Photon Upconversion in a Vapor Deposited 2D Inorganic-Organic Semiconductor Heterostructure.

Energy transfer processes may be engineered in van der Waals heterostructures by taking advantage of the atomically abrupt, Å-scale, and topologically tailorable interfaces within them. Here, we prepare heterostructures comprised of 2D WSe2 monolayers interfaced with dibenzotetraphenylperiflanthene (DBP)-doped rubrene, an organic semiconductor capable of triplet fusion. We fabricate these heterostructures entirely through vapor deposition methods. Time-resolved and steady-state photoluminescence measurements reveal rapid subnanosecond quenching of WSe2 emission by rubrene and fluorescence from guest DBP molecules at 612 nm (λexc = 730 nm), thus providing clear evidence of photon upconversion. The dependence of the upconversion emission on excitation intensity is consistent with a triplet fusion mechanism, and maximum efficiency (linear regime) of this process occurs at threshold intensities as low as 110 mW/cm2, which is comparable to the integrated solar irradiance. This study highlights the potential for advanced optoelectronic applications employing vdWHs which leverage strongly bound excitons in monolayer TMDs and organic semiconductors.

Sequence-Defined Conjugated Oligomers in Donor-Acceptor Dyads.

Conjugated macromolecules have a rich history in chemistry, owing to their chemical arrangements that intertwine physical and electronic properties. The continuing study and application of these systems, however, necessitates the development of atomically precise models that bridge the gap between molecules, polymers, and/or their blends. One class of conjugated polymers that have facilitated the advancement of structure-property relationships is discrete, precision oligomers that have remained an outstanding synthetic challenge with only a handful of reported examples. Here we show the first synthesis of molecular dyads featuring sequence-defined oligothiophene donors covalently linked a to small-molecule acceptor. These dyads serve as a platform for probing complex photophysical interactions involving sequence-defined oligomers. This assessment is facilitated through the unprecedented control of oligothiophene length- and sequence-dependent arrangement relative to the acceptor unit, made possible by the incorporation of hydroxyl-containing side chains at precise positions along the backbone through sequence-defined oligomerizations. We show that both the oligothiophene sequence and length play complementary roles in determining the transfer efficiency of photoexcited states. Overall, the work highlights the importance of the spatial arrangement of donor-acceptor systems that are commonly studied for a range of uses, including light harvesting and photocatalysis.

The Effect of Linker-to-Metal Energy Transfer on the Photooxidation Performance of an Isostructural Series of Pyrene-Based Rare-Earth Metal-Organic Frameworks.

The tetratopic linker, 1,3,6,8-tetrakis(p-benzoic acid)pyrene (H4 TBAPy) along with rare-earth (RE) ions is used for the synthesis of 9 isostructures of a metal-organic framework (MOF) with shp topology, named RE-CU-10 (RE = Y(III), Gd(III), Tb(III), Dy(III), Ho(III), Er(III), Tm(III), Yb(III), and Lu(III)). The synthesis of each RE-CU-10 analogue requires different reaction conditions to achieve phase pure products. Single crystal X-ray diffraction indicates the presence of a RE9 -cluster in Y- to Tm-CU-10, while a RE11 -cluster is observed for Yb- and Lu-CU-10. The photooxidation performance of RE-CU-10 analogues is evaluated, observing competition between linker-to-metal energy transfer versus the generation of singlet oxygen. The singlet oxygen produced is used to detoxify a mustard gas simulant 2-chloroethylethyl sulfide, with half-lives ranging from 4.0 to 5.8 min, some of the fastest reported to date using UV-irradiation and < 1 mol% catalyst, in methanol under O2 saturation.

The smallest PbS nanocrystals pervasively show decreased brightness, linked to surface-mediated decay on the average particle.

PbS semiconductor nanocrystals (NCs) have been heavily explored for infrared optoelectronics but can exhibit visible-wavelength quantum-confined optical gaps when sufficiently small (⌀ = 1.8-2.7 nm). However, small PbS NCs traditionally exhibited very broad ensemble absorption linewidths, attributed to poor size-heterogeneity. Here, harnessing recent synthetic advances, we report photophysical measurements on PbS ensembles that span this underexplored size range. We observe that the smallest PbS NCs pervasively exhibit lower brightness and anomalously accelerated photoluminescence decays-relative to the idealized photophysical models that successfully describe larger NCs. We find that effects of residual ensemble size-heterogeneity are insufficient to explain our observations, so we explore plausible processes that are intrinsic to individual nanocrystals. Notably, the anomalous decay kinetics unfold, surprisingly, over hundreds-of-nanosecond timescales. These are poorly matched to effects of direct carrier trapping or fine-structure thermalization but are consistent with non-radiative recombination linked to a dynamic surface. Thus, the progressive enhancement of anomalous decay in the smallest particles supports predictions that the surface plays an outsized role in exciton-phonon coupling. We corroborate this claim by showing that the anomalous decay is significantly remedied by the installation of a rigidifying shell. Intriguingly, our measurements show that the anomalous aspect of these kinetics is insensitive to temperature between T = 298 and 77 K, offering important experimental constraint on possible mechanisms involving structural fluctuations. Thus, our findings identify and map the anomalous photoluminescence kinetics that become pervasive in the smallest PbS NCs and call for targeted experiments and theory to disentangle their origin.

PbS Nanocrystal Emission Is Governed by Multiple Emissive States.

Lead chalcogenide colloidal nanocrystals (NCs) are promising materials for solution processable optoelectronics. However, there is little agreement on the identity and character of PbS NC emission for different degrees of quantum confinement-a critical parameter for realizing applications for these nanocrystals. In this work, we combine ensemble and single NC spectroscopies to interrogate preparations of lead sulfide NCs. We use solution photon correlation Fourier spectroscopy (S-PCFS) to measure the average single NC linewidth of near-infrared-emitting PbS quantum dots and find it to be dominated by homogeneous broadening. We further characterize PbS NCs using temperature-dependent linear and time-resolved emission spectroscopy which demonstrate that a kinetically accessed defect state dominates room temperature emission of highly confined emitting NCs. These experiments, taken together, demonstrate that the linewidth and Stokes shift of PbS NCs are the result of emission from two states: a thermally accessed defect-with an energetically pinned charge carrier-and an inhomogeneously broadened band-edge state.

Methylammonium Bismuth Iodide as a Lead-Free, Stable Hybrid Organic-Inorganic Solar Absorber.

Methylammonium lead halide (MAPbX3 ) perovskites exhibit exceptional carrier transport properties. But their commercial deployment as solar absorbers is currently limited by their intrinsic instability in the presence of humidity and their lead content. Guided by our theoretical predictions, we explored the potential of methylammonium bismuth iodide (MBI) as a solar absorber through detailed materials characterization. We synthesized phase-pure MBI by solution and vapor processing. In contrast to MAPbX3, MBI is air stable, forming a surface layer that does not increase the recombination rate. We found that MBI luminesces at room temperature, with the vapor-processed films exhibiting superior photoluminescence (PL) decay times that are promising for photovoltaic applications. The thermodynamic, electronic, and structural features of MBI that are amenable to these properties are also present in other hybrid ternary bismuth halide compounds. Through MBI, we demonstrate a lead-free and stable alternative to MAPbX3 that has a similar electronic structure and nanosecond lifetimes.

Energy harvesting of non-emissive triplet excitons in tetracene by emissive PbS nanocrystals.

Triplet excitons are ubiquitous in organic optoelectronics, but they are often an undesirable energy sink because they are spin-forbidden from emitting light and their high binding energy hinders the generation of free electron-hole pairs. Harvesting their energy is consequently an important technological challenge. Here, we demonstrate direct excitonic energy transfer from 'dark' triplets in the organic semiconductor tetracene to colloidal PbS nanocrystals, thereby successfully harnessing molecular triplet excitons in the near infrared. Steady-state excitation spectra, supported by transient photoluminescence studies, demonstrate that the transfer efficiency is at least (90 ± 13)%. The mechanism is a Dexter hopping process consisting of the simultaneous exchange of two electrons. Triplet exciton transfer to nanocrystals is expected to be broadly applicable in solar and near-infrared light-emitting applications, where effective molecular phosphors are lacking at present. In particular, this route to 'brighten' low-energy molecular triplet excitons may permit singlet exciton fission sensitization of conventional silicon solar cells.

Deconstructing the photon stream from single nanocrystals: from binning to correlation.

Prior to the advent of single-molecule fluorescence spectroscopy, many of the fundamental optical properties of colloidal semiconductor nanocrystal quantum dots were obscured by ensemble averaging over their inherent inhomogeneities. Single quantum dot spectroscopy has become a leading technique for the unambiguous determination of the governing excitonic physics of these quantum-confined systems. The analysis and interpretation of the timing and energies of photons emitted from individual nanocrystals have uncovered unexpected and fundamental electronic processes at the nanoscale. We review several different paradigms for deconstructing the photon stream from single nanocrystals, ranging from intensity "binning" techniques to more sophisticated methods based on single-photon counting. In particular, we highlight photon correlation - a powerful developing paradigm in single-nanocrystal studies. The application of photon-correlation techniques to single nanocrystals is changing the study of multiexcitonic recombination dynamics, uncovering the basic processes governing spectral linewidths and spectral diffusion, and enabling the extraction of single-nanocrystal properties directly from an ensemble with high statistical significance. These single-molecule techniques have proven invaluable for understanding the physics of nanocrystals and can provide unique insight into other heterogeneous and dynamical systems.

Singlet exciton fission in polycrystalline pentacene: from photophysics toward devices.

Singlet exciton fission is the process in conjugated organic molecules bywhich a photogenerated singlet exciton couples to a nearby chromophore in the ground state, creating a pair of triplet excitons. Researchers first reported this phenomenon in the 1960s, an event that sparked further studies in the following decade. These investigations used fluorescence spectroscopy to establish that exciton fission occurred in single crystals of several acenes. However, research interest has been recently rekindled by the possibility that singlet fission could be used as a carrier multiplication technique to enhance the efficiency of photovoltaic cells. The most successful architecture to-date involves sensitizing a red-absorbing photoactive layer with a blue-absorbing material that undergoes fission, thereby generating additional photocurrent from higher-energy photons. The quest for improved solar cells has spurred a drive to better understand the fission process, which has received timely aid from modern techniques for time-resolved spectroscopy, quantum chemistry, and small-molecule device fabrication. However, the consensus interpretation of the initial studies using ultrafast transient absorption spectroscopy was that exciton fission was suppressed in polycrystalline thin films of pentacene, a material that would be otherwise expected to be an ideal model system, as well as a viable candidate for fission-sensitized photovoltaic devices. In this Account, we review the results of our recent transient absorption and device-based studies of polycrystalline pentacene. We address the controversy surrounding the assignment of spectroscopic features in transient absorption data, and illustrate how a consistent interpretation is possible. This work underpins our conclusion that singlet fission in pentacene is extraordinarily rapid (∼80 fs) and is thus the dominant decay channel for the photoexcited singlet exciton. Further, we discuss our demonstration that triplet excitons generated via singlet fission in pentacene can be dissociated at an interface with a suitable electron acceptor, such as fullerenes and infrared-absorbing inorganic semiconducting quantum dots. We highlight our recent reports of a pentacene/PbSe hybrid solar cell with a power conversion efficiency of 4.7% and of a pentacene/PbSe/amorphous silicon photovoltaic device. Although substantive challenges remain, both to better our understanding of the mechanism of singlet exciton fission and to optimize device performance, this realization of a solar cell where photocurrent is simultaneously contributed from a blue-absorbing fission-capable material and an infrared-absorbing conventional cell is an important step towards a dual-bandgap, single-junction, fission-enhanced photovoltaic device, which could one day surpass the Shockley-Queisser limit.

Temperature-independent singlet exciton fission in tetracene.

We use transient absorption spectroscopy to demonstrate that the dynamics of singlet exciton fission in tetracene are independent of temperature (10­270 K). Low-intensity, broad-band measurements allow the identification of spectral features while minimizing bimolecular recombination. Hence, by directly observing both species, we find that the time constant for the conversion of singlets to triplet pairs is ~90 ps. However, in contrast to pentacene, where fission is effectively unidirectional, we confirm that the emissive singlet in tetracene is readily regenerated from spin-correlated "geminate" triplets following fission, leading to equilibrium dynamics. Although free triplets are efficiently generated at room temperature, the interplay of superradiance and frustrated triplet diffusion contributes to a nearly 20-fold increase in the steady-state fluorescence as the sample is cooled. Together, these results require that singlets and triplet pairs in tetracene are effectively degenerate in energy, and begin to reconcile the temperature dependence of many macroscopic observables with a fission process which does not require thermal activation.

Singlet exciton fission-sensitized infrared quantum dot solar cells.

We demonstrate an organic/inorganic hybrid photovoltaic device architecture that uses singlet exciton fission to permit the collection of two electrons per absorbed high-energy photon while simultaneously harvesting low-energy photons. In this solar cell, infrared photons are absorbed using lead sulfide (PbS) nanocrystals. Visible photons are absorbed in pentacene to create singlet excitons, which undergo rapid exciton fission to produce pairs of triplets. Crucially, we identify that these triplet excitons can be ionized at an organic/inorganic heterointerface. We report internal quantum efficiencies exceeding 50% and power conversion efficiencies approaching 1%. These findings suggest an alternative route to circumvent the Shockley-Queisser limit on the power conversion efficiency of single-junction solar cells.

Sequential Carrier Transfer Can Accelerate Triplet Energy Transfer from Functionalized CdSe Nanocrystals.

Nanocrystal (NC)-sensitized triplet-fusion upconversion is a rising strategy to convert long-wavelength, incoherent light into higher-energy output photons. Here, we chart the photophysics of tailor-functionalized CdSe NCs to understand energy transfer to surface-anchored transmitter ligands, which can proceed via correlated exciton transfer or sequential carrier hops. Varying NC size, we observe a pronounced acceleration of energy transfer (from kquench = 0.0096 ns-1 ligand-1 to 0.064 ns-1 ligand-1) when the barrier to hole-first sequential transfer is lowered from 100 ± 25 meV to 50 ± 25 meV. This acceleration is 5.1× the expected effect of increased carrier wave function leakage, so we conclude that sequential transfer becomes kinetically dominant under the latter conditions. Last, transient photoluminescence shows that NC band-edge and trap states are comparably quenched by functionalization (up to ∼98% for sequential transfer) and exhibit matched dynamics for t > 300 ns, consistent with a dynamic quasi-equilibrium where photoexcitations can ultimately be extracted even when a carrier is initially trapped.

Sequential Co-Passivation in InAs Colloidal Quantum Dot Solids Enables Efficient Near-Infrared Photodetectors.

III-V colloidal quantum dots (CQDs) are promising materials for optoelectronic applications, for they avoid heavy metals while achieving absorption spanning the visible to the infrared (IR). However, the covalent nature of III-V CQDs requires the development of new passivation strategies to fabricate conductive CQD solids for optoelectronics: this work shows herein that ligand exchanges, previously developed in II-VI and IV-VI quantum dots and employing a single ligand, do not fully passivate CQDs, and that this curtails device efficiency. Guided by density functional theory (DFT) simulations, this work develops a co-passivation strategy to fabricate indium arsenide CQD photodetectors, an approach that employs the combination of X-type methyl ammonium acetate (MaAc) and Z-type ligands InBr3 . This approach maintains charge carrier mobility and improves passivation, seen in a 25% decrease in Stokes shift, a fourfold reduction in the rate of first-exciton absorption linewidth broadening over time-under-stress, and leads to a doubling in photoluminescence (PL) lifetime. The resulting devices show 37% external quantum efficiency (EQE) at 950 nm, the highest value reported for InAs CQD photodetectors.

Synthesis and optoelectronic properties of radical conjugated polyfluorenes.

A novel redox-active fluorene monomer is synthesized and copolymerized with 9,9-dioctylfluorene and benzo[c][1,2,5]thiadiazole via Suzuki cross-coupling to produce alternating and tertiary copolymers. Electrochemical and chemical reduction of the copolymers generates organic polymeric radical anions. Electrochemical, spectroscopic, and photophysical characterization grant insight into the structure-property relationship for open-shell conjugated polymers.

Ultrafast dynamics of exciton fission in polycrystalline pentacene.

We use ultrafast transient absorption spectroscopy with sub-20 fs time resolution and broad spectral coverage to directly probe the process of exciton fission in polycrystalline thin films of pentacene. We observe that the overwhelming majority of initially photogenerated singlet excitons evolve into triplet excitons on an ∼80 fs time scale independent of the excitation wavelength. This implies that exciton fission occurs at a rate comparable to phonon-mediated exciton localization processes and may proceed directly from the initial, delocalized, state. The singlet population is identified due to the brief presence of stimulated emission, which is emitted at wavelengths which vary with the photon energy of the excitation pulse, a violation of Kasha's Rule that confirms that the lowest-lying singlet state is extremely short-lived. This direct demonstration that triplet generation is both rapid and efficient establishes multiple exciton generation by exciton fission as an attractive route to increased efficiency in organic solar cells.

Ultra-small PbS nanocrystals as sensitizers for red-to-blue triplet-fusion upconversion.

Photon upconversion is a strategy to generate high-energy excitations from low-energy photon input, enabling advanced architectures for imaging and photochemistry. Here, we show that ultra-small PbS nanocrystals can sensitize red-to-blue triplet-fusion upconversion with a large anti-Stokes shift (ΔE = 1.04 eV), and achieve max-efficiency upconversion at near-solar fluences (I th = 220 mW cm-2) despite endothermic triplet sensitization. This system facilitates the photo-initiated polymerization of methyl methacrylate using only long-wavelength light (λ exc: 637 nm); a demonstration of nanocrystal-sensitized upconversion photochemistry. Time-resolved spectroscopy and kinetic modelling clarify key loss channels, highlighting the benefit of long-lifetime nanocrystal sensitizers, but revealing that many (48%) excitons that reach triplet-extracting carboxyphenylanthracene ligands decay before they can transfer to free-floating acceptors-emphasizing the need to address the reduced lifetimes that we determine for molecular triplets near the nanocrystal surface. Finally, we find that the inferred thermodynamics of triplet sensitization from these ultra-small PbS quantum dots are surprisingly favourable-completing an advantageous suite of properties for upconversion photochemistry-and do not vary significantly across the ensemble, which indicates minimal effects from nanocrystal heterogeneity. Together, our demonstration and study of red-to-blue upconversion using ultra-small PbS nanocrystals in a quasi-equilibrium, mildly endothermic sensitization scheme offer design rules to advance implementations of triplet fusion, especially where large anti-Stokes wavelength shifts are sought.

Binary Cu2-xS Templates Direct the Formation of Quaternary Cu2ZnSnS4 (Kesterite, Wurtzite) Nanocrystals.

Kesterite Cu2ZnSnS4 (k-CZTS) nanocrystals have received attention for their tunable optoelectronic properties, as well as the earth abundance of their constituent atoms. However, the phase-pure synthesis of these quaternary NCs is challenging due to their polymorphism, as well as the undesired formation of related binary and ternary impurities. A general synthetic route to tackle this complexity is to pass through intermediate template nanocrystals that direct subsequent cation exchange toward the desired quaternary crystalline phase, particularly those that are thermodynamically disfavored or otherwise synthetically challenging. Here, working within this model multinary system, we achieve control over the formation of three binary copper sulfide polymorphs, cubic digenite (Cu1.8S), hexagonal covellite (CuS), and monoclinic djurleite (Cu1.94S). Controlled experiments with Cu0 seeds show that selected binary phases can be favored by the identity and stoichiometry of the sulfur precursor alone under otherwise comparable reaction conditions. We then demonstrate that the nature of the Cu2-xS template dictates the final polymorph of the CZTS nanocrystal products. Through digenite, the cation exchange reaction readily yields the k-CZTS phase due to its highly similar anion sublattice. Covellite nanocrystals template the k-CZTS phase but via major structural rearrangement to digenite that requires elevated temperatures in the absence of a strong reducing agent. In contrast, we show that independently synthesized djurleite nanorods template the formation of the wurtzite polymorph (w-CZTS) but with prominent stacking faults in the final product. Applying this refined understanding to the standard one-pot syntheses of k- and w-CZTS nanocrystals, we identify that these reactions are each effectively templated by binary intermediates formed in situ, harnessing their properties to guide the overall synthesis of phase-pure quaternary materials. Our results provide tools for the careful development of tailored nanocrystal syntheses in complex polymorphic systems.

Exciton fission and charge generation via triplet excitons in pentacene/C60 bilayers.

Organic photovoltaic devices are currently studied due to their potential suitability for flexible and large-area applications, though efficiencies are presently low. Here we study pentacene/C(60) bilayers using transient optical absorption spectroscopy; such structures exhibit anomalously high quantum efficiencies. We show that charge generation primarily occurs 2-10 ns after photoexcitation. This supports a model where charge is generated following the slow diffusion of triplet excitons to the heterojunction. These triplets are shown to be present from early times (<200 fs) and result from the fission of a spin-singlet exciton to form two spin-triplet excitons. These results elucidate exciton and charge generation dynamics in the pentacene/C(60) system and demonstrate that the tuning of the energetic levels of organic molecules to take advantages of singlet fission could lead to greatly enhanced photocurrent in future OPVs.

Sub-Bandgap Optical Modulation of Quantum Dot Blinking Statistics.

Colloidal quantum dots (QDs) suffer from pervasive photoluminescence intermittency that frustrates applications and correlates with irreversible photodegradation. In single-QD spectroscopies, blinking manifests as sporadic switching between ON and OFF states without a characteristic time scale, and the longstanding search for mechanisms has been recently accelerated by techniques to controllably modulate the QD environment. Here, we develop an all-optical modulation scheme and demonstrate that sub-bandgap light tuned to the stimulated emission transition perturbs the blinking statistics of individual CdSe/ZnS core/shell QDs. Resonant optical modulation progressively suppresses long-duration ON events, quantified by a power-law slope that is more negative on average (ΔαON = 0.46 ± 0.09), while OFF distributions and truncation times are unaffected. This characteristic effect is robust to choices in background subtraction and statistical analysis but supports mechanistic descriptions beyond first-order kinetics. This demonstration of all-optical perturbation of QD blinking dynamics provides an experimental avenue to disentangle the complex photophysics of photoluminescence intermittency.

PbS Nanocrystals Made with Excess PbCl2 Have an Intrinsic Shell that Reduces Their Stokes Shift.

The use of excess PbCl2 in the synthesis of PbS nanocrystals has become a convenient route to produce narrow-line-width infrared emitters. However, these materials have found limited adoption in optoelectronic devices-even compared to PbS nanocrystals prepared with lead oleate. Here, using both transmission electron microscopy and small-angle X-ray scattering, we show that excess PbCl2 results in larger-diameter PbS nanocrystals for the same excitonic features, which is consistent with the formation of an intrinsic insulating shell. We observe further differences in excess-lead-chloride nanocrystals consistent with a shell, including lattice strain and smaller Stokes shifts for intermediate sizes (⌀: 4.8-6.8 nm) that match the passivation/rigidification predicted for a chloride-terminate surface. Our results clarify and rationalize the divergent properties of PbS nanocrystals prepared using different synthetic methodologies, give guidance for device implementation, and offer a new target for synthetic control.