Guest talks

Dr. Wenxin Mao (Guest talk, Monash University)

i-MEET, seminar room 3.71
Martensstraße 7, 91058 Erlangen

07.10.2019, 11:00

Towards Single-Crystalline Perovskite Devices

Organo-lead halide perovskites (OHPs) have recently emerged as a new class of exceptional optoelectronic materials. However, despite the thorough studies into lead halide perovskites during over past 10 years, there are still many unknowns concerning both the device performance and the stability.
The first part of this presentation introduces the fabrication of a single-crystalline perovskite based electric-optical modulator device through a novel solution-processed perovskite crystal growth method. The as-synthesized MAPbBr3 microplatelets were tailored into a waveguide-based photonic device, of which an efficient active electro-optical modulator (AEOM) consisting of a MAPbBr3 non-linear arc waveguide was demonstrated, exhibiting a 98.4% PL intensity modulation with an external voltage of 45 V. This novel synthetic approach as well as the demonstration of effective waveguiding will pave the way for developing a wide range of photonic devices based on organo-lead halide perovskites.
The second part of this presentation explores the much-debated mechanism of light induced phase segregation in mixed halide perovskite single crystals. Herein, a method for growing large (30×30×1 μm) monocrystalline MAPb(BrxI1-x)3 single crystals is presented. The direct visualisation of the growth of nanocluster-like I-rich domains throughout the entire crystal revealed grain boundaries are not required for this transformation. Narrowband fluorescence imaging and time-resolved spectroscopy provided new insight into the nature of phase segregated domains and the collective impact on optoelectronic properties.

Vincent M. Le Corre ( Zernike Institute for Advanced Materials University of Groningen )

i-MEET, seminar room 3.71
Martensstraße 7, 91058

16.10.2019, 11:00

Lessons learned from device modeling of perovskite solar cells

In the past decade, perovskite solar cells (PSCs) have been one of the major research interests of the photovoltaic community. This technology is now on the brink of catching up with the classical inorganic solar cells with efficiency now reaching up to 25%. Further improvements in efficiency require a thorough understanding of the remaining loss mechanisms.

In this work, we have been able to reproduce experimental current-voltage characteristics using drift-diffusion modeling. The simulations allow us to pinpoint the main losses in a device for a wide range active materials, deposition techniques and device structures. Using this simulation on a larger scale, we have been able to derive two criteria that help to optimize the charge transport layers prior to making the solar cell stack to avoid transport losses. Next, we have revisited the analysis of space-charge-limited measurements on perovskite single-crystals. Such measurements have been reported in the literature to extract trap densities. However, our simulations clearly show that this analysis is severely complicated by the presence of ions. Finally, we used large-scale simulations and trained machine learning algorithms to identify the dominant recombination processes in PSCs based on easily accessible experimental parameters.

Dr. Fabian Panzer (Guest talk, University of Bayreuth)

30.10.2019, 11:00

Helmholtz-Institut Erlangen-Nürnberg (HI ERN)
Fürther Str. 248, 90429 Nürnberg

Understanding and controlling perovskite film formation – optical in situ characterizations during solution processing and a solvent free route for film processing based on pressure treatment of perovskite powders

Optoelectronic devices based on hybrid perovskites have undergone a remarkable development in recent years mainly due to enormous efforts in optimizing their film properties. However, the relatively complex film formation dynamics as they occur, e.g. during solution processing, has not yet been addressed and understood in detail. In the first part of my talk I discuss measurements of absorption and photoluminescence (PL) during a solution based two-step synthesis of the model hybrid perovskite CH3NH3PbI3 in situ while spin coating. We observe changes in both, absorption and PL during the spin-coating process which allows us to identify different time regions with specific spectral changes. Our results suggest that prior to film formation, a variety of intermediate states exist in the electronic structure to which the transition rates can be altered with temperature.

In the second part of my talk I will present an alternative, entirely dry processing approach, decoupling perovskite crystallization and film formation, by using readily prepared perovskite powders and produce films by appropriate mechanical pressure treatment. We show how a mechanochemical synthesis by ball milling allows to produce a wide range of phase pure and highly stable perovskite powders with a high flexibility in processing and we address the impact of milling parameters on the powder properties. Using these powders, we demonstrate how the used pressure and the powder microstructure, i.e. particle size and stoichiometry affect the mechanical stability, compactness and surface roughness of the pressed layers. We further address how specific temperature treatment during the pressing step can improve the properties of the pressed layer, and show their capability to be used in perovskite based optoelectronic devices.

Short Biography:

Dr. Fabian Panzer (born in Marktredwitz, Germany, 1988) is a Junior Researcher (Habilitand) at the Chair for Soft Matter Optoelectronics at the University of Bayreuth. He studied Physics at the University of Bayreuth where he obtained his PhD in 2016 with honors. A major interest of his work is to understand how changes in the conformation or structure of organic or hybrid semiconductors affect their optical properties. With these findings he addresses technical and application-oriented questions of these semiconductor materials, with a focus on their film formation process, to advance their successful processing in an industrial context.

Dr. Sudhanshu Shukla (Guest talk, Laboratory for Photovoltaics, University of Luxembourg )

i-MEET, seminar room 3.71
Martensstraße 7, 91058 Erlangen

20.11.2019, 11:00

Defects, disorder and phase instabilities in next generation energy materials

Material properties such as high optical absorption, long minority carrier diffusion lengths and reasonable carrier mobilities are desirable but not sufficient for high performing and enduring photovoltaic (PV) device. Iron pyrite (FeS2) is a “near ideal” low-cost semiconductor but it fails to perform well, mainly due to its problem of low photovoltage. To address this problem, optoelectronic properties of pyrite thin films prepared by different routes will be shown and the role of phase impurities, defects and disorder will be emphasized via temperature dependent charge transport studies and carrier dynamics, revealing the hopping transport mechanism and carrier recombination through mid-gap states associated with the intrinsic defects in pyrite thin films. In the second part of the talk, promising results on the impact of defect passivation and phase stabilization in halide perovskites will be presented in the context of environmental and operational stability. Device architecture, composition engineering and additive enabled crystallization control helps in achieving better stability.