Photovoltaic cells, available for nearly half a century, convert sunlight to electricity. Simple versions are found in watches and calculators, while more complex arrays power the Hubble Space Telescope and the International Space Station. Crystalline silicon is the material used in these cells, which are relatively expensive to manufacture, and cost is the issue which has prevented solar cells from being more widely used. Thin-film photovoltaic cells use layers of semiconductor material attached to an inexpensive backing such as glass, stainless steel, or flexible plastic. Thin films require much less semiconductor than traditional cells, and offer a route to drive down costs. However, silicon itself is not suitable for thin-film technology because in its amorphous form it is gradually degraded by exposure to sunlight. Instead, photovoltaics researchers have investigated the Group III and V elements in the periodic table, some of which show very high efficiencies for sunlight conversion. And this brings us to paper #7.

Miguel Contreras and his colleagues at the National Renewable Energy Laboratory (NREL), Golden, Colorado, describe the properties of a polycrystalline thin-film which uses copper, gallium, indium, and diselenide. The energy absorber is Cu(In,Ga)Se₂, on a substrate of glass or steel, and an insulating ZnO layer is deposited on top; the three-stage process to make the films is patented by NREL. In laboratory conditions this device achieved 18.8% conversion, which represents a new record for such devices and makes the desired 20% performance level much closer to reality. The NREL team hint that they can get closer to 20% by optimizing the window material and anti-reflective coatings and tweaking the bandgap of the absorber. For Science Watch, Contreras says "The reasons for the strong interest in my work is because the efficiency is the highest reported for thin-films, which are now established as serious competitors for silicon."

Hot Paper #8, from a team led by Jun Akimitsu of Aoyama-Gakuin University, Tokyo, reports a superconducting transition temperature (T_C) of 39 K for MgB₂. This is generating huge excitement: it is the highest T_C yet observed for a Type-II superconductor. Magnesium diboride is a common laboratory chemical, a black powdery material you can buy for $2/g. What’s surprising is that physicists have taken so long to discover that it is superconducting. Back in the 1930s in Berlin, Walter Meissner found superconducting compounds of transition metals with carbides and borides. Interest has heightened as well because MgB₂ is much easier to understand than the fiendishly complex high-T_C cuprate ceramics. MgB₂is a "normal" superconductor governed by the BCS theory.

Much interest is unfolding in the literature already on the practicality of using MgB₂ in real superconducting devices. If the mechanical properties are controllable it could contribute significantly to the production of superconducting applications. The signs are already amazingly good. David Caplin and a team at Imperial College London have discovered that proton irradiation enhances the critical current density (see Y. Bugoslavsky, et al., Nature, 411[6837]:561-3, 2001). Physicists at Lucent Technologies have made iron-clad MgB₂ wires carrying 30,000 A cm at a temperature of 25 K, which is almost high enough for power transmission cables cooled by hydrogen gas (see S. Jin, et al., Nature, 411[6837]: 563-5, 2001). A group based at Oak Ridge National Laboratory, Tennessee, has devised a method to make superconducting films using pulsed laser deposition (see H.M. Christen, et al., Physica C, 353[3-4]:157-61, 2001). It is unprecedented in the history of superconductivity that such a range of applications is produced within weeks of the publication of the discovery paper.

Finally, #10 is in the emerging field of "plastic electronics," so Science Watch asked Lucent's Hendrik Schon for a comment. "Organic semiconductors are particularly interesting in low-cost, large-area, flexible electronic and optoelectronic applications. Organic transistors are close to commercial availability. We made high-quality devices of one of the most promising candidates, pentacene. Our work shows that performance similar to amorphous silicon is possible. Pentacene shows ambipolar behavior, which is important for complementary logic applications."

So where do we go from here? An entirely plastic radio powered by a solar cell with current from a superconducting wire as a backup for cloudy days? Well, maybe!