E. Butler-Caddle, K.D.G.I. Jayawardena, A. Wijesekara, R.L. Milot and J. Lloyd-Hughes Phys. Rev. Applied 22024103 (Aug 2024)
In perovskite solar cells, photovoltaic action is created by charge transport layers (CTLs) either side of the light-absorbing metal halide perovskite semiconductor. Hence, the rates for desirable charge extraction and unwanted interfacial recombination at the perovskite-CTL interfaces play a critical role for device efficiency. Here, the electrical properties of perovskite-CTL bilayer heterostructures are obtained using ultrafast terahertz and optical studies of the charge carrier dynamics after pulsed photoexcitation, combined with a physical model of charge carrier transport that includes the prominent Coulombic forces that arise after selective charge extraction into a CTL, and cross-interfacial recombination. The charge extraction velocity at the interface and the ambipolar diffusion coefficient within the perovskite are determined from the experimental decay profiles for heterostructures with three of the highest-performing CTLs, namely C60, PCBM and Spiro-OMeTAD. Definitive targets for the further improvement of devices are deduced: fullerenes deliver fast electron extraction, but suffer from a large rate constant for cross-interface recombination or hole extraction. Conversely, Spiro-OMeTAD exhibits slow hole extraction but does not increase the perovskite’s surface recombination rate, likely contributing to its success in solar cell devices.
Justas Deveikis, Marcin Giza, David Walker, Jie Liu, Claire Wilson, Nathaniel P. Gallop, Pablo Docampo, James Lloyd-Hughes and Rebecca L. Milot J. Phys. Chem. C, (July 2024)
Improved knowledge of the influence of temperature upon layered perovskites is essential to enable perovskite-based devices to operate over a broad temperature range and to elucidate the impact of structural changes upon the optoelectronic properties. We examined the Ruddlesden–Popper layered perovskite 2-thiophenemethylammonium lead iodide (ThMA2PbI4) and observed a structural phase transition between a high- and a low-temperature phase at 220 K using temperature-dependent X-ray diffraction, UV–visible absorption, and photoluminescence (PL) spectroscopy. The structural phase transition altered the tilt pattern of the inorganic octahedra layer, modifying the absorption and PL spectra. Further, we found a narrow and intense additional PL peak in the low-temperature phase, which we assigned to radiative emission from a defect-bound exciton state. In both phases we determined the thermal expansion coefficient and found values similar to those of cubic 3D perovskites, i.e., larger than those of typical substrates such as glass. These results demonstrate that the organic spacer plays a critical role in controlling the temperature-dependent structural and optoelectronic properties of layered perovskites and suggests more widely that strain management strategies may be needed to fully utilize layered perovskites in device applications.
BFM Healy, SL Pain, J. Lloyd-Hughes, NE Grant and JD Murphy Adv. Mater. Interfaces 11 015002 (Jul 2024)
Monolayer molybdenum disulfide (1L MoS2), a promising optoelectronic material, emits strong visible photoluminescence (PL). Systematic control of the intensity, energy, and spectral width of PL from 1L MoS2 on silicon dioxide/silicon (SiO2/Si) is demonstrated via simple external treatments. Treating MoS2 with solutions formed from the superacid bis-(trifluoromethanesulfonyl)amide (TFSA) enhances, blueshifts, and sharpens the PL. Treatments with solutions from structurally analogous chemicals that lack sulfur, in the case of bis(trifluoroacetamide) (BTFA), or lack fluorine, in the case of methanesulfonamide (MSA), show the same trend, suggesting a two-component mechanism for TFSA involving the presence of electronegative species and sulfur vacancy passivation. Up to ≈100× enhancement of the PL intensity is achieved, with the peak blueshifted by ≈30 meV and the spectral linewidth halved. Conversely, direct thermal atomic layer deposition (ALD) of aluminum oxide (Al2O3) or hafnium oxide (HfO2) is found to suppress the PL by up to a factor of ≈3, redshift by up to ≈70 meV, and broaden by ≈3×. Single-spot and mapping Raman/PL techniques are combined in a robust characterization process to associate changes in the PL character to charge doping. This work demonstrates the convenient tunability of the optical behavior of 1L MoS2 by varying the electron density.
A.I. Hernandez-Serrano, X. Ding, G. Costa, G. Nurumbetov, D.M. Haddleton and E. Pickwell-MacPherson Biomedical Optics Express 15, 3064 (May 2024)
Transdermal drug delivery patches are a good alternative to hypodermic drug injection. The drug delivery efficiency depends strongly on the hydration of the skin under treatment, and therefore, it is essential to study the effects on the skin induced by the application of these medical-grade patches. Terahertz (THz) spectroscopy shows great promise for non-invasive skin evaluation due to its high sensitivity to subtle changes in water content, low power and non-ionizing properties. In this work, we study the effects of transdermal drug delivery patches (three fully occlusive and three partially occlusive) applied on the upper arms of ten volunteers for a maximum period of 28 h. Three different levels of propylene glycol (0 %, 3 % and 6 %) are added to the patches as excipient. By performing multilayer analysis, we successfully retrieve the water content of the stratum corneum (SC) which is the outermost layer of skin, as well as its thickness at different times before and after applying the patches. This study demonstrates the potential of using THz sensing for non invasive skin monitoring and has wide applications for skin evaluation as well as the development of skin products.
A.I. Hernandez-Serrano, X. Ding, J. Young, G. Costa, A. Dogra, J. Hardwicke and E. Pickwell-MacPherson Advanced Photonics Nexus 3, 016012 (Feb 2024)
This study introduces a handheld terahertz (THz) scanner designed to quantitatively evaluate human skin hydration levels and thickness. This device, through the incorporation of force sensors, demonstrates enhanced repeatability and accuracy over traditional fixed THz systems. The scanner was evaluated in the largest THz skin study to date, assessing 314 volunteers, successfully differentiating between individuals with dry skin and hydrated skin using a numerical stratified skin model. The scanner measures and displays skin hydration dynamics within a quarter of a second, indicating its potential for real-time, noninvasive examinations, opening up opportunities for in vivo and ex vivo diagnosis during patient consultations. Furthermore, the portability and ease of use of our scanner enable its widespread application for in vivo and ex vivo diagnosis during patient consultations, potentially allowing in situ biopsy evaluation and elimination of histopathology processing wait times, thereby improving patient outcomes by facilitating simultaneous tumor diagnosis and removal.
H. Ou, R.I. Stantchev, X. Chen, T. Blu, M. Semtsiv, W.T. Masselink, A. Hernandez Serrano, G. Costa, J. Young, N. Chopra, J. Lloyd-Hughes, and E. Pickwell-MacPherson Optics Express 32, 5567 (Feb 2024)
We propose a polarization sensitive terahertz time-domain spectrometer that can record orthogonally polarized terahertz fields simultaneously, using fibre-coupled photoconductive antennas and a scheme that modulated the emitter’s polarization. The s and p channels of the multi-pixel terahertz emitter were modulated at different frequencies, thereby allowing orthogonal waveforms to be demultiplexed from the recorded signal in post-processing. The performance of the multi-pixel emitter used in this multiplexing scheme was comparable to that of a commercial single-polarization H-dipole antenna. The approach allowed two orthogonally polarized terahertz pulses to be recorded with good signal to noise (>1000:1) within half a second. We verified the capability of the spectrometer by characterizing a birefringent crystal and by imaging a polarization-sensitive metamaterial. This work has significant potential to improve the speed of terahertz polarization sensitive applications, such as ellipsometry and imaging.
BFM Healy, SL Pain, J. Lloyd-Hughes, NE Grant and JD Murphy Materials Research Express 11 015002 (Jan 2024)
Monolayer molybdenum disulfide (MoS2) is a promising candidate for inclusion in optoelectronic technologies, owing to its two-dimensional (2D) nature and resultant novel photoluminescence (PL). Chemical vapour deposition (CVD) is an important method for the preparation of large-area films of monolayer MoS2. The PL character of as-prepared monolayer MoS2 must be well understood to facilitate detailed evaluation of any process-induced effects during device fabrication. We comparatively explore the PL emission from four different commercially available CVD-grown MoS2 monolayer films. We characterize the samples via Raman and PL spectroscopy, using both single-spot and mapping techniques, while atomic force microscopy (AFM) is applied to map the surface structure. Via multipeak fitting, we decompose the PL spectra into constituent exciton and trion contributions, enabling an assessment of the quality of the MoS2 monolayers. We find that the PL character varies significantly from sample to sample. We also reveal substantial inhomogeneity of the PL signal across each individual MoS2 film. We attribute the PL variation to non-uniform MoS2 film morphologies that result from the nucleation and coalescence processes during the CVD film development. Understanding the large variability in starting PL behaviour is vital to optimize the optoelectronic properties for MoS2-based devices.
T.J. Keat, D. J. L. Coxon, R.J. Cruddace, V. G. Stavros, M. E. Newton, and J. Lloyd-Hughes Diamond and Related Materials 141110661 (Jan 2024)
The dynamics of the 3237 cm−1 local vibrational mode in diamond, associated with an unknown defect, was investigated using ultrafast infrared pump-probe spectroscopy. When pumped at 3237 cm−1, a degenerate probe was used to study the ground state's recovery, while a non-degenerate probe tracked excited state absorption at 3029 cm−1, corresponding to the 1 → 2 vibrational state transition. The similar population lifetimes for the ground state recovery and excited state absorption suggests a single population decay pathway, with a lifetime of T1=2.2+-0.1ps. Perturbed free induction decay signals observed in negative time delays gave the dephasing time of the coherent state between the 0 and 1 vibrational states, and further predicted the 3029 cm−1 transition. Images from FTIR microscopy show that the 3237 cm−1 feature and the 3107 cm−1 absorption line from the N3VH0 defect are not correlated, and our pump-probe study shows the 3237 cm−1 feature does not share a common ground state with the N3VH0 defect, both of which suggest that this local vibrational mode does not originate from the N3VH0 defect. A calibration factor was obtained via a Morse potential model constrained by the observed transition energies, which relates the concentration of the defect producing the 3237 cm−1 feature to its absorption coefficient measured by FTIR spectroscopy. Based on FTIR absorption spectroscopy under uniaxial stress, we further assign a trigonal symmetry character to the defect that gives the 3237 cm−1 feature. The results presented are consistent with the theory that the 3237 cm−1 feature originates from the N4VH defect, the quantification of which allows better tracking of the nitrogen content in diamond.