Richard L. Brutchey

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, DPTs 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.

Tin and germanium monochalcogenide IV–VI semiconductor nanocrystals for use in solar cells

The incorporation of colloidal semiconductor nanocrystals into the photoabsorbant material of photovoltaic devices may reduce the production costs of solar cells since nanocrystals can be readily synthesized on a large scale and are solution processable. While the lead chalcogenide IV-VI nanocrystals have been widely studied in a variety of photovoltaic devices, concerns over the toxicity of lead have motivated the exploration of less toxic materials. This has led to the exploration of tin and germanium monochalcogenide IV-VI semiconductors, both of which are made up of earth abundant elements and possess properties similar to the lead chalcogenides. This feature article highlights recent efforts made towards achieving synthetic control over nanocrystal size and morphology of the non-lead containing IV-VI monochalcogenides (i.e., SnS, SnSe, SnTe, GeS and GeSe) and their application toward photovoltaic devices.

Synthesis and Characterization of Wurtzite-Phase Copper Tin Selenide Nanocrystals

A new wurtzite phase of copper tin selenide (CTSe) was discovered, and the resulting nanocrystals were synthesized via a facile solution-phase method. The wurtzite CTSe nanocrystals were synthesized with dodecylamine and 1-dodecanethiol as coordinating solvents and di-tert-butyl diselenide ((t)Bu(2)Se(2)) as the selenium source. Specific reaction control (i.e., a combination of 1-dodecanethiol with (t)Bu(2)Se(2)) was proven to be critical in order to obtain this new phase of CTSe, which was verified by powder X-ray diffraction and selected area electron diffraction. The wurtzite CTSe nanocrystals possess an optical and electrochemical band gap of 1.7 eV and display an electrochemical photoresponse indicative of a p-type semiconductor.

Improving Open Circuit Potential in Hybrid P3HT:CdSe Bulk Heterojunction Solar Cells via Colloidal tert-Butylthiol Ligand Exchange

Organic ligands have the potential to contribute to the reduction potential, or lowest unoccupied molecular orbital (LUMO) energy, of semiconductor nanocrystals. Rationally introducing small, strongly binding, electron-donating ligands should enable improvement in the open circuit potential of hybrid organic/inorganic solar cells by raising the LUMO energy level of the nanocrystal acceptor phase and thereby increasing the energy offset from the polymer highest occupied molecular orbital (HOMO). Hybrid organic/inorganic solar cells fabricated from blends of tert-butylthiol-treated CdSe nanocrystals and poly(3-hexylthiophene) (P3HT) achieved power conversion efficiencies of 1.9%. Compared to devices made from pyridine-treated and nonligand exchanged CdSe, the thiol-treated CdSe nanocrystals are found to consistently exhibit the highest open circuit potentials with V(OC) = 0.80 V. Electrochemical determination of LUMO levels using cyclic voltammetry and spectroelectrochemistry suggest that the thiol-treated CdSe nanocrystals possess the highest lying LUMO of the three, which translates to the highest open circuit potential. Steady-state and time-resolved photoluminescence quenching experiments on P3HT:CdSe films provide insight into how the thiol-treated CdSe nanocrystals also achieve greater current densities in devices relative to pyridine-treated nanocrystals, which are thought to contain a higher density of surface traps.

Ligand Exchange on Colloidal CdSe Nanocrystals Using Thermally Labile tert-Butylthiol for Improved Photocurrent in Nanocrystal Films

As-prepared CdSe nanocrystals were ligand exchanged using tert-butylthiol, which yielded stable CdSe nanocrystal inks in the strong donor solvent tetramethylurea. The efficacy of ligand exchange was probed by thermogravimetric analysis (TGA) and FT-IR spectroscopy. By studying sequential exchanges of tetradecylphosphonic acid and then tert-butylthiol, TGA and energy dispersive X-ray spectroscopic evidence clearly demonstrated that the ligand exchange is essentially quantitative. The resulting tert-butylthiol-exchanged CdSe nanocrystals undergo facile thermal ligand expulsion (≤200 °C), which was studied by TGA-mass spectrometry. Mild thermal treatment of tert-butylthiol-exchanged CdSe nanocrystal films was found to induce loss of quantum confinement (as evidenced by UV-vis spectroscopy) and provided for increased electrochemical photocurrent, electron mobility, and film stability. Pyridine-exchanged CdSe nanocrystals were employed as a control system throughout to demonstrate the beneficial attributes of tert-butylthiol exchange; namely, lower organic content, better colloidal stability, improved interparticle coupling, and vastly increased electrochemical photocurrent response upon illumination.

Alkahest for V2VI3 Chalcogenides: Dissolution of Nine Bulk Semiconductors in a Diamine-Dithiol Solvent Mixture

The ability to solution deposit semiconductor films has received a great deal of recent attention as a way to potentially lower costs for many optoelectronic applications; however, most bulk semiconductors are insoluble in common solvents. Here we describe a novel and relatively nonhazardous binary solvent mixture comprised of 1,2-ethanedithiol and 1,2-ethylenediamine that possesses the remarkable ability to rapidly dissolve a series of nine bulk V2VI3 chalcogenides (V = As, Sb, Bi; VI = S, Se, Te) at room temperature and atmospheric pressure. After solution deposition and low-temperature annealing, the chalcogenides can be fully recovered as good quality, highly crystalline thin films with negligible organic content, as demonstrated for Sb2Se3 and Bi2S3.

Two-phase microfluidic droplet flows of ionic liquids for the synthesis of gold and silver nanoparticles.

Droplet-based microfluidic platforms have the potential to provide superior control over mixing as compared to traditional batch reactions. Ionic liquids have advantageous properties for metal nanoparticle synthesis as a result of their low interfacial tension and complexing ability; however, droplet formation of ionic liquids within microfluidic channels in a two-phase system has not yet been attained because of their complex interfacial properties and high viscosities. Here, breakup of an imidazolium-based ionic liquid into droplets in a simple two-phase system has for the first time been achieved and characterized by using a microchannel modified with a thin film fluoropolymer. This microfluidic/ionic liquid droplet system was used to produce small, spherical gold (4.28 ± 0.84 nm) and silver (3.73 ± 0.77 nm) nanoparticles.

Effect of surface modification on the dielectric properties of BaTiO3 nanocrystals.

We present the first direct comparison of the dielectric properties of organically modified BaTiO(3) nanocrystals with unmodified BaTiO(3) nanocrystals. Well-defined 6 nm BaTiO(3) nanocrystals were functionalized with n-hexylphosphonic acid (HPA) to give a surface coverage of 2.4 phosphonate groups/nm(2). Chemisorption of HPA to the oxide surface occurs mainly via tridentate bonding of the deprotonated phosphonate, as determined by (31)P MAS NMR, FT-IR, and XPS spectroscopies. The HPA-modified BaTiO(3) (HPA-BaTiO(3)) nanocrystals possess improved dielectric properties, as demonstrated by decreased sensitivity to temperature and frequency for both the dielectric constant and dielectric loss. HPA-BaTiO(3) had a much lower dielectric loss than unmodified BaTiO(3), which also indicates an improvement in the dielectric quality of the material. Such improvements are of potential importance for the fabrication of high energy density nanocomposites.

Structural Evolution of BaTiO3 Nanocrystals Synthesized at Room Temperature

Sub-10 nm BaTiO(3) nanocrystals were synthesized at room temperature via the vapor diffusion sol-gel method, and their structural evolution during nucleation and growth stages was followed using a series of techniques that probe the atomic structure on different length and time scales. Special emphasis was placed on assessing the evolution of the local symmetry and structural coherence of the resulting nanocrystals, as these are the structural bases for cooperative properties such as ferroelectricity. Although the room-temperature crystal structure of the fully grown nanocrystals appears cubic to Rietveld analysis of synchrotron X-ray diffraction data, Raman spectroscopy and pair distribution function analysis demonstrate the presence of non-centrosymmetric regions arising from the off-centering of the titanium atoms. This finding demonstrates that accounting for diffuse scattering is critical when attempting the structural characterization of nanocrystals with X-ray diffraction. The local symmetry of acentric regions present in BaTiO(3) nanocrystals, particularly structural correlations within an individual unit cell and between two adjacent unit cells, is best described by a tetragonal P4mm space group. The orthorhombic Amm2 space group also provides an adequate description, suggesting both types of local symmetry can coexist at room temperature. The average magnitude of the local off-center displacements of the titanium atoms along the polar axis is comparable to that observed in bulk BaTiO(3), and their coherence length is on the order of 16 Å. The presence of local dipoles suggests that a large amount of macroscopic polarization can be achieved in nanocrystalline BaTiO(3) if the coherence of their ferroelectric coupling is further increased.

Facile dissolution of selenium and tellurium in a thiol–amine solvent mixture under ambient conditions

Despite their extremely low solubility in most solvents, hexagonal grey selenium and tellurium are shown to be remarkably soluble in binary mixtures of thiols and ethylenediamine (en) at room temperature and ambient pressure. A 1 : 4 vol/vol mixture of ethanethiol (EtSH) and en gave saturated solutions of 38 and 9.3 wt% for grey selenium and tellurium, respectively. Crystalline and phase-pure chalcogen is easily recovered from solution by drying and mild heat treatment at 250 °C (for selenium) or evaporation at room temperature (for tellurium). To demonstrate utility for these dissolved chalcogens, it was shown that elemental antimony readily reacts with the dissolved selenium to give a stable, solution processable Sb–Se precursor solution. In the same way, elemental tin reacts with the dissolved tellurium to generate a Sn–Te precursor solution. Upon solution deposition and heat treatment to 250 °C, these precursor solutions yielded crystalline Sb2Se3 and SnTe.