Syntheses, Morphology, and Optoelectronic Device Applications of Thiophene and Imide based Donor-Acceptor Polymer Systems
Date Issued
2012
Date
2012
Author(s)
Lai, Yi-Cang
Abstract
Donor-acceptor (D-A) polymers have attracted significant scientific interest recently because their electronic and optoelectronic properties can be manipulated through intramolecular charge transfer (ICT). However, the correlation between the chemical structure, morphology, and optoelectronic properties has not been fully explored yet. In this thesis, we address the above issue by exploring the following subjects:
I. Morphology control of poly(3-hexylthiophene)/PCBM blends for enhancing the
solar cell characteristics using thiophene-based surfactant:
We used the triblock copolymer PTPA-P3HT-PTPA, diblock copolymer P3HT-b-P3PyT, small molecules PCBTE, PCBBTE, and PCBTTE as surfactants to control the morphology of PCBM in P3HT/PCBM blend and enhance the long-term stability of solar cell devices. The optimized PCE of the PTPA-P3HT-PTPA, P3HT-b-P3PyT, PCBTE, PCBDTE, and PCBTTE blended system reached up to 4.4%, 3.95%, 3.86%, 4.08%, and 4.37% under illumination of AM 1.5G (100mW/cm2). DSC, TEM, optical microscopy, and AFM were used to confirm these surfactants could reduce the interfacial energy to prevent domain coarsening and macrophase separation in an active layer depending on their contents. The increased PCE, combined with good air and thermal stability of solar cell devices by using the surfactants, indicates their promising potential for polymer solar cells. This best performance (stability and PCE value) among solar cell devices is using PCBTTE as surfactants blended with P3HT/PCBM, which is due to PCBTTE is an effective interfacial agent for dispersing and fixing the P3HT and PCBM domains in thin films and the minimal insulating linker between the electrically active moieties.
II. Design and synthesis of new D-A polymeric systems for nonvolatile memory device applications.
We have successfully designed and synthesized a series of conjugated polymers P3HT-b-P3PT and random copolyimides PI-NTCDIX, and used them for resist-type memories. For the P3HT-b-P3PT, the charge trapping of the P3HT-b-P3PT based memory devices may be occurred within the amorphous P3PT domains dispersed in the block copolythiophene by preventing charge transport. P3HT52-b-P3PT39 and P3HT102-b-P3PT37 exhibited the DRAM behaviours, suggesting that the significant effect of the amorphous P3PT segments on the electrical switching behavior. By blending a small amount of PCBM into P3HT-b-P3PT, the memory devices showed a WORM behavior. The mechanism associated with the memory characteristics was the charge transfer from the P3HT-b-P3PT donor to the PCBM acceptor, which stabilized the charge separated state for a long time during the ON stage. For the PI-NTCDIX (where X=1, 2.5, 5 and 10 for NTCDI molar composition), varying the feed ratio of NTCDI in random copolyimides, the memory devices exhibited the tunable electrical bistability from the volatile dynamic random access memory (DRAM) to nonvolatile write once read many (WORM) memory characteristics as the NTCDI composition increased. The OFF/ON electrical switching transition was mainly attributed to the charge transfer (CT) mechanism for charge separated high conductance. Also, the volatility of PI-NTCDIX device depended on the stability of CT complex. The long conjugation and high electron affinity of the NTCDI moiety stabilize the radical anion generated in the CT complex and prevented recombination of segregated radical species even through applying the high negative and positive voltage. The stability of the charge transfer between the donor and acceptor moieties could control the volatility of the memory devices.
Subjects
solar cell
memory
morphology
polymer
compatibilizer
Type
thesis
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