How the next generation of photovoltaics will soak up the light their predecessors can't see...
Sat, 14th Jul 2012
The World needs energy, and if we want to keep the climate stable, then it had better be clean energy. At the same time, the Sun is bathing us in thousands of times more[pic] energy than we currently consume, and we know how to turn the Sun's energy into electricity with solar cells.
But, the solar cells on the market today, although they are getting cheaper, are far from being as efficient as they could be in terms of their ability to turn light into electricity. In fact, the efficiency of the best cells is significantly less than 30%.
To address this problem of poor efficiency, we first need a crash course in what light is and how a solar cell works. Light consists of tiny energy particles called photons. For example, red light consists of “red” photons with a specific amount of energy. When these photons hit the solar cell, electrons inside the cell get knocked loose and, tapped off into a circuit, it's these electrons that give us the current.
But in order for a photon to knock loose an electron, it needs to have a minimum amount of energy. On the other hand, if the photon packs a bigger energy punch than this minimum, the surplus is just wasted as heat. And here lies the problem.
Sunlight contains photons with a wide range of energies. It can be divided into three parts. The visible light we can see; the high energy ultraviolet (UV) light that, amonst other things causes sunburn, and low-energy infrared (IR) light, which we experience as heat.
For silicon solar cells, red photons have just a bit more energy than is needed to knock loose an electron. Red photons are therefore used very efficiently by such silicon-based cells. On the other hand, blue and UV photons have much more energy and could knock loose two or even three electrons per photon respectively. But since we lose all the surplus energy from these high energy photons as heat, the efficiency is cut in half for blue light and cut to a third for UV light. Lastly, we have the low energy part of the sunlight. The IR photons, or heat rays, have less energy than red photons. Most of them don’t have enough energy to be used by the solar cell and pass through it like visible light passes through your house window.
So the efficiency of a silicon solar cell to convert red light into electricity is extremely good, even for poor cells. But the efficiency then drops steadily with increasing photon energy. At opposite end of the scale, the efficiency is zero for most of the IR part.
This is why the solar cells today are only around 20% efficient. Had the sun shone pure red instead of the full spectrum, the energy crisis would [pic]have been solved years ago! But what if we could just turn the sunlight red?
This is not as far-fetched as it might seem. In fact, all you need is something that absorbs the light that you want to change, and something that shines with the colour you want. And we have been doing this for decades. Fluorescent lights contain a UV light source (mercury atoms in a vapour inside the tube) and the white coating inside the glass converts the UV light into visible light. Optical brighteners used in laundry detergents and paper also utilise this effect, converting UV light into blue light to make your clothes appear shining white.
This is a one-to-one conversion (called down-shifting), turning one UV photon into one visible photon. But solar cells will not benefit much from doing this, because the extra energy is still being lost as heat. Instead what we need is a one-to-two conversion achieved by splitting one high energy photon into two medium energy photons (called down-conversion), and a two-to-one conversion by combining two low energy photons into one medium [pic]energy photon (called up-conversion). Up- and down- conversion are a lot trickier than...