Meetup Presentation

This online seminar brings the scientific conference gathering to the desktops or smartphones of scientists worldwide. Researchers can present their work and keep up with cutting-edge research in the field while reducing their carbon footprint, improve the work-life balance and keep the sense of community.

It will consist of two parts, in which interaction will be the main force:

  - The Oral Session will consist on a few short broadcasted talks led by Invited Speakers, followed by a time for questions from the public driven by a moderator.

  - ePoster Session, where attendees and authors can share their ideas and get feedback from their colleagues worldwide through a chatroom.

All participants can join and present here their work. Due to the short format of the meeting (with few oral contributions), we encourage senior researchers to present ePoster, and of course we expect enthusiastic participation of junior researchers.

Defects are an important topic for every photovoltaic material, because they reduce the open-circuit voltage and thereby also the power conversion efficiency. While metal-halide perovskites are considered to be defect tolerant materials, they are not immune to the influence of defects. The talk attempts to answer three questions, namely do defects matter in lead-halide perovskite solar cells, do they matter in the bulk or at interfaces and finally, where in the band gap are these defects? The first question of the importance of defects in general is easily studied by looking at current high V_oc and high luminescence perovskite solar cells.^1-3 So far perovskite solar cells haven’t beaten the level of 10% external luminescence quantum efficiency, which implies that still 90% of recombination events eventually produce heat. Therefore, non-radiative recombination is still highly relevant, at least at one sun conditions. The second question of where the defects matter has no generic answer. However, for many practical situations, the interfaces between absorber and electron or hole transfer layers limit the open-circuit voltage.^4-5 This implies that it is defects at these interfaces that are most important for further improving efficiencies. The final question deals with the question of where the defects are in energy. For a defect to be highly recombination active, it has to capture electrons and holes efficiently with the slower of the two capture processes limiting the total rate. The classical models of recombination assuming a parabolic potential energy surface (harmonic approximation) predict that defects with energy levels extremely close to midgap should be the most recombination active.^6-7 However, to fully understand the situation in perovskite solar cells, calculations including anharmonicity⁸ have to be performed in the future. 

1.  Krückemeier, L.; Rau, U.; Stolterfoht, M.; Kirchartz, T., How to Report Record Open-Circuit Voltages in Lead-Halide Perovskite Solar Cells. Advanced Energy Materials 2020, 10 (1), 1902573.

2.  Liu, Z.; Krückemeier, L.; Krogmeier, B.; Klingebiel, B.; Marquez, J. A.; Levcenko, S.; Öz, S.; Mathur, S.; Rau, U.; Unold, T.; Kirchartz, T., Open-Circuit Voltages Exceeding 1.26 V in Planar Methylammonium Lead Iodide Perovskite Solar Cells. ACS Energy Letters 2019, 4, 110-117.

3.  Jiang, Q.; Zhao, Y.; Zhang, X.; Yang, X.; Chen, Y.; Chu, Z.; Ye, Q.; Li, X.; Yin, Z.; You, J., Surface passivation of perovskite film for efficient solar cells. Nature Photonics 2019, 13, 460-466.

4.  Stolterfoht, M.; Caprioglio, P.; Wolff, C. M.; Marquez, J. A.; Nordmann, J.; Zhang, S.; Rothhardt, D.; Hörmann, U.; Amir, Y.; Redinger, A.; Kegelmann, L.; Zu, F.; Albrecht, S.; Koch, N.; Kirchartz, T.; Saliba, M.; Unold, T.; Neher, D., The impact of energy alignment and interfacial recombination on the internal and external open-circuit voltage of perovskite solar cells. Energy & Environmental Science 2019, 12 (9), 2778-2788.

Prof. Aron Walsh holds the Chair in Materials Design at Imperial College London. He was awarded his Ph.D in Chemistry from Trinity College Dublin and later worked at the National Renewable Energy Laboratory, University College London, and University of Bath.

His research involves cutting-edge materials theory and simulation applied to problems across solid-state chemistry and physics, including materials for solar cells and fuels, batteries, thermoelectrics, and solid-state lighting. He has an expertise in the theory of semiconductors and dielectrics, and is developing innovative solutions for materials data, informatics and design. His group published a review on machine learning for molecules and materials in Nature.

Here, first I will summarize our understanding of the nature of defects and their photo-chemistry, which leverages on the cooperative action of density functional theory investigations and accurate experimental design. Then, I will show the correlation between the nature of defects and the observed semiconductor instabilities. We identify photo-instabilities related to competing light-induced formation and annihilation of trap states, disclosing their characteristic length and time scales and the factors responsible for both processes. We show that short range/short time defect annihilation can prevail over defect formation, happening on longer scales, when effectively blocking undercoordinated surface sites, which act as a defect reservoir. Finally, based on such knowledge, I will discuss different synthetic and passivation strategies which are able to stabilize the perovskite layer towards such photo-induced instabilities, leading to improved optoelectronic material quality and enhanced photo-stability in a working solar cell.