Matthew J. Crane

Beyond Fullerenes: Design of Nonfullerene Acceptors for Efficient Organic Photovoltaics

New electron-acceptor materials are long sought to overcome the small photovoltage, high-cost, poor photochemical stability, and other limitations of fullerene-based organic photovoltaics. However, all known nonfullerene acceptors have so far shown inferior photovoltaic properties compared to fullerene benchmark [6,6]-phenyl-C60-butyric acid methyl ester (PC60BM), and there are as yet no established design principles for realizing improved materials. Herein we report a design strategy that has produced a novel multichromophoric, large size, nonplanar three-dimensional (3D) organic molecule, DBFI-T, whose π-conjugated framework occupies space comparable to an aggregate of 9 [C60]-fullerene molecules. Comparative studies of DBFI-T with its planar monomeric analogue (BFI-P2) and PC60BM in bulk heterojunction (BHJ) solar cells, by using a common thiazolothiazole-dithienosilole copolymer donor (PSEHTT), showed that DBFI-T has superior charge photogeneration and photovoltaic properties; PSEHTT:DBFI-T solar cells combined a high short-circuit current (10.14 mA/cm(2)) with a high open-circuit voltage (0.86 V) to give a power conversion efficiency of 5.0%. The external quantum efficiency spectrum of PSEHTT:DBFI-T devices had peaks of 60-65% in the 380-620 nm range, demonstrating that both hole transfer from photoexcited DBFI-T to PSEHTT and electron transfer from photoexcited PSEHTT to DBFI-T contribute substantially to charge photogeneration. The superior charge photogeneration and electron-accepting properties of DBFI-T were further confirmed by independent Xenon-flash time-resolved microwave conductivity measurements, which correctly predict the relative magnitudes of the conversion efficiencies of the BHJ solar cells: PSEHTT:DBFI-T > PSEHTT:PC60BM > PSEHTT:BFI-P2. The results demonstrate that the large size, multichromophoric, nonplanar 3D molecular design is a promising approach to more efficient organic photovoltaic materials.

Laser refrigeration of hydrothermal nanocrystals in physiological media

Significance Although the laser refrigeration of bulk crystals has recently shown to cool below cryogenic temperatures (∼90 K) in vacuum, to date the laser refrigeration of physiological media has not been reported. In this work, a low-cost hydrothermal synthetic approach is used to prepare nanocrystals that are capable of locally refrigerating physiological buffers (PBS, DMEM) upon near-infrared illumination. Optical tweezers are used in tandem with cold Brownian motion analysis to observe the refrigeration of individual (Yb3+)-doped nanocrystals >10 °C below ambient conditions. The ability to optically generate local refrigeration fields around individual nanocrystals promises to enable precise optical temperature control within integrated electronic/photonic/microfluidic circuits, and also thermal modulation of basic biomolecular processes, including the dynamics of motor proteins. Coherent laser radiation has enabled many scientific and technological breakthroughs including Bose–Einstein condensates, ultrafast spectroscopy, superresolution optical microscopy, photothermal therapy, and long-distance telecommunications. However, it has remained a challenge to refrigerate liquid media (including physiological buffers) during laser illumination due to significant background solvent absorption and the rapid (∼ps) nonradiative vibrational relaxation of molecular electronic excited states. Here we demonstrate that single-beam laser trapping can be used to induce and quantify the local refrigeration of physiological media by >10 °C following the emission of photoluminescence from upconverting yttrium lithium fluoride (YLF) nanocrystals. A simple, low-cost hydrothermal approach is used to synthesize polycrystalline particles with sizes ranging from <200 nm to >1 μm. A tunable, near-infrared continuous-wave laser is used to optically trap individual YLF crystals with an irradiance on the order of 1 MW/cm2. Heat is transported out of the crystal lattice (across the solid–liquid interface) by anti-Stokes (blue-shifted) photons following upconversion of Yb3+ electronic excited states mediated by the absorption of optical phonons. Temperatures are quantified through analysis of the cold Brownian dynamics of individual nanocrystals in an inhomogeneous temperature field via forward light scattering in the back focal plane. The cold Brownian motion (CBM) analysis of individual YLF crystals indicates local cooling by >21 °C below ambient conditions in D2O, suggesting a range of potential future applications including single-molecule biophysics and integrated photonic, electronic, and microfluidic devices.

Annealing temperature dependence of the efficiency and vertical phase segregation of polymer/polymer bulk heterojunction photovoltaic cells

We report observation of annealing temperature-induced simultaneous vertical phase segregation and large enhancement of power conversion efficiency (PCE) of all-polymer bulk heterojunction (BHJ) solar cells composed of a poly(3-hexylthiophene) (P3HT) donor and a naphthalene diimide-selenolo[3,2-b]selenophene copolymer (PNDISS) acceptor. The PCE of P3HT:PNDISS BHJ devices increased over 50-fold from 0.04% to 2.03% when the annealing temperature was increased from 50 to 150 °C. Absorption spectroscopy and photoluminescence quenching experiments provide evidence of increasing phase segregation of the polymer/polymer blend films with increasing annealing temperature. Field-effect charge transport, contact angle, surface energy, and variable angle ellipsometry measurements on the P3HT:PNDISS blend films showed that thermal annealing induced vertical phase segregation, whereby the low surface energy polymer (P3HT) migrated to the bulk, while the high surface energy polymer (PNDISS) enriches at the substrate/ble...

Rapid synthesis of transition metal dichalcogenide–carbon aerogel composites for supercapacitor electrodes

Transition metal dichalcogenide (TMD) materials have recently demonstrated exceptional supercapacitor properties after conversion to a metallic phase, which increases the conductivity of the network. However, freestanding, exfoliated transition metal dichalcogenide films exhibit surface areas far below their theoretical maximum (1.2 %), can fail during electrochemical operation due to poor mechanical properties, and often require pyrophoric chemicals to process. On the other hand, pyrolyzed carbon aerogels exhibit extraordinary specific surface areas for double layer capacitance, high conductivity, and a strong mechanical network of covalent chemical bonds. In this paper, we demonstrate the scalable, rapid nanomanufacturing of TMD (MoS2 and WS2) and carbon aerogel composites, favoring liquid-phase exfoliation to avoid pyrophoric chemicals. The aerogel matrix support enhances conductivity of the composite and the synthesis can complete in 30 min. We find that the addition of transition metal dichalcogenides does not impact the structure of the aerogel, which maintains a high specific surface area up to 620 m2 g−1 with peak pore radii of 10 nm. While supercapacitor tests of the aerogels yield capacitances around 80 F g−1 at the lowest applied currents, the aerogels loaded with TMD’s exhibit volumetric capacitances up to 127% greater than the unloaded aerogels. In addition, the WS2 aerogels show excellent cycling stability with no capacitance loss over 2000 cycles, as well as markedly better rate capability and lower charge transfer resistance compared to their MoS2-loaded counterparts. We hypothesize that these differences in performance stem from differences in contact resistance and in the favorability of ion adsorption on the chalcogenides.

Photothermal Heating and Cooling of Nanostructures

A vast range of insulating, semiconducting, and metallic nanomaterials have been studied over the past several decades with the aim of understanding how continuous-wave or pulsed laser radiation can influence their chemical functionality and local environment. Many fascinating observations have been made during laser irradiation including, but not limited to, the superheating of solvents, mass-transport-mediated morphology evolution, photodynamic therapy, morphology dependent resonances, and a range of phase transformations. In addition to laser heating, recent experiments have demonstrated the laser cooling of nanoscale materials through the emission of upconverted, anti-Stokes photons by trivalent rare-earth ions. This Focus Review outlines the analytical modeling of photothermal heat transport with an emphasis on the experimental validation of anti-Stokes laser cooling. This general methodology can be applied to a wide range of photothermal applications, including nanomedicine, photocatalysis, and the synthesis of new materials. The review concludes with an overview of recent advances and future directions for anti-Stokes cooling.

Laser-refrigeration of rare-earth-doped nanocrystals in water

Single-beam laser-tweezers have been demonstrated over the past several decades to confine nanometer-scale particles in three dimensions with sufficient sensitivity to measure the spring constants of individual biological macromolecules including DNA. Large laser-irradiance values (on the order of MW/cm2) commonly are used to generate laser traps which can lead to significant laser-heating within the 3D optical potential well. To date, laser-refrigeration of particles within an aqueous medium has not been reported stemming primarily from the large near-infrared (NIR) optical absorption coefficient of liquid water (0.2 cm-1 at lambda = 1020nm). In this paper we will detail the methods on how single-beam laser-traps can be used to induce and quantify the refrigeration of optically trapped nanocrystals in an aqueous medium. Analysis of the Brownian dynamics of individual nanocrystals via forward light scattering provides a way to determine both a relative and absolute measurement of particle’s temperature. Signal analysis considerations to interpreting Brownian motion data of trapped particles in nonisothermal aqueous environments, or so-called hot Brownian motion, are detailed. Applications of these methods to determining local laser-refrigeration of laser trapped nanoparticles in water show promise at realizing the first observation of particles undergoing cold Brownian motion.