arXiv subjects

Alexander J. Sneyd

Publications and source records attributed to Alexander J. Sneyd.

4 recordsLinked to original sources

arXiv2022

Direct Observation of Ultrafast Singlet Exciton Fission in Three Dimensions

We present quantitative ultrafast interferometric pump-probe microscopy capable of tracking of photoexcitations with sub 10 nm spatial precision in three dimensions with 15 fs temporal resolution, through retrieval of the full transient photoinduced complex refractive index. We use this methodology to study the spatiotemporal dynamics of the quantum coherent photophysical process of ultrafast singlet exciton fission. Measurements on microcrystalline pentacene films grown on glass (SiO$_2$) and boron nitride (hBN) reveal a 25 nm, 70 fs expansion of the joint-density-of-states along the crystal a,c-axes accompanied by a 6 nm, 115 fs change in the exciton density along the crystal b-axis. We propose that photogenerated singlet excitons expand along the direction of maximal orbital $π$-overlap in the crystal a,c-plane to form correlated triplet pairs, which subsequently electronically decouples into free triplets along the crystal b-axis due to molecular sliding motion of neighbouring pentacene molecules. This methodology opens a new route for the study of three dimensional transport on ultrafast timescales.

physics.optics↗

arXiv2021

The critical role of the donor polymer in the stability of high-performance non-fullerene acceptor organic solar cells

Driven by the rapid development of non-fullerene electron acceptors (NFAs), the power conversion efficiencies of organic solar cells (OSCs) have reached levels suitable for commercial applications. However, the poor operational stability of high-performance NFA OSCs is a remaining fundamental challenge that must be addressed. Whilst previous studies have primarily focused on the NFA component, we consider here the degradation pathways of both the donor and acceptor materials in the benchmark PM6:Y6 blend. Here, we show that light soaking greatly increases the energetic disorder and trap state density in PM6, with little effect on Y6. This is corroborated by electron paramagnetic resonance spectroscopy, which reveals increased recombination via trapped polarons on PM6 after light soaking. In addition, ultrafast optical spectroscopy studies on light-soaked samples show that PM6 singlet excitons are rapidly converted into interchain polaron pairs on sub-100 fs timescales; this process outcompetes electron transfer to Y6, significantly reducing the charge generation yield of the blend. We make similar observations in the parent polymer, PBDB-T, indicating that this class of donor materials, used in most high-performance OSCs to date, are intrinsically unstable to light soaking. Thus, we reveal that the donor polymer can be a further critical weak link in efficient OSC systems, whose degradation mechanism needs to be addressed collectively with NFAs.

cond-mat.mtrl-sci↗

arXiv2021

Dielectric control of reverse intersystem crossing in thermally-activated delayed fluorescence emitters

Thermally-activated delayed fluorescence (TADF) enables organic semiconductors with charge transfer (CT)-type excitons to convert dark triplet states into bright singlets via a reverse intersystem crossing (rISC) process. Here, we consider the role of the dielectric environment in a range of TADF materials with varying changes in dipole moment upon optical excitation. In a dipolar reference emitter, TXO-TPA, environmental reorganisation after excitation in both solution and doped films triggers the formation of the full CT product state. This lowers the singlet excitation energy by 0.3 eV and minimises the singlet-triplet energy gap (ΔEST). Using impulsive Raman measurements, we observe the emergence of two (reactant-inactive) modes at 412 and 813 cm-1 as a vibrational fingerprint of the CT product. In contrast, the dielectric environment plays a smaller role in the electronic excitations of a less dipolar material, 4CzIPN. Quantum-chemical calculations corroborate the appearance of these new product modes in TXO-TPA and show that the dynamic environment fluctuations are large compared to ΔEST. The analysis of the energy-time trajectories and the corresponding free energy functions reveals that the dielectric environment significantly reduces the activation energy for rISC, thus increasing the rISC rate by up to three orders of magnitude when compared to a vacuum environment.

physics.app-ph↗

arXiv2021

Efficient Energy Transport in an Organic Semiconductor Mediated by Transient Exciton Delocalization

Efficient energy transport is highly desirable for organic semiconductor (OSC) devices such as photovoltaics, photodetectors, and photocatalytic systems. However, photo-generated excitons in OSC films mostly occupy highly localized states over their lifetime. Energy transport is hence thought to be mainly mediated by the site-to-site hopping of localized excitons, limiting exciton diffusion coefficients to below ~10^{-2} cm^2/s with corresponding diffusion lengths below ~50 nm. Here, using ultrafast optical microscopy combined with non-adiabatic molecular dynamics simulations, we present evidence for a new highly-efficient energy transport regime: transient exciton delocalization, where energy exchange with vibrational modes allows excitons to temporarily re-access spatially extended states under equilibrium conditions. In films of highly-ordered poly(3-hexylthiophene) nanofibers, prepared using living crystallization-driven self-assembly, we show that this enables exciton diffusion constants up to 1.1+-0.1 cm^2/s and diffusion lengths of 300+-50 nm. Our results reveal the dynamic interplay between localized and delocalized exciton configurations at equilibrium conditions, calling for a re-evaluation of the basic picture of exciton dynamics. This establishes new design rules to engineer efficient energy transport in OSC films, which will enable new devices architectures not based on restrictive bulk heterojunctions.

cond-mat.mtrl-sci↗