Aron
Walsh
Department of Materials, Imperial College London, London SW7 2AZ, UK. E-mail: a.walsh@imperial.ac.uk
First published on 18th August 2022
A personal perspective is given on the major results, themes and trends from the Faraday Discussion on emerging materials for solar energy technologies. This covers research progress into adamantine semiconductors, the narrowing divide between materials modelling and measurements of solar cells, as well as the control of defects in novel absorber materials that include Cu2ZnSnS4, Zn3P2, Se, GeSe, Sb2Se3 and BaZrS3. This paper is adapted from a transcript of the closing lecture.
On the train from London to Bath, I collected the abstracts from the event programme and generated a word cloud (see Fig. 1) to have a feeling for what I could expect. In the text, you can see many of the points that have been discussed. Of course, the main focus is solar energy. There is coverage of kesterites, some mention of perovskites, and an emphasis on efficiency, performance and stability. It reflects what we have been actively discussing.
Fig. 1 A word cloud generated from the abstracts of papers presented at the Faraday Discussion meeting on emerging materials for solar energy technologies. |
To begin, the topic of this Faraday Discussion is emerging photovoltaic (PV) materials. We have heard that many of the systems studied have a long history. Some for many decades. So by “emerging”, we don’t necessarily mean that these are new materials or new technologies. One way to classify them is according to their performance with respect to the detailed balance limit (see Fig. 2). In most cases, we’ve heard about materials that fall in the bottom 50%. Materials that don’t yet realise half of their potential. Following this definition, we are making life difficult for ourselves. We’re not using the highest performing materials and looking for incremental improvements. We’re choosing technologies that have potential, but fundamental science needs to be completed to understand the current bottlenecks and to engineer higher performance devices. It is a difficult topic by definition. But it has been inspiring to see the progress being made and the exciting ideas that people shared over the past few days.
Fig. 2 The champion light-to-electricity conversion efficiencies for a range of photovoltaic technologies with respect to the detailed balance limit. The image is reproduced with permission from https://www.lmpv.nl/db/ and an earlier version was published in ref. 1. |
Fig. 3 A map of the relationship between adamantine crystal structures adapted with permission from ref. 3. |
Pamplin considered building blocks based on face-centred cubic and hexagonal close packing. He thought of stoichiometric combinations, as well as ordered vacancy systems. When you follow the two strands for binary compounds, you have zinc blende versus wurtzite, with ternary and quaternary equivalents, for example, stannite and wurtz-stannite (these are closely related to the kesterite and wurtz-kesterite structures4). For certain compositions, such as Cu2CdSiS4, the hexagonal stacking sequences are favoured.5 In fact, when you look at high-resolution microscopy over large areas, often the competition between AB and ABC close-packing leads to stacking faults, a type of planar defect.6 We once performed some simulations of this phenomenon,7 but I recall that their connection to photovoltaic efficiency is still not well understood. It is nice to think that decades ago at this location, someone was concerned with similar problems to us now.
Actually a lot of my post-doctoral work was inspired by the pioneering work of Pamplin and others working in a similar direction such as Colin Goodman.8 At the time I was working at the National Renewable Energy Laboratory and had access to some quite large Department of Energy computers. We decided to map out this crystal space systematically, looking at many chemistries of going from the elemental to quaternary systems in terms of the structures that emerge and also the property trends.9 This was largely the PhD work of my collaborator Shiyou Chen. It is a field that I have enjoyed working in since then. This type of materials workflow is attractive because you can control the complexity, adding one more component when you go down a series. But there is an associated cost. In each session of this meeting, we have heard about defects in crystals. In quaternary systems, the situation becomes incredibly difficult because so many point defects (vacancies, interstitials, anti-sites), complexes that form between neighbouring sites, and extended defects can form in tandem.10
In Cu2ZnSnS4, Cu–Zn disorder is an issue, but unlikely to be the bottleneck for photovoltaic performance.11 It is really the Sn-related defects that act as killer centres. They result in deep levels that are the strongest traps for recombining electrons and holes.12 The challenge here is that often the Sn-related defects are hidden beneath the Cu–Zn disorder. Because you have such high levels of cation site mixing, it’s very difficult to see the small changes. Even if some of these defects are present in parts per million, they could still be responsible for large voltage deficits. That’s one of the challenges in trying to get to grips with these systems. From the kesterite presentations that we heard, and the community feeling in general, a big open question remains: is there a future for kesterites? What we have on our side is materials engineering. So I think there is always hope. But we need a breakthrough in this field such as a new processing step that suddenly activates the photovoltaic performance of Cu2ZnSnS4 absorbers to approach 20% light-to-electricity conversion efficiency.
In the past, if you’re concerned about materials’ properties such as electrical conductivity, chemical stability, photogenerated carrier lifetimes, it would be quite common to use a method like density functional theory (DFT) to make predictions. You calculate the electronic band structure to get an effective mass, then wave your hands and say this should be a good p-type conductor or that should be a good n-type conductor. But now we can actually calculate carrier mobilities, by estimating the electron and hole scattering rates, which don’t always follow the underlying effective masses.13 You might have a system that has low lying vibrational states that scatter strongly and limit mobility at room temperature. Then for stability fields and materials processing, the standard practice was to work with athermal internal energies from DFT. But when you heat materials, the thermodynamic balance can change. You may have a phase that would be stable at 0 K, but at room temperature is no longer accessible. It’s now possible to go from internal energies to free energies and so include temperature and pressure dependence in phase diagrams. That’s becoming increasingly standard and we previously developed models for kesterites.14 The third point, which we heard about this morning from Seán Kavanagh (https://doi.org/10.1039/D2FD00043A), is defects. In the past it was standard practice to calculate defect levels, a series of charge state transitions between the valence and conduction bands of a host crystal. These allowed for qualitative comparison with a range of optical and electronic probes but there was no direct connection for unambiguous assignments. We heard that we are now closer to calculating the spectroscopic features of particular defects. A move in that direction is the calculation of carrier capture coefficients,15 considering the interaction of defects with electron and hole and carriers, which has been a focus for my research group over the past few years. A talented researcher Sunghyun Kim led the development of the CarrierCapture package16 and we have applied it to a range of cases from Cu2ZnSnS4 (ref. 17) to GaAs.18
This transition to a higher complexity in the modelling does not come for free. These are not calculations that you can run on your laptop. They often use large-scale national supercomputing resources. The website Top500 (https://www.top500.org) provides a ranking of the fastest public supercomputers in the world. And now in the U.S., they’ve launched a supercomputer that has 8.7 million cores. One computer with 8.7 million processing units. That’s amazing! There are now several public systems in the world with over 1 million cores. But you also have to pay attention to the power consumption. The top entry requires 21000 kW to operate. The equivalent of an old generation coal power plant just running that one computer. One active question is the sustainability of computational research. Should we do everything brute force at the highest level of theory possible, or should we try to be smarter in what we do and how we do it? That’s the subject of ongoing debate and developments. We have methods that are very accurate, but it’s not feasible to run them out for every possible material because you’re just wasting so much energy and generating tons of CO2 along the way.
Fig. 4 (a) Crystal structure (Pnma space group) and (b) histogram of Sb–X distances of the photovoltaic absorber materials Sb2S3 and Sb2Se3. The figure is reproduced from ref. 20 under a Creative Commons CC-BY license. |
A question debated earlier is how we can emulate the defect tolerance of metal halide perovskites in other materials. I have a particular opinion that not everybody would share, but I would say the number one reason for perovskites being so tolerant is simply dielectric screening. This property is related both to the perovskite structural flexibility and the chemical softness of the constituent ions, which give rise to large Born effective charges and in turn a sizable ionic contribution to the dielectric response. Why is this relevant for solar cells you ask? In the screening of electrostatic interactions, whether it be electron–hole separation or electron-charged defect scattering, it is beneficial to have a large dielectric constant.22 Many relevant interaction terms depend on the square of the dielectric constant. As halide perovskites generally have large dielectric constants, they have small carrier capture cross-sections. Of course there are other factors to consider, one of which is lattice thermal conductivity. Non-radiative losses are facilitated by heating and if the host crystal is a thermal insulator then any recombination cycle will be supressed. I don’t believe this has been well explored for photovoltaics except for the slowing of hot carrier cooling in nanostructured materials and quantum dot solar cells.
In conclusion, I’ll restate several opportunities. For emerging materials, David Mitzi outlined the balance between simplicity and tuneability in his opening lecture (https://doi.org/10.1039/D2FD00132B). We heard about Se in the final morning’s session. In principle, you can’t get simpler than one element but even Se has many structural forms to control. Perhaps simplicity is not just the number of elements and we need to develop better structure and property features. The need for new electrical contacts for emerging technologies was highlighted. CdS may have a refined deposition procedure in this community, but it is not optimal in terms of performance and also for its composition. What has been used a lot in the organic photovoltaic community is modification layers. For example, while TiO2 may not have an appropriate conduction band for your technologies, an ultra-thin molecular or polymeric dipole layer could alter the contact behaviour with shifts on the order of eV possible. It is a trick that could be useful here, instead of going directly to alloys and trying to balance band gap and resistivity changes. Finally, I will echo Thomas Unold’s call for better use and sharing of data concerning both PV materials and devices (https://doi.org/10.1039/D2FD00085G). So much information gets hidden in the literature that you can’t access or reproduce. I look forward to a future where a photovoltaic information file (.pif) becomes standard with each new publication.
Our understanding of emerging photovoltaic materials continues to grow and I hope that we can learn from past experiences to accelerate the development of the exciting technologies that have been covered at this event.
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