Self-Assembled Framework Enhances Electronic Communication of Ultrasmall-Sized Nanoparticles for Exceptional Solar Hydrogen Evolution.

Colloidal quantum dots (QDs) have demonstrated great promise in artificial photosynthesis. However, the ultrasmall size hinders its controllable and effective interaction with cocatalysts. To improve the poor interparticle electronic communication between free QD and cocatalyst, we design here a self-assembled architecture of nanoparticles, QDs and Pt nanoparticles, simply jointed together by molecular polyacrylate to greatly enhance the rate and efficiency of interfacial electron transfer (ET). The enhanced interparticle electronic communication is confirmed by femtosecond transient absorption spectroscopy and X-ray transient absorption. Taking advantage of the enhanced interparticle ET with a time scale of ∼65 ps, 5.0 mL of assembled CdSe/CdS QDs/cocatalysts solution produces 94 ± 1.5 mL (4183 ± 67 µmol) of molecular H2 in 8 h, giving rise to an internal quantum yield of ∼65% in the first 30 min and a total turnover number of >1.64â€¯× 107 per Pt nanoparticle. This study demonstrates that self-assembly is a promising way to improve the sluggish kinetics of the interparticle ET process, which is the key step for advanced H2 photosynthesis.

Side-chain effects on the solution-phase conformations and charge photogeneration dynamics of low-bandgap copolymers.

Solution-phase conformations and charge photogeneration dynamics of a pair of low-bandgap copolymers based on benzo[1,2-b:4,5-b(')]dithiophene (BDT) and thieno[3,4-b]thiophene (TT), differed by the respective carbonyl (-C) and ester (-E) substituents at the TT units, were comparatively investigated by using near-infrared time-resolved absorption (TA) spectroscopy at 25 °C and 120 °C. Steady-state and TA spectroscopic results corroborated by quantum chemical analyses prove that both PBDTTT-C and PBDTTT-E in chlorobenzene solutions are self-aggregated; however, the former bears a relatively higher packing order. Specifically, PBDTTT-C aggregates with more π-π stacked domains, whereas PBDTTT-E does with more random coils interacting strongly at the chain intersections. At 25 °C, the copolymers exhibit comparable exciton lifetimes (~1 ns) and fluorescence quantum yields (~2%), but distinctly different charge photogeneration dynamics: PBDTTT-C on photoexcitation gives rise to a branching ratio of charge separated (CS) over charge transfer (CT) states more than 20% higher than PBDTTT-E does, correlating with their photovoltaic performance. Temperature and excitation-wavelength dependent exciton∕charge dynamics suggest that the CT states localize at the chain intersections that are survivable up to 120 °C, and that the excitons and the CS states inhabit the stretched strands and the also thermally robust orderly stacked domains. The stable self-aggregation structures and the associated primary charge dynamics of the PBDTTT copolymers in solutions are suggested to impact intimately on the morphologies and the charge photogeneration efficiency of the solid-state photoactive layers.

Subnanosecond charge recombination dynamics in P3HT/PC61BM films.

Ultrafast near-infrared absorption spectroscopy was used to investigate the influence of film morphology and excitation photon energy on the charge recombination (CR) dynamics in the initial nanosecond timescale in the P3HT/PC(61)BM blend films. With reference to the CS(2)-cast films, the solvent vapor annealed (SVA) ones show 2­3-fold improvement in hole mobility and more than 5-fold reduction in the polymer-localized trap states of holes. At Dt = 70 ps, the hole mobility (m(h)) and the bimolecular CR rate (γ(bi)) of the SVA films are µ(h) = 8.7 × 10(−4) cm2 × s(−1) × V(−1) and γ(bi) = 4.5 × 10(−10) cm3 × s(−1), whereas at Δt = 1 ns they drop to 8.7 × 10(−5) cm2 × s(−1) × V(−1) and 4.6 × 10(−11) cm3 × s(−1), respectively. In addition, upon increasing the hole concentration, the hole mobility increases substantially faster under the above-gap photoexcitation than it does under the band-gap photoexcitation, irrespective of the film morphologies. The results point to the importance of utilizing the photogenerated free charges in the early timescales.

A femtosecond transient absorption study of charge photogeneration and recombination dynamics in photovoltaic polymers with different side-chain linkages.

A pair of 9-arylidene-9H-fluorene and benzothiadiazole based, low-bandgap copolymers differing merely in the para or meta substitution of alkoxy groups to the arylidene linkages, i.e. p-PAFDTBT and m-PAFDTBT respectively, were comparatively investigated by using morphological characterization, ultrafast spectroscopy and quantum chemical calculations. Despite the subtle difference in the alkoxy substitution patterns, p-PAFDTBT molecules in photoactive films were shown to have a higher degree of crystallinity owing to the relatively less rotational torsion of the arylidene linkages. As a result, in either neat or fullerene-blended films, p-PAFDTBT compared to m-PAFDTBT gave rise to a substantially higher charge yield and much slower charge recombination. This work demonstrates that the alkoxy substitution pattern and the arylidene linkage are highly influencing on the morphology of the photoactive layers and thereby on the photovoltaic performance of the semiconducting copolymers.

Dependence of Excited-State Properties of a Low-Bandgap Photovoltaic Copolymer on Side-Chain Substitution and Solvent.

The excited-state properties and chain conformations of a new low-bandgap copolymer based on benzo[1,2-b:4,5-b']dithiophene (BDT) and thieno[3,4-b]thiophene with meta-alkoxyphenyl-substituted side chains in solution were investigated comprehensively. Time-resolved spectroscopy suggested that the excited-state properties were sensitive to the conformations of the copolymer in solution. In addition, excited-state dynamics analyses revealed the photogeneration of triplet excited states by intersystem crossing (ISC) at a rate constant of ∼0.4×10(9)  s(-1) as a result of direct meta-alkoxyphenyl connection to the donor unit BDT irrespective to the macromolecular conformations. According to El-Sayed's rule, the fast ISC herein is correlated with the change of orbital types between singlet and triplet excited states as also shown by quantum chemical calculations. Our studies may shed light on the structure-property relationships of photovoltaic materials.

Controlled Growth of Well-Defined Conjugated Polymers from the Surfaces of Multiwalled Carbon Nanotubes: Photoresponse Enhancement via Charge Separation.

The installation of heterojunctions on the surfaces of carbon nanotubes (CNTs) is an effective method for promoting the charge separation processes needed for CNT-based electronics and optoelectronics applications. Conjugated polymers are proven state-of-the-art candidates for modifying the surfaces of CNTs. However, all previous attempts to incorporate conjugated polymers to CNTs resulted in unordered interfaces. Herein we show that well-defined chains of regioregular poly(3-hexylthiophene) (P3HT) were successfully grown from the surfaces of multiwalled CNTs (MWNTs) using surface-initiated Kumada catalyst-transfer polycondensation. The polymerization was found to proceed in a controlled manner as chains of tunable lengths were prepared through variation of the initial monomer-to-initiator ratio. Moreover, it was determined that large-diameter MWNTs afforded highly ordered P3HT aggregates, which exhibited a markedly bathochromically shifted optical absorption due to a high grafting density induced planarization of the polymer chains. Using ultrafast spectroscopy, the heterojunctions formed between the MWNTs and P3HT were shown to effectively overcome the binding energy of excitons, leading to photoinduced electron transfer from P3HT to MWNTs. Finally, when used as prototype devices, the individual MWNT-g-P3HT core-shell structures exhibited excellent photoresponses under a low illumination density.

Thermal Adaptability of the Light-Harvesting Complex 2 from Thermochromatium tepidum: Temperature-Dependent Excitation Transfer Dynamics.

The photosynthetic purple bacterium Thermochromatium (Tch.) tepidum is a thermophile that grows at an optimal temperature of ∼50 °C. We have investigated, by means of steady-state and time-resolved optical spectroscopies, the effects of temperature on the near-infrared light absorption and the excitation energy transfer (EET) dynamics of its light-harvesting complex 2 (LH2), for which the mesophilic counterpart of Rhodobacter (Rba.) sphaeroides 2.4.1 (∼30 °C) was examined in comparison. In a limited range around the physiological temperature (10-55 °C), the B800-to-B850 EET process of the Tch. tepidum LH2, but not the Rba. sphaeroides LH2, was found to be characteristically temperature-dependent, mainly because of a temperature-tunable spectral overlap. At 55 °C, the LH2 complex from Tch. tepidum maintained efficient near-infrared light harvesting and B800-to-B850 EET dynamics, whereas this EET process was disrupted in the case of Rba. sphaeroides 2.4.1 owing to the structural distortion of the LH2 complex. Our results reveal a remarkable thermal adaptability of the light-harvesting function of Tch. tepidum, which could enhance our understanding of the survival strategy of this thermophile in response to environmental challenges.