The importance of lead iodide stoichiometry when preparing perovskite solar cells
Alan Dunbar a, Kostas Tsevas a, Maria Vasilopoulou b, Mohammad Nazeeruddin c, David Lidzey a, Connie Rodenburg a
a Chemical and Biological Engineering, The University of Sheffield, Mappin St Sheffield City Centre, Sheffield, United Kingdom
b Institute of Nanoscience and Nanotechnology, NCSR Demokritos, Patriarchou Grigoriou & Neapoleos Str., Agia Paraskevi, Athens, 15310, Greece
c Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland.
International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV22)
València, Spain, 2022 May 19th - 25th
Organizers: Pablo Docampo, Eva Unger and Elizabeth Gibson
Oral, Alan Dunbar, presentation 040
Publication date: 20th April 2022

Perovskite solar cells have made remarkable progress in the field of photovoltaics over the past decade. The optoelectronic properties of the perovskite absorber are of paramount importance in determining the device performance, and the stoichiometry of the starting materials plays a key role in this, but little published work has focused on the synthesis of fully stoichiometric precursor materials of high purity and at high yield. We have investigated planetary ball milling as a low-cost, energy-efficient, and solvent-free synthesis route for lead iodide. It enables low-oxygen, single or multiple polytypic phase PbI2 with tuneable stoichiometry to be prepared [1]. We compared our PbI2 alongside commercially available materials, using X-ray diffraction, X-ray photoelectron spectroscopy, and Rutherford backscattering spectroscopy. Both the stoichiometric PbI2 we synthesized and a sub-stoichiometric commercially available PbI2 (where the iodide content is below the optimum Pb:I atomic ratio of 1:2) were used to prepare methylammonium lead iodide microcrystals. Perovskite solar cells were produced using stoichiometric and sub-stoichiometric PbI2 mixed with an equimolar amount of methylammonium iodide and compared to devices produced from re-dissolved microcrystals. We found that the device performance is strongly dependent upon the stoichiometry of the lead iodide precursor, with champion efficiencies achieving a power conversion efficiency of over 17%, with no obvious correlation with its polytypic phases. The formation of intermediate methylammonium lead iodide microcrystals was found to correct the iodide stoichiometry permitting high efficiency devices to be made from either PbI2 source. This work highlights the critical role of the initial PbI2 stoichiometry in hybrid perovskites as well as demonstrating synthesis methods and perovskite layer fabrication protocols suitable for low-cost solar energy harvesting.


This work was funded by the UK Engineering and Physical Sciences Research Council (EPSRC) via a PhD studentship for K. Tsevas at the University of Sheffield from the Centre for Doctoral Training in New and Sustainable Photovoltaics, grant code EP/L01551X/1. A.D.F Dunbar wishes to acknowledge support from the EPSRC through Supergen Solar Challenge Grant: EP/M025020/1. K. Tsevas wishes to acknowledge the access provided through the RADIATE GATE scheme for RBS measurements, proposal number 19001741-ST. We also appreciate the assistance from Hans-Arno Synal, Christof Vockenhuber and colleagues from the Laboratory of Ion Beam Physics at the ETH Zurich facilities in Switzerland, who carried out the RBS measurements and analysis of PbI2 samples. M. Vasilopoulou acknowledges support of this work by the project “Development of Materials and Devices for Industrial, Health, Environmental and Cultural Applications” (MIS 5002567), which is implemented under the “Action for the Strategic Development on the Research and Technological Sector”, funded by the Operational Program "Competitiveness, Entrepreneurship and Innovation" (NSRF 2014-2020) and co-financed by Greece and the European Union (European Regional Development Fund).

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