Engineering Metal Impurities in Multicrystalline Silicon Solar Cells

Transition metals are one of the main culprits in degrading the efficiency of multicrystalline solar cells. With a suite of x-ray microprobe techniques, a multi-institutional collaboration led by researchers from the University of California, Berkeley, and Berkeley Lab studied the distribution of metal clusters in a variety of multicrystalline solar cells before and after processing. Their discovery that the size, spatial distribution, and chemical binding of metals within clusters is just as important as the total metal concentration in limiting the performance of multicrystalline silicon solar cells led to the concept of defect engineering by optimizing growth and processing sequences to trap metals in their least harmful state.

A Path to Cheaper Solar Energy

Solar energy is often touted as the most promising and secure alternative energy source, capable of reducing both our dependence on foreign fuels and the emission of dangerous pollutants that alter the world's climate. Although its current market share is small, in the last several years the solar cell industry experienced explosive growth at a rate of 25 to 45% per year. The market share could substantially increase, if the cost of installed residential systems decreased from the current price of approximately $16,000–$25,000. One way to lower the cost is to use less expensive, low-grade raw silicon instead of the currently used ultrahigh-purity, electronic-grade material, which is currently in short supply, owing to the parallel growth of the semiconductor and solar cell industries and the resulting pressure on relatively limited supplies of high-quality silicon.

Buonassisi et al. used an x-ray analytical technique to find and analyze metal clusters as small as a few tens of nanometers, dispersed tens of micrometers apart in solar cells. By this means, they established correlations between the size and distribution of the nanoscale clusters and the efficiency of the cells. Based on their findings, they suggested a novel concept of metal defect engineering to decrease the negative impact of transition metals on solar cells by transforming them to a less harmful state. This approach may lead to making cheaper solar cells from inexpensive low-grade silicon.

Artist’s impression of an intense beam of synchrotron light striking a solar cell and the resulting fluorescence image of the distribution of iron impurities.

Materials alternatives for solar cells range from single-crystal electronic-grade silicon, which yields solar cells with efficiencies close to the theoretical limit but at a prohibitive price, to dirty metallurgical-grade silicon, which has failed to produce working solar cells. Some manufacturers experiment with blending semiconductor-grade silicon and metallurgical-grade silicon; others discuss the concept of cheap "solar-grade silicon" that is purified only to the extent necessary to make working solar cells. In practice, nearly 90% of commercial solar cells are made of highly purified silicon.

The key factor that determines the quality of raw silicon used for crystal growth is its transition metal content. Multicrystalline silicon (mc-Si) solar cells can tolerate iron, copper, or nickel in concentrations up to 10¹⁴–10¹⁵ cm^–3 because metals in mc-Si are often found in less electrically active inclusions or precipitates at structural defects (e.g., grain boundaries) rather than being atomically dissolved. Since there is no simple correlation between the total metal content and cell efficiency, there is a strong need to understand the physics and the properties of metal clusters in solar cells.

The researchers used a combination of x-ray fluorescence (μ-XRF), x-ray absorption, and x-ray-beam-induced-current techniques at ALS Beamline 10.3.2 and Advanced Photon Source SRICAT and PNCCT beamlines to study the distribution, chemical state, and electron-hole recombination activity of metal clusters in mc-Si. Two types of metal clusters were found: large (greater than a micron in size) inclusions, often containing a variety of metals in oxidized chemical states, and small (several tens of nanometers in size) metal-silicide precipitates.

Two types of metal defects in commercial solar cell material. Top: Iron silicide nanoprecipitates with radii of about 20–30 nm. Bottom: Iron oxide inclusion, several microns in diameter. X-ray fluorescence (left) maps the defect distribution, while x-ray absorption spectra (right) determine their chemical states.

Metal clusters were found predominantly at boundaries between grains, dislocations, and, in some types of materials, at defect clusters within grains. Materials that had metal clusters confined to specific structural defects performed better in solar cells than materials in which metals were dispersed through the wafer. This led the researchers to the conclusion that the impact of metals on solar cells is determined not only by the total metal content but also by their distribution within the cell.

Material performance as measured by minority carrier diffusion length in three differently cooled samples (quench–gray, quench and re-anneal–blue, slow cool–orange) and size and spatial distributions of metal defects (insets). The slowly cooled material with microdefects in lower spatial densities clearly outperforms materials with smaller nanodefects in higher spatial densities, even though all materials contain the same total amount of metals.

The existing technologies of gettering (removal from the wafers) and hydrogen passivation, routinely used to reduce the impact of metals on cell efficiency, can improve the cell performance by several percent, but they are insufficient if low-grade silicon is used. The researchers suggested that instead of taking the impurities out, one can manipulate them in a way that reduces their detrimental impact on the solar cell efficiency. This concept was dubbed "defect engineering of metal nanodefects."

For example, a simple variation of the cooling sequence of a sample that was intentionally contaminated with iron, copper, and nickel to simulate low-grade mc-Si improved the minority carrier diffusion length, the parameter directly linked to the cell efficiency, by a factor of four. This improvement was caused by a change in metal distribution from a high density of small clusters and complexes to a low density of larger but isolated precipitates. Defect engineering of metal clusters, when optimized, could lead to new cost-efficient solar cell technologies.

Research conducted by T. Buonassisi, A.A. Istratov, and E.R. Weber (University of California, Berkeley, and Berkeley Lab); M.A. Marcus (ALS); B. Lai and Z. Cai (Argonne National Laboratory); and S.M. Heald (Pacific Northwest National Laboratory).

Research funding: National Renewable Energy Laboratory. Operation of the ALS and the APS is supported by the U.S. Department of Energy, Office of Basic Energy Sciences.

Publications about this research: T. Buonassisi, A.A. Istratov, M.A. Marcus, B. Lai, Z. Cai, S.M. Heald, and E.R. Weber, "Engineering metal-impurity nanodefects for low-cost solar cells," Nature Materials 4, 676 (2005); T. Buonassisi, A.A. Istratov, M. Heuer, M.A. Marcus, R. Jonczyk, J. Isenberg, B. Lai, Z. Cai, S. Heald, W. Warta, R. Schindler, and E.R. Weber, "Synchrotron-based investigations of the nature and impact of iron contamination in multicrystalline silicon solar cell materials," J. Appl. Phys. 97, 074901 (2005); T. Buonassisi, M.A. Marcus, A.A. Istratov, M. Heuer, T. F. Ciszek, B. Lai, Z. Cai, and E.R. Weber, "Analysis of copper-rich precipitates in silicon: Chemical state, gettering, and impact on multicrystalline silicon solar cell material," J. Appl. Phys.97, 063503 (2005).