High-temperature defect engineering for silicon solar cells : predictive process simulation and synchrotron-based microcharacterization

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (pages 175-203).Efficiency is a major lever for cost reduction in crystalline silicon solar cells, which dominate the photovoltaics market but cannot yet compete subsidy-free in most areas. Iron impurities are a key performance-limiting defect present in commercial and precommercial silicon solar cell materials, affecting devices at concentrations below even one part per billion. The lack of process simulation tools that account for the behavior of such impurities hinders efforts at increasing efficiency in commercial materials and slows the time-to-market for novel materials. To address the need for predictive process modeling focused on the impact of impurities, the Impurity-to-Efficiency kinetics simulation tool is developed to predict solar cell efficiency from initial iron contamination levels. The modeling effort focuses on iron because it is known to limit most industrial solar cells. The simulation models phosphorus diffusion, the coupled diffusion and segregation of iron to the high phosphorus concentration emitter, and the dissolution and growth of iron-silicide precipitates. The ID process simulation can be solved in about 1 minute assuming standard processing conditions, allowing for rapid iteration. By wrapping the kinetics simulation tool with a genetic algorithm, global optima in the high-dimensional processing parameter space can be pursued for a given starting metal concentration and distribution. To inform and test the model, synchrotron-based X-ray fluorescence is employed with beam spot sizes less than 200 nm to identify iron-rich precipitates down to 10 nm in radius in industrial and research materials. Experimental X-ray fluorescence data confirm model predictions that iron remains in heavily-contaminated multicrystalline materials after a typical industrial phosphorus diffusion. Similar measurements of the iron-silicide precipitate distribution in multicrystalline silicon samples before and after higher-temperature gettering steps confirm that the higher the process temperature, the larger the reduction in precipitated iron, leading to marked lifetime improvement. By combining the impurity kinetics modeling with the experimental assessment of metal distribution, design guidelines for process improvement are proposed: the high-temperature portion of the process can be designed to enhance dissolution of precipitated iron, while the cooldown from the high-temperature process is crucial to the reduction of the interstitial iron concentration. Finally, while precipitated iron reduction improves with higher temperatures, some regions of multicrystalline silicon samples degrade with higher-temperature gettering steps. To investigate the effect of gettering temperature on the remaining lifetime-limiting defects, spatially-resolved lifetime, interstitial iron concentration, and dislocation density are measured. The detailed defect characterization and analysis provide insight into the limitations of high-temperature phosphorus diffusion gettering.by David P. Fenning.Ph. D

Retrograde melting in transition metal-silicon systems : thermodynamic modeling, experimental verification, and potential application

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 97-103).A theoretical framework is presented in this work for retrograde melting in silicon driven by the retrograde solubility of low-concentration metallic solutes at temperatures above the binary eutectic. High enthalpy of formation of point defects in silicon leads to retrograde solubility for a number of solutes, including many 3d transition metals. The Ni-Si system is used to demonstrate that in silicon under supersaturated conditions, such solutes precipitate out into liquid droplets. Synchrotron-based Xray Absorption Microspectroscopy measurements provide experimental confirmation of such phase transitions and the underlying thermodynamics. Finally, the potential for using retrograde melting to improve the electronic minority carrier lifetime of low quality silicon solar cell materials is considered.by David P. Fenning.S.M

Impurity-to-efficiency simulator: Predictive simulation of solar cell efficiencies based on measured metal distribution and cell processing conditions

We present a fast and simple 1D simulation tool to predict solar cell performance as a function of the initial iron content and distribution in the as-grown silicon wafer, the time-temperature profiles applied during the fabrication process, and several parameters related to cell architecture. The applied model consists of three parts that are validated by comparison to experimental results from literature. Assuming a time-temperature profile of a standard solar cell fabrication process, we calculate the redistribution of iron and the evolution of minority carrier lifetime for different as-grown Fe distributions. The solar cell performance as a function of the total iron concentration and the final lifetime distribution is also simulated and compared to experimental results for multicrystalline Si. Keywords: simulation, crystalline silicon solar cell, getterin

Simulated Co-Optimization of Crystalline Silicon Solar Cell Throughput and Efficiency Using Continuously Ramping Phosphorus Diffusion Profiles

http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6318036Defect engineering is essential for the production of high-performance silicon photovoltaic (PV) devices with cost-effective solar-grade Si input materials. Phosphorus diffusion gettering (PDG) can mitigate the detrimental effect of metal impurities on PV device performance. Using the Impurity-to-Efficiency (I2E) simulator, we investigate the effect of gettering temperature on minority carrier lifetime while maintaining an approximately constant sheet resistance. We simulate a typical constant temperature plateau profile and an alternative “volcano” profile that consists of a ramp up to a peak temperature above the typical plateau temperature followed by a ramp down with no hold time. Our simulations show that for a given PDG process time, the “volcano” produces an increase in minority carrier lifetime compared to the standard plateau profile for as-grown iron distributions that are typical for multicrystalline silicon. For an initial total iron concentration of 5×1013 cm-3, we simulate a 30% increase in minority carrier lifetime for a fixed PDG process time and a 43% reduction in PDG process cost for a given effective minority carrier lifetime while achieving a constant sheet resistance of 100 Ω/□.National Science Foundation (U.S.)United States. Dept. of Energy (NSF CA No. EEC-1041895)Massachusetts Institute of Technology. School of Engineering (SMA2 Fellowship)National Science Foundation (U.S.) (NSF Graduate Research Fellowship)Alexander von Humboldt-Stiftung (Feodor Lynen Fellowship Program)United States. Dept. of Defense (National Defense Science and Engineering Graduate Fellowship

Effective lifetimes exceeding 300 μs in gettered p-type epitaxial kerfless silicon for photovoltaics

We evaluate defect concentrations and investigate the lifetime potential of p-type single-crystal kerfless silicon produced via epitaxy for photovoltaics. In gettered material, low interstitial iron concentrations (as low as (3.2 ± 2.2) × 10[superscript 9] cm[superscript −3]) suggest that minority-carrier lifetime is not limited by dissolved iron. An increase in gettered lifetime from 300 μs is observed after increasing growth cleanliness. This improvement coincides with reductions in the concentration of Mo, V, Nb, and Cr impurities, but negligible change in the low area-fraction (23%.United States. Dept. of Energy (Contract DE-EE0005314)National Science Foundation (U.S.) (United States. Dept. of Energy NSF CA EEC-1041895)American Society for Engineering Education. National Defense Science and Engineering Graduate FellowshipAlexander von Humboldt-Stiftung (Feodor Lynen Postdoctoral Fellowship

TCAD for PV: a fast method for accurately modelling metal impurity evolution during solar cell processing

Coupled device and process silumation tools, collectively known as technology computer-aided design (TCAD), have been used in the integrated circuit industry for over 30 years. These tools allow researchers to quickly converge on optimized devide designs and manufacturing processes with minimal experimental expenditures. The PV industry has been slower to adopt these tools, but is quickly developing competency in using them. This paper introduces a predictive defect engineering paradigm and simulation tool, while demonstrating its effectiveness at increasing the performance and throughput of current industrial processes. the impurity-to-efficiency (I2E) simulator is a coupled process and device simulation tool that links wafer material purity, processing parameters and cell desigh to device performance. The tool has been validated with experimental data and used successfully with partners in industry. The simulator has also been deployed in a free web-accessible applet, which is available for use by the industrial and academic communities

Synchrotron-based investigation of transition-metal getterability in n-type multicrystalline silicon

Solar cells based on n-type multicrystalline silicon (mc-Si) wafers are a promising path to reduce the cost per kWh of photovoltaics; however, the full potential of the material and how to optimally process it are still unknown. Process optimization requires knowledge of the response of the metal-silicide precipitate distribution to processing, which has yet to be directly measured and quantified. To supply this missing piece, we use synchrotron-based micro-X-ray fluorescence (μ-XRF) to quantitatively map >250 metal-rich particles in n-type mc-Si wafers before and after phosphorus diffusion gettering (PDG). We find that 820 °C PDG is sufficient to remove precipitates of fast-diffusing impurities and that 920 °C PDG can eliminate precipitated Fe to below the detection limit of μ-XRF. Thus, the evolution of precipitated metal impurities during PDG is observed to be similar for n- and p-type mc-Si, an observation consistent with calculations of the driving forces for precipitate dissolution and segregation gettering. Measurements show that minority-carrier lifetime increases with increasing precipitate dissolution from 820 °C to 880 °C PDG, and that the lifetime after PDG at 920 °C is between the lifetimes achieved after 820 °C and 880 °C PDG

Discovery of the Zintl-phosphide BaCd2_{2}P2_{2} as a long carrier lifetime and stable solar absorber

Thin-film photovoltaics offers a path to significantly decarbonize our energy production. Unfortunately, current materials commercialized or under development as thin-film solar cell absorbers are far from optimal as they show either low power conversion efficiency or issues with earth-abundance and stability. Entirely new and disruptive materials platforms are rarely discovered as the search for new solar absorbers is traditionally slow and serendipitous. Here, we use first principles high-throughput screening to accelerate this process. We identify new solar absorbers among known inorganic compounds using considerations on band gap, carrier transport, optical absorption but also on intrinsic defects which can strongly limit the carrier lifetime and ultimately the solar cell efficiency. Screening about 40,000 materials, we discover the Zintl-phosphide BaCd$_{2}$P$_{2}$ as a potential high-efficiency solar absorber. Follow-up experimental work confirms the predicted promises of BaCd$_{2}$P$_{2}$ highlighting an optimal band gap for visible absorption, bright photoluminescence, and long carrier lifetime of up to 30 ns even for unoptimized powder samples. Importantly, BaCd$_{2}$P$_{2}$ does not contain any critical elements and is highly stable in air and water. Our work opens an avenue for a new family of stable, earth-abundant, high-performance Zintl-based solar absorbers. It also demonstrates how recent advances in first principles computation can accelerate the search of photovoltaic materials by combining high-throughput screening with experiment