Quantitative Transient Absorption Measurements of Polaron Yield and Absorption Coefficient in Neat Conjugated Polymers

Transient absorption methods are crucial for probing photogenerated polaron dynamics in conjugated polymers but are usually limited to qualitative studies because the polaron absorption coefficient is unknown. Herein, we quantify polaron absorption coefficients by exploiting the parasitic exciton–polaron quenching process, which appears in transient absorption experiments as a decrease in polaron yield at high fluence. We modulate the charge density in neat polymer films and measure the exciton–polaron quenching rate constant and dopant density via time-resolved photoluminescence. Using these parameters, we fit relative yield–fluence curves obtained from transient absorption, quantifying the yield and absorption coefficient of the polarons. We use time-resolved microwave conductivity as the transient probe and present results for the GHz mobility and polaron yield in films of three common conjugated polymers that are consistent with previous reports where they exist. These experiments demonstrate a new, generally accessible spectroscopic method for quantitative study of polaron dynamics in conjugated polymers

Resonance Energy Transfer Enables Efficient Planar Heterojunction Organic Solar Cells

Poor energy transport in disordered organic materials is one of the key problems that must be overcome to produce efficient organic solar cells. Usually, this is accomplished by blending the donor and acceptor molecules into a bulk heterojunction. In this article, we investigate an alternative approach to cell design: planar mulitilayer hetrojunctions with efficient energy transport to a central reaction center. We use an experimentally verified Monte Carlo model of energy transport to show that an appropriately engineered planar multilayer stack can achieve power conversion efficiencies comparable to those of the best bulk heterojunction devices. The key to this surprising performance is careful control of the optical properties and thicknesses of each layer to promote Förster resonance energy transfer from antenna/transport layers to a central reaction center. We provide detailed design rules for fabricating efficient planar heterojunction organic cells

Photoinduced Carrier Generation and Recombination Dynamics of a Trilayer Cascade Heterojunction Composed of Poly(3-hexylthiophene), Titanyl Phthalocyanine, and C₆₀

We use flash-photolysis time-resolved microwave conductivity experiments (<i>FP</i>-TRMC) and femtosecond–nanosecond pump–probe transient absorption spectroscopy to investigate photoinduced carrier generation and recombination dynamics of a trilayer cascade heterojunction composed of poly­(3-hexylthiophene) (P3HT), titanyl phthalocyanine (TiOPc), and fullerene (C₆₀). Carrier generation following selective photoexcitation of TiOPc is independently observed at both the P3HT/TiOPc and TiOPc/C₆₀ interfaces. The transient absorption results indicate that following initial charge generation processes to produce P3HT^•+/TiOPc^•– and TiOPc^•+/C₆₀^•– at each interface from (P3HT/TiOPc*/C₆₀), the final charge-separated product of (P3HT^•+/TiOPc/C₆₀^•–) is responsible for the long-lived photoconductance signals in <i>FP</i>-TRMC. At the P3HT/TiOPc interface in both P3HT/TiOPc and P3HT/TiOPc/C₆₀ samples, the electron transfer appears to occur only with the crystalline (weakly coupled H-aggregate) phase of the P3HT

Delocalization Drives Free Charge Generation in Conjugated Polymer Films

We demonstrate that the product of photoinduced electron transfer between a conjugated polymer host and a dilute molecular sensitizer is controlled by the structural state of the polymer. Ordered semicrystalline solids exhibit free charge generation, while disordered polymers in the melt phase do not. We use photoluminescence (PL) and time-resolved microwave conductivity (TRMC) measurements to sweep through polymer melt transitions in situ. Free charge generation measured by TRMC turns off upon melting, whereas PL quenching of the molecular sensitizers remains constant, implying unchanged electron transfer efficiency. The key difference is the intermolecular order of the polymer host in the solid state compared to the melt. We propose that this order–disorder transition modulates the localization length of the initial charge-transfer state, which controls the probability of free charge formation

Missing Excitons: How Energy Transfer Competes with Free Charge Generation in Dilute-Donor/Acceptor Systems

Energy transfer across the donor–acceptor interface in organic photovoltaics is usually beneficial to device performance, as it assists energy transport to the site of free charge generation. Here, we present a case where the opposite is true: dilute donor molecules in an acceptor host matrix exhibit ultrafast excitation energy transfer (EET) to the host, which suppresses the free charge yield. We observe an optimal photochemical driving force for free charge generation, as detected via time-resolved microwave conductivity (TRMC), but with a low yield when the sensitizer is excited. Meanwhile, transient absorption shows that transferred excitons efficiently produce charge-transfer states. This behavior is well described by a competition for the excited state between long-range electron transfer that produces free charge and EET that ultimately produces only localized charge-transfer states. It cannot be explained if the most localized CT states are the intermediate between excitons and the free charge in this system

Excited-State Electronic Properties in Zr-Based Metal–Organic Frameworks as a Function of a Topological Network

Molecular assemblies in metal–organic frameworks (MOFs) are reminiscent of natural light-harvesting (LH) systems and considered as emerging materials for energy conversion. Such applications require understanding the correlation between their excited-state properties and underlying topological net. Two chemically identical but topologically different tetraphenylpyrene (1,3,6,8-tetrakis­(<i>p</i>-benzoicacid)­pyrene; H₄TBAPy)-based Zr^IV MOFs, NU-901 (<b><i>scu</i></b>) and NU-1000 (<b><i>csq</i></b>), are chosen to computationally and spectroscopically interrogate the impact of topological difference on their excited-state electronic structures. Time-dependent density functional theory-computed transition density matrices for selected model compounds reveal that the optically relevant S₁, S₂, and S_<i>n</i> states are delocalized over more than four TBAPy linkers with a maximum exciton size of ∼1.7 nm (i.e., two neighboring TBAPy linkers). Computational data further suggests the evolution of polar excitons (hole and electron residing in two different linkers); their oscillator strengths vary with the extent of interchromophoric interaction depending on their topological network. Femtosecond transient absorption (fs-TA) spectroscopic data of NU-901 highlight instantaneous spectral evolution of an intense S₁ → S_<i>n</i> transition at 750 nm, which diminishes with the emergence of a broad (580–1100 nm) induced absorption originating from a fast excimer formation. Although these ultrafast spectroscopic data reveal the first direct spectral observation of fast excimer formation (τ = 2 ps) in MOFs, the fs-TA features seen in NU-901 are clearly absent in NU-1000 and the free H₄TBAPy linker. Furthermore, transient and steady-state fluorescence data collected as a function of solvent dielectrics reveal that the emissive states in both MOF samples are electronically nonpolar; however, low-lying polar excited states may get involved in the excited-state decay processes in polar solvents. The present work shows that the topological arrangement of the linkers critically controls the excited-state electronic structures

Chiral-Selective Protection of Single-walled Carbon Nanotube Photoluminescence by Surfactant Selection^†

We study the effects of adding H2O2 to acid-purified and unpurified single-walled carbon nanotubes (SWNTs) in aqueous suspensions using photoluminescence (PL) and optical absorption spectroscopies. The addition of H2O2 to suspensions of unpurified SWNTs results in a rapid (1−2 h) quenching of the photoluminescence from all tubes, whereas H2O2 addition to acid-purified SWNTs causes the nanotube PL to grow in intensity over a period of several days before decaying in a tube-specific manner that depends on the binding strength of the surfactant sheath. With the appropriate choice of surfactants, the PL for specific acid-purified SWNTs can be protected such that novel mid-gap and phonon-assisted absorption and emission transitions can be observed without the obscuring effects associated with emission from other nanotubes. The H2O2 treatment also results in a reduction of the high-energy absorption background that has been associated with either carbonaceous impurities or the SWNT π-plasmon oscillation. An understanding of the related mechanisms leads to a new method for separating nanotubes by type based on selective oxidation followed by selective precipitation. These findings offer the possibility of efficiently separating large quantities of nanotubes by chirality

Controlled Assembly of Hydrogenase-CdTe Nanocrystal Hybrids for Solar Hydrogen Production

We present a study of the self-assembly, charge-transfer kinetics, and catalytic properties of hybrid complexes of CdTe nanocrystals (nc-CdTe) and Clostridium acetobutylicum [FeFe]-hydrogenase I (H2ase). Molecular assembly of nc-CdTe and H2ase was mediated by electrostatic interactions and resulted in stable, enzymatically active complexes. The assembly kinetics was monitored by nc-CdTe photoluminescence (PL) spectroscopy and exhibited first-order Langmuir adsorption behavior. PL was also used to monitor the transfer of photogenerated electrons from nc-CdTe to H2ase. The extent to which the intramolecular electron transfer (ET) contributed to the relaxation of photoexcited nc-CdTe relative to the intrinsic radiative and nonradiative (heat dissipation and surface trapping) recombination pathways was shown by steady-state PL spectroscopy to be a function of the nc-CdTe/H2ase molar ratio. When the H2ase concentration was lower than the nc-CdTe concentration during assembly, the resulting contribution of ET to PL bleaching was enhanced, which resulted in maximal rates of H2 photoproduction. Photoproduction of H2 was also a function of the nc-CdTe PL quantum efficiency (PLQE), with higher-PLQE nanocrystals producing higher levels of H2, suggesting that photogenerated electrons are transferred to H2ase directly from core nanocrystal states rather than from surface-trap states. The duration of H2 photoproduction was limited by the stability of nc-CdTe under the reactions conditions. A first approach to optimization with ascorbic acid present as a sacrificial donor resulted in photon-to-H2 efficiencies of 9% under monochromatic light and 1.8% under AM 1.5 white light. In summary, nc-CdTe and H2ase spontaneously assemble into complexes that upon illumination transfer photogenerated electrons from core nc-CdTe states to H2ase, with low H2ase coverages promoting optimal orientations for intramolecular ET and solar H2 production

Confirmation of K-Momentum Dark Exciton Vibronic Sidebands Using ¹³C-labeled, Highly Enriched (6,5) Single-walled Carbon Nanotubes

A detailed knowledge of the manifold of both bright and dark excitons in single-walled carbon nanotubes (SWCNTs) is critical to understanding radiative and nonradiative recombination processes. Exciton–phonon coupling opens up additional absorption and emission channels, some of which may “brighten” the sidebands of optically forbidden (dark) excitonic transitions in optical spectra. In this report, we compare ¹²C and ¹³C-labeled SWCNTs that are highly enriched in the (6,5) species to identify both absorptive and emissive vibronic transitions. We find two vibronic sidebands near the bright ¹E₁₁ singlet exciton, one absorptive sideband ∼200 meV above, and one emissive sideband ∼140 meV below, the bright singlet exciton. Both sidebands demonstrate a ∼50 cm^–1 isotope-induced shift, which is commensurate with exciton–phonon coupling involving phonons of A₁^′ symmetry (D band, ω ∼ 1330 cm^–1). Independent analysis of each sideband indicates that both sidebands arise from the same dark exciton level, which lies at an energy approximately 25 meV above the bright singlet exciton. Our observations support the recent prediction of, and mounting experimental evidence for, the dark K-momentum singlet exciton lying ∼25 meV (for the (6,5) SWCNT) above the bright Γ-momentum singlet. This study represents the first use of ¹³C-labeled SWCNTs highly enriched in a single nanotube species to unequivocally confirm these sidebands as vibronic sidebands of the dark K-momentum singlet exciton

Chiral-Selective Protection of Single-walled Carbon Nanotube Photoluminescence by Surfactant Selection^†

We study the effects of adding H2O2 to acid-purified and unpurified single-walled carbon nanotubes (SWNTs) in aqueous suspensions using photoluminescence (PL) and optical absorption spectroscopies. The addition of H2O2 to suspensions of unpurified SWNTs results in a rapid (1−2 h) quenching of the photoluminescence from all tubes, whereas H2O2 addition to acid-purified SWNTs causes the nanotube PL to grow in intensity over a period of several days before decaying in a tube-specific manner that depends on the binding strength of the surfactant sheath. With the appropriate choice of surfactants, the PL for specific acid-purified SWNTs can be protected such that novel mid-gap and phonon-assisted absorption and emission transitions can be observed without the obscuring effects associated with emission from other nanotubes. The H2O2 treatment also results in a reduction of the high-energy absorption background that has been associated with either carbonaceous impurities or the SWNT π-plasmon oscillation. An understanding of the related mechanisms leads to a new method for separating nanotubes by type based on selective oxidation followed by selective precipitation. These findings offer the possibility of efficiently separating large quantities of nanotubes by chirality