----------------- #1212 - How to Make Solar Cells more Efficient?
- Photovoltaic solar cells are still too expensive for the average homeowner. A 10 year payback on your electric bill is too much to justify the upfront costs. But, today’s commercial solar cells are only about 15% efficient. With 1000 watts of sun power incident on the solar panel only 150 watts of electric power is output. The average homeowner uses about 5,000 watt hours. So he needs 33 solar panels on the roof. At an installed cost of $1,000 per panel that is $30,000. Or, if you just cut your PG&E electric bill in half the cost is $15,000. Still a 10 year payback on your electric bill. I have some people brag that they pay no PG&E electric bill at all. Yeah, but for what you paid for that installation it will take you 30 years to get your money back. They did not understand that. They thought paying nothing was a good deal. Please teach math and science in public schools.
- Dr. Zongfu Yu of Stanford University presented some of the technology improvements being studied that would make solar cells cheaper and more efficient. 10-18-10 lecture at SSU “Nanophotonic Light-trapping Solar Cells”. The technology focused on increasing the amount of photon absorption and increasing the electric output per cell.
- The way that a silicon oxide, SiO2, substrate works is that a light photon with a wavelength from 400 to 1200 nanometers incident on the surface is absorbed by the silicon. The photon energy knocks out an electron from the atom creating a free electron and a hole, equivalent to a positive electron. The electron travels down the semiconductor’s energy gap to the n-doped material and negative electrode. The hole travels up the energy gap to the p-doped material and the positive electrode The electron and hole are separated by the voltage potential between the “n” substrate and the “p” substrate. A conductor connected between them will carry a direct current. Solar cells produce direct current. An inverter is added to a home installation to convert the DC to AC for use in the home. The inverter lasts about 10 years and cost $2,000. However, if the solar panels are used to charge batteries, say in an electric car, then no inverter is needed.
- If you plot the light absorption with frequency you will find that the amount of absorption decays from 600 to 1200 nanometers. The goal of a good design is to shift that decay to the right so that absorption continues into the lower infrared frequencies. This can be accomplished by making the silicon substrate thicker. But, the free electron will only travel about 200 nanometers before it recombines with a hole and creates heat instead of electricity. So, make the substrate thinner. The best trade off between thicker and thinner is a function of the refraction index of silicon. The formula for the tradeoff is:
------------------- Absorption = 4*n^2*d = 10
------------------ d is the thickness
------------------ n is the refraction index
----------------- The theoretical limit for silicon is 4n^2d = 50
- Another approach is to create light scattering inside the substrate to cause more absorption inside the cell. This can be done by texturing the surface of the substrate or putting a reflective mirror on the substrate. By putting two mirrors on the substrate you can reflect the photons back and forth increasing the amount of absorption and thus the efficiency of the cell.
- To maximize this light trapping effect of reflective surfaces on the substrate the wavelength separation of the surfaces must match the resonant frequency of the photons. At one particular frequency the resonance curve peaks at that frequency and falls off at the higher and lower frequencies. So, a good technique is to have more than one resonance frequency. To do this a thick layer of amorphous silicon is mounted to a thin layer of crystalline silicon. This has the effect of broadening the resonance curve over a wider range of frequencies. The best broadening occurs when the reflective index of the substrate is much greater then the absorption index of the material.
- Nanotechnology can greatly increase the absorption index by constructing nano-particles of tiny pyramids as the textured surface on the substrate. The silicon surface turns from an shiny blue mirror surface to a totally deep black as all of the light is absorbed. These nano-pyramids greatly improve the amount of photon absorption.
- Another efficiency improvement considers the fact that the Sun arcs across the solar panel from sunrise to sunset. The reflection angle of maximum absorption occurs at high noon. By constructing a half dome lens over the cell the reflection angle can be focused on the center of the cell throughout the day. The period of repeating the half domes across the surface is optimized at 500 nanometers.
- Amorphous silicon is much cheaper to produce than crystalline silicon. The ultimate goal of fabrication would have these layers of transparent half domes -------- nanowire electrodes --------- 200 nanometer amorphous-silicon thick substrate --------- nanoparticle textured surface ------ 5 nanometer crystalline-silicon thin substrate ----- nanowire electrode ----- reflective mirror ---------- solar cells all rolling off in flexible sheets like paper from a printing press.
- Such fabrications have been made in the lab increasing absorption from 4n^2 = 10 to 60n^2 = 119. A ten fold improvement in efficiency. Going from the lab to full production needs a lot of science and engineering. However, super efficient solar cells are in our future. When nanotechnology gets perfected every house will have a solar cell roof, or solar cells windows that are transparent but producing electricity, or solar cell paint, or, who knows what nanotechnology breakthroughs will come from those science and math students that are in schools today.
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home: 707-539-6291 , mobile 536-3272, [email protected] Friday, October 22, 2010