Browsing by Author "Hu, Zhiqi"

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    An experimental and computational study of donor–linker–acceptor block copolymers for organic photovoltaics
    (Wiley, 2018) Hu, Zhiqi; Jakowski, Jacek; Zheng, Chenyu; Collison, Christopher J.; Strzalka, Joseph; Sumpter, Bobby G.; Verduzco, Rafael
    Block copolymers with donor and acceptor conjugated polymer blocks provide an approach to dictating the donor–accepter interfacial structure and understanding its relationship to charge separation and photovoltaic performance. We report the preparation of a series of donor‐linker‐acceptor block copolymers with poly(3‐hexylthiophene) (P3HT) donor blocks, poly((9,9‐dioctylfluorene)‐2,7‐diyl‐alt‐[4,7‐bis(thiophen‐5‐yl)‐2,1,3‐benzothiadiazole]‐2′,2″‐diyl) (PFTBT) acceptor blocks, and varying lengths of oligo‐ethylene glycol (OEG) chains as the linkers. Morphological analysis shows that the linkers increase polymer crystallinity while a combination of optical and photovoltaic measurements shows that the insertion of a flexible spacer reduces fluorescence quenching and photovoltaic efficiencies of solution processed photovoltaic devices. Density functional theory (DFT) simulations indicate that the linking groups reduce both charge separation and recombination rates, and block copolymers with flexible linkers will likely rotate to assume a nonplanar orientation, resulting in a significant loss of overlap at the donor–linker–acceptor interface. This work provides a systematic study of the role of linker length on the photovoltaic performance of donor–linker–acceptor block copolymers and indicates that linkers should be designed to control both the electronic properties and relative orientations of conjugated polymers at the interface. 
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    Molecular Engineering of Efficient and Robust Organic Photovoltaics
    (2019-11-22) Hu, Zhiqi; Verduzco, Rafael; Vargas, Francisco
    Organic photovoltaics (OPVs) are appealing alternatives to conventional silicon based solar cells due to their advantages of low-cost, light weight, high flexibility and solution processability. Those properties are of great interest to researchers and make OPV possible candidates as future source of clean energy. However, the low power conversion efficiency and poor mechanical stability prevent the realization of OPV devices for industrial applications. In this thesis, we will demonstrate methods to address the challenges mentioned above. Through investigation of light sensitive polymer materials and optimization of processing conditions, we are able to obtain a better understanding in how OPV performance is related to charge transfer at polymer interface and fabricate OPV devices with improved mechanical stability under stretching/bending induced deformation. Future work will focus on mechanical analysis and molecular simulation of stabilized OPV films. This will allow a better understanding in stability improvement of OPV film on a microscale level. Finally, I wish this thesis can provide insight to those who are interested in the fabrication of OPVs for real world applications.
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    Patterning, Transfer, and Tensile Testing of Covalent Organic Framework Films with Nanoscale Thickness
    (American Chemical Society, 2021) Zhu, Dongyang; Hu, Zhiqi; Rogers, Tanya K.; Barnes, Morgan; Tseng, Chia-Ping; Mei, Hao; Sassi, Lucas M.; Zhang, Zhuqing; Rahman, Muhammad M.; Ajayan, Pulickel M.; Verduzco, Rafael
    Covalent organic frameworks (COFs) are promising materials for a variety of applications, including membrane-based separations, thin-film electronics, and as separators for electrochemical devices. Robust mechanical properties are critical to these applications, but there are no reliable methods for patterning COFs or producing free-standing thin films for direct mechanical testing. Mechanical testing of COFs has only been performed on films supported by a rigid substrate. Here, we present a method for patterning, transferring, and measuring the tensile properties of free-floating nanoscale COF films. We synthesized COF powders by condensation of 1,3,5-tris(4-aminophenyl)benzene (TAPB) and terephthalaldehyde (PDA) and prepared uniform thin films by spin casting from a mixture of trifluoroacetic acid and water. The COF films were then reactivated to recover crystallinity and patterned by plasma etching through a poly(dimethylsiloxane) (PDMS) mask. The films were transferred to the surface of water, and we performed direct tensile tests. We measured a modulus of approximately 1.4 GPa for TAPB-PDA COF and a fracture strain of 2.5%, which is promising for many applications. This work advances the development of COFs for thin-film applications by demonstrating a simple and generally applicable approach to cast, pattern, and transfer COF thin films and to perform direct mechanical analysis.