As effective building blocks for high-mobility transistor polymers, oligothiophenes are receiving attention for polymer solar cells (PSCs) because the resulting polymers can effectively suppress ...charge recombination. Here we investigate two series of in-chain donor–acceptor copolymers, PTPDnT and PBTInT, based on thieno3,4-cpyrrole-4,6-dione (TPD) or bithiopheneimide (BTI) as electron acceptor units, respectively, and oligothiophenes (nTs) as donor counits, for high-performance PSCs. Intramolecular S···O interaction leads to more planar TPD polymer backbones, however backbone torsion yields greater open-circuit voltages for BTI polymers. Thiophene addition progressively raises polymer HOMOs but marginally affects their band gaps. FT-Raman spectra indicate that PTPDnT and PBTInT conjugation lengths scale with nT catenation up to n = 3 and then saturate for longer oligomer. Furthermore, the effects of oligothiophene alkylation position are explored, revealing that the alkylation pattern greatly affects film morphology and PSC performance. The 3T with “outward” alkylation in PTPD3T and PBTI3T affords optimal π-conjugation, close stacking, long-range order, and high hole mobilities (0.1 cm2/(V s)). These characteristics contribute to the exceptional ∼80% fill factors for PTPD3T-based PSCs with PCE = 7.7%. The results demonstrate that 3T is the optimal donor unit among nTs (n = 1–4) for photovoltaic polymers. Grazing incidence wide-angle X-ray scattering, transmission electron microscopy, and time-resolved microwave conductivity measurements reveal that the terthiophene-based PTPD3T blend maintains high crystallinity with appreciable local mobility and long charge carrier lifetime. These results provide fundamental materials structure-device performance correlations and suggest guidelines for designing oligothiophene-based polymers with optimal thiophene catenation and appropriate alkylation pattern to maximize PSC performance.
The helical organization of oligo‐p‐phenylene‐based organogelators has been investigated by atomic force microscopy, circular and vibrational circular dichroism, and Raman techniques. Whilst OPPs ...with more than two phenyl rings in the core self‐assemble into left‐handed helices, that with a biphenyl core shows an inversion of the supramolecular helicity depending on the formation conditions through the atropisomerism of the biphenyl central unit. The results presented herein outline a new example of kinetically controlled modulation of supramolecular helicity.
Twist of fate: The helical organization of oligo‐p‐phenylene‐based organogelators has been investigated spectroscopically. Whilst OPPs 2 and 3 self‐assemble into left‐handed helices, an inversion of the supramolecular helicity of 1 occurs depending on the formation conditions because of the atropisomerism of the biphenyl central unit.
The combination of a bis‐alkynyl‐helicene moiety with two iron centers leads to novel electroactive species displaying unprecedented redox‐triggered chiroptical switching. Upon oxidation, strong ...changes of vibrational modes (either local or extended coupled modes) are detected by vibrational circular dichroism and Raman optical activity. Remarkably, the sign of the optical rotation at 1.54 µm (that is, at wavelengths typically used for telecommunications) changes upon oxidation while the topology and stereochemistry of the helicene remain unchanged.
Chiroptical switch: Iron alkynyl6helicene complexes have been synthesized. Vibrational circular dichroism and Raman optical activity spectra are used to investigate how the oxidation of the complexes to form radical cations influences their vibrational modes. The optical rotation of these chiral species (Φ1.54 µm23; measured in ° cm2 dmol−1) changes sign at wavelengths typically used for telecommunications (1.54 µm).
We report the process of singlet exciton fission with high-yield upon photoexcitation of a quinoidal thiophene molecule. Efficient ultrafast triplet photogeneration and its yield are determined by ...photoinduced triplet–triplet absorption, flash photolysis triplet lifetime measurements, as well as by femtosecond time-resolved transient absorption and fluorescence methods. These experiments show that optically excited quinoidal bithiophene molecule undergoes ultrafast formation of the triplet-like state with the lifetime ∼57 μs. CASPT2 and RAS-SF calculations have been performed to support the experimental findings. To date, high singlet fission rates have been reported for crystalline and polycrystalline materials, whereas for covalently linked dimers and small oligomers it was found to be relatively small. In this contribution, we show an unprecedented quantum yield of intramolecular singlet exciton fission of ∼180% for a quinoidal bithiophene system.
Conducting polymers can be synthesized by irreversible diradical monomer polymerization. A reversible version of this reaction consisting of the formation/dissociation of σ‐dimers and σ‐polymers from ...a stable quinonoidal diradical precursor is described. The reaction reversibility is made by a quinonoidal molecule which changes its structure to an aromatic species by forming weak and long intermolecular C−C single bonds. The reaction provokes a giant chromic effect of about 2.5 eV. The two opposite but complementary quinonoidal and aromatic tautomers provide the Janus faces of the reactants and products which produces the observed chromic effect. A reaction mechanism is proposed to explain the variety of final products starting with structurally very similar reactants. These reversible reactions, covering an unusual regime of weak covalent supramolecular bonding, yield products which might be envisaged as novel molecular and polymeric soft matter phases.
Long and weak C−C bonds are formed from a quinonoidal diradical molecule and lead to either double σ‐bonded cyclophane dimers or σ‐bonded polymers. The highly strained character of the C−C bonds makes these reactions reversible and the quinonoidal–aromatic transformation results in a large change in the optical properties and thus large chromic effects.
Rational creation of polymeric semiconductors from novel building blocks is critical to polymer solar cell (PSC) development. We report a new series of bithiopheneimide-based donor–acceptor ...copolymers for bulk-heterojunction (BHJ) PSCs. The bithiopheneimide electron-deficiency compresses polymer bandgaps and lowers the HOMOsessential to maximize power conversion efficiency (PCE). While the dithiophene bridge progression R2Si→R2Ge minimally impacts bandgaps, it substantially alters the HOMO energies. Furthermore, imide N-substituent variation has negligible impact on polymer opto-electrical properties, but greatly affects solubility and microstructure. Grazing incidence wide-angle X-ray scattering (GIWAXS) indicates that branched N-alkyl substituents increased polymer π–π spacings vs linear N-alkyl substituents, and the dithienosilole-based PBTISi series exhibits more ordered packing than the dithienogermole-based PBTIGe analogues. Further insights into structure–property–device performance correlations are provided by a thieno3,4-cpyrrole-4,6-dione (TPD)–dithienosilole copolymer PTPDSi. DFT computation and optical spectroscopy show that the TPD-based polymers achieve greater subunit–subunit coplanarity via intramolecular (thienyl)S···O(carbonyl) interactions, and GIWAXS indicates that PBTISi-C8 has lower lamellar ordering, but closer π–π spacing than does the TPD-based analogue. Inverted BHJ solar cells using bithiopheneimide-based polymer as donor and PC71BM as acceptor exhibit promising device performance with PCEs up to 6.41% and V oc > 0.80 V. In analogous cells, the TPD analogue exhibits 0.08 V higher V oc with an enhanced PCE of 6.83%, mainly attributable to the lower-lying HOMO induced by the higher imide group density. These results demonstrate the potential of BTI-based polymers for high-performance solar cells, and provide generalizable insights into structure–property relationships in TPD, BTI, and related polymer semiconductors.
Recent advances in polymer solar cell (PSC) performance have resulted from compressing the bandgap to enhance the short-circuit current while lowering the highest occupied molecular orbital to ...increase the open-circuit voltage. Nevertheless, PSC power conversion efficiencies are still constrained by low fill factors, typically below 70%. Here, we report PSCs with exceptionally high fill factors by combining complementary materials design, synthesis, processing and device engineering strategies. The donor polymers, PTPD3T and PBTI3T, when incorporated into inverted bulk-heterojunction PSCs with a PC71 BM acceptor, result in PSCs with fill factors of 76-80%. The enhanced performance is attributed to highly ordered, closely packed and properly oriented active-layer microstructures with optimal horizontal phase separation and vertical phase gradation. The result is efficient charge extraction and suppressed bulk and interfacial bimolecular recombination. The high fill factors yield power conversion efficiencies of up to 8.7% from polymers with suboptimal bandgaps, suggesting that efficiencies above 10% should be realizable by bandgap modification.
Evidence for the surprising formation of polymeric phases under high pressure for conjugated nanohoop molecules was found. This paper represents one of the unique cases, in which the molecular‐level ...effects of pressure in crystalline organic solids is addressed, and provides a general approach based on vibrational Raman spectroscopy combining experiments and computations. In particular, we studied the structural and supramolecular chemistry of the cyclic conjugated nanohoop molecule 5cyclo‐para‐phenylene (5CPP) under high pressures up to 10 GPa experimentally and up to 20 GPa computationally. The theoretical modeling for periodic crystals predicts good agreements with the experimentally obtained Raman spectra in the molecular phase. In addition, we have discovered two stable polymeric phases that arise in the simulation. The critical pressures in the simulation are too high, but the formation of polymeric phases at high pressures provides a natural explanation for the observed irreversibility of the Raman spectra upon pressure release between 6 and 7 GPa. The geometric parameters show a deformation toward quinonoid structures at high pressures accompanied by other deformations of the 5CPP nanohoops. The quinonoidization of the benzene rings is linked to the systematic change of the bond length alternation as a function of the pressure, providing a qualitative interpretation of the observed spectral shifts of the molecular phase.
Hula hoop: The behavior of 5cyclo‐para‐phenylene (5CPP) under high pressures up to 10 GPa experimentally and up to 20 GPa computationally was investigated. A good accordance of both results was found. Additionally, we found a polymerization of the compound under high pressure, which explains the impossibility to recover the Raman spectra after a pressure release (see scheme).
We present a systematic study of the morphology and absorption properties of a typical donor–acceptor polymer (PCPDTBT) with semicrystalline behavior in solution and in thin films. In-situ ...spectroelectrochemical data give information about the evolution of the absorption spectra from neutral to charged species. The experimental data are supported by theoretical calculations in the framework of the density functional theory (DFT). Regarding thin film structures, we show that the choice of the solvent has significant influence on the morphology in thin films: whereas CS2 and CHCl3 give rather structureless (amorphous) morphologies, films from 1-CN exhibit a clear crystalline nanofiber morphology. Accompanying UV/vis/NIR spectra of films are highly dependent on the morphology and therefore on the choice of the processing solvent. The absorption of fiber morphologies is strongly red-shifted compared to the structureless films.
Long CC bonds: Analysis of the 1064 nm Raman vibrational spectrum of K2TCNE2 possessing isolated π‐TCNE22− (TCNE=tetracyanoethylene) dimers shows several low‐energy symmetric intradimer breathing ...modes at 198, 173, 155, 131, 107, and 85 cm−1. These data confirm the presence of a long two‐electron/four‐center CC bond (see picture).
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