Feature article written for BENG0027 Tech Journalism, aimed at Chemistry world. Chemistry undergraduate / general knowledge but not research-field specific.

Understanding intervalence charge transfer for better solar cell materials

An unusually stable, tin doped lead-free perovskite inspired material (PIM) behaviour was examined by University of London scientists. Their paper “Enhanced visible light absorption in layered Cs3Bi2Br9 through mixed-valence Sn(II) / Sn(IV) doping” expands current understanding of the mechanism of intervalence charge transfer (IVCT) dopants in band-lowering PIMs. Seán Kavanagh (Department of Chemistry (UCL), Department of Materials (Imperial College London) & CDT-ACM) states, “This work demonstrates that mixed-valent doping can be employed to favourably tune the optical properties of potential photovoltaic materials, but that the optical transitions introduced must be appropriately aligned in order to yield extractable charge carriers (i.e. electricity).”

Solar energy is an underutilised resource. Worldwide, our total annual energy usage is less than 0.01% of the 1500 exawatt hours energy incident on the earth. Therefore photovoltaics (PVs) could be our ‘get out of jail free card’ in the green energy crisis. However, current silicon-based solar cells are inefficient, and the sand required to synthesise these is limited.

Many alternative technologies present acceptable efficiencies. For example, thin-film PVs such as GaAs or CdTe perform well, but are regrettably toxic and rare. Dye sensitised solar cells are appealingly low-cost, whilst colour gives them aesthetic advantages for architectural use. However, their photocatalytic efficiency has plateaued at 13%.

A novel class of PV material was discovered in dye sensitised solar cell research. CH₃NH₃PbI₃, more commonly known as MAPI, is a very effective inorganic dye; researchers investigated its ability as an absorber on its own and found that the efficiency was improved. This guided research into lead halide perovskite (LHP) as an absorber. Of all materials investigated, LHPs have had the steepest trajectory, with 3.8% efficiency in 2009 they skyrocketed to a certified efficiency of 25.5% in 2020. Simple to synthesise as a thin-film, the perovskite structure allows for easy tuning of the ‘bandgap’.

The bandgap is the energy difference between the highest occupied energy level of the material and the lowest empty energy level. Bandgap energy tuning capability is crucial for PV applications. With conversion of energy to heat, photons passing through the solar cell, and energy being lost to recombination effects, there is an absolute limit on the efficiency for a solar cell. The commonly referenced model is the Shockley-Queisser (SQ) limit. The SQ limit calculates a maximum efficiency of 33% is possible for bandgap values of 1.1 to 1.7 electron volts. Therefore, most PV research focuses on optimising bandgap energy.

Although LHPs possess ideal bandgap tunability, they are currently unsuitable for widespread commercial use, primarily due to their instability in ambient air and the toxicity of lead. A recent study found that up to “40% of lead in airborne particles today comes from the legacy of leaded petrol despite having been banned over 20 years ago”. Lead’s high bioavailability, toxicity, and ability to persist within the environment, justifies hesitancy towards its use in industrial scale manufacture.

Several key strategies are being explored to replace lead, as UCL’s Prof Robert Palgrave described during his recent talk at the 15^th International Conference on Materials Chemistry, entitled “Pb or not Pb?”. PIM research aims to exploit the tuneable nature of the perovskite structure whilst replacing the lead component. An initial approach is to replace lead with an element of the same charge such as Sn(II) or Ge(II). A second method involves cation mutation, where you replace lead with two cations whose average charge is the same as lead. For example, one might pair Na(I) or Ag(I) with Bi(III) or Sb(III), and over the bulk structure, one would have an average charge of +2. method of averaging can also work by combining vacancies with a 4+ charge such as Sn(IV), overall giving a 2+ charge.

Palgrave explained that for Cs₃Bi₂Br₉, another strategy is at play. This material is a vacancy-ordered triple perovskite – “Instead of doing cation mutation … [one can] simply chop up the perovskite structure and remove some of the B site cations. If we remove one third of the B site cations… so we remove every third row… we can get a structure… which has … the formula A₃B₂X₉… It’s no longer a three dimensional material, it’s now a 2 dimensional layered material.” These materials tend to be non-toxic, air stable and have long charge-carrier lifetimes. Additionally, their synthesis is scalable, being solution based and low temperature.

Despite its many desirable properties, Cs₃Bi₂Br₉‘s band gap (2.6 eV) was too big for PVs. One can reduce the band gap of LHPs by substituting a heavier halogen, like iodine. However, Palgrave explained that in this case “when we make this iodide rich, we do narrow the band gap but we actually change the structure”. The new structure lost conductive properties, instead producing localised charges making it unsuitable for PV applications.

Therefore, the group investigated cation mutation, replacing a percentage of 2Bi(III) with Sn(II), and Sn(IV) via a heterovalent doping strategy. The optical absorption dramatically altered with a relatively small amount of Sn, changing from yellow to pitch back; so dramatic a redshift was unexpected. As Kavanagh (Computational Chemist working on the project) explained, “similar absorption enhancements have been obtained upon doping in other ‘perovskite-inspired materials’, but the changes are typically not as substantial…. in other studies, only slight colour changes are typically found.”

Figure 1: Illustration of redshift transition between the doped and undoped PIM. (From C. Krajewska et al’s pre-print paper.)

Kavanagh explained, “The combination of theory and experiment in studies like these is crucial to obtain comprehensive understanding of the chemistry at play”. By computational examination of the defect formation energy for the Sn species in the material, it appeared that Sn disproportionated into +4 and +2 oxidation states. Consequently, it was expected that the Sn would cluster, the two oxidation states being most stable when adjacent to each other. This hypothesis was verified by simulation of the formation energies for the clustered and non-clustered cases, finding the clustered arrangement the most favourable.

This affected the doping behaviour as, rather than creating isolated defects, the tin behaved as clustered double substitution complexes. When defect formation energy calculations were revisited, they revealed a self-consistent fermi level, with the midpoint precisely matching in situ experimental results. The electronic structure was further examined by computational methods; this analysis revealed that doping produced a filled and unfilled state within the conduction and valence bands of the material. From these results, three possible optical transitions were calculated, only made possible by the introduction of the intervalence states. Again, this was verified by the experimental results.

Sn-based compounds usually suffer from similar stability issues as LHPs. However, this material (Sn: Cs₃Bi₂Br₉) was stable in air for more than 12 months. Kavanagh says they “believe this unusual stability is driven by the disproportionation and resulting Sn2+-Sn4+ interaction”. Despite improved stability, this material is some way off commercial application. The group are currently investigating methods to shift synthesis from powder to thin-film structures, as required in solar cells. A substantial obstacle in achieving this is the increased likelihood of “Sn oxidation prior to incorporation in the host material”; additionally, the charge carriers are localised, hindering performance in a PV application context.

Regarding future research, Kavanagh stated –

“I would see this more as a stepping stone to understanding the behaviour of ‘IVCT’ dopants…By obtaining a clear picture of the atomic mechanisms behind this behaviour in our study, we can successfully engineer this behaviour in other ‘perovskite-inspired materials’. The ability to control material properties in this manner allows us to effectively design materials which are perfectly suited for the desired application.”

This investigation is a perfect example of in silico techniques providing deeper understanding of in situ results. As Kavanagh (somewhat emphatically) states “the synergistic combination of theory and experiment [is vital] … without theory, we wouldn’t know exactly why the material properties change, and without experiment, we wouldn’t have proof it actually works! This combinatorial approach is really important for rapidly expanding our understanding of doping behaviour (and the properties of PIMs in general), thereby accelerating us towards the goal of controlled materials design.”

References

Huang, Y. T., Kavanagh, S. R., Scanlon, D. O., Walsh, A. & Hoye, R. L. Z. Perovskite-inspired materials for photovoltaics and beyond-from design to devices. Nanotechnology 32, (2021).

Krajewska, C. et al. Enhanced visible light absorption in layered Cs₃Bi₂Br₉ through mixed-valence Sn(II) / Sn(IV) doping. ChemRxiv Pre-print, 1–9 (2021).

Additional information taken from 15^th International Conference on Materials Chemistry talk: “Pb or not Pb? Control of Structure and Optical Properties in Lead Free Halides for Photovoltaic applications” by Robert Palgrave, University College London, UK

Background based on CHEM0030 content delivered by Prof David Scanlon.

News Article written for BENG0027 Tech Journalism, aimed at Chemistry world readers. Most have chemistry undergraduate knowledge but articles are suitable for chemists not working directly in the area of research.

Figure 1: Pictured above, Professor Yaghi in the celebratory graphic from the Royal Society of Chemistry website. [RSC copyright]

Omar Yaghi Wins 2020 Sustainable Water Prize from Royal Society of Chemistry

The Royal Society of Chemistry Sustainable Water 2020 prize has been awarded to Professor Omar Yaghi for “the impactful development of water harvesting from desert air using metal–organic frameworks”. Recognised ‘as the father of reticular chemistry’, Yaghi defined reticular chemistry as “linking molecular building blocks by strong bonds to make extended crystalline structures”. This definition now encompasses the millions of structures falling under the categories of zeolites, metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs).

Despite 70% of our planet’s surface being covered by water, only 3% of this is potable. World Wildlife Fund state 1.1 billion people lack access to water, 2.7 billion face frequent water scarcity. By 2025, two thirds of the world’s population may face water shortages – currently, underground reserves are being consumed faster than they are replenished. Climate change will impact potable water scarcity. Yaghi states this technology is “making water independent of the [local] grid” and his research aims “to give citizens of the world water independence”.

Figure 2: Infographic illustrating the current MOF-based water harvesting research position.

Yaghi’s recent research proves scalability for his metal-organic framework (MOF) water-harvesting devices. Hanikel et al’s nature nanotechnology review presents a roadmap for future testing, upscaling, and optimisation of their technology. In particular their MOF framework is at an advanced stage (as illustrated in figure 2), and the device is at an exciting turning point.

Yaghi’s foray into water harvesting technology proved serendipitous. In 2014, whilst researching MOF-801’s potential as a water separator in carbon capture, Yaghi noted that this material achieved remarkable uptake of water at low humidity. Examining MOF-801’s isotherm, clearly this material had potential, achieving water uptake of 42% with respect to weight. Additionally, it adsorbs and desorbs at reasonably low temperatures (25° C for adsorption, 45° C for desorption).

Desalination provides large-scale freshwater but is unsustainable due to its high energy costs and environmental impact. Fog harvesting is sustainable and on an industrial scale, is appealing for its energy-passivity but requires a consistently high humidity environment for efficiency. Direct air cooling, cannot function in arid climates – the very locations with the greatest water harvesting requirement.

Effective desiccants, salt and zeolite-based materials have been tested as potential water harvesting candidates. Though both function at low humidity, neither has the level of capacity or kinetics for viability. The quick kinetics of MOF-801 in low humidity conditions, indicated that MOF materials might hold the solution; MOFs have high capacities and can be modified for property optimisation. MOF-801 produced good cycling performance, but showed an initial drop in capacity after the first cycle. Investigations showed cycle water binding in tetrahedral or cubic arrangements to the MOF within the initial cycle, then continuing to act as seeds for water uptake within the pores. This water remains in place and affects the kinetics, as subsequent water molecules bind to the water seed structures rather, than directly to the MOF.

In situ passive MOF water harvesting was initially demonstrated in 2017 (the device only used 2 g of zirconium-based MOF-801, but proved the concept). In 2018, a similar larger device was tested in Berkeley. This first-generation of harvester was energy passive, low-tech and cheap and in lab conditions, it produced up to 0.3 L kg_MOF^-1. The water produced was pure, requiring only mineralisation to achieve potability. Zirconium is expensive, so lower cost alternatives were investigated. Aluminium is cheap and abundant, but vulnerable to water corrosion, yet when aluminium is deployed in a rod structure, it is water resistant. Using this principle MOF-303 was developed, which achieved 0.33 L kg_MOF^-1.

The group’s first energy-active water harvester was developed in 2018. This second-generation harvester was rapid cycling, and harvested day and night at humidity as low as 7%. Rapid cycling produced up to 1 L per kg of MOF. The device was further optimised to produce a third generation harvester; this latest assembly uses 100 g of MOF-303, and cycles 200-250 times per day producing in the most arid conditions, a minimum of 5 L kg_MOF^-1 daily, to a maximum of 100 L kg_MOF^-1 daily in more humid conditions. MOF-303 has run over 50,000 cycles, without any degradation. The MOF is expected to last (or even outlast) the water harvesting unit itself.

Figure 3: Pictured above the third (and latest) generation of MOF based water harvester and a video of the device working (53:53 timestamp). [Copyright Omar Yaghi]

Yaghi envisages “the next generation device will be a nice tabletop device that delivers around five litres per day, based on 100 grams [of MOF], of water”. He dismisses concerns that atmospheric water harvesting could affect the environment, pointing out the statistic – “If we served 50L [of water] to all 6.7 billion people on the planet, we would use just 0.002% of the water in the atmosphere on any given day.”

Spanning his work in reticular chemistry, to his current device testing, clearly Yaghi’s contribution to sustainable potable water technology has been pivotal. Yaghi has maintained his research interest in this area, alongside other reticular chemistry interests, most recently examining xenon separation.

References

https://www.rsc.org/awards-funding/awards/2020-winners/professor-omar-yaghi/#undefined

https://orcid.org/0000-0002-5611-3325

https://youtu.be/nIK96K3biNA

Hanikel, N., Prévot, M. S. & Yaghi, O. M. MOF water harvesters. Nat. Nanotechnol. 15, 348–355 (2020).

Xu, W. & Yaghi, O. M. Metal-Organic Frameworks for Water Harvesting from Air, Anywhere, Anytime. ACS Cent. Sci. 6, 1348–1354 (2020).

Hanikel, N. et al. Rapid Cycling and Exceptional Yield in a Metal-Organic Framework Water Harvester. ACS Cent. Sci. 5, 1699–1706 (2019).

Article written for BENG0027 Tech Journalism:

Book Review of Discovering Cosmetic Science, ISBN: 9781782624721

Target audience: Chemistry World readers . Some chemistry knowledge, but non-specific.

Quotes taken from the book and these videos:
https://www.youtube.com/watch?v=IFA8RYorxbk
https://ifscc.org/videos_and_webinars/discovering-cosmetic-science-panel-discussion-of-authors-2-dec-2020/

Discovering Cosmetic Science

“Discovering Cosmetic Science” is described as a “novice’s guide” to the science of cosmetics industry, which bridges the gap between basic popular science books and academic textbooks. Whilst providing satisfying scientific detail, it is pitched so “…you won’t need a PhD to understand the science”. Cosmetics – whether makeup, facial masks, perfume, shampoo, sunscreen, soaps, toothpaste or nail varnish – attract high value research investment.

Broadly, cosmetics are categorised as sun care, hair care, skin care, body care, oral care, perfume, and decorative cosmetics. Although “care” is emphasised, whilst medicines provide actual therapeutic effect, cosmetics “provide a combination of sensorial and functional benefits”. Medicines interact chemically to treat a condition, whereas a cosmetic has a shorter term external effect. As a science student interested in cosmetics, I searched for years for books on cosmetics; this book is the first accessible example which covers the basics of the cosmetic industry. A great general cosmetic science book, it demystifies the jargon of active ingredients, explains what ‘free-from’ and ‘natural’ claims REALLY mean, leaving one feeling empowered as a consumer and satisfied as a reader.

Each chapter covers one aspect of cosmetic science, following a narrative, but which can be dipped into individually to answer specific questions. So the format is handy for reference. In compiling her chapter, Nichola Roberts asked a focus group of new technicians from the Society of Cosmetic Scientists, “If you were in the room with the skincare expert for 10 minutes what questions would you ask?” and responses defined the areas which are covered. As Roberts’ co-author, Robin Parker explained, “the idea was that you would have an expert collaborating with someone who was less experienced in the industry to challenge some of the questions that might be asked by the audience”. This collaborative approach results in a nuanced, authoritative tone.

Roberts asks “how and why does the skins appearance change with age”? The insidious effect of sun damage is amply illustrated in the photograph below. Her chapter addresses questions including “are there differences between men’s and women’s skin”, “how do cosmetics make a difference to skin appearance”, and “why do we have day and night moisturizers, and are they different”?

Figure 1: Photograph of a Mexican lorry driver, showing dramatic asymmetric skin damage. This was the result of driving with the sun on the left hand side of his face for years.

This book is not a guide to make-up manufacture. Whilst decorative cosmetics are covered in the chapter “More Than a Smudge of Colour – The Science Behind Colour Cosmetics”, formulation is only briefly touched upon and pigment dispersion is explained in a basic manner, appropriate for the target reader.

Although targeted at non-scientific readers, cosmetic scientist Perry Romanowski describes this book as a “a great primer for people who are in the industry”. During an authors’ panel discussion, Rachael Polowyj (co-author of the “Myths and Scares – Science in Perspective” chapter with Emma Meredith), revealed: “A lot of the people I know in this industry have kind of fallen into it… But this book is just great because there’s a section at the back and it tells you the different careers that you could have in cosmetic science.” References for further reading, coupled with explanatory sidebars, are useful for readers interested in cosmetics as a career path.

Romanowski commented on the book, “One of the biggest problems we face in the industry is that [it] is really run by the marketing departments … [this book] gets the real science out there it… cuts through a lot of the marketing flair.” “Discovering Cosmetic Science” also sets the record straight. As Emma Meredith explains – the authors’ collective aim was to “bring the science down to a level that people can try and understand” and to restore faith in the industry. She also cites the damage ubiquitous and misleading ‘free-from’ claims have wrought on consumer confidence. So regardless of any interest in the science, this book is a must read for anyone who wants to understand what they are actually buying. British beauty blogger, Cunningham, sums up the book well – “More than anything this book gives you a sort of armour I suppose, to go about looking at your beauty products in a different light and understanding them a bit better”.