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The website contains (relatively- Jan 2019) up-to-date information about our Research Group and our research activities.
We are based in the Materials Department, Imperial College London and together with a number of other research groups are a part of the Centre for Plastic Electronics. Our research interests are focussed on identifying, preparing and characterising electrically and/or optically active thin films, the incorporation of these materials into a variety of device platforms and characterisation of the interfaces created as these and other functional materials are combined into devices.
You are welcome to contact Martyn directly with any enquiries you may have about our research, facilities, current or future vacancies
Some recent work from our research group and our collaborators, for a full list of publications see the menu bar at the top of the page.
Here, previously unobserved nanoscale defects residing within individual grains of solution-processed methylammonium lead tri-iodide (CH3NH3PbI3, MAPI) thin films are identified. Using scanning transmission electron microscopy (STEM), the defects inherently associated with the established solution-processing methodology are identified, and a facile processing modification to eliminate these defects is introduced. Specifically, defect elimination is achieved by coannealing the as-deposited MAPI layer with the electron transport layer (phenyl-C61-butyric acid methyl, PCBM) resulting in devices that significantly outperform devices prepared using the established methodology—with power conversion efficiencies increasing from 13.6% to 17.4%. The use of transmission electron microscopy allows the correlation of performance enhancements to improved intragrain crystallinity and shows that highly coherent crystallographic orientation results within individual grains when processing is modified. Detailed optoelectronic characterization reveals that the improved intragrain crystallinity drives an improvement of charge collection and a reduction of PEDOT:PSS/perovskite interfacial recombination. The study suggests that the microstructural defects in MAPI, owing to a lack of structural coherence throughout the thickness of thin film, are a significant cause of interfacial recombination.
Recombination via subgap trap states is considered a limiting factor in the development of organometal halide perovskite solar cells. Here, 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) is demonstrated. It is shown that MAPI crystallinity can be systematically tailored by modulating the stoichiometry of the precursor mix, where small quantities of excess methylammonium iodide (MAI) improve crystallinity, increasing device Voc by ≈200 mV. Using in situ differential charging and transient photovoltage measurements, charge density and charge carrier recombination lifetime are determined under operational conditions. Increased Voc is correlated to improved active layer crystallinity and a reduction in the density of trap states in MAPI. Photo-luminescence spectroscopy shows that an increase in trap state density correlates with faster carrier trapping and more nonradiative recombina-tion pathways. Fundamental insights into the origin of Voc in perovskite photovoltaics are provided and it is demonstrated why highly crystalline perovskite films are paramount for high-performance devices