Solar energy is undoubtedly one of the main candidates for our future renewable energy providers. The sun is essentially inexhaustible, and a very abundant source of energy: the rate of solar irradiation incident on the earth is 10 000 times greater than the rate at which people use energy . Solar cell technology - that is, technology for direct conversion of sunlight into electricity – is currently one of the fastest emerging technologies.
There is a wide variety of possible solar cell technologies, the most common still being a simple planar silicon (Si) solar cell based on the junction between p- and n-doped silicon – even though already in the 1970s it was thought that Si would eventually give way to other technologies with higher efficiency and less energy-demanding production. The drastic price reduction of the raw material for Si solar cells, and the entry of Chinese solar-cell manufacturers into the market, has however brought the price of Si solar panels rapidly down. This has been the death strike for most Norwegian solar cell producers – but it has of course been very positive for increasing the usage of solar panels on roof tops world wide. Nevertheless, a Si solar cell is far from being optimal, and better alternatives are constantly searched for.
The efficiency of a solar cell - defined as the ratio between the cell power output and the power of the incident irradiance - is limited by several factors. Firstly, the solar radiation consists of photons at different wavelengths, with a wavelength-intensity distribution known as the solar spectrum. Only photons with energy higher than or equal to a materials property called band gap energy can create electron-hole pairs to produce current in the solar cell; this effect alone limits the maximum (Si) solar cell efficiency to 44% . Secondly, not all the incident photons are absorbed by the material; a great deal of incident radiation is lost due to reflection. Thirdly, not all the generated electron-hole pairs contribute to the current: they might recombine before being swept apart by the electric field of the pn-junction. Considering all these effects, the maximum laboratory efficiency of a Si solar cell is limited to 24% .
'Nano' has been a magic work in science during the past decade – not least in the funding applications. But nanotechnology has certainly a lot to offer, also for solar cells, nanowire based solar cells being one option. Nanowires are a couple of micrometers long, in the order of 10-100 nm thick wires, which grow vertically up from a substrate. One common way to produce these wires is molecular beam epitaxy, in which case the growth occurs simply by heating up the substrate to a certain temperature, and exposing it to the constituent gases, which may be for instance gallium (Ga) and arsenic (As), or indium (In) and phosphorus (P). Sounds simple!
|One possible design for a solar cell based on GaAs nanowires .|