November 9, 2020 — 4:30 p.m.

Prof. Dr. Michael Saliba
Institute for Photovoltaics, University of Stuttgart & Research Center Jülich

"The Versatility of Perovskite Materials for Optoelectronics"

 

Perovskite solar cells (PSCs) have created much excitement in the past years and attract spotlight attention. This talk will provide an overview of the reasons for this development highlighting the historic development as well as the specific material properties that make perovskites so attractive for the research community [1–3].

The current challenges are exemplified using a high-
performance model system for PSCs (multication Rb, Cs, methylammonium (MA), formamidinium (FA) perovskites) [2,3]. The triple cation (Cs, MA, FA) achieves power conversion efficiencies (PCEs) close to 21% due to suppressed phase impurities. This results in more robust materials enabling breakthrough reproducibility.

Through multication engineering, the usually not-considered Rb can be studied (unsuited as single-cation perovskite) [4]. This results in a stabilized efficiency of 21.6% with one of the smallest differences between bandgap and voltage ever measured for any PV material. Polymer-coated cells maintained 95% of their initial performance at elevated temperature for 500 hours under working conditions, a crucial step towards industrialisation of PSCs.

To explore the theme of multicomponent perovskites further, molecular cations were re-evaluated using a globularity factor. With this, we calculated that ethylammonium (EA) has been misclassified as too large. Using the multication strategy, we studied an EA-containing compound that yielded a high open-circuit voltage of 1.59 V. Moreover, using EA, we demonstrate a continuous fine-tuning for perovskites in the "green gap" which is relevant for lasers and display technology [5].

The last part elaborates on a roadmap on how to extend the multication to multicomponent engineering providing a series of new compounds that are highly relevant candidates for the coming years, also in areas beyond photovoltaics, for example for medical scintillation detectors [5,6].

[1] N. Jeon et al., Nature (2015)

[2] J. Lee et al., Advanced Energy Materials (2015)

[3] D. McMeekin et al., Science (2016)

[4] M. Saliba et al., Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance, Science (2016)

[5] S. Gholipour et al., Globularity-Selected Large Molecules for a New Generation of Multication Perovskites, Advanced Materials (2017)

[6] S. Turren-Cruz et al., Methylammonium-free, high-performance and stable perovskite solar cells on a planar architecture, Science (2018)

Professor Michael Saliba is the director of the Institute for Photovoltaics (ipv) at the University of Stuttgart, with a dual appointment at the Research Center Jülich, Germany. His research focuses on a deeper understanding and improvement of optoelectronic properties of photovoltaic materials with an emphasis on emerging perovskites for a sustainable energy future.

Previously, he was at TU Darmstadt, Fribourg University and a Marie Curie Fellow at EPFL, Switzerland. He obtained his PhD at Oxford University and MSc degree at Stuttgart University in conjunction with the Max Planck Institute for Solid State Research.

Michael Saliba has published numerous works in the fields of plasmonics, lasers, LEDs, and perovskite optoelectronics. He has been an ISI Highly Cited Researcher in 2018 and 2019. He was awarded the Young Scientist Award of the German University Association, the Heinz-Maier-Leibnitz prize by the German Research Foundation (DFG) and named as one of the World’s 35 Innovators Under 35 by the MIT Technology Review. Since 2018, he is a Member of the German National Young Akademy ( Junge Akademie).

September 17, 2020 — 2 p.m.

Prof. Dr. Peter G. Bruce
University of Oxford

"Lithium Transition Metal Oxides
  What happens when electrons are removed from O2-?"

 

Lithium transition metal oxide intercalation compounds are the basis of cathodes in Li-ion batteries. On charging, Li+ is removed from the lattice with charge compensation by oxidation of the transition metal ion, e.g. Li+ is extracted from LiMn2O4 while Mn3+ is oxidised to Mn4+. Limiting oxidation to the transition metal limits the capacity to store charge and hence the energy density of the Li-ion battery. For more than 20 years it has been known that more Li+ can be removed from certain compounds than is charge compensated by Tm oxidation. The archetypal examples are the layers compounds: Li[Li0.2Ni0.2Mn0.6]O2 and Li[Li0.2Ni0.13Co0.13Mn0.54]O2. Understanding the origin of this phenomenon has proved difficult. It is now known that electrons are removed from the O2- ions (holes in the O valance band) but the nature of the hole states, their link to the resulting structural changes and how understanding this might be harnessed to increase the energy storage of Li-ion batteries is only now crystallising. [1–3]

I shall show, using a range of techniques including XAS, RIXS, STEM, NMR, diffraction and DFT, that across a wide range of alkali metal rich transition metal oxides, O2- is oxidised to O2 [4,5]. The O2 is either evolved from the surface or trapped in voids formed in the bulk by reorganisation of vacancies within the structure. Although O2 can be reduced back to O2-, the process is not energetically reversible, explaining the much lower voltage on discharge compare with the first charge (approx. 1 eV less), the phenomenon of so-called voltage hysteresis. Furthermore, it is possible to suppress O2 formation, trapping hole states on O2- and obtaining energetic (voltage) and structural reversibility [4]. Such behaviour points the way to awards high energy density cathodes for Li-ion batteries.

[1] Luo, K. et al. Charge-compensation in 3d-transition-metal-oxide intercalation cathodes through the generation of localized electron holes on oxygen. Nature Chemistry 8, 684-691 (2016).

[2] Maitra, U. et al. Oxygen redox chemistry without excess alkali-metal ions in Na2/3[Mg0.28Mn0.72]O2. Nature Chemistry 10, 288-295 (2018).

[3] Boivin, E. et al. The Role of Ni and Co in Suppressing O‐Loss in Li‐Rich Layered Cathodes. Advanced Functional Materials 2020, 2003660 (2020).

[4] House, R. A. et al. Superstructure control of first-cycle voltage hysteresis in oxygen-redox cathodes. Nature 577, 502-508 (2020).

[5] House, R. A. et al. First cycle voltage hysteresis in Li-rich 3d cathodes associated with molecular O2 trapped in the bulk. Nature Energy, in press (2020).

July 23, 2020 — 2 p.m.

Prof. Dr. Jürgen Janek
Justus Liebig University Gießen

"Solid State Ionics and its Future Directions"

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