THESIS
2019
2 unnumbered pages, xiv, 96 pages : illustrations ; 30 cm
Abstract
Organic solar cells were considered a promising alternative to the traditional fossil fuels due to
their unique advantages including environmental friendliness, low production cost, excellent
mechanical flexibility and compatibility with printing production. Although great progress has
been made in the past few years, the development of new materials was always based on a trial-and-error manner. To achieve high power conversion efficiency, several factors need to be
considered including the energy level alignment, the light absorption range, and the final
morphology in active layer. Therefore, a good understanding of the structure-property-performance
relationship is critical to guide the rational design of novel materials.
In Chapter 2, a hydrocarbon solvents-based processing sy...[
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Organic solar cells were considered a promising alternative to the traditional fossil fuels due to
their unique advantages including environmental friendliness, low production cost, excellent
mechanical flexibility and compatibility with printing production. Although great progress has
been made in the past few years, the development of new materials was always based on a trial-and-error manner. To achieve high power conversion efficiency, several factors need to be
considered including the energy level alignment, the light absorption range, and the final
morphology in active layer. Therefore, a good understanding of the structure-property-performance
relationship is critical to guide the rational design of novel materials.
In Chapter 2, a hydrocarbon solvents-based processing system was demonstrated and yielded
even better OSC morphology and performance than that obtainable with conventional
halogenated solvents. The record high efficiency was attributed to the synergetic effects of 4
factors. The temperature dependent aggregation property of polymers enables a good balance
between crystallinity and domain size. The hydrocarbon solvent reduces the domain size
compared to halogenated solvents. The high boiling point additive PN promotes the formation
of pure fullerene domains with moderate size. And finally, adjusting alkyl chains could help
fine tune the molecular packing orientation and domain size.
In Chapter 3, we first explored the effect of linker group in PDI-based polymer acceptors and
found that the thiophene linker is the best choice among the 4 competitors. Although PffBT4T-2DT has been proven to be an excellent donor for fullerene systems, it did not work well with
the polymer acceptors, revealing the importance of controlling morphology in all-polymer solar
cells. By alleviating the steric hindrance near the bay region of PDI, a novel non-fullerene
polymer acceptor PDI-V was synthesized and applied in BHJ organic solar cells. The all-polymer
solar cell devices based on PDI-V could exhibit a high PCE up to 7.57%. Furthermore,
the device processed in ambient air can still reach PCE over 7%, which is a very attractive
property for application to the industrial processing. To further reduce the conformational
disorder of the previous PDI-V acceptor, larger, shape-persistent naphthodiperylenediimide
(NDP) units were used to replace the single PDI unit. The solar cell characteristics suggest that
more favorable morphology is shown by the polymer blend of PTB7-Th:NDP-V, as evidenced
by both increased hole and electron mobilities, as well as an improved FF. Nice reproducibility
of the cell performance is demonstrated by a high average PCE of 8.48%, which is among the
highest values so reported for all-PSCs and a recorder for PDI-based polymers.
Chapter 4 concentrates on three pairs of PDI-based SMAs with and without ring fusion and
found that ring-fusion and domain purity are the key structural and morphological factors
determining the FFs and efficiencies of PDI-based non-fullerene OSCs. Non-fullerene OSCs
based on the ring-fused PDI-based SMAs exhibit much higher average domain purity and thus
increased charge mobilities, which lead to enhanced fill factors compared to those solar cells
based on non-fused PDI. This is explained by higher Flory Huggins interaction parameters as
observed by melt depression measurements. This study suggests that increasing repulsive
molecular interactions to lower the miscibility between the polymer donor and PDI acceptor is
the key to improve the FF and performance of PDI-based devices
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