Isoindigo Based Donor-Acceptor Conjugated Polymers: Synthesis, Properties and Solar Cell Application
Date Issued
2012
Date
2012
Author(s)
Huang, Tzu-Chia
Abstract
Isoindigo based conducting polymers have attracted extensive interests for polymer solar cell application due to the isoindigo is a renewable raw material and available from plant. We have synthesized a series of soluble low band gap isoindigo based polymers (PCI) with cyclopentadithiophene (CPDT) as weak donor unit and isoindigo (I) as strong acceptor unit, decorated with two kinds of side chains, octyl and 2-ethylhexyl, via the Stille cross-coupling reaction. The side chain effect of this conducting polymer on the performance of polymer solar cell has been quantitatively investigated. By changing the side chain of polymers from linear to branch, the λmax of absorption can be blue-shifted for more than 30 nm, the energy level and electronic band gap can be varied by more than 0.2 eV. In addition, the weight ratio with fullerene of optimized efficiency varies with the shape of side chains. Higher ratio (66 %) of fullerene could be observed from polymers with linear side chain on isoindigo unit whereas low ratio of fullerene (50%) was observed in polymers with branched side chains on isoindigo unit. The relationships of pre-exponential dark current term Jso, which accounts for the intermolecular interactions in the polymer/PCBM blends, to Voc and Jsc respectively, were quantitatively determined. The Voc is negatively proportional to Jso, whereas Jsc is positively proportional to Jso. The relationships were applied to the elaboration of the impact on photovoltaic performance done by side chain. The Jso could be improved when linear side chains are attached to polymers but weakened when branched side chains are attached. Therefore, the side chain design of polymer PCeI8, with branched side chains on CPDT units and linear side chains on isoindigo units, could optimize Jso to achieve the balance between Voc and Jsc and thus results in the highest power conversion efficiency (PCE) of 3.6 % by far. The power conversion of efficiency of this type polymer could be further improved by optimizing the fabrication condition.
Subjects
isoindigo
donor-acceptor conjugated polymer
side chain effect
low band gap
polymer solar cell
SDGs
Type
thesis
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