Impacts of side chain and excess energy on the charge photogeneration dynamics of low-bandgap copolymer-fullerene blends

Abstract

Primary charge photogeneration dynamics in neat and fullerene-blended films of a pair of alternating benzo[1,2-b:4,5-b(')]dithiophene (BDT) and thieno[3,4-b]thiophene (TT) copolymers are comparatively studied by using near-infrared, time-resolved absorption (TA) spectroscopy under low excitation photon fluence. PBDTTT-E and PBDTTT-C, differed merely in the respective TT-substituents of ester (-E) and carbonyl (-C), show distinctly different charge photogeneration dynamics. The pair of neat PBDTTT films show exciton lifetimes of ∼0.1 ns and fluorescence quantum yields below 0.2%, as well as prominent excess-energy enhanced exciton dissociation. In addition, PBDTTT-C gives rise to >50% higher P(•+) yield than PBDTTT-E does irrespective to the excitation photon energy. Both PBDTTT-E:PC61BM and PBDTTT-C:PC61BM blends show subpicosecond exciton lifetimes and nearly unitary fluorescence quenching efficiency and, with respect to the former blend, the latter one shows substantially higher branching ratio of charge separated (CS) state over interfacial charge transfer (ICT) state, and hence more efficient exciton-to-CS conversion. For PBDTTT-C:PC61BM, the ultrafast charge dynamics clearly show the processes of ICT-CS interconversion and P(•+) migration, which are possibly influenced by the ICT excess energy. However, such processes are relatively indistinctive in the case of PBDTTT-E:PC61BM. The results strongly prove the importance of ICT dissociation in yielding free charges, and are discussed in terms of the film morphology and the precursory solution-phase macromolecular conformation.

Charge Photogeneration and Recombination in Fluorine-Substituted Polymer Solar Cells

“…As for the neat PBDTTT-E layer, it exhibits a main absorption band in the wavelength range of 500–800 nm with two absorption peaks at 680 and 626 nm. The former absorption peak is often referred to as the vibronic progression of the electronic state (0′-0), whereas the latter peak is considered as the (1′-0) vibronic absorption of electronic transition, and they both are in relation to the aggregation state of co-polymers ( Clark et al, 2009 ; Hu et al, 2014 ; Huo et al, 2014 ; Fauvell et al, 2016 ). Herein, the relative absorption intensity of the 626 nm peak of the neat PTB7 film ( A 1´-0 / A 0´-0 ) is slightly enhanced comparing with that of the PBDTTT-E film, suggesting that the fluorine substituent can influence the co-planar conformation and optical absorption of the polymer ( Leclerc et al, 1993 ; Huo et al, 2013 ).…”

Enhancement of photovoltaic performance by two-step dissolution processed photoactive blend in polymer solar cells

“…After single dissolution, the polymer and fullerene are evenly distributed in the solvent. Only some polymers, however, are unfolded and kinked polymer chains remain in solution, which has been observed in PBDTTT systems [28,29]. The unfolded polymer configuration will remain in polymer:fullerene films after solvent volatilization.…”

“…The unfolded polymer configuration will remain in polymer:fullerene films after solvent volatilization. Moreover, it has been found that the ratio of kinked conformation depends on the solvents [28,29]. When the polymer:fullerene film is redissolved in another solvent, the configuration of original kinked polymers can be further extended to the unfolded mode by solvent-induced regulation.…”

“…Recently, we found that polymers undergo self-aggregation even in the solution-phase, which strongly depends on the solvents. Such a self-aggregated conformation of polymers can inhabit the photo-induced charge generation process in PSCs [28][29][30]. Therefore, regulating such self-aggregation by adjusting the dissolution processes of polymers is expected to enhance the device performance of PSCs.…”

Excitation Energy Transfer Dynamics in a Low-Band-Gap Copolymer: Two-Dimensional Electronic Spectroscopy of PTB7 in Solution

“…Thin-film organic solar cells have attracted a lot of interests because organic cells are expected as replacements for silicon solar cells and inorganic electronic materials owing to higher processability and lower-cost devices. − Conjugated polymers have extensively been used in thin-film organic solar cells, − and they act as electron donors and hole transporters in bulk heterojunction (BHJ) films. Homopolymers, poly­(3-hexylthiophene) (P3HT) − and poly­[2-methoxy-5-(2′-ethylhexyloxy)- p -phenylene vinylene] (MEH-PPV), , have rigorously been studied as electron donors in solar cells.…”