Instilling Defect-Tolerance in ABZ2 Photovoltaic Materials
Lead Research Organisation:
University of Oxford
Department Name: Oxford Chemistry
Abstract
This project aims to develop a new class of semiconductors for photovoltaics (PVs) that can tolerate defects to achieve high efficiencies when manufactured by low capital-intensity and scalable methods.
PVs produce clean electricity from sunlight, and their deployment in the UK needs to accelerated by over an order of magnitude so that we can meet our legislated net-zero CO2 emissions target by 2050. New thin film PV materials are urgently needed. Thin film PVs can be used in tandem device structures, in which they are deposited on top of silicon PVs (which dominate the market) or smaller-bandgap thin film PVs. These tandem devices convert a larger fraction of the solar spectrum into electrical energy and can achieve efficiencies surpassing the best single-junction devices, which will be vital for accelerating utility-scale PV deployment. Thin film PVs can also be used as energy-harvesting roof-tiles, windows or cladding to enable sustainable carbon-neutral buildings.
But across all applications, it is essential that the materials are efficient when made with by low cost manufacturing methods. The limiting factor is the deleterious role of point defects, such as vacancies. In traditional semiconductors, these point defects introduce energy levels deep within the bandgap and cause irreversible losses in energy. Minimising the density of these defects often requires expensive manufacturing routes. Defect-tolerant semiconductors circumvent these limitations by forming defect levels close to the band-edges (i.e., shallow), where they are less harmful. Such materials were rare until the recent serendipitous discovery of the lead-halide perovskites. Grown cheaply by solution-processing, these polycrystalline materials have over a million times more defects than silicon but are already more efficient in PVs than multi-crystalline silicon. A critical question is whether defect-tolerance can be found in other classes of materials that are free from the toxicity burden of the halide perovskites.
This work aims to develop a set of design rules to pinpoint lead-free defect-tolerant semiconductors, and systematically develop these materials into efficient, stable PVs that can be deployed on the terawatt scale. The materials focussed on are ABZ2 compounds, where A is a monovalent cation, B a divalent cation and Z a divalent anion. These materials already show promising signs hinting at defect-tolerance.
My approach draws off my experimental strengths in the control of complex thin films. I hypothesise that materials forming shallow traps can be identified through their crystal structure, band-edge orbital composition and degree of cation-anion orbital overlap. I will experimentally elucidate the role of each property by tuning the composition of a small set of ABZ2 materials to vary one property at a time. Defect tolerance will be measured by intentionally inducing vacancies and measuring their effect on charge-carrier lifetime and electronic structure. These design rules will be applied to identify the most promising ABZ2 materials, which will be grown by scalable solution- and vapour-based methods. I will optimise their growth using a fast experimental feedback loop to achieve materials with promising bulk properties for solar absorbers. Such materials will be developed into PVs, drawing off my skills and experience in device engineering.
This work is extremely timely and will lead the emerging area of defect-tolerant semiconductors away from toxic perovskites. The new materials can ultimately become commercial contenders for tandem or building-integrated PVs, and therefore impact on the £120B PV industry. These new materials can also have much broader impact and be used, for example, as cheap but efficient materials for clean solar fuel production or biosensors. This project sets the key foundations for achieving these exciting possibilities and will enable me to set-up my group with a cutting-edge programme.
PVs produce clean electricity from sunlight, and their deployment in the UK needs to accelerated by over an order of magnitude so that we can meet our legislated net-zero CO2 emissions target by 2050. New thin film PV materials are urgently needed. Thin film PVs can be used in tandem device structures, in which they are deposited on top of silicon PVs (which dominate the market) or smaller-bandgap thin film PVs. These tandem devices convert a larger fraction of the solar spectrum into electrical energy and can achieve efficiencies surpassing the best single-junction devices, which will be vital for accelerating utility-scale PV deployment. Thin film PVs can also be used as energy-harvesting roof-tiles, windows or cladding to enable sustainable carbon-neutral buildings.
But across all applications, it is essential that the materials are efficient when made with by low cost manufacturing methods. The limiting factor is the deleterious role of point defects, such as vacancies. In traditional semiconductors, these point defects introduce energy levels deep within the bandgap and cause irreversible losses in energy. Minimising the density of these defects often requires expensive manufacturing routes. Defect-tolerant semiconductors circumvent these limitations by forming defect levels close to the band-edges (i.e., shallow), where they are less harmful. Such materials were rare until the recent serendipitous discovery of the lead-halide perovskites. Grown cheaply by solution-processing, these polycrystalline materials have over a million times more defects than silicon but are already more efficient in PVs than multi-crystalline silicon. A critical question is whether defect-tolerance can be found in other classes of materials that are free from the toxicity burden of the halide perovskites.
This work aims to develop a set of design rules to pinpoint lead-free defect-tolerant semiconductors, and systematically develop these materials into efficient, stable PVs that can be deployed on the terawatt scale. The materials focussed on are ABZ2 compounds, where A is a monovalent cation, B a divalent cation and Z a divalent anion. These materials already show promising signs hinting at defect-tolerance.
My approach draws off my experimental strengths in the control of complex thin films. I hypothesise that materials forming shallow traps can be identified through their crystal structure, band-edge orbital composition and degree of cation-anion orbital overlap. I will experimentally elucidate the role of each property by tuning the composition of a small set of ABZ2 materials to vary one property at a time. Defect tolerance will be measured by intentionally inducing vacancies and measuring their effect on charge-carrier lifetime and electronic structure. These design rules will be applied to identify the most promising ABZ2 materials, which will be grown by scalable solution- and vapour-based methods. I will optimise their growth using a fast experimental feedback loop to achieve materials with promising bulk properties for solar absorbers. Such materials will be developed into PVs, drawing off my skills and experience in device engineering.
This work is extremely timely and will lead the emerging area of defect-tolerant semiconductors away from toxic perovskites. The new materials can ultimately become commercial contenders for tandem or building-integrated PVs, and therefore impact on the £120B PV industry. These new materials can also have much broader impact and be used, for example, as cheap but efficient materials for clean solar fuel production or biosensors. This project sets the key foundations for achieving these exciting possibilities and will enable me to set-up my group with a cutting-edge programme.
Publications
Barma S
(2024)
Synthesis and Interface Engineering in Heterojunctions of Tin-Selenide-Based Nanostructures for Photoelectrochemical Water Splitting
in ACS Applied Nano Materials
Blakesley J
(2024)
Roadmap on established and emerging photovoltaics for sustainable energy conversion
in Journal of Physics: Energy
Fu Y
(2025)
Structural and electronic features enabling delocalized charge-carriers in CuSbSe2
in Nature Communications
Grandhi G
(2023)
Wide-Bandgap Perovskite-Inspired Materials: Defect-Driven Challenges for High-Performance Optoelectronics
in Advanced Functional Materials
Guo X
(2023)
Air-stable bismuth sulfobromide (BiSBr) visible-light absorbers: optoelectronic properties and potential for energy harvesting
in Journal of Materials Chemistry A
Hoye R
(2023)
Preventing lead release from perovskites
in Nature Sustainability
Huang Y
(2024)
Elucidating the Role of Ligand Engineering on Local and Macroscopic Charge-Carrier Transport in NaBiS 2 Nanocrystal Thin Films
in Advanced Functional Materials
Related Projects
| Project Reference | Relationship | Related To | Start | End | Award Value |
|---|---|---|---|---|---|
| EP/V014498/1 | 30/06/2021 | 29/09/2022 | £437,299 | ||
| EP/V014498/2 | Transfer | EP/V014498/1 | 01/01/2023 | 29/06/2025 | £304,319 |
| Description | This work aimed to investigate ABZ2 materials for photovoltaics. We focussed on NaBiS2 and AgBiS2, and successfully developed synthesis protocols to achieve phase-pure materials through nanocrystal synthesis routes. We found that these materials are stable in air, without encapsulation, for months. Remarkably, we found that these materials are very strong light absorbers, such that only 30 nm thick films are adequate for achieving >20%-efficient photovoltaic devices (as calculated based on the optical absorption profile). Originally, the aim of this work was to develop defect-tolerant semiconductors and new insights into defect tolerance. As we began investigating these materials, we realised that these materials have much more interesting properties, and the charge-carrier kinetic properties are not limited by the effects of defects, but rather the effects of electron-phonon coupling. We found that NaBiS2 has strong carrier localisation, such that electrons and holes are localised in different regions in the lattice (https://doi.org/10.1038/s41467-022-32669-3 and https://doi.org/10.1002/adfm.202310283). We showed that intentionally increasing the defect density does not in fact cause a reduction in lifetime, which remained on the microsecond timescale. The in-depth experimental-computational investigation performed not only shed important light on these novel materials, but also emphasised the importance of understanding electron-phonon coupling in novel bismuth-based semiconductors in order to design efficient solar absorbers. This realisation led us to investigate a related material, BiOI, through in-depth spectroscopy and computations (https://doi.org/10.1038/s41467-023-38008-4). From these investigations we found that this bismuth-halide-based material avoids carrier localisation, which deviates from the behaviour of all other bismuth-based perovskite-inspired materials explored thus far. By exhibited band-like transport, we found that BiOI has high mobilities >80 cm^2/Vs, allowing this material to work effectively as X-ray detectors. This led to two new grants from the EPSRC and Royal Academy of Engineering to separately investigate this new line of research in depth. In addition, we put forward the hypothesis that BiOI exhibits band-like transport because of the layered nature of its crystal structure, as well as the thick nature of each layer, such that electrons are not strongly confined within each layer. Inspired by this, we investigated CuSbSe2, which is also comprised of thick layers, and indeed we found that this material also exhibits delocalised charge-carriers. Through in-depth spectroscopy and computations, we found that the reasons for this are: 1) distortions to the lattice are mostly relaxed in the interlayer gaps rather than changes in bond length, lowering the deformation potentials, 2) there is strong electronic coupling across the interlayer gaps, increasing the electronic dimensionality, and 3) the ionic dielectric constant is small relative to the electronic dielectric constant, giving a low Fröhlich coupling constant. These important new insights now allow us to test how widely applicable these principles are, and whether we can use these to design next-generation solar absorbers with band-like transport (https://doi.org/10.1038/s41467-024-55254-2). This is the focus of a new project funded by First Solar, which will began in Oct. 2024, and will run for a year. Separately, we have also investigated the applications of AgBiS2 for near-infrared photodetectors, achieving cut-off frequencies reaching 500 kHz (https://doi.org/10.1002/smll.202310199). This makes these materials among the fastest inorganic absorbers for near-infrared photodetection, which is important for a wide range of applications, from optical communications to bio imaging. We investigated in-depth the underlying reasons for the fast nature of these devices, finding that it was because the thickness of the absorber layers required is very low (due to the high absorption coefficients), such that the transport lengths required a very short. |
| Exploitation Route | The outcomes of this project are much more important than the original vision of the project. Originally, the project focussed on defects, but carrier localisation fundamentally limits materials, even if the material were defect-free. This project has pioneered the understanding of this phenomenon in ABZ2 materials, which can be more broadly applied to perovskite-inspired materials. The findings made will change the direction of travel in the wider academic field, and are critical for discovering the next generation of efficient and stable solar absorbers. As a result of the outstanding results of this project, First Solar, the largest manufacturer in the world of thin film photovoltaics, approached us to start a collaboration focussed on discovering the next generation of absorbers for tandem photovoltaics. This project began recently, and will make use of the findings from this project. It is expected that the new materials designed will be patented. |
| Sectors | Electronics Energy |
| URL | https://doi.org/10.1038/s41467-024-55254-2 |
| Description | As part of our pioneering efforts in developing next-generation photovoltaic materials, we have engaged widely with the UK photovoltaics community. In 2020, we co-led a roadmapping effort on the photovoltaics landscape in the UK, and what investment is required to help us reach net-zero by 2050: https://www.royce.ac.uk/materials-for-the-energy-transition-photovoltaic-systems/ This has informed the strategic goals of the Henry Royce Institute. Last year, we took a led a photovoltaics roadmap that brought together >80 groups from across the UK industry, academia and national labs to discuss the key challenges across the main technologies in the photovoltaics field, and what opportunities are emerging that will allow us to overcome these challenges. This has now been published as an invited roadmap article to Journal of Physics: Energy (https://doi.org/10.1088/2515-7655/ad7404). |
| First Year Of Impact | 2023 |
| Sector | Energy |
| Impact Types | Economic Policy & public services |
| Description | Input into Scottish Draft Energy Policy |
| Geographic Reach | National |
| Policy Influence Type | Contribution to a national consultation/review |
| Impact | Hoye's feedback to the Scottish Energy Plan has led to the proposed ideas being more realistic and more attuned to the specific needs of the UK |
| Description | Roadmap for photovoltaics research |
| Geographic Reach | Multiple continents/international |
| Policy Influence Type | Contribution to a national consultation/review |
| Impact | This roadmap is written in an accessible way, and is available to the whole public. By providing a comprehensive discussion of the status and challenges across the breadth of the photovoltaics field, we communicated to the wider public, both within the community and in the wider energy community, the key areas to focus on for photovoltaics, and key areas requiring further investment. To make this roadmap more widely known, we spoke about it in a podcast with Physics World, which is available here: https://physicsworld.com/a/how-to-boost-the-sustainability-of-solar-cells/ |
| URL | https://doi.org/10.1088/2515-7655/ad7404 |
| Description | Tutorials on photovoltaic materials and ultrafast spectroscopy |
| Geographic Reach | National |
| Policy Influence Type | Influenced training of practitioners or researchers |
| Impact | These courses provide a strong foundation in the fundamentals of photovoltaics and spectroscopy, upon which our research is based. Following the delivery of these courses, there was an improved level of understanding, as evidenced in the ability of the students to engage with the concepts, and ability to analyse the data obtained. The improved skills of the wider cohort of students taught from the IMAT CDT was evidenced from the work they produced in the workshops. |
| Description | ECCS-EPSRC: A new generation of cost-effective, scalable and stable radiation detectors with ultrahigh detectivity |
| Amount | £766,208 (GBP) |
| Funding ID | EP/Y032942/1 |
| Organisation | Engineering and Physical Sciences Research Council (EPSRC) |
| Sector | Public |
| Country | United Kingdom |
| Start | 03/2023 |
| Description | John Fell Fund |
| Amount | £71,984 (GBP) |
| Funding ID | DPD00380 |
| Organisation | University of Oxford |
| Department | Department of Chemistry |
| Sector | Academic/University |
| Country | United Kingdom |
| Start | 01/2023 |
| End | 12/2023 |
| Title | CSD 2201891: Experimental Crystal Structure Determination |
| Description | Related Article: Robert A. Jagt, Ivona Bravic, Lissa Eyre, Krzysztof Galkowski, Joanna Borowiec, Kavya Reddy Dudipala, Michal Baranowski, Mateusz Dyksik, Tim W. J. van de Goor, Theo Kreouzis, Ming Xiao, Adrian Bevan, Paulina Plochocka, Samuel D. Stranks, Felix Deschler, Bartomeu Monserrat, Judith L. MacManus-Driscoll, Robert L. Z. Hoye |2023|Nat.Commun.|14|2452|doi:10.1038/s41467-023-38008-4 |
| Type Of Material | Database/Collection of data |
| Year Produced | 2023 |
| Provided To Others? | Yes |
| URL | http://www.ccdc.cam.ac.uk/services/structure_request?id=doi:10.25505/fiz.icsd.cc2cx7r9&sid=DataCite |
| Title | Elucidating the Role of Ligand Engineering on Local and Macroscopic Charge-Carrier Transport in NaBiS2 Nanocrystal Thin Films |
| Description | The experimental raw data for an accepted paper. The data is stored in Excel spreadsheet. |
| Type Of Material | Database/Collection of data |
| Year Produced | 2024 |
| Provided To Others? | Yes |
| Impact | This dataset supports our publication in press on ligand engineering NaBiS2 to understand its macroscopic charge-carrier transport. All raw data is contained in this dataset. In this work, we discovered that while the macroscopic charge-carrier transport of NaBiS2 could be improved by exchanging the original long-chain ligands with short ligands, carrier localisation still takes place. We also revealed NaBiS2 to be a strong ionic conductor, which causes hysteresis in its photovoltaic performance and makes the material better suited to applications required ionic conduction, such as energy storage devices. |
| URL | https://ora.ox.ac.uk/objects/uuid:0868d86d-e133-4ea4-90cb-2a10da359dcf |
| Title | Fast near-infrared photodetectors based on nontoxic and solution-processable AgBiS2 |
| Description | Data relating to 'Fast near-infrared photodetectors based on nontoxic and solution-processable AgBiS2'. |
| Type Of Material | Database/Collection of data |
| Year Produced | 2023 |
| Provided To Others? | Yes |
| Impact | In this work, AgBiS2 near-infrared photodetectors are developed with a cut-off frequency exceeding 500 kHz, making these among the fastest inorganic detectors. We demonstrate the applicability of these devices for practical applications through heartbeat monitoring, and show the underlying reasons behind the rapid operation comes from the high absorption strength, enabling very thin absorbers to be used, which fall entirely within the built in field of the device. |
| URL | https://ora.ox.ac.uk/objects/uuid:d6816230-7d6c-4a6d-a3e2-12281404a3ab |
| Description | Collaboration on computational materials discovery |
| Organisation | Imperial College London |
| Department | Faculty of Engineering |
| Country | United Kingdom |
| Sector | Academic/University |
| PI Contribution | We synthesised novel ternary chalcogenide materials, including NaBiS2 and AgBiS2, and characterised their properties by ultrafast spectroscopy techniques. |
| Collaborator Contribution | Prof. Aron Walsh's group provided in-depth computations to understand the properties of the materials investigated. For example, they calculated the electronic structure, Fröhlich coupling constant, phonon dispersion curve and the deformation potentials of the materials investigated. This allowed us to understand the factors that affect carrier localisation in these materials. Through this collaboration, we have been able to offer to a DPhil student, Hugh Lohan, the opportunity to learn from both of us on computations as well as materials chemistry. This is a unique experience, since normally groups would only offer one of these two aspects. |
| Impact | Joint publications in progress. We worked together prior to 2023 and have published 6 papers together, including in Nature Communications (x2) on defect tolerance and carrier localisation in NaBiS2. This collaboration is multi-disciplinary, involving materials chemistry and density functional theory calculations. |
| Start Year | 2023 |
| Description | Collaboration on photodetector development |
| Organisation | Imperial College London |
| Department | Department of Chemistry |
| Country | United Kingdom |
| Sector | Academic/University |
| PI Contribution | Worked together to develop AgBiS2 photodetectors. We developed the AgBiS2 nanocrystals, and the preparation of thin films from these nanocrystals (including the optimal ligand exchange process). |
| Collaborator Contribution | The Gasparini group in the Department of Chemistry at Imperial College London developed the optimal device architecture for the photodetectors, and characterised their performance, as well as demonstrated their practical applications as heartbeat sensors. |
| Impact | Joint publication: https://doi.org/10.1002/smll.202310199 This collaboration is multi-disciplinary, involving chemistry, materials science and electrical engineering. |
| Start Year | 2023 |
| Description | Collaboration on terahertz spectroscopy measurements |
| Organisation | University of Warwick |
| Department | Warwick Centre for Ultrafast Spectroscopy |
| Country | United Kingdom |
| Sector | Academic/University |
| PI Contribution | Optical pump terahertz probe spectroscopy measurements are essential for us to determine whether carrier localisation takes place in the materials we are investigating. This facility is available in the Centre for Ultrafast Spectroscopy at Warwick. We provide the samples, with optimised phase purity, and provide the research hypotheses. |
| Collaborator Contribution | The facility manager in the Centre provides us with access to the facility, for a standard user fee, and provides us with training to conduct the measurements. |
| Impact | We have several ongoing projects involving this collaboration. The materials and hypotheses we are currently investigating are: 1) BiOX compounds (X = I, Br, Cl); understanding the role of Bi-X ionicity and frontier orbital energy alignment on carrier localisation. 2) Cu3SbSe3, CuSbSe2, along with the S analogs; understanding the role of structure (layered vs. 3D) on carrier localisation. This is to test the generalisability of the design rules for band-like transport we developed by investigating CuSbSe2, which we recently published in Nature Communications 3) BiSBr thin films vs. nanocrystals; understand extrinsic self-trapping in this system 4) AgBiS2 thin films and nanocrystals, with hexagonal vs. disordered rock salt structures; understanding the effect of grain size and structure on carrier localisation. The 3rd and 4th areas are the furtherest advanced, while the first area is still under development. We anticipate to close these projects this year and publish on them. |
| Start Year | 2023 |
| Title | NON-TOXIC X-RAY DETECTORS WITH LOW DETECTION LIMITS AND X-RAY PANELS FOR USE IN THE SAME |
| Description | This invention relates to a panel for an X-ray detector comprising a bismuth oxyiodide (BiOl) single crystal material, or a material derived therefrom, as well as a bismuth oxyiodide single crystal material useful in such an X-ray detector, as well as a method for preparing the same. In one aspect, the present invention provides a bismuth oxyiodide (BiOl) single crystal material for use in a panel for an X-ray detector having a length (L) dimension of at least 1 mm, a width (W) dimension of at least 1 mm, and a thickness (T) dimension of at least 0.12 mm. |
| IP Reference | WO2023180733 |
| Protection | Patent / Patent application |
| Year Protection Granted | 2023 |
| Licensed | No |
| Impact | Through this patent, I have attracted commercial interest from 5N Plus, who are interested in the potential to license this patent and commercialise the BiOI X-ray detectors. I also secured funding from the EPSRC-ECCS scheme, as well as from the Royal Academy of Engineering to develop this technology further, with the hope of ultimately creating a spin off company. |
| Description | Symposium organisation at 2024 Spring MRS Meeting and Exhibit (Seattle, USA) |
| Form Of Engagement Activity | Participation in an activity, workshop or similar |
| Part Of Official Scheme? | No |
| Geographic Reach | International |
| Primary Audience | Postgraduate students |
| Results and Impact | Prof. Hoye is co-organising a symposium on earth-abundant materials for solar fuels, held at the 2024 Spring MRS Meeting & Exhibit. This includes emerging photovoltaic materials being used for solar fuel applications. In particular, Prof. Hoye is aiming to bridge the divide between the photovoltaics and photo electrochemistry communities. |
| Year(s) Of Engagement Activity | 2024 |
| URL | https://www.mrs.org/meetings-events/spring-meetings-exhibits/2024-mrs-spring-meeting |
| Description | Talk at London International Science Youth Forum |
| Form Of Engagement Activity | A talk or presentation |
| Part Of Official Scheme? | No |
| Geographic Reach | International |
| Primary Audience | Schools |
| Results and Impact | I was invited to give a specialists talk on photovoltaics at the London International Science Youth Forum. This is a prestigious STEM outreach event that takes place each year, located at Imperial College London. I spoke about my research on developing novel materials for indoor light harvesting, as part of this project, and this inspired students to think more broadly about the exciting opportunities of photovoltaics. |
| Year(s) Of Engagement Activity | 2023 |
| Description | Talk at STEM Ambassador's event hosted by Royal Academy of Engineering |
| Form Of Engagement Activity | Participation in an activity, workshop or similar |
| Part Of Official Scheme? | No |
| Geographic Reach | National |
| Primary Audience | Professional Practitioners |
| Results and Impact | The Royal Academy of Engineering organises workshops to train their STEM Ambassadors to engage with their outreach mission to raise the profile of engineering nationally. I delivered a talk to inspire them and share my experience as a STEM Ambassador. |
| Year(s) Of Engagement Activity | 2023 |