Souped-up solar absorption with perovskites

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This image shows perovskite photovoltaics in the background with individual perovskite crystals shown as the colourful units. (Credit: CUBE3D Graphic)

Researchers at MIT have developed a fresh approach to designing perovskite solar cells that pushes the material to match, and even exceed, the efficiency of typical silicon solar cells used today.

Perovskites are a leading candidate for eventually replacing silicon as the material of choice for solar panels. They offer the potential for low-cost, low-temperature manufacturing of ultrathin, lightweight flexible cells, but so far their efficiency at converting sunlight to electricity has lagged behind that of silicon and some other alternatives.

While silicon solar cells offer efficiency ranging from 20 to 22 per cent, the MIT scientists have boosted the efficiency of perovskite as a solar cell to 25.2 per cent, which they say has laid the groundwork for further improvements in the future.

The efficiency boost, which eclipses that of many existing solar panels, was achieved by modifying the perovskite formula, as well as adding a specially treated conductive layer of tin dioxide bonded to the perovskite material, which provides an improved path for the charge carriers in the cell. The findings are described in a paper in Nature.

A strong contender

Perovskites are a broad class of materials defined by the fact that they have a particular kind of molecular arrangement, or lattice, that resembles that of the naturally occurring mineral perovskite. There are vast numbers of possible chemical combinations that can make perovskites, with these materials attracting worldwide interest due to them being, at least on paper, much cheaper to produce than silicon or gallium arsenide (one of the other leading material contenders for solar cells). This is partly because they are much simpler to manufacture and process, as to produce and process silicon or gallium arsenide, sustained heat of over 1,000°C is required. In contrast, perovskites can be processed at less than 200°C, either in solution or by vapour deposition.

The other major advantage of perovskite over silicon, or many other candidate replacements, is that it forms extremely thin layers while still efficiently capturing solar energy.

‘Perovskite cells have the potential to be lightweight compared to silicon, by orders of magnitude,’ said Moungi Bawendi, a professor of chemistry at MIT and an author of the Nature paper.

Perovskites have a higher bandgap than silicon, which means they absorb a different part of the light spectrum and thus can complement silicon cells to provide even greater combined efficiencies. But even using only perovskite, according to co-author Jason Yoo, ‘what we’re demonstrating is that even with a single active layer, we can make efficiencies that threaten silicon, and hopefully within punching distance of gallium arsenide. And both of those technologies have been around for much longer than perovskites have.’

Layered development

One of the keys to the team’s improvement of the material’s efficiency, Bawendi explained, was in the precise engineering of one layer of the sandwich that makes up a perovskite solar cell – the electron transport layer. The perovskite itself is layered with a transparent conductive layer used to carry an electric current from the cell out to where it can be used. However, if the conductive layer is directly attached to the perovskite itself, the electrons and their counterparts, called holes, simply recombine on the spot and no current flows. In the researchers’ design, the perovskite and the conductive layer are separated by an improved type of intermediate layer that can let the electrons through while preventing the recombination. This middle electron transport layer, and especially the interfaces where it connects to the layers on each side of it, tend to be where inefficiencies occur.

By studying these mechanisms and designing a layer consisting of tin oxide, which more perfectly conforms with those adjacent to it, the researchers were able to greatly reduce the losses. The development of the new layer was combined with an optimisation of the perovskite layer itself. The researchers used a set of additives to the perovskite recipe to improve its stability, which had been tried before but had an undesired effect on the material’s bandgap, making it a less efficient light absorber. The team found that by adding much smaller amounts of these additives — less than 1 per cent — they could still get the beneficial effects without altering the bandgap.

The resulting improvement in efficiency has already driven the perovskite material to over 80 per cent of the theoretical maximum efficiency that such materials could have.

While these high efficiencies were demonstrated in tiny lab-scale devices, ‘the kind of insights we provide in this paper, and some of the tricks we provide, could potentially be applied to the methods that people are now developing for large-scale, manufacturable perovskite cells, and therefore boost those efficiencies,’ said Bawendi.

Future avenues

In pursuing the research further, there are two important avenues, according to Bawendi: to continue pushing the limits on better efficiency, and to focus on increasing the perovskite material’s long-term stability, which is currently measured in months, compared to decades for silicon cells. But for some purposes, Bawendi noted, longevity may not be so essential. This is because many electronic devices such as cellphones, for example, tend to be replaced in a few years, so there may be some useful applications even for relatively short-lived solar cells.

‘I don’t think we’re there yet with these cells, even for these kinds of shorter-term applications,’ he said. ‘But people are getting close, so combining our ideas in this paper with ideas other people have with increasing stability, could lead to something really interesting.’

Robert Hoye, a lecturer in materials at Imperial College London, who was not part of the study, said: ‘This is excellent work by an international team. This could lead to greater reproducibility and excellent device efficiencies achieved in the lab translating to commercialised modules. In terms of scientific milestones, not only do they achieve an efficiency that was the certified record for perovskite solar cells for much of last year, they also achieve open-circuit voltages up to 97 per cent of the radiative limit. An astonishing achievement for solar cells grown from solution.’ 

Paper in Nature

New analysis of 2D perovskites could boost the development of next-generation solar cells

Three-dimensional perovskites have proved themselves remarkably successful materials for solar panels in the past decade. One key issue with these materials, however, is their stability, with device performance decreasing quicker than other state-of-the-art materials. Researchers from the University of Surrey believe that the 2D variant of perovskites could provide answers to these performance issues.

 

2D perovskites could provide answers to the performance issues of perovskite materials for solar cells. (Credit: University of Surrey)

In a study in The Journal of Physical Chemistry Letters, researchers from the university’s Advanced Technology Institute (ATI) have shown how to improve the physical properties of a 2D perovskite called Ruddlesden-Popper. 

Their study analysed the effects of combining lead with tin inside the Ruddlesden-Popper structure to reduce the toxic lead quantity. This also allows for the tuning of key properties such as the wavelengths of light that the material can absorb at the device level, which could improve the performance of photovoltaics. 

‘There is rightly much excitement about the potential of 2D perovskites, as they could inspire a sustainability revolution in many industries,’ said Cameron Underwood, lead author of the research paper.

‘We believe our analysis of strengthening the performance of perovskite can play a role in improving the stability of low-cost solar energy.’

Professor Ravi Silva, a co-author of the research and director of the ATI, added: ‘As we wean ourselves away from fossil energy sources to more sustainable alternatives, we are starting to see innovative and ground-breaking uses of materials such as perovskites. The ATI is dedicated to being a strong voice in shaping a greener and more sustainable future in electronics – and our new analysis is part of this continuing discussion.’

Paper in The Journal of Physical Chemistry Letters

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