Morphological characterization of a new low-bandgap thermocleavable polymer showing stable photovoltaic properties

Abstract

The stability of the morphology of bulk heterojunction photovoltaic cells employing a novel low-bandgap polymer as donor is studied. The polymer is based on a poly(thieno[3,4-b]thiophene-benzo[1,2-b:4,5-b 0 ]dithiophene) (PTB) backbone with tetrahydropyranyl (THP) terminated side chains that are cleaved upon thermal treatment, which leads to stable performance of the photovoltaic properties during an accelerated aging process achieved by thermal annealing. The morphology of films made from blends of the polymer with PCBM is investigated before (PTB(THP):PCBM) and after (dPTB:PCBM) cleaving at micro and nanoscale using optical microscopy, transmission electron microscopy, and grazing incidence small angle X-ray scattering. Results are compared to films made from blends of poly-(3-hexylthiophene) (P3HT:PCBM) and the version of the PTB series whose structure is closest to that of PTB(THP), PTB4 (PTB4:PCBM). All three techniques demonstrate that phase separation is suppressed at micro and nanoscale in the dPTB:PCBM films, while large micron-sized PCBM aggregates develop during thermal annealing in P3HT:PCBM, PTB4:PCBM and PTB(THP):PCBM films. Our studies show that the removal of THP on the side chains can lead to stable morphology, and result in stable performance of photovoltaic cells. While we have focused on comparison to PTB4 blends, our results should be transferable to other polymers in the PTB series.

Morphological, Chemical, and Electronic Changes of the Conjugated Polymer PTB7 with Thermal Annealing

“…The double transition in TGA is consistent with the previous literature on side-chain-cleavable materials, in which the first TGA mass loss is attributed to side chain cleavage ( Guo et al., 2015 , Helgesen et al., 2011 , Vahdani et al., 2016 , Kuhn et al., 2015 ). Furthermore, the solubility of PTB7 powder dropped significantly after annealing at 290°C, with visible precipitates remaining in a 10-mg/mL solution in chlorobenzene after stirring for several hours at 60°C.…”

“…Because of this, there has been considerable interest in eliminating these side chains through thermal cleavage ( Sun et al., 2012 ). Thermo-cleavable side chains have been shown to have several advantages: increase in backbone planarity ( Guo et al., 2015 ), increase in efficiency in some devices, improvement in device stability ( Manceau et al., 2010 , Helgesen et al., 2011 ), suppression of PCBM aggregation in blends ( Vahdani et al., 2016 ), and decrease in solubility of the film, allowing multilayer processing ( Kuhn et al., 2015 ). In many cases, cleavage occurs at the ester group, leaving a carboxyl group ( Manceau et al., 2010 , Helgesen et al., 2011 ) or removing the ester group entirely ( Guo et al., 2015 , Kuhn et al., 2015 , Höfle et al., 2017 , Hillebrandt et al., 2016 ).…”

“…In many cases, cleavage occurs at the ester group, leaving a carboxyl group ( Manceau et al., 2010 , Helgesen et al., 2011 ) or removing the ester group entirely ( Guo et al., 2015 , Kuhn et al., 2015 , Höfle et al., 2017 , Hillebrandt et al., 2016 ). In polymers based on the poly(thieno[3,4-b]thiophene-benzo[1,2-b:4,5-b′]dithiophene) (PTB) backbone, cleavage occurs at the ether group, yielding a hydroxyl group ( Vahdani et al., 2016 ). Although the annealing temperatures used in this work are high, the literature has shown that the cleavage temperature can be reduced through inclusion of a catalyst, as in the case of Vahdani et al.…”

“…In polymers based on the poly(thieno[3,4-b]thiophene-benzo[1,2-b:4,5-b′]dithiophene) (PTB) backbone, cleavage occurs at the ether group, yielding a hydroxyl group ( Vahdani et al., 2016 ). Although the annealing temperatures used in this work are high, the literature has shown that the cleavage temperature can be reduced through inclusion of a catalyst, as in the case of Vahdani et al. (2016) , or through engineering of side chains ( Hillebrandt et al., 2016 ).…”

Synthesis and Characterization of Isoindigo‐Based Polymers with Thermocleavable Side Chains

“…A different method to stabilize polymer films makes use of cleavable moieties that solubilize the polymer until removed. The cleavable chains can be designed to cleave by several external stimuli, such as heat, UV‐light, acid, or base . Many of these make use of large side chains, which means that the cleaved off groups stay in the film.…”