THESIS
2016
xi, 133 pages : illustrations ; 30 cm
Abstract
In the past two decades, polymer solar cell (PSC) field has seen tremendous
development and the certified power conversion efficiency (PCE) has quickly
increased from around 1% to 11.5%. Before the development of PffBT4T-2OD, PSC
field was dominated by two types of polymers: P3HT and PTB7, and the best
performing PSC devices are based on PTB7-Th. Due to the sensitivity of PTB7-based
polymers, the electron acceptor of PSCs is constraint to one specific fullerene
derivative: PC
71BM. Also, because of the regio-irregularity and poor crystallinity of
PTB7, the thickness of the active layer is limited to less than 100 nm in order to
maintain high FFs, and systematic studies on the control of donor:acceptor blend
morphology have rarely been reported. A brief history of organic solar c...[
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In the past two decades, polymer solar cell (PSC) field has seen tremendous
development and the certified power conversion efficiency (PCE) has quickly
increased from around 1% to 11.5%. Before the development of PffBT4T-2OD, PSC
field was dominated by two types of polymers: P3HT and PTB7, and the best
performing PSC devices are based on PTB7-Th. Due to the sensitivity of PTB7-based
polymers, the electron acceptor of PSCs is constraint to one specific fullerene
derivative: PC
71BM. Also, because of the regio-irregularity and poor crystallinity of
PTB7, the thickness of the active layer is limited to less than 100 nm in order to
maintain high FFs, and systematic studies on the control of donor:acceptor blend
morphology have rarely been reported. A brief history of organic solar cells has been
presented in Chapter I.
In the following chapters, a series of semi-crystalline polymers and dozens of
fullerenes and non-fullerene acceptors have been described.
In Chapter II, a family of polythiophene (PffBT4T-2OD, PBTff4T-2OD and
PNT4T-2OD) based low-bandgap polymers are introduced. The semi-crystalline
nature of these polymers endowed them with high charge carrier mobility, and thus
thicker active layers in PSCs become feasible. The key 4T-2OD moiety enables them
gradual temperature-dependent aggregation behavior that allows the processing of the
polymer solutions at elevated temperature and controlled aggregation of the polymer
during cooling and drying. A near-ideal polymer:fullerene morphology could be
efficiently controlled by polymer aggregation during pre-heating and spin casting. As
a result, the donor:acceptor blend morphology is insensitive to the choice of fullerenes.
The morphology control by temperature-dependent aggregation behavior allows the
PSC community to explore more polymer:fullerene combinations to achieve higher
PCEs. The related paper published on Nature Communications has been cited over
900 times upon finishing of this Ph.D. thesis since published in the year 2014.
In Chapter III-V, a family of 3D structure PDI-based low-cost non-fullerene
acceptors (TPE-PDI4, TPC-PDI4, TPSi-PDI4, TPGe-PDI4) have been described.
Since fullerenes are produced by costly procedures and are extremely hard to purify,
research interest has been focused on low-cost non-fullerene substitutions. Like ball-shaped
fullerenes, 3D-structure non-fullerene acceptors own the advantage that they
can form readily 3D charge transporting network. More importantly, the introduction
of a tetraphenylethylene core to the 3D non-fullerene acceptor could efficiently reduce
the aggregation tendency of the acceptors yet maintain sufficient electron mobility.
These Chapters provides an attractive strategy to design novel efficient 3D structure
non-fullerene acceptors by using inexpensive industrial dyes.
In chapter VI, an IDT-based large bandgap non-fullerene acceptor (IDTT-4CN)
is described. Since the introduction of IDT into non-fullerene acceptors, the PCE of
IDT-based PSCs has quickly become comparable to that of fullerene-based devices.
Currently all reported high efficiency IDT-based non-fullerene acceptors are low
bandgap materials, which could suffer spectrum overlap with state-of-art high
performing low-bandgap polymers. IDTT-4CN provides a facile approach to design
IDT-based large bandgap non-fullerene acceptors, and could possibly find their
applications in future higher efficiency tandem organic solar cells.
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