Perovskites and quantum dots: New materials for solar cells and LED lights

Creating new materials that turn light into electricity—or electricity into light. That is, in a nutshell, what physicist Maria Antonietta Loi, Professor of Photophysics and Optoelectronics at the University of Groningen, does. She focuses on new types of semiconductors with exotic structures and properties, and uses them to create cheaper, thinner, and more flexible solar cells and light-emitting and light-sensing devices.

FSE Science Newsroom | Text René Fransen | Photography Leoni von Ristok

In 2009, a new type of solar cell was described that contained a material with a special crystal structure: perovskite solar cells. Solar panels using this technology are already on the market; consumers can buy them from the Chinese company Renshine Solar. Loi shows a small panel with a faux-marble look: ‘This could be placed on a building as cladding.’ Another example on the Renshine website is that of headphones with flexible perovskite cells on the headband.

Perovskite solar cells have a high efficiency that—in the laboratory—equals that of silicon solar cells. However, they have more advantages: The panels are made from thin films, which are much lighter than silicon, and they are flexible. These films are made by coating a substrate with a solution containing the molecules from which perovskite crystals form. This procedure is simpler than the production of silicon solar cells. Furthermore, perovskite solar cells can be produced in different colours.

However, there is a drawback: the perovskites that are currently used are not very stable in air. ‘You can seal off the cells on a panel, but you still need to be careful during production,’ explains Loi. Another drawback is that the best perovskites at present are based on lead, a metal that is toxic for humans and bad for the environment.

An unofficial world record

To overcome the drawback of using lead, Loi is working on perovskites in which lead is partly replaced by the more environmentally friendly metal tin. This is not a trivial thing to do: ‘Changing perovskite formulations presents us with technical difficulties that we have to solve,’ says Loi. ‘For example, many formulations are unstable.’ She did find one rather mundane key factor required to produce well-functioning cells: work in a very, very clean manner. ‘We had a visiting postdoctoral researcher, who came to us because he couldn’t reproduce our results. I told him to clean the glove box where he was making the devices.’ The visitor thought she was having him on, then found out she wasn’t.

At a recent conference, Loi announced that her group made lead/tin perovskite that converts light into electricity with 25.9 per cent efficiency. ‘This is an unofficial world record for lead/tin perovskites, and it is very close to the 27 per cent conversion efficiency of the lead-only cells. Our aim for the next five years is to bring lead/tin perovskites to the same efficiency as lead-only cells and to make them more stable.’ Perovskite solar cells are very important because they can be combined with silicon solar cells. ‘Such tandem solar cells, when tested in the laboratory, are 10 per cent more efficient than silicon-only devices. So, to make the next products, companies are concentrating on tandem solar cells.’

Quantum dots

As a physicist, Loi is always playing around with materials in order to give them the properties she wants. ‘We start by taking lots of measurements to understand the materials’ properties. Then we figure out how we can give a material the right properties. And finally, we turn these materials into devices.’ Loi is also working on quantum dots: tiny particle clusters that act as one super atom. By precisely adjusting their sizes, Loi can tune them to have specific desired properties.

On her desk are some near-infrared photo detectors. These could be useful for making cameras that see through fog or for measuring distances. Loi: ‘You can use a laser of a wavelength that is not harmful for our eyes and detectors that register the reflected light, allowing the distance of objects to be determined.’ One of her former PhD students has started the company QDI Systems in Groningen, to develop quantum dot sensors not only for infrared light but also for X-rays.

A problem with quantum dots is that it is hard to make them in a reproducible way. And, as with the perovskite solar panels, the best dots contain lead. ‘My colleague, Professor Loredana Protesescu, is working on this; she develops quantum dots based on the elements indium and antimony. These should have the same properties as the current lead sulphide dots.’

The devices Loi’s group produced are already inspiring products. Former group members have started their own companies. Loi: ‘At the end of the day, we want to produce something that is useful, that makes people’s lives easier, and that also contributes to our local economy.’