tirsdag 4. juni 2013

Say nano for better solar cells?

by Hanne Kauko, PhD Candidate at Department of Physics, NTNU

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 [1]. 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% [3]. 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% [3].

'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 [7]. 
Nanowire-based solar cells can beat the traditional, planar solar cells in many of the factors listed that limit the efficiency of conventional, planar solar cells. The vertical geometry of the nanowires enable light absorption superior to that of planar device: with optimal spacing, the nanowire array itself creates a light trap, and no anti-reflective coatings are needed. Furthermore, in nanowires the pn-junction can be created axially. In this case the electron-hole pairs are extracted radially across the nanowire, while photons are absorbed along the length of the nanowire. This reduces the recombination of electrons and holes. Finally, with the wire-like geometry, the material demand is significantly reduced. If a 1 cm2 large and 5 μm thick layer is instead made of 20 nm diameter nanowires with 20 nm spacing in between, the surface area becomes three times the original area, while using only one-fifth of the material [4]!

A group at NTNU is working hard towards a nanowire solar cell device based on GaAs nanowires (see http://www.iet.ntnu.no/projects/nanordsun/, a site for NaNordSun, a Nordic collaboration project for nanowire based solar cells).  GaAs has properties that make it a superior solar cell material compared to Si, and it has long been used to make high-efficiency solar cells for space applications. With thin film GaAs, 28.3% efficiency has been reached in the laboratory [5]. Ga and As are however much less abundant on earth than Si, hence the price of these high-efficiency solar cells is very high. GaAs nanowires could be a way to go to produce less costly, high-efficiency solar cells due to the reduced demand for the material. 

Alternatively, a group in Lund (also a part of the NaNordSun project) is working with InP nanowires and recently published a paper in Science reporting on an InP nanowire-based solar cell reaching 13.8% efficiency [6]. Not a bad start for a technology just taking its first steps! Even though GaAs might have better photovoltaic properties and yield higher efficiencies in the end, from environmental point of view InP might be the way to go: arsenic, and hence also the compound GaAs is considered highly toxic and dangerous for the environment. To conclude: nanowire-based solar cells - yes please, but please do some life-cycle analysis first.

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