Defects in Solar-Cell Materials Enhance Efficiency
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structures, but also to arbitrary shapes. Currently, Zhang and colleagues are developing fully parallel arbitrary patterning plasmonic lithography. ALFRED A. ZINN
Defects in Solar-Cell Materials Enhance Efficiency Scientists working to improve solar cells have traditionally sought to eliminate defects found in their materials. However, it has been known for some time that polycrystalline solar cells with grain-boundary defects can be more efficient than their single-crystal counterparts. I. Visoly-Fisher, D. Cahen, and S. Cohen of the Weizmann Institute in Israel and A. Ruzin of Tel Aviv University have recently used a number of scanning probe techniques to determine how grainboundary defects improve the performance of one type of polycrystalline solar cell. As reported in the June issue of Advanced Materials (p. 879), the researchers characterized the electrical properties of single grain boundaries and grain surfaces of ~1 µm CdTe grains in p-CdTe/n-CdS polycrystalline thin-film solar cells. The tools used included scanning capacitance microscopy, scanning Kelvin probe microscopy, atomic force microscopy (AFM), and conductive probe AFM. Most commercial solar cells are made of single-crystal semiconductors such as silicon, but single crystals are expensive to manufacture. The more cost-effective solar cells made of polycrystalline films were found to outperform their single-crystal counterpart cells, which was an unexpected outcome. When light strikes the solar cell, the semiconductor within it serves as an antenna, absorbing the light energy and allowing the free flow of electrons
Figure 1. Grain boundaries within solarcell films can improve the light-toelectricity conversion because they provide a path along which free electrons are efficiently collected and channeled.
MRS BULLETIN/SEPTEMBER 2004
present in the semiconductor. These electrons are then harvested as an electric current for external use. The researchers at Weizmann Institute demonstrated that grain boundaries within CdTe/CdS polycrystalline solar cells improve the light-toelectricity conversion because they provide a path along which the free electrons are efficiently collected and channeled on their way out (see Figure 1). The researchers indicated that the efficient collection of electrons is due to the beneficial effects of the CdCl2 vapor treatment that occurs during processing of these cells. The treatment getters both defects and impurities along the grain boundaries and alters the doping profile to facilitate spatial separation of electrons and holes, thus reducing recombination.
Large-Scale Synthesis of Nearly Monodisperse Au and Ag Nanoparticles Achieved Gold and silver nanoparticles are important in diverse areas ranging from fundamental research to applications in electronics (single-electron transistors), catalysis, chemical sensors, and biosensors. These applications require nanoparticles in the 2–100 nm range that are surfacederivatized with hydrophobic and hydrophilic surfactants. H. Hiramatsu and F.E. Osterloh of the University
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