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Title: Photochemical stability of organic solar cells : the role of electron acceptors
Author: Speller, Emily M.
ISNI:       0000 0004 7967 4461
Awarding Body: Swansea University
Current Institution: Swansea University
Date of Award: 2018
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Environmental stability remains to be a critical barrier for the commercialisation (along with cost) of organic solar cells, and understanding the roles of material degradation is the key to address this challenge. The thesis investigates the photochemical stability (namely under illumination and air) of well established fullerene-based acceptors and some cutting edge non-fullerene acceptors, and their impact upon organic solar cell performance. Fullerene photo-oxidation was found to have a general and detrimental effect to polymer:fullerene organic cell performance for a range of device architectures, benchmark polymers and benchmark fullerenes. This photo-oxidation was found to correlate to the degree of aggregation, whereby the more aggregated the fullerene the more resistant it was to photo-oxidation. The photo-oxidation was found to substantially reduce electron mobility through the formation of trap states. In donor polymer:fullerene films the photochemical stability of both the polymer and fullerene were related to the degree of aggregation, and correlated with a decrease in device stability. Transient absorption spectroscopy (TAS) revealed fullerene photo-oxidation occurs primarily due to singlet oxygen generation via the fullerene triplet states for photo-inactive polymer:fullerene films and via the polymer triplet states for photo-active polymer:fullerene films. An energetic origin of acceptor photochemical stability was also determined. Generally, it was found that fullerenes with a lower lowest unoccupied molecular orbital (LUMO) underwent less photo-oxidation and device performance degraded less. For both fullerene and non-fullerene acceptors, the lower the LUMO level of the acceptor, the more stable the polymer:acceptor film. This relationship was shown to be strongly mediated by the yield of superoxide formation via the acceptor LUMO level. The work presented herein establishes relationships between electron acceptor structural/nanomorphological/electronic properties with material/device stability, thereby paving the way toward achieving long-term environmental stability of low-cost organic solar cells with minimal encapsulation.
Supervisor: Tsoi, Wing C. ; Durrant, James R. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral