Understanding intervalence charge transfer for better solar cell materials
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 15th 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 15th 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.