Design and synthesis of electron acceptor materials for high-performance organic solar cells

Abstract

Organic solar cells (OSCs) are an attractive technology due to the advantages of low production cost, mechanical flexibility and compatibility with roll-to-roll printing. Recent advances in small molecular acceptors (SMAs) and all-polymer solar cells (all-PSCs) have brought a “paradigm shift” from fullerene devices to non-fullerene devices. However, the structure-property relationships and donor-acceptor interactions according to morphological and photophysical aspects are still not clear enough. In this thesis, I will introduce my research focused on the molecular design of small molecule and polymer acceptor materials aiming to provide comprehensions into highly efficient non-fullerene OSCs. Chapter II highlights the strategy of fluorination on end group moieties for high-performance polymer acceptors thus promoting efficiencies of all-PSCs. Three examples of my research work on all-PSCs are summarized, including: 1) fluorinated end group on polymer acceptors enable all-PSCs with red-shifted absorption; 2) regio-regular polymer acceptors enabled by determined fluorination on end groups for all PSCs; 3) a difluoro-monobromo end group enables superior polymer acceptors for high-performance all-PSCs. Systematic study among a series of polymer acceptors is made regarding optoelectronic properties, device performances, photophysics and morphology. Record efficiencies of up to 17% are achieved despite of low voltage losses of ~0.5 V for all PSCs. Chapter III concentrates on enhancing intramolecular charge transfer (ICT) effect to promote performances of Y-series SMAs via central molecule backbone modulation. One case is the introduction of selenium-incorporated heterocycles with stronger electron-donating ability and quinoid propertiy, enabling red-shifted absorption and stronger intermolecular packing relative to the orginal Y6 counterpart. The benzoselenathiazole substituted one exhibited stronger effect and better device performance than the selenothiophene-substituted one, resulting an absorption onset extended to ~950 nm and a efficienc over 16%. Chapter IV investigates the structure-performance relationship for a pair of isomeric perylene-diimide (PDI) SMAs with a [2.2]paracyclophane core. Systematic comparisons are processed regarding optoelectronic properties, device performances and morphology. The one with better transannularly conjugation in molecule geometry exhibited a better efficiency over 9%.

Date of Award	2021
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	He YAN (Supervisor)