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Photochemical Stability of Organic Solar Cells: The Role of Electron Acceptors / Emily M. Speller
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DOI (Published version): 10.23889/Suthesis.50959
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 ai...
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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 poly-mer: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 elec-tron 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 spec-troscopy (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 struc-tural/nanomorphological/electronic properties with material/device stability, thereby pav-ing the way toward achieving long-term environmental stability of low-cost organic solar cells with minimal encapsulation.
A selection of third party content is redacted or is partially redacted from this thesis.
Organic photovoltaics, third generation photovoltaics, photochemical stability, fullerene receptors, non fullerene receptors
College of Engineering