Ultrafast Spectroscopic Studies of Solution-Processing Organic Photovoltaic Materials and Devices
Creators: name: Zhang, Jiangbin
Publisher: name: Apollo - University of Cambridge Repository
Year: 2020
Subjects: organic solar cells
ultrafast spectroscopy
pump-push photocurrent spectroscopy
non-fullerene acceptor
sequential deposition
temperature-dependent ultrafast spectroscopy
binding energy
charge-transfer state

description: Organic solar cells (OSCs) have potential applications in wearable electronics as well as in-door and building-integrated photovoltaics, owing to features such as low-temperature processing, light weight and flexibility, and synthetic versatility. Due to the development of new materials systems with unique optoelectronic properties, a breakthrough in the OSC performance has been achieved. However, knowledge of the underlying mechanism still lags behind. Such understanding is essential for further development of organic photovoltaics. The needed comprehensive investigation mainly comes from studying the carrier dynamics on promising OSC systems. To this aim, the used tool kits, ultrafast spectroscopic methods, are the subject of this thesis. First three chapters of the thesis present the general paradigm of photoconversion in OSCs and outlines the key models used to describe their photovoltaic performance. The ability of OSCs to generate electrical current in response to absorbing sunlight greatly differs, depending on materials and the following device fabrication methods. In the device operation, the performance is related to the conversion efficiency from molecular excited states into charge carriers. Chapter 2 presents a historical review of different material combination, device architecture and key processes relevant for solar cell performance. This leads to the key questions that this thesis addresses: which molecular properties are most important for photoconversion in OSCs? How much energy is required to convert from molecular excited states to free charges? What are the optimal preparation procedure and microstructure of OSC devices? Chapter 3 gives an overview of spectroscopic tools used to address these questions. Chapters 4 to 8 target above questions in application to different material systems, from the ‘classical’ broadly studied polythiophene-fullerene blends to the high-performance OSC devices based on non-fullerene acceptors (NFAs). Firstly, the rate of exciton dissociation is studied in state-of-the-art organic blends with NFAs. The charge generation is found to be slow (ps), in sharp contrast with the ultrafast timescales in traditional blends based on fullerene acceptors. Secondly, a new technique is developed, namely temperature-dependent pump-push photocurrent spectroscopy, which is able to measure the strength of interaction between charges in working OSCs. This technique is applied to research which factors govern the dissociation of molecular excited states in fullerene-based blends. Thirdly, recombination processes are studied using a set of ultrafast techniques, focusing on their effect on device performance, and the dependence on processing conditions and used materials. Finally, a new method ‘via sequential deposition’ is demonstrated for fabrication of efficient NFA-based devices. We discuss the properties of OSCs fabricated by this method and the potential for its commercialisation.
description: China Scholarship Council (No. 201503170255)