The living world is solar-powered and people are looking for this unlimited source of power to be turn into their lifeline i.e., electricity in the form of solar electricity. People are looking for affordable clean electricity with solar-cell technology of distinctly superior cost efficiency, versatility, and availability. Solar cells development is almost stagnated in terms of their energy (light into electricity) conversion efficiency, rigidity, limitations with light frequencies and high cost.
In a nanotechnology era, an individual nanoelectronic device will indeed consume very little power, but to do something interesting will require many interconnected devices and thus the power requirement even for nanosystems can be a challenge. Two hundred billionths of a watt may not seem much, but at nanoscale it is enough to provide a steady output of electricity to run ultralow power electronics, including some that could be worn on or even inside the body.
The photocell operates by absorption of light particles in a medium that promotes electron-hole (or free electron- electron bond) separation. The well-designed photocell system has efficient absorption and then sweeps out the resulting carriers by making the collecting conductors ubiquitous. Otherwise, electrons may get trapped inside the semconductor, ending the process of electron liberation that would have provided useful current. It is the nanoscale presence of carrier collectors that represents the means to get efficient electron capturing in systems that otherwise would reabsorb much photo-generated current. This capture efficiency is what makes nanotechnology able to open wide the possibilities for material systems which can effectively convert sunlight to useful electrical power. Right now, silicon-based solar cells are expensive to make, and replacing the silicon with nanomaterials promises to lower costs. Several groups are exploring approaches to improve the collection of electrons within a cell, including forming titanium-oxide nanotubes or complex branching structures made of various semiconductors.
Recent science advances in nanostructured materials are helping in the development of high-yield high-throughput thin-film process technology based on:
- Flexible polymer-based photovoltaics
- Particle solar cells using nanotech
- Sprayable self-assembling photocells
This makes it possible to simply print solar cells that are efficient and durable, and fundamentally change the cost efficiency and volume availability of solar electricity. The nano solar cells could be rolled out, ink-jet printed, or even painted onto surfaces, so that any thing any where can become a solar collector. Companies have developed nano solar technology that makes it possible to simply roll-print high-performance thin-film solar cells -- that is, produce solar cells 100x thinner 100x faster. Nano technology dramatically lowers the materials cost and the process complexity involved in the production of solar cells and make it possible to scale production very rapidly. These developments set the standard for cost-efficient solar electricity. Here some different approaches towards the development of nano solar cells are highlighted.
Using Carbon Nanotubes
The approach, developed by Prof. Prashant Kamat, University of Notre Dame (USA) and his colleagues, addresses one of the most significant limitations of solar cells based on nanoparticles. Such cells are appealing because nanoparticles have a great potential for absorbing light and generating electrons. But so far, the efficiency of actual devices made of such nanoparticles has been considerably lower than that of conventional silicon solar cells. That's largely because it has proved difficult to harness the electrons that are generated to create a current. The researchers added single-walled carbon nanotubes to a film made of titanium-dioxide nanoparticles, doubling the efficiency of converting ultraviolet light into electrons when compared with the performance of the nanoparticles alone. Titanium oxide is a main ingredient in white paint. According to them, carbon nanotubes could help make nanoparticle-based solar cells more efficient and practical. Researchers have demonstrated a way to significantly improve the efficiency of solar cells made using low-cost, readily available materials, including a chemical commonly used in paints. Indeed, without the carbon nanotubes, electrons generated when light is absorbed by titanium-oxide particles have to jump from particle to particle to reach an electrode. Many never make it out to generate an electrical current. The carbon nanotubes "collect" the electrons and provide a more direct route to the electrode, improving the efficiency of the solar cells. The nanotubes serve as a scaffold on which the titanium-oxide particles are deposited.
The new carbon-nanotube and nanoparticle system is not yet a practical solar cell. That's because titanium oxide only absorbs ultraviolet light; most of the visible spectrum of light is reflected rather than absorbed. But researchers have already demonstrated ways to modify the nanoparticles to absorb the visible spectrum. In one strategy, a one-molecule-thick layer of light-absorbing dye is applied to the titanium-dioxide nanoparticles. Another approach, which has been demonstrated experimentally by Kamat, is to coat the nanoparticles with quantum dots--tiny semiconductor crystals. Unlike conventional materials in which one photon generates just one electron, quantum dots have the potential to convert high-energy photons into multiple electrons. The nanosize particles of titanium dioxide are spread out on a crystalline structure; this is then dipped into the colour agent. This process yields a very large active surface that may catch solar energy. Yet, as these surfaces are not completely flat, the molecular complexes may react in various ways with each other – this is what the scientists want to know more about.
In a step toward cheaper and more efficient solar cells, a new approach may be to use billions of nanowires, said researchers led by chemist Peidong Yang from the University of California, Berkeley (USA). The nanowires, about 60 nanometers in diameter and 20 micrometers in length made of zinc oxide and coated in a light-absorbing dye, conducted electrons from one end of the cell to the other about 100 times more efficiently than other nanoparticle-based solar cells currently under development. By replacing the nanoparticles with long single-crystal nanowires that run between the cell's electrodes, the researchers were able to get the electrons moving through the solar cell more efficiently. This advance could ultimately lead to more-efficient nano solar cells. Researchers made nanowire arrays by coating a conductive glass surface with zinc oxide dots three to four nanometers in diameter. The dots served as seeds for the subsequent growth of the wires and then immersed the glass in a solution of zinc oxide for two-and-a-half hours. A polymer in the solution controlled the rate and direction of the wires' growth, ensuring they remained perpendicular to the surface of the glass. Researchers dipped the array in a dye solution, placed the array between two electrodes, and filled the internal space with a liquid electrolyte. They then shone light with the same spectrum as sunlight onto the cells and measured the electrical output. While the cells' electron transport was better, their overall light conversion efficiency was low compared to that of other nanoparticle-based solar cells.
Zinc oxide harvests electrons from the dye less efficiently than titanium dioxide, a material more commonly used in nano solar cells. The researchers are now making nanowires out of titanium dioxide, a more challenging manufacturing process. The nanowires also have a smaller surface area than a network of nanoparticles, so they carry less light-absorbing dye. Researchers are consequently shrinking their nanowires to 10 nanometers in diameter so they can fit more nanowires onto their arrays and increase the total surface area. With thinner and more numerous titanium wires, it will be possible to achieve a conversion efficiency of 10% or more, which could make these nano solar cells a viable source of energy.