Prof. Martyn A. McLachlan Profile

Prof. Martyn A. McLachlan

Professor at Imperial College

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Publications (5)

Proceedings Article | 10 September 2019 Presentation

Morphology, microstructure and stability in perovskite photovoltaics (Conference Presentation)

Martyn McLachlan

Proceedings Volume 11094, 110940Z (2019) https://doi.org/10.1117/12.2530213

KEYWORDS: Crystals, Perovskite, Photovoltaics, Solar cells, Crystallography, Optoelectronics, Interfaces, Scanning transmission electron microscopy, Lead, Modulation

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A significant volume of literature exists that describe minor changes in composition and/or microstructure in perovskite solar cells (PSCs) as the driving force for incremental improvements in device performance. Many authors cite crystallinity as a fundamental driver of performance improvements, yet often do so without quantitatively defining crystallinity or, importantly, addressing the important questions behind their observations. Here I will discuss two recent case studies where processing modifications have been investigated as a means of controllably varying active layer crystallinity and describe the impact on device performance. i) We demonstrate the impact of active layer crystallinity on the accumulated charge and open-circuit voltage (Voc) in solar cells based on methylammonium lead triiodide (CH3NH3PbI3,MAPI). We show that MAPI crystallinity can be systematically tailored by modulating the stoichiometry of the precursor mix, where a small excess of methylammonium iodide (MAI) improves crystallinity increasing device Voc by ~200 mV and, in parallel, that the photoluminescence (PL) yield increases 15x, indicative of a suppression of non-radiative recombination pathways. This is coupled with the development of crystallographic texture (110) in the MAPI. In-situ transient optoelectronic measurements of the charge concentration in PSCs under operation suggest that the concentration of trapped charges (either at interfaces or in the bulk) is some 5x lower at matched Voc. We believe these trap states originate in/near the disordered or amorphous areas between MAPI crystallites, resulting at least in part from orientation mismatch between crystalline domains. ii) Secondly, we identify previously unobserved nanoscale defects residing within individual grains of solution processed MAPI thin films. Using scanning transmission electron microscopy (STEM) we identify the defects to be inherently associated with the established solution-processing methodology and introduce a facile processing modification to eliminate such defect formation. Specifically, defect elimination is achieved by co-annealing the as deposited MAPI layer with the electron transport layer PCBM resulting in devices that significantly outperform devices prepared using the established methodology, achieving PCE increases from 13.6 % to 17.7 %. The use of TEM allows us to correlate the performance enhancements to improved intra-grain crystallinity and show that highly coherent crystallographic orientation results within individual grains when processing is modified. Detailed optoelectronic characterization reveals that the improved intra-grain crystallinity drives an improvement of charge collection, a reduction of surface recombination at the MAPI/PCBM interface and a change in the density of local sub-gap states.

Proceedings Article | 4 March 2019 Presentation

High throughput fabrication of nanoscale optoelectronic devices on large area flexible substrates using adhesion lithography (Conference Presentation)

Dimitra Georgiadou, Martyn McLachlan, Thomas Anthopoulos

Proceedings Volume 10930, 1093003 (2019) https://doi.org/10.1117/12.2511163

KEYWORDS: Nanolithography, Optoelectronic devices, Lithography, Electrodes, Fabrication, Lead, Telecommunications, Consumer electronics, Medicine, Electron beam lithography

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SPIE Journal Paper | 1 January 2011 Open Access

Solution processed hybrid photovoltaics: preparation of a standard ZnO template

Jonathan Downing, Mary Ryan, Natalie Stingelin-Stutzmann, Martyn McLachlan

JPE, Vol. 1, Issue 1, 011117, (January 2011) https://doi.org/10.1117/12.10.1117/1.3569104

KEYWORDS: Zinc oxide, Photovoltaics, Scanning electron microscopy, Polymers, Crystals, Standards development, X-ray diffraction, Excitons, Zinc, Nanorods

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Proceedings Article | 26 August 2008 Paper

Colloidal crystals as nanostructured templates for organic solar cells

Sarah Berhanu, Martyn McLachlan, David McComb, Tim Jones

Proceedings Volume 7052, 70521H (2008) https://doi.org/10.1117/12.803554

KEYWORDS: Optical spheres, Nanocomposites, Crystals, Excitons, Absorption, Composites, Scanning electron microscopy, Diffusion, Interfaces, Nanostructuring

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Proceedings Article | 15 September 2004 Paper

Application of pressure to shift the bandgap in polystyrene-based photonic crystals

Nigel Johnson, Ali Khokhar, Martyn McLachlan, David McComb, Richard De La Rue

Proceedings Volume 5450, (2004) https://doi.org/10.1117/12.546533

KEYWORDS: Optical spheres, Photonic crystals, Reflectivity, Objectives, Neck, Silica, Capillaries, Scanning electron microscopy, Lanthanum, Visible radiation

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