Ultrafast Spectroscopic Studies of Solution-Processing Organic Photovoltaic Materials and Devices

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.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2020

Author dc:creator

• Zhang, Jiangbin

Advisors dc:contributor.advisor

• Friend, Richard
• Bakulin, Artem

• organic solar cells
• ultrafast spectroscopy
• pump-push photocurrent spectroscopy
• non-fullerene acceptor
• sequential deposition
• temperature-dependent ultrafast spectroscopy
• binding energy
• charge-transfer state

Statement dc:rights

• p.44, figure of efficiency and number of publications of non-fullerene acceptor-based organic solar cells over years, copyright holder is Elsevier. p.44, figure of relationship between energy loss, efficiency, and absorption wavelength, copyright holder is Springer Nature. p.59, diagram of UV-Vis spectroscopy, copyright holder is HP. p.62, diagram of photothermal deflection spectroscopy, copyright holder is The Optical Society. p.64, diagram of PLQY setup, copyright holder is Wiley‐VCH Verlag GmbH & Co. KGaA. p.68, figure of transient absorption signatures, copyright holder is Dr. Andreas Jakowetz, used with permission. chapter 5 permission obtained from American Chemical Society for reusing the figures, diagrams etc. in the thesis. chapter 6 permission obtained from Wiley for reusing the figures, diagrams etc.in the thesis.

Licence

• https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/87df4095-41f5-4a37-901e-727ef20909fb/download
• https://www.rioxx.net/licenses/all-rights-reserved/

Language dc:language

en

Author Identifier

0000-0001-6565-5962
0000-0001-6565-6308
0000-0002-3998-2000

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/304206