By 2050, world annual energy consumption is predicted to grow from the present 13 terawatt - years (TWyr) to nearly 30 TWyr. Although all energy sources must be considered in meeting this challenge, solar energy may arguably be the only carbon-free source capable of supplying a significant fraction of energy at these levels. This issue of MRS Bulletin reviews the status and future development of solar photovoltaic technologies based on inorganic materials. The discussion begins with materials and cell designs for second-generation photovoltaics based on thin films [a-Si:H, Si, Cu(In, Ga)(Se, S)2, CdTe]. Recent advances in tandem cells and concentrators are alsoreported, along with photovoltaic approaches involving nanoscale materials such as quantum dot arrays. Finally, work on transparent conducting oxides that are critical to nearly all cell designs are discussed.

Thin-film solar cell technologies based on Si with a thickness of less than a few micrometers combine the low-cost potential of thin-film technologies with the advantages of Si as an abundantly available element in the earth's crust and a readily manufacturable material for photovoltaics (PVs). In recent years, several technologies have been developed that promise to take the performance of thin-film silicon PVs well beyond that of the currently established amorphous Si PV technology. Thin-film silicon, like no other thin-film material, is very effective in tandem and triple-junction solar cells. The research and development on thin crystalline silicon on foreign substrates can be divided into two different routes: a low-temperature route compatible with standard float glass or even plastic substrates, and a high-temperature route (>600°C). This article reviews the material properties and technological challenges of the different thin-film silicon PV materials.

The record laboratory cell (∼1 cm2 area) efficiency for thin-film cadmium telluride (CdTe) is 16.5%, and that for a copper indium diselenide (CuInSe2) thin-film alloy is 19.5%. Commercially produced CdTe and CuInSe2 modules (0.5–1 m2 area) have efficiencies in the 7–11% range. Research is needed both to increase laboratory cell efficiencies and to bring those small - area efficiencies to large-area production. Increases in laboratory CdTe cell efficiency will require increasing open-circuit voltage, which will allow cells to harvest more energy from each absorbed photon. This will require extending the minority carrier lifetime from its present τ ≤ 2 ns to τ ≥ 10 ns and increasing hole concentration in the CdTe beyond 1015 cm2, which appears to be limited by compensating defects. Increasing laboratory CuInSe2-based cell efficiency significantly beyond 19.5% will also require increasing the open-circuit voltage, either by increasing the bandgap, the doping level, or the minority carrier lifetime. The photovoltaic cells in commercial modules occupy tens of square centimeters, and both models and experiments have shown that low-performing regions in small fractions of a cell can significantly reduce the overall cell per formance. Increases in commercial module efficiency will require control of materials properties across large deposition areas in a high-throughput environment to minimize such non-uniformities. This article discusses approaches used and research needed to increase the ultimate efficiencies of CdTe- and CuInSe2-based devices and translate these gains to commercial photovoltaic modules.

The efficiency of a solar cell can be increased by stacking multiple solar cells with a range of bandgap energies, resulting in a multijunction solar cell with a maximum the oretical efficiency limit of 86.8% III–V compound semiconductors are good candidates for fabricating such multijunction solar cells for two reasons: they can be grown with excellent material quality; and their bandgaps span a wide spectral range, mostly with direct bandgaps, implying a high absorption coefficient. These factors are the reason for the success of this technology, which has achieved 39% efficiency, the highest solar-to-electric conversion efficiency of any photovoltaic device to date. This article explores the materials science of today's high-efficiency multijunction cells and describes challenges associated with new materials developments and how they may lead to next-generation, multijunction solar cell concepts.

Semiconductor quantum dots may be used in so-called third-generation solar cells that have the potential to greatly increase the photon conversion efficiency via two effects: (1) the production of multiple excitons from a single photon of sufficient energy and (2) the formation of intermediate bands in the bandgap that use sub-bandgap photons to form separable electron–hole pairs. This is possible because quantization of energy levels in quantum dots produces the following effects: enhanced Auger processes and Coulomb coupling between charge carriers; elimination of the requirement to conserve crystal momentum; slowed hot electron–hole pair (exciton) cooling; multiple exciton generation; and formation of minibands (delocalized electronic states) in quantum dot arrays. For exciton multiplication, very high quantum yields of 300–700% for exciton formation in PbSe, PbS, PbTe, and CdSe quantum dots have been reported at photon energies about 4–8 times the HOMO–LUMO transition energy (quantum dot bandgap), respectively, indicating the formation of 3–7 excitons/photon, depending upon the photon energy. For intermediate-band solar cells, quantum dots are used to create the intermediate bands from the con fined electron states in the conduction band. By means of the intermediate band, it is possible to absorb below-bandgap energy photons. This is predicted to produce solar cells with enhanced photocurrent without voltage degradation.

Transparent conducting oxides (TCOs) are an increasingly important component of photovoltaic (PV) devices, where they act as electrode elements, structural templates, and diffusion barriers, and their work function controls the open-circuit device voltage. They are employed in applications that range from crystalline-Si heterojunction with intrinsic thin layer (HIT) cells to organic PV polymer solar cells. The desirable characteristics of TCO materials that are common to all PV technologies are similar to the requirements for TCOs for flat-panel display applications and include high optical transmissivity across a wide spectrum and low resistivity. Additionally, TCOs for terrestrial PV applications must use low-cost materials, and some may require device-technology-specific properties. We review the fundamentals of TCOs and the matrix of TCO properties and processing as they apply to current and future PV technologies.

The pipe organ is the king of musical instruments. No other instrument can compare with the pipe organ in power, timbre, dynamic range, tonal complexity, and sheer majesty of sound. The art of organ building reached its peak in the Baroque Age (∼1600–1750); with the industrial revolution in the 19th century, organ building shifted from a traditional artisans' work to factory production, changing the aesthetic concept and design of the organ so that the profound knowledge of the organ masters passed down over generations was lost.

This knowledge is being recreated via close collaborations between research scientists, musicians, and organ builders throughout Europe. Dozens of metallic samples taken from 17th- to 19th-century organ pipes have been investigated to determine their composition, microstructure, properties, and manufacturing processes using sophisticated methods of materials science. Based upon these data, technologies for casting, forming, hammering, rolling, filing, and annealing selected leadtin pipe alloys and brass components for reed pipes have been reinvented and customized to reproduce those from characteristic time periods and specific European regions. The new materials recreated in this way are currently being processed and used by organ builders for the restoration of period organs and the manufacture of new organs with true Baroque sound.