The dependence of the thin film morphology and excited-state dynamics for the low-bandgap donor-acceptor copolymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) in pristine films and in blends (1:2) with [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) on the use of the solvent additive 1,8-octanedithiol (ODT) is studied by solid-state nuclear magnetic resonance (NMR) spectroscopy and broadband visible and near-infrared pump-probe transient absorption spectroscopy (TAS) covering a spectral range from 500-2000 nm. The latter allows monitoring of the dynamics of excitons, bound interfacial charge-transfer (CT) states, and free charge carriers over a time range from femto- to microseconds. The broadband pump-probe experiments reveal that excitons are not only generated in the polymer but also in PCBM-rich domains. Depending on the morphology controlled by the use of solvent additives, polymer excitons undergo mainly ultrafast dissociation (<100 fs) in blends prepared without ODT or diffusion-limited dissociation in samples prepared with ODT. Excitons generated in PCBM diffuse slowly to the interface in both samples and undergo dissociation on a time scale of several tens of picoseconds up to hundreds of picoseconds. In both samples a significant fraction of the excitons creates strongly bound interfacial CT states, which exhibit subnanosecond geminate recombination. The total internal quantum efficiency loss due to geminate recombination is estimated to be 50% in samples prepared without ODT and is found to be reduced to 30% with ODT, indicating that more free charges are generated in samples prepared with solvent additives. In samples prepared with ODT, the free charges exhibit clear intensity-dependent recombination dynamics, which can be modeled by Langevin-type recombination with a bimolecular recombination coefficient of 6.3 × 10(-11) cm(3) s(-1). In samples prepared without ODT, an additional nanosecond recombination of polaron pairs is observed in conjunction with an increased intensity-independent trap-assisted nongeminate recombination of charges. Furthermore, a comparison of the triplet-induced absorption spectra of PCPDTBT with the charge-induced absorption in PCPDTBT:PCBM blends reveals that triplets have a very similar excited-state absorption spectrum compared to the free charge carriers, however, in contrast have a distinct intensity-independent lifetime. Overall, our results suggest that whether free charges or strongly bound CT states are created upon dissociation of excitons at the PCPDTBT:PCBM interface is determined instantaneously upon exciton dissociation and that once formed strongly bound CT states rapidly recombine and thus are unlikely to dissociate into free charges. The observation of a significantly larger bimolecular recombination coefficient than previously determined for poly(3-hexylthiophen-2,5-diyl):PCBM (P3HT:PCBM) and PCDTBT:PCBM samples indicates that nongeminate recombination of free charges considerably ...
Porous polymers [1] with porosity at the nanoscale have attracted tremendous attention since their porous features are associated with prominent physical properties and since they have potential applications in, for example, light harvesting, [2] sensing, [3] gas separation [4] and storage, [5] catalysis, [6] and energy storage and conversion. [7] There are several classes of micro-/mesoporous polymers, such as hyper-crosslinked polymers (HCPs), polymers of intrinsic microporosity (PIMs), and covalent organic frameworks (COFs). Porous polymers can be also classified according to their structural conformations as amorphous-(HCPs and PIMs) or crystalline-type (COFs) materials. [1e] Conjugated microporous polymers (CMPs) represent one of the fastest developing types of porous materials not only because of their efficient synthesis by conventional metal-catalyzed polymerization techniques and the availability of a large number of commercially available functional monomers but also due to their controllable and adaptable physical properties. [1d] Unlike COFs, CMPs are formed under kinetic control, and thus are amorphous and show no longrange structural order. [8] For this reason, most of the previous work on CMPs has been focused on developing new chemical strategies and tuning the pore size distribution and surface area of these polymers by varying the length of the organic linkers rather than through morphology control. Very recently, efforts have been made to synthesize CMPs with controlled nanostructures, such as quasi-zero-dimensional microspheres, [9] and one-dimensional nanofibers [10] and nanotubes [11] as well as three-dimensional monoliths. [12] However, the synthesis of porous polymers with two-dimensional (2D) sheet structures remains little explored. Dichtel et al. employed a solvothermal method to grow oriented 2D COF thin films on substrate-supported graphene by the dynamic assembly of boronic acid and hexahydroxytriphenylene monomers, [13] but the large-scale production of free-standing 2D porous polymer networks has not yet been achieved.Graphene, because of its single-atom thickness, large aspect ratio, high surface area, and many intriguing physical properties, has proved to be a promising template for the highly efficient construction of 2D porous nanohybrid materials, such as 2D porous silica, [14] metal oxides, [15] metal sulfides, [16] carbon nitride, [17] and carbon-coated graphene/ metal oxide sheets. [18] All these approaches typically rely on the use of a graphene-based porous silica template in nanocasting technology or the nucleation of metal oxide or sulfide nanostructures on the graphene surface. Nevertheless, the porous structures of these graphene-based hybrid materials cannot be tailored at the molecular level, as has been done for organic porous materials. [1] We report herein on a graphene-inspired synthetic approach to the large-scale production of 2D sandwichlike conjugated microporous polymers in which each graphene sheet is fully separated by a porous polymer shell. Thiophene...
Dimensionality for conjugated micro-porous polymers (CMP-nD, n = 0, 1, 2) is proven to be of great importance for tailoring their photophysical properties. Moreover, CMP-nD can further be converted into boron and nitrogen co-doped porous carbons (nDBN, n = 0, 1, 2) with maintained 0D, 1D, and 2D nano-structures and highly efficient catalytic performance.
The solid-state morphology and photo-generated charge carrier dynamics in low-bandgap polymer:fullerene bulk heterojunction photovoltaic blends using the donor-acceptor type copolymers PCPDTBT or its siliconsubstituted analogue PSBTBT as donors are compared by two-dimensional (2D) solid-state nuclear magnetic resonance (NMR) and femto-to microsecond broadband Vis-NIR transient absorption (TA) pump-probe spectroscopy. The 2D solid-state NMR experiments demonstrate that the film morphology of PCPDTBT:PC 60 BM blends processed with additives such as octanedithiol (ODT) are similar to those of PSBTBT:PC 60 BM blends in terms of crystallinity, phase segregation, and interfacial contacts. The TA experiments and analysis of the TA data by multivariate curve resolution (MCR) reveal that after exciton dissociation and free charge formation, fast sub-nanosecond non-geminate recombination occurs which leads to a substantial population of the polymer's triplet state. The extent to which triplet states are formed depends on the initial concentration of free charges, which itself is controlled by the microstructure of the blend, especially in case of PCPDTBT:PC 60 BM. Interestingly, PSBTBT:PC 70 BM blends show a higher charge generation efficiency, but less triplet state formation at similar free charge carrier concentrations. This indicates that the solid-state morphology and interfacial structures of PSBTBT:PC 70 BM blends reduces non-geminate recombination, leading to superior device performance compared to optimized PCPDTBT:PC 60 BM blends. Broader contextThe growing demand of electrical power paired with the limited abundance of fossil energy sources steadily increases the importance of alternative and sustainable energy sources such as solar power. In this respect a very promising technology is organic photovoltaics, since large area devices can be produced at low costs. While in the first generation of organic photovoltaic materials the absorption was mostly limited to the visible part of the solar spectrum thereby significantly limiting the solar photon harvesting, the development of novel donor-acceptor type materials has recently pushed efficiencies towards 10%. However, many of the novel low-bandgap systems underperform as additional loss channels such as triplet state formation exist. Sophisticated transient spectroscopy studies paired with advanced data analysis methods can help to unravel these loss processes and to develop meaningful structure-property relations for a more guided material design. In fact, a careful control of interfacial and bulk morphology appears to be essential to suppress these undesired loss channels.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.