Ryan D. Pensack

Structure-Tuned Lead Halide Perovskite Nanocrystals.

Colloidally stable suspensions of lead halide perovskite nanocrystals are prepared from high-quality lead halide nanocrystal seeds. Perovskite nanocrystals with different layered crystal structures are reported. These systems are well suited for investigations of the intrinsic photophysics and spectroscopy of organic-inorganic metal halide perovskites.

Barrierless Free Carrier Formation in an Organic Photovoltaic Material Measured with Ultrafast Vibrational Spectroscopy

The dynamics of free carrier formation following photoinduced electron transfer from the conjugated polymer, CN-MEH-PPV, to the electron-accepting functionalized fullerene, PCBM, are directly measured using ultrafast vibrational spectroscopy. The vibrational frequency of the carbonyl (C horizontal lineO) stretch of PCBM is sensitive to the location of the molecules relative to the interfaces formed between PCBM clusters and CN-MEH-PPV. The correlation between the carbonyl frequency and the proximity to the interfaces provides the ability to directly measure the escape of electrons from their Coulombically bound radical pairs. The data indicate that the rate of free carrier formation is temperature independent from 200 to 350 K suggesting that excess vibrational energy resulting from the electron transfer reaction enables electrons to escape their Coulombic potentials on ultrafast time scales.

Exciton delocalization drives rapid singlet fission in nanoparticles of acene derivatives.

We compare the singlet fission dynamics of five pentacene derivatives precipitated to form nanoparticles. Two nanoparticle types were distinguished by differences in their solid-state order and kinetics of triplet formation. Nanoparticles that comprise primarily weakly coupled chromophores lack the bulk structural order of the single crystal and exhibit nonexponential triplet formation kinetics (Type I), while nanoparticles that comprise primarily more strongly coupled chromophores exhibit order resembling that of the bulk crystal and triplet formation kinetics associated with the intrinsic singlet fission rates (Type II). In the highly ordered nanoparticles, singlet fission occurs most rapidly. We relate the molecular packing arrangement derived from the crystal structure of the pentacene derivatives to their singlet fission dynamics and find that slip stacking leads to rapid, subpicosecond singlet fission. We present evidence that exciton delocalization, coincident with an increased relative admixture of charge-transfer configurations in the description of the exciton wave function, facilitates rapid triplet pair formation in the case of single-step singlet fission. We extend the study to include two hexacene derivatives and find that these conclusions are generally applicable. This work highlights acene derivatives as versatile singlet fission chromophores and shows how chemical functionalization affects both solid-state order and exciton interactions and how these attributes in turn affect the rate of singlet fission.

Vibrational coherence probes the mechanism of ultrafast electron transfer in polymer–fullerene blends

The conversion of photoexcitations into charge carriers in organic solar cells is facilitated by the dissociation of excitons at the donor/acceptor interface. The ultrafast timescale of charge separation demands sophisticated theoretical models and raises questions about the role of coherence in the charge-transfer mechanism. Here, we apply two-dimensional electronic spectroscopy to study the electron transfer process in poly(3-hexylthiophene)/PCBM (P3HT/PCBM) blends. We report dynamics maps showing the pathways of charge transfer that clearly expose the significance of hot electron transfer. During this ultrafast electron transfer, vibrational coherence is directly transferred from the P3HT exciton to the P3HT hole polaron in the crystalline domain. This result reveals that the exciton converts to a hole with a similar spatial extent on a timescale far exceeding other photophysical dynamics including vibrational relaxation.

Observation of Two Triplet-Pair Intermediates in Singlet Exciton Fission

Singlet fission is an excitation multiplication process in molecular systems that can circumvent energy losses and significantly boost solar cell efficiencies; however, the nature of a critical intermediate that enables singlet fission and details of its evolution into multiple product excitations remain obscure. We resolve the initial sequence of events comprising the fission of a singlet exciton in solids of pentacene derivatives using femtosecond transient absorption spectroscopy. We propose a three-step model of singlet fission that includes two triplet-pair intermediates and show how transient spectroscopy can distinguish initially interacting triplet pairs from those that are spatially separated and noninteracting. We find that the interconversion of these two triplet-pair intermediates is limited by the rate of triplet transfer. These results clearly highlight the classical kinetic model of singlet fission and expose subtle details that promise to aid in resolving problems associated with triplet extraction.

Evidence for the Rapid Conversion of Primary Photoexcitations to Triplet States in Seleno- and Telluro- Analogues of Poly(3-hexylthiophene)

Broadband pump-probe spectroscopy is used to examine the ultrafast photophysics of the π-conjugated polymers poly(3-hexylselenophene) (P3HS) and poly(3-hexyltellurophene) (P3HTe) in solution. An excited-state absorption feature that we attribute to a transition in the triplet manifold appears on the picosecond time scale. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations support this assignment. The formation of triplets is consistent with significant fluorescence quenching observed in solutions of the neat polymers. Triplet formation occurs in ~26 and ~1.8 ps (upper limit) for P3HS and P3HTe, respectively. The successive decrease in fluorescence quantum efficiency and triplet formation time are consistent with intersystem crossing facilitated by the heavier selenium and tellurium atoms. These results strongly suggest that primary photoexcitations are rapidly converted into triplet states in P3HS and P3HTe.

Vibrational solvatochromism in organic photovoltaic materials: method to distinguish molecules at donor/acceptor interfaces

We report the observation of vibrational solvatochromism in organic photovoltaic materials. The frequency of the carbonyl (C=O) stretch of the methyl ester group of the functionalized fullerene, PCBM, is sensitive to the local phase separated morphology of polymer blends and bilayers incorporating the fullerene. In particular, PCBM molecules at interfaces with conjugated polymers exhibit higher frequency carbonyl stretch vibrations in comparison to molecules imbedded in the interiors of PCBM clusters/layers. The resulting frequency gradient was recently used to examine the dynamics of charge photogeneration in a blend of the conjugated polymer, CN-MEH-PPV, with PCBM. In this contribution, we explore the origin of the frequency shift and show that it arises from variations in the inhomogeneous solvent environments experienced by PCBM molecules as a result of phase separation in the polymer blend.

Vibrational spectroscopy of electronic processes in emerging photovoltaic materials.

Molecules affect the electronic properties of many emerging materials, ranging from organic thin film transistors and light emitting diodes for flexible displays to colloidal quantum dots (CQDs) used in solution processed photovoltaics and photodetectors. For example, the interactions of conjugated molecules not only influence morphological and charge transport properties of organic photovoltaic (OPV) materials, but they also determine the primary photophysical events leading to charge generation. Ligand-nanocrystal interactions affect the density and energetic distributions of trap states, which in turn influence minority carrier transport in CQD photovoltaics. Therefore, it is critical for scientists to understand how the underlying molecular structures and morphologies determine the electronic properties of emerging materials. Recently, chemists have used vibrational spectroscopy to study electronic processes in emerging materials, and been able to directly measure the influence molecular properties have on those processes. Time-resolved vibrational spectroscopy is uniquely positioned to examine molecular species involved in electronic processes because it combines ultrafast time resolution with measurement of the vibrational spectra of molecules. For instance, molecules at the electron donor/acceptor interfaces in OPV materials have unique vibrational features because vibrational frequencies of molecules are sensitive to their local molecular environments. Through ultrafast vibrational spectroscopy, researchers can directly examine the dynamics of charge transfer (CT) state formation and dissociation to form charge separated states specifically at donor/acceptor interfaces. Vibrational modes of ligands are also sensitive to their bonding interactions with nanocrystal surfaces, which enables chemists to directly probe the molecular nature of charge trap states in colloidal quantum dot solids. Because of the ability to connect electrical properties with the underlying molecular species, scientists can use ultrafast vibrational spectroscopy to address fundamental challenges in the development of emerging electronic materials. In this Account, we focus on two applications of vibrational spectroscopy to examine electronic processes in OPV and CQD photovoltaic materials. In the first application, we examine archetypal classes of electron acceptors in OPV materials and reveal how their molecular structures influence the dynamics and energetic barriers to CT state formation and dissociation. In the second application, we discuss the surface chemistry of ligand-nanocrystal interactions and how they impact the density and energetic distribution of charge trap states in CQD photovoltaic materials. Through direct observations of the vibrational features of ligands attached to surface trap states, we can obtain valuable insights into the nature of charge traps and begin to understand pathways for their elimination. We expect that further examination of electronic processes in materials using ultrafast vibrational spectroscopy will lead to new design rules in support of continued materials development efforts.

Temperature-independent vibrational dynamics in an organic photovoltaic material.

Ultrafast orientational motion and spectral diffusion of the carbonyl stretch vibration of the functionalized fullerene, PCBM, blended with the conjugated polymer, CN-MEH-PPV, are examined with two-dimensional infrared and polarization-resolved IR pump probe spectroscopy. In previous contributions from our group, the carbonyl stretch frequency of PCBM has been used as a local vibrational reporter to measure the temperature dependence of the time scale for dissociation of charge transfer excitons in CN-MEH-PPV:PCBM polymer blends. It was found that the rate of charge separation is independent of temperature, indicating that charge separation occurs through an activationless pathway. This assignment was supported by the observation at room temperature that thermal fluctuations do not give rise to spectral diffusion of the carbonyl stretch vibration on the picosecond and longer time scale. In this contribution, we examine the temperature dependence of the carbonyl vibrational dynamics to determine whether thermal fluctuations might give rise to spectral diffusion at other temperatures. We find that the time scale for fast wobbling-in-cone orientational motion is independent of temperature on the subpicosecond time scale. Similarly, spectral diffusion is not observed on the picosecond and longer time scale at all temperatures examined confirming our earlier interpretation of the frequency shift dynamics exclusively in terms of charge separation. Interestingly, the half angle characterizing the wobbling-in-cone orientational motion does increase at higher temperature due to increased free-volume resulting from thermal expansion of the polymer blend.

Vibrational Energy Mediates Charge Separation in Organic Photovoltaic Materials

Charge separation and charge trapping following photoinduced electron transfer from the conjugated polymer, poly (2-methoxy-5-ethylhexyloxy-1,4-phenylenecyanovinylene (CN-MEH-PPV), to the electron accepting functionalized fullerene, [6,6]-phenyl C61-butyric acid methyl ester (PCBM), is directly measured using ultrafast vibrational spectroscopy. Our group previously demonstrated that the vibrational frequency of the carbonyl (C=O) stretch of PCBM is sensitive to the location of the molecules relative to the interfaces formed between PCBM clusters and CN-MEH-PPV. The correlation between the carbonyl frequency and the proximity of PCBM molecules to the interfaces enables the time evolution of the frequency of the bleach peak to be interpreted in terms of dissociation of charge transfer states into charge separated states. Temperature-dependent measurements of the rate of this charge separation process indicate that excess vibrational energy in “hot” charge transfer states resulting from the electron transfer reaction enables electrons to escape their Coulombic potentials at the interfaces on ultrafast timescales. Furthermore, temperature-dependent measurements on longer timescales indicate that after the initial charge separation reaction, electrons enter shallow trap states that, on the basis of the radial variation of vibrational frequencies, lie in the interior of PCBM clusters. The vibrational spectra suggest that these interior regions have higher intermolecular order in comparison to the interfaces.