Synthesis and Characterization of Acceptor Polymers for All-Polymer Solar Cells and Photodetectors
Sammanfattning: The development of polymer semiconductors has become an important topic due to its advantages of low cost, easy fabrication, light weight, and capability to fabricate flexible large-area devices. For example, as the need for new clean energy sources is increasing, polymer solar cells (PSCs) are being developed rapidly and becoming a promising alternative to silicon solar cells. This thesis focuses on the applications of polymer semiconductors in two active fields of polymeric optoelectronics: PSCs and polymer photodetectors (PPDs). Heretofore, PSCs and PPDs were fabricated commonly using a blend of a conjugated polymer and a fullerene derivative as the active layer. Despite the wide use of fullerene derivatives, their limitations such as low absorption, morphological instability, and high costs, created a strong need to develop new acceptor materials. Therefore, all-polymer solar cells (all-PSCs) and allpolymer photodetectors (all-PPDs) based on a blend of conjugated polymers acting as both electron donor and acceptor are being actively pursued. We have made concerted efforts to prepare high-performance all-PSCs and all-PPDs, by specifically modifying the acceptor molecular structure, and rationally choosing suitable donor and acceptor combinations. This aspect of our work had two main facets: ' Material synthesis: the design, synthesis and characterization of novel acceptor polymers. ' Device engineering: the fabrication, optimization and characterization of all-PSCs and all-PPDs. Our efforts in the design of novel acceptor polymers focused on crystallinity and energy level engineering via structural modifications like backbone and sidechain modulation. Also, a comprehensive comparison of the characteristic functional properties of acceptor polymers was undertaken. Binary devices using donor and acceptor polymers with complementary absorption or suitable energy level offset, and ternary devices were studied to further improve the performance of all- PSCs. High efficiencies of 8.0% and 9.0% are achieved for binary all-PSCs and ternary all-PSCs, respectively. Additionally, high-performance all-PPDs exhibiting low dark current density (Jd) and high responsivity (R) under -5 V bias were demonstrated. Based on the results presented herein, we are now moving closer to understanding the correlation between the polymer structure, blend morphology, and device performance. This thesis also provides a guideline for developing all-PSCs and all-PPDs with improved performance.
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