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Bulk and Interfacial Processes in (Doped) Organic Semiconductors

English title Bulk and Interfacial Processes in (Doped) Organic Semiconductors
Applicant Banerji Natalie
Number 184819
Funding scheme Project funding (Div. I-III)
Research institution Departement für Chemie und Biochemie Universität Bern
Institution of higher education University of Berne - BE
Main discipline Physical Chemistry
Start/End 01.04.2019 - 31.03.2023
Approved amount 1'044'574.00
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All Disciplines (2)

Discipline
Physical Chemistry
Material Sciences

Keywords (10)

Organic solar cells; Ultrafast spectroscopy; Excited-state properties; Conjugated molecules/polymers; Terahertz conductivity; Doped organic materials; Charge separation dynamics; Sum frequency generation; Non-fullerene acceptors; Charge transport

Lay Summary (German)

Lead
Wir untersuchen organische Solarzellen mit Kurzzeit Spektroskopie, um die Funktions-Mechanismen besser zu verstehen und ihre Effizienz zu steigern.
Lay summary

Die Sonne ist eine unumgängliche Quelle, um die wachsenden Energie-Bedürfnisse der Menschheit zu erfüllen. Eine Möglichkeit, diese Energie zu nutzen, ist sie in Solarzellen in Elektrizität umzuwandeln. Forscher arbeiten daran, die jetzigen teuren Silizium-Solarzellen durch neue Technologien zu ersetzen. Zum Beispiel können leitfähige organische Moleküle dazu benützt werden, sehr leichte und flexible Solarzellen herzustellen. Dazu wird typischerweise ein Elektron-Donor (ein konjugiertes Polymer) mit einem Elektron-Akzeptor zu einem dünnschichtigen Film verarbeitet. Bis jetzt wurden hauptsächlich Fullerene (C60-Derivate) als Akzeptoren verwendet, neuerlich werden diese aber durch nicht-Fulleren Akzeptoren (NFAs) ersetzt, welche eine viel höhere Effizienz in Solarzellen zeigen. In meiner Forschungsgruppe arbeiten wir daran, die Mechanismen zu verstehen, mit denen Licht in diesen organischen Solarzellen zu Elektrizität umgewandelt wird. Da es sich dabei um sehr schnelle Prozesse handelt, die in einem Sekunden-Bruchteil stattfinden, benützen wir für unsere Experimente Laser, die ganz kurze Lichtimpulse erzeugen, sogenannte ultraschnelle Laser. In diesem Projekt nützen wir verschiedene spektroskopische Experimente im sichtbaren und terahertz Bereich, um organische Solarzellen zu untersuchen. Insbesondere wollen wir erforschen:

- Warum Solarzellen mit NFAs eine höhere Effizienz haben und wie man diese weiter steigern kann.

- Welche Prozesse an Grenzflächen in den organischen Solarzellen stattfinden.

- Wie man die Leitfähigkeit von organischen Halbleitern durch Dotierung erhöhen kann.

Direct link to Lay Summary Last update: 01.04.2019

Responsible applicant and co-applicants

Employees

Associated projects

Number Title Start Funding scheme
150536 A Mechanistic Approach to Organic Electronics 01.09.2014 SNSF Professorships

Abstract

Organic semiconductors are pi-conjugated small molecules or polymers that can be processed into thin films and used in a variety of electronic applications, including solar cells, light-emitting diodes or field-effect transistors. They have many advantages compared to conventional inorganic materials, since they are cost-efficient, non-toxic, chemically tunable and mechanically flexible. Great progress has been made in recent years to push the efficiency of organic electronic devices towards and beyond the one of other technologies. This has led to the development of new organic materials with complex optoelectronic properties, offering great opportunities to address fundamental scientific questions and to gain essential feedback about the underlying working principles of the applications. Here, we plan to investigate particularly timely and topical aspects of organic electronics, exploiting our exceptional expertise with ultrafast spectroscopy to bring unique insights to a broad and diverse field. Specifically, we will focus on the photophysics of non-fullerene acceptors and interfacial processes in organic solar cells and study the conductivity of doped organic semiconductors. In the last couple of years, non-fullerene acceptors have replaced fullerene acceptors in photovoltaic blends with electron-donating conjugated materials. Their superior performance has allowed record power conversion efficiencies, re-launching organic photovoltaics to the forefront of attention. Non-fullerene solar cells are characterized by fundamentally different photophysics compared to fullerene-based systems, for example extremely small driving forces for charge transfer lead to high open circuit voltages. Our first objective will be to relate charge transfer rates and recombination mechanisms to these small driving forces in non-fullerene systems using transient absorption spectroscopy from the femtosecond to microsecond range. Our second objective will be to include dispersive charge transport in the picture, measuring it from the few-nanometer scale to macroscopic device distances with a combination of terahertz, electro-absorption and transient photocurrent experiments. We expect a comprehensive picture of charge generation, recombination and transport as a function of energetic and morphological parameters. Since short-range mobility also plays an essential role in the separation of charges from interfacial charge transfer states (which we directly visualize using electro-modulated differential absorption spectroscopy), we envisage to find correlations between the charge separation efficiency and the terahertz short-range mobility. Another crucial parameter for this dissociation is the molecular-level interfacial structure (conformations, orientations, packing), which has been little investigated due to the experimental challenge to address the interface. Our third objective will be to use (interface-specific) vibrational sum-frequency generation spectroscopy to establish structure-property relations in model donor:fullerene and donor:non-fullerene bilayers. We expect to unravel links between interfacial structure, energetics, charge dynamics and short-range mobility. Finally, doped organic semiconductors are increasingly used as conductive interlayers in solar cells, light-emitting diodes and transistors. Our fourth objective is therefore to bring new insights to the electronic structure, doping mechanisms and charge transport in doped organic materials using transient absorption and terahertz spectroscopy. We foresee to determine the optimal parameters governing high doping efficiency and conductivity, with particularly novel insights for self-doped conjugated polyelectrolytes that are conductive without addition of external dopants. Overall, the outcome of our research provides essential guidelines to the organic electronics community for design strategies of highly efficient devices, contributing on the mid-term to large-scale implementation of flexible electronics and cheap solar energy conversion. We address important scientific challenges such as electron transfer, interfacial processes and doping mechanisms, reaching beyond organic electronics and leaving a broad impact on key concepts of science.
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