To enable solar electricity from photovoltaics to be competitive with, or cheaper than, present fossil fuel electricity costs likely requires devices that operate above the existing performance limit of energy conversion efficiency of 32% calculated for single-junction cells. At present, the best single-junction solar cells have efficiencies of 2025%. New concepts, structures, and methods of capturing the energy from sunlight without thermalization of carriers are required to break through this barrier and enable solar cells having efficiencies of greater than 50%.
     Mature energy conversion technologies typically operate close to their maximum thermodynamic efficiency. For solar energy conversion, this efficiency is between 66% and 87%, depending on the concentration and the spectrum. A grand challenge for photovoltaics is the development of high-efficiency, low-cost photovoltaic structures that can reach these ultimate thermodynamic efficiency limits. Existing photovoltaic devices, which are based primarily on single-junction silicon, have made dramatic improvements over the 50 years of their development, and these solar cells now achieve about three-quarters of the Shockley-Queisser efficiency limit of ~32%. Discovering new technologies, processes, and materials that allow photovoltaic devices to substantially exceed this efficiency while maintaining low cost are critical research goals for photovoltaics.
     The viability of achieving these goals has been dramatically increased in the last few years due to the combination of theoretical and material advances, particularly improved understanding of materials and their interaction with growth and defects; and through new approaches, materials, and concepts relying on phenomena allowed by low-dimensional structures. Report on the Basic Energy Sciences Workshop
on Solar Energy Utilization. Chmn. S. LewisThe latter include approaches such as multiple junctions (tandems), optical spectrum shifting, multiple electron/exciton generation, multiple energy level solar cells, and hot carrier solar cells. Substantial scientific challenges exist in each of these approaches, relating to understanding, modeling, and controlling the basic physical mechanisms, as well as to incorporating these physical phenomena into highperformance solar cells (see right figure). The development of solar cells based on such principles would revolutionize photovoltaics by allowing high-efficiency, cost-effective solar cells, and further, contribute directly to fundamental scientific advances. Moreover, since many solar energy utilization technologies depend on the understanding and control of these physical phenomena, advances in such high-efficiency photovoltaic devices contribute directly toward enhanced understanding that underpins other solar conversion technologies, including organic and photochemical conversion as well as biologically based solar conversion systems. Several paths exist toward the realization of photovoltaic devices with efficiency greater than 50%, including multiple junction solar cells (tandems), solar cells using optical frequency shifting (such as up/down conversion or thermophotonics), multiple exciton generation (MEG) from a single photon, multiple energy level solar cells (such as intermediate band solar cells), and hot carrier solar cells (Marti and Luque 2003; Green 2004). In addition to high efficiency, such cells must also be low in cost, made of polycrystalline thin films grown on inexpensive substrates.
     The present photovoltaic market is dominated by "first generation" product based in silicon wafers, either single-crystalline as in microelectronics, or lower grade multicrystalline wafer. More exptnsive monocrystalline solar cells made on the base of heterojunctions. The second generation devices are thin film SC. The main aim of these cells low price is reaced by lowering their efficiency. The latest generations of solar cells use new physical phenomena in order to get high efficiency devices.
      The picture at the left shows the nanostuctured Si-based tandem cell. The material is engineered using a quantum dot nanostructure of silicon in silicon based dielectric matrix. The confined energy levels in quantum dots will increase the lowest absorption edge of the material compared to bulk silicon. If the quqntum dot (QD) density is high enough, the wavefunctions of quantum dots will overlap to create true superlattice minibands and increase effective band gap of the material. The main chalenge for a nanostructure engineered material is to acheave sufficient carrier mobility and hence a reasonable conductivity. For nanostructure, this generally requires formation of a supetlattice; which in turn requires either close spacing between QDs or QWs or low barrier height. Another requirement for a tandem cell element is the presence of some form of junction for carrier separation. This can either be a grown or diffised pn-junction or pin-junction, formed in superlattice as the i-region.
Transport properties are expected to depend on the marix in which the silicon quantum dots are embeded. As shown at the right figure? different matrices produce different transport barriers between Si dot and the matrix, with tunneling probability heavily dependent on barreiers between Si dot and the matrix, with tunneling probability heavily dependent on the height of the barrie: Si3N4 and SiC give lower barriers than SiO2 allow larger dot spacing for a given tunneling current. Transport between dots can be significantly increased as the barrier height decreases with alternative materials.
   The picture below shows TEM (medium resolution 120 kV) image of multylayer Si nanostructure in a-SiO2 matrix (a); YRTEM (at 300 kV) image of Si quantum dots - hoghlited by ellipses.
                   a                                       b
This equation gives probability (Te)of tunneling through the barrier. Here m* - effective mass of carrier in conduction band, ΔE is the energy difference between conduction band and the first confined minband energy level in the QD superrlattice.