Flexible solar cell implant could restore vision

The first flexible retinal implant could restore some vision to people with certain forms of visual impairment.

Conditions such as age-related macular degeneration occur when some of the photoreceptors in the eye stop functioning properly. But as other parts of the eye still work, it should be possible to restore vision using an implant that mimics the photoreceptor layer, says Rostam Dinyari at Stanford University in California.

To achieve this, an implant needs to convert a light signal into an electrical pulse – in other words, perform like a solar cell.

But most solar cells are rigid, which makes them far from ideal for use inside the eye. "If you have a lens, the focal plane is always curved and the best picture forms on a spherical surface," Dinyari says. This is why the retina is curved.

Rigid implants

Using rigid chips, a large number of small implants must be fitted in order to approximate the curve of the retina. A flexible implant would simplify matters.

"You would need a lot of surgery to implant a large enough number [of rigid implants] to cover the retina," says Dinyari. A flexible implant "would use just one surgical procedure".

While several companies are developing rigid implants, Dinyari and colleagues have designed a flexible silicon implant. They did so by carving deep grooves into the silicon between adjacent solar cell pixels that are each just 115 micrometres across.

The implant would be inserted over the most damaged part of the retina. A glasses-mounted camera would capture video, convert it to near-infrared signals and project it directly onto the implant.

Projecting images

When hit by the light, the solar cells inject current patterns corresponding to the projected images into neural tissue, which ultimately arrive at the visual cortex via the optic nerve. Near-infrared signals are used as they do not interfere with the surrounding intact photoreceptor cells, which send signals to the brain as normal.

Initial trials using retinas extracted from pigs showed that the implant could be inserted without damaging the fragile solar cell array. The team hope to implant the device into a live pig soon, before testing it in humans.

Jason Dowling at the Australian eHealth Research Centre in Herston, Queensland, thinks the approach is interesting. "To the best of my knowledge I think this is the first implant which is shaped to the curved surface and this [approach] makes a lot of sense," he says.

Dinyari presented his work at the 2009 IEDM conference in Baltimore, Maryland, last week.

Thin film solar cells

The high cost of crystalline silicon wafers (they make up 40-50% of the cost of a finished module) has led the industry to look at cheaper materials to make solar cells.

The selected materials are all strong light absorbers and only need to be about 1micron thick, so materials costs are significantly reduced. The most common materials are amorphous silicon (a-Si, still silicon, but in a different form), or the polycrystalline materials: cadmium telluride (CdTe) and copper indium (gallium) diselenide (CIS or CIGS).

Each of these three is amenable to large area deposition (on to substrates of about 1 meter dimensions) and hence high volume manufacturing. The thin film semiconductor layers are deposited on to either coated glass or stainless steel sheet.

The semiconductor junctions are formed in different ways, either as a p-i-n device in amorphous silicon, or as a hetero-junction (e.g. with a thin cadmium sulphide layer) for CdTe and CIS. A transparent conducting oxide layer (such as tin oxide) forms the front electrical contact of the cell, and a metal layer forms the rear contact.

Thin film technologies are all complex. They have taken at least twenty years, supported in some cases by major corporations, to get from the stage of promising research (about 8% efficiency at 1cm2 scale) to the first manufacturing plants producing early product.

Amorphous silicon is the most well developed of the thin film technologies. In its simplest form, the cell structure has a single sequence of p-i-n layers. Such cells suffer from significant degradation in their power output (in the range 15-35%) when exposed to the sun.

The mechanism of degradation is called the Staebler-Wronski Effect, after its discoverers. Better stability requires the use of a thinner layers in order to increase the electric field strength across the material. However, this reduces light absorption and hence cell efficiency.
his has led the industry to develop tandem and even triple layer devices that contain p-i-n cells stacked one on top of the other. In the cell at the base of the structure, the a-Si is sometimes alloyed with germanium to reduce its band gap and further improve light absorption. All this added complexity has a downside though; the processes are more complex and process yields are likely to be lower.

In order to build up a practically useful voltage from thin film cells, their manufacture usually includes a laser scribing sequence that enables the front and back of adjacent cells to be directly interconnected in series, with no need for further solder connection between cells.

As before, thin film cells are laminated to produce a weather resistant and environmentally robust module. Although they are less efficient (production modules range from 5 to 8%), thin films are potentially cheaper than c-Si because of their lower materials costs and larger substrate size.

However, some thin film materials have shown degradation of performance over time and stabilized efficiencies can be 15-35% lower than initial values. Many thin film technologies have demonstrated best cell efficiencies at research scale above 13%, and best prototype module efficiencies above 10%. The technology that is most successful in achieving low manufacturing costs in the long run is likely to be the one that can deliver the highest stable efficiencies (probably at least 10%) with the highest process yields.

Amorphous silicon is the most well-developed thin film technology to-date and has an interesting avenue of further development through the use of "microcrystalline" silicon which seeks to combine the stable high efficiencies of crystalline Si technology with the simpler and cheaper large area deposition technology of amorphous silicon.

However, conventional c-Si manufacturing technology has continued its steady improvement year by year and its production costs are still falling too.

The emerging thin film technologies are starting to make significant in-roads in to grid connect markets, particularly in Germany, but crystalline technologies still dominate the market. Thin films have long held a niche position in low power (<50W) and consumer electronics applications, and may offer particular design options for building integrated applications.

Crystalline silicon solar cells

Historically, crystalline silicon (c-Si) has been used as the light-absorbing semiconductor in most solar cells, even though it is a relatively poor absorber of light and requires a considerable thickness (several hundred microns) of material. Nevertheless, it has proved convenient because it yields stable solar cells with good efficiencies (11-16%, half to two-thirds of the theoretical maximum) and uses process technology developed from the huge knowledge base of the microelectronics industry.

Two types of crystalline silicon are used in the industry. The first is monocrystalline, produced by slicing wafers (up to 150mm diameter and 350 microns thick) froma high-purity single crystal boule. The second is multicrystalline silicon, made by sawing a cast block of silicon first into bars and then wafers. The main trend in crystalline silicon cell manufacture is toward multicrystalline technology.

For both mono- and multicrystalline Si, a semiconductor homojunction is formed by diffusing phosphorus (an n-type dopant) into the top surface of the boron doped (p-type) Si wafer. Screen-printed contacts are applied to the front and rear of the cell, with the front contact pattern specially designed to allow maximum light exposure of the Si material with minimum electrical (resistive) losses in the cell.

The most efficient production cells use monocrystalline c-Si with laser grooved, buried grid contacts for maximum light absorption and current collection.

Some companies are productionizing technologies that by-pass some of the inefficiencies of the crystal growth/casting and wafer sawing route. One route is to grow a ribbon of silicon, either as a plain two-dimensional strip or as an octagonal column, by pulling it from a silicon melt.

Another is to melt silicon powder on a cheap conducting substrate. These processes may bring with them other issues of lower growth/pulling rates and poorer uniformity and surface roughness.

Each c-Si cell generates about 0.5V, so 36 cells are usually soldered together in series to produce a module with an output to charge a 12V battery. The cells are hermetically sealed under toughened, high transmission glass to produce highly reliable, weather resistant modules that may be warrantied for up to 25 years

How to Build Your Own Solar Cell

Step 1 -
Stain the Titanium Dioxide with the Natural Dye: Stain the white side of a glass plate which has been coated with titanium dioxide (TiO). This glass has been previously coated with a transparent conductive layer (SnO), as well as a porous TiOfilm. Crush fresh (or frozen) blackberries, raspberries, pomegranate seeds, or red Hibiscus tea in a tablespoon of water. Soak the film for 5 minutes in this liquid to stain the film to a deep red-purple color. If both sides of the film are not uniformly stained, then put it back in the juice for 5 more minutes. Wash the film in ethanol and gently blot it dry with a tissue.


Step 2 -
Coat the Counter Electrode: The solar cell needs both a positive and a negative plate to function. The positive electrode is called the counter electrode and is created from a "conductive" SnO coated glass plate. A Volt - Ohm meter can be used to check which side of the glass is conductive. When scratched with a finger nail, it is the rough side. The "non-conductive" side is marked with a "+." Use a pencil lead to apply a thin graphite (catalytic carbon) layer to the conductive side of plate's surface.

Steps 3 & 4 -
Add the Electrolyte and Assemble the Finished Solar Cell: The Iodide solution serves as the electrolyte in the solar cell to complete the circuit and regenerate the dye. Place the stained plate on the table so that the film side is up and place one or two drops of the iodide/iodine electrolyte solution on the stained portion of the film. Then place the counter electrode on top of the stained film so that the conductive side of the counter electrode is on top of the film. Offset the glass plates so that the edges of each plate are exposed. These will serve as the contact points for the negative and positive electrodes so that you can extract electricity and test your cell.
Use the two clips to hold the two electrodes together at the corner of the plates.

The output is approximately 0.43 V and 1 mA/cm2 when the cell is illuminated in full sun through the TiO side.

Solar Cells

Solar cells (as the name implies) are designed to convert (at least a portion of) available light into electrical energy. They do this without the use of either chemical reactions or moving parts.

History
The development of the solar cell stems from the work of the French physicist Antoine-César Becquerel in 1839. Becquerel discovered the photovoltaic effect while experimenting with a solid electrode in an electrolyte solution; he observed that voltage developed when light fell upon the electrode. About 50 years later, Charles Fritts constructed the first true solar cells using junctions formed by coating the semiconductor selenium with an ultrathin, nearly transparent layer of gold. Fritts's devices were very inefficient, transforming less than 1 percent of the absorbed light into electrical energy.

By 1927 another metalÐsemiconductor-junction solar cell, in this case made of copper and the semiconductor copper oxide, had been demonstrated. By the 1930s both the selenium cell and the copper oxide cell were being employed in light-sensitive devices, such as photometers, for use in photography. These early solar cells, however, still had energy-conversion efficiencies of less than 1 percent. This impasse was finally overcome with the development of the silicon solar cell by Russell Ohl in 1941. In 1954, three other American researchers, G.L. Pearson, Daryl Chapin, and Calvin Fuller, demonstrated a silicon solar cell capable of a 6-percent energy-conversion efficiency when used in direct sunlight. By the late 1980s silicon cells, as well as those made of gallium arsenide, with efficiencies of more than 20 percent had been fabricated. In 1989 a concentrator solar cell, a type of device in which sunlight is concentrated onto the cell surface by means of lenses, achieved an efficiency of 37 percent due to the increased intensity of the collected energy. In general, solar cells of widely varying efficiencies and cost are now available.

Structure
Modern solar cells are based on semiconductor physics -- they are basically just P-N junction photodiodes with a very large light-sensitive area. The photovoltaic effect, which causes the cell to convert light directly into electrical energy, occurs in the three energy-conversion layers.
The first of these three layers necessary for energy conversion in a solar cell is the top junction layer (made of N-type semiconductor ). The next layer in the structure is the core of the device; this is the absorber layer (the P-N junction). The last of the energy-conversion layers is the back junction layer (made of P-type semiconductor).

As may be seen in the above diagram, there are two additional layers that must be present in a solar cell. These are the electrical contact layers. There must obviously be two such layers to allow electric current to flow out of and into the cell. The electrical contact layer on the face of the cell where light enters is generally present in some grid pattern and is composed of a good conductor such as a metal. The grid pattern does not cover the entire face of the cell since grid materials, though good electrical conductors, are generally not transparent to light. Hence, the grid pattern must be widely spaced to allow light to enter the solar cell but not to the extent that the electrical contact layer will have difficulty collecting the current produced by the cell. The back electrical contact layer has no such diametrically opposed restrictions. It need simply function as an electrical contact and thus covers the entire back surface of the cell structure. Because the back layer must be a very good electrical conductor, it is always made of metal.

Operation
Solar cells are characterized by a maximum Open Circuit Voltage (Voc) at zero output current and a Short Circuit Current (Isc) at zero output voltage. Since power can be computed via this equation:

P = I * V
Then with one term at zero these conditions (V = Voc / I = 0, V = 0 / I = Isc ) also represent zero power. As you might then expect, a combination of less than maximum current and voltage can be found that maximizes the power produced (called, not surprisingly, the "maximum power point"). Many BEAM designs (and, in particular, solar engines) attempt to stay at (or near) this point. The tricky part is building a design that can find the maximum power point regardless of lighting conditions.

Passive Solar House Plans

We offer a wide array of solar house plans and energy efficient home plans. Our solar home plans were created by architects who are well known and respected in the passive solar community. Properly oriented to the sun, homes built from passive solar house plans require much less energy for heating and cooling.


Common Characteristics of Passive Solar Home Plans:
Refers to both design (Passive) or construction (Active) features
Passive Solar orients home to take advantage of site
Takes advantage of clean, healthy and free solar energy
Proper design is warm in winter and cool in summer
Abundant windows on south side typical
Operable windows to control heat gain

How Does Solar Energy for Home Use Work?

One nice thing about the advancement of technology is how things that were expensive within the past, become quite affordable as years go by. A perfect example of this is panels that convert solar energy for home use, or solar panels. These solar panels were quite expensive, but since the value has dramatically decreased, more and more people have considered cashing in or investing on this kind of energy.

Although solar electrical systems have been around for quite some time already, not everybody knows exactly how solar panels work and convert sunlight into electricity. The science behind it is complicated, but the method, however, is amazingly simple. What is commonly known as solar panels are "photovoltaic" panels, which means electricity from light. These panels contain plates that are coated with a substance that reacts with the collected sunshine, and creates a flow of electrons. The electricity is then converted by an inverter within the system to Direct Current (DC), which makes it safe to be used at home. The exceedingly solar electric system creates electricity, or solar energy for home, can be used for just about anything. It can run your lights, power your tools, and operate appliances such as your TV and radio, basically anything that needs electricity.

When the sun goes down and the solar panels can no longer collect sunlight and produce solar energy for home use, there are still other options to choose from for electricity. You can either switch back to using the facility from your electrical company, or have storage units that uses batteries that continuously enables to supply your electrical needs. However, if the system is working properly and is able to manufacture electricity, some of that electricity is diverted to your storage units. This will keep the batteries charged and ready for use. So once the solar panels are not producing electricity, then the storage system kicks in and provides you with solar energy for home use.

The electricity that these solar panels provide is safe and fairly easy to use. Once you've learned the basic knowledge of exactly what it is and the way it works, then you are well on your way to breaking your dependence on fossil fuels for your electrical needs. So why not start to take advantage of the free energy from the sun, and start saving money from your monthly electricity bill? With the way our economy is going now and prices for our necessities are continuously increasing, then the solar energy for home use will surely benefit you. But also remember that this is not the only way you can cut down on your expenses. There are plenty more ways that you can take advantage of what our environment provides us.

Solar energy panels are becoming easier to find. Chances are, you can probably find a provider that is fairly close to where you live. If not, then the web is a valuable resource for finding suppliers and even helps you find the lowest rates.