitivity of the capture cross-section ratio to variations in the measured lifetime curve and in the surface recombination. We find that the responsible defect most likely has an energy level between 0.3 and 0.7 eV above the valence band and a capture cross-section ratio between 26 and 36. Additionally, we calculate the concentrations of several candidate impurities that may cause the degradation.
Tabula Rasa for n‐Cz silicon‐based photovoltaics LaSalvia, Vincenzo; Youssef, Amanda; Jensen, Mallory A. ...
Progress in photovoltaics,
February 2019, 2019-02-00, 20190201, Letnik:
27, Številka:
2
Journal Article
Recenzirano
Odprti dostop
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.
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 and process‐induced lifetime degradation. Without dissolving grown‐in oxygen precipitate nuclei, high‐temperature solar cell processing sequence severely degrades the minority carrier lifetimes to below 0.1 ms, while TR‐treated n‐Cz wafers after the cell processing steps exhibit carrier lifetimes >2.2 ms.
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.
The noncontact crucible (NOC) method was proposed for obtaining Si single bulk crystals with a large diameter and volume using a cast furnace and solar cells with high conversion efficiency and ...yield. This method has several novel characteristics that originate from its key feature that ingots can be grown inside a Si melt without contact with a crucible wall. Si ingots for solar cells were grown by utilizing the merits resulting from these characteristics. Single ingots with high quality were grown by the NOC method after furnace cleaning, and the minority carrier lifetime was measured to investigate reduction of the number of impurities. A p-type ingot with a convex growth interface in the growth direction was also grown after furnace cleaning. For p-type solar cells prepared using wafers cut from the ingot, the highest and average conversion efficiencies were 19.14% and 19.0%, respectively, which were obtained using the same solar cell structure and process as those employed to obtain a conversion efficiency of 19.1% for a p-type Czochralski (CZ) wafer. Using the cast furnace, solar cells with a conversion efficiency and yield as high as those of CZ solar cells were obtained by the NOC method.
•The noncontact crucible method was proposed for obtaining Si single ingots.•The conversion efficiency was 19.1% for p-type CZ wafers as the reference.•The highest conversion efficiency was 19.14% for p-type full wafers (15.6cm2).•The average conversion efficiency were 19.0% under the scattering width of 0.34%.•Solar cells with as high conversion efficiency and yield as CZ ones can be obtained.
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.
Optimizing photovoltaic (PV) devices requires characterization and optimization across several length scales, from centimeters to nanometers. Synchrotron-based micro-X-ray fluorescence ...spectromicroscopy (μ-XRF) is a valuable link in the PV-related material and device characterization suite. μ-XRF maps of elemental distributions in PV materials have high spatial resolution and excellent sensitivity and can be measured on absorber materials and full devices. Recently, we implemented on-the-fly data collection (flyscan) at Beamline 2-ID-D at the Advanced Photon Source at Argonne National Laboratory, eliminating a 300 ms per-pixel overhead time. This faster scanning enables high-sensitivity (~10 14 atoms/cm 2 ), large-area (10 000s of μm 2 ), high-spatial resolution (<;200 nm scale) maps to be completed within a practical scanning time. We specifically show that when characterizing detrimental trace metal precipitate distributions in multicrystalline silicon wafers for PV, flyscans can increase the productivity of μ-XRF by an order of magnitude. Additionally, flyscan μ-XRF mapping enables relatively large-area correlative microscopy. As an example, we map the transition metal distribution in a 50 μm-diameter laser-fired contact of a silicon solar cell before and after lasing. While we focus on μ-XRF of mc-Si wafers for PV, our results apply broadly to synchrotron-based mapping of PV absorbers and devices.
While progress has been made in developing engineering solutions and understanding light- and elevated temperature-induced degradation (LeTID) in p-type multicrystalline silicon (mc-Si), open ...questions remain regarding the root cause of LeTID. Previously, lifetime spectroscopy of multicrystalline silicon (mc-Si) passivated emitter and rear cell semifabricates in the unaffected and the degraded states enabled identification of the effective recombination parameters of the responsible defect. To gain further insight into the root cause of LeTID, in this paper, we measure the injection-dependent lifetime throughout degradation and regeneration and perform lifetime spectroscopy at several time points. Our analysis indicates that the change in lifetime during most of the process can be described by a corresponding change in the concentration of a single responsible defect. We also explore further exposure to light and temperature after nearly complete regeneration and a subsequent dark anneal to demonstrate that the behavior is no longer consistent with LeTID and the same defect is not detected by lifetime spectroscopy at maximum degradation. We consider our results in the context of the proposed hypotheses for LeTID and conclude that both hydrogenation and precipitate dissolution during firing are consistent with our results.
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