Defects Cause Subgap Luminescence from a Crystalline Tetracene Derivative.

We use steady-state and ultrafast nonlinear spectroscopies in combination with density functional theory calculations to explain light emission below the optical gap energy (Eo) of crystalline samples of 5,12-diphenyl tetracene (DPT). In particular, the properties of vibrational coherences imprinted on a probe pulse transmitted through a DPT single crystal indicate discrete electronic transitions below Eo of this organic semiconductor. Analysis of coherence spectra leads us to propose structural defect states give rise to these discrete transitions and subgap light emission. We use the polarization dependence of vibrational coherence spectra to tentatively assign these defects in our DPT samples. Our results provide fundamental insights into the properties of midgap states in organic materials important for their application in next-generation photonics and optoelectronics technologies.

Influence of moisture on the preparation, crystal structure, and photophysical properties of organohalide perovskites.

The effect of preparing lead-based organohalide perovskites under inert conditions has been investigated. We find that when prepared under anhydrous conditions, only poorly crystalline powders were obtained. On exposure to small amounts of moisture a rapid crystallization into the expected cubic unit cell for CH3NH3PbBr3 and tetragonal cell for CH3NH3PbI3 is observed. While the as-prepared iodide phase is non-emissive, the lifetime of the emission for the bromide is found to be much longer when prepared under atmospheric conditions.

Aqueous colloidal acene nanoparticles: a new platform for studying singlet fission.

Singlet fission is a process that occurs in select molecular systems wherein a singlet excited state divides its energy to form two triplet excitations on neighboring chromophores. While singlet fission has been largely studied in molecular crystals, colloidal nanoparticles offer the ability to investigate fission using liquid suspensions, allowing questions regarding the importance of molecular arrangement and charge transfer to be assessed. Herein, we report the synthesis of aqueous colloidal nanoparticles of 5,12-diphenyltetracene (DPT), a material recently demonstrated to undergo singlet fission in disordered films. Upon synthesis, nanoparticles display absorption features that lie between those of monomeric DPT and disordered DPT films. These features evolve over a few days in a manner that suggests an increase in the degree of association between neighboring molecules within the nanoparticles. Transient absorption and time-resolved emission experiments indicate that photoexcited DPT nanoparticles undergo fission, but produce a lower triplet yield than disordered films.

Efficient singlet fission discovered in a disordered acene film.

Singlet exciton fission is a process that occurs in select organic semiconductors and entails the splitting of a singlet excited state into two lower triplet excitons located on adjacent chromophores. Research examining this phenomenon has recently seen a renaissance due to the potential to exploit singlet fission within the context of organic photovoltaics to prepare devices with the ability to circumvent the Shockley-Queisser limit. To date, high singlet fission yields have only been reported for crystalline or polycrystalline materials, suggesting that molecular disorder inhibits singlet fission. Here, we report the results of ultrafast transient absorption and time-resolved emission experiments performed on 5,12-diphenyl tetracene (DPT). Unlike tetracene, which tends to form polycrystalline films when vapor deposited, DPT's pendant phenyl groups frustrate crystal growth, yielding amorphous films. Despite the high level of disorder in these films, we find that DPT exhibits a surprisingly high singlet fission yield, with 1.22 triplets being created per excited singlet. This triplet production occurs over two principal time scales, with ~50% of the triplets appearing within 1 ps after photoexcitation followed by a slower phase of triplet growth over a few hundred picoseconds. To fit these kinetics, we have developed a model that assumes that due to molecular disorder, only a subset of DPT dimer pairs adopt configurations that promote fission. Singlet excitons directly excited at these sites can undergo fission rapidly, while singlet excitons created elsewhere in the film must diffuse to these sites to fission.

Observation of Triplet Exciton Formation in a Platinum-Sensitized Organic Photovoltaic Device.

Organic photovoltaics (OPVs) constitute a promising new technology due to their low production costs. However, OPV efficiencies remain low because excitons typically diffuse only ∼5-20 nm during their lifetime, limiting the effective thickness of the light-absorbing layer. One strategy to improve OPVs is to increase exciton lifetimes by converting them into triplet states, which typically persist 10(3)-10(5) times longer than singlet excitons. We present femtosecond transient absorption and steady-state photovoltaic measurements of a model OPV system consisting of diphenyltetracene (DPT) films doped with platinum tetraphenylbenzoporphyrin (Pt(TPBP)). Photoexcitation of Pt(TPBP) creates a singlet excitation that rapidly intersystem crosses to a triplet state before transferring to the DPT host matrix. This transfer is rapid and efficient, occurring in 35 ps with an 85% conversion ratio of porphyrin singlets to DPT triplets. These triplet excitons lead to enhanced photocurrent response that increases with device thickness.