Opto-electronic Characterization of Perovskite Solar Cells Arman Mahboubi Soufiani A THESIS IN FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHYLOSOPHY School of Photovoltaic and Renewable Energy Engineering Faculty of Engineering The University of New South Wales May 2017 THE UNIVERSITY OF NEW SOUTH WALES Thesis/Dissertation Sheet Surname or Family name: Mahboubi Soufiani First name: Arman Other name/s: Abbreviation for degree as given in the University calendar: PhD School: School of Photovoltaic and Renewable Energy Engineering Faculty: Engineering Title: Opto-electronic Characterization of Perovskite Solar Cells Abstract 350 words maximum: Since the first report, in 2009, on solar cells based on organic-inorganic metal halide perovskites as the active light absorber, intensive research on performance improvement of these types of solar cells has led to a dramatic rise in their power conversion efficiencies from 3-4% to the recently certified efficiency of 22.1% for laboratory-scale devices. Researchers from various fields of physics, chemistry, material science and engineering have been attempting to understand the opto-electronic properties of these materials to (i) develop new fabrication techniques of the perovskite layer to optimise their photovoltaic characteristics; (ii) synthesize stable and energetically appropriate carrier selective contacts; (iii) unravel the underlying reasons of the current-voltage hysteresis in these solar cells; and (iv) overcome their long-term operational stability and to lessen the toxicity issues of the lead component. In two separate sections, this thesis is dedicated to understand both the optical characteristics of most studied photovoltaic perovskites, with the focus being on the exciton binding energy influencing light absorption, and also to develop fast, reliable and standard characterization technique for electrical examination of the corresponding solar cells which is based on the luminescence properties of perovskites. The absorption spectrum of these materials with and without the inclusion of excitonic effects is elaborated. The experimental values for the binding energies extracted from temperature-dependent absorption spectra of high-quality CH3NH3PbI3 and CH3NH3PbBr3 thin films are compared with theory. In the theory employed, the interaction of electrons and holes with an effective longitudinal optical phonon was considered for the first time in these semiconductors to better estimate the theoretical values. Good agreement between theory and experiments was shown to be achievable. Performance characterization of perovskite solar cells was initiated in this thesis by exploration of the generalized Planck’s radiation law for planar structured devices both under light- and electrical-bias conditions (photoluminescence, PL, and electroluminescence, EL, respectively). In hysteresis affected devices, light-soaking was found to be an essential prerequisite in order to be able to correlate luminescence intensity and the device internal voltage. Long-term behaviour of planar solar cells with CH3NH3PbI3 active layer was investigated using full device EL imaging and it was realized that interfacial deterioration even when the device is stored in dark under glovebox condition (devices only used for light current-voltage measurements) is the critical factor suppressing the fill factor. In the last section, using PL and EL imaging, the immediate response and long-term electrical evolution of the perovskite bulk and compact TiO2/perovskite and perovskite/Spiro-OMeTAD interfaces after the initial short and prolonged selective illumination of the device is elaborated. Perovskite structure buckling over time resulting in the interfacial decoupling at the titanium dioxide/perovskite is demonstrated and the associated mechanism is discussed in detail. The latter is explained by the intrinsic ion migration feature of CH3NH3PbI3 perovskite which is influenced principally due to the change in the internal electric field of the device. Declaration relating to disposition of project thesis/dissertation I hereby grant to the University of New South Wales or its agents the right to archive and to make available my thesis or dissertation in whole or in part in the University libraries in all forms of media, now or here after known, subject to the provisions of the Copyright Act 1968. I retain all property rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertation. I also authorise University Microfilms to use the 350 word abstract of my thesis in Dissertation Abstracts International (this is applicable to doctoral theses only). …………………………………………………………… ……………………………………..……………… ……….……………………...…….… Signature Witness Signature Date The University recognises that there may be exceptional circumstances requiring restrictions on copying or conditions on use. Requests for restriction for a period of up to 2 years must be made in writing. Requests for a longer period of restriction may be considered in exceptional circumstances and require the approval of the Dean of Graduate Research. FOR OFFICE USE ONLY Date of completion of requirements for Award: COPYRIGHT STATEMENT ‘I hereby grant the University of New South Wales or its agents the right to archive and to make available my thesis or dissertation in whole or part in the University libraries in all forms of media, now or here after known, subject to the provisions of the Copyright Act 1968. I retain all proprietary rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertation. I also authorise University Microfilms to use the 350 word abstract of my thesis in Dissertation Abstract International (this is applicable to doctoral theses only). I have either used no substantial portions of copyright material in my thesis or I have obtained permission to use copyright material; where permission has not been granted I have applied/will apply for a partial restriction of the digital copy of my thesis or dissertation.' Signed ……………………………………………........................... Date ……………………………………………........................... AUTHENTICITY STATEMENT ‘I certify that the Library deposit digital copy is a direct equivalent of the final officially approved version of my thesis. No emendation of content has occurred and if there are any minor variations in formatting, they are the result of the conversion to digital format.’ Signed ……………………………………………........................... Date ……………………………………………........................... ORIGINALITY STATEMENT ‘I hereby declare that this submission is my own work and to the best of my knowledge it contains no materials previously published or written by another person, or substantial proportions of material which have been accepted for the award of any other degree or diploma at UNSW or any other educational institution, except where due acknowledgement is made in the thesis. Any contribution made to the research by others, with whom I have worked at UNSW or elsewhere, is explicitly acknowledged in the thesis. I also declare that the intellectual content of this thesis is the product of my own work, except to the extent that assistance from others in the project's design and conception or in style, presentation and linguistic expression is acknowledged.’ Signed …………………………………………….............. Date …………………………………………….............. Acknowledgements Below, I would like to gratefully and humbly express my gratitude to those who greatly contributed to the journey that led to the accomplishment of an important step in my career life but not in the traditional form. I start my acknowledgment being firstly and deeply grateful to my lovely parents, Raihan and Yousof, who dedicated their life to grow and educate me as a person who aims to be useful both locally and globally and to love surrounding people unconditionally. The enthusiasm to start and fulfil PhD study could not happen without cultivating the seed of passion for research continuously throughout my educational life. So, I would like to thank all my teachers and lecturers from my very first level teacher at the Isfahan University of Technology primary school, Mrs. Goli, to my last lecturer of the last course I took with Dr. Alison Lennon at the University of New South Wales. Thanks to an intellectual and supportive friend, Omid Doostmohammadi, and also Masoud Salehi and Dr. Abas Mohsenzadeh who encouraged me to move out of my comfort zone and follow my passion for my PhD. The three and half years of my PhD journey began with my first contact with Dr. Richard Corkish who warmly and kindly, as has always been since then, brought me in touch with Prof. Gavin Conibeer who accepted and supported me as his student. I am very much thankful to both of them for giving me this opportunity. Being directly supervised by Prof. Gavin Conibeer and Dr. Murad Tayebjee during the first year of my PhD, while working on molecular organic semiconductors, I learnt a lot from both. I am humbly thankful to both of them for their guidance and insightful discussions. My special thanks goes to Murad to whom, with his generosity, kindness and great programing skills helped me to develop various skills such as persistence in research and also coding. i I will never forget my first meeting with Prof. Martin Green in the first month of my PhD during which I understood just a fraction of his scientific points. A year after that,