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.

Annotation The Photovoltaics (PV) Program at the National Renewable Energy Laboratory (NREL) is part of The US Dept. of Energy's National Photovoltaics Program. These proceedings of the 12th NREL PV Program Review Meeting, comprise presentations by invited speakers from private industry, universities, federal laboratories, and representatives of other countries, on such topics as material growth and characterization; single and multijunction devices; cell processing; PV manufacturing; module reliability and field testing; system engineering and applications; and markets and user perspectives. No subject index. Annotation c. by Book News, Inc., Portland, Or.

Annotation The Photovoltaics (PV) Program at the National Renewable Energy Laboratory (NREL) is part of The US Dept. of Energy's National Photovoltaics Program. These proceedings of the 12th NREL PV Program Review Meeting, comprise presentations by invited speakers from private industry, universities, federal laboratories, and representatives of other countries, on such topics as material growth and characterization; single and multijunction devices; cell processing; PV manufacturing; module reliability and field testing; system engineering and applications; and markets and user perspectives. No subject index. Annotation c. by Book News, Inc., Portland, Or.

The major objective of this study, conducted from October 1988 to September 1991, was to gain an understanding of the behavior of impurities in polycrystalline silicon and the influence of these impurities on solar cell efficiency. The authors studied edge-defined film-fed growth (EFG) and cast poly-Si materials and solar cells. With EFG Si they concentrated on chromium-doped materials and cells to determine the role of Cr on solar cell performance. Cast poly-Si samples were not deliberately contaminated. Samples were characterized by cell efficiency, current-voltage, deep-level transient spectroscopy (DLTS), surface photovoltage (SPV), open-circuit voltage decay, secondary ion mass spectrometry, and Fourier transform infrared spectroscopy measurements. They find that Cr forms Cr-B pairs with boron at room temperature and these pairs dissociate into Cr{sub i} and B− during anneals at 210°C for 10 min. Following the anneal, Cr-B pairs reform at room temperature with a time constant of 230 h. Chromium forms CrSi2 precipitates in heavily contaminated regions and they find evidence of CrSi2 gettering, but a lack of chromium segregation or precipitation to grain boundaries and dislocations. Cr-B pairs have well defined DLTS peaks. However, DLTS spectra of other defects are not well defined, giving broad peaks indicative of defects with a range of energy levels in the band gap. In some high-stress, low-efficiency cast poly-Si they detect SiC precipitates, but not in low-stress, high-efficiency samples. SPV measurements result in nonlinear SPV curves in some materials that are likely due to varying optical absorption coefficients due to locally varying stress in the material.

Foundations for the reality of a broadly based, large scale deployment of photovoltaics in commercial applications are described. Research, development, and applications experience and efforts are presented. Special sessions on the problems relating to financing, installing, and operating photovoltaic power generating systems are given. Production problems and techniques are described.