Tabletop technology is helping researchers to develop better-quality materials for transforming light energy into electricity (semiconductors), such as solar cells. Dr Aditya Sadhanala from the University of Cambridge has created the device involved, using readily available, affordable technology to detect minute defects in materials with extremely high levels of accuracy.
The device is dubbed the Mirage Maker but the official, less glamorous, name for it is a photo-thermal deflection spectrometer (PDS). Sadhanala is the first to admit that the set-up looks slightly ramshackle – he winds a pulley to lift an MDF box to reveal lasers, deflectors and sensors that surround a piece of glass no bigger than a contact lens. However, appearances can be deceiving, and the accuracy of the device – the only one of its kind in the UK – has seen materials researchers from around the globe send samples to be tested at the university.
“Essentially we make mirages, but we make them in the lab,” says Sadhanala. “We use it as a sensitive thermometer which can measure very low changes in temperature. You’re talking about putting six to nine zeros in after a decimal point – that sensitive. That is why we need a mirage to do that – a normal thermometer doesn’t give that
level of sensitivity.”
The kind of mirages that Sadhanala and his colleagues create are smaller than the heat-wave effects you’d see on hot tarmac from afar on a hot summer’s day, but the process is the same. A mirage forms when the road ahead is hot but the surrounding air is colder, causing the light rays travelling at a shallow angle towards your eyes to refract and create an illusion, such as a pool of water on the ground.
“That illusion is because of the temperature difference. We use the same concept. We go for heating of the material created by light absorption,” says Sadhanala.
By turning on a powerful white lamp aimed through a monochromator, a miniature mirage is created no more than the width of a human hair in distance from the small disc of glass holding the material sample, which is suspended in a liquid. This effect occurs because the light absorbed by the solar material is released as heat and interacts with the cold air surrounding it, causing refraction.
Sadhanala can select one ‘colour’, or wavelength, at a time for heightened precision and to carry this through an optic fibre to shine on the sample.
“When the light is shone on the sample, it tends to absorb as much as it can of that particular colour,” he says. “When it absorbs that light, there are two physical processes that can happen. The absorbed light creates some charges in the sample and those charges can recombine after you switch off the light.” There are two kinds of recombination: one is radiative and results in photo-luminescence; the second is a non-radiative recombination that produces heat.
The heat is what Sadhanala measures, determining how much the sample is heating up for each wavelength that passes through it. The team uses mirages instead of using costly, sensitive thermometers, he says. “If the sample gets hot and there is a heat gradient around it, then we will have a refractive index change and the laser will be veered off its spot. We measure that deviation, and we can do it very sensitively.”
Plotting absorption profile
Sadhanala and his team can then change the kind of wavelength they direct at the material sample, measure the deviation in the laser beam for each wavelength, and plot the data on a computer to get the absorption profile of the sample.
Sadhanala uses the PDS to look for very low absorptions – compared with standard spectrometer systems – in semiconductors that come from defects, or dopant molecules. He can also spot special states that arise from mixing two different semiconductors, which wouldn’t show up in normal spectrometers.
Using this technique, the team can measure with up to five orders of magnitude more sensitivity than those obtained with traditional methods – other spectrometers achieve, at most, around two orders of magnitude. “You are not doing a fancy, low-temperature measurement; it is done at room temperature. The only difference is the concept,” he says. “It makes it more powerful. It is really simple.”
The method’s simplicity is its greatest advantage over other methods; there is no need for complex measurements or interpretation of the data. “Before, we would have had to make a whole solar device and spend months testing it to find out how efficient the material is,” says Sadhanala. “Now you can measure a new material in half a day, and you don’t even need to make the whole device – you just need to coat the material onto glass and make a mirage.” Simple measurements, he adds, can even take as little as half an hour.
The PDS has already helped researchers at Imperial College London to make a breakthrough in their materials research into solution processable polymers. Polymers have low-to-decent moderate charge mobility, meaning the speed at which electrical charges move in the material is slow compared with standard inorganic materials used in electronics. “If we want solution processor printable materials to match current electronics, we need to make them better in terms of the speed at which they operate. Otherwise, we’ll have things such as slowly operating smartphone screens,” he says.
Usually, organic materials such as polymers are high in defects, which means they cannot conduct electricity as well as inorganic materials. However, Imperial College came across a polymer that performed better than most. Testing with the PDS confirmed that it had a very low defect state density, resulting in its very high mobility. This discovery has led to a host of other researchers around the country focusing their attention on this class of material, which has since been used for transistors, electronics and solar cells.
“Now the properties are matching the inorganic semiconductors in electronics,” says Sadhanala. “Having a solution process printable polymer gives you an advantage in making flexible electronics. That’s where all these things will go, as you want low-cost manufacturing.”
Another area where the technology is proving useful is solar cell research, which badly needs to improve the mobility and conductivity of its semiconductors.
“For solar cells you need semiconductors that have very few defects because defects mean you lose your current and therefore the energy you want to get out of the technology,” he says. “The PDS acts as a fast screening tool to narrow down on the better-performing materials, rather than having to build entire devices and conduct costly, time-consuming experiments in search of defects.”
Dr Marcus Boehm, researcher in the optoelectronics group at the University of Cambridge, says: “The PDS became a key technique that we’ve been using for our solar cell research over the past three years.” Current solar cells, made from materials such as silicon and gallium arsenide, can operate at around 25% photo-conversion efficiency, he says. However, there is a cap on the light-to-electricity efficiency of 33%.
Current solar cells work relatively efficiently if they have no sub-band gap states – also called energy gaps. These sub-band gap states originate mainly from material defects. “When we hit the semiconductor with a photon, we excite the electrons sitting in the valence band up into the conduction band,” says Boehm. “This creates charges that we then want to extract in the solar cell as electricity.”
Diverted energy
However, if there are many of these sub-band gap states, the electrons in the conduction band can move slowly back down to the valence band. In most materials this is an undesirable outcome, as the gained photon energy is used to heat up the solar cell, and not to create electricity.
But now, using the PDS, Boehm and his team can quantify how many of these sub-band gap states are formed. Normally they would have used an ultraviolet-visible absorption spectrometer, but this is a much rougher way to determine these states and is prone to problems such as scattering.
“With the PDS, we are not limited on the scattering effects because it measures second-order heat effects and not first-order light-matter interactions,” he says. “With that, we can get sensitive readings. We can detect the smallest amount of acceptor states in the bulk: islands where the energy losses are most severe. As far as I know, it is the only method to really quantify what holds solar cells back.”
All the information that Boehm’s team get from the PDS is fed back to the chemistry labs, where researchers synthesise quantum dots or organic materials for solar cell applications, which then may not be held back by such sub-band gap states.
While the measurement to analyse band gaps is relatively easy to carry out, before the team had the PDS they lacked a convenient tool to do so, says Boehm. “To measure these states is labour-intensive and expensive. We would have to use big, powerful lasers that cost millions, or would have to go to the synchrotron radiation facilities. However, even the synchrotron doesn’t always get the core features that you get out of a PDS.
“The great thing is that the PDS is basically a one-man show. Aditya can measure a sample in a couple of hours, and then we have a clearer picture on the material quality. It is speeding up the process; it is cheaper; it doesn’t take up a lot of room. It is a neat technique.”