Harnessing the power of the sun to replace the use of fossil fuels holds tremendous promise. One way to do this is through the use of solar, or photovoltaic, cells. Large-scale installation already show the technical feasibility of this technology although the major problem of photovoltaic solar energy - its relative inefficiency - still needs to be overcome to make the cost of electricity produced by solar cells equal or less than electricity produced by nuclear or fossil fuels. Until now, solar cells that convert sunlight to electric power have been dominated by solid state junction devices, often made of silicon wafers. Efforts are being made in laboratories worldwide to design ordered assemblies of semiconductor nanostructures, metal nanoparticles and carbon nanotubes for constructing next generation solar energy conversion devices. Quantum dots have been identified as important light harvesting material for building highly efficient solar cells. Quantum dots are nanoscale semiconductor structures which, when exposed to light at certain wavelengths, can generate free electrons and create an electrical current. Quantum dot technology represents an exciting field of research in solar energy yet the actual research results to use them in solar cells are relatively limited. By combining spectroscopic and photoelectrochemical techniques, researchers now have demonstrated size-dependent charge injection from different-sized cadmium selenide (CdSe) quantum dots into titanium dioxide nanoparticles and nanotubes, showing a way to maximize the light absorption of quantum dot-based solar cells. Termed 'rainbow solar cells', these next-generation solar cells consist of different size quantum dots assembled in an orderly fashion. Just as a rainbow displays multiple colors of the visible light spectrum, the 'rainbow solar cell' has the potential to simultaneously absorb multiple wavelengths of light and convert it to electricity in a very efficient manner.
Nanotechnology is already part of the process through which silicon-based photovoltaic solar cells - which make up some 95 percent of the solar cell market today - produce electricity. Nanotechnology is also the focus of research and development of a new generation of solar power technology that includes organic and inorganic solar cells derived from nanocrystals that can convert sunlight into electricity at a fraction of the cost of silicon solar cells. Silicon-based solar cells are made from a refined, highly purified silicon crystal, similar to those used in the manufacture of integrated circuits and computer chips. The high cost of these silicon solar cells and their complex production process has generated interest in developing alternative photovoltaic technologies. Compared to silicon-based devices, polymer solar cells are lightweight (which is important for small autonomous sensors), disposable, inexpensive to fabricate, flexible, designable on the molecular level, and have little potential for negative environmental impact. These solar nanocells are so small and pliable that they can be painted onto physical structures so that the windows and walls of a building may one day soon be able to generate electricity. The big question today is to what degree polymer solar cells will be able to commercially compete with silicon solar cells. There are two major issues that need to be solved: 1) The present efficiency of organic solar cells lies at only around 6 percent, compared to up to 30 percent for the most efficient silicon cells. 2) Polymer solar cells suffer from huge degradation effects: the efficiency is decreased over time due to environmental effects such as water, oxygen or UV rays.
Titanium oxide - due to its versatile optical, electrical and photochemical properties, its relative abundance and low cost, and its non-toxicity - is an important ceramic material with numerous applications as pigments; powders for catalytic or photocatalytic applications; colloids and thin films for photovoltaic, electrochromic, photochromic, electroluminescence devices and sensors; components for antireflecting coatings; or porous membranes for ultrafiltration. Nanocrystalline titania has become a prominent material for dye-sensitized solar cells (DSSCs, also known as 'Grätzel cells' after their inventor), which are photoelectrochemical cells that use photo-sensitization of wide-band-gap mesoporous oxide semiconductors. One major problem with the use of titania in solar cells is that its bandgap does not match that of visible light and titania therefore can only absorb 3-4% of the energy from sunlight. Grätzel cells decrease the bandgap of titania by using dye-absorbed TiO2 nanocrystals as one of the electrodes, resulting in a higher solar energy conversion of 10% or more. Other methods use doping and indeed the application of nitrogen-doped titania as photocatalyst has received increasing attention over the last years because N-doping is found to be particularly effective in decreasing the bandgap of anatase (many of the properties of titania depend on the structure of the TiO2 phase - mainly anatase, brookite and rutile). In order for photocatalysis-based applications to become commercially viable, it will be critical to design low-cost, reproducible, synthetic methods that yield controlled, reproducible, and easy-to-handle nanomaterials processed as coatings with high surface area and high porosity. Researchers in France and Spain now describe for the first time nanostructured coatings that fulfill all these requirements.
OLEDs - organic light-emitting diodes - are full of promise for a range of practical applications not too far into the future. Today, OLEDs are used in small electronic device displays in mobile phones, MP3 players, digital cameras, etc. With more efficient and cheaper OLED technologies we soon will see ultraflat, very bright and power-saving OLED televisions, windows that could be used as light source at night, and large-scale organic solar cells. In contrast to regular LEDs, the emissive electroluminescent layer of an OLED consists of a thin-film of organic compounds. What makes OLEDs so attractive is that they do not require a backlight to function. Thus they draw far less power and, when powered from a battery, can operate longer on the same charge. OLED devices can be made thinner and lighter than comparable LED devices. Last but not least, OLEDs can be printed onto almost any substrate with inkjet printer technology, making new applications like displays embedded in clothes or roll-up displays possible. Unfortunately there are also drawbacks to this technology. Apart from its currently high manufacturing cost, the major problem is device degradation and the limited lifetime of organic materials. In particular, the most commonly used material for the anode, ITO (indium tin oxide), is a less than optimal material for future high-performance OLEDs. New research indicates that nanoimprinted semitransparent metal electrodes, replacing ITO electrodes, are an attractive and potentially practical solution for OLEDs and other organic devices.
Nanowires have received a great deal of interest in recent years. As quasi one-dimensional systems they may overcome the fundamental difficulty in the electronic transport encountered when attempting to use quasi zero-dimensional structures (quantum dots) while still being able to take advantage of the effects related to quantum confinement. In a conventional semiconductor, electrons and holes typically stay in the same region after photoexcitation, which is very desirable for certain applications, light-emitting devices for instance. However, for a number of key renewable energy applications, including hydrogen generation via photoelectrochemical water splitting, dye-sensitized solar cells, and even regular solar cells, the efficient charge separation of the electron and hole after photoexcitation is instead highly preferred, although not readily available in the existing repertory of materials. In this context, researchers have explored the use of core-shell nanowires for such applications. However, for the material combinations that have been investigated, the energy alignments between the core and the shell are typically type I, and thus, the role of the shell is to either provide quantum confinement to both electrons and holes in the core or a protective cladding to the core to reduce sensitivity to the environment. Researchers at the National Renewable Energy Laboratory and Lawrence Berkeley National Laboratory designed a new class of core-shell semiconductor nanowires with strong type II band alignments. The photo-generated electron and hole in such nanowires are naturally separated in the core and shell, respectively, making them much more suitable for the solar energy applications than type I core-shell nanowires.
The photocatalytic activity of nanostructured semiconductor films has been widely explored in designing solar cells, solar hydrogen production, and environmental remediation. Of particular interest is the dye-sensitized solar cell (DSSC) which uses nanostructured titanium dioxide films modified with sensitizing dyes. Despite the initial success of achieving 10% solar conversion efficiency, the effort to further improve their performance has not been very successful. A major hurdle in attaining higher photoconversion efficiency in such nanostructured electrodes is the transport of electrons across the particle network. The photogenerated electrons in nanostructured films for example have to travel through the network of semiconductor particles and encounter many grain boundaries during the transit. Such a random transit path for the photogenerated electrons increases the probability of their recombination with oxidized sensitizer. With the recent advance in the design of nanotube and nanowire architecture, it should be possible to use such one-dimensional nanostructures to direct the flow of photogenerated charge carriers. The obvious challenge is to use nanowire or nanotube networks as support to anchor light-harvesting semiconductor particles and facilitate the electron transport to the collecting electrode surface in a solar cell. Researchers now have demonstrated that single wall carbon nanotube (SWCNT) architecture when employed as conducting scaffolds in a titanium dioxide semiconductor based photoelectrochemical cell can boost the photoconversion efficiency by a factor of 2.
With an increased focus on alternative sources of cheap, abundant, clean energy, solar cells are receiving lots of attention. Harnessing the power of the sun to replace the use of fossil fuels holds tremendous promise. One way to do this is through the use of solar, or photovoltaic, cells. Until now, solar cells that convert sunlight to electric power have been dominated by solid state junction devices, often made of silicon wafers. Thanks to nanotechnology, this is now being challenged by the development of a new generation of solar cells based on thin film materials, nanocrystalline materials and conducting polymeric films. These offer the prospects of cheaper materials, higher efficiency and flexible features. This has opened up new opportunities in solar cell research and development and, consequently, there is considerable investor interest in solar nanotechnology startups. Both inventors and investors are betting that flexible sheets of solar cells used to harness the sun's strength will ultimately provide a cheap and efficient source of energy.
A group of Chinese researchers prepared dye-sensitized solar cells using micro/nanocomposite TiO2 porous films, resulting in cells with enhanced light collection. They applied a technique which can produce a large area in continuous fabrication. This technique opens an alternative way for manufacturing solar cells on an industrial scale.