Laboratory for Surface Modification (LSM)

Seminars Archives

May 2020 | February 2021 | March 2021

Thursday, February 11, 2021
Assembling Designer Solids from Molecular Building Blocks: Principles, Prospects, and Problems
Christof Wöll
Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology, North Campus, 76021 Karlsruhe
12:00 PM Eastern Time (US and Canada)

Realizing molecular “Designer Solids” by programmed assembly of building units taken form libraries is a very appealing objective. Recently, metal-organic frameworks (MOFs) have attracted a huge interest in this context. Here, we will focus on MOF-based electrochemical, photoelectron-chemical, photovoltaic, and sensor devices. Internal interfaces in MOF heterostructures are also of interest with regard to photon-upconversion and the fabrication of diodes.

Since the fabrication of reliable and reproducible contacts to MOF-materials represent a major challenge, we have developed a layer-by-layer (lbl) deposition method to produce well-defined, highly oriented and monolithic MOF thin films on appropriately functionalized substrates. The resulting films are referred to as SURMOFs [1,2]. The fabrication of hetero-multilayers (see Fig. 1) is rather straightforward with this lbl method. In this talk, we will describe the principles of SURMOF fabrication as well as the results of systematic investigations of electrical and photophysical properties exhibited by empty MOFs and after loading their pores with functional guests. We will close with discussing further applications realized by loading MOFs with nanoparticles or quantum dots.

[1] J. Liu, Ch. Wöll, Chem. Soc. Rev. 46, 5730-5770 (2017)
[2] L. Heinke, Ch. Wöll, Advanced Materials 31 (26), 1970184 (2019)
Thursday, February 18, 2021
Devices Emerging from Controlling Organic and Metal Halide Perovskite Energetics and Morphology
Barry Rand
Princeton University
Department of Electrical Engineering & the Andlinger Center for Energy and the Environment
12:00pm ZOOM

In this seminar, we will focus on our recent work on two different thin film systems – metal halide perovskites and organic semiconductors.

For organic semiconductors, through proper control of processing, we are able to realize pinhole free films with grains of up to 500 μm in extent. we have found that charge transfer (CT) states incorporating these long-range-ordered films can be highly delocalized, contributing to noticeably lower energy losses. We will discuss these aspects and their implications for more efficient organic solar cell function.

Hybrid inorganic-organic perovskite materials, most commonly methylammonium lead triiodide (MAPbI3), have captured significant interest in the thin film optoelectronics community due to their impressive optical and electrical properties. For light emitting diodes (LEDs), we have established a general protocol for preparing ultrathin, smooth, passivated, and pinhole free films of metal halide perovskites with various compositions, by incorporating bulky organoammonium halide additives to the stoichiometric 3D perovskite precursors. Here, we will present this approach as well as our understanding for how to select bulky organoammonium additives. LEDs produced in this way are capable of exceeding 17% external quantum efficiency, exhibit significantly improved stability, and are capable of being as flexible as organic electronic thin films. Finally, they allow for stabilizing mixed halide (I and Br) and mixed Pb-Sn stoichiometries such that we can tune emission from the green to near infrared. Also, we will show how these smooth films can be employed in optically pumped laser structures that tunable and capable of sustaining cw emission
Thursday, February 25, 2021
Imaging Defects and Electronic Disorder in Organic Semiconductors
Dan Frisbie
Department of Chemical Engineering and Materials Science
University of Minnesota
12:00pm ZOOM

The central thesis of this talk is that many structural defects in crystalline organic semiconductors have surface potential signatures that can be recorded and imaged by scanning Kelvin probe microscopy (SKPM) with sub-100 nm resolution.[1-5] This allows straightforward visualization of defects that are difficult to detect by other methods. Additionally, we argue that surface potential fluctuations are a direct measure of static electronic disorder, namely band edge variations, that will impact electron and hole transport. Thus, surface potential imaging not only reveals defects in crystalline organic semiconductors but importantly provides a direct link to electronic disorder (e.g., traps, scattering centers) that degrade transport performance. This talk will focus on three illustrative examples based on thin films and single crystals of benchmark organic semiconductors,[2,4,5] including one case where we can make a thorough connection between structure, surface potential, and field effect transport.[5] We propose that in many cases the surface potential contrast associated with a given defect arises due to inhomogeneous strain around the defect. To support this, we further describe the first direct measurements of the strain-surface potential relationship for macroscopic single crystals of rubrene.[3] Overall, we suggest that surface potential measurements are a powerful approach to understanding correlated structural and electronic disorder in soft organic semiconductors.

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