Unravelling ultrafast charge recombination and transport dynamics in hybrid perovskites.

Lead Research Organisation: University of Cambridge
Department Name: Physics

Abstract

The photoluminescence (PL) efficiency of the hybrid perovskites are exceptionally high for a poly-crystalline semiconductor. I was the first to report this, as exemplified by lasing in vertical cavity structure, and I have gone on to demonstrate that the strong radiative emission is key for the perovskites' exceptional performance in solar cells (power conversion efficiency >20% reported) and light-emitting diodes (external quantum efficiency now >18% in our labs). Future gains will require understanding and control of the non-radiative losses both in the bulk and at the electrodes. Our initial results showed that the PL arises from the recombination of free charges, but new physical concepts are now required to understand how this process can give such unexpectedly high radiative yields, while maintaining low non-radiative losses. The reported long lifetimes bring much longer length scales into play, compared to typical high-emission III-V quantum-well systems. Material inhomogeneity is a central parameter, since electrons and holes sample large volumes before recombination, which potentially changes their physical properties and interactions.

State-of-the art PL studies recorded spatially-averaged information with time-resolution around 100 ps, due to challenges in recording local signals on short timescales. Yet, we have found carrier interactions already on sub-picosecond times, which are likely to affect recombination at longer time scales. Probing PL at these fast timescales, will now give insights into a new physical regime. For this, our new technique will combine ultrafast spectrally-resolved PL with spatial microscopy, which will advance the state-of-the art in temporal and spatial resolution by an order of magnitude to sub-picosecond and sub-micrometre regimes. We will use this new setup to study carrier recombination and diffusion in a regime dominated by intrinsic properties (controlled by the carrier-carrier interactions), which will allow us to untangle extrinsic effects (controlled by material properties such as trapping). Simultaneously, local probing will resolve the impact of material morphology on recombination and localisation.

Our study will give unprecedented insights into the photo-physics of hybrid perovskites in previously inaccessible, yet highly relevant, temporal and spatial regimes. Our findings will establish a new picture on the physical and material origin enabling the exceptionally-efficient radiative recombination in hybrid perovskites, which will be crucial for unlocking gains in device performance.

Planned Impact

Despite the impressive progress in photovoltaic performance (PCE ~ 21%) and LED efficacies (EQE ~12%), the development of low-cost, renewable optoelectronic applications based on hybrid perovskites, in particular for solar cells and LEDs, is a current important research theme. Within this context, this project seeks insights into the material effect on key processes that control device performance, such as charge recombination and transport. A range of beneficiaries of the project can be identified and we will make every effort to directly engage and connect with them, to maximize the impact of the project. We have identified three primary directions in which the results from this project will have impact beyond the immediate academic field: People and Skills, Economy and Industry, Society and Environment.

People and Skills: One PDRA will be hired during the course of the project, and two PhD students will be directly involved in the research. The developed techniques will establish a powerful research tool, that will be used by PhD students joining in the next academic year (from 10/2018, one student funded from EPSRC PV CDT confirmed) and beyond the duration of this project. Everyone on the project will work in a highly interdisciplinary environment at the interface between physics, material science and industry. The PI will improve his skills in running a successful research group, and will rely on the available training from the University of Cambridge for starting group leaders to make this a success. These actions will generate highly-skilled researchers, with strong inter-personal skills, who are crucial for a working society and economy.

Economy and Industry: The global optoelectronics market will reach $55bn by 2020, with LEDs expected to lead this market. It is crucial that the UK picks up on new technologies to hold and extend its position in this field. The UK has gained a leading role in the development and exploration of hybrid perovskite optoelectronics, with several spin-off companies developing products for market-entry (Oxford PV, Heliochrome). The results from this project will generate intellectual property (IP) with value for the emerging field of hybrid perovskites optoelectronics. This IP will grow existing and promote the development of new companies, which will attract intellectual and investment capital. It is expected that novel insights and IP will be created for the development of new commercial products in the field of spintronics and quantum technologies, which was highlighted as an important segment for UK technologies. Novel insights can substantially change the economics of future technologies, and could open up a wider range of materials with equally promising properties, yet with more economical, and more sustainable, properties. The results from this project will ensure that the relevant IP is made available to industrial stakeholders.

Society and Environment: Climate change is the greatest global challenge we currently face. After transportation, lighting is the largest contributor to global energy consumption, with the majority of this energy still generated from fossil fuels. Failure to find and employ solutions to reduce global warming through more sustainable energy production will have large negative impacts on the quality and standard of life of future generations. Our results will help solving this challenge by developing concepts for novel sustainable energy production and consumption. It is crucial that the public is informed about these challenges and the measures taken to tackle them. We will interact with the public through schools and community out-reach events at the University of Cambridge, and nation-wide, where possible. We will use the high public visibility of the Cavendish Laboratory and the University of Cambridge to interact with policy makers and industrial stakeholders at events such as the Winton Symposia to convey the importance of taking action now.

Publications

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Description Lead halide perovskite semiconductors show real promise for use in solar cells and in LEDs. The photoluminescence from these materials provides very direct information about the unusual electronic properties of these materials. In this process of electron-hole capture followed by photon emission, the kinetics for capture and for emission are important to understand. We have important findings on the process by which one of the carriers (the holes) can be trapped at a locally lower bandgap, recently published in Nature Photonics.
Exploitation Route grant still in progress
Sectors Energy