Sam L. Bayliss

Solution-Processable Singlet Fission Photovoltaic Devices

We demonstrate the successful incorporation of a solution-processable singlet fission material, 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene), into photovoltaic devices. TIPS-pentacene rapidly converts high-energy singlet excitons into pairs of triplet excitons via singlet fission, potentially doubling the photocurrent from high-energy photons. Low-energy photons are captured by small-bandgap electron-accepting lead chalcogenide nanocrystals. This is the first solution-processable singlet fission system that performs with substantial efficiency with maximum power conversion efficiencies exceeding 4.8%, and external quantum efficiencies of up to 60% in the TIPS-pentacene absorption range. With PbSe nanocrystal of suitable bandgap, its internal quantum efficiency reaches 170 ± 30%.

Triplet diffusion in singlet exciton fission sensitized pentacene solar cells

Singlet fission sensitized photovoltaics have the potential to surpass the Shockley-Queisser limit for a single-junction structure. We investigate the dynamics of triplet excitons resulting from singlet fission in pentacene and their ionization at a C60 heterojunction. We model the generation and diffusion of excitons to predict the spectral response. We find the triplet diffusion length in polycrystalline pentacene to be 40 nm. Poly(3-hexylthiophene) between the electrode and pentacene works both to confine triplet excitons and also to transfer photogenerated singlet excitons into pentacene with 30% efficiency. The lower bound for the singlet fission quantum efficiency in pentacene is 180 ± 15%.

The Entangled Triplet Pair State in Acene and Heteroacene Materials

Entanglement of states is one of the most surprising and counter-intuitive consequences of quantum mechanics, with potent applications in cryptography and computing. In organic materials, one particularly significant manifestation is the spin-entangled triplet-pair state, which mediates the spin-conserving fission of one spin-0 singlet exciton into two spin-1 triplet excitons. Despite long theoretical and experimental exploration, the nature of the triplet-pair state and inter-triplet interactions have proved elusive. Here we use a range of organic semiconductors that undergo singlet exciton fission to reveal the photophysical properties of entangled triplet-pair states. We find that the triplet pair is bound with respect to free triplets with an energy that is largely material independent (∼30 meV). During its lifetime, the component triplets behave cooperatively as a singlet and emit light through a Herzberg–Teller-type mechanism, resulting in vibronically structured photoluminescence. In photovoltaic blends, charge transfer can occur from the bound triplet pairs with >100% photon-to-charge conversion efficiency.

Spin-dependent recombination probed through the dielectric polarizability.

Despite residing in an energetically and structurally disordered landscape, the spin degree of freedom remains a robust quantity in organic semiconductor materials due to the weak coupling of spin and orbital states. This enforces spin-selectivity in recombination processes which plays a crucial role in optoelectronic devices, for example, in the spin-dependent recombination of weakly bound electron-hole pairs, or charge-transfer states, which form in a photovoltaic blend. Here, we implement a detection scheme to probe the spin-selective recombination of these states through changes in their dielectric polarizability under magnetic resonance. Using this technique, we access a regime in which the usual mixing of spin-singlet and spin-triplet states due to hyperfine fields is suppressed by microwave driving. We present a quantitative model for this behaviour which allows us to estimate the spin-dependent recombination rate, and draw parallels with the Majorana–Brossel resonances observed in atomic physics experiments.

In Situ Optical Measurement of Charge Transport Dynamics in Organic Photovoltaics

We present a novel experimental approach which allows extraction of both spatial and temporal information on charge dynamics in organic solar cells. Using the wavelength dependence of the photonic structure in these devices, we monitor the change in spatial overlap between the photogenerated hole distribution and the optical probe profile as a function of time. In a model system we find evidence for a buildup of the photogenerated hole population close to the hole-extracting electrode on a nanosecond time scale and show that this can limit charge transport through space-charge effects under operating conditions.

Site-selective measurement of coupled spin pairs in an organic semiconductor

Significance Pairs of spins in molecular materials have attracted significant interest as intermediates in photovoltaic devices and light-emitting diodes. However, isolating the local spin and electronic environments of such intermediates has proved challenging due to the complex structures in which they reside. Here we show how exchange coupling can be used to select and characterize multiple coexisting pairs, enabling joint measurement of their exchange interactions and optical profiles. We apply this to spin-1 pairs formed by photon absorption whose coupling gives rise to total-spin S=0,1 and 2-pair configurations with drastically different properties. This presents a way of identifying the molecular conformations involved in spin-pair processes and generating design rules for more effective use of interacting spins. From organic electronics to biological systems, understanding the role of intermolecular interactions between spin pairs is a key challenge. Here we show how such pairs can be selectively addressed with combined spin and optical sensitivity. We demonstrate this for bound pairs of spin-triplet excitations formed by singlet fission, with direct applicability across a wide range of synthetic and biological systems. We show that the site sensitivity of exchange coupling allows distinct triplet pairs to be resonantly addressed at different magnetic fields, tuning them between optically bright singlet (S=0) and dark triplet quintet (S=1,2) configurations: This induces narrow holes in a broad optical emission spectrum, uncovering exchange-specific luminescence. Using fields up to 60 T, we identify three distinct triplet-pair sites, with exchange couplings varying over an order of magnitude (0.3–5 meV), each with its own luminescence spectrum, coexisting in a single material. Our results reveal how site selectivity can be achieved for organic spin pairs in a broad range of systems.

Room temperature magneto-optic effect in silicon light-emitting diodes

In weakly spin–orbit coupled materials, the spin-selective nature of recombination can give rise to large magnetic-field effects, e.g. on the electro-luminescence of molecular semiconductors. Although silicon has weak spin–orbit coupling, observing spin-dependent recombination through magneto-electroluminescence is challenging: silicon’s indirect band-gap causes an inefficient emission and it is difficult to separate spin-dependent phenomena from classical magneto-resistance effects. Here we overcome these challenges and measure magneto-electroluminescence in silicon light-emitting diodes fabricated via gas immersion laser doping. These devices allow us to achieve efficient emission while retaining a well-defined geometry, thus suppressing classical magnetoresistance effects to a few percent. We find that electroluminescence can be enhanced by up to 300% near room temperature in a seven Tesla magnetic field, showing that the control of the spin degree of freedom can have a strong impact on the efficiency of silicon LEDs.Silicon is an important material in spintronics but its inefficiency in light emission limits the optical probes for spin transport. Here Chiodi et al. develop ultra-doped silicon light-emitting devices and show that electroluminescence can be used to probe spin phenomena in silicon even at room temperature.

Research data supporting: The Entangled Triplet Pair State in Acene and Heteroacene Materials

The data comprise the underpinning data of the main text figures and supplementary figures of a paper accepted for publication in Nature Communications. The paper investigates the role of an entangled triplet pair state in the singlet fission process occurring in organic semiconductors.

Research data supporting “Strongly exchange-coupled triplet pairs in an organic semiconductor”

Text files of (1) transient electron spin resonance (ESR) of TIPS-Tetracene as a function of temperature taken at X-band frequencies (~9.6 GHz) in a home-build transient ESR spectrometer, time slices are taken at 300 ns after laser flash in a TIPS-Tetracene film (2) Rabi nutation time traces of Hahn echo intensity as a function of microwave pulse length taken on a Bruker E580 spectrometer configured for X-band using a dielectric resonator for a TIPS-Tetracene film and macrocrystalline sample (3) Hahn echo decay time traces showing the decay of the Hahn echo as a function of delay between a 90 degree and 180 degree microwave pulse for a TIPS-Tetracene film and macrocrystalline sample. (4) Transient ESR intensity time traces taken at 10K in a TIPS-Tetracene film (5) metadata file with any necessary information to understand what each file corresponds to. Details of the experimental setup and corresponding figures can be found in the corresponding publication.