Comparing blends and blocks: Synthesis of partially fluorinated diblock polythiophene copolymers to investigate the thermal stability of optical and morphological properties

Abstract

SummaryThe microstructure of the active blend layer has been shown to be a critically important factor in the performance of organic solar devices. Block copolymers provide a potentially interesting avenue for controlling this active layer microstructure in solar cell blends. Here we explore the impact of backbone fluorination in block copolymers of poly(3-octyl-4-fluorothiophene)s and poly(3-octylthiophene) (F-P3OT-b-P3OT). Two block co-polymers with varying block lengths were prepared via sequential monomer addition under Kumada catalyst transfer polymerisation (KCTP) conditions. We compare the behavior of the block copolymer to that of the corresponding homopolymer blends. In both types of system, we find the fluorinated segments tend to dominate the UV–visible absorption and molecular vibrational spectral features, as well as the thermal behavior. In the block copolymer case, non-fluorinated segments appear to slightly frustrate the aggregation of the more fluorinated block. However, in situ temperature dependent Raman spectroscopy shows that the intramolecular order is more thermally stable in the block copolymer than in the corresponding blend, suggesting that such materials may be interesting for enhanced thermal stability of organic photovoltaic active layers based on similar systems.

The Influence of Backbone Fluorination on the Dielectric Constant of Conjugated Polythiophenes

“…This is apparent when com paring the differences between the normalized absorbance spectra of P3OTbFP3OT with that obtained from a linear combination of the two constituent polymers, P3OT and FP3OT ( Figure S3, Supporting Information). In common to other diblock polymers, such as thiopheneselenophenebased systems, there is a reasonable match between the spectra and the linear combination of the two constituent polymers, [40,41] although similar to the 3:1 diblock [37] polymer it appears that the enhanced backbone planarity and elongated chain of the fluorinated block is partially inhibiting the ordering of the non fluorinated block in this 1:1 copolymer.…”

“…P3OTbFP3OT was also synthesized by GRIM polymerization, using the method we recently reported, with the more soluble P3OT block grown first from the activated monomer 8 fol lowed by the addition of activated monomer 9 to the P3OT mac roinitiator. [37] The polymer was purified by Soxhlet extraction as with P3OT. However, a further wash with dichloromethane was performed before extraction with chloroform in order to remove traces of P3OT homopolymer.…”

“…The presence of P3OT homopoly mer within P3OTbFP3OT is typically characterized by melt and crystallization peaks (at 198 and 155 °C, respectively) observed in the differential scanning calorimetry thermograms (see Figure S1, Supporting Information). [37] After purification, 1 H NMR analysis showed that the two blocks were of approxi mately equal length. 19 F NMR only afforded a single peak, again suggestive of very high degrees of backbone regioregularity.…”

Group 16 conjugated polymers based on furan, thiophene, selenophene, and tellurophene

“…Previous syntheses of BCP containing P3AT and P3AS blocks indicate that BCPs are successfully prepared when the more soluble polymer is polymerized first. 103,106,107 The polymerization of second blocks may sometimes also necessitate higher temperatures to maintain the solubility of growing BCPs, 108,109 especially when including P3ATe, and because magnesium salts by-products can partially inhibit Kumada CTP. 11 Expanding the idea of chain extension, triblock copolymers are synthesized through the repeated addition of activated monomers to polychalcogenophenes with living ends.…”

“…For example, the forced planarization/increased rigidity of one block in a poly(3-octylthiophene)-based (P3OT-based) BCP with a block of poly(3-octyl-4-fluorothiophene) (P3OFT) can stabilize P3OT domains better than analogous blends, suggesting that backbone fluorination may be a viable strategy in stabilizing BHJ active layers. 107 For P3AT- b -P3AT where blocks differ only in their side chain, a difference of more than 2 carbons can easily induce microphase separation into lamellae, as with P3BT- b -P3OT or P3HT- b -P3DDT. Otherwise, co-crystallization occurs readily between blocks.…”