Conference Programme

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Overview
Session
U-04: Symp U
Time:
Tuesday, 20/Jun/2017:
1:30pm - 3:30pm

Session Chair: Maksym Kovalenko, ETH Zurich
Session Chair: Solenn Berson, University Grenoble Alpes
Location: Rm 331

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Presentations
1:30pm - 2:00pm
Invited

Polarons, Polaritons, and the Lead Halide Perovskite Puzzle

Xiaoyang ZHU

Columbia University, United States

Hybrid lead halide perovskites exhibit carrier properties that resemble those of pristine nonpolar semiconductors despite static and dynamic disorder, but how carriers are protected from efficient scattering with charged defects and optical phonons is unknown. We have recently put forward the large polaron model to explain the carrier protection (J. Phys. Chem. Lett. 2015, 6, 4758; Science, 2016, 353, 1409). We find that nascent charge carriers are screened by “solvation” or large-polaron formation on time scales ≤ 250 fs, leading to protected carriers with dramatic suppression of electron- LO phonon scattering. This results in long-lived energetic electrons with excess energy ~ 0.25 eV above the conduction band minimum and with lifetime on the order of 100 ps, which is three-orders of magnitude longer than those in conventional semiconductors (Science, 2016, 353, 1409; J. Am. Chem. Soc. 2016, 2016, 138, 15717). The exceptionally long-lived energetic carriers lead to hot fluorescence emission. The protection of energetic carriers is directly correlated with the liquid-like motion of the lattice, as revealed by femtosecond Kerr-effect spectroscopy. We will discuss strong light-matter interaction leading to efficient exciton-polariton formation and our most recent observation of coherent light emission (Nature Mater. 2015, 14, 636) without population inversion.


2:00pm - 2:30pm
Invited

Dynamics of Carrier Separation and Accumulation at Interfaces in Perovskite Thin-Film Solar Cells

Jacques-E. MOSER

École Polytechnique Fédérale de Lausanne, Switzerland

Organo-lead trihalide perovskite solar cells (PSCs) belong to the family of electron donor-acceptor heterojunction systems, where photocarriers do not only need to be quickly transported across the light-absorbing semiconductor layer, but must also be injected efficiently into the respective charge-extracting contact materials.

The origin of the performance inhomogeneity observed among PSCs of various compositions and architectures is unveiled by probing separately the dynamics of three essential processes: i) Carrier separation and transport across planar multigrain or homogeneous perovskite films, ii) Carrier trapping and recombination, and iii) Transmission of carriers through heterojunctions with hole- and electron-transporting materials.

Field-induced dynamic phenomena such as carrier separation and charge accumulation at interfaces were monitored in real time by use of time-resolved electroabsorption spectroscopy (TREAS). In combination with ultrafast THz spectroscopy, this technique allowed to monitor the time-evolution of the mobility of carriers and to unravel the mechanisms of charge trapping and recombination in the active layer.

Transport and interfacial charge transfer problems that could occur in non-optimum devices was diagnosed by application of TREAS. A significantly more efficient electron injection at the (MA, FA)PbI3-xBrx | SnO2 interface was observed in particular, together with a decreased bulk recombination rate, compared to that of the standard MAPbI3 material. These results account for the reported higher open circuit voltage and altogether better photovoltaic performance of solar cells based on a planar mixed organic cations, mixed halide-perovskite semiconducting film.


2:30pm - 2:45pm
Oral

Controlling Ionic Motion in Hybrid Perovskite Solar Cells by Temperature and Electrical Poling

Annalisa BRUNO1, Daniele CORTECCHIA2, Xin Yu CHIN1,3, Francesco MADDALENA3, Subodh MHAISALKAR1, Cesare SOCI3

1Energy Research Institute @ NTU (ERI@N), Research Techno Plaza, Nanyang Technological University, Singapore; 2Interdisciplinary Graduate School, Nanyang Technological University, Singapore; 3Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore

Despite their excellent power conversion efficiency, hybrid organic-inorganic perovskite (HOIP) solar cells exhibit significant hysteresis effects that hinder reliable device operation [1,2]. In this work we show that ionic motion is the dominant mechanism underlying hysteresis in pure MAPbI3 and in mixed HOIP (e.g.: mixed MA+, Cs+ and FA+and mixed I-/Br-) solar cells by studying temperature and electrical poling effects on solar cell performance. We observe that the hysteresis in the I-V curves is completely suppressed below 160 K and at the same time the effects of electrical poling are strongly temperature dependent. The initial decrease of power conversion efficiency observed while lowering the operating temperature is completely recovered, and even enhanced up to 20% of its original value, when electrical poling is performed at temperatures below 160 K. This behavior is consistent with temperature activated diffusion of halide anions and/or the motion of the cations that drive charge accumulation at TiO2/HOIP interface, leading to the reduction of the electron extraction barrier. The specific condition of enhancement strongly depends on the perovskites compositions. Studying HOIP solar cells with different contents of MA+ and FA+ and Br- and I- allows discriminating the contribution of specific ions and cations to the ionic motion within the HOIP and at the TiO2/HOIP interfaces. This is particularly important to develop effective strategies to mitigate operational instability of HOIP solar cells.

References:

[1] D. A. Egger, A. M. Rappe, L. Kronik, Accounts of chemical research 2016, 49, 573

[2] Y. Yuan, J. Huang, Accounts of chemical research 2016, 49, 286


2:45pm - 3:00pm
Oral

Atomic Structure And Electronic Properties Of Interfaces Formed By Perovskites

Susi LINDNER, Martin FRANZ, Andrea LENZ, Holger EISELE

Technical University of Berlin, Germany

Most electronic and optoelectronic properties of materials and devices are dominated by the atomic order of their surface and interfaces. Due to the major surface to volume ratio of nano particles, the functionalization of surfaces plays an important role in the world of nanotechnology. The search of new material compositions, so-called heterostructures, whichoffers a broad range of unexpected and fascinating properties, increases.

Perovskites have a long history beeing used as substrates for oxide semiconductor devices. Strontiumtitanate (SrTiO3) is a promising candidate of the perovskite material class. Due to its great variety of optical, electronic, as well as chemical properties, it is in focus of many research activities. An important role plays the usage of SrTiO3 as substrate or as active medium. SrTiO3 is intensivly studied and can be used as modell system for the group of perovskites.

In this work we present the surface preparation of Nb-doped SrTiO3(100) for further deposition of organic adsorbates, which is studied by means of scanning tunnling microscopy and -spectroscopy (STM/STS) and low-energy electron diffraction (LEED). After repeated annealing a flat surface is obtained, showing a highly ordered surface with different reconstructions. The STM images with atomic resolution will be discussed in detail. Furthermore STS data reveal electronic properties, which can be used to engineer heterostructures formed by organic charge-transfer material on top of the perovskite surface.


3:00pm - 3:15pm
Oral

Effects of Lead Sulphide Buffer Layer on the Property of MAPbl3 Organic Perovskite

Yung Fu CHEN1, Chuan Feng SHIH1, Ching Chich LEU2

1National Cheng Kung University, Taiwan; 2National University of Kaohsiung, Taiwan

A new generation of solid-state photovoltaics is being made possible by the use of organometal-trihalide perovskite materials. Due to the suitable direct band gap (around 1.5 eV), large extinction coefficient and long electron-hole diffusion length (>175 mm in single crystal), methylammonium lead (MAI) halide (MAPbX3, X=Cl, Br, I) based solar cell became a very promising device to achieve excellent performance in converting solar energy into electricity. Several high-performance perovskite cells were reported to have {002}- or {110}- oriented perovskite films, meaning the texture of organic perovskite is critical for device performance. However, control of the preferred orientation of a film on a polycrystalline substrate is a challenge. In this study, lead sulphide thin film was employed as a buffer layer to control the structure of organic perovskite.

Lead sulphide buffer layer with (100) preferred orientation was deposited via chemical bath deposition (CBD) process. The reactive substances used to obtained this lead sulphide buffer layer were (Pb(NO3)2), (NaOH), (SC(NH2)2) and deionized water. The lead sulphide buffer layer was then iodization to form the lead iodide thin film. After spin coating of MAI on top of the lead iodide thin film, MAPbl3 organic perovskite was prepared. With the measurements of X-Ray Diffraction, Scanning Electron Microscope, Current (Capacitance)–Voltage curve and Polarization-Electric curve, the influence of lead sulphide buffer layer on the property of MAPbl3 organic perovskite was discussed. Based on the study results, we tried to find out the favorable buffer layer process conditions for high performance devices.


3:15pm - 3:30pm
Oral

The Interplay of Crystal Chemistry and Mechanical Flexibility in Hybrid Organic-Inorganic Perovskites (HOIPs)

Shijing SUN1, Gregor KIESLICH2, Yanan FANG3, Furkan ISIKGOR4, Yue WU1, Fengxia WEI1, Michael GAULTOIS5, Jianyong OUYANG4, Tim WHITE3, Anthony CHEETHAM1

1University of Cambridge, United Kingdom; 2Department of Chemistry, Technical University of Munich, Germany; 3School of Materials Science and Engineering, Nanyang Technological University, Singapore; 4Department of Materials Science and Engineering, National University of Singapore, Singapore; 5Department of Chemistry, University of Cambridge, United Kingdom

In the past a few years, the light-harvesting, semiconducting hybrid organic–inorganic perovskites (HOIPs) have attracted a great deal of attention in the photovoltaic community, as the efficiency of perovskite solar cells roared from 3.8% to over 22% within just six years of intensive research.[1,2]

Although the outstanding optoelectronic properties of HOIPs led to scientific advances, there is concern that the applications of HOIPs in devices may be limited by their lack of robustness. In particular, the stress state and the crystallinity of the perovskite layer have a strong impact on the absorption performance. However, despite the growing interest in making the perovskite based flexible and wearable devices, there were few studies on the mechanical flexibility and how it is influenced by the crystal chemistry in HOIP systems.

We studied the mechanical properties and the structural-property relationships of APbX3 perovskites via nanoindentaion and X-ray diffraction, and reported that the hardness of CH3NH3PbX3 single crystals show that iodide based lead perovskites are more resistant to permanent deformation than the bromide perovskites. Further studies on CN2H5PbX3 revealed that how the compressibility are influenced by the structural evolution. The three key factors here are the strength of Pb–X bonds, the relative packing and phase stability, as well as the hydrogen bonding interactions. We also investigated the effects of dimensionality on the mechanical response, and for materials with lower dimensionality, the elastic properties tend to be more anisotropic. If we replace the lead with one monovalent (e.g. potassium) and a trivalent ion (e.g. bismuth) and form a double perovskite, the elastic moduli decreased and the structures were more compliant.

References:

[1] M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J. Snaith, Science 2012, 338, 643–647.

[2] C. C. Stoumpos, C. D. Malliakas, M. G. Kanatzidis, Inorg. Chem. 2013, 52, 9019–38.



 
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