Resume

• 2008

  French

  Ph.D.

  Physics

  University of Cambridge

• 2002

  M.Sc.

  B.Sc.

  B.A.

  Engineering Physics

  History

  Queen's University

• 2

  Mengfei Wu

  Nature Photonics

  Optical upconversion via sensitized triplet–triplet exciton annihilation converts incoherent low-energy photons to shorter wavelengths under modest excitation intensities1

  3. Here

  we report a solid-state thin film for infrared-to-visible upconversion that employs lead sulphide colloidal nanocrystals as a sensitizer. Upconversion is achieved from pump wavelengths beyond λ = 1 μm to emission at λ = 612 nm. When excited at λ = 808 nm

  two excitons in the sensitizer are converted to one higher-energy state in the emitter at a yield of 1.2 ± 0.2%. Peak efficiency is attained at an absorbed intensity equivalent to less than one sun. We demonstrate that colloidal nanocrystals are an attractive alternative to existing molecular sensitizers

  given their small exchange splitting

  wide wavelength tunability

  broadband infrared absorption

  and our transient observations of efficient energy transfer. This solid-state architecture for upconversion may prove useful for enhancing the capabilities of solar cells and photodetectors.

  Solid-state infrared-to-visible upconversion sensitized by colloidal nanocrystals

  Neil C. Greenham

  Richard H. Friend

  We demonstrate an organic/inorganic hybrid photovoltaic device architecture that uses singlet exciton fission to permit the collection of two electrons per absorbed high-energy photon while simultaneously harvesting low-energy photons. In this solar cell

  infrared photons are absorbed using lead sulfide (PbS) nanocrystals. Visible photons are absorbed in pentacene to create singlet excitons

  which undergo rapid exciton fission to produce pairs of triplets. Crucially

  we identify that these triplet excitons can be ionized at an organic/inorganic heterointerface. We report internal quantum efficiencies exceeding 50% and power conversion efficiencies approaching 1%. These findings suggest an alternative route to circumvent the Shockley-Queisser limit on the power conversion efficiency of single-junction solar cells.

  Singlet exciton fission-sensitized infrared quantum dot solar cells

  Dassia Egorova

  Donatas Zigmantas

  Sarah E. Morgan

  Singlet fission is the spin-allowed conversion of a spin-singlet exciton into a pair of spin-triplet excitons residing on neighbouring molecules. To rationalize this phenomenon

  a multiexcitonic spin-zero triplet-pair state has been hypothesized as an intermediate in singlet fission. However

  the nature of the intermediate states and the underlying mechanism of ultrafast fission have not been elucidated experimentally. Here

  we study a series of pentacene derivatives using ultrafast two-dimensional electronic spectroscopy and unravel the origin of the states involved in fission. Our data reveal the crucial role of vibrational degrees of freedom coupled to electronic excitations that facilitate the mixing of multiexcitonic states with singlet excitons. The resulting manifold of vibronic states drives sub-100 fs fission with unity efficiency. Our results provide a framework for understanding singlet fission and show how the formation of vibronic manifolds with a high density of states facilitates fast and efficient electronic processes in molecular systems.

  Real-time observation of multiexcitonic states in ultrafast singlet fission using coherent 2D electronic spectroscopy

  Richard H. Friend

  Jenny Clark

  Riccardo di Pietro

  We use transient absorption spectroscopy to demonstrate that the dynamics of singlet exciton fission in tetracene are independent of temperature (10–270 K). Low-intensity

  broad-band measurements allow the identification of spectral features while minimizing bimolecular recombination. Hence

  by directly observing both species

  we find that the time constant for the conversion of singlets to triplet pairs is ∼90 ps. However

  in contrast to pentacene

  where fission is effectively unidirectional

  we confirm that the emissive singlet in tetracene is readily regenerated from spin-correlated “geminate” triplets following fission

  leading to equilibrium dynamics. Although free triplets are efficiently generated at room temperature

  the interplay of superradiance and frustrated triplet diffusion contributes to a nearly 20-fold increase in the steady-state fluorescence as the sample is cooled. Together

  these results require that singlets and triplet pairs in tetracene are effectively degenerate in energy

  and begin to reconcile the temperature dependence of many macroscopic observables with a fission process which does not require thermal activation.

  Temperature-Independent Singlet Exciton Fission in Tetracene

  Richard Friend

  Heinz Bassler

  Organic photovoltaic devices are currently studied due to their potential suitability for flexible and large-area applications

  though efficiencies are presently low. Here we study pentacene/C60 bilayers using transient optical absorption spectroscopy; such structures exhibit anomalously high quantum efficiencies. We show that charge generation primarily occurs 2−10 ns after photoexcitation. This supports a model where charge is generated following the slow diffusion of triplet excitons to the heterojunction. These triplets are shown to be present from early times (<200 fs) and result from the fission of a spin-singlet exciton to form two spin-triplet excitons. These results elucidate exciton and charge generation dynamics in the pentacene/C60 system and demonstrate that the tuning of the energetic levels of organic molecules to take advantages of singlet fission could lead to greatly enhanced photocurrent in future OPVs.

  Exciton fission and charge generation via triplet excitons in pentacene/C60 bilayers

  Richard H Friend

  Giulio Cerullo

  Daniele Brida

  R.Sai Santosh Kumar

  Jenny Clark

  Akshay Rao

  We use ultrafast transient absorption spectroscopy with sub-20 fs time resolution and broad spectral coverage to directly probe the process of exciton fission in polycrystalline thin films of pentacene. We observe that the overwhelming majority of initially photogenerated singlet excitons evolve into triplet excitons on an ∼80 fs time scale independent of the excitation wavelength. This implies that exciton fission occurs at a rate comparable to phonon-mediated exciton localization processes and may proceed directly from the initial

  delocalized

  state. The singlet population is identified due to the brief presence of stimulated emission

  which is emitted at wavelengths which vary with the photon energy of the excitation pulse

  a violation of Kasha's Rule that confirms that the lowest-lying singlet state is extremely short-lived. This direct demonstration that triplet generation is both rapid and efficient establishes multiple exciton generation by exciton fission as an attractive route to increased efficiency in organic solar cells.

  Ultrafast dynamics of exciton fission in polycrystalline pentacene

  Neil C. Greenham

  Richard H. Friend

  Singlet exciton fission-sensitized solar cells have the potential to exceed the Shockley–Queisser limit by generating additional photocurrent from high-energy photons. Pentacene is an organic semiconductor that undergoes efficient singlet fission—the conversion of singlet excitons into pairs of triplets. However

  the pentacene triplet is non-emissive

  and uncertainty regarding its energy has hindered device design. Here we present an in situ measurement of the pentacene triplet energy by fabricating a series of bilayer solar cells with infrared-absorbing nanocrystals of varying bandgaps. We show that the pentacene triplet energy is at least 0.85 eV and at most 1.00 eV in operating devices. Our devices generate photocurrent from triplets

  and achieve external quantum efficiencies up to 80%

  and power conversion efficiencies of 4.7%. This establishes the general use of nanocrystal size series to measure the energy of non-emissive excited states

  and suggests that fission-sensitized solar cells are a favourable candidate for third-generation photovoltaics.

  In situ measurement of exciton energy in hybrid singlet-fission solar cells

  Richard H. Friend

  In this Account

  we review the results of our recent transient absorption and device-based studies of polycrystalline pentacene. We address the controversy surrounding the assignment of spectroscopic features in transient absorption data

  and illustrate how a consistent interpretation is possible. This work underpins our conclusion that singlet fission in pentacene is extraordinarily rapid (∼80 fs) and is thus the dominant decay channel for the photoexcited singlet exciton. Further

  we discuss our demonstration that triplet excitons generated via singlet fission in pentacene can be dissociated at an interface with a suitable electron acceptor

  such as fullerenes and infrared-absorbing inorganic semiconducting quantum dots. We highlight our recent reports of a pentacene/PbSe hybrid solar cell with a power conversion efficiency of 4.7% and of a pentacene/PbSe/amorphous silicon photovoltaic device. Although substantive challenges remain

  both to better our understanding of the mechanism of singlet exciton fission and to optimize device performance

  this realization of a solar cell where photocurrent is simultaneously contributed from a blue-absorbing fission-capable material and an infrared-absorbing conventional cell is an important step towards a dual-bandgap

  single-junction

  fission-enhanced photovoltaic device

  which could one day surpass the Shockley–Queisser limit.

  Singlet exciton fission in polycrystalline pentacene: From photophysics toward devices

  Jeffery C Grossman

  Donghun Kim

  Chemical oxidation of under-charged Pb atoms reduces the density of trap states by a factor of 40 in films of colloidal PbS quantum dots for devices. These emissive sub-bandgap states are a byproduct of several standard ligand-exchange procedures. X-ray photoelectron spectro­scopy measurements and density function theory simulations demonstrate that they are associated with under-charged Pb.

  Identifying and Eliminating Emissive Sub-bandgap States in Thin Films of PbS Nanocrystals

  Troy Van Voorhis

  Marc A. Baldo

  Richard H. Friend

  Timothy Swager

  Matthew Y. Sfeir

  Exciton fission is a process that occurs in certain organic materials whereby one singlet exciton splits into two independent triplets. In photovoltaic devices these two triplet excitons can each generate an electron

  producing quantum yields per photon of >100% and potentially enabling single-junction power efficiencies above 40%. Here

  we measure fission dynamics using ultrafast photoinduced absorption and present a first-principles expression that successfully reproduces the fission rate in materials with vastly different structures. Fission is non-adiabatic and Marcus-like in weakly interacting systems

  becoming adiabatic and coupling-independent at larger interaction strengths. In neat films

  we demonstrate fission yields near unity even when monomers are separated by >5 Å. For efficient solar cells

  however

  we show that fission must outcompete charge generation from the singlet exciton. This work lays the foundation for tailoring molecular properties like solubility and energy level alignment while maintaining the high fission yield required for photovoltaic applications.

  A transferable model for singlet-fission kinetics

  Prior to the advent of single-molecule fluorescence spectroscopy

  many of the fundamental optical properties of colloidal semiconductor nanocrystal quantum dots were obscured by ensemble averaging over their inherent inhomogeneities. Single quantum dot spectroscopy has become a leading technique for the unambiguous determination of the governing excitonic physics of these quantum-confined systems. The analysis and interpretation of the timing and energies of photons emitted from individual nanocrystals have uncovered unexpected and fundamental electronic processes at the nanoscale. We review several different paradigms for deconstructing the photon stream from single nanocrystals

  ranging from intensity “binning” techniques to more sophisticated methods based on single-photon counting. In particular

  we highlight photon correlation – a powerful developing paradigm in single-nanocrystal studies. The application of photon-correlation techniques to single nanocrystals is changing the study of multiexcitonic recombination dynamics

  uncovering the basic processes governing spectral linewidths and spectral diffusion

  and enabling the extraction of single-nanocrystal properties directly from an ensemble with high statistical significance. These single-molecule techniques have proven invaluable for understanding the physics of nanocrystals and can provide unique insight into other heterogeneous and dynamical systems.

  Deconstructing the photon stream from single nanocrystals: from binning to correlation

  Mark

  Wilson

  MIT

  University of Toronto

  AECL

  Chalk River Laboratories

  Numerical simulations and experimental testing of the thermalhydraulic performance of an all-passive cooling system for next-generation CANDU reactors.

  Research Assistant

  AECL

  Cambridge

  MA

  Studies of singlet exciton fission\nSpectroscopy on semiconducting quantum dots\nResearch into new technologies for renewable energy

  Postdoctoral Associate

  MIT

  Toronto

  Ontario

  Canada

  Assistant Professor

  University of Toronto

• Home - The Wilson Lab

  Welcome to the Wilson Lab

  the newest research effort in experimental physical chemistry & nanoscience in the Department of Chemistry at the University of Toronto. Starting in the summer of 2016

  we will use spectroscopy to reveal and understand the properties of excitonicmaterials

  and chart the flow of excitonic energy in multi-component architectures.

  The Wilson Lab - Research Group Website

  Public Speaking

  Physics

  Materials Science

  Organic Electronics

  Research

  Spectroscopy

  Teaching/mentoring

  Nanotechnology

  Optics

  Data Analysis

  Experimentation

  Energy harvesting of non-emissive triplet excitons in tetracene by emissive PbS nanocrystals

  Marc A. Baldo

  Vladimir Bulovic

  Mengfei Wu

  Patrick R. Brown

  Nicholas J. Thompson

  Nature Materials

  Triplet excitons are ubiquitous in organic optoelectronics

  but they are often an undesirable energy sink because they are spin-forbidden from emitting light and their high binding energy hinders the generation of free electron–hole pairs. Harvesting their energy is consequently an important technological challenge. Here

  we demonstrate direct excitonic energy transfer from ‘dark’ triplets in the organic semiconductor ​tetracene to colloidal ​PbS nanocrystals

  thereby successfully harnessing molecular triplet excitons in the near infrared. Steady-state excitation spectra

  supported by transient photoluminescence studies

  demonstrate that the transfer efficiency is at least (90 ± 13)%. The mechanism is a Dexter hopping process consisting of the simultaneous exchange of two electrons. Triplet exciton transfer to nanocrystals is expected to be broadly applicable in solar and near-infrared light-emitting applications

  where effective molecular phosphors are lacking at present. In particular

  this route to ‘brighten’ low-energy molecular triplet excitons may permit singlet exciton fission sensitization of conventional silicon solar cells.

  Energy harvesting of non-emissive triplet excitons in tetracene by emissive PbS nanocrystals

  Marc A. Baldo

  Troy Van Voorhis

  Matthew Welborn

  Nadav Geva