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

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

Controlling Long-Lived Triplet Generation from Intramolecular Singlet Fission in the Solid State

The conjugated polymer poly­(benzothiophene dioxide) (PBTDO1) has recently been shown to exhibit efficient intramolecular singlet fission in solution. We investigate the role of intermolecular interactions in triplet separation dynamics after singlet fission. We use transient absorption spectroscopy to determine the singlet fission rate and triplet yield in two polymers differing only by side-chain motif in both solution and the solid state. Whereas solid-state films show singlet fission rates identical to those measured in solution, the average lifetime of the triplet population increases dramatically and is strongly dependent on side-chain identity. These results show that it may be necessary to carefully engineer the solid-state microstructure of these “singlet fission polymers” to produce the long-lived triplets needed to realize efficient photovoltaic devices

Charge Separation in P3HT:SWCNT Blends Studied by EPR: Spin Signature of the Photoinduced Charged State in SWCNT

Single-wall carbon nanotubes (SWCNTs) could be employed in organic photovoltaic (OPV) devices as a replacement or additive for currently used fullerene derivatives, but significant research remains to explain fundamental aspects of charge generation. Electron paramagnetic resonance (EPR) spectroscopy, which is sensitive only to unpaired electrons, was applied to explore charge separation in P3HT:SWCNT blends. The EPR signal of the P3HT positive polaron increases as the concentration of SWCNT acceptors in a photoexcited P3HT:SWCNT blend is increased, demonstrating long-lived charge separation induced by electron transfer from P3HT to SWCNTs. An EPR signal from reduced SWCNTs was not identified in blends due to the free and fast-relaxing nature of unpaired SWCNT electrons as well as spectral overlap of this EPR signal with the signal from positive P3HT polarons. However, a weak EPR signal was observed in chemically reduced SWNTs, and the <i>g</i> values of this signal are close to those of C₇₀-PCBM anion radical. The anisotropic line shape indicates that these unpaired electrons are not free but instead localized

Through-Space Ultrafast Photoinduced Electron Transfer Dynamics of a C₇₀-Encapsulated Bisporphyrin Covalent Organic Polyhedron in a Low-Dielectric Medium

Ultrafast photoinduced electron transfer (PIET) dynamics of a C₇₀-encapsulated bisporphyrin covalent organic polyhedron hybrid (C₇₀@COP-5) is studied in a nonpolar toluene medium with fluorescence and transient absorption spectroscopies. This structurally rigid donor (D)–acceptor (A) molecular hybrid offers a new platform featuring conformationally predetermined cofacial D–A orientation with a fixed edge-to-edge separation, <i>R</i>_EE (2.8 Å), without the aid of covalent bonds. Sub-picosecond PIET (τ_ET ≤ 0.4 ps) and very slow charge recombination (τ_CR ≈ 600 ps) dynamics are observed. The origin of these dynamics is discussed in terms of enhanced D–A coupling (<i>V</i> = 675 cm^–1) and extremely small reorganization energy (λ ≈ 0.18 eV), induced by the intrinsic structural rigidity of the C₇₀@COP-5 complex