Reduced Recombination and Capacitor-like Charge Buildup in an Organic Heterojunction

Organic photovoltaic (OPV) efficiencies continue to rise, raising their prospects for solar energy conversion. However, researchers have long considered how to suppress the loss of free carriers by recombination—poor diffusion and significant Coulombic attraction can cause electrons and holes to encounter each other at interfaces close to where they were photogenerated. Using femtosecond transient spectroscopies, we report the nanosecond grow-in of a large transient 20 Stark effect, caused by nanoscale electric fields of ~487 kV/cm between photogenerated free carriers in the device active layer. We find that particular morphologies of the active layer lead to an energetic cascade for charge carriers, suppressing pathways to recombination, which is ~2000 times less than predicted by Langevin theory. This in turn leads to the build-up of electric charge in donor and acceptor domains—away from the interface—resistant to bimolecular recombination. 25 Interestingly, this signal is only experimentally obvious in thick films, due to the different scaling of electro-absorption and photo-induced absorption signals in transient absorption spectroscopy. Rather than inhibiting device performance, we show that devices up to 600 nm thick maintain efficiencies of > 8 % because domains can afford much higher carrier densities. These observations suggest that with particular nanoscale morphologies, the bulk heterojunction can go beyond its established role in charge photogeneration, and can act as a capacitor, where adjacent free charges are held away from the interface and can be protected from bimolecular recombination

Organizing crystalline functionalized pentacene using periodicity of poly(vinyl alcohol)

Nanoparticles of acenes exhibit highly efficient intermolecular singlet fission (SF). Recent reports indicate that altering the morphology of 6,13-(triisopropylsilylethynyl)pentacene (TIPS-Pn) nanoparticles has a profound influence on their SF dynamics. Here, we show that poly(vinyl alcohol) (PVA) induces a phase transition in pre-formed TIPS-Pn nanoparticles. These nanoparticles are amorphous when initially formed, but crystalline after addition of PVA. Surface characterization indicates that a diffuse PVA layer surrounds the nanoparticles. We propose that a periodic interaction between the hydroxyl groups of PVA and TIPS groups of TIPS-Pn on the nanoparticle surface induces a large-scale structural rearrangement to yield crystalline TIPS-Pn. Such reorganization in pre-formed organic nanoparticles is unprecedented, and we believe that this is the first report of such an effect induced by polymer adsorption. Transient absorption spectroscopic results reveal that SF within these nanoparticles is accelerated by an order of magnitude upon structural rearrangement.Rohan J. Hudson, Jessica M. de la Perrelle, Ryan D. Pensack, Bryan Kudisch, Gregory D. Scholes, David M. Huang, Tak W. Ke

Polariton Transitions in Femtosecond Transient Absorption Studies of Ultrastrong Light-Molecule Coupling

Strong light-matter coupling is emerging as a fascinating way to tune optical properties and modify the photophysics of molecular systems. In this work, we studied a molecular chromophore under strong coupling with the optical mode of a Fabry-Perot cavity resonant to the first electronic absorption band. Using femtosecond pump-probe spectroscopy, we investigated the transient response of the cavity-coupled molecules upon photoexcitation resonant to the upper and lower polaritons. We identified an excited state absorption from upper and lower polaritons to a state at the energy of the second cavity mode. Quantum mechanical calculations of the many-molecule energy structure of cavity polaritons suggest assignment of this state as a two-particle polaritonic state with optically allowed transitions from the upper and lower polaritons. We provide new physical insight into the role of two-particle polaritonic states in explaining transient signatures in hybrid light-matter coupling systems consistent with analogous many-body systems

Ring currents modulate optoelectronic properties of aromatic chromophores at 25 T

The properties of organic molecules can be influenced by magnetic fields, and these magnetic field effects are diverse. They range from inducing nuclear Zeeman splitting for structural determination in NMR spectroscopy to polaron Zeeman splitting organic spintronics and organic magnetoresistance. A pervasive magnetic field effect on an aromatic molecule is the aromatic ring current, which can be thought of as an induction of a circular current of π-electrons upon the application of a magnetic field perpendicular to the π-system of the molecule. While in NMR spectroscopy the effects of ring currents on the chemical shifts of nearby protons are relatively well understood, and even predictable, the consequences of these modified electronic states on the spectroscopy of molecules has remained unknown. In this work, we find that photophysical properties of model phthalocyanine compounds and their aggregates display clear magnetic field dependences up to 25 T, with the aggregates showing more drastic magnetic field sensitivities depending on the intermolecular interactions with the amplification of ring currents in stacked aggregates. These observations are consistent with ring currents measured in NMR spectroscopy and simulated in time-dependent density functional theory calculations of magnetic field-dependent phthalocyanine monomer and dimer absorption spectra. We propose that ring currents in organic semiconductors, which commonly comprise aromatic moieties, may present new opportunities for the understanding and exploitation of combined optical, electronic, and magnetic properties

Binary small molecule organic nanoparticles exhibit both direct and diffusion-limited ultrafast charge transfer with NIR excitation

Here we describe a facile, one-step synthesis of a binary organic nanoparticle composed completely of NIR-absorbing small molecules, a quatterylene diimide and a vanadyl napthalocyanine, using Flash Nanoprecipitation. We show that the molecules are co-encapsulated within an amphiphilic block copolymer shell by observing distinct ultrafast dynamics in the binary nanoparticles compared to nanoparticles of their individual components, which we rationalize as a photoinduced charge transfer. We then draw similarities between the charge transfer dynamics studied in our system and the charge dissociation process in macroscale organic bulk heterojunction blends for OPV applications by assigning the ultrafast time component (∼10 ps) to direct interfacial charge transfer and the slow component (70-200 ps) to diffusion limited charge transfer. This discovery can inspire the development of mixed-composition nanoparticles with new functionality for optoelectronic and theranostic applications

Ultrafast Dynamics of Nonrigid Zinc-Porphyrin Arrays Mimicking the Photosynthetic "Special Pair"

Conjugated porphyrin arrays are heavily investigated as efficient molecular systems for photosynthesis and photocatalysis. Recently, a series of one-, two-, and six-zinc-porphyrin arrays, noncovalently linked through benzene-based hubs, have been synthesized with the aim of mimicking the structure and function of the bacteriochlorophyll "special pair" in photosynthetic reaction centers. The excitonically coupled porphyrin subunits are expected to activate additional excited state relaxation channels with respect to the monomer. Here, we unveil the appearance of such supramolecular electronic interactions using ultrafast transient absorption spectroscopy with sub-25 fs time resolution. Upon photoexcitation of the Soret band, we resolve energy trapping within ∼150 fs in a delocalized dark excitonic manifold. Moreover, excitonic interactions promote an additional fast internal conversion from the Q-band to the ground state with an efficiency of up to 60% in the hexamer. These relaxation pathways appear to be common loss channels that limit the lifetime of the exciton states in noncovalently bound molecular aggregates

Long-term intraocular pressure reduction with intracameral polycaprolactone glaucoma devices that deliver a novel anti-glaucoma agent

Long-term treatment of glaucoma, a major leading cause of blindness, is challenging due to poor patient compliance. Therefore, a drug delivery device that can achieve drug release over several months can be highly beneficial for glaucoma management. Here, we evaluate the long-term pharmacokinetics and therapeutic efficacy of polycaprolactone intracameral drug delivery devices in rabbit eyes. Our study showed that a single drug delivery device loaded with a proprietary hypotensive agent, DE-117, reduced intraocular pressure in normotensive rabbits significantly for 23weeks. In addition, we demonstrated that concentration of DE-117 and its hydrolyzed active form (hDE-117) was maintained in the aqueous humor and the target tissue (iris-ciliary body) up to 24weeks. Our proof-of-concept glaucoma implant shows potential as a long-term treatment that circumvents patient compliance barriers compared to current treatment via eye drops

In vivo and in vitro sustained release of ranibizumab from a nanoporous thin-film device.

Current administration of ranibizumab and other therapeutic macromolecules to the vitreous and retina carries ocular risks, a high patient treatment burden, and compliance barriers that can lead to suboptimal treatment. Here we introduce a device that produces sustained release of ranibizumab in the vitreous cavity over the course of several months. Composed of twin nanoporous polymer thin films surrounding a ranibizumab reservoir, these devices provide release of ranibizumab over 16 weeks in vitro and 12 weeks in vivo, without exhausting the initial drug payload. Following implantation in vivo, devices were well-tolerated and showed no sign of immune response. This platform presents a potential solution to the challenge of delivering protein therapeutics to the vitreous and retina for sustained periods of time