Edward Butler-Caddle

Background:

I studied Natural Sciences (BA and MSci) at the University of Cambridge, specialising in physics.

For my master’s project I investigated the photo-stability of mixed-halide perovskites, supervised by Dr Aditya Sadhanala. I spin-coated samples using different excesses of precursor solution to determine the effect on photo-stability, specifically the light-induced phase segregation which is thought to occur via halide ion migration.

Research: Understanding Charge Carrier Dynamics in Metal-Halide Perovskites

I am investigating the charge carrier dynamics in metal-halide perovskites which is a class of semiconductors which show great potential for use in photovoltaic and light emitting applications.

I use time-resolved terahertz, time-resolved photoluminescence and time-resolved absorption spectroscopies to probe the carrier lifetimes and mobilities in these materials (see gifs on right).

I also perform steady state photoluminescence and absorption measurements to characterise the optical properties of these materials.

Recent Work: Charge Extraction by Charge Transport Layers

Recently I have been using the spectroscopy techniques outlined above to observe the extraction of electrons and holes out of the perovskite layer by a charge transport layer deposited on top. In a solar cell, the perovskite layer is sandwiched between charge transport layers (CTLs) that extract electrons and holes out of opposite sides of the perovskite layer. Without these CTLs there will be no current extraction so the extraction performance of these layers is crucial to the overall device efficiency.

Teaching:

I am a problems class tutor for the first year undergraduate physics course “Mathematics for Physicists”.

Interests:

In my spare time I enjoy playing a variety of ball sports.

THz spectroscopy

Time-resolved THz spectroscopy involves measuring a short (almost single cycle) pulse of THz frequency radiation in the time domain, and observing how it changes when it passes through a sample. This can reveal information about the charge carrier density and mobility in the sample. Measuring in the time domain means both amplitude and phase information of the transmission spectrum is recorded.
Optical pump terahertz probe (OPTP) spectroscopy can measure the conductivity and population of optically excited carriers as they evolve over time after optical excitation. This is done by comparing the transmission of the THz pulse through the sample with and without optical excitation of the sample.

Metal Halide Perovskites

In the last decade there has been a dramatic surge of research into the properties and synthesis of metal-halide perovskites (MHPs), and their application in photovoltaic (PV) and light emitting devices.
This interest is driven by the rapid rise in the efficiencies of solar cells based on these materials - reaching 25.2% (not stabilised) in 2019 [1] – coupled with the fact that the fabrication methods for MHPs are relatively simple, fast and inexpensive eg spin-coating from solution.
The term Metal-Halide Perovskite actually refers to a broad range of materials which assume the perovskite crystal structure with an ABX3 composition where A is an organic or alkali metal cation, B is a metal ion and X is a halide ion. The X anion is typically I-, Br- or Cl-, the B cation is typically Pb2+ or Sn2+, and the A cation is typically methylammonium(+), formamidinium(+) or Cs(+). The broad range of possible compositions means there is a vast swathe of materials needing to be understood.

[1] - https://www.nrel.gov/pv/cell-efficiency.html

Position: PhD Student

Started: October 2018

Supervisor:

James Lloyd-Hughes

Group:

Ultrafast & THz photonics

Research Interests: perovskite semiconductors, THz spectroscopy, time-resolved spectroscopy, photoluminescence and absorption, solar cells

Funding: EPSRC

Contact

Email: Edward.Butler-Caddle@warwick.ac.uk

Office: MAS 3.09 (map)

A gif showing photoluminescence. A layer of perovskite material is sandwiched between two discs of quartz. A pulse of green light hits the sample from above. This creates an excited region in the perovskite layer containing excited electrons and holes. This region then emits red light called photoluminescence.

Above: A cartoon of measuring photoluminescence from a metal halide perovskite thin film. CTL is the charge transport layer used to extract electrons or holes (e- or h+).

A gif showing transient absorption. A layer of perovskite material is sandwiched between two discs of quartz. A pulse of red light hits the sample from the left. This creates an excited region in the perovskite layer containing excited electrons and holes. A pulse of white light then enters the sample from the left and exits on the right, smaller in amplitude.

Above: A cartoon of a transient absorption measurement of a metal halide perovskite thin film.

A gif showing optical pump terahertz probe. A layer of perovskite material is sandwiched between two discs of quartz. A pulse of red light hits the sample from the left. This creates an excited region in the perovskite layer containing excited electrons and holes. A pulse of THz radiation then enters the sample from the left and exits on the right, smaller in amplitude.

Above: A cartoon of an optical pump terahertz probe measurement of a metal halide perovskite thin film.