Charge Trapping in Bright and Dark States of Coupled PbS Quantum Dot Films

Analysis of photoluminescence (PL) from chemically treated lead sulfide (PbS) quantum dot (QD) films versus temperature reveals the effects of QD size and ligand binding on the motion of carriers between bright and dark trap states. For strongly coupled QDs, the PL exhibits temperature-dependent quenching and shifting consistent with charges residing in a shallow exponential tail of quasi-localized states below the band gap. The depth of the tail varies from 15 to 40 meV, similar to or smaller than exponential band tail widths measured for polycrystalline Si. The trap state distribution can be manipulated with QD size and surface treatment, and its characterization should provide a clearer picture of charge separation and percolation in disordered QD films than what currently exists

Singlet Fission and Excimer Formation in Disordered Solids of Alkyl-Substituted 1,3-Diphenylisobenzofurans

We describe the preparation and excited state dynamics of three alkyl derivatives of 1,3-diphenylisobenzofuran (<b>1</b>) in both solutions and thin films. The substitutions are intended to disrupt the slip-stacked packing observed in crystals of <b>1</b> while maintaining the favorable energies of singlet and triplet for singlet fission (SF). All substitutions result in films that are largely amorphous as judged by the absence of strong X-ray diffraction peaks. The films of <b>1</b> carrying a methyl in the para position of one phenyl ring undergo SF relatively efficiently (≥75% triplet yield, Φ_T) but more slowly than thin films of <b>1</b>. When the methyl is replaced with a <i>t</i>-butyl, kinetic competition in the excited state favors excimer formation rather than SF (Φ_T = 55%). When <i>t</i>-Bu groups are placed in both meta positions of the phenyl substituent, SF is slowed further and Φ_T = 35%

Ultrafast Spectroscopic Signature of Charge Transfer between Single-Walled Carbon Nanotubes and C₆₀

The time scales for interfacial charge separation and recombination play crucial roles in determining efficiencies of excitonic photovoltaics. Near-infrared photons are harvested efficiently by semiconducting single-walled carbon nanotubes (SWCNTs) paired with appropriate electron acceptors, such as fullerenes (<i>e</i>.<i>g</i>., C₆₀). However, little is known about crucial photochemical events that occur on femtosecond to nanosecond time scales at such heterojunctions. Here, we present transient absorbance measurements that utilize a distinct spectroscopic signature of charges within SWCNTs, the absorbance of a trion quasiparticle, to measure both the ultrafast photoinduced electron transfer time (τ_pet) and yield (ϕ_pet) in photoexcited SWCNT–C₆₀ bilayer films. The rise time of the trion-induced absorbance enables the determination of the photoinduced electron transfer (PET) time of τ_pet ≤ 120 fs, while an experimentally determined trion absorbance cross section reveals the yield of charge transfer (ϕ_pet ≈ 38 ± 3%). The extremely fast electron transfer times observed here are on par with some of the best donor:acceptor pairs in excitonic photovoltaics and underscore the potential for efficient energy harvesting in SWCNT-based devices

Charge Generation in PbS Quantum Dot Solar Cells Characterized by Temperature-Dependent Steady-State Photoluminescence

Charge-carrier generation and transport within PbS quantum dot (QD) solar cells is investigated by measuring the temperature-dependent steady-state photoluminescence (PL) concurrently during <i>in situ</i> current–voltage characterization. We first compare the temperature-dependent PL quenching for PbS QD films where the PbS QDs retain their original oleate ligand to that of PbS QDs treated with 1,2-ethanedithiol (EDT), producing a conductive QD layer, either on top of glass or on a ZnO nanocrystal film. We then measure and analyze the temperature-dependent PL in a completed QD-PV architecture with the structure Al/MoO₃/EDT-PbS/ZnO/ITO/glass, collecting the PL and the current simultaneously. We find that at low temperatures excitons diffuse to the ZnO interface, where PL is quenched <i>via</i> interfacial charge transfer. At high temperatures, excitons dissociate in the bulk of the PbS QD film <i>via</i> phonon-assisted tunneling to nearby QDs, and that dissociation is in competition with the intrinsic radiative and nonradiative rates of the individual QDs. The activation energy for exciton dissociation in the QD-PV devices is found to be ∼40 meV, which is considerably lower than that of the electrodeless samples, and suggests unique interactions between injected and photogenerated carriers in devices

Emission Quenching in PbSe Quantum Dot Arrays by Short-Term Air Exposure

Clear evidence for two emitting states in PbSe nanocrystals (NCs) has been observed. The flow of population between these two states as temperature increases is interrupted by the presence of nonradiative trap states correlated with the exposure of the NC film to air. Quenching of the higher-energy emission begins after only seconds of exposure, with the effect saturating after several days. Unlike short-term oxygen-related effects in solution, the emission quenching appears to be irreversible, signaling a distinction between surface reactivity in NCs in films and that in solution. The origin of the two emissive centers and the impact of trapping on other NC film properties (e.g., electron/hole mobilities) remain important issues to be resolved

Coherent Exciton Delocalization in Strongly Coupled Quantum Dot Arrays

Quantum dots (QDs) coupled into disordered arrays have exhibited the intriguing property of bulk-like transport while maintaining discrete excitonic optical transitions. We have utilized ultrafast cross-polarized transient grating (CPTG) spectroscopy to measure electron–hole wave function overlap in CdSe QD films with chemically modified surfaces for tuning inter-QD electronic coupling. By comparing the CPTG decays with those of isolated QDs, we find that excitons coherently delocalize to form excited states more than 200% larger than the QD diameter

Solvent-Controlled Branching of Localized versus Delocalized Singlet Exciton States and Equilibration with Charge Transfer in a Structurally Well-Defined Tetracene Dimer

A detailed photophysical picture is elaborated for a structurally well-defined and symmetrical bis-tetracene dimer in solution. The molecule was designed for interrogation of the initial photophysical steps (S₁ → ¹TT) in intramolecular singlet fission (SF). (Triisopropylsilyl)­acetylene substituents on the dimer TIPS-BT1 as well as a monomer model TIPS-Tc enable a comparison of photophysical properties, including transient absorption dynamics, as solvent polarity is varied. In nonpolar toluene solutions, TIPS-BT1 decays via radiative and nonradiative pathways to the ground state with no evidence for dynamics related to the initial stages of SF. This contrasts with the behavior of the previously reported unsubstituted dimer BT1 and is likely a consequence of energetic perturbations to the singlet excited-state manifold of TIPS-BT1 by the (trialkylsilyl)­acetylene substituents. In polar benzonitrile, two key findings emerge. First, photoexcited TIPS-BT1 shows a bifurcation into both arm-localized (S_1‑loc) and dimer-delocalized (S_1‑dim) singlet exciton states. The S_1‑loc decays to the ground state, and weak temperature dependence of its emissive signatures suggests that once it is formed, it is isolated from S_1‑dim. Emissive signatures of the S_1‑dim state, on the other hand, are strongly temperature-dependent, and transient absorption dynamics show that S_1‑dim equilibrates with an intramolecular charge-transfer state in 50 ps at room temperature. This equilibrium decays to the ground state with little evidence for formation of long-lived triplets nor ¹TT. These detailed studies spectrally characterize many of the key states in intramolecular SF in this class of dimers but highlight the need to tune electronic coupling and energetics for the S₁ → ¹TT photoreaction

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

Covalently Bound Nitroxyl Radicals in an Organic Framework

A series of covalent organic framework (COF) structures is synthesized that possesses a tunable density of covalently bound nitroxyl radicals within the COF pores. The highest density of organic radicals produces an electron paramagnetic resonance (EPR) signal that suggests the majority of radicals strongly interact with other radicals, whereas for smaller loadings the EPR signals indicate the radicals are primarily isolated but with restricted motion. The dielectric loss as determined from microwave absorption of the framework structures compared with an amorphous control suggests that free motion of the radicals is inhibited when more than 25% of available sites are occupied. The ability to tune the mode of radical interactions and the subsequent effect on redox, electrical, and optical characteristics in a porous framework may lead to a class of structures with properties ideal for photoelectrochemistry or energy storage

Coupling between a Molecular Charge-Transfer Exciton and Surface Plasmons in a Nanostructured Metal Grating

The interaction of molecular excitons in organic thin films with surface plasmon polaritons (SPPs) in nanostructured metal electrodes represents a unique opportunity for enhancing the properties of the active layer of a photoconversion device. We present evidence of hybridization between charge-transfer excitons (CTEs) in 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and SPP modes in silver grating nanostructures. Molecular and SPP absorption peaks exhibit avoided crossings in angle-dependent reflectivity experiments, which are verified by electromagnetic-field simulations of the PTCDA-grating structure. Photoluminescence measurements indicate that the radiative decay of the CTE is enhanced. Besides energetic resonance, selective coupling between the SPP and the exciton in this unique case may be aided by the oriented nature of PTCDA into 1-D “molecular stacks” as well as the delocalized nature of the CTE