Professor Adélio Mendes speaks to SciTech Europa Quarterly about the challenges involved in boosting the efficiency of perovskite solar cells.
During the European Materials Research Society’s (E-MRS) 2017 Fall Meeting, Professor Adélio Mendes took part in a symposium on perovskite solar cells, which was designed to provide an overview on the most advanced studies and recent trends in perovskite solar cells science and technology in order to give relevant answers to key challenges involved in ensuring that this technology can be successfully commercialised.
Speaking to SciTech Europa, Mendes picked up on a few of the themes which ran through the session, and answers questions around areas such as lead-free perovskite solar cells and the use of atomic wires.
Perovskite solar cells (PSCs) are the fastest-advancing solar technology to date. What would you say are the biggest reasons for that?
There are several reasons for PSCs become a promising PV technology. First of all, they have experienced the fastest power conversion efficiency growth we have ever seen, improving from 3.8 % in 2009 to 23.3 % of 2018. The very thin light absorber layer of 400-500nm, made of an organic-inorganic perovskite is also very interesting because of the low content of raw materials.
Perovskite solar cells are also cheap to produce and can be produced in relatively small plants; this will allow the existence of various plants across Europe.
There are still hurdles to overcome to bring this new technology to the market. Among them, the lead which exists in the currently best-performing perovskite absorbers presents a challenge. Though very small, it is water soluble and then more dangerous. A great effort is now being undertaking for bringing the lead content below the 0.1% mass fraction allowed in the EU.
When it comes to knowledge gaps in the fundamentals of PSC operation, where do you feel research priorities should lie (function modelling, stability, life cycle assessments etc.)?
Right now, the greatest challenge is the development of suitable production and encapsulation methods. Heretofore, most of the developments concerned materials and arrangements and only very small cells were considered, with areas of ca. 0.2cm2.
The upscale of perovskite solar cells began only recently, and many challenges have to be addressed before they can be properly commercialised. This includes the full glass encapsulation of the glass substrates. This was envisioned by the GOTSolar project but, despite the advancements obtained, it still need to be improved. Patterning the PSC modules also remains a challenge.
Regarding new materials, what materials would you say hold the biggest promise moving forwards?
Despite the low incorporation of lead in PSCs, which in principle comply with the EU legislation, this is still a major cause for concern. Research on perovskite solar cells containing low levels of lead, or indeed perovskite solar cells free of lead, are now being investigated. However, lead-free PSCs are still underperforming significantly when compared with convectional PSCs.
What are your thoughts on replacing indium-tin-oxide with graphene in solar cells moving forwards? Wat would be the benefits of this? And, similarly, what are your thoughts on technologies such as carbon atom wires (i.e., polyyne) for thin films applied in the PV space?
A great effort for improving/replacing the so called transparent conducting oxide layer (TCO) is now in progress. ITO (indium tin oxide) is one of the TCOs used within the perovskite solar cells field. Despite great performance, any TCO displays low electrical conductivity, and so alternatives are now being proposed.
One of the most recent alternative is the so-called ETCO (embedded transparent conducting oxide), where very thin metal lines are inserted into grooves performed in the TCO layer. This approach has been demonstrated to display transparency at the same level of the best TCO substrates, and one order of magnitude higher electrical conductivity. These developments are now being led by Faculdade de Engenharia da Universidade do Porto within the GOTSolar project.
Given that your own research interests also focus on water splitting, what do you feel have been the biggest advances with regard to catalysis technologies and how could these be applies to the (renewable) energy sector?
The state-of-the-art of the so-called photoelectrochemical water splitting uses tandem electrodes made of a PV coupled with a photoelectrochemical electrode. This strategy could prove to be better than electrolysis driven by renewable electricity or thermochemical water splitting. For this to become a reality, however, we still need to develop a >8 mA/cm2 photoelectrode coupled with a large photopotential PV cell such as a PSC or HIT cell, delivering >1.1 V.
Hematite semiconductors have several good properties which could be of use here, such as stability, cost and photopotential. However, they also lack large photocurrents; the highest reported photocurrent for hematite is 6 mA/cm2.
The other challenge is to develop an efficient photoelectrochemical cell, suitable for working with tandem photoelectrodes and concentrated solar radiation.
In light of the advances and areas discussed, where will your own research priorities lie moving forwards?
In order to be able to financially support my research it is of critical importance to ensure that industry is interested in the work and the developments coming out of it. This year, two of my previous PhD students started PixelVoltaic, a spin-off company for the development, production and commercialisation of PV panels, and they are helping here.
My research in the area of solar technologies will follow the research money from one side, and what I believe will be critical technologies in the future. As such, I will continue working on perovskite solar cells, dye sensitised solar cells and photoelectrochromic cells as well as on solar redox flow cells – a topic pioneered by my good friend at the University of Aarhus, Anders Bentien, and I, and the electro- and photoelectroreduction of CO2 and electro- and photoelectrochemical water splitting.
Professor Adélio Mendes
Faculdade de Engenharia da Universidade do Porto
+351 22041 3612
As Professor Mendes has mentioned in the article proper, the efficiency of solar cells has been boosted in recent years. However, challenges around materials (Mendes focused on lead content in his article) persist and these may have a potential impact on any further growth in efficiency levels, which, of course, must be avoided.
Since perovskite solar cells absorb blue and green light more efficiently and silicon cells absorb red and infrared light more efficiently, a perovskite cell is applied over a silicon cell, in a so-called tandem arrangement. The resulting device has the potential to efficiently absorb the whole sunlight spectrum.
Perovskite/silicon tandem solar cells are now being researched, aiming to overpass 30% of power conversion efficiency at a reasonable cost.
While in PSCs carbon nanotubes have been used mostly as counter-electrodes and carbon nanowires are used as electrodes and, mostly, as counter-electrodes, research by a team at the University of Illinois, USA, as early as 2011 found that nanoscale wires can overcome many of the challenges encountered when, for instance, attempting to integrate semiconductors in the III-V group with silicon – this integration can never be seamless due to the lattice constant (that is, each material has a specific distance between the atoms in the crystal). Due to the fact that silicon is the most ubiquitous device platform this is a challenge that needs to be overcome, and research has shown that nanowire geometry can not only solve this problem but can also enhance solar cell efficiency.
In this study, led by electrical and computer engineering Professor Xiuling Li, the team developed a technique to integrate compound semiconductor nanowires on silicon wafers, overcoming key challenges in device production.
Nanowires made from carbon nanotubes have also been demonstrated to increase the flexibility and lower the cost of solar cells, as well as having the potential to further boost efficiency (such as, for instance, in research back in 2013 at MIT, USA).
Graphene has been demonstrated to be able to increase the flexibility and lower the cost of solar cells, as well as having the potential to further boost efficiency, especially when used as hole and electron transport media (HTM and ETM).
Given the emphasis being placed on graphene – not least through the EU’s Graphene Flagship project, which has already demonstrated that the issue of perovskite-based solar cells suffering from efficiency reductions at large areas can be overcome with graphene, thus making large area solar cells with superior performances a reality – there is a sense that we can perhaps expect further carbon-based developments to the PV field in the months and years to come.