Imperial College London

Dr Xingyuan Shi

Faculty of Natural SciencesDepartment of Physics

Research Associate in Materials Chemistry







H704Huxley BuildingSouth Kensington Campus





Publication Type

17 results found

Hou X, Coker JF, Yan J, Shi X, Azzouzi M, Eisner FD, McGettrick JD, Tuladhar SM, Abrahams I, Frost JM, Li Z, Dennis TJS, Nelson Jet al., 2024, Structure–Property Relationships for the Electronic Applications of Bis-Adduct Isomers of Phenyl-C₆₁ Butyric Acid Methyl Ester, Chemistry of Materials, Vol: 36, Pages: 425-438, ISSN: 0897-4756

Higher adducts of a fullerene, such as the bis-adduct of PCBM (bis-PCBM), can be used to achieve shallower molecular orbital energy levels than, for example, PCBM or C60. Substituting the bis-adduct for the parent fullerene is useful to increase the open-circuit voltage of organic solar cells or achieve better energy alignment as electron transport layers in, for example, perovskite solar cells. However, bis-PCBM is usually synthesized as a mixture of structural isomers, which can lead to both energetic and morphological disorder, negatively affecting device performance. Here, we present a comprehensive study on the molecular properties of 19 pure bis-isomers of PCBM using a variety of characterization methods, including ultraviolet photoelectron spectroscopy, thermal gravimetric analysis, differential scanning calorimetry, single crystal structure, and (time-dependent) density functional theory calculation. We find that the lowest unoccupied molecular orbital of such bis-isomers can be tuned to be up to 170 meV shallower than PCBM and up to 100 meV shallower than the mixture of unseparated isomers. The isolated bis-isomers also show an electron mobility in organic field-effect transistors of up to 4.5 × 10⁻² cm²/(Vs), which is an order of magnitude higher than that of the mixture of bis-isomers. These properties enable the fabrication of the highest performing bis-PCBM organic solar cell to date, with the best device showing a power conversion efficiency of 7.2%. Interestingly, we find that the crystallinity of bis-isomers correlates negatively with electron mobility and organic solar cell device performance, which we relate to their molecular symmetry, with a lower symmetry leading to more amorphous bis-isomers, less energetic disorder, and higher dimensional electron transport. This work demonstrates the potential of side chain engineering for optimizing the performance of fullerene-based organic electronic devices.

Journal article

Hillman SAJ, Sprick RS, Pearce D, Woods DJ, Sit W-Y, Shi X, Cooper AI, Durrant JR, Nelson Jet al., 2022, Why do sulfone-containing polymer photocatalysts work so well for sacrificial hydrogen evolution from water?, Journal of the American Chemical Society, Vol: 144, Pages: 19382-19395, ISSN: 0002-7863

Many of the highest-performing polymer photocatalysts for sacrificial hydrogen evolution from water have contained dibenzo[b,d]thiophene sulfone units in their polymer backbones. However, the reasons behind the dominance of this building block are not well understood. We study films, dispersions, and solutions of a new set of solution-processable materials, where the sulfone content is systematically controlled, to understand how the sulfone unit affects the three key processes involved in photocatalytic hydrogen generation in this system: light absorption; transfer of the photogenerated hole to the hole scavenger triethylamine (TEA); and transfer of the photogenerated electron to the palladium metal co-catalyst that remains in the polymer from synthesis. Transient absorption spectroscopy and electrochemical measurements, combined with molecular dynamics and density functional theory simulations, show that the sulfone unit has two primary effects. On the picosecond timescale, it dictates the thermodynamics of hole transfer out of the polymer. The sulfone unit attracts water molecules such that the average permittivity experienced by the solvated polymer is increased. We show that TEA oxidation is only thermodynamically favorable above a certain permittivity threshold. On the microsecond timescale, we present experimental evidence that the sulfone unit acts as the electron transfer site out of the polymer, with the kinetics of electron extraction to palladium dictated by the ratio of photogenerated electrons to the number of sulfone units. For the highest-performing, sulfone-rich material, hydrogen evolution seems to be limited by the photogeneration rate of electrons rather than their extraction from the polymer.

Journal article

Hou X, Clarke AJ, Azzouzi M, Yan J, Eisner F, Shi X, Wyatt MF, Dennis TJS, Li Z, Nelson Jet al., 2022, Relationship between molecular properties and degradation mechanisms of organic solar cells based on bis-adducts of phenyl-C₆₁ butyric acid methyl ester, Journal of Materials Chemistry C, Vol: 10, Pages: 7875-7885, ISSN: 2050-7526

Environmental stability remains a major challenge for the commercialisation of organic solar cells and degradation pathways remain poorly understood. Designing materials for improved device stability requires an understanding of the relationship between the properties of the donor or acceptor molecule and different degradation mechanisms. Here we study the correlations between various molecular parameters of the fullerene derivative bis-PCBM and the degradation rate of polymer:bis-PCBM organic solar cells, based on the same carbazole-alt-benzothiadiazole polymer, in aerobic and anaerobic conditions. We compare eight high purity bis-PCBM isomers with different electronic, chemical and packing properties along with PCBM and the mixture of bis isomers. In the case of aerobic photodegradation, we find that device degradation rate is positively correlated to the LUMO energy of the bis-PCBM isomer and to the degree of crystallinity of the isomer, while the correlation of degradation with driving force for epoxide formation is unclear. These results support the idea that in these samples, aerobic photodegradation proceeds via superoxide formation by the photogenerated polaron on the fullerene, followed by further chemical reaction. In the absence of air, photodegradation rate is correlated with molecular structure, supporting the mechanism of microstructural degradation via fullerene dimerization. The approach and findings presented here show how control of specific molecular parameters through chemical design can serve as a strategy to enhance stability of organic solar cells.

Journal article

Ward MD, Wade J, Shi X, Nelson J, Campbell AJ, Fuchter MJet al., 2022, Highly selective high-speed circularly polarized photodiodes based on π-conjugated polymers, Advanced Optical Materials, Vol: 10, ISSN: 2195-1071

Chiral π-conjugated molecular systems that are intrinsically sensitive to the handedness of circularly polarized (CP) light potentially allow for miniaturized, low-cost CP detection devices. Such devices promise to transform several technologies, including biosensing, quantum optics, and communication of data encrypted by exploiting the spin angular momentum of light. Here a simple, bilayer organic photodiode (CP OPD) comprising an achiral π-conjugated polymer–chiral additive blend as the electron donor layer and an achiral C60 electron acceptor layer is realized. These devices exhibit considerable photocurrent dissymmetry gph, with absolute values as high as 0.85 and dark currents as low as 10 pA. Impressively, they showcase a linear dynamic range of 80 dB, and rise and fall times of ≈7 µs, which significantly outperforms all previously reported CP selective photodetectors. Mechanistically, it is shown that the gph is sensitive to the thickness of both the chiral donor and achiral acceptor layers and that a trade-off exists between the external quantum efficiency and gph. The fast-switching speeds of these devices, coupled with their large dynamic range and highly selective response to CP light, opens up the possibility of their direct application in CP sensing and optical communications.

Journal article

Laidlaw B, Eng J, Wade J, Shi X, Salerno F, Fuchter MJ, Penfold TJet al., 2021, On the factors influencing the chiroptical response of conjugated polymer thin films, Chemical Communications, Vol: 57, Pages: 9914-9917, ISSN: 1359-7345

We study the influence of the physical and chemical structure on the chiroptical response of fluorene-based polymeric systems, namely poly(9,9-dioctylfluorene) (PFO) and the donor–acceptor type copolymer poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT). We reveal the significance of electric-magnetic coupling, at both short (molecular-level) and intermediate (delocalised over multiple polymer chains) length scales, on the magnitude of the dissymmetry. These findings provide a framework for the design of new materials with enhanced chiroptical response.

Journal article

Greenfield JL, Wade J, Brandt JR, Shi X, Penfold TJ, Fuchter MJet al., 2021, Pathways to increase the dissymmetry in the interaction of chiral light and chiral molecules, Chemical Science, Vol: 12, Pages: 8589-8602, ISSN: 2041-6520

The dissymmetric interaction between circularly polarised (CP) light and chiral molecules is central to a range of areas, from spectroscopy and imaging to next-generation photonic devices. However, the selectivity in absorption or emission of left-handed versus right-handed CP light is low for many molecular systems. In this perspective, we assess the magnitude of the measured chiroptical response for a variety of chiral systems, ranging from small molecules to large supramolecular assemblies, and highlight the challenges towards enhancing chiroptical activity. We explain the origins of low CP dissymmetry and showcase recent examples in which molecular design, and the modification of light itself, enable larger responses. Our discussion spans spatial extension of the chiral chromophore, manipulation of transition dipole moments, exploitation of forbidden transitions and creation of macroscopic chiral structures; all of which can increase the dissymmetry. Whilst the specific strategy taken to enhance the dissymmetric interaction will depend on the application of interest, these approaches offer hope for the development and advancement of all research fields that involve interactions of chiral molecules and light.

Journal article

Wan L, Shi X, Wade J, Campbell AJ, Fuchter MJet al., 2021, Strongly Circularly Polarized Crystalline and β-Phase Emission from Poly(9,9-dioctylfluorene)-Based Deep-Blue Light-Emitting Diodes, Advanced Optical Materials, Vol: 9, ISSN: 2195-1071

A key challenge in the realization of circularly polarized polymer light-emitting diodes (CP-PLEDs) is the generation of highly circularly polarized deep-blue electroluminescence (EL). Here, by blending the achiral luminescent polymer poly(9,9-dioctylfluorene) (PFO) with a helically chiral molecule 1-aza[6]helicene the authors present CP-PLEDs with state-of-the-art device performance for deep-blue CP emission: for an inverted device with a semicrystalline microstructure a current efficiency (CE) of 1.13 cd A−1, a power efficiency (PE) of 0.81 lm W−1, and an EL dissymmetry (gEL) of −0.42 are achieved; for the planarized and extended “β-phase” chain conformation a CE of 1.23 cd A−1, a PE of 0.63 lm W−1, and a gEL of −0.44 are achieved. While these two phases achieve both high CE, as well as gEL, the latter affords the first demonstration of chiral β-phase emission from solid-state PFO devices. Such strongly circularly polarized light is generated from a supramolecular assembly of interacting planar polymer backbones. The authors rationalize that the strong chiroptical effects observed within such chiral β-phase PFO domains originate from coupled interchain aggregates. The findings not only demonstrate efficient deep-blue CP-PLEDs, but also provide insight into the mechanisms that underpin the strong CP emission from excitonically coupled polymer chains.

Journal article

Wade J, Hilfiker J, Brandt J, Liirò-Peluso L, Wan L, Shi X, Salerno F, Ryan S, Schöche S, Arteaga O, Jávorfi T, Siligardi G, Wang C, Amabilino D, Beton P, Campbell A, Fuchter Met al., 2020, Natural optical activity as the origin of the large chiroptical properties in π-conjugated polymer thin films, Nature Communications, Vol: 11, ISSN: 2041-1723

Polymer thin films that emit and absorb circularly polarised light have been demonstrated with the promise of achieving important technological advances; from efficient, high-performance displays, to 3D imaging and all-organic spintronic devices. However, the origin of the large chiroptical effects in such films has, until now, remained elusive. We investigate the emergence of such phenomena in achiral polymers blended with a chiral small-molecule additive (1-aza[6]helicene) and intrinsically chiral-sidechain polymers using a combination of spectroscopic methods and structural probes. We show that – under conditions relevant for device fabrication – the large chiroptical effects are caused by magneto-electric coupling (natural optical activity), not structural chirality as previously assumed, and may occur because of local order in a cylinder blue phase-type organisation. This disruptive mechanistic insight into chiral polymer thin films will offer new approaches towards chiroptical materials development after almost three decades of research in this area.

Journal article

Perevedentsev A, Francisco-López A, Shi X, Braendle A, Caseri WR, Goñi AR, Campoy-Quiles Met al., 2020, Homoconjugation in Light-Emitting Poly(phenylene methylene)s: Origin and Pressure-Enhanced Photoluminescence, Macromolecules, Vol: 53, Pages: 7519-7527, ISSN: 0024-9297

The surprising optical properties of the non-π-conjugated polymer poly(phenylene methylene) (PPM) and its derivatives—that is, absorption in the 350–450 nm and photoluminescence (PL) in the 400–600 nm spectral regions—have been attributed to chromophores formed by homoconjugation along the polymer chain. The enabling role of homoconjugation, however, was hitherto ascertained primarily by excluding alternative origins of luminescence. The present study offers direct evidence for homoconjugation by employing optical and vibrational spectroscopy to investigate the interplay between the microstructure and solid-state optical properties of PPM and its derivative poly(2,4,6-trimethylphenylene methylene). In particular, polarized Raman and PL spectroscopy of melt-drawn fibers reveal a preferentially perpendicular orientation of the phenylene rings relative to the fiber axis and, simultaneously, a preferentially parallel orientation of the transition dipole moment. PL spectroscopy under applied hydrostatic pressure yields a nearly fourfold increase in PL intensity at 8 GPa, together with a surprising absence of excimer emission. These characteristics, being highly atypical of conventional π-conjugated polymers, highlight the different origin of the optical properties of PPMs and unique opportunities for applications.

Journal article

Wan L, Wade J, Shi X, Xu S, Fuchter MJ, Campbell AJet al., 2020, Highly Efficient Inverted Circularly Polarized Organic Light-Emitting Diodes, ACS Applied Materials & Interfaces, Vol: 12, Pages: 39471-39478, ISSN: 1944-8244

Circularly polarized (CP) electroluminescence has been demonstrated as a strategy to improve the performance of organic light-emitting diode (OLED) displays. CP emission can be generated from both small-molecule and polymer OLEDs (SM-OLEDs and PLEDs), but to date, these devices suffer from low dissymmetry factors (g-factor < 0.1), poor device performance, or a combination of the two. Here, we demonstrate the first CP-PLED employing an inverted device architecture. Through this approach, we demonstrate a highly efficient CP-PLED, with a current efficiency of 16.4 cd/A, a power efficiency of 16.6 lm/W, a maximum luminance of over 28,500 cd/m2, and a high EL dissymmetry (gEL) of 0.57. We find that the handedness of the emitted light is sensitive to the PLED device architecture: the sign of CP-EL from an identically prepared active layer reverses between inverted and conventional devices. The inverted structure affords the first demonstration of CP-PLEDs exhibiting both high efficiency and high dissymmetry—the two figures of merit which, until now, have been difficult to achieve at the same time. We also highlight device architecture and associated internal electric field to be a previously unexplored means to control the handedness of CP emission. Our findings significantly broaden the versatility of CP emissive devices and should enable their further application in a variety of other CP-dependent technologies.

Journal article

Güsken NA, Nielsen MP, Nguyen NB, Shi X, Dichtl P, Oulton RFet al., 2020, Efficient four wave mixing and low-loss adiabatic in-coupling in hybrid gap plasmonic waveguides

We show four-wave-mixing over 2 µm with 1% signal-to-idler conversion efficiency enabled by strong non-linearities and highly confined fields. We also demonstrate low-loss in-coupling into nanometer gaps with an efficiency of 80%.

Conference paper

Shi X, Nádaždy V, Perevedentsev A, Frost J, Wang X, von Hauff E, Mackenzie R, Nelson Jet al., 2019, Relating Chain Conformation to the Density of States and Charge Transport in Conjugated Polymers: The Role of the β-phase in Poly(9,9-dioctylfluorene), Physical Review X, Vol: 9, ISSN: 2160-3308

Charge transport in π-conjugated polymers is characterised by a strong degree of disorder in both the energy of conjugated segments and the electronic coupling between adjacent sites. This disorder arises from variations in the structure and conformation of molecular units, as well as the weak inter-molecular binding interactions. Although disorder in molecular conformation can be expected to influence the density of states (DoS) distribution, and hence optoelectronic properties of the material, until now, there has been no direct study of the relationship between a distinct conformational defect and the charge transport properties of a conjugated polymer. Here, we investigate the impact of introducing an extended, planarised chain geometry, known as the ‘β-phase’, on hole transport through otherwise amorphous films of poly(9,9-dioctylfluorene) (PFO). We show that whilst β-phase introduces a striking ~hundredfold drop in time-of-flight (ToF) hole mobility (μh) at room temperature, it reduces the steady-state μh measured from hole-only devices by a factor of less than ~5. In order to reconcile these observations, we combine high-dynamic-range ToF photocurrent spectroscopy and energy-resolved electrochemical impedance spectroscopy to extract the hole DoS of the conjugated polymer. Both methods show that the effect of the β-phase content is to introduce a sharp sub-bandgap feature into the DoS of glassy PFO lying ~0.3 eV above the highest occupied molecular orbital. The observed energy of the conformational trap is consistent with electronic structure calculations using a tight-binding approach. Using the obtained DoS with a drift-diffusion model capable of resolving charge carriers in both time and energy, we show how the seemingly contradictory transport phenomena obtained via the time-resolved, frequency-resolved, and steady-state methods are reconciled. The results highlight the significance of energetic redistribut

Journal article

Sachs M, Sprick RS, Pearce D, Hillman SAJ, Monti A, Guilbert AAY, Brownbill NJ, Dimitrov S, Shi X, Blanc F, Zwijnenburg MA, Nelson J, Durrant JR, Cooper AIet al., 2018, Understanding structure-activity relationships in linear polymer photocatalysts for hydrogen evolution, Nature Communications, Vol: 9, ISSN: 2041-1723

Conjugated polymers have sparked much interest as photocatalysts for hydrogen production. However, beyond basic considerations such as spectral absorption, the factors that dictate their photocatalytic activity are poorly understood. Here we investigate a series of linear conjugated polymers with external quantum efficiencies for hydrogen production between 0.4 and 11.6%. We monitor the generation of the photoactive species from femtoseconds to seconds after light absorption using transient spectroscopy and correlate their yield with the measured photocatalytic activity. Experiments coupled with modeling suggest that the localization of water around the polymer chain due to the incorporation of sulfone groups into an otherwise hydrophobic backbone is crucial for charge generation. Calculations of solution redox potentials and charge transfer free energies demonstrate that electron transfer from the sacrificial donor becomes thermodynamically favored as a result of the more polar local environment, leading to the production of long-lived electrons in these amphiphilic polymers.

Journal article

Rohr J, Shi X, Haque S, Kirchartz T, Nelson Jet al., 2018, Charge transport in Spiro-OMeTAD investigated through space-charge-limited current measurements, Physical Review Applied, Vol: 9, ISSN: 2331-7019

Extracting charge-carrier mobilities for organic semiconductors from space-charge-limited conduction measurements is complicated in practice by nonideal factors such as trapping in defects and injection barriers. Here, we show that by allowing the bandlike charge-carrier mobility, trap characteristics, injection barrier heights, and the shunt resistance to vary in a multiple-trapping drift-diffusion model, a numerical fit can be obtained to the entire current density–voltage curve from experimental space-charge-limited current measurements on both symmetric and asymmetric 2,2′,7,7′-tetrakis(N,N-di-4-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) single-carrier devices. This approach yields a bandlike mobility that is more than an order of magnitude higher than the effective mobility obtained using analytical approximations, such as the Mott-Gurney law and the moving-electrode equation. It is also shown that where these analytical approximations require a temperature-dependent effective mobility to achieve fits, the numerical model can yield a temperature-, electric-field-, and charge-carrier-density-independent mobility. Finally, we present an analytical model describing trap-limited current flow through a semiconductor in a symmetric single-carrier device. We compare the obtained charge-carrier mobility and trap characteristics from this analytical model to the results from the numerical model, showing excellent agreement. This work shows the importance of accounting for traps and injection barriers explicitly when analyzing current density–voltage curves from space-charge-limited current measurements.

Journal article

Nielsen MP, Shi X, Dichtl P, Maier SA, Oulton RFet al., 2017, Giant nonlinear response at a plasmonic nanofocus drives efficient four-wave mixing, Science, Vol: 358, Pages: 1179-1181, ISSN: 0036-8075

Efficient optical frequency mixing typically must accumulate over large interaction lengths because nonlinear responses in natural materials are inherently weak. This limits the efficiency of mixing processes owing to the requirement of phase matching. Here, we report efficient four-wave mixing (FWM) over micrometer-scale interaction lengths at telecommunications wavelengths on silicon. We used an integrated plasmonic gap waveguide that strongly confines light within a nonlinear organic polymer. The gap waveguide intensifies light by nanofocusing it to a mode cross-section of a few tens of nanometers, thus generating a nonlinear response so strong that efficient FWM accumulates over wavelength-scale distances. This technique opens up nonlinear optics to a regime of relaxed phase matching, with the possibility of compact, broadband, and efficient frequency mixing integrated with silicon photonics.

Journal article

Rodriguez-Martinez X, Vezie MS, Shi X, McCulloch I, Nelson J, Goni AR, Campoy-Quiles Met al., 2017, Quantifying local thickness and composition in thin films of organic photovoltaic blends by Raman scattering, Journal of Materials Chemistry C, Vol: 5, Pages: 7270-7282, ISSN: 2050-7526

We report a methodology based on Raman spectroscopy that enables the non-invasive and fast quantitative determination of local thickness and composition in thin films (from a few monolayers to hundreds of nm) of one or more components. We apply our methodology to blends of organic conjugated materials relevant in the field of organic photovoltaics. As a first step, we exploit the transfer-matrix formalism to describe the Raman process in thin films including reabsorption and interference effects of the incoming and scattered electric fields. This allows determining the effective solid-state Raman cross-section of each material by studying the dependence of the Raman intensity on film thickness. These effective cross sections are then used to estimate the local thickness and composition in a series of polymer:fullerene blends. We find that the model is accurate within ±10 nm in thickness and ±5 vol% in composition provided that (i) the film thickness is kept below the thickness corresponding to the first maximum of the calculated Raman intensity oscillation; (ii) the materials making up the blend show close enough effective Raman cross-sections; and (iii) the degree of order attained by the conjugated polymer in the blend is similar to that achieved when cast alone. Our methodology opens the possibility of making quantitative maps of composition and thickness over large areas (from microns to centimetres squared) with diffraction-limited resolution and in any multi-component system based thin film technology.

Journal article

Yang Y, Rice B, Shi X, Brandt JR, Correa da Costa R, Hedley GJ, Smilgies D-M, Frost JM, Samuel IDW, Otero-de-la-Roza A, Johnson ER, Jelfs KE, Nelson J, Campbell AJ, Fuchter MJet al., 2017, Emergent Properties of an Organic Semiconductor Driven by its Molecular Chirality, ACS Nano, Vol: 11, Pages: 8329-8338, ISSN: 1936-0851

Chiral molecules exist as pairs of nonsuperimposable mirror images; a fundamental symmetry property vastly underexplored in organic electronic devices. Here, we show that organic field-effect transistors (OFETs) made from the helically chiral molecule 1-aza[6]helicene can display up to an 80-fold difference in hole mobility, together with differences in thin-film photophysics and morphology, solely depending on whether a single handedness or a 1:1 mixture of left- and right-handed molecules is employed under analogous fabrication conditions. As the molecular properties of either mirror image isomer are identical, these changes must be a result of the different bulk packing induced by chiral composition. Such underlying structures are investigated using crystal structure prediction, a computational methodology rarely applied to molecular materials, and linked to the difference in charge transport. These results illustrate that chirality may be used as a key tuning parameter in future device applications.

Journal article

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