The Origin of Non-Radiative Losses in Metal Halide Perovskites

Lead Research Organisation: University of Cambridge
Department Name: Physics

Abstract

Solar cells and light-emitting diodes (LEDs) made from novel, inexpensive materials have the potential to be low-cost, clean and scalable solutions to supply our growing electricity and lighting demands. While solar cells convert sunlight into electrical energy, LEDs are the reverse, with electrical energy transformed into emitted light. Metal halide perovskites are extremely promising materials for both applications. Perovskite solar cells have improved their power conversion efficiency from 3% to 22% in just three years, approaching that of the market-leading technology, silicon (25%). Early reports of perovskite LEDs are also encouraging though relatively unexplored. Perovskite ingredients are abundant and can be combined inexpensively into thin films with a crystalline structure similar to silicon. Rolls of thin, flexible perovskite film could one day be rapidly spooled from a special printer to make lightweight, bendable, and colourful solar and light-emitting sheets.

Nevertheless, the full potential of perovskites has not yet been realised. Strong light emission is essential for both solar cells and LEDs to reach their theoretical efficiency limits, but emission and therefore performance is still limited by parasitic emission loss pathways that are still poorly understood. The films are made up of densely packed crystals (grains) and we hypothesise that each grain has slightly different local chemistry and structural properties, some of which are defective. The ultimate aim of this work is to determine the fundamental origin of these loss pathways in perovskite films and full devices by elucidating which are the optimal chemical and structural properties, and using this information to achieve optimal films.

This aim will be achieved by measuring the grain-to-grain emission using a novel microscope system which will allow rapid imaging of the emission with high spatial resolution. Most microscopic emission measurements on perovskites to date have employed confocal microscopes in which the emission is mapped by taking sequential measurements of the spectra of adjacent regions and moving the sample point by point until the region of interest has been covered. On the other hand, imaging consists of focusing the image of a sample on a detector and measuring for each pixel the intensity of light at one particular wavelength, much like taking a photograph, but at a single wavelength. In some applications, the power of the laser used in imaging can be orders of magnitude higher than in mapping, since the power is spread over the whole region instead of a single point, thus allowing measurement under device-like conditions. Imaging also permits a higher resolution and reduces the acquisition time by orders of magnitude.

Emission images under both light (photoluminescence) and when applying an electrical bias (electroluminescence) will be acquired. The emission images of the same scan area will then be directly correlated with maps of the local grain-to-grain chemistry using electron microscopy techniques including energy-dispersive X-Ray (EDX) spectroscopy and local structural measurements using a nano-X-Ray Diffraction (n-XRD) beamline at the Diamond synchrotron.

The work is highly timely and the results will provide a platform for efforts to take perovskites to their efficiency limits. The work will reveal the specific preferred chemistry and structural properties which must be targeted for growth of higher performing perovskite films and also reveal insights into potential post-treatments capable of healing defects in the perovskite materials. This will be of strong interest to a range of academic researchers in the perovskite field as well as industrial entities such as UK-based Oxford PV, which is leading the current commercialisation efforts of this exciting technology. Finally, the project will allow the PI to establish his team as a world-leading group with a cutting-edge programme and toolset.

Planned Impact

The work has the potential for significant economic impact through important fundamental science discoveries relating to the origins of loss mechanisms in perovskite films and devices. These results will lead directly into wider programmes to exploit this knowledge to rationally eliminate the losses through passivation treatments and/or improved crystal growth. Many of these efforts will be UK-based owing to significant relevant local experience. It is likely that several routes to enhance perovskite performance along these lines will be discovered in the proposed work alone. These findings and any associated IP will be of strong interest to companies focusing on commercialisation of the perovskite technology, and the PI and the University will undertake licensing discussions with relevant entities such as the UK-based companies Oxford PV and Heliochrome. Success in the project and any subsequent licensing agreement will push the perovskite technology ever closer to commercialisation, increasing the value of these indigenous UK companies by attracting further international investment. The technology has enormous scope for international scale and could therefore generate substantial value for the UK economy.

The work would provide the fundamental science that, through the above commercialisation routes, could lead to low-cost, clean renewable solar power and lighting technologies. This would have enormous benefits for UK society by generating strong PV and lighting industries with significant job creation. The next decade will be decisive for the future of the UK PV industry and this project is suitably timely to provide an important contribution to this sector and to energy-based policy. Low-cost power and light-emission technologies will also help the UK and other countries to meet their emissions and renewable energy target, limiting the damaging effects of climate change for future generations. The move from resource-based fossil fuels to sustainable renewables will have positive implications for energy security and the geo-politics. Furthermore, a low-cost energy source could provide power for the 1.3 billion people in the developing world who currently lack access to electricity. Solar power has the potential to do for developing communities what mobile phones have done for telecommunications: they allow several phases of infrastructure to be bypassed, permitting communities without grid electricity to realise dramatic improvements in quality of life. Likewise, demonstration of perovskite lighting at quality comparable to today's technologies will lead to opportunities for deployment of low-cost lighting solutions.

Publications

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Andaji-Garmaroudi Z (2020) Elucidating and Mitigating Degradation Processes in Perovskite Light-Emitting Diodes in Advanced Energy Materials

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Andaji-Garmaroudi Z (2019) A Highly Emissive Surface Layer in Mixed-Halide Multication Perovskites. in Advanced materials (Deerfield Beach, Fla.)

 
Description The work funded through this award has led to a number of new breakthroughs in methodology, understanding and device improvement for halide perovskite optoelectronic devices. These are summarised in three key findings:

i) Visualising local luminescence and carrier transport in halide perovskites. Luminescence - for example through excitation of photons (photoluminescence, PL) or through injection of charges (electroluminescence, EL) - is an excellent probe of power losses in a semiconductor material. We developed a wide-field PL technique to image the local micro-scale PL in halide perovskites, allowing extraction of key parameters for devices including quasi-fermi-level splitting and Urbach energy (measure of disorder). We further identified that luminescence yields are high in the most promising alloyed perovskite photovoltaic materials because carriers funnel onto specific nanoscale sites, and this provides a mechanism to avoid local carrier trap states (Nature Nanotechnology, 2022). We have also modelled recycling of photons in films and devices - unveiling important understanding of how to maximise each photon.

ii) Multimodal microscopy approaches to reveal origin of non-radiative losses in halide perovskites. We have developed a series of techniques to correlate local optoelectronic properties (including luminescence) with local structural, chemical and morphological properties to unveil direct relationships between these important properties - working closely with national facilities including Diamond Light Source and ePSIC to push the envelope on experimental capabilities. The results have revealed the important role of strain and local distorted structural phases in forming nanoscale trap clusters at grain boundaries that act as non-radiative recombination sites and explain the PL map heterogeneity seen in these materials (Nature 2020). These findings have led to a profound change in the community's understanding of deep trap states in perovskites, spurring new work to manage nanoscale structure to achieve optimal performance.

iii) High-performance optoelectronic devices. We have used the enhanced nanoscale understanding to integrate new and passivated materials into promising photovoltaic and LED devices. Examples include blue LEDs with nanoplatelets exhibiting encouraging initial efficiencies, extremely emissive bulk halide perovskite films for solar cells (Nature 2018) and LEDs through use of potassium salts to manage surface halides and mitigate trap states, and record solar cells using vapour deposition methods to control phases.
Exploitation Route These techniques will be widely applicable to both academic and industry users seeking to understand the nanoscale heterogeneity in semiconductors. We envisage two paths forward. First, exploring a wide range of halide perovskite materials beyond those studied in the project together with collaborators. We are starting conversations with industry partners about how to use these approaches to guide technology development. We expect other academic users with similar hyperspectral microscope setups can employ our methodology. Second, exploring a range of other semiconductor materials where nanoscale heterogeneity may be important, including quantum materials (eg 2D transition metal dichalcogendides, CdTe solar cell absorbers). We demonstrated applicability in the project, and we expect this will now be more widely adopted - both by academics and industry.
Sectors Electronics,Energy

 
Description STFC Physical Sciences & Engineering Advisory Panel
Geographic Reach National 
Policy Influence Type Participation in a advisory committee
Impact Improving national STFC facilities access, availability and diversity of equipment
 
Description Bright & Breezy: Exploring next-generation energy solutions with the next generation
Amount £4,000 (GBP)
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 11/2020 
 
Description ECCS-EPSRC Superlattice Architectures for Efficient and Stable Perovskite LEDs
Amount £1,095,855 (GBP)
Funding ID EP/V06164X/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 08/2021 
End 09/2025
 
Description High-Efficiency Flexible and Scalable Halide-Perovskite Solar Modules
Amount £2,271,562 (GBP)
Funding ID EP/V027131/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 01/2022 
End 06/2025
 
Description Marie Sklodowska-Curie Actions Individual Fellowship
Amount € 212,933 (EUR)
Organisation European Commission H2020 
Sector Public
Country Belgium
Start 07/2019 
End 08/2021
 
Title Wide-field hyperspectral imaging of thin films and devices 
Description We have customised a hyperspectral imaging setup to measure photoluminescence, electroluminescence, transmission and reflection of thin film samples under operating conditions relevant to devices (for example photovoltaic or light-emitting diodes). More recently, this has been demonstrated on barrert structures 
Type Of Material Improvements to research infrastructure 
Year Produced 2020 
Provided To Others? No  
Impact We are currently utilising the instrument for a variety of ongoing projects and we hope to have further academic publications on these to report soon. We hope to extend its capabilities to a range of samples from collaborators. 
URL https://www.nature.com/articles/s41565-021-01019-7
 
Title Research data supporting "Nanoscale Chemical Heterogeneity Dominates the Optoelectronic Response of Alloyed Perovskite Solar Cells" 
Description This repository contains the data required to reproduce the figures from the associated manuscript. This data includes hyperspectral optical imaging cubes, nano X-ray fluorescence and diffraction maps and transient absorption microscopy data. 
Type Of Material Database/Collection of data 
Year Produced 2021 
Provided To Others? Yes  
URL https://www.repository.cam.ac.uk/handle/1810/329908
 
Company Name SUSTAIN/EDUCATION LTD 
Description Sustain Education is a UK registered charity (Registered Charity Number 1197693). We are developing climate change teaching resources for primary school children to understand the impacts of climate change and the possible solutions. 
Year Established 2021 
Impact Currently running the modules in 14 schools across the UK, and planning to expand across the country in the coming year.
Website https://sustaineducation.org/
 
Company Name SUSTAIN/EDUCATION LTD 
Description Sustain Education is a UK registered charity (Registered Charity Number 1197693). We are developing climate change teaching resources for primary school children to understand the impacts of climate change and the possible solutions. 
Year Established 2021 
Impact Currently running the modules in 14 schools across the UK, and planning to expand across the country in the coming year.
Website https://sustaineducation.org/
 
Description Annual Cambridge Science Festival 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact We performed a hands-on activity: building dye-sensitised solar cells with frozen berries, and other low-cost materials.
The attendees were given instruction sheets detailing how to build a dye sensitised solar cell, and they were also taught the physics behind how a solar cell works. For this event, we hoped to promote the different applications of solar cells in the world.
Year(s) Of Engagement Activity 2019,2020
URL https://www.sciencefestival.cam.ac.uk/events/ceb-visions
 
Description Article in the Conversation 
Form Of Engagement Activity Engagement focused website, blog or social media channel
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact Article describing latest on perovskite technology and commercialisation pursuits. There have been over 35,000 reads of the article from international audience
Year(s) Of Engagement Activity 2020
URL https://theconversation.com/how-a-new-solar-and-lighting-technology-could-propel-a-renewable-energy-...
 
Description Cambridge Festival of Ideas 2019 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact This activity is a boardgame we invented to teach the general public about what sacrafices will need to be made in order to prevent climate change in our local areas. We ask questions like: Is your community future proof? How will we adapt to climate change? What would you be willing to change for a resilient life?
Year(s) Of Engagement Activity 2019,2020
URL https://www.festivalofideas.cam.ac.uk/events/2050-new-world
 
Description Erasumus programme workshop 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Schools
Results and Impact Students came to visit that are apart of the Erasmus programme that is based around sustainable energy. Their teachers also hoped to get a bit of an understanding of current trends in technologies related to this, alongside a little cultural experience from bringing them to Cambridge. The group was also given a Cavendish Laboratory museum tour and then took part of the physics workshop linked to the sustainability and energy storage. We discussed renewable energy usage in the EU, discussed how we make earth observations to monitor data on climate change, solar energy, and batteries..
Year(s) Of Engagement Activity 2018
 
Description IoP Physics Fest Pop-up in Ipswich 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Schools
Results and Impact The objective is to raise the profile of physics in this region and to try to inspire more children to study physics at GCSE and A level. Our way to do this is to go there with enthusiastic people and do lectures and demonstrations for them in the hope that they may have a 'Wow' moment!
Year(s) Of Engagement Activity 2018
URL https://www.youtube.com/watch?v=C9vryORKevs&index=4&list=PLnpAhkiu3jSY1ZmoAlsuFy6nTfZFxk8ac
 
Description Physics at Work Exhibition 2018 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Schools
Results and Impact The primary aim of the exhibition is to stimulate interest and encourage wider participation in physics amongst 14-16 year olds by showcasing the many and varied ways in which physics is used in the everyday world. The Physics at Work Exhibition is an integral part of the Cavendish Laboratory's Aspiration Raising activities, funded by the Higher Education Funding Council for England.
Year(s) Of Engagement Activity 2018
URL https://outreach.phy.cam.ac.uk/programme/physicsatwork
 
Description Physics at Work annual 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Schools
Results and Impact The primary aim of the exhibition is to stimulate interest and encourage wider participation in physics amongst 14-16 year olds by showcasing the many and varied ways in which physics is used in the everyday world. The Physics at Work Exhibition is an integral part of the Cavendish Laboratory's Aspiration Raising activities, funded by the Higher Education Funding Council for England.
Year(s) Of Engagement Activity 2019,2020
 
Description Primary School Energy Mapping Challenge 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Schools
Results and Impact As scientific researchers at the University of Cambridge one of our jobs is to discover and develop novel materials that will help provide green energy for future generations, with a focus on producing low-cost solar cell materials. We also have a responsibility to share our research and expertise with others in order to accelerate the uptake of green energy sources and spread awareness of the issues faced by society.

The Primary School Energy Mapping Challenge is one such way to reach out and share the knowledge we have gained from our studies and experiments. With this project we aim to teach budding young scientists at Primary Schools across the UK, about the benefits of renewable energy and its potential to permanently replace fossil fuels.

In this programme, we are asking students to measure the sunlight and wind-speed in their school yards, each day, for a six-week period. By providing measurement instruments and demonstrations we will facilitate development of their investigative skills and scientific mindedness. Pupils will log their data on this online portal which not only describes and contextualises the amount of energy that could be generated by a small wind turbine or solar installation on their school grounds but allows them to compare data with other schools around the country.

This project hopes to give awareness to the youth that will be affected by the climate crisis, while also teaching them important critical thinking and data acquisition skills that are appropriate for this age group (Year 6, Primary 7). Equipped with wind and solar measurement tools, the students will take complete ownership of the project. Throughout, they will be exposed to concepts such as energy, power, electricity, and scientific units.

We are thrilled with the energy and enthusiasm shown by the students so far and hope to keep this project growing across the UK. If your Primary School is interested in joining in on this unique challenge, please do not hesitate to contact us!
Year(s) Of Engagement Activity 2020
URL https://energymap.oe.phy.cam.ac.uk/