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...[ Read more ]

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