Resolving the Molecular Origin of Mechanical Relaxations in Donor–Acceptor Polymer Semiconductors

Abstract

The thermomechanical behavior of polymer semiconductors plays an important role in the processing, morphology, and stability of organic electronic devices. However, donor-acceptor-based copolymers exhibit complex thermal relaxation behavior that is not well understood. This study uses dynamic mechanical analysis (DMA) to probe thermal relaxations of a systematic set of polymers based around the benzodithiophene (BDT) moiety. The loss tangent curves are resolved by fitting Gaussian functions to assign and distinguish different relaxations. Three prominent transitions are observed that correspond to: i) localized relaxations driven primarily by the side chains (γ ), ii) relaxations along the polymer backbone (β ), and iii) relaxations associated with aggregates (α ). The side chains are found to play a clear role in dictating T γ , and that mixing the side chain chemistry of the monomer to include alkyl and oligo(ethylene glycol) moieties results in splitting the γ -relaxation. The β relaxations are shown to be associated with backbone elements along with the monomer. In addition, through processing, it is shown that the α-relaxation is due to aggregate formation. Finally, it is demonstrated that the thermal relaxation behavior correlates well with the stress-strain behavior of the polymers, including hysteresis and permanent set in cyclically stretched films.

Achieving High-Efficiency Organic Photovoltaics from a New Completely Amorphous Donor Polymer

“…In order to compare PIDT-TXBT with the reported high-performing donor polymers, we also performed DMTA measurement on PM6 57 and J71 58 (DMA plots shown in the Supporting Information, Figure S18). However, these two high-performing donor polymers only show sub-T g transitions and no detectable T g , 59 which we believe is a result of the "hairy aggregates" phase described in our previous work. 40 In short, these "hairy aggregates", which are neither amorphous nor crystalline, are revealed from some polymers that only show sub-T g transitions with no detectable glass transition.…”

“…Consequently, an amorphous material is defined as a material with a clear T g while lacking the order present in aggregated and crystalline structures. These definitions will be used throughout the paper to describe the materials; however, we acknowledge that different interpretations for amorphous, aggregated, and crystalline structures exist in the literature. − According to IUPAC Recommendations 2013, an amorphous polymer is defined to be generally isotropic at a large scale and may have some degree of local order about 1 nm in length . The amorphous-phase behavior of PIDT-TXBT brings up a question: can completely amorphous donor polymers be used to make high-performing photovoltaics?…”

Mechanically robust all-polymer solar cells enabled by polymerized small molecule acceptors featuring flexible siloxane-spacers

“…This testing method enables us to obtain the intrinsic tensile properties of thin lms, excluding effects from thick substrates. [70][71][72] All blend lms for the tensile testing were prepared under the same conditions as those of device fabrication. The PYSiO-0 blend showed COS and toughness values of 9.6% and 3.2 MJ m À3 , respectively.…”

Sequentially regular polymer acceptors featuring flexible spacers for high-performance and mechanically robust all-polymer solar cells

“…All-polymer solar cells (all-PSCs), consisting of polymer donors (P D s) and polymer acceptors (P A s), are considered the most promising candidates for constructing highly durable IS-PSCs because they have superior mechanical robustness compared with their SMA-based counterparts and other inorganic material-based solar cells. [14][15][16][17][18][19][20][21] The recent development of various polymerized small-molecule acceptors (PSMAs) is particularly encouraging as all-PSCs with PCE values of over 17-18% are reported, owing to their superior light absorption and fast charge transport. [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38] Despite the significant improvement in device efficiency, the blend films containing PSMAs are still mechanically fragile, showing limited crack onset strain (COS) values (o10%).…”