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

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

Beyond the Adiabatic Limit: Charge Photogeneration in Organic Photovoltaic Materials

Mounting evidence suggests that excess energy in charge-transfer (CT) excitonic states facilitates efficient charge separation in organic solar cells. Experimental and theoretical studies have revealed that this excess energy may reside in phonon modes or in electronic coordinates of organic photovoltaic materials that are directly excited by the transition from Frenkel to CT excitons. Despite their strong Coulombic attraction, electron−hole pairs in hot CT excitons are able to undergo activationless separation because the rate of separation competes with thermalization of electronic and nuclear degrees of freedom. We argue that these observations indicate strong coupling of the dynamics of electronic and nuclear coordinates in organic photovoltaic materials. Thus, a nonadiabatic description is needed to properly understand the mechanism of charge photogeneration in organic solar cells. Such a description will support continuing efforts toward the development of low-band-gap organic solar cells that efficiently generate photocurrent with minimal energy losses

Influence of Acceptor Structure on Barriers to Charge Separation in Organic Photovoltaic Materials

Energetic barriers to charge separation are examined in photovoltaic polymer blends based on regioregular-poly­(3-hexylthiophene) (P3HT) and two classes of electron acceptors: a perylene diimide (PDI) derivative and a fullerene (PCBM). Temperature-dependent measurements using ultrafast vibrational spectroscopy are used to directly measure the free energy barriers to charge separation. Charge separation in P3HT:PDI polymer blends occurs through activated pathways, whereas P3HT:PCBM blends exhibit activationless charge separation. X-ray scattering measurements reveal that neither the PDI derivative nor PCBM form highly crystalline domains in their polymer blends with P3HT. The present findings suggest that fullerenes are able to undergo barrierless charge separation even in the presence of structural disorder. In contrast, perylene diimides may require greater molecular order to achieve barrierless charge separation

Singlet Fission in Core–Shell Micelles of End-Functionalized Polymers

Singlet fission is the process in aggregates of molecular semiconductors where the initial product of light absorption (a singlet exciton) is converted into two correlated spin-triplet excitons. While most studies of singlet fission are conducted on assemblies of small molecule singlet fission chromophores, polymer self-assembly has yet to be explored as a means of creating nanostructures conducive for singlet fission. In this work, we use solution self-assembly of mono- and difunctionalized polymers to create core–shell micelles that display efficient singlet fission. The polymers are synthesized by copper­(I)-catalyzed “click” chemistry between a 6,13-bis­(triisopropyl­silylethynyl)­pentacene (TIPS-Pn) alkyne precursor and the corresponding azide-terminated poly­(ethylene glycol) (PEG) polymer. Spontaneous solution self-assembly creates starlike and flowerlike core–shell micelles that are characterized by dynamic light scattering (DLS) and transmission electron microscopy (TEM) experiments. Ultrafast transient absorption spectroscopy and time-resolved fluorescence experiments evidence nearly equivalent singlet fission dynamics in starlike and flowerlike micelles. Studies on mixed micelles of the Pn-functionalized polymer with a C₁₆-PEG surfactant reveal how triplet pair formation and decay rates vary with micelle composition. The core–shell micelles developed herein demonstrate the potential of polymer self-assembly for creating functional singlet fission nanostructures and provide insight into how secondary components and solubilizing blocks influence singlet fission dynamics and triplet pair losses in self-assembled systems

Ultrafast Triplet Formation in Thionated Perylene Diimides

Perylene diimides (PDIs) are versatile n-type materials showing great promise in a number of optoelectronic applications. While the singlet manifold of PDI can be readily populated, triplet excited states are only accessible through complex multistep energy cascades or bimolecular sensitization. In this work, we have synthesized a series of thionated PDIs that display rapid intersystem crossing to triplet states. Significantly, the thionated PDIs are synthesized in one step from the parent compound using commercially available Lawesson’s reagent. Electrochemical and steady state optical absorption measurements show that the electron affinity and ionization potentials can be systematically tuned through successive sulfur atom substitution. Thin-film optical absorption measurements show how the number and regiochemistry of the thiocarbonyl groups influence π–π interactions in the solid state. Ultrafast transient absorption spectroscopy reveals rapid triplet formation that is independent of the degree of thionation, highlighting this approach as a facile means of accessing the triplet manifold of PDI

Ultrafast Triplet Formation in Thionated Perylene Diimides

Perylene diimides (PDIs) are versatile n-type materials showing great promise in a number of optoelectronic applications. While the singlet manifold of PDI can be readily populated, triplet excited states are only accessible through complex multistep energy cascades or bimolecular sensitization. In this work, we have synthesized a series of thionated PDIs that display rapid intersystem crossing to triplet states. Significantly, the thionated PDIs are synthesized in one step from the parent compound using commercially available Lawesson’s reagent. Electrochemical and steady state optical absorption measurements show that the electron affinity and ionization potentials can be systematically tuned through successive sulfur atom substitution. Thin-film optical absorption measurements show how the number and regiochemistry of the thiocarbonyl groups influence π–π interactions in the solid state. Ultrafast transient absorption spectroscopy reveals rapid triplet formation that is independent of the degree of thionation, highlighting this approach as a facile means of accessing the triplet manifold of PDI

Triplet Energy Transfer Governs the Dissociation of the Correlated Triplet Pair in Exothermic Singlet Fission

Singlet fission is a spin-allowed process of exciton multiplication that has the potential to enhance the efficiency of photovoltaic devices. The majority of studies to date have emphasized understanding the first step of singlet fission, where the correlated triplet pair is produced. Here, we examine separation of correlated triplet pairs. We conducted temperature-dependent transient absorption on 6,3-bis­(tri<i>iso</i>propylsilylethynyl)­pentacene (TIPS-Pn) films, where singlet fission is exothermic. We evaluated time constants to show that their temperature dependence is inconsistent with an exclusively thermally activated process. Instead, we found that the trends can be modeled by a triplet–triplet energy transfer. The fitted reorganization energy and electronic coupling agree closely with values calculated using density matrix renormalization group quantum-chemical theory. We conclude that dissociation of the correlated triplet pair to separated (but spin-entangled) triplet excitons in TIPS-Pn occurs by triplet–triplet energy transfer with a hopping time constant of approximately 3.5 ps at room temperature

Direct Synthesis of CdSe Nanocrystals with Electroactive Ligands

We report the synthesis and characterization of cadmium selenide nanocrystals with electroactive ligands directly attached to the surface. The conventional surfactant-assisted synthesis yields nanocrystals with surfaces functionalized with insulating organic ligands. These insulating ligands act as a barrier for charge transport between nanocrystals. Electroactive (reducing/oxidizing) ligands like ferrocene and cobaltocene have potential for applications as photoexcited hole conductors and photoredox systems. Although ferrocene ligands anchored to the nanocrystal surface through insulating long-chain hydrocarbon spacers have previously been reported, this approach is limited because the charge transfer between nanocrystal and ferrocene is highly sensitive to their separation. We report here ferrocene directly bound to the inorganic core of the nanocrystal, and as a result the distance between the nanocrystals and the electroactive moiety is minimized

Excited-State Dynamics of 5,14- vs 6,13-Bis(trialkylsilylethynyl)-Substituted Pentacenes: Implications for Singlet Fission

Singlet fission is a process in conjugated organic materials that has the potential to considerably improve the performance of devices in many applications, including solar energy conversion. In any application involving singlet fission, efficient triplet harvesting is essential. At present, not much is known about molecular packing arrangements detrimental to singlet fission. In this work, we report a molecular packing arrangement in crystalline films of 5,14-bis­(triisopropyl­silylethynyl)-substituted pentacene, specifically a local (pairwise) packing arrangement, responsible for complete quenching of triplet pairs generated via singlet fission. We first demonstrate that the energetic condition necessary for singlet fission is satisfied in amorphous films of the 5,14-substituted pentacene derivative. However, while triplet pairs form highly efficiently in the amorphous films, only a modest yield of independent triplets is observed. In crystalline films, triplet pairs also form highly efficiently, although independent triplets are not observed because triplet pairs decay rapidly and are quenched completely. We assign the quenching to a rapid nonadiabatic transition directly to the ground state. Detrimental quenching is observed in crystalline films of two additional 5,14-bis­(trialkyl­silylethynyl)-substituted pentacenes with either ethyl or isobutyl substituents. Developing a better understanding of the losses identified in this work, and associated molecular packing, may benefit overcoming losses in solids of other singlet fission materials