Electron spin is a key consideration for the function of organic semiconductors in light-emitting diodes and solar cells, as well as spintronic applications relying on organic magnetoresistance. A ...mechanism for triplet excited state generation in such systems is by recombination of electron-hole pairs. However, the exact charge recombination mechanism, whether geminate or nongeminate and whether it involves spin-state mixing is not well understood. In this work, the dynamics of free charge separation competing with recombination to polymer triplet states is studied in two closely related polymer-fullerene blends with differing polymer fluorination and photovoltaic performance. Using time-resolved laser spectroscopic techniques and quantum chemical calculations, we show that lower charge separation in the fluorinated system is associated with the formation of bound electron-hole pairs, which undergo spin-state mixing on the nanosecond timescale and subsequent geminate recombination to triplet excitons. We find that these bound electron-hole pairs can be dissociated by electric fields.
This paper is concerned with the photophysics of triplet excitons in conjugated donor polymers, and their quenching by molecular oxygen. These photophysics are assayed by transient absorption ...spectroscopy, and correlated with X‐ray diffraction measurements of relative material crystallinity. Eleven different donor polymers are considered, including representatives from several classes of donor polymers recently developed for organic solar cell applications. Triplet lifetimes in an inert (nitrogen) environment range from <100 ns to 5 μs. A remarkably quantitative correlation is observed between these triplet lifetimes and polymer XRD strength, with more crystalline polymers exhibiting shorter triplet lifetimes. Given the broad range of polymers considered, this correlation indicates that material crystallinity is the dominant factor determining triplet lifetime for the polymers studied herein. The rate constant for oxygen quenching of these triplet states, determined from a comparison of transient absorption data under inert and oxygen environments, also show a correlation with material crystallinity. Overall these dependencies result in the yield of oxygen quenching of polymer triplet states increasing strongly as the crystallinity of the polymer is reduced. These photophysical data are compared with photochemical stability of these donor polymers, assayed by photobleaching studies of polymer films under continuous light exposure in an oxygen environment. A partial correlation is observed, with the most stable polymers being the most crystalline, exhibiting negligible oxygen quenching yields. These results are discussed in terms of the likely origins of the correlations between material crystallinity and photophysics, and in terms of their implications for the environmental stability of such donor polymers in optoelectronic devices.
Triplet lifetime is found to strongly correlate with relative material crystallinity for a broad range of semiconducting polymers, with more amorphous/disordered polymer films exhibiting longer triplet lifetimes.
The solid‐state packing and polymer orientation relative to the substrate are key properties to control in order to achieve high charge carrier mobilities in organic field effect transistors (OFET). ...Intuitively, shorter side chains are expected to yield higher charge carrier mobilities because of a denser solid state packing motif and a higher ratio of charge transport moieties. However our findings suggest that the polymer chain orientation plays a crucial role in high‐performing diketopyrrolopyrrole‐based polymers. By synthesizing a series of DPP‐based polymers with different branched alkyl side chain lengths, it is shown that the polymer orientation depends on the branched alkyl chain lengths and that the highest carrier mobilities are obtained only if the polymer adopts a mixed face‐on/edge‐on orientation, which allows the formation of 3D carrier channels in an otherwise edge‐on‐oriented polymer chain network. Time‐of‐flight measurements performed on the various polymer films support this hypothesis by showing higher out‐of‐plane carrier mobilities for the partially face‐on‐oriented polymers. Additionally, a favorable morphology is mimicked by blending a face‐on polymer into an exclusively edge‐on oriented polymer, resulting in higher charge carrier mobilities and opening up a new avenue for the fabrication of high performing OFET devices.
The solid state packing and morphology of semiconducting polymers play a key role in achieving high charge carrier mobilities in field effect transistors. In this work, the creation of 3D transport pathways in organic semiconductors is found to be beneficial for charge transport, and can be artificially engineered by blending polymers with different solid state packing motifs.
Four new conjugated microporous polymers (CMPs) were synthesized by a Buchwald–Hartwig (BH) cross-coupling reaction of tri- and tetrafunctionalized precursors to yield materials with tunable surface ...area and pore size distribution. This approach yielded LPCMP1–4, CMPs with significantly higher Brunauer–Emmett–Teller (BET) surface areas (more than 5 times higher) than other related BH-based CMPs. These CMPs possess not only high BET specific surface areas and high chemical and thermal stabilities, but also exhibit outstanding swellability. To the best of our knowledge, swellable behavior was studied in great detail for CMPs for the first time, with the greatest degree of swelling for methanol reaching 16.5 and 16.3 mL g–1 for LPCMP1 and LPCMP3, respectively. Owing to their excellent swellability, we further studied the adsorption capacity of these CMPs for different toxic organic vapors (including toluene and methanol). LPCMP1 and LPCMP3 adsorbed 124 and 117 mg g–1 toluene, respectively, at saturated vapor pressure. For methanol, the adsorption capacities of LPCMP1 and LPCMP3 were up to 250 and 215 mg g–1, respectively, which are the highest recorded values when compared with published data for CMPs, HCPs, MOFs, and porous carbons. These materials are promising candidates for the removal and elimination of hazardous organic vapors and chemical warfare agents. Moreover, all the polymers show high sensitivity to nitroaromatic explosives. LPCMP2 and LPCMP4 exhibit high selectivity for TNT and may be suitable as new candidates to selectively detect TNT for security or environmental applications.
Cost‐effective, solution‐processable organic photovoltaics (OPV) present an interesting alternative to inorganic silicon‐based solar cells. However, one of the major remaining challenges of OPV ...devices is their lack of long‐term operational stability, especially at elevated temperatures. The synthesis of a fullerene dumbbell and its use as an additive in the active layer of a PCDTBT:PCBM‐based OPV device is reported. The addition of only 20 % of this novel fullerene not only leads to improved device efficiencies, but more importantly also to a dramatic increase in morphological stability under simulated operating conditions. Dynamic secondary ion mass spectrometry (DSIMS) and TEM are used, amongst other techniques, to elucidate the origins of the improved morphological stability.
Fullerene dumbbells: Long‐term operational stability of organic photovoltaics remains a challenge, especially at high temperatures. A novel fullerene dumbbell was synthesized and used as an additive in the active layer. The addition of the dumbbell not only leads to improved device efficiencies, but also to a dramatic increase in morphological stability (see picture) under simulated operating conditions.
Organometallic coordination polymers (OMCPs) are a promising class of thermoelectric materials with high electrical conductivities and thermal resistivities. The design criteria for these materials, ...however, remain elusive and so far material modifications have been focused primarily on the nature of the metal cation to tune the thermoelectric properties. Herein, an alternative approach is described by synthesizing new organic ligands for OMCPs, allowing modulation of the thermoelectric properties of the novel OMCP materials over several orders of magnitude, as well as controlling the polarity of the Seebeck coefficient. Extensive material purification combined with spectroscopy experiments and calculations furthermore reveal the charge‐neutral character of the polymer backbones. In the absence of counter‐cations, the OMCP backbones are composed of air‐stable, ligand‐centered radicals. The findings open up new synthetic possibilities for OMCPs by removing structural constraints and putting significant emphasis on the molecular structure of the organic ligands in OMCP materials to tune their thermoelectric properties.
The thermoelectric properties of novel organometallic coordination polymers are tuned via the development of novel organic ligands. Depending on the nature of the ligand, the new materials exhibit opposite sign Seebeck coefficients. Theoretical and spectroscopic analysis reveal the open‐shell nature of the ligands and confirm the existence of air‐stable, open‐shell biradicals in the polymer backbone.
The tight regulation of the glucose concentration in the body is crucial for balanced physiological function. We developed an electrochemical transistor comprising an n-type conjugated polymer film ...in contact with a catalytic enzyme for sensitive and selective glucose detection in bodily fluids. Despite the promise of these sensors, the property of the polymer that led to such high performance has remained unknown, with charge transport being the only characteristic under focus. Here, we studied the impact of the polymer chemical structure on film surface properties and enzyme adsorption behavior using a combination of physiochemical characterization methods and correlated our findings with the resulting sensor performance. We developed five n-type polymers bearing the same backbone with side chains differing in polarity and charge. We found that the nature of the side chains modulated the film surface properties, dictating the extent of interactions between the enzyme and the polymer film. Quartz crystal microbalance with dissipation monitoring studies showed that hydrophobic surfaces retained more enzymes in a densely packed arrangement, while hydrophilic surfaces captured fewer enzymes in a flattened conformation. X-ray photoelectron spectroscopy analysis of the surfaces revealed strong interactions of the enzyme with the glycolated side chains of the polymers, which improved for linear side chains compared to those for branched ones. We probed the alterations in the enzyme structure upon adsorption using circular dichroism, which suggested protein denaturation on hydrophobic surfaces. Our study concludes that a negatively charged, smooth, and hydrophilic film surface provides the best environment for enzyme adsorption with desired mass and conformation, maximizing the sensor performance. This knowledge will guide synthetic work aiming to establish close interactions between proteins and electronic materials, which is crucial for developing high-performance enzymatic metabolite biosensors and biocatalytic charge-conversion devices.
The heterocyclic thiazole unit has been extensively used as electron‐deficient building block in π‐conjugated materials over the last decade. Its incorporation into organic semiconducting materials ...is particularly interesting due to its structural resemblance to the more commonly used thiophene building block, thus allowing the optoelectronic properties of a material to be tuned without significantly perturbing its molecular structure. Here, we discuss the structural differences between thiazole‐ and thiophene‐based organic semiconductors, and the effects on the physical properties of the materials. An overview of thiazole‐based polymers is provided, which have emerged over the past decade for organic electronic applications and it is discussed how the incorporation of thiazole has affected the device performance of organic solar cells and organic field‐effect transistors. Finally, in conclusion, an outlook is presented on how thiazole‐based polymers can be incorporated into all‐electron deficient polymers in order to obtain high‐performance acceptor polymers for use in bulk‐heterojunction solar cells and as organic field‐effect transistors. Computational methods are used to discuss some newly designed acceptor building blocks that have the potential to be polymerized with a fused bithiazole moiety, hence propelling the advancement of air‐stable n‐type organic semiconductors.
Heterocyclic thiazole units can be incorporated into organic semiconducting materials, thus allowing the optoelectronic properties of a material to be tuned without significantly perturbing its molecular structure and affording high‐performance acceptor polymers for use in bulk‐heterojunction solar cells and as organic field‐effect transistors.
Silaindacenodithiophene is copolymerized with benzoc1,2,5thiadiazole (BT) and 4,7‐di(thiophen‐2‐yl)benzoc1,2,5thiadiazole (DTBT), respectively their fluorinated counter parts ...5,6‐difluorobenzoc1,2,5thiadiazole (2FBT) and 5,6‐difluoro‐4,7‐di(thiophen‐2‐yl) benzoc1,2,5thiadiazole (2FDTBT). The influence of the thienyl spacers and fluorine atoms on molecular packing and active layer morphology is investigated with regard to device performances. bulk heterojunction (BHJ) solar cells based on silaindacenodithiophene donor‐acceptor polymers achieved PCE's of 4.5% and hole mobilities of as high as 0.28 cm2/(V s) are achieved in an organic field‐effect transistor (OFET).
Fluorine is introduced into silaindacenodithiophene based semiconducting polymers with the aim to improve the photovoltaic as well as the charge carrier properties. The influence of the thienyl spacers and fluorine atoms on molecular packing and active layer morphology of the new polymers is investigated with regard to device performance.
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