Extracting in Situ Charge Carrier Diffusion Parameters in Perovskite Solar Cells with Light Modulated Techniques.

Frequency resolved methods are widely used to determine device properties of perovskite solar cells. However, obtaining the electronic parameters for diffusion and recombination by impedance spectroscopy has been so far elusive, since the measured spectra do not present the diffusion of electrons. Here we show that intensity modulated photocurrent spectroscopy (IMPS) displays a high frequency spiraling feature determined by the diffusion-recombination constants, under conditions of generation of carriers far from the collecting contact. We present models and experiments in two different configurations: the standard sandwich-contacts solar cell device and the quasi-interdigitated back-contact (QIBC) device for lateral long-range diffusion. The results of the measurements produce the hole diffusion coefficient of D p = 0.029 cm2/s and lifetime of τ p = 16 μs for one cell and D p = 0.76 cm2/s and τ p = 1.6 μs for the other. The analysis in the frequency domain is effective to separate the carrier diffusion (at high frequency) from the ionic contact phenomena at a low frequency. This result opens the way for a systematic determination of transport and recombination features in a variety of operando conditions

Production of Magnetic Arsenic–Phosphorus Alloy Nanoribbons with Small Band Gaps and High Hole Conductivities

Quasi-1D nanoribbons provide a unique route to diversifying the properties of their parent 2D nanomaterial, introducing lateral quantum confinement and an abundance of edge sites. Here, a new family of nanomaterials is opened with the creation of arsenic–phosphorus alloy nanoribbons (AsPNRs). By ionically etching the layered crystal black arsenic–phosphorus using lithium electride followed by dissolution in amidic solvents, solutions of AsPNRs are formed. The ribbons are typically few-layered, several micrometers long with widths tens of nanometers across, and both highly flexible and crystalline. The AsPNRs are highly electrically conducting above 130 K due to their small band gap (ca. 0.035 eV), paramagnetic in nature, and have high hole mobilities, as measured with the first generation of AsP devices, directly highlighting their properties and utility in electronic devices such as near-infrared detectors, quantum computing, and charge carrier layers in solar cells

Room Temperature Optically and Magnetically Active Edges in Phosphorene Nanoribbons

Nanoribbons - nanometer wide strips of a two-dimensional material - are a unique system in condensed matter physics. They combine the exotic electronic structures of low-dimensional materials with an enhanced number of exposed edges, where phenomena including ultralong spin coherence times, quantum confinement and topologically protected states can emerge. An exciting prospect for this new material concept is the potential for both a tunable semiconducting electronic structure and magnetism along the nanoribbon edge. This combination of magnetism and semiconducting properties is the first step in unlocking spin-based electronics such as non-volatile transistors, a route to low-energy computing, and has thus far typically only been observed in doped semiconductor systems and/or at low temperatures. Here, we report the magnetic and semiconducting properties of phosphorene nanoribbons (PNRs). Static (SQUID) and dynamic (EPR) magnetization probes demonstrate that at room temperature, films of PNRs exhibit macroscopic magnetic properties, arising from their edge, with internal fields of ~ 250 to 800 mT. In solution, a giant magnetic anisotropy enables the alignment of PNRs at modest sub-1T fields. By leveraging this alignment effect, we discover that upon photoexcitation, energy is rapidly funneled to a dark-exciton state that is localized to the magnetic edge and coupled to a symmetry-forbidden edge phonon mode. Our results establish PNRs as a unique candidate system for studying the interplay of magnetism and semiconducting ground states at room temperature and provide a stepping-stone towards using low-dimensional nanomaterials in quantum electronics.Comment: 18 pages, 4 figure

Microcavity-like exciton-polaritons can be the primary photoexcitation in bare organic semiconductors.

Strong-coupling between excitons and confined photonic modes can lead to the formation of new quasi-particles termed exciton-polaritons which can display a range of interesting properties such as super-fluidity, ultrafast transport and Bose-Einstein condensation. Strong-coupling typically occurs when an excitonic material is confided in a dielectric or plasmonic microcavity. Here, we show polaritons can form at room temperature in a range of chemically diverse, organic semiconductor thin films, despite the absence of an external cavity. We find evidence of strong light-matter coupling via angle-dependent peak splittings in the reflectivity spectra of the materials and emission from collective polariton states. We additionally show exciton-polaritons are the primary photoexcitation in these organic materials by directly imaging their ultrafast (5 × 106 m s-1), ultralong (~270 nm) transport. These results open-up new fundamental physics and could enable a new generation of organic optoelectronic and light harvesting devices based on cavity-free exciton-polaritons.EPSRC (EP/R025517/1), EPSRC (EP/M025330/1), ERC Horizon 2020 (grant agreements No 670405 and No 758826), ERC (ERC-2014-STG H2020 639088), Netherlands Organisation for Scientific Research, Swedish Research Council (VR, 2014-06948), Knut and Alice Wallenberg Foundation 3DEM-NATUR (no. 2012.0112), Royal Commission for the Exhibition of 1851, CNRS (France), US Department of Energy, Office of Science, Basic Energy Sciences, CPIMS Program, Early Career Research Program (DE-SC0019188)

Investigating Quantum Processes in Disordered Materials

The last two decades have witnessed incredible progress in realising quantum phenomena, from Bose-Einstein condensation to quantum teleportation. The future holds the exciting prospect of applying this knowledge to technologies based on real materials such as transistors, light emitting diodes and solar cells. The central impediment to this is that real materials are disordered; characterised by crystalline imperfections, atomic vibrations and amorphous microstructures. The effect of disorder on a quantum system can generally be thought of as a perturbation that disrupts the material's underlying structure on some length and timescale, causing the electrons to change their behaviour. This change in the behaviour can range from trivial (trapping on a low energy site or a loss of quantum coherence), to rich phenomena (quantum interference suppressed propagation), possibly even violating basic laws of thermodynamics (systems that never reach thermal equilibrium). It is precisely this duality of richness and unpredictability of the physics of electrons in the presence of disorder that this dissertation investigates, bridging theory, experiment and new techniques. In the first part of this dissertation, we develop the techniques needed to investigate such processes through quantitative ultrafast pump-probe microscopy. These chapters serve as the background and theory chapters typical to most dissertations, but cover original work. In our endeavour to reveal quantum coherent processes in disordered materials we need to solve three key questions - can we tie ultrafast spectral features to the underlying electronic structure of the material, can we image these processes on their native length and timescales, and what do we expect to see when imaging processes at the intersection of quantum coherence and stochastic physics? We address these questions by first establishing a quantitative spectroscopic approach to studying femtosecond (10-15 s) processes, with which one can unambiguously determine the optical effects of a photoinduced perturbation on the electronic structure of a material system. By leveraging the information gained through this quantitative spectroscopy, we build the full optical theory of femtosecond optical microscopy, enabling us to image quantum phenomena with sub-10 nm precision in all three dimensions for the first time. Having demonstrated that we can capture quantum processes in a pump-probe microscope in three-dimensions, we then predict theoretical signatures of quantum coherent transport and the crossover through scattering into a stochastic regime, which we subsequently experimentally observe in hybrid light-matter states measured in our pump-probe microscope. Using this new toolkit, we go on to explore the ultrafast physics of a few material systems in detail, unveiling hitherto inaccessible quantum processes. First, we examine the well-studied photophysical process of singlet fission with our new three-dimensional ultrafast microscope, where we are able to visualise the first three-dimensional picture of the quantum coherent formation of entangled electronic states. Next, in a polycrystalline semiconductor, we find that structural disorder can drive the formation of photoexcited spin domains through local inversion symmetry breaking, exemplifying the unexpected and rich physics that can arise in disordered material systems. In the vein of exploring the coupling of optical excitations to magnetism, we then investigate a new two-dimensional material, monolayer nanoribbons of black phosphorus, where we find that the ribbons show signatures of strong ground state magnetism and on picosecond timescales, the photoexcitation couples to the magnetic edges. Finally, we examine the role of `dynamic disorder' that manifests through the ever-present lattice vibrations, an area that has historically been challenging to study. We approach this problem computationally using first principle density functional theory (DFT) by capturing the empirically well-established phenomenology of an exponential sub-gap absorption tail known as the Urbach tail. By laying the foundational principles of capturing Urbach tails using DFT in two well-studied semiconductors, we retrieve the role of particular phonon modes, the localisation of Urbach tail states, the role of long wavelength phonon vibrations and finally, a surprising Urbach tail even at 0 K due to zero-point phonons, forcing a serious reconsideration of the physicality of an Urbach tail at 0 K.Gates Cambridge Trust Winton Program for the Physics of Sustainabilit

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 reso- lution, 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 (SiO2) 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 max- imal 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. Our methodology lays the foundation for the study of three dimensional transport on ultrafast timescales

Data For:Tuning the coherent propagation of organic exciton-polaritons through dark state delocalization

Raw data associated with Figures 1 and 2 of main text and supporting information. Origin/Excel files containing raw data underlying figures. Absorption, reflectivity spectra of cavities, angle depdt. reflectivity, fs-TA spectra and associated fits from kinetics, raw TA microscopy data as well as processed kinetics and mean square displacement plots

Thermodynamic Limits of Photon-Multiplier Luminescent Solar Concentrators

Luminescent solar concentrators (LSCs) are theoretically able to concentrate both direct and diffuse solar radiation with extremely high efficiencies. Photon-multiplier luminescent solar concentrators (PM-LSCs) contain chromophores which exceed 100\% photoluminescence quantum efficiency. PM-LSCs have recently been experimentally demonstrated and hold promise to outcompete traditional LSCs. However, we find that the thermodynamic limits of PM-LSCs are different and are sometimes more extreme relative to traditional LSCs. As might be expected, to achieve very high concentration factors a PM-LSC design must also include a free energy change, analogous to the Stokes shift in traditional LSCs. Notably, unlike LSCs, the maximum concentration ratio of a PM-LSC is dependent on brightness of the incident photon field. For some brightnesses, but equivalent energy loss, the PM-LSC has a greater maximum concentration factor than that of the traditional LSC. We find that the thermodynamic requirements to achieve highly concentrating PM-LSCs differ from traditional LSCs. The new model gives insight into the limits of concentration of PM-LSCs and may be used to extract design rules for further PM-LSC design

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Direct Imaging of Carrier Funneling in a Dielectric Engineered 2D Semiconductor.

Publication status: PublishedIn atomically thin transition-metal dichalcogenides (TMDCs), the environmental sensitivity of the strong Coulomb interaction offers promising approaches to create spatially varying potential landscapes in the same continuous material by tuning its dielectric environment. Thus, allowing for control of transport. However, a scalable and CMOS-compatible method for achieving this is required to harness these effects in practical applications. In addition, because of their ultrashort lifetime, observing the spatiotemporal dynamics of carriers in monolayer TMDCs, on the relevant time scale, is challenging. Here, we pattern and deposit a thin film of hafnium oxide (HfO2) via atomic layer deposition (ALD) on top of a monolayer of WSe2. This allows for the engineering of the dielectric environment of the monolayer and design of heterostructures with nanoscale spatial resolution via a highly scalable postsynthesis methodology. We then directly image the transport of photoexcitations in the monolayer with 50 fs time resolution and few-nanometer spatial precision, using a pump probe microscopy technique. We observe the unidirectional funneling of charge carriers, from the unpatterned to the patterned areas, over more than 50 nm in the first 20 ps with velocities of over 2 × 103 m/s at room temperature. These results demonstrate the possibilities offered by dielectric engineering via ALD patterning, allowing for arbitrary spatial patterns that define the potential landscape and allow for control of the transport of excitations in atomically thin materials. This work also shows the power of the transient absorption methodology to image the motion of photoexcited states in complex potential landscapes on ultrafast time scales