Instilling Defect-Tolerance in ABZ2 Photovoltaic Materials

Lead Research Organisation: Imperial College London
Department Name: Materials

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.

Publications

10 25 50
 
Description This project is a New Investigator Award that Prof. Hoye obtained when he started as a Lecturer at Imperial College London in 2020. He moved to the University of Oxford in 2022, part way through this project, and this award was moved with him. This project is therefore continuing, but for administrative purposes, this part of the project, covering the time when Prof. Hoye was at Imperial, is covered separately. Please refer to the entry for this project at Oxford (EP/V014498/2) for full details.

In the portion of the project at Imperial College, Prof. Hoye's group established the synthesis of NaBiS2 and AgBiS2 materials phase-pure through nanocrystal synthesis. Prof. Hoye investigated NaBiS2 in detail, originally with the aim of understanding the effects of band-edge orbital compositions on defect tolerance. Instead, he found that the charge-carrier kinetics of this material is dominated by carrier localisation. This is a more important finding, since carrier localisation is intrinsic to the material and not dependent on extrinsic factors, such as defects. By taking an joint experimental-computational approach, it was found that carrier localisation likely takes place in this material due to the localised S 3p states that form close to the valence band maximum, which capture holes and favour the formation of small hole polarons. Electrons are also partially localised at Bi3+ sites. The spatial separation of electrons and holes as small polarons accounts for the slow decay in the population of photo excited charge-carriers.
Exploitation Route This project is being continued at Oxford. Please refer to the detailed in the entry for EP/V014498/2
Sectors Energy

URL https://doi.org/10.1038/s41467-022-32669-3
 
Description Course on controlling the properties of thin films to taught postgraduate students
Geographic Reach Multiple continents/international 
Policy Influence Type Influenced training of practitioners or researchers
Impact 50 students attended this course each year. The knowledge imparted improves their skills in thin film growth and control by solution and vapour-based processing.
 
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 Input into UK Blue-Skies Research Funding
Geographic Reach National 
Policy Influence Type Contribution to a national consultation/review
Impact Hoye's input influenced the government's plans for their proposed alternative to Horizon Europe, such as having a global outlook to strengthen the UK's influence in the world
 
Description ERC Starting Grant
Amount € 1,500,000 (EUR)
Funding ID 101039982 
Organisation European Research Council (ERC) 
Sector Public
Country Belgium
Start 09/2022 
End 09/2027
 
Description Henry Royce Institute Industrial Collaboration Programme
Amount £153,713 (GBP)
Funding ID ICP002 (EPSRC reference no: EP/X527257/1) 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 11/2022 
End 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
 
Description Welcome Grant
Amount £12,491 (GBP)
Organisation University of Oxford 
Department St John's College Oxford
Sector Academic/University
Country United Kingdom
Start 03/2023 
End 12/2023
 
Title Grain engineering of Sb2S3 thin films to enable efficient planar solar cells with high open-circuit voltage 
Description Research data for publication 
Type Of Material Database/Collection of data 
Year Produced 2023 
Provided To Others? Yes  
URL https://ora.ox.ac.uk/objects/uuid:125d2a30-3f2e-4624-9ceb-7a8014a98341
 
Title Layered BiOI single crystals capable of detecting low dose rates of X-rays 
Description Data are the experimental raw data, and the files to support the computations for the research paper. Details can be found in the readme files included in this repository 
Type Of Material Database/Collection of data 
Year Produced 2023 
Provided To Others? Yes  
URL https://ora.ox.ac.uk/objects/uuid:28d4c4f4-fb79-4ed3-89af-c179680ee01a
 
Title Strong Absorption and Ultrafast Localisation in NaBiS2 Nanocrystals with Slow Charge-Carrier Recombination 
Description Authors: Yi-Teng Huang,† Seán R. Kavanagh,† Marcello Righetto, Marin Rusu, Igal Levine, Thomas Unold, Szymon J. Zelewski, Alexander J. Sneyd, Kaiwen Zhang, Linjie Dai, Andrew J. Britton, Junzhi Ye, Jaakko Julin, Mari P. Napari, Zhilong Zhang, James Xiao, Mikko Laitinen, Laura Torrente-Murciano, Samuel D. Stranks, Akshay Rao, Laura M. Herz, David O. Scanlon, Aron Walsh, and Robert L. Z. Hoye* †Joint first authors * Corresponding author 
Type Of Material Database/Collection of data 
Year Produced 2022 
Provided To Others? Yes  
Impact This dataset corresponds to all of the experimental and computational data associated with Nat. Commun., 2022, 13, 4960. This work has contributed to the PhD thesis of one the students in Hoye Group (Yi-Teng Huang), who has completed their PhD and will stay as a PDRA, working on this project. This work also led to a successful application for beamtime on the ISIS Neutron and Muon source to perform inelastic neutron scattering measurements. The work also set the research direction for three PhD students working in Hoye Group. 
URL https://data.hpc.imperial.ac.uk/resolve/?doi=10614
 
Description Academic collaboration with Helmholtz Zentrum Berlin 
Organisation Helmholtz Association of German Research Centres
Department Helmholtz-Zentrum Berlin for Materials and Energy
Country Germany 
Sector Academic/University 
PI Contribution We developed novel NaBiS2 absorbers for photovoltaics, devising a route to synthesise phase pure nanocrystals that we then integrated into thin films. We also devised the route to exchange ligands and understand their effect on charge-carrier transport.
Collaborator Contribution Helmholtz zentrum Berlin (group led by Prof. Dr. Thomas Unold) provided surface photovoltage, Kelvin probe, time-resolved microwave conductance, optical pump terahertz probe spectroscopy, and photo-Hall measurements to understand the charge-carrier transport properties and band positions of NaBiS2.
Impact Y.-T. Huang,† S. R. Kavanagh,† M. Righetto, M. Rusu, I. Levine, T. Unold, S. J. Zelewski, A. J. Sneyd, K. Zhang, L. Dai, A. J. Britton, J. Ye, J. Julin, M. P. Napari, Z. Zhang, J. Xiao, M. Laitinen, L. Torrente-Murciano, S. D. Stranks, A. Rao, L. M. Herz, D. O. Scanlon, A. Walsh, R. L. Z. Hoye.* Strong Absorption and Ultrafast Localisation in NaBiS2Nanocrystals with Slow Charge-Carrier Recombination. Nature Communications, 2022, 13, 4960 More publications in the pipeline
Start Year 2021
 
Description Collaboration on THz measurements with Prof. Laura Herz (Oxford) 
Organisation University of Oxford
Department Department of Physics
Country United Kingdom 
Sector Academic/University 
PI Contribution Since beginning this project, a critical factor that has emerged is the role of carrier-phonon coupling. This can prolong charge-carrier lifetimes and screen carriers from defects, thus imparting a form of defect tolerance that is different to the Shockley-Read-Hall model. To truly understand the role of carrier-phonon coupling, it is necessary to track the photoconductivity of the samples on a picosecond timescale. We did this in collaboration with Prof. Laura Herz at the University of Oxford, who provided these measurements. We have performed these studies on NaBiS2 so far, and identified carrier localisation to be responsible for the microsecond lifetimes in these materials. We have plans to conduct these THz measurements on AgBiS2 and CuSbSe2.
Collaborator Contribution Prof. Laura Herz provided optical pump terahertz probe spectroscopy measurements. We are currently preparing a publication on our results with NaBiS2
Impact A paper is underway
Start Year 2021
 
Description Collaboration with Dr. Akshay Rao at the University of Cambridge 
Organisation University of Cambridge
Department Department of Physics
Country United Kingdom 
Sector Academic/University 
PI Contribution My collaboration with Dr. Rao was stated in the original project proposal. He is official supervisor for one of my PhD students (Yi-Teng Huang), who is working (non-costed) on this project. Yi-Teng is leading the work on NaBiS2 and AgBiS2 nanocrystals, and these projects are at an advanced stage, ready to submit as a publication. Yi-Teng is also conducting the transient absorption spectroscopy measurements, which are needed to understand the charge-carrier kinetics of the ABZ2 materials. He is providing these measurements for my team working on this project. In other words, my contribution to this partnership are the ideas and manpower, and I am directly supervising Yi-Teng.
Collaborator Contribution Dr. Rao has provided the necessary official supervisor of Yi-Teng Huang. He has also provided lab-space for Yi-Teng to synthesise the ABZ2 nanocrystals, as well as access to the transient absorption spectroscopy setup.
Impact This collaboration is multi-disciplinary between Physics and Materials. Joint authorship publications are under preparation at the moment.
Start Year 2016
 
Description Early Career Researchers Forum on Horizon Europe 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Policymakers/politicians
Results and Impact A workshop with Minister George Freeman, MP, to discuss the path forward for the UK's investment in blue-skies research. This was primarily to discuss what the alternative to Horizon Europe would be if the UK does not become an Associate Country. The ideas contributed from us were on what makes the UK research environment world class, and how this could best be supported and strengthened in the future.
Year(s) Of Engagement Activity 2022
 
Description Outreach talk (Sutton Trust) 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Schools
Results and Impact A presentation on my research to high school students visiting the Department of Materials at Imperial College London through the Sutton Trust scheme
Year(s) Of Engagement Activity 2021
 
Description Presentation on research at the Materials Society seminar series 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Undergraduate students
Results and Impact Presentation on my group's research into bismuth-based solar cells. This not only taught students the cutting edge in novel solar research, but also encouraged several students to approach my group for summer internships and to write about this as part of their 'Art of Research' project.
Year(s) Of Engagement Activity 2021
 
Description Press release on research from EPSRC project 
Form Of Engagement Activity A magazine, newsletter or online publication
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact Hoye gave an interview to the editor of Materials World (magazine for IOM3) about his recent work into ABZ2 materials from this EPSRC project. This led to a press release about the impact and future opportunities opened up by the work. As a result of this press release, Hoye received enquiries from the companies 5N Plus and Nissan, who are keen to form partnerships with his group.
Year(s) Of Engagement Activity 2022
URL https://www.iom3.org/resource/ultra-light-material-energises-solar-potential.html
 
Description School Visit (Eton College) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Schools
Results and Impact I was invited by Eton College to give a talk as part of their Science seminar series. c. 50 students and three teachers (including head of science) attended this evening talk, where I discussed my research activities in bismuth-based photovoltaics. This stimulated a wide interested from the students on photovoltaic technology, and the potential for indoor light harvesting for the Internet of Things. Importantly, it stimulated interest, awareness and excitement around Materials Science, with several students stating that they are keen on getting to know more about this subject.
Year(s) Of Engagement Activity 2021
 
Description Visit to Eton College 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Schools
Results and Impact Hoye was invited to give an outreach talk to the scientific society of Eton College. He discussed with them his research into sustainable materials and future technologies for green energy harvesting for producing clean electricity or fuels.
Year(s) Of Engagement Activity 2022