Transient Photophysics of Bismuth Halide Semiconductors for Optoelectronic Applications

This thesis describes a series of spectroscopic studies of three different semiconductors based on bismuth halides in order to discover their fundamental photophysical properties. The materials investigated — the double perovskite Cs2AgBiBr6, bismuth iodide, and bismuth oxyiodide — have all been demonstrated in thin film photovoltaic devices fabricated at low temperatures. Propelled by the success of lead halide perovskites, this work forms part of the search for defect tolerant, non-toxic and stable materials for next-generation solar cells. I use transient absorption spectroscopy to show that the charge carrier lifetime in Cs2AgBiBr6 thin films is 1.4 μs. This is significantly longer than previous estimates based on time-resolved photoluminescence measurements, which measure the radiatively decaying carriers, but is a less sensitive probe of the high proportion of carriers which recombine non-radiatively. I propose a radiative recombination mechanism via defects, based on the detection of mid-gap electronic states in the transient absorption spectra. Coherent phonon transients are also measured on ultrafast timescales showing strong electron-phonon coupling in the material, which may contribute to the slow recombination in this material. The recombination dynamics of excitons in the layered material bismuth iodide, BiI3, are investigated using temperature dependent photoluminescence and transient absorption. I show that coupling to interlayer and intralayer phonon modes strongly affects the direct exciton decay pathway through scattering and modulation of the band gap energy. In a single crystal of BiI3, the room temperature exciton lifetime is 72 ns, which indicates this material’s potential for photovoltaics if the defects in thin films can be passivated. I show that the excited states of thin films of layered bismuth oxyiodide, BiOI, have a great degree of energetic disorder, most likely due to the presence of self-trapped excitons and defect states. Transient absorption measurements give an excited state lifetime of 47 ps, which would make efficient photovoltaic performance of BiOI thin film absorbers very unlikely. On the basis of my findings, all three materials show strong coupling between phonons and electronic states, but have excited state lifetimes varying over many orders of magnitude. This should guide the direction of future research towards different applications: the most promising candidate for photovoltaic applications is therefore Cs2AgBiBr6, whereas BiI3 and BiOI have more potential for use in nanostructured devices, such as ultrathin photodetectors, which can exploit their anisotropic nature

Photoexcitation Control of Excitation Relaxation in Mixed‐Phase Ruddlesden‐Popper Hybrid Organic‐Inorganic Lead‐Iodide Perovskites

AbstractThe electronic states and exciton binding energies of layered Ruddlesden‐Popper (RP) metal‐halide perovskites can be tailored through changes of their chemical composition, yielding multi‐phase systems with complex energy cascades. Ultrafast photoexcitation relaxation with transfer dynamics into domains of increasing layer number has been reported for these materials. Here, ultrafast optical spectroscopy is used to report an unexpected excitation energy dependence of photoexcitation relaxation dynamics in mixed‐dimensional benzylammonium cesium lead iodide RP perovskite (BeA2CsPb2I7) thin films, which gives rise to spectrally broadband luminescence over the visible region. Using transient absorption and photoluminescence spectroscopy it is found that excitations, which are formed in the n  =  2 RP‐phase after photoexcitation with ≈0.2 electron volt excess energy, transfer to higher layer number RP‐phases on unexpectedly slow timescales of tens of picoseconds. Further, it is observed that such excitations are initially optically passive. Notably, luminescence occurs under these conditions from multiple RP‐phases with optical bandgaps across the visible range, yielding broadband luminescence. The results hold potential for realization of broadband white‐light emitters and other light‐emitting devices.</jats:p

Research data supporting "Electronic Structure and Optoelectronic Properties of Bismuth Oxyiodide Robust Against Percent-Level Iodine-, Oxygen- and Bismuth-Related Surface Defects"

See the README file for a detailed description of the dataset. The data has been collected using a number of techniques. Figure 1 contains data gathered using photoelectron spectroscopy and have been saved in a .vms format. The data has been analysed and processed using CasaXPS and Origin respectively. Figure 2 contains data from photoluminescence and Transient absorbance spectroscopy and have been saved as .asc and .txt files. Figure 3 contains plots that have been gathered via X-ray diffraction and Time-of-flight Secondary Ion Mass spectroscopy techniques (ToF-SIMs). The ToF-SIMs data includes the response from all the species that evolves with ion bombardment but the species of interest are I2- and O2-. Figure 4 contains all the photovoltaic performance data in .txt format which can be opened and plotted using the Matlab script provided. The photovoltaic metrics, PCE, Jsc, VOC and FF have been generated with the Matlab script which can be taken forward for the box plot plotting as shown in Figure 4. Figure 5 is a summary of the PICTs data which has been derived from the raw data in Figure S11. Please see the ReadME files for more details

Research data supporting "Electronic Structure and Optoelectronic Properties of Bismuth Oxyiodide Robust Against Percent-Level Iodine-, Oxygen- and Bismuth-Related Surface Defects"

See the README file for a detailed description of the dataset. The data has been collected using a number of techniques. Figure 1 contains data gathered using photoelectron spectroscopy and have been saved in a .vms format. The data has been analysed and processed using CasaXPS and Origin respectively. Figure 2 contains data from photoluminescence and Transient absorbance spectroscopy and have been saved as .asc and .txt files. Figure 3 contains plots that have been gathered via X-ray diffraction and Time-of-flight Secondary Ion Mass spectroscopy techniques (ToF-SIMs). The ToF-SIMs data includes the response from all the species that evolves with ion bombardment but the species of interest are I2- and O2-. Figure 4 contains all the photovoltaic performance data in .txt format which can be opened and plotted using the Matlab script provided. The photovoltaic metrics, PCE, Jsc, VOC and FF have been generated with the Matlab script which can be taken forward for the box plot plotting as shown in Figure 4. Figure 5 is a summary of the PICTs data which has been derived from the raw data in Figure S11. Please see the ReadME files for more details

Strong Induced Circular Dichroism in a Hybrid Lead‐Halide Semiconductor Using Chiral Amino Acids for Crystallite Surface Functionalization

Chirality is a desired property in functional semiconductors for optoelectronic, catalytic, and spintronic applications. Here, introducing enantiomerically-pure 3-aminobutyric acid (3-ABA) into thin films of the 1D semiconductor dimethylammonium lead iodide (DMAPbI$_3$) is found to result in strong circular dichroism (CD) in the optical absorption. X-ray diffraction and grazing incidence small angle X-ray scattering (GISAXS) are applied to gain molecular-scale insights into the chirality transfer mechanism, which is attributed to a chiral surface modification of DMAPbI$_3$ crystallites. This study demonstrates that the CD signal strength can be controlled by the amino-acid content relative to the crystallite surface area. The CD intensity is tuned by the composition of the precursor solution and the spin-coating time, thereby achieving anisotropy factors (g$_{abs}$) as high as 1.75 × 10$^{–2}$. Grazing incidence wide angle scattering reveals strong preferential ordering that can be suppressed via tailored synthesis conditions. Different contributions to the chiroptical properties are resolved by a detailed analysis of the CD signal utilizing an approach based on the Mueller matrix model. This report of a novel class of chiral hybrid semiconductors with precise control over their optical activity presents a promising approach for the design of circularly polarized light detectors and emitters

Exploiting Excited-State Aromaticity To Design Highly Stable Singlet Fission Materials.

Singlet fission, the process of forming two triplet excitons from one singlet exciton, is a characteristic reserved for only a handful of organic molecules due to the atypical energetic requirement for low energy excited triplet states. The predominant strategy for achieving such a trait is by increasing ground state diradical character; however, this greatly reduces ambient stability. Herein, we exploit Baird's rule of excited state aromaticity to manipulate the singlet-triplet energy gap and create novel singlet fission candidates. We achieve this through the inclusion of a [4n] 5-membered heterocycle, whose electronic resonance promotes aromaticity in the triplet state, stabilizing its energy relative to the singlet excited state. Using this theory, we design a family of derivatives of indolonaphthyridine thiophene (INDT) with highly tunable excited state energies. Not only do we access novel singlet fission materials, they also exhibit excellent ambient stability, imparted due to the delocalized nature of the triplet excited state. Spin-coated films retained up to 85% activity after several weeks of exposure to oxygen and light, while analogous films of TIPS-pentacene showed full degradation after 4 days, showcasing the excellent stability of this class of singlet fission scaffold. Extension of our theoretical analysis to almost ten thousand candidates reveals an unprecedented degree of tunability and several thousand potential fission-capable candidates, while clearly demonstrating the relationship between triplet aromaticity and singlet-triplet energy gap, confirming this novel strategy for manipulating the exchange energy in organic materials

Layered BiOI single crystals capable of detecting low dose rates of X-rays

Acknowledgements: We would like to thank Prof. Richard Phillips (University of Cambridge) for useful feedback on the manuscript and help with optical measurements. The authors also thank Zhuotong (Thomas) Sun (University of Cambridge) for assistance on the powder X-ray diffraction measurements, and Prof. James Marrow and Marcus Williamson (University of Oxford) for assistance in taking radiographs. R.A.J. acknowledges funding from an EPSRC Department Training Partnership studentship (no. EP/N509620/1), as well as Bill Welland and the Winton Programme for the Physics of Sustainability. L.E. and T.V.D.G. acknowledge support from the EPSRC Cambridge NanoDTC (no. EP/L015978/1). L.E. acknowledges funding by the DFG (project no. 387651688). T.V.D.G. also acknowledges financial support from the Schiff Foundation. K.G. and S.D.S. acknowledge the EPSRC (no. EP/R023980/1) for funding. S.D.S. acknowledges the Royal Society and Tata Group (no. UF150033) and EPSRC (no. EP/W004445/1) for funding. The work has received funding from the European Research Council under the European Union’s Horizon 2020 research and innovation programme (HYPERION - grant agreement no. 756962; PEROVSCI - 957513). The work was supported by a Royal Society International Exchanges Cost Share award (no. IEC\R2\170108) and the Alliance Hubert Curien Programme of the British Council (no. 608412749). K.R.D. thanks the Department of Chemistry at the University of Oxford for a studentship. P.P. appreciates support from National Science Centre Poland within the OPUS program (no. 2019/33/B/ST3/01915). This work was partially supported by OPEP project, which received funding from the ANR-10-LABX-0037-NEXT. The Polish participation in European Magnetic Field Laboratory is supported by the DIR/WK/2018/07 grant from Ministry of Science and Higher Education, Poland. F.D. acknowledges support from the DFG Emmy Noether Programme (project no. 387651688) and the Winton Programme for the Physics of Sustainability. J.L.M.-D. acknowledges funding from the Royal Academy of Engineering under the Chair in Emerging Technologies Scheme (no. CIET1819_24). R.L.Z.H. acknowledges support from the Royal Academy of Engineering under the Research Fellowship scheme (no. RF\201718\1701), the Isaac Newton Trust (Minute 19.07(d)), Downing College Cambridge through the Kim and Juliana Silverman Research Fellowship, and an EPSRC grant (no. EP/V014498/2). I.B. and B.M. acknowledge support from the Winton Programme for the Physics of Sustainability. B.M. also acknowledges support from a UKRI Future Leaders Fellowship (no. MR/V023926/1) and from the Gianna Angelopoulos Programme for Science, Innovation and Technology. The calculations are conducted using resources provided by the Cambridge Tier-2 system, operated by the University of Cambridge Research Computing Service (www.hpc.cam.ac.uk) and funded by EPSRC Tier-2 capital grant (no. EP/P020259/1).AbstractDetecting low dose rates of X-rays is critical for making safer radiology instruments, but is limited by the absorber materials available. Here, we develop bismuth oxyiodide (BiOI) single crystals into effective X-ray detectors. BiOI features complex lattice dynamics, owing to the ionic character of the lattice and weak van der Waals interactions between layers. Through use of ultrafast spectroscopy, first-principles computations and detailed optical and structural characterisation, we show that photoexcited charge-carriers in BiOI couple to intralayer breathing phonon modes, forming large polarons, thus enabling longer drift lengths for the photoexcited carriers than would be expected if self-trapping occurred. This, combined with the low and stable dark currents and high linear X-ray attenuation coefficients, leads to strong detector performance. High sensitivities reaching 1.1  × 103 μC Gyair−1 cm−2 are achieved, and the lowest dose rate directly measured by the detectors was 22 nGyair s−1. The photophysical principles discussed herein offer new design avenues for novel materials with heavy elements and low-dimensional electronic structures for (opto)electronic applications.</jats:p